Variable oil flow of gas turbine

Abstract

The invention discloses a method 200 of operating a gas turbine engine for an aircraft comprising: an engine core comprising a turbine, a compressor, a combustion chamber for burning fuel, and a core shaft connecting the turbine to the compressor; a fan located upstream of the engine core; a reduction gear which receives an input from the core shaft and provides a drive to the fan; an oil loop system for supplying oil to the reduction gear; and a heat exchange system comprising an air-oil heat exchanger through which oil flows; a fuel-oil heat exchanger through which oil and fuel flow;and a valve for varying a proportion of the oil sent through at least one of the heat exchangers, the method 200 comprising controlling 202 the valve such that, under idle conditions, an oil flow ratio of: is in the range of 0.62 to 5.29. Figure for abstract: Fig. 13;

Description

Title of the invention: Variable oil flow of gas turbine

[0001] The present description relates to aircraft propulsion systems, as well as to methods of operating aircraft involving the management of different fluids and the transfer of heat between them, and in particular to the management of oil and / or air flows within an aircraft engine.

[0002] There is an expectation in the aviation industry regarding a trend towards the use of fuels different from the traditional kerosene-based jet fuels generally used today. The fuels may have different fuel characteristics compared to petroleum-based hydrocarbon fuels. Thus, there is a need to take into account fuel properties for these new fuels and to adjust both the own gas turbine engines and the operating processes of the gas turbine engines.

[0003] According to a first aspect, a method of operating a gas turbine engine of an aircraft is provided, the gas turbine engine comprising:

[0004] an engine core comprising a turbine, a compressor, a combustion chamber for burning fuel, and a core shaft connecting the turbine to the compressor;

[0005] a blower located upstream of the engine core;

[0006] a reducer which receives an input from the core shaft and provides a drive to the blower so as to drive the blower at a lower rotational speed than the core shaft;

[0007] an oil loop system for supplying oil to the reducer; and

[0008] a heat exchange system comprising:

[0009] an air-oil heat exchanger through which the oil in the oil loop system flows;

[0010] a fuel-oil heat exchanger through which the oil in the oil loop system and the fuel flow such that heat is transferred between the oil and the fuel; and

[0011] at least one valve making it possible to vary the proportion of oil sent via at least one of the heat exchangers.

[0012] The method comprises controlling the at least one valve such that, under cruising conditions, an oil flow ratio of:

[0013] oil flow rate in the air-oil heat exchanger (m3s4 or legs-') oil flow rate in the fuel-oil heat exchanger (mV1 or kgsr1)

[0014] is in the range of 0 to 0.59.

[0015] The inventors were aware that the use of fuels different from traditional kerosene-based jet fuels, such as sustainable aviation fuels, may lead to different fuel properties, and that cruise parameters may be adjusted to utilize the different fuel properties. In particular, certain fuels may be heated to higher temperatures in one or more fuel-oil heat exchangers than traditional fuels, without significantly increasing the deposition of fuel degradation products, for example, by coking or varnishing.This can enable a process providing improved oil cooling (the fuel being able to absorb more heat) and can also improve the overall thermal / thermodynamic efficiency of the engine, with less heat lost to the environment and potentially also more power recovered in the thermodynamic cycle. Controlling the oil flow in the engine has a key role to play in managing heat transfer. The inventors were aware that these principles can be applied both to engines with branched oil loop paths with different heat exchangers on different branches (parallel arrangements) and to engines with serial, substantially linear arrangements of heat exchangers and in which the use of one or more bypass pipes is made as an alternative to a main branch path.The introduction of one or more controllable oil valves and / or improved control of existing oil valves may therefore enable a method providing improved oil cooling (since the fuel to be used can absorb more heat than traditional fuels) and may also improve the overall thermal efficiency of the engine. The controllable oil valve(s) allows the oil flow ratio to be adjusted as appropriate for a given fuel.

[0016] Although the oil flow ratio is dimensionless, units (cubic meters per second or kilograms per second) are given above as examples. The numerator and denominator must have the same units to provide a dimensionless ratio. The flow rates used can therefore be either volumetric or gravimetric (i.e., mass) flow rates, provided that the numerator and denominator are consistent.

[0017] The oil loop system may branch such that a proportion of the oil may flow along each branch. The air-oil and fuel-oil heat exchangers may be arranged in a parallel configuration on different branches of an oil loop system. The at least one valve for varying the proportion of oil sent through at least one of the heat exchangers may be or include a modulating valve for to allow the proportion of oil sent through each branch to be varied.

[0018] The oil loop system may include at least one bypass pipe for allowing a proportion of the oil to bypass at least one of the air-to-oil heat exchanger and the fuel-to-oil heat exchanger. The at least one valve for allowing the proportion of the oil sent through at least one of the heat exchangers to be varied may be or include a bypass valve for allowing a proportion of the oil to bypass the at least one heat exchanger. The heat exchange system may include multiple bypass pipes each arranged to allow the oil to bypass a heat exchanger - in such implementations, controlling the at least one valve may be or include controlling at least two bypass valves.In particular, the oil loop system may include at least one bypass pipe for allowing a proportion of the oil to bypass the air-oil heat exchanger.

[0019] The air-oil and fuel-oil heat exchangers of the implementations with one or more bypass pipes may be arranged in series in the oil loop system (such that a main oil flow path passes through both, one after the other) or in parallel in the oil loop system (such that a main oil flow path splits, with a branch passing through each). In implementations in which the air-oil and fuel-oil heat exchangers are arranged in parallel, on different branches of the oil loop system, the step of controlling the at least one valve may comprise controlling one or more bypass valves and a modulating valve to allow the proportion of oil sent through each branch to be varied.

[0020] The method may include controlling the at least one valve such that, under cruising conditions, the oil flow ratio is in the range of 0 to 0.50, optionally 0 to 0.40, and further optionally 0 to 0.30, 0 to 0.20, or optionally 0 to 0.10.

[0021] The step of controlling the at least one valve so as to adjust the oil flow ratio may comprise decreasing the amount of oil sent through the at least one air-oil heat exchanger when the oil flow ratio is too high. Additionally or alternatively, the amount of oil sent through the at least one fuel-oil heat exchanger may be increased when the oil flow ratio is too high.

[0022] The method may comprise determining the fuel temperature downstream of the fuel-oil heat exchanger (and optionally at the inlet of the combustion chamber) and adjusting the oil flow ratio control - if necessary - as a function of this fuel temperature. The method may comprise controlling the at least one valve under cruising conditions such that:

[0023] (i) the oil flow ratio is in the range of 0 to 0.4, optionally from 0 to 0.3, and possibly also between 0 and 0.25, provided that the fuel temperature downstream of the fuel-oil heat exchanger is at least 140°C;

[0024] (ii) the oil flow ratio is in the range from 0 to 0.3, optionally from 0 to 0.2, and further optionally between 0 and 0.15, provided that the fuel temperature downstream of the fuel-oil heat exchanger is at least 160°C; and / or

[0025] (iii) the oil flow ratio is in the range from 0 to 0.2, optionally from 0 to 0.1, and further optionally between 0 and 0.075, provided that the fuel temperature downstream of the fuel-oil heat exchanger is at least 180°C.

[0026] The method may include determining one or more fuel characteristics and adjusting the oil flow ratio control - if necessary - based on the determined one or more fuel characteristics. For example, the method may include controlling the at least one valve such that the oil flow ratio is in the range of 0 to 0.3, optionally 0 to 0.2, and further optionally 0 to 0.15, provided that the fuel is composed of at least 70% sustainable aviation fuel (SAF). The proportion of SAF (X%) may be volumetric.

[0027] The heat exchange system may include a refrigeration cycle apparatus for providing thermal elevation by transferring additional heat from the oil to the fuel beyond that transferred by the fuel-oil heat exchanger. The method may include controlling the refrigeration cycle apparatus to adjust the amount of additional heat transferred to the fuel. In this case, the fuel temperature may be raised above the oil temperature.

[0028] According to a second aspect, there is provided a gas turbine engine for an aircraft, comprising:

[0029] an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor;

[0030] a fan located upstream of the engine core; and

[0031] a reducer that receives an input from the core shaft and provides a drive to the blower so as to drive the blower at a lower rotational speed than the core shaft;

[0032] an oil loop system for supplying oil to the reducer; and

[0033] a heat exchange system comprising:

[0034] an air-oil heat exchanger through which the oil in the oil loop system flows;

[0035] a fuel-oil heat exchanger through which the oil in the oil loop system and the fuel flow such that heat is transferred between the oil and the fuel; and

[0036] at least one valve making it possible to vary the proportion of oil sent via at least one of the heat exchangers.

[0037] The at least one valve is intended to be controlled such that, under cruising conditions, an oil flow ratio of:

[0038] oil flow rate in 1 air-oil heat exchanger (m^'1 or kgr1) oil flow rate in 1 fuel-oil heat exchanger (mV1 or kgr1)

[0039] is in the range of 0 to 0.59.

[0040] The gas turbine engine may further comprise a controller for controlling the valve. The gas turbine engine may further comprise one or more oil flow sensors. The controller may be configured to receive outputs from the one or more oil flow sensors and to make control decisions based on those outputs. The oil flow rate may be detected directly or inferred from one or more other measurements, e.g., using pressure drop measurements across an orifice.

[0041] The heat exchange system may further include a refrigeration cycle apparatus for providing thermal elevation by transferring more heat from the oil to the fuel, possibly such that the fuel temperature is raised above the oil temperature.

[0042] The heat exchange system may further include branched fuel return paths and at least one valve controlling a fuel flow split, the branched paths being configured to return fuel from the heat exchange system to at least two different locations along a main fuel path from the fuel inlet in the gas turbine engine to the combustion chamber.

[0043] The turbine may be a first turbine, the compressor may be a first compressor, and the core shaft may be a first core shaft. The engine core may further comprise a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor. The second turbine, the second compressor, and the second core shaft may be configured to rotate at a higher rotational speed than the first core shaft.

[0044] The motor of the second aspect may be for carrying out the method of the first aspect, and may have any of the characteristics described in relation to the first aspect.

[0045] According to a third aspect, a method of operating a gas turbine engine on an aircraft is provided, the gas turbine engine comprising:

[0046] an engine core comprising a turbine, a compressor, a combustion chamber for burning fuel, and a core shaft connecting the turbine to the compressor;

[0047] a blower located upstream of the engine core;

[0048] a reducer that receives an input from the core shaft and provides a drive to the blower so as to drive the blower at a lower rotational speed than the core shaft;

[0049] an oil loop system for supplying oil to the reducer; and

[0050] a heat exchange system comprising:

[0051] an air-oil heat exchanger through which the oil in the oil loop system flows;

[0052] a fuel-oil heat exchanger through which the oil in the oil loop system and the fuel flow such that heat is transferred between the oil and the fuel; and

[0053] at least one valve making it possible to vary the proportion of oil sent via at least one of the heat exchangers.

[0054] The method comprises controlling the at least one valve such that, under idle conditions, an oil flow ratio of:

[0055] oil flow in 1 air-oil heat exchanger (nps-1 or kg.v!) oil flow in 1 fuel-oil heat exchanger (mV1 or kg.v9

[0056] is in the range of 0.62 to 5.29.

[0057] As mentioned for the first aspect, the inventors were aware that the use of fuels different from traditional kerosene-based jet fuels, such as sustainable aviation fuels, may lead to different fuel properties, and that operating parameters may be adjusted to utilize the different fuel properties. In particular, certain fuels may be heated to higher temperatures in one or more fuel-oil heat exchangers than traditional fuels, without significantly increasing the deposition of fuel degradation products, for example, by coking or veering. This may allow a process providing improved oil cooling (the fuel being able to absorb more heat) and may also improve the overall thermal efficiency of the engine, with less heat lost to the environment. The oil valve(s) controllable have a key role to play in managing the oil flow ratio. Furthermore, the inventors were aware that while cruise conditions typically represent a much larger proportion of an aircraft engine's operating time, idling operation is also significant. Since the fuel mass flow rate is much lower at idle than at cruise, even a relatively low heat load on the fuel can result in a large temperature increase - the use of non-traditional fuels may therefore have an even greater effect on optimal approaches to heat management under idle conditions - for example, at ground idle, during aircraft start-up, during aircraft stationary operation during boarding and taxiing (to a runway or hangar, or between other ground locations), or at in-flight idle, such as during the initiation of descent.Since the operating conditions are very different between cruising and idling - particularly in terms of the desired thrust output of the engine - the oil flow must be regulated differently.

[0058] With respect to the first and second aspects, although the oil flow ratio is dimensionless, units (cubic meters per second or kilograms per second) are provided above for example and to demonstrate that the numerator and denominator should have the same units. Idling operation when the aircraft is operating on the ground may be referred to as "ground idle." Idling operation when the aircraft is in flight may be referred to as "flight idle." All of the options described below for this aspect may be considered relevant for ground idle conditions unless otherwise stated. Flight idle is typically at slightly higher thrust than ground idle. In some implementations, only the least restrictive ranges may apply to flight idle for a particular engine.In other implementations, all of the options described below for this aspect may also apply in in-flight idle conditions.

[0059] The oil loop system may branch such that a proportion of the oil may flow along each branch. The air-oil and fuel-oil heat exchangers may be arranged in a parallel configuration on different branches of an oil loop system. The at least one valve for varying the proportion of oil sent through at least one of the heat exchangers may be or include a modulating valve for varying the proportion of oil sent through each branch.

[0060] The oil loop system may include at least one bypass pipe for allowing a proportion of the oil to bypass at least one of the air-oil heat exchanger and the fuel-oil heat exchanger. The at least one valve for allowing variation of the proportion of oil sent through at least one of the heat exchangers may be or comprise a bypass valve for allowing a proportion of the oil to bypass the at least one heat exchanger. The heat exchange system may comprise multiple bypass pipes each arranged to allow oil to bypass a heat exchanger - in such implementations, controlling the at least one valve may be or comprise controlling at least two bypass valves.

[0061] The air-oil and fuel-oil heat exchangers of the implementations with one or more bypass pipes may be arranged in series in the oil loop system (such that a main oil flow path passes through both, one after the other) or in parallel in the oil loop system (such that a main oil flow path splits, with a branch passing through each). In implementations in which the air-oil and fuel-oil heat exchangers are arranged in parallel, on different branches of the oil loop system, the step of controlling the at least one valve may comprise controlling one or more bypass valves and a modulating valve to allow the proportion of oil sent through each branch to be varied.

[0062] The method may include controlling the at least one valve such that, under idle conditions, the oil flow ratio is less than 5.50, optionally less than 5.0, optionally less than 4.5, and further optionally less than 4.0.

[0063] The step of controlling the at least one valve to adjust the oil flow ratio may include decreasing the amount of oil sent through the at least one air-oil heat exchanger when the oil flow ratio is too high.

[0064] The method may comprise determining the fuel temperature downstream of the fuel-oil heat exchanger (and optionally at the inlet to the combustion chamber) and adjusting the oil flow ratio control - if necessary - as a function of this fuel temperature. The method may comprise controlling the at least one valve under idle conditions such that:

[0065] (i) the oil flow ratio is in the range of 0.62 to 4.00 when the fuel temperature downstream of the fuel-oil heat exchanger is above 140°C;

[0066] (ii) the oil flow ratio is in the range of 0.62 to 3.00 when the fuel temperature downstream of the fuel-oil heat exchanger is above 160°C; and / or

[0067] (iii) the oil flow ratio is in the range of 0.62 to 2.00 when the fuel temperature downstream of the fuel-oil heat exchanger is above 180°C.

[0068] The method may include determining one or more fuel characteristics and adjusting the oil flow ratio control - if necessary - based on the determined one or more fuel characteristics. For example, the method may include, under idle conditions, controlling the at least one valve such that the oil flow ratio is in the range of 0.62 to 3.67, provided that the fuel is comprised of at least 70% sustainable aviation fuel (SAF).

[0069] The heat exchange system may include a refrigeration cycle apparatus for providing thermal elevation by transferring additional heat from the oil to the fuel beyond that transferred by the fuel-oil heat exchanger. The method may include controlling the refrigeration cycle apparatus to adjust the amount of additional heat transferred to the fuel. In this case, the fuel temperature may be raised above the oil temperature.

[0070] The methods of the first and third aspects may be complementary, and may be performed together in various implementations. The method of the third aspect may be performed using the engine of the second aspect.

[0071] According to a fourth aspect, a gas turbine engine for an aircraft is provided, the gas turbine engine comprising:

[0072] an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor;

[0073] a fan located upstream of the engine core; and

[0074] a reducer that receives an input from the core shaft and provides a drive to the blower so as to drive the blower at a lower rotational speed than the core shaft;

[0075] an oil loop system for supplying oil to the reducer; and

[0076] a heat exchange system comprising:

[0077] an air-oil heat exchanger through which the oil in the oil loop system flows;

[0078] a fuel-oil heat exchanger through which the oil in the oil loop system and the fuel flow such that heat is transferred between the oil and the fuel; and

[0079] at least one valve making it possible to vary the proportion of oil sent via at least one of the heat exchangers.

[0080] The at least one valve is intended to be controlled such that, under idle conditions, an oil flow ratio of:

[0081] oil flow rate in the air-oil heat exchanger (m¥ or kgv'l oil flow rate in the fuel-oil heat exchanger (m3s-' or kgrO

[0082] is in the range of 0.62 to 5.29.

[0083] The heat exchange system may further include a refrigeration cycle apparatus for providing thermal elevation by transferring more heat from the oil to the fuel, possibly such that the fuel temperature is raised above the oil temperature.

[0084] The turbine may be a first turbine, the compressor may be a first compressor, and the core shaft may be a first core shaft. The engine core may further comprise a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor. The second turbine, the second compressor, and the second core shaft may be configured to rotate at a higher rotational speed than the first core shaft.

[0085] The heat exchange system may further include branched fuel return paths and at least one valve controlling a fuel flow split, the branched paths being configured to return fuel from the heat exchange system to at least two different locations along a main fuel path from the fuel inlet in the gas turbine engine to the combustion chamber.

[0086] The motor of the fourth aspect may be for carrying out the method of the first and / or third aspect, and may have any of the characteristics described in relation to the first, second or third aspect.

[0087] According to a fifth aspect, a method of operating a gas turbine engine of an aircraft is provided, the gas turbine engine comprising:

[0088] an engine core comprising a turbine, a compressor, a combustion chamber for burning a fuel, and a core shaft connecting the turbine to the compressor;

[0089] a blower located upstream of the engine core;

[0090] a reducer that receives an input from the core shaft and provides a drive to the blower so as to drive the blower at a lower rotational speed than the core shaft;

[0091] an oil loop system for supplying oil to the reducer; and

[0092] a heat exchange system comprising:

[0093] an air-oil heat exchanger through which the oil in the oil loop system flows;

[0094] a fuel-oil heat exchanger through which the oil in the oil loop system and the fuel flow such that heat is transferred between the oil and the fuel; and

[0095] an air valve for controlling an air flow through the air-oil heat exchanger.

[0096] The method comprises:

[0097] determining at least one fuel characteristic of the fuel to be burned by the combustion chamber; and

[0098] controlling the air valve based on the at least one fuel characteristic so as to adjust the air flow through the air-oil heat exchanger.

[0099] The inventors were aware that the use of fuels different from traditional kerosene-based jet fuels, such as sustainable aviation fuels, may lead to different fuel properties, and that operational parameters may be adjusted to utilize the different fuel properties. In particular, certain fuels may be heated to higher temperatures in one or more fuel-oil heat exchangers than traditional fuels, without significantly increasing the deposition of fuel degradation products, e.g., by coking or varnishing, thereby allowing the fuel to absorb more heat and thereby reducing a need for air cooling. This may improve the combustion efficiency of the fuel and may also improve the overall efficiency of the engine, with less heat lost to the environment (via the air-oil heat exchanger).Higher fuel temperatures at the combustion chamber inlet, when fuel characteristics permit, can therefore enable a process enhancing improved oil cooling. For example, fuel thermal stability (a fuel characteristic) affects the amount of heat the fuel can accept / the temperature to which the fuel can be brought without forming deposits within pipes, burners and / or a hydromechanical unit or other engine component. Taking fuel thermal stability into account and reducing the air flow rate through the air-oil heat exchanger (possibly to zero) when fuel thermal stability is higher can therefore provide a more thermally efficient engine while avoiding excessive coking or varnishing, thereby improving aircraft performance.Using the fuel to absorb more heat from the oil, rather than relying on heat transfer from the oil to the surrounding environment / ambient air (in the air-to-oil heat exchanger) can therefore both provide a more efficient engine. thermodynamically efficient and improve the cooling of the oil before it is returned to the rest of the turbine engine.

[0100] In a transition period, where available aviation fuels change over the lifetime of a gas turbine engine, and also depend on the geographical location of a refueling point (among other variables) it is important to determine a relevant fuel characteristic and control the airflow to be achieved based on the specific fuel used. Thus, the operation of gas turbines can be adapted to make the most of a wide variety of fuels. The controllable airflow valve for adjusting the airflow through the air-oil heat exchanger has a key role to play in this adaptation of engine performance.

[0101] The method may comprise, subject to the suitability of the at least one determined fuel characteristic, controlling the air valve under idle conditions such that:

[0102] (i) the air flow rate in the air-oil heat exchanger is reduced to less than 60% of what the flow rate would be with the valve fully open when the engine is operating at idle conditions; and / or

[0103] (ii) the air flow rate in the air-oil heat exchanger is reduced to less than 40% of what the flow rate would be with the valve fully open when the engine is operating at idle conditions.

[0104] The method may comprise, subject to the suitability of the at least one determined fuel characteristic, controlling the air valve under cruising conditions such that:

[0105] (i) the air flow rate in the air-oil heat exchanger is reduced to less than 20%, and possibly less than 17% of what the flow rate would be with the valve fully open when the engine is operating at idle conditions; and / or

[0106] (ii) the air flow rate in the air-oil heat exchanger is reduced to at least substantially zero when the engine is operating under cruising conditions.

[0107] The suitability of the at least one determined fuel characteristic may be determined by comparing one or more fuel characteristics to a threshold value set for the respective fuel characteristic(s). For example, a fuel with a SAF content (%SAF) above 60%, 65%, 70%, 75%, or 80% may be considered suitable for airflow reduction compared to conventional aviation fuels. Multiple fuel characteristics may be considered together in some implementations—for example, with a lower threshold for one fuel characteristic being used when another fuel characteristic is within of a specific range. Fuel characteristics can be considered independently / isolated in other implementations.

[0108] The methods of the first, third, and fifth aspects may be complementary, and any two or more thereof may be performed together in various implementations. For example, both the oil flow and the air flow may be regulated, at cruise and / or at idle. The method of the fifth aspect may be performed using the engine of the second or fourth aspect.

[0109] The at least one determined fuel characteristic may be or include thermal stability. In such implementations, the air flow rate through the air-oil heat exchanger may be reduced, at cruise, to less than 15% of what the flow rate would be with the valve fully open provided the fuel is stable in operation at temperatures above 160°C. Alternatively or in addition, the air flow rate through the air-oil heat exchanger may be reduced, at cruise, to less than 5% of what the flow rate would be with the valve fully open provided the fuel is stable in operation at temperatures above 180°C. In some implementations, the air flow rate may be reduced with increasing thermal stability, possibly linearly.

[0110] The at least one determined fuel characteristic may be or include an aromatic hydrocarbon content of the fuel. In such implementations, the air flow rate through the air-oil heat exchanger may be reduced, at cruise, to less than 5% of what the flow rate would be with the valve fully open provided that the fuel has a mole percentage of aromatic hydrocarbons below 12%, and optionally below 10% or below 5%.

[0111] The at least one determined fuel characteristic may be or include a percentage of sustainable aviation fuel - S AF - in the fuel (%SAF). In such implementations, the air flow rate through the air-oil heat exchanger may be reduced, at cruise, to less than 5% of what the flow rate would be with the valve fully open provided the fuel has a SAF content above 60%. Alternatively or in addition, the air flow rate through the air-oil heat exchanger may be reduced, at cruise, to less than 2% of what the flow rate would be with the valve fully open provided the fuel has a SAF content above 80%. In some implementations, the air flow rate may be reduced, possibly linearly, with increasing %SAF once the fuel exceeds 60%, 65%, 70%, 75% or 80% SAF.

[0112] The at least one determined fuel characteristic may be or include a heating value of the fuel. In such implementations, the air flow rate in the air-oil heat exchanger may be reduced, at cruise, to less than 4% of what would be the flow rate with the valve fully open provided that the fuel has a calorific value of at least 43.5 MJ / kg. The at least one determined fuel characteristic may be or include thermal stability of the fuel.

[0113] Options relating to fuel characteristics, as well as their determination, are indicated below. These options are given for example purposes only and are not intended to be limiting.

[0114] According to a sixth aspect, a gas turbine engine for an aircraft is provided, the gas turbine engine comprising:

[0115] an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor;

[0116] a blower located upstream of the engine core;

[0117] a reducer that receives an input from the core shaft and provides a drive to the blower so as to drive the blower at a lower rotational speed than the core shaft;

[0118] an oil loop system for supplying oil to the reducer; and

[0119] a heat exchange system comprising:

[0120] an air-oil heat exchanger through which the oil in the oil loop system flows;

[0121] a fuel-oil heat exchanger through which the oil in the oil loop system and the fuel flow such that heat is transferred between the oil and the fuel; and

[0122] an air valve for controlling an air flow rate through the air-oil heat exchanger; and

[0123] a fuel composition determination module, intended to determine at least one fuel characteristic of the fuel intended to be burned by the combustion chamber.

[0124] The air valve is to be controlled based on the at least one fuel characteristic so as to adjust the air flow through the air-oil heat exchanger.

[0125] A control device may be provided to control the valve.

[0126] The turbine may be a first turbine, the compressor may be a first compressor, and the core shaft may be a first core shaft. The engine core may further comprise a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor. The second turbine, the second compressor, and the second core shaft may be configured to rotate at a higher rotational speed than the first core shaft.

[0127] The heat exchange system may further comprise branched fuel return paths and at least one valve controlling a division fuel flow paths, the branched paths being adapted to return fuel from the heat exchange system to at least two different locations along a main fuel path from the fuel inlet to the gas turbine engine to the combustion chamber.

[0128] The motor of the sixth aspect may be for carrying out the method of the first, third, and / or fifth aspect, and may have any of the characteristics described in relation to any preceding aspect.

[0129] According to a seventh aspect, a method of operating a gas turbine engine of an aircraft is provided, the gas turbine engine comprising:

[0130] an engine core comprising a turbine, a compressor, a combustion chamber for burning a fuel, and a core shaft connecting the turbine to the compressor;

[0131] a blower located upstream of the engine core;

[0132] a reducer that receives an input from the core shaft and provides a drive to the blower so as to drive the blower at a lower rotational speed than the core shaft;

[0133] an oil loop system for supplying oil to the reducer;

[0134] a heat exchange system comprising:

[0135] an air-oil heat exchanger through which the oil in the oil loop system flows;

[0136] a fuel-oil heat exchanger through which the oil in the oil loop system and the fuel flow such that heat is transferred between the oil and the fuel; and

[0137] at least one valve for varying at least one of the oil flow rate and the air flow rate through at least one of the fuel-oil heat exchanger and the air-oil heat exchanger; and

[0138] a temperature sensor for providing an indication of fuel temperature downstream of the fuel-oil heat exchanger (possibly at the inlet of the combustion chamber),

[0139] the method comprising:

[0140] determining whether the fuel temperature has increased above a set threshold under cruise conditions, based on an output of the temperature sensor; and

[0141] in response to determining that the fuel temperature has increased above the set threshold under cruise conditions, controlling the at least one valve to alter the at least one flow rate through the at least one heat exchanger.

[0142] For example, the at least one valve may be or comprise a valve intended to allow a proportion of the oil sent through the exchanger to be varied. fuel-oil heat exchanger, and the method may include, in response to determining that the fuel temperature has increased above a set threshold under cruise conditions, controlling the at least one valve to send less oil through the fuel-oil heat exchanger.

[0143] The inventors were aware that it is important that the oil temperature remains within a desired range and that the fuel temperature does not exceed a limit, so in some implementations the focal point of the flow rate adjustment may be the air-oil heat exchanger. The at least one valve may therefore be or include a valve for varying a proportion of the oil sent through the air-oil heat exchanger, and the method may include, in response to determining that the fuel temperature has increased above a set threshold under cruising conditions, controlling the at least one valve to send more oil through the air-oil heat exchanger.

[0144] A lack of an oil bypass on the air-oil heat exchanger could result in excessive cooling of the oil (e.g. it freezes or, in less extreme cases, excessive cooling may result in less efficient operation of the gearbox, e.g. with more heat losses and, in general, an efficiency penalty for the entire engine cycle) under certain conditions. A bypass pipe may therefore be provided for both heat exchangers. In some implementations, at least some of the oil may bypass both heat exchangers.

[0145] The at least one valve may be or include a valve for regulating a flow rate of air through the air-oil heat exchanger. The method may include sending more air through the air-oil heat exchanger in response to determining that the fuel temperature has increased above the set threshold under cruise conditions.

[0146] The method may include controlling multiple valves in some implementations, for example: • controlling the flow of oil through each of the fuel-oil heat exchanger and the air-oil heat exchanger separately; and / or • control of both air flow and oil flow.

[0147] As described for the preceding aspects, the inventors were aware that the use of fuels different from traditional kerosene-based jet fuels, such as sustainable aviation fuels, may lead to different fuel properties, and that operating parameters may be adjusted to utilize the different fuel properties. In particular, some fuels may be heated to higher temperatures in a or more fuel-oil heat exchangers than conventional fuels, without significantly increasing the deposition of fuel degradation products, e.g., by coking or vemissge, which allows the fuel to absorb more heat and thus reduces a need for air cooling and improves engine efficiency. However, allowing unsuitable fuels to reach these higher temperatures could adversely affect engine performance and even, in some cases, lead to the obstruction of fuel injection nozzles (or other components such as small filters, orifices and any small passages, such as those typically found within the hydromechanical unit and / or fuel heat exchangers). Checks and balances are therefore desired to ensure that engine performance is optimized for a given fuel.The method described in this seventh aspect includes monitoring the fuel temperature to control any excessively high fuel temperature and taking steps to reduce fuel heating when appropriate. The fuel temperatures downstream of the fuel-oil heat exchanger (e.g., at the combustion chamber inlet) under cruise conditions may be defined as an average for at least 1, 2, 3, 4, or 5 minutes, and optionally for ten minutes, twenty minutes, or thirty minutes, under steady-state cruise conditions. These average temperatures do not include transient temperature spikes, which may be defined as fluctuations in fuel temperature during operation, often a rise in temperature.A transient spike - for example, a spike lasting only a few seconds or minutes - at a higher temperature may therefore not be sufficient to trigger an oil flow change.

[0148] In a transition period, where available aviation fuels change over the lifetime of a gas turbine engine, and also depend on the geographical location of a refueling point (among other variables) it is important to decide the level of fuel heating based on the specific fuel used. Sending less oil through the fuel-oil heat exchanger results in less heat being transferred to the fuel, thus providing a lower fuel temperature as it approaches the combustion chamber. Thus, the operation of gas turbines can be adapted to take best advantage of a wide variety of fuels. The controllable oil flow valve for adjusting the oil flow rate through the fuel-oil heat exchanger has a key role to play in this adaptation of engine performance.

[0149] The inventors were aware that these principles can be applied both to engines with branched oil loop paths with different heat exchangers on different branches (parallel arrangements) and to engines with serial, substantially linear arrangements of heat exchangers and in which the use of one or more branch pipes is made as an alternative to a main branching path.

[0150] The oil loop system branches such that a proportion of the oil can flow along each branch and the air-oil and fuel-oil heat exchangers may be arranged in a parallel configuration on different branches of the oil loop system. In such implementations, the at least one valve for allowing the proportion of oil sent through the fuel-oil heat exchanger to be varied may be or include a modulating valve for allowing the proportion of oil sent through each branch to be varied.

[0151] The oil loop system may include at least one oil bypass pipe for allowing a proportion of the oil to bypass at least one of the fuel-oil heat exchanger and the air-oil heat exchanger. In such implementations, the at least one valve may be or include at least one bypass valve for allowing a proportion of the oil to bypass the respective heat exchanger / for regulating the proportion of oil sent via the bypass pipe. In such implementations, the air-oil and fuel-oil heat exchangers may be arranged in series or in parallel in the oil loop system.In implementations in which the air-oil and fuel-oil heat exchangers are arranged in parallel, on different branches of the oil loop system, and in which there is additionally at least one bypass pipe, the method may comprise controlling both the bypass valve and a modulating valve to allow the proportion of oil sent through each branch to be varied.

[0152] In implementations in which the method is intended to adjust the flow rate of oil through the fuel-oil heat exchanger, at least a portion of the oil diverted from the fuel-oil heat exchanger may be sent into the air-oil heat exchanger. In some implementations, the flow of air through the air-oil heat exchanger may be increased when more oil is sent through the air-oil heat exchanger.

[0153] In implementations in which the method is intended to adjust the flow rate of oil through the fuel-oil heat exchanger or the air-oil heat exchanger, at least a portion of the oil diverted from the respective heat exchanger may be sent via a bypass pipe around the respective heat exchanger.

[0154] The method may further comprise determining the set threshold based on at least one fuel characteristic of the fuel. In such implementations, the at least one fuel characteristic of the fuel may be or include at least one of: thermal stability of the fuel, nitrogen content of the fuel, sulfur content of the fuel, and sustainable aviation fuel (SAF) content of the fuel.

[0155] The step of determining the set threshold may comprise increasing the set threshold, possibly linearly, with increasing thermal stability of the fuel.

[0156] The step of determining the fixed threshold may comprise increasing the fixed threshold, possibly linearly, with the increase in the S AF content of the fuel, for fuels whose SAF content is greater than 70%.

[0157] The method may further comprise determining the at least one fuel characteristic of the fuel, optionally by any of the methods described below.

[0158] The heat exchange system may include an air valve for regulating the flow of air through the air-oil heat exchanger. The method may further include, in response to determining that the fuel temperature has increased beyond a set threshold under cruising conditions, controlling the air valve to send more air through the air-oil heat exchanger.

[0159] The heat exchange system may include a refrigeration cycle apparatus for providing thermal elevation by transferring additional heat from the oil to the fuel beyond that transferred by the fuel-oil heat exchanger. The method may further include controlling the refrigeration cycle apparatus to reduce the amount of additional heat transferred to the fuel in response to determining that the fuel temperature has increased beyond a set threshold under cruise conditions. For example, the refrigeration cycle apparatus may be turned off / disabled.

[0160] The heat exchange system may include at least one bypass pipe, and the at least one valve may be or include a bypass valve for regulating flow through that bypass pipe. The heat exchange system may include multiple bypass pipes, each for allowing oil to bypass a heat exchanger (e.g., the fuel-to-oil heat exchanger or air-to-oil heat exchanger described above, or a secondary fuel-to-oil heat exchanger, an oil-to-oil heat exchanger for transferring heat between two separate oil loops of the heat exchange system, or any other suitable heat exchanger).The method may include controlling at least two bypass valves - for example, a bypass valve for the air-oil heat exchanger may be adjusted to send more oil to the air-oil heat exchanger when the bypass valve for . The fuel-oil heat exchanger is adjusted to send less oil to the fuel-oil heat exchanger. The same valve (e.g., a three-way valve) can adjust the oil flow to both heat exchangers in some implementations.

[0161] The set threshold may be in the range of 140°C to 300°C, and optionally 250°C to 300°C. The set threshold may be 140°C, 180°C, 200°C, 210°C, 220°C, 230°C, 240°C, 250°C, 260°C, 270°C, 275°C, 280°C, 290°C, or 300°C.

[0162] The determining step may be performed at regular intervals during operation of the aircraft in cruise.

[0163] The methods of the first, third, fifth, and seventh aspects may be complementary, and any two or more thereof may be performed together in various implementations. The method of the seventh aspect may be performed using the engine of the second, fourth, or sixth aspect.

[0164] According to an eighth aspect, a gas turbine engine for an aircraft is provided, the gas turbine engine comprising:

[0165] an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor;

[0166] a blower located upstream of the engine core;

[0167] a reducer that receives an input from the core shaft and provides a drive to the blower so as to drive the blower at a lower rotational speed than the core shaft;

[0168] an oil loop system for supplying oil to the reducer;

[0169] a heat exchange system comprising:

[0170] an air-oil heat exchanger through which the oil in the oil loop system flows;

[0171] a fuel-oil heat exchanger through which the oil in the oil loop system and the fuel flow such that heat is transferred between the oil and the fuel; and

[0172] at least one valve for varying at least one of the oil flow rate and the air flow rate through at least one of the fuel-oil heat exchanger and the air-oil heat exchanger;

[0173] a temperature sensor for providing an indication of fuel temperature downstream of the fuel-oil heat exchanger (possibly at the inlet of the combustion chamber); and

[0174] a controller for receiving an output from the cruise temperature sensor, determining whether the fuel temperature has increased above a set threshold based on that output, and in response to determining that the fuel temperature has increased above a set threshold under cruise conditions, cruise, control the at least one valve so as to modify the at least one flow rate through the at least one heat exchanger.

[0175] For example, the at least one valve may be intended to allow a proportion of the oil sent through the fuel-oil heat exchanger to be varied; and the control device may be intended to:

[0176] receive an output from the temperature sensor under cruising conditions;

[0177] determining whether the fuel temperature has increased above a set threshold based on this output; and

[0178] in response to determining that the fuel temperature has increased above a set threshold under cruising conditions, controlling the at least one valve to send less oil through the fuel-oil heat exchanger.

[0179] The controller, or other processing module, may be configured to determine the set threshold based on one or more fuel characteristics of the fuel.

[0180] The heat exchange system may include a refrigeration cycle apparatus for providing thermal elevation by transferring more heat from the oil to the fuel beyond that transferred by the fuel-oil heat exchanger. Optionally, the refrigeration apparatus may be operable to raise the fuel temperature above the oil temperature. The controller may be operable to deactivate the refrigeration cycle apparatus in response to determining that the fuel temperature has increased above a set threshold during cruise conditions.

[0181] The turbine may be a first turbine, the compressor may be a first compressor, and the core shaft may be a first core shaft. The engine core may further comprise a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor. The second turbine, the second compressor, and the second core shaft may be configured to rotate at a higher rotational speed than the first core shaft.

[0182] The heat exchange system may further include branched fuel return paths and at least one valve controlling a fuel flow split, the branched paths being configured to return fuel from the heat exchange system to at least two different locations along a main fuel path from the fuel inlet in the gas turbine engine to the combustion chamber.

[0183] The motor of the eighth aspect may be for carrying out the method of the first, third, fifth and / or seventh aspect, and may have any of the characteristics described in relation to any preceding aspect.

[0184] According to a ninth aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

[0185] an engine core comprising a turbine, a compressor, a combustion chamber for burning a fuel, and a core shaft connecting the turbine to the compressor;

[0186] a blower located upstream of the engine core;

[0187] an oil loop system for supplying oil to cool at least one engine component;

[0188] a fuel-oil heat exchanger through which the oil in the oil loop system and the fuel flow such that heat is transferred from the oil to the fuel; and

[0189] a refrigeration cycle apparatus through which the oil in the oil loop system and the fuel flow, the refrigeration cycle apparatus being adapted to transfer more heat from the oil to the fuel.

[0190] The method includes controlling the refrigeration cycle apparatus such that the fuel temperature at the inlet to the combustion chamber is higher than a maximum oil temperature in the oil loop system.

[0191] The inventors were aware that the use of fuels different from traditional kerosene-based jet fuels, such as sustainable aviation fuels, may lead to different fuel properties, and that parameters under cruise conditions may be adjusted to utilize the different fuel properties. In particular, some fuels may be heated to higher temperatures than traditional fuels, without significantly increasing the deposition of fuel degradation products, for example by coking or varnishing, and thermal elevation to bring the fuel above the oil temperature may be used to maximize fuel performance in some cases.This may also allow a method providing improved oil cooling (since the use of a refrigeration cycle apparatus allows heat to be pumped away from the oil even when the oil is lowered to fuel temperature) and may therefore improve the overall thermal efficiency of the engine, as the fuel is burned hotter. Control of the refrigeration cycle apparatus allows thermal lift to be provided and, possibly, also adjusts the level of thermal lift provided as appropriate for the fuel and engine operating conditions.

[0192] As used herein, the maximum oil temperature within the oil loop system indicates the highest oil temperature at any point in the oil loop system at a given time, i.e., at the time when the comparison with the fuel temperature is made. It will be kept in mind that the oil temperature generally increases as it passes through one or more engine components to be cooled by oil, and then decreases as it passes through the at least one fuel-oil heat exchanger (and any other heat exchanger intended to transfer heat from the oil - e.g., an air-oil heat exchanger). A temperature sensor intended to provide an indication of the maximum oil temperature within the oil loop system may therefore be located downstream of the engine component(s) to be cooled and upstream of the fuel-oil heat exchanger(s) (and any other heat exchanger intended to transfer heat from the oil). The fuel and oil temperatures may be evaluated as described above; in particular, ignoring transient peaks. An average may be calculated over a period of time, e.g., one minute or a few minutes.

[0193] The engine component(s) to be cooled may include one or more of: a main power reducer - i.e., a reducer that receives input from the core shaft and provides drive to the blower, an auxiliary reducer, one or more bearings (e.g., shaft bearings, e.g., for the core shaft), one or more generators, electrical wiring, and / or one or more pumps. It will be appreciated that any structure within the engine that is desired to be cooled may be considered an engine component.

[0194] The motor may include a reduction gear that receives an input from the core shaft and provides a drive to the blower so as to drive the blower at a lower rotational speed than the core shaft, and the oil loop system may supply oil to the reduction gear. The refrigeration cycle apparatus may therefore be used to provide supplemental cooling to the main reduction gear of a gear motor.

[0195] The refrigeration cycle apparatus may contain a refrigerant, the refrigerant being for transferring heat from the oil to the fuel. The refrigerant may be 1,1,1,2-tetrafluoroethane.

[0196] The refrigeration cycle apparatus may comprise an evaporator, a refrigerant compressor and a condenser. The method may comprise:

[0197] transferring heat from the oil to the refrigerant using the evaporator so as to evaporate the liquid in the refrigerant to form a saturated vapor;

[0198] compressing the saturated vapor to a superheated vapor using the refrigerant compressor, and sending the superheated vapor to the condenser; and

[0199] transferring heat from the superheated vapor to the fuel using the condenser, at least partially converting the superheated vapor to liquid in the process.

[0200] The refrigerant compressor may also act as a pump, circulating the refrigerant around the refrigeration cycle apparatus. A separate refrigerant pump may be provided in some implementations.

[0201] The method may include converting the superheated vapor to a saturated liquid in the condenser (a saturated liquid, in the context of thermodynamics / refrigeration cycles, being a liquid that contains as much thermal energy as it can without boiling). The condenser may form a nearly saturated liquid in some implementations.

[0202] The refrigeration cycle apparatus may also include an expansion valve located between the condenser and the evaporator. The method may include using the expansion valve to convert the liquid from the condenser (which may be a saturated liquid) into a reduced temperature liquid and vapor mixture (i.e., a liquid and gas mixture at a lower temperature than the liquid delivered from the condenser). The refrigerant may then be returned to the evaporator and the cycle may begin again.

[0203] The engine may further include a temperature sensor for measuring the temperature of fuel downstream of the refrigeration cycle apparatus. The fuel temperature sensor may be located at or near an inlet of the combustion chamber. The engine may further include a temperature sensor for providing an indication of the maximum oil temperature within the oil loop system. The oil temperature sensor may be located at or near an inlet of the fuel-oil heat exchanger, or at or near an outlet of the one or more engine components to be cooled. The method may further include comparing the oil and fuel temperatures and adjusting control of the refrigeration cycle apparatus based on the comparison.

[0204] The method may include controlling one or more additional components of the engine heat exchange system as well as controlling the refrigeration cycle apparatus - the multiple components may work together to bring the fuel temperature to the desired level.

[0205] For example, the method may include controlling the flow of fluid through the fuel-oil heat exchanger so as to increase heat transfer to the fuel by:

[0206] (i) increasing the oil flow rate through the fuel-oil heat exchanger, e.g. by controlling a bypass valve, a modulating valve or a recirculation valve as described elsewhere herein, or by regulating the pump speed of an oil pump of the oil loop system; and / or

[0207] (ii) recirculating fuel through the fuel-oil heat exchanger, e.g. using a recirculation valve as described elsewhere herein.

[0208] In some implementations, the engine further includes an air-to-oil heat exchanger located upstream of the fuel-to-oil heat exchanger with respect to the oil flow. In such implementations, the method may include increasing heat transfer from the oil to the fuel in the fuel-to-oil heat exchanger by reducing air cooling of the oil before the oil enters the fuel-to-oil heat exchanger.

[0209] The method may be performed while cruising. The engine may be controlled to operate with a combustion chamber inlet fuel temperature higher than the maximum oil temperature for at least 10%, 20%, or 30% of the time while cruising.

[0210] The method may include controlling the refrigeration cycle apparatus such that the fuel temperature at the inlet to the combustion chamber is higher than the maximum oil temperature within the oil loop system by at least 2°C, 5°C, 10°C, 15°C, 20°C or 25°C. The method may include controlling the refrigeration cycle apparatus such that the fuel temperature at the inlet to the combustion chamber is higher than the maximum oil temperature within the oil loop system by between 2°C and 50°C.

[0211] The method may include controlling the refrigeration cycle apparatus such that the fuel temperature at the inlet to the combustion chamber is higher than the maximum oil temperature within the oil loop system by a margin determined based on at least one fuel characteristic of the fuel.

[0212] The methods of the first, third, fifth, seventh, and ninth aspects may be complementary, and any two or more thereof may be performed together in various implementations. The method of the ninth aspect may be performed using the engine of the second, fourth, sixth, or eighth aspect.

[0213] According to a tenth aspect, there is provided a gas turbine engine for an aircraft comprising:

[0214] an engine core comprising a turbine, a compressor, a combustion chamber for burning a fuel, and a core shaft connecting the turbine to the compressor;

[0215] a blower located upstream of the engine core;

[0216] an oil loop system for supplying oil to cool at least one engine component;

[0217] a fuel-oil heat exchanger through which the oil in the oil loop system and the fuel flow such that heat is transferred from the oil to the fuel; and

[0218] a refrigeration cycle apparatus through which the oil in the oil loop system and the fuel flow, the refrigeration cycle apparatus being adapted to transfer more heat from the oil to the fuel.

[0219] The fuel-oil heat exchanger and the refrigeration cycle apparatus are intended to transfer heat to the fuel such that the fuel temperature at the inlet to the combustion chamber is higher than a maximum oil temperature within the oil loop system.

[0220] The gas turbine engine may further comprise a reduction gear that receives an input from a core shaft and provides a drive to the fan so as to drive the fan at a lower rotational speed than that of the core shaft. The gas turbine engine may therefore be a reduction geared gas turbine engine. The oil loop system may be for supplying oil to the reduction gear.

[0221] The turbine may be a first turbine, the compressor may be a first compressor, and the core shaft may be a first core shaft. The engine core may further comprise a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor. The second turbine, the second compressor, and the second core shaft may be configured to rotate at a higher rotational speed than the first core shaft.

[0222] The gas turbine engine may further include branched fuel return paths and at least one valve controlling a fuel flow split along the branched fuel return paths. The branched paths may be configured to return fuel from the fuel-oil heat exchanger to at least two different locations along a primary fuel path from the fuel inlet of the gas turbine engine to the combustion chamber.

[0223] The motor of the tenth aspect may be for carrying out the method of the first, third, fifth, seventh and / or ninth aspect, and may have any of the characteristics described in relation to any preceding aspect.

[0224] In any of the above aspects:

[0225] In implementations in which one or more fuel characteristics of the fuel are used, the at least one fuel characteristic of the fuel may include at least one of:

[0226] i. a percentage of sustainable aviation fuel in the fuel (%SAF);

[0227] ii. a concentration of heteroatomic species of the fuel;

[0228] iii. an aromatic hydrocarbon content of the fuel;

[0229] iv. a multi-aromatic hydrocarbon content of the fuel;

[0230] v. a percentage of nitrogen-containing species in the fuel;

[0231] vi. a presence or percentage of a tracer species or trace element in fuel;

[0232] vii. a hydrogen / carbon ratio of the fuel;

[0233] viii. a distribution of hydrocarbons in the fuel;

[0234] ix. a level of emissions of non-volatile particulate matter during combustion;

[0235] x. a naphthalene content of the fuel;

[0236] xi. a sulfur content of the fuel;

[0237] xii. a cycloparaffin content of the fuel;

[0238] xiii. an oxygen content of the fuel;

[0239] xiv. thermal stability of the fuel (e.g., degradation temperature thermal); a thermal stability can be assigned a numerical value at any temperature by taking the inverse of a fuel degradation product deposition rate at the given temperature);

[0240] xv. a level of coking of the fuel or, more generally, the level of deposition of fuel degradation products;

[0241] xvi. an indication that the fuel is a fossil fuel;

[0242] xvii. at least one of a density, a viscosity, a calorific value and a thermal capacity.

[0243] The method may further comprise chemically or physically detecting one or more fuel-related parameters in the fuel tank after refueling. The detected parameters may be fuel characteristics, or may be used to calculate or derive fuel characteristics - for example, the detected parameters may be a shaft speed and a fuel mass flow rate, from which a calorific value (a fuel characteristic) may be determined, or the detected parameters may be a fuel density and / or the presence of a tracer, both of which are fuel characteristics. Determining at least one fuel characteristic may comprise obtaining stored data on the fuel characteristics.The chemical and / or physical determination of one or more fuel parameters in the fuel tank can be accomplished by extracting a sample of the fuel from the fuel tank for off-wing testing.

[0244] Determining the at least one fuel characteristic of the fuel may include obtaining at least one fuel characteristic of any fuel already present in the fuel tank prior to refueling; determining at least one fuel characteristic of an added fuel to the fuel tank upon refueling; and calculating at least one fuel characteristic of the resulting fuel in the fuel tank after refueling (based on this information).

[0245] The determination of the at least one fuel characteristic may be performed based on a detection of at least one fuel property. The fuel property may be the fuel characteristic, or may be used to calculate or otherwise determine (e.g., by retrieving from a lookup table) the fuel characteristic. The detection may be performed on the wing.

[0246] The determination of the at least one fuel characteristic may be performed based on received fuel composition data. The fuel composition data may be provided to the aircraft during refueling. The fuel composition data may be entered manually.

[0247] At least one fuel characteristic may be derived from the performance of the gas turbine engine during at least one of taxi, takeoff, and climb of the aircraft.

[0248] It will be noted that a feature described in relation to one aspect may be used in combination with any aspect, mutatis mutandis.

[0249] As indicated elsewhere herein, the present disclosure may apply to any relevant configuration of a gas turbine engine. Such a gas turbine engine may be, for example, a bypass gas turbine engine, an open rotor gas turbine engine (in which the propeller is not surrounded by a nacelle), a turboprop engine, or a turbojet engine. Any such engine may or may not be provided with an afterburner. Such a gas turbine engine may be, for example, designed for land-based or marine power generation applications.

[0250] A gas turbine engine in accordance with any aspect of the present disclosure may include an engine core comprising a turbine, a combustor, a compressor, and a core shaft connecting the turbine to the compressor. Such a gas turbine engine may include a fan (having fan blades). Such a fan may be located upstream of the engine core. Alternatively, in some examples, the gas turbine engine may include a fan located downstream of the engine core, for example in the case where the gas turbine engine is an open rotor or turboprop engine (in which case the fan may be referred to as a propeller).

[0251] Where the gas turbine engine is an open rotor or turboprop engine, the gas turbine engine may include two counter-rotating propeller stages attached to and driven by a free power turbine via a shaft. The propellers may rotate in opposite directions such that one rotates clockwise. and the other counterclockwise around the engine's axis of rotation. Alternatively, the gas turbine engine may include a propeller stage and a guide vane stage designed downstream of the propeller stage. The guide vane stage may be of variable pitch. Thus, the high-pressure, intermediate-pressure, and free-power turbines may respectively drive high-pressure and intermediate-pressure propellers and compressors through suitable interconnecting shafts. Thus, the propellers may provide the majority of the propulsion thrust.

[0252] In the case where the gas turbine engine is an open rotor or turboprop engine, one or more of the propeller stages may be driven by a reduction gear. The reduction gear may be of the type described herein.

[0253] An engine according to the present disclosure may be a bypass engine. Such an engine may be a direct-drive bypass engine in which the fan is directly connected to the fan drive turbine, for example without a reduction gear, via a core shaft. In such a direct-drive bypass engine, the fan may be said to rotate at the same rotational speed as the fan drive turbine. Strictly by way of example, the fan drive turbine may be a first turbine, the core shaft may be a first core shaft, and the gas turbine engine may further comprise a second turbine and a second core shaft connecting the second turbine to the compressor. The second turbine, the compressor, and the second core shaft may be intended to rotate at a higher rotational speed than the first core shaft.In such an arrangement, the second turbine may be positioned axially upstream of the first turbine.

[0254] An engine according to the present disclosure may be a geared turbofan engine. In such an arrangement, the engine has a fan that is driven via a gear reducer. Accordingly, such a gas turbine engine may include a gear reducer that receives an input from the core shaft and provides a drive to the fan so as to drive the fan at a lower rotational speed than the core shaft. The input to the gear reducer may be directly from the core shaft, or indirectly from the core shaft, for example via a spur shaft and / or pinion. The core shaft may interlock the turbine and compressor, such that the turbine and compressor rotate at the same speed (with the fan rotating at a lower speed).

[0255] The gas turbine engine as described and / or claimed herein may have any suitable general architecture. For example, the gas turbine engine may have any desired number of shafts that connect turbines and compressors, e.g., one, two, or three shafts. By way of example only, the turbine connected to the core shaft may be a first turbine, the compressor connected to the core shaft may be a first compressor, and the core shaft may be a first core shaft. The engine core may further comprise a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor. The second turbine, the second compressor, and the second core shaft may be configured to rotate at a higher rotational speed than the first core shaft.

[0256] In such an arrangement, the second compressor may be positioned axially downstream of the first compressor. The second compressor may be adapted to receive (e.g., directly receive, e.g., via a generally annular conduit) a flow from the first compressor.

[0257] The reducer may be intended to be driven by the core shaft that is designed to rotate (e.g., in use) at the lowest rotational speed (e.g., the first core shaft in the example above). For example, the reducer may be intended to be driven only by the core shaft that is designed to rotate (e.g., in use) at the lowest rotational speed (e.g., only by the first core shaft, and not the second core shaft, in the example above). Alternatively, the reducer may be intended to be driven by any one or more shafts, e.g., the first and / or second shafts in the example above.

[0258] The reducer may be a reduction gear (in that the output to the fan has a lower rotational speed than the input from the core shaft). Any type of reducer may be used. For example, the reducer may be a "planetary" or "star" reducer, as described in more detail elsewhere herein. Such a reducer may be a single stage. Alternatively, such a reducer may be a compound reducer, for example a compound planetary reducer (which may have the input on the sun pinion and the output on the ring gear, and thus be referred to as a "compound star" reducer), for example with two reduction stages.

[0259] The reducer may have any desired reduction ratio (defined as the rotational speed of the input shaft divided by the rotational speed of the output shaft), for example greater than 2.5, for example in the range of 3 to 4.2, or 3.2 to 3.8, for example in the range of or at least 3, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4, 4.1 or 4.2. The reduction ratio may be, for example, between any two of the values ​​in the preceding sentence. Strictly by way of example, the reducer may be a "star" reducer having a reduction ratio in the range of 3.1 or 3.2 to 3.8. Strictly by way of further example, the reducer may be a "star" reducer having a reduction ratio in the range of 3.0 to 3.1. Strictly by way of example, the reducer may be a "planetary" reducer. having a reduction ratio in the range of 3.6 to 4.2. In some arrangements, the reduction ratio may be outside these ranges.

[0260] In any gas turbine engine as described and / or claimed herein, a fuel of a given composition or mixture is supplied to a combustor, which may be provided downstream of the fan and the compressor(s) with respect to the flow path (e.g., axially downstream). For example, the combustor may be directly downstream of (e.g., at the outlet of) the second compressor, when a second compressor is provided. As a further example, the outlet flow to the combustor may be provided at the inlet of the second turbine, when a second turbine is provided. The combustor may be provided upstream of the turbine(s).

[0261] The or each compressor (e.g., the first compressor and the second compressor as described above) may comprise any number of stages, e.g., multiple stages. Each stage may comprise a row of rotor blades and a row of stator blades, which may be variable stator blades (in that their angle of incidence may be variable). The row of rotor blades and the row of stator blades may be axially offset from each other. For example, the gas turbine engine may be a direct-drive, bypass gas turbine engine comprising 13 or 14 compressor stages (in addition to the fan). Such an engine may, for example, comprise 3 stages in the first (or "low pressure") compressor and either 10 or 11 stages in the second (or "high pressure") compressor.As a further example, the gas turbine engine may be a geared gas turbine engine (in which the fan is driven by a first core shaft through a reduction gearbox) having 11, 12, or 13 compressor stages (in addition to the fan). Such an engine may have 3 or 4 stages in the first (or "low pressure") compressor and 8 or 9 stages in the second (or "high pressure") compressor. As a further example, the gas turbine engine may be a geared gas turbine engine having 4 stages in the first (or "low pressure") compressor and 10 stages in the second (or "high pressure") compressor.

[0262] The or each turbine (e.g., the first turbine and the second turbine as described above) may comprise any number of stages, e.g., multiple stages. Each stage may comprise a row of rotor blades and a row of stator blades, or vice versa, as required. The respective rows of rotor blades and stator blades may be axially offset from each other. The second (or "high pressure") turbine may comprise 2 stages in any arrangement (e.g., regardless of whether it is a geared or direct-drive engine). The gas turbine engine may be a direct drive gas turbine engine comprising a first (or "low pressure") turbine having 5, 6 or 7 stages. Alternatively, the gas turbine engine may be a geared gas turbine engine comprising a first (or "low pressure") turbine having 3 or 4 stages.

[0263] Each fan blade may be defined as having a radial throw extending from a root (or hub) at a radially inner gas-washed location, or a 0% throw position, to a tip at a 100% throw position. The ratio of the fan blade radius at the hub to the fan blade radius at the tip may be less than (or in the order of) any of: 0.4, 0.39, 0.38, 0.37, 0.36, 0.35, 0.34, 0.33, 0.32, 0.31, 0.3, 0.29, 0.28, 0.27, 0.26, or 0.25. The ratio of the fan blade radius at the hub to the fan blade radius at the tip may be included in an inclusive range bounded by any two of the values ​​in the preceding sentence (i.e., the values ​​may form upper or lower limits), for example, in the range 0.28 to 0.32, or 0.29 to 0.30.These ratios can be commonly referred to as the hub-to-tip ratio. Both the radius at the hub and the radius at the tip can be measured at the leading edge (or axially forward-most) portion of the blade. The hub-to-tip ratio refers, of course, to the gas-washed portion of the fan blade, i.e., the portion radially outboard of any platform.

[0264] The fan radius can be measured between the engine centerline and the tip of a fan blade at its leading edge. The blower diameter (which may simply be twice the blower radius) may be greater than (or of the order of) any of: 140 cm, 170 cm, 180 cm, 190 cm, 200 cm, 210 cm, 220 cm, 230 cm, 240 cm, 250 cm (about 100 inches), 260 cm, 270 cm (about 105 inches), 280 cm (about 110 inches), 290 cm (about 115 inches), 300 cm (about 120 inches), 310 cm, 320 cm (about 125 inches), 330 cm (about 130 inches), 340 cm (about 135 inches), 350 cm, 360 cm (about 140 inches), 370 cm (about 145 inches), 380 cm (approximately 150 inches), 390 cm (approximately 155 inches), 400 cm, 410 cm (approximately 160 inches) or 420 cm (approximately 165 inches).The blower diameter may be in an inclusive range bounded by any two of the values ​​in the preceding sentence (i.e., the values ​​may form upper or lower limits), for example, in the range of 210 cm to 240 cm, or 250 cm to 280 cm, or 320 cm to 380 cm. Strictly by way of non-limiting example, the blower diameter may be in the range of 170 cm to 180 cm, 190 cm to 200 cm, 200 cm to 210 cm, 210 cm to 230 cm, 290 cm to 300 cm, or 340 cm to 360 cm.

[0265] The rotational speed of the fan may vary during use. Generally, the rotational speed is lower for fans with a larger diameter. Strictly by way of non-limiting example, the rotational speed of the fan under cruising conditions may be lower than 3500 rpm, for example lower than 2600 rpm, or lower than 2500 rpm, or lower than 2300 rpm. Strictly by way of further non-limiting example, the rotational speed of the fan under cruising conditions for a geared gas turbine engine having a fan diameter in the range of 200 cm to 210 cm may be in the range of 2750 to 2900 rpm.Strictly by way of further non-limiting example, the fan rotational speed under cruising conditions for a geared gas turbine engine having a fan diameter in the range of 210 cm to 230 cm may be in the range of 2500 to 2800 rpm. Strictly by way of further non-limiting example, the fan rotational speed under cruising conditions for a geared gas turbine engine having a fan diameter in the range of 340 cm to 360 cm may be in the range of 1500 to 1800 rpm. Strictly by way of further non-limiting example, the fan rotational speed under cruising conditions for a direct drive engine having a fan diameter in the range of 190 cm to 200 cm may be in the range of 3600 to 3900 rpm.Strictly by way of further non-limiting example, the fan rotational speed under cruising conditions for a direct drive engine having a fan diameter in the range of 300 cm to 340 cm may be in the range of 2000 to 2800 rpm.

[0266] During operation of the gas turbine engine, the fan (with associated fan blades) rotates about an axis of rotation. This rotation results in a displacement of the fan blade tip with a velocity Utip. The work done by the fan blades on the flow results in an enthalpy increase dH of the flow. A fan tip load can be defined as dH / UpOint2, where dH is the enthalpy increase (e.g., the 1-D average enthalpy increase) across the fan and Utip is the (translational) velocity of the fan tip, e.g., at the leading edge of the tip (which can be defined as the fan tip radius at the leading edge multiplied by the angular velocity).The peak fan load at cruise conditions may be greater than (or of the order of) any one of: 0.28, 0.29, 0.30, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39, or 0.4 (all values ​​being dimensionless). The peak fan load may be within an inclusive range bounded by any two of the values ​​in the preceding sentence (i.e. . i.e. the values ​​may form upper or lower limits), for example in the range from 0.28 to 0.31 or from 0.29 to 0.3 (e.g. for a geared gas turbine engine).

[0267] Gas turbine engines in accordance with the present disclosure may have any desired bypass ratio (BPR), where the bypass ratio is defined as the ratio of the mass flow rate of the flow through the bypass duct to the mass flow rate of the flow through the core. In some arrangements the cruise bypass ratio may be greater than (or of the order of) any of the following: 9, 9.5, 10, 10.5, 11, 11.5, 12, 12.5, 13, 13.5, 14, 14.5, 15, 15.5, 16, 16.5, 17, 17.5, 18, 18.5, 19, 19.5, or 20. The cruise bypass ratio may be within an inclusive range bounded by any two of the values ​​in the preceding sentence (i.e., the values ​​may form upper or lower limits), for example, within the range 12 to 16, 13 to 15, or 13 to 14.Strictly by way of non-limiting example, the cruise bypass ratio of a direct drive gas turbine engine according to the present disclosure may be in the range of 9:1 to 11:1. Strictly by way of further non-limiting example, the cruise bypass ratio of a geared gas turbine engine according to the present disclosure may be in the range of 12:1 to 15:1. The bypass duct may be substantially annular. The bypass duct may be radially outboard of the core engine. The radially outer surface of the bypass duct may be defined by a nacelle and / or a fan case.

[0268] The overall pressure ratio (OPR) of a gas turbine engine as described and / or claimed herein may be defined as the ratio of the stagnation pressure at the outlet of the highest pressure compressor (before an inlet into the combustor) to the stagnation pressure upstream of the fan. By way of non-limiting example, the overall pressure ratio of a gas turbine engine as described and / or claimed herein under cruise conditions may be greater than (or in the order of) any of the following: 35, 40, 45, 50, 55, 60, 65, 70, 75. The overall pressure ratio may be within an inclusive range bounded by any two of the values ​​in the preceding sentence (i.e., the values ​​may form upper or lower limits), for example, in the range of 50 to 70.Strictly by way of non-limiting example, the overall pressure ratio at cruise conditions of a geared gas turbine engine having a fan diameter in the range of 200 cm to 210 cm may be in the range of 40 to 45. Strictly by way of non-limiting example, the overall pressure ratio at cruise conditions of a geared gas turbine engine having a fan diameter in the range of 210 cm to 230 cm may be in the range of 45 to 55. Strictly at . As a non-limiting example, the overall pressure ratio at cruise conditions of a geared gas turbine engine having a fan diameter in the range of 340 cm to 360 cm may be in the range of 50 to 60. Strictly as a non-limiting example, the overall pressure ratio at cruise conditions of a direct drive gas turbine engine having a fan diameter in the range of 300 cm to 340 cm may be in the range of 50 to 60.

[0269] The specific thrust of an engine may be defined as the net thrust of the engine divided by the total mass flow rate through the engine. In some examples, specific thrust may depend, for a given thrust condition, on the specific fuel composition supplied to the combustion chamber. At cruise conditions, the specific thrust of an engine described and / or claimed herein may be lower than (or on the order of) any of the following: 110 Nkg 's, 105 Nkg's, 100 Nkg's, 95 Nkg's, 90 Nkg's, 85 Nkg's or 80 Nkg's. The specific thrust may be in an inclusive range bounded by any two of the values ​​in the preceding sentence (i.e., the values ​​may form upper or lower limits), for example, in the range of 80 Nkg 's to 100 Nkg 's, or of 85 Nkg 's to 95 Nkg 's. Such engines may be particularly efficient compared to conventional gas turbine engines.Strictly by way of non-limiting example, the specific thrust of a geared gas turbine engine having a fan diameter in the range of 200 cm to 210 cm may be in the range of 90 Nkg 's to 95 Nkg 's. Strictly by way of non-limiting example, the specific thrust of a geared gas turbine engine having a fan diameter in the range of 210 cm to 230 cm may be in the range of 80 Nkg 's to 90 Nkg 's. Strictly by way of non-limiting example, the specific thrust of a geared gas turbine engine having a fan diameter in the range of 340 cm to 360 cm may be in the range of 70 Nkg 's to 90 Nkg 's. Strictly by way of non-limiting example, the specific thrust of a direct drive gas turbine engine having a fan diameter in the range of 300 cm to 340 cm may be in the range of 90 Nkg's to 120 Nkg's.

[0270] A gas turbine engine as described and / or claimed herein may have any desired maximum thrust. Strictly by way of non-limiting example, a gas turbine as described and / or claimed herein may be capable of producing a maximum thrust of at least (or in the order of) any of the following: 100 kN, 110 kN, 120 kN, 130 kN, 135 kN, 140 kN, 145 kN, 150 kN, 155 kN, 160 kN, 170 kN, 180 kN, 190 kN, 200 kN, 250 kN, 300 kN, 350 kN, 400 kN, 450 kN, 500 kN, or 550 kN. The maximum thrust may be in an inclusive range bounded by any two of the values ​​in the preceding sentence (i.e., the values ​​may form upper or lower limits). By way of non-limiting example only, a gas turbine as described and / or claimed herein may be capable of producing a maximum thrust in the range of 155 kN to 170 kN, 330 kN to 420 kN, or 350 kN to 400 kN. Strictly by way of non-limiting example, the maximum thrust of a geared gas turbine engine having a fan diameter in the range of 200 cm to 210 cm may be in the range of 140 kN to 160 kN. Strictly by way of non-limiting example, the maximum thrust of a geared gas turbine engine having a fan diameter in the range of 210 cm to 230 cm may be in the range of 150 kN to 200 kN. Strictly by way of non-limiting example, the maximum thrust of a geared gas turbine engine having a fan diameter in the range of 340 cm to 360 cm may be in the range of 370 kN to 500 kN.Strictly by way of non-limiting example, the maximum thrust of a direct drive gas turbine engine having a fan diameter in the range of 300 cm to 340 cm may be in the range of 370 kN to 500 kN. The thrust referred to above may be the maximum net thrust under typical atmospheric conditions at sea level plus 15 degrees C (ambient pressure of 101.3 kPa, temperature of 30 degrees C), with the engine static.

[0271] In use, the temperature of the flow at the inlet of the high pressure turbine may be particularly high. This temperature, which may be referred to as TET, may be measured at the outlet of the combustion chamber, for example immediately upstream of the first turbine blade, which itself may be referred to as the nozzle guide vane. In some examples, the TET may depend, for a given thrust condition, on the specific composition of fuel supplied to the combustion chamber. Under cruise conditions, the TET may be at least (or in the order of) any of the following values: 1400 K, 1450 K, 1500 K, 1520 K, 1530 K, 1540 K, 1550 K, 1600 K or 1650 K. Thus, by way of non-limiting example only, the TET under cruise conditions of a geared gas turbine engine having a fan diameter in the range of 200 cm to 210 cm may be in the range of 1540 K to 1600 K.Strictly by way of non-limiting example, the TET at cruise conditions of a geared gas turbine engine having a fan diameter in the range of 210 cm to 230 cm may be in the range of 1590 K to 1650 K. Strictly by way of non-limiting example, the TET at cruise conditions of a geared gas turbine engine having a fan diameter in the range of 340 cm to 360 cm may be in the range of 1600 K to 1660 K. Strictly by way of non-limiting example, the TET at cruise conditions of a direct drive gas turbine engine having a fan diameter in the range of 300 cm to 340 cm may be in the . range from 1590 K to 1650 K. Strictly by way of non-limiting example, the TET under cruise conditions of a direct drive gas turbine engine having a fan diameter in the range from 300 cm to 340 cm may be in the range from 1570 K to 1630 K.

[0272] The TET under cruise conditions may be within an inclusive range bounded by any two of the values ​​in the preceding sentence (i.e., the values ​​may form upper or lower limits), for example, 1530 K to 1600 K. The maximum TET under engine operation may be, for example, at least (or in the order of) any one of the following values: 1700 K, 1750 K, 1800 K, 1850 K, 1900 K, 1950 K, 2000 K, 2050 K, or 2100 K. Thus, solely by way of non-limiting example, the maximum TET of a geared gas turbine engine having a fan diameter in the range of 200 cm to 210 cm may be in the range of 1890 K to 1960 K. Strictly by way of example non-limiting, the maximum TET of a geared gas turbine engine having a fan diameter in the range of 210 cm to 230 cm may be in the range of 1890 K to 1960 K.Strictly by way of non-limiting example, the maximum TET of a geared gas turbine engine having a fan diameter in the range of 340 cm to 360 cm may be in the range of 1890 K to 1960 K. Strictly by way of non-limiting example, the maximum TET of a direct-drive gas turbine engine having a fan diameter in the range of 300 cm to 340 cm may be in the range of 1935 K to 1995 K. Strictly by way of non-limiting example, the maximum TET of a direct-drive gas turbine engine having a fan diameter in the range of 300 cm to 340 cm may be in the range of 1890 K to 1950 K. The maximum TET may be in an inclusive range bounded by any two of the values ​​in the preceding sentence. (i.e., the values ​​may form upper or lower limits), for example in the range from 1800 K to 1950 K, or from 1900 K to 2000 K.Maximum TET may occur, for example, under a high thrust condition, such as a maximum takeoff thrust (MTTP) condition.

[0273] A fan blade and / or airfoil portion of a fan blade described and / or claimed herein may be fabricated from any suitable material or combination of materials. For example, at least a portion of the fan blade and / or airfoil may be fabricated at least in part from a composite, for example, a metal matrix composite and / or an organic matrix composite, such as a carbon fiber composite. As a further example, at least a portion of the fan blade and / or airfoil may be fabricated at least in part from a metal, such as a titanium-based metal or an aluminum-based material (such as an aluminum- lithium) or a steel-based material. The fan blade may include two or more regions manufactured using different materials. For example, the fan blade may have a protective leading edge, which may be manufactured using a material that is more able to withstand impact (e.g., by birds, ice, or another material) than the rest of the blade. Such a leading edge may, for example, be manufactured using titanium or a titanium-based alloy. Thus, strictly by way of example, the fan blade may have a carbon fiber or aluminum-based (such as an aluminum-lithium alloy) body with a titanium leading edge.

[0274] A fan as described and / or claimed herein may include a central portion, from which the fan blades may extend, for example in a radial direction. The fan blades may be connected to the central portion in any desired manner. For example, each fan blade may include a fastening element that may engage a corresponding notch in the hub (or disc). Strictly by way of example, such a fastening element may be in the form of a dovetail that may notch into and / or engage a corresponding notch in the hub / disc to secure the fan blade to the hub / disc. As a further example, the fan blades may be formed integrally with a central portion. Such an arrangement may be referred to as a bladed disc or a bladed ring.Any suitable method may be used to manufacture such a bladed disc or bladed ring. For example, at least a portion of the fan blades may be machined from a block and / or at least a portion of the fan blades may be joined to the hub / disc by welding, such as a linear friction stir weld.

[0275] The gas turbine engines described and / or claimed herein may or may not be provided with a variable area nozzle (VAN). Such a variable area nozzle may allow the bypass duct outlet area to be varied during use. The general principles of the present disclosure may be applied to engines with or without a VAN.

[0276] The fan of a gas turbine as described and / or claimed herein may have any desired number of fan blades, for example, 14, 16, 18, 20, 22, 24, or 26 fan blades. When the fan blades have a carbon fiber composite body, there may be 16 or 18 fan blades. When the fan blades have a metal body (for example, aluminum-lithium or titanium alloy), there may be 18, 20, or 22 fan blades.

[0277] As used herein, the terms idle, taxi, takeoff, climb, cruise, descent, approach and landing (or one or more parts thereof) have the meaning conventional and would be readily understood by those skilled in the art. Thus, for a given gas turbine engine for an aircraft, those skilled in the art would immediately recognize that each term refers to the entirety, or one or more parts, of a phase of operation of the engine within a given mission of an aircraft to which the gas turbine engine is designed to be attached.

[0278] As such, ground idling may refer to a phase of engine operation where the aircraft is stationary and in contact with the ground, but where there is a need for the engine to taxi. At idle, the engine may produce between 3% and 9% of the available engine thrust. In other non-limiting examples, the engine may produce between 5% and 8% of the available thrust. In other non-limiting examples, the engine may produce between 6% and 7% of the available thrust. Taxiing may refer to a phase of engine operation where the aircraft is propelled along the ground by the thrust produced by the engine. When taxiing, the engine may produce between 5% and 15% of the available thrust. In other non-limiting examples, the engine may produce between 6% and 12% of the available thrust. In other non-limiting examples, the engine may produce between 7% and 10% of the available thrust.Takeoff may refer to a phase of engine operation where the aircraft is propelled by thrust produced by the engine. At an initial stage in the takeoff phase, the aircraft may be propelled while the aircraft is in contact with the ground. At a later stage in the takeoff phase, the aircraft may be propelled while the aircraft is not in contact with the ground. During takeoff, the engine may produce between 90% and 100% of the available thrust. In other non-limiting examples, the engine may produce between 95% and 100% of the available thrust. In other non-limiting examples, the engine may produce 100% of the available thrust.

[0279] Climb may refer to a phase of engine operation where the aircraft is propelled by thrust produced by the engine. During climb, the engine may produce between 75% and 100% of the available thrust. In other non-limiting examples, the engine may produce between 80% and 95% of the available thrust. In other non-limiting examples, the engine may produce between 85% and 90% of the available thrust. In this regard, climb may refer to an operational phase within an aircraft flight cycle between takeoff and arrival at cruise conditions, with arrival at cruise conditions thus defining the start of the cruise phase, or a portion thereof, of the aircraft flight.Additionally or alternatively, climb may refer to a nominal point in, or one or more nominal periods during, an aircraft flight cycle between takeoff and landing, where a relative increase in altitude is required, which may necessitate an additional demand for engine thrust.

[0280] As used herein, cruise conditions, which may define the cruise phase (or a portion thereof) of aircraft flight, have a conventional meaning and will be readily understood by those skilled in the art. In some examples, for a given gas turbine engine for an aircraft, cruise conditions may refer to an engine operating point at mid-cruise of a given mission (which may be referred to in the industry as an "economic mission") of an aircraft to which the gas turbine engine is designed to be attached. In this sense, mid-cruise may be considered the point in an aircraft flight cycle at which 50% of the total fuel that is burned between the end of climb and the beginning of descent has been burned (which may be approximated by the midpoint—in terms of time and / or distance—between the end of climb and the beginning of descent).Cruise conditions may therefore define an operating point, phase, or part thereof, of flight that provides thrust that would ensure steady-state operation (i.e., maintaining a constant altitude and / or a constant Mach number), or at least substantially steady-state operation (i.e., maintaining at least a substantially constant altitude and / or at least a substantially constant Mach number) of an aircraft to which it is designed to be attached, taking into account the number of engines provided on that aircraft. For example, where an engine is designed to be attached to an aircraft that has two engines of the same type, in cruise conditions the engine may provide half the total thrust that would be required for steady-state operation, or at least substantially steady-state operation, of that aircraft at mid-cruise.

[0281] In other words, for a given gas turbine engine for an aircraft, cruise conditions can be defined as the operating point of the engine that provides a specified thrust (required to provide - in combination with any other engines on the aircraft - steady-state operation, or at least substantially steady-state operation, of the aircraft to which it is designed to be attached at a given mid-cruise Mach number) at mid-cruise atmospheric conditions (defined by the International Standard Atmosphere according to ISO 2533 at the mid-cruise altitude). For any given gas turbine engine for an aircraft, the mid-cruise thrust, atmospheric conditions and Mach number are known, and thus the operating point of the engine at cruise conditions can be clearly defined.

[0282] Strictly by way of example, the forward speed at the cruise condition may be any point in the range of Mach 0.7 to 0.9, e.g., 0.75 to 0.85, e.g., 0.76 to 0.84, e.g., 0.77 to 0.83, e.g., 0.78 to 0.82, e.g., 0.79 to 0.81, e.g., of the order of Mach 0.8, of the order of Mach 0.85, or in the range of 0.8 to 0.85. Any single speed within these ranges may be part of the cruise condition. For a given aircraft, cruise conditions may be outside these ranges, for example below Mach 0.7 or above Mach 0.9.

[0283] Strictly by way of example, the cruising conditions may correspond to standard atmospheric conditions (according to the International Standard Atmosphere, ISA) at an altitude which is in the range of 10,000 m to 15,000 m, for example in the range of 10,000 m to 12,000 m, for example in the range of 10,400 m to 11,600 m (approximately 38,000 feet), for example in the range of 10,500 m to 11,500 m, for example in the range of 10,600 m to 11,400 m, for example in the range of 10,700 m (approximately 35,000 feet) to 11,300 m, for example in the range of 10,800 m to 11,200 m, for example in the range from 10,900 m to 11,100 m, for example of the order of 11,000 m. Cruise conditions may correspond to typical atmospheric conditions at any given altitude within these ranges.

[0284] Strictly by way of example, the cruise conditions may correspond to a forward Mach number of 0.8 and standard atmospheric conditions (according to the International Standard Atmosphere) at an altitude of 35,000 feet (10,668 m). At such cruise conditions, the engine may provide a known required net thrust level. The known required net thrust level is of course dependent on the engine and its intended application and may be, for example, a value ranging from 20 kN to 40 kN.

[0285] Strictly by way of further example, cruising conditions may correspond to a forward Mach number of 0.85 and standard atmospheric conditions (according to the International Standard Atmosphere) at an altitude of 38,000 feet (11,582 m). At such cruising conditions, the engine can provide a known required net thrust level. The known required net thrust level is of course dependent on the engine and its intended application and may be, for example, a value ranging from 35 kN to 65 kN.

[0286] In use, a gas turbine engine described and / or claimed herein may operate at cruise conditions defined elsewhere herein. Such cruise conditions may be determined by the cruise conditions (e.g., mid-cruise conditions) of an aircraft to which at least one (e.g., 2 or 4) gas turbine engines may be mounted to provide propulsive thrust.

[0287] Further, one skilled in the art would readily recognize that either or both of a descent and an approach refers to a phase of operation within an aircraft flight cycle between cruise and landing of the aircraft, with an approach in particular being part of the landing and takeoff (LTO) phase. During either or both of the descent and approach, the engine may produce between 0% and 50% of the thrust available. In other non-limiting examples, the engine may produce between 25% and 40% of the available thrust. In other non-limiting examples, the engine may produce between 30% and 35% of the available thrust. Additionally or alternatively, descent may refer to a nominal point in an aircraft flight cycle between takeoff and landing, where a relative decrease in altitude is required, and which may necessitate a reduced thrust demand from the engine.

[0288] According to one aspect, there is provided an aircraft comprising a gas turbine engine as described and / or claimed herein. The aircraft according to this aspect is the aircraft to which the gas turbine engine has been designed to be attached. Accordingly, the cruise conditions according to this aspect may correspond to an operating point, a phase, or a portion thereof, of the flight of the aircraft, as defined elsewhere herein.

[0289] According to one aspect, there is provided a method of operating a gas turbine engine as described and / or claimed herein. The operation may be at any suitable condition, which may be as defined elsewhere herein (e.g., in terms of thrust, atmospheric conditions, and Mach number).

[0290] According to one aspect, there is provided a method of operating an aircraft comprising a gas turbine engine as described and / or claimed herein. Operation according to this aspect may comprise (or may be) operation at any suitable condition, for example at mid-cruise of the aircraft, as defined elsewhere herein.

[0291] Those skilled in the art would understand that, unless mutually exclusive, any feature or parameter described in connection with any of the above aspects may be applied to any other aspect. Furthermore, unless mutually exclusive, any feature or parameter contained or described herein may be applied to any aspect and / or in association with any other feature or parameter contained or described herein.

[0292] Except where mutually exclusive, any parameter or value contained or described herein may be applied and / or combined with any one or more parameters and / or values ​​contained or described herein. For example, a first parameter or value contained or described herein (e.g., parameter A) may be applied and / or combined with any one or more other parameters and / or values ​​contained or described herein (e.g., any one or more of parameter B; parameter C; and parameter D, and so on) to express a product of their relationship.For example, one skilled in the art would understand that when parameter A is described separately from parameter B, a product of their relationship may be expressed as, for example, A / B, B / A, B*A, or any other application, combination, or function of parameter A with respect to parameter B, as needed.

[0293] Embodiments will now be described by way of example only, with reference to the Figures, in which:

[0294] [Fig.l] is a sectional side view of a gas turbine engine;

[0295] [Fig.2] is a close-up sectional side view of an upstream portion of a motor geared gas turbine;

[0296] [Fig.3] is a partially cutaway view of a reduction gear for a gas turbine engine;

[0297] [Fig.4] is a representation of an aircraft with a propulsion system comprising two gas turbine engines;

[0298] [Fig.5] is a schematic representation of an exemplary fuel system;

[0299] [Fig.6] is a representation of an alternative example of a fuel system;

[0300] [Fig.7] is a representation of a portion of an exemplary oil system recirculation (a primary oil loop system);

[0301] [Fig.8] is a representation of another portion of an exemplary recirculating oil system (a secondary oil loop system);

[0302] [Fig.9] is a representation of a portion of an alternative example of a recirculating oil system, also showing features of the air-oil heat exchanger;

[0303] [Fig. 10] is a representation of a portion of the exemplary fuel system of [Fig. 5] and the exemplary recirculating oil system of Figures 7 and 8;

[0304] [Fig.l 1] is a representation of another example of a recirculating oil system, illustrating all of the main components of a heat exchange system;

[0305] [Fig. 12] illustrates an exemplary method of operating a gas turbine engine;

[0306] [Fig. 13] illustrates another example of a method of operating a gas turbine engine;

[0307] [Fig. 14] illustrates another example of a method of operating a gas turbine engine;

[0308] [Fig. 15] illustrates another example of a method of operating a gas turbine engine;

[0309] [Fig. 16] illustrates another example of a recirculating oil system of a gas turbine engine;

[0310] [Fig. 17] illustrates another example of a method of operating a gas turbine engine; and

[0311] [Fig. 18] illustrates a refrigeration cycle apparatus incorporated in a gas turbine engine.

[0312] [Fig.l] illustrates a gas turbine engine 10 having a main rotational axis 9. The engine 10 includes an air intake 12 and a propulsion fan 23 that generates two airflows: a core airflow A and a bypass airflow B. The gas turbine engine 10 includes a core 11 that receives the core airflow A. The engine core 11 includes, in axial flow series, a low-pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, a low-pressure turbine 19, and a core exhaust nozzle 20. A nacelle 21 surrounds the gas turbine engine 10 and defines a bypass duct 22 and a bypass exhaust nozzle 18. The bypass airflow B flows through the bypass duct 22. The blower 23 is attached to, and driven by, the low pressure turbine 19 by means of a shaft 26 and an epicyclic reduction gear 30.

[0313] In use, the core airflow A is accelerated and compressed by the low pressure compressor 14 and directed into the high pressure compressor 15 where further compression takes place. The compressed air discharged from the high pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel F and the mixture is combusted. The combustion equipment 16 may be referred to as the combustor 16, with the terms "combustion equipment 16" and "combustion chamber 16" used interchangeably herein. The resulting hot combustion products then expand, and thereby drive, the high pressure and low pressure turbines 17, 19 before being discharged through the nozzle 20 to provide some propulsive thrust. The high pressure turbine 17 drives the high pressure compressor 15 by a suitable interconnecting shaft 27.The blower 23 generally acts to impart increased pressure to the bypass airflow B flowing through the bypass duct 22, such that the bypass airflow B is exhausted through the bypass exhaust nozzle 18 to generally provide the majority of the propulsive thrust. The epicyclic reduction gear 30 is a reduction box.

[0314] An exemplary arrangement for a geared fan gas turbine engine 10 is illustrated in [Fig. 2]. The low pressure turbine 19 (see [Fig. 1]) drives the shaft 26, which is coupled to a sun gear, or sun pinion, 28 of the epicyclic reduction gear 30. Radially outwardly of the sun gear 28 and meshing therewith are a plurality of planet gears 32 which are coupled together by a planet carrier 34. The planet carrier 34 forces the gears 32 to change orientation about the sun gear 28 in synchronism while allowing each planet gear 32 to rotate about its own axis. The planet carrier 34 is coupled by means of links 36 to the fan 23 in order to cause its rotation around the engine axis 9. Radially outwardly of the planet gears 32 and meshing with these, there is a ring or toothed crown 38 which is coupled, by means of links 40, to a stationary support structure 24.

[0315] It should be noted that the terms "low pressure turbine" and "low pressure compressor" as used herein may be taken to indicate the lowest pressure turbine stages and the lowest pressure compressor stages (i.e., not including the fan 23) respectively and / or the turbine and compressor stages that are connected together by the interconnecting shaft 26 with the lowest rotational speed in the engine (i.e., not including the reduction gear output shaft that drives the fan 23). In some literature, the "low pressure turbine" and "low pressure compressor" referred to herein may alternatively be known as an "intermediate pressure turbine" and "intermediate pressure compressor".When such alternative nomenclature is used, the blower 23 may be designated as the first compression stage or the lower pressure compression stage.

[0316] The epicyclic reduction gear 30 is shown by way of example in more detail in [Fig. 3]. Each of the sun gear 28, the planet gears 32 and the ring gear 38 includes teeth around its periphery for meshing with the other gears. However, for clarity, only exemplary portions of the teeth are illustrated in [Fig. 3]. There are four planet gears 32 illustrated, although it will be apparent to the skilled reader that more or fewer planet gears 32 may be provided within the scope of the claimed invention. Practical applications of an epicyclic planetary reduction gear 30 generally include at least three planet gears 32.

[0317] The epicyclic reduction gear 30 illustrated by way of example in Figures 2 and 3 is of the planetary type, in that the planet carrier 34 is coupled to an output shaft via links 36, with the ring gear 38 fixed. However, any other suitable type of epicyclic reduction gear 30 may be used. As a further example, the epicyclic reduction gear 30 may be a star arrangement, in which the planet carrier 34 is held fixed, with the ring gear 38 allowed to rotate. In such an arrangement, the fan 23 is driven by the ring gear 38. As another alternative example, the reduction gear 30 may be a differential reduction gear in which both the ring gear 38 and the planet carrier 34 are allowed to rotate.

[0318] It will be appreciated that the arrangement illustrated in Figures 2 and 3 is by way of example only, and that various alternatives are within the scope of this disclosure. Strictly by way of example, any suitable arrangement may be used to position the reducer 30 in the motor 10 and / or to connect the reducer 30 to the motor 10. By way of further example, the connections (such as links 36, 40 in the example of [Fig. 2]) between the reducer 30 and other parts of the motor 10 (such as the input shaft 26, the output shaft and the fixed structure 24) may have any desired degree of rigidity or flexibility. As a further example, any suitable arrangement of bearings between rotating and stationary parts of the motor (e.g. between the input and output shafts from the reducer and fixed structures, such as the reducer housing) may be used, and the description is not limited to the exemplary arrangement of [Fig. 2]. For example, where the reducer 30 has a star arrangement (described above), one skilled in the art would readily understand that the arrangement of the output and support links and bearing locations would typically be different from that shown as an example in [Fig. 2].

[0319] Thus, the present disclosure extends to a gas turbine engine having any arrangement of reduction gear styles (e.g., star or planetary), support structures, input and output shaft arrangements, and bearing locations.

[0320] Optionally, the reducer may drive additional and / or alternative components (e.g. the intermediate pressure compressor and / or a booster).

[0321] Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. For example, such engines may have a different number of compressors and / or turbines and / or a different number of interconnecting shafts. As a further example, the gas turbine engine illustrated in [Fig.l] has a split-flow nozzle 18, 20 which means that the flow through the bypass duct 22 has its own nozzle 18 which is independent of, and radially outward of, the core engine nozzle 20. However, this is not limiting, and any aspect of the present disclosure may also apply to engines in which the flow through the bypass duct 22 and the flow through the core 11 are mixed, or combined, before (or upstream of) a single nozzle, which may be referred to as a mixed-flow nozzle.The nozzle(s) (whether mixed or split flow) may have a fixed or variable area.

[0322] As a further example, other gas turbine engines to which the present disclosure may be applied may not have a reduction gear for the main shaft(s), being instead direct drive engines.

[0323] Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. For example, such engines may have an alternative number of interconnecting shafts (e.g., two) and / or an alternative number of compressors and / or turbines. Further, the engine may include a reducer provided in the drive train from a turbine to a compressor and / or a blower.

[0324] While the described example relates to a bypass engine, the description may apply, for example, to any type of gas turbine engine, such as an open rotor (in which the fan stage is not surrounded by a nacelle) or a turboprop, for example. In some arrangements, the gas turbine engine 10 may not include a reduction gear 30.

[0325] The geometry of the gas turbine engine 10, and components thereof, is defined by a conventional axis system, comprising an axial direction (which is aligned with the rotation axis 9), a radial direction (in the bottom-up direction in [Fig.l]) and a circumferential direction (perpendicular to the page in the view of [Fig.l]). The axial, radial and circumferential directions are mutually perpendicular.

[0326] The fuel F supplied to the combustion equipment 16 may comprise a fossil-based hydrocarbon fuel, such as kerosene. Thus, the fuel F may comprise molecules from one or more of the chemical families of n-alkanes, isoalkanes, cycloalkanes, and aromatics. In addition or alternatively, when cut with, or blended with, or substituted for an alternative fuel, the fuel F may comprise renewable hydrocarbons produced from biological or non-biological resources, otherwise known as sustainable aviation fuel (SAF). In each of the examples provided, the fuel F may comprise one or more trace elements including, for example, sulfur, nitrogen, oxygen, inorganics, and metals.

[0327] SAF is understood by those skilled in the art to refer to, for example, a biofuel, a renewable aviation fuel, a renewable jet fuel, an alternative fuel or a biofuel, produced from biological or non-biological resources. SAF is understood to be generally synthesized from carbon-containing gases extracted from the atmosphere and / or captured during industrial processes; or from a wide range of sustainable feedstocks such as, for example, used oils and greases; municipal solid waste; cellulosic waste (such as corn stover); cover crops such as camelina, carinata and stinkweed; non-biogenic alternative fuels; jatropha; halophytes and algae, rather than fossil-based hydrocarbons. SAF is not understood to encompass fossil fuels.

[0328] The operational performance of a given fuel composition, or fuel mixture F for use in a given mission, may be defined, at least in part, by the fuel's ability to handle the Brayton cycle of the gas turbine engine 10. The performance-defining parameters functionalities may include, for example, specific energy; energy density; thermal stability; and emissions including gaseous and / or particulate matter emissions. In this regard, particulate matter emissions may include soot particles created by the combustion of said fuel F, also known as non-volatile particulate matter (nvPM). Any reference made herein to soot or smoke may also apply to other types of particulate matter emissions known in the art. The gaseous emissions may include any one or more of nitrogen oxides (NOx); carbon monoxide (CO); carbon dioxide (CO2); unburned hydrocarbons (UHC); sulfur oxides (SO), including, for example, sulfur dioxide (SO2) and / or sulfur trioxide (SO3); and volatile organic compounds (VOCs) created by the combustion of said fuel F.Any reference made herein to gaseous emissions may also apply to other types of gaseous emissions known in the art.

[0329] A relatively higher specific energy (i.e., energy per unit mass), expressed in MJ / kg, may at least partially reduce takeoff weight, potentially providing a relative improvement in fuel efficiency. A relatively higher energy density (i.e., energy per unit volume), expressed in MJ / L, may at least partially reduce fuel takeoff volume, which may be particularly important for volume-limited missions or military operations involving refueling. A relatively higher thermal stability (i.e., inhibiting fuel degradation or coking under thermal stress) may allow the fuel to experience elevated temperatures in the engine and fuel injectors, potentially providing relative improvements in combustion efficiency.Reduced emissions, including particulate matter, can allow for reduced contrail formation, while reducing the environmental impact of a given mission.

[0330] Other fuel properties may also be critical to functional performance. For example, a relatively lower freezing point (°C) may allow long-range missions to optimize flight profiles; minimum aromatic concentrations (%) may ensure sufficient swelling of certain materials used in the construction of O-rings and seals previously exposed to high aromatic fuels; and, maximum surface tension (mN / m) may ensure sufficient spray breakup and atomization of the fuel.

[0331] The ratio of the number of hydrogen atoms to the number of carbon atoms in a molecule can influence the specific energy of a given composition, or fuel mixture. Fuels with higher ratios of hydrogen atoms to carbon atoms can have higher specific energies in the absence of bonding strain. For example, fossil-based hydrocarbon fuels may comprise molecules with approximately 7 to 18 carbons, with a significant portion of a given composition derived from molecules of 9 to 15 carbons, with an average of 12 carbons.

[0332] A number of sustainable aviation fuel blends have been approved for use. For example, some approved blends include blend ratios of up to 10% sustainable aviation fuel, while other approved blends include blend ratios of between 10% and 50% sustainable aviation fuel (the remainder comprising one or more fossil-based hydrocarbon fuels, such as kerosene), with further compositions pending approval. However, there is an anticipation in the aviation industry that sustainable aviation fuel blends comprising up to (and including) 100% sustainable aviation fuel (SAF) will eventually be approved for use.

[0333] Sustainable aviation fuels may comprise one or more of n-alkanes, iso-alkanes, cycloalkanes, and aromatics, and may be produced, for example, from one or more of synthesis gas (syngas); lipids (e.g., fats, oils, and greases); sugars; and alcohols. Thus, sustainable aviation fuels may comprise lower aromatic and / or sulfur contents, relative to fossil-based hydrocarbon fuels. Additionally or alternatively, sustainable aviation fuels may comprise one and / or the other of higher iso-alkane content and cycloalkane content, relative to fossil-based hydrocarbon fuels. In some examples, sustainable aviation fuels may include a density between 90% and 98% of that of kerosene and / or a heating value between 101% and 105% of that of kerosene.

[0334] In some examples, the sustainable aviation fuel(s), or blend(s) supplied to the combustion equipment 16 may have a relatively lower aromatics and / or other non-paraffinic content than kerosene. The sustainable aviation fuel may comprise an aromatics content of, for example, 30%, 20%, 15%, 10%, 8%, 5% or less than 5%; for example, 4%, 3%, 2%, 1% or less than 1%; for example, 0.75%, 0.5%, 0.25% or less than 0.25%; for example, 0.2%, 0.1% or less than 0.1%; for example, 0.01%, 0.001% or 0%. The aromatics content of sustainable aviation fuel may be within an inclusive figure or range bounded by any two or more of the values ​​in the preceding sentence (i.e. the values ​​may form upper limits or lower), for example 13.5%, 8.5%, 2.5%, 0.35%, 0.15%, 0.05%, 0.005% or 0%; or from 0% to 0.75%, from 0% to 0.5%, or from 0.1% to 0.25%; or from 0.15% to 0.65%, from 0.35% to 0.55%, or from 0.035% to 0.055%; depending on one or more preferences, stock or fuel supplier, and composition variations.

[0335] Due at least in part to the molecular structure of sustainable aviation fuels, sustainable aviation fuels may provide beneficial effects including, for example, one or more of higher specific energy (despite, in some examples, lower energy density); higher specific heat capacity; higher thermal stability; higher lubricity; lower viscosity; lower surface tension; lower freezing point; lower soot emissions; lower NOx; and, lower CO2 emissions, compared to fossil-based hydrocarbon fuels (e.g., when burned in combustion equipment 16).Thus, compared to fossil-based hydrocarbon fuels, such as kerosene, sustainable aviation fuels can lead to either or both a relative decrease in specific fuel consumption, and a relative decrease in maintenance costs.

[0336] As shown in [Fig.4], an aircraft 1 may include a plurality of fuel tanks 50, 53; for example, a larger primary fuel tank 50 located in the aircraft fuselage, and a smaller fuel tank 53a, 53b located in each wing. In other examples, an aircraft 1 may have only one fuel tank 50, and / or the wing fuel tanks 53 may be larger than the center fuel tank 50, or no center fuel tank may be provided (with all fuel being stored instead in the wings of the aircraft) - it will be understood that many different tank arrangements are contemplated and that the examples that have been illustrated are provided for ease of description and not intended to be limiting.

[0337] [Fig. 4] shows an aircraft 1 with a propulsion system 2 comprising two gas turbine engines 10. The gas turbine engines 10 are supplied with fuel from a fuel supply system on board the aircraft 1. The fuel supply system 1000 of the illustrated example comprises a single fuel source. For the purposes of the present application, the term "fuel source" means either 1) a single fuel tank or 2) a plurality of fuel tanks that are fluidically interconnected. Each fuel source is intended to provide a distinct source of fuel, i.e., a first fuel source may contain a first fuel having a different characteristic or characteristics than a second fuel contained in a second fuel source. The first and second fuel sources are therefore not coupled. fluidly to each other so as to separate the different fuels (at least under normal operating conditions). The use of multiple fuel sources allows an aircraft 1 to carry multiple different fuels and to change the fuel used during operation, and possibly even during cruise or when transitioning from one stage of operation to another during flight.

[0338] In the present examples, the first (and, in these examples, only) fuel source comprises a central fuel tank 50, located primarily in the fuselage of the aircraft 1 and a plurality of wing fuel tanks 53a, 53b, where at least one wing fuel tank is located in the port wing and at least one wing fuel tank is located in the starboard wing for balancing. All tanks 50, 53 are fluidly connected in the example shown, thus forming a single fuel source.Each of the center fuel tank 50 and the wing fuel tanks 53 may include a plurality of fluidly interconnected fuel tanks.

[0339] In another example, the wing fuel tanks 53a, 53b may not be fluidly connected to the center tank 50, thereby forming a second, separate fuel source. For balancing purposes, one or more port wing fuel tanks may be fluidly connected to one or more starboard wing fuel tanks. This may be done either through a center fuel tank (if that tank is not part of the other fuel source), or by bypassing the center fuel tank(s), or both (for maximum flexibility and safety). In another example, the first fuel source includes wing fuel tanks 53 and a center fuel tank 50, while a second fuel source includes another, separate center fuel tank.

[0340] Fluid interconnection between the wing fuel tanks and the center fuel tank of the first fuel source may be provided for balancing the aircraft 1. In the aircraft 1 with multiple fuel sources, two or more of the fuel sources may therefore contain different fuels from each other, so that the aircraft 1 may change fuels during flight. It may therefore be more complex to determine which fuel is supplied to the combustion chamber 16 than simply recording a single identity of a fuel on board the aircraft 1 or performing a check at the time of start-up.

[0341] In some examples, the distribution of fuel tanks 50, 53 available on the aircraft 1 may be limited such that the first fuel source and the second fuel source are each substantially symmetrical about the aircraft centerline. In cases where an asymmetrical distribution of the fuel tanks fuel is permitted, a suitable means of fuel transfer is generally provided between the fuel tanks of the first fuel source and / or between the fuel tanks of the second fuel source such that the position of the center of mass of the aircraft can be maintained within acceptable lateral limits throughout the duration of the flight.

[0342] Aircraft typically refuel at several different airports, for example at the beginning and end of a long-haul flight. While there are standards that all aviation fuels must comply with, as mentioned above, different aviation fuels have different compositions, for example depending on their source (e.g., different sources of petroleum, biofuels, or other synthetic aviation fuels (often described as sustainable aviation fuels - SAF), and / or blends of petroleum-based fuels and other fuels) and any included additives (e.g., antioxidants and metal deactivators, biocides, static reducers, icing inhibitors, corrosion inhibitors) and any impurities.The composition of available aviation fuel may not only vary from airport to airport and fuel supplier to fuel supplier, but also from batch to batch, even for a given airport or fuel supplier. Furthermore, the fuel tanks 50, 53 of the aircraft 1 are generally not emptied before being refilled for a subsequent flight, resulting in mixtures of different fuels in the tanks - fuel with a different composition effectively resulting from the mixture.

[0343] The inventors were aware that, as different fuels may have different properties, while still remaining compliant with standards, knowledge of the fuel(s) available for an aircraft 1 may allow for more efficient and tailored control of the aircraft 1, and more specifically of the aircraft's propulsion system 2 (i.e., the gas turbine engine(s) 10 of the aircraft 1, and associated controls and components). Knowledge of the fuel may therefore be used as a tool to improve aircraft performance, such that determining or monitoring the fuel composition may have advantages.In particular, determining one or more characteristics of the fuel to be supplied to the combustion chamber 16 - whether fuel from a single fuel source or a mixture of one or more fuels from different sources - is therefore important in determining the operation of the engine. A key element of engine operation is heat management - engine heat management is primarily achieved using heat transfer to and from the oil and fuel in the engine, so that controlling a heat exchange system 3000 - for example by . regulating oil flow, regulating fuel flow, and / or controlling one or more heat exchangers 1004, 1006, 2020, 2030 or other heat exchange components in other ways - may optimize the performance of the engine 10 for a fuel with particular fuel characteristics.

[0344] As used herein, the term "fuel characteristics" refers to intrinsic or inherent properties of the fuel, such as fuel composition, and not to variable properties such as volume or temperature. One or more fuel characteristics may be determined, and this data used to adjust control of the engine 10, and in particular the heat management system 3000. Examples of fuel characteristics include one or more of:

[0345] i. the percentage of sustainable aviation fuel (SAF) in the fuel, or an indication that the fuel is a fossil fuel, e.g., fossil kerosene, or that the fuel is pure SAF fuel;

[0346] ii. parameters of a distribution of hydrocarbons in the fuel, such as:

[0347] • the aromatic hydrocarbon content of the fuel, and possibly also / alternatively the multi-aromatic hydrocarbon content of the fuel;

[0348] • the hydrogen / carbon (H / C) ratio of the fuel;

[0349] • composition information in % for some or all hydrocarbons present;

[0350] iii. the presence or percentage of a particular element or species, such as:

[0351] • a nitrogen content of the fuel / the percentage of nitrogen-containing species in fuel;

[0352] • the presence or percentage of a tracer species or trace element in the fuel ;

[0353] • a naphthalene content of the fuel;

[0354] • a sulfur content of the fuel / the percentage of sulfur-containing species in fuel;

[0355] • a cycloparaffin content of the fuel;

[0356] • an oxygen content of the fuel;

[0357] iv. one or more properties of the fuel used in a gas turbine engine 10, such as:

[0358] • a level of emissions of non-volatile particulate matter or CO2 during the combustion;

[0359] • a level of coking or varnishing of the fuel (or other measure of the fuel degradation product depot);

[0360] v. one or more properties of the fuel itself, independent of its use in an engine 10 or combustion, such as:

[0361] • thermal stability of the fuel (e.g., degradation temperature thermal); and

[0362] • one or more physical properties such as a density, a viscosity, a calorific value, freezing temperature and / or heat capacity.

[0363] The fuel characteristic(s) to be determined may be chosen based on the fuel properties most relevant to modifications that may be made to the heat management system 3000. Determining fuel characteristics may include obtaining fuel characteristics of any fuel already present in the fuel tank 50, 53 prior to refueling and an indication of the amount of fuel remaining, and then combining this information with information regarding new fuel added to the tank 50, 53 upon refueling.

[0364] Obtaining fuel characteristics of any fuel already present in the fuel tank 50, 53 prior to refueling, and / or obtaining fuel characteristics of a fuel supplied during refueling, may include one or more of the following:

[0365] (i) the physical and / or chemical detection of one or more particularities or fuel composition parameters (which may allow direct detection of fuel characteristics and / or may allow determination of fuel characteristics using the detection results), and / or the detection of one or more tracer elements or compounds added to the fuel to facilitate its identification (e.g., a dye);

[0366] (ii) retrieving fuel characteristic information from an integrated memory or data store; and / or

[0367] (iii) receiving data, for example from an input provided in a user interface, or data transmitted to the aircraft 1.

[0368] In some examples, one or more fuel characteristics may be determined during operation of the gas turbine 10, for example by deriving fuel characteristics of the fuel supplied to the combustor 16 in operation from engine performance measurements, or by performing on-wing sensing.

[0369] In some examples, multiple different methods may be performed to obtain the fuel characteristics - e.g., different methods may be used for different characteristics, and / or different methods may be used for the same characteristics as a control. For example, stored or otherwise provided data on the fuel characteristics may be compared with the results of chemical or physical detection of one or more fuel parameters. In the event of a mismatch between the stored fuel characteristic and the corresponding detected parameter, an alert can be provided.

[0370] Fuel characteristics may be determined by physically and / or chemically sensing one or more features of the fuel composition (e.g., in an off-wing test unit, or when the fuel is transported to an on-wing fuel tank, or when used in the gas turbine engine 10), thereby enabling the fuel characteristics to be directly sensed or data to be provided from which they may be determined, as discussed above, and / or detecting one or more tracer elements or compounds added to the fuel to aid in its identification (e.g., a dye); or by receiving data, e.g., from an input provided in a user interface, or data transmitted to the aircraft, e.g., by scanning a barcode associated with the fuel delivery.

[0371] When a physical and / or chemical determination is used, the fuel characteristics may be detected in various ways, both directly (e.g., from sensor data corresponding to the fuel characteristic in question) and indirectly (e.g., by inference or calculation from other characteristics or measurements, or by reference to data of a specific tracer detected in the fuel). The characteristics may be determined as relative values ​​by comparison with another fuel or as absolute values. For example, one or more of the following detection methods may be used: • The aromatic or cycloparaffin content of the fuel can be determined based on measurements of the swelling of a sensor component made from a sealing material such as a nitrile sealing material. • Trace substances or species, either naturally occurring in the fuel or added as a tracer, can be used to determine fuel characteristics such as the percentage of sustainable aviation fuel in the fuel or whether the fuel is kerosene. • Measurements of the vibrational mode of a piezoelectric crystal exposed to fuel can be used as a basis for determining various fuel characteristics, including the aromatic content of the fuel, the oxygen content of the fuel, and the thermal stability or coking / spitting level of the fuel - for example, by measuring the accumulation of surface deposits on the piezoelectric crystal, which will cause a change in vibrational mode. Various fuel characteristics may be determined by collecting performance parameters of the gas turbine engine 10 during a first operating period (e.g., during takeoff), and possibly also during a second operating period (e.g., during cruise), and comparing these collected parameters to expected values ​​using a fuel of known properties. Various fuel characteristics, including the aromatic hydrocarbon content of the fuel, may be determined based on sensor measurements of the presence, absence, or degree of condensation trail formation by the gas turbine 10 during operation. Fuel characteristics, including aromatic hydrocarbon content, can be determined based on a UV-Vis spectroscopy measurement performed on the fuel. Various fuel characteristics, including sulfur content, naphthalene content, aromatic hydrogen content, and hydrogen-to-carbon ratio, may be determined by measuring substances present in the exhaust gases emitted by the gas turbine engine 10 during operation. The calorific value of the fuel may be determined during operation of the aircraft 1 based on measurements taken as the fuel is burned - for example using the fuel flow rate and shaft speed or the temperature change in the combustion chamber 16. Various fuel characteristics may be determined by making an operational change to affect the operation of the gas turbine engine 10, and detecting a response to the operational change; and determining the one or more fuel characteristics based on the response to the operational change. Various fuel characteristics may be determined relative to the fuel characteristics of a first fuel by replacing the first fuel supplied to the gas turbine engine 10 with a second fuel, and determining the fuel characteristic(s) of the second fuel based on a change in a relationship between T30 and one of T40 and T41 (the relationship indicating the temperature increase in the combustion chamber 16). The characteristics may be determined as relative values ​​by comparison with the first fuel, or as absolute values, e.g. as a reference to known values ​​for the first fuel.

[0372] As used herein, T30, T40, and T41, and any other numbered pressures and temperatures, are defined using the station numbering listed in the SAE AS755 standard, specifically:

[0373] • T30 = Total outlet temperature of the high pressure compressor (HPC);

[0374] • T40 = Total temperature at the combustion outlet;

[0375] • T41 = Total temperature at the inlet of the high pressure turbine (HPT) rotor.

[0376] Any suitable approach known in the art may be used, and the determination fuel characteristics will not be discussed further here.

[0377] The aircraft 1 may therefore comprise a fuel composition determination module 57 for determining at least one fuel characteristic of the fuel. The determination may be made by obtaining a value from a data store (e.g., from data provided to the aircraft 1 during refueling), or by performing one or more calculations based on data provided by one or more sensors or other engine components, using any of the methods described above. In the example of [Fig. 4], a fuel composition determination module 57 is integrated into each engine 10, where the fuel enters the engine 10. In other implementations, the module 57 may be located differently within the engine 10 or located elsewhere on the aircraft 1, e.g., in, on, or adjacent to a fuel tank 50, 53.In implementations with a fuel composition determination module 57, the module 57 may be intended to provide an output to an electronic engine control (EEC), or the module 57 may be provided as part of an EEC. The fuel characteristic(s) determined by the fuel composition determination module 57 may therefore be used to influence control of the engine 10. A specialized controller 58 may be provided to process the output of the fuel composition determination module 57 and provide control instructions to the controllable engine components based on the fuel characteristic information. In other implementations, the control functionality 58 may be provided by the EEC and no separate unit or module may be provided.

[0378] An exemplary fuel system 1000 for a geared gas turbine engine 10 is illustrated in [Fig. 5], the fuel system 1000 including a fuel flow path from the fuel tank 50 to the combustion chamber 16 of the gas turbine engine 10 of the aircraft 1. The fuel system 1000 includes both the fuel supply system 50, 1002 (which supplies fuel to the engine 10) and the fuel management system 1500 (which operates within the engine 10) of the aircraft 1. The fuel management system 1500 has a role in fuel temperature management as well as fuel flow, directing the fuel through one or more heat exchangers 1004, 1006 of the engine heat exchange system 3000. The heat exchange system 3000 includes portions of the fuel management system 1500, as well as the recirculating oil system 2000, 2000' (which is described in more detail below). The heat exchange system 3000 is the general term used for the systems and components used to transfer heat between fluids (in particular, oil and fuel) within the engine 10, and includes heat exchangers, valves, connecting piping and associated components, e.g., pumps, refrigeration units, etc.

[0379] Returning to the fuel system 1000, fuel is pumped from the fuel tank 50 to the gas turbine engine 10 by a low-pressure fuel feed pump 1002. The fuel then flows through a secondary fuel-oil heat exchanger 1004 and a primary fuel-oil heat exchanger 1006. The primary fuel-oil heat exchanger 1006 may be referred to as the primary fuel-oil heat exchanger because the oil flowing therethrough may be used to cool and lubricate the primary reduction gear 30 of the engine 10. The gas turbine engine 10 of the implementations being described further includes a generator (particularly, an integrated drive generator) and a secondary oil loop system for supplying oil to the generator.

[0380] The secondary fuel-oil heat exchanger 1004 may be referred to as an integrated drive generator fuel-oil heat exchanger because oil flowing therethrough may be used to cool and / or lubricate one or more components of the integrated drive generator (IDG) of the engine 10. In other implementations, a different type of generator may be used in place of an IDG—for example, a variable frequency generator (VFG) or a variable frequency starter generator (VFSG). The fuel system 1000 of such implementations may be otherwise equivalent.

[0381] The engine 10 of the example being described thus comprises two fuel-oil heat exchangers 1004, 1006. More or fewer fuel-oil heat exchangers may be provided in other implementations. The illustrated fuel management system 1500 is arranged so that the fuel reaches the secondary fuel-oil heat exchanger 1004 before the primary fuel-oil heat exchanger 1006. After leaving the primary fuel-oil heat exchanger 1006, the fuel then passes through an engine fuel pump 1003 and then to the combustion chamber 16. The engine fuel pump 1003 may be described as a primary fuel pump. In other implementations, the fuel pump engine 1003 may be upstream of one or more of the heat exchangers 1004, 1006.

[0382] In the example illustrated in [Fig.5], the fuel system 1000 further comprises a temperature sensor 1009 adapted to sense the temperature of the fuel approaching or reaching the combustion chamber 16. The temperature sensor 1009 is shown adjacent an inlet of the combustion chamber 16 in the illustrated example, but in various examples, the sensor 1009 may be placed anywhere downstream of the primary fuel-oil heat exchanger 1006 (or the most downstream fuel-oil heat exchanger in other implementations), on the fuel side, so as to provide a measurement of the fuel temperature.In gas turbine engines 10 such as those described herein, fuel passes through fuel spray nozzles before entering the combustion chamber proper - these nozzles can be very sensitive to thermal degradation of fuel resulting in obstructions in relatively narrow passages (e.g., metering slots); therefore, it is advisable in some implementations to locate the sensor 1009 as close as reasonably possible to the nozzle inlet. In some implementations, multiple temperature sensors 1009 may be used. The one or more temperature sensors 1009 are intended to provide an output (temperature data) to a controller 58.

[0383] The primary fuel-oil heat exchanger 1006 and the secondary fuel-oil heat exchanger 1004 are configured such that an oil flow is routed through each in addition to the fuel flow therethrough. The primary fuel-oil heat exchanger 1006 and the secondary fuel-oil heat exchanger 1004 are configured such that heat can be transferred between the oil and fuel flowing therethrough.In standard operation of the engine 10, for example, during cruising conditions, the average temperature of the oil flow entering the primary fuel-oil heat exchanger 1006 is higher than the average temperature of the fuel entering the primary fuel-oil heat exchanger 1006, and the average temperature of the oil flow entering the secondary fuel-oil heat exchanger 1004 is higher than the average temperature of the fuel entering the secondary fuel-oil heat exchanger 1004. In this manner, the primary fuel-oil heat exchanger 1006 and the secondary fuel-oil heat exchanger 1004 are each configured to transfer thermal energy from an oil flow to a fuel flow flowing therethrough in operation.

[0384] The two oil flows (oil flow through the primary heat exchanger and oil flow through the secondary heat exchanger) can be separated - physically separate and possibly also chemically distinct oils, and / or having a different flow rate. A different oil may therefore flow through the primary fuel-oil heat exchanger 1006 than that flowing through the secondary fuel-oil heat exchanger 1004. Each heat exchanger 1004, 1006 may be on a separate closed-loop oil system 2000, 2000'.

[0385] In general, at least the majority of the fuel that passes through the secondary fuel-oil heat exchanger 1004 also passes through the primary fuel-oil heat exchanger 1006. The two heat exchangers 1004, 1006 may therefore be described as being in series with each other, with respect to the fuel flow, and as being along a primary fuel flow path from the tank 50 to the combustion chamber 16. However, either or each heat exchanger 1004, 1006 may be provided with a bypass to allow some of the fuel to avoid passing through the respective heat exchanger, for example in the form of a bypass pipe 1005, as illustrated in [Fig. 5]. A valve (not shown) can determine what proportion of the fuel passes through the heat exchanger 1004 and what proportion passes through the bypass pipe 1005.In various implementations, a bypass pipe may be provided for each heat exchanger 1004, 1006, allowing a portion of the fuel to bypass one or both heat exchangers. In addition or alternatively, one or more bypass pipes 2005, 2005' may be provided for the oil for one or both fuel-oil heat exchangers 1004, 1006, thereby allowing a portion of the oil to bypass one or more heat exchangers. A valve for regulating the flow of a fluid through a bypass pipe 1005 may be referred to as a bypass valve.

[0386] The secondary fuel-oil heat exchanger 1004 and the primary fuel-oil heat exchanger 1006 are thus designed such that an oil flow is routed through each in addition to the fuel flow - the oil flowing through one is different from the oil flowing through the other in the implementation being described, although it will be appreciated that the same oil may flow through one fuel-oil heat exchanger and then through another fuel-oil heat exchanger in other implementations.

[0387] The two heat exchangers 1004, 1006 are thus in separate closed loop systems 2000, 2000', (Figures 7, 8) with respect to the oil flow, in the implementation being described, i.e. the oils flowing through the primary and secondary fuel-oil heat exchangers are fluidically separated, and may be chemically distinct from each other. The two oil loop systems 2000, 2000' serve to circulate oil through their heat exchangers respective fuel-oil systems 1006, 1004, and optionally also through one or more other heat exchangers, for example air-oil or oil-oil heat exchangers as described below. The two oil loop systems 2000, 2000' together can be described as providing a recirculating oil system for the engine 10.

[0388] [Fig. 6] shows an exemplary alternative fuel system 6000, including a fuel delivery system and a fuel management system 6500 including a fuel flow path from the fuel tank 50 to the combustion chamber 16 of the gas turbine engine 10 of the aircraft 1. As with the fuel system 1000, fuel is pumped from the fuel tank 50 by the low-pressure fuel feed pump 1002. The fuel then passes through a secondary fuel-oil heat exchanger 1004 and then a primary fuel-oil heat exchanger 1006 before reaching an engine fuel pump 1003, which pumps the fuel along its flow path to the combustion chamber 16.The fuel system 6000 differs from the fuel system 1000 in that the fuel system 6000 includes a recirculation valve 6010 located downstream of the primary fuel-oil heat exchanger 1006 and adapted to recirculate at least a portion of the fuel that has left the primary fuel-oil heat exchanger back to the inlet 1006a of the primary fuel-oil heat exchanger 1006, allowing additional heat transfer between the oil of the primary loop system and the fuel. The fuel temperature sensor 1009 is located downstream of the starting point of the recirculation valve 6010, and ideally adjacent to the combustion chamber 16, so as to provide a more accurate indication of the temperature at the inlet of the combustion chamber 16 (another temperature sensor may be provided between the heat exchanger outlet and the starting point of the recirculation valve 6010 in some implementations).The recirculation valve 6010 may determine what proportion of the fuel is recirculated, via the recirculation pipe 6011 and what proportion continues more directly to the combustion chamber 16. In the illustrated example, the recirculation valve 6010 is located downstream of the primary fuel-oil heat exchanger 1006. In the illustrated example, the recirculation valve 6010 is positioned upstream of the engine fuel pump 1003. An additional recirculation pump (not shown) may be provided in some implementations in which the recirculation valve 6010 is positioned upstream of the engine fuel pump 1003, to provide a positive pressure gradient. A fuel return-to-tank (FRTT) pump may be present and also assist in recirculation in some implementations. The recirculation valve 6010 is intended to allow . a controlled amount of fuel to be returned to the inlet 1006a of the primary heat exchanger 1006, thereby flowing through the primary heat exchanger 1006 multiple times before reaching the pump 1003 and the combustion chamber 16. It is contemplated that the recirculation valve may be positioned downstream of the engine fuel pump 1003 in alternative implementations, for example as illustrated in [Fig. 10]. In such implementations, the recirculation valve 6010 would be intended to allow a controlled amount of fuel to be returned to the inlet 1006a of the primary heat exchanger 1006, thereby flowing through the primary heat exchanger 1006 and the pump 1003 multiple times before reaching the combustion chamber 16.This recirculation provides a mechanism for regulating the flow of fuel within the fuel management system 6500, as well as within the heat management system 3000, without altering the flow of fuel from the tank 50 to the engine 10.

[0389] A pipe 6011, which may be referred to as a recirculation pipe because it transports fuel from a point along the main flow path through the engine 10 to an earlier point along that flow path such that the fuel must pass through the portion of the flow path between them again, may therefore be provided, leading from the recirculation valve to a point on the flow path upstream of the inlet 1006a to the primary heat exchanger 1006. In some implementations, a recirculation pipe 6011 and a bypass pipe 1005 may be provided for any given heat exchanger 1004, 1006. In some implementations, the same pipe 1005, 6011 may serve as both a recirculation pipe and a bypass pipe - one or more valves may be used to control the direction of fluid flow therethrough.

[0390] In the example illustrated in [Fig. 10], unlike that in [Fig. 6], the recirculation valve 6010 is located downstream of the pump 1003, such that the recirculated fuel has already passed through both the primary heat exchanger 1006 and the pump 1003, rather than just the primary heat exchanger 1006. The recirculation pipe 6011 returns the recirculated fuel to a point in the flow path upstream of both the primary pump 1003 and the primary heat exchanger 1006, such that the recirculated fuel passes through both of these components once more. The recirculated fuel through the pump may allow more adjustable control of the fuel flow to the combustion chamber 16 for a given shaft speed of the engine 10, noting that the pump speed (or a limited group of pump speed options) is often fixed by the shaft speed.

[0391] Regulation of the fuel flow in the fuel circuit 6000 - by recirculation and / or bypass of one or more heat exchangers - may contribute to heat management (for example, by influencing the fuel temperature at the inlet of the combustion chamber 16 or the pump 1003, or the amount of heat transferred to the fuel). Part of the control of the heat exchange system 3000 may therefore be the control of one or more valves regulating the flow of oil and / or fuel through one or more recirculation and / or bypass pipes 6011, 1005.

[0392] The gas turbine engine 10 of the aircraft 1 being described also includes a recirculating oil system for supplying oil to lubricate and remove heat from a plurality of components. In the implementation being described herein, the recirculating oil system includes a primary oil loop system 2000 and a secondary oil loop system 2000', each of which is a closed loop oil system. An example of a closed loop primary oil system 2000 is schematically illustrated in [Fig.7] - as with the heat exchangers, this oil loop system is referred to as "primary" because it is responsible for oiling and cooling the main reduction gear 30 and, generally, is responsible for the main / primary cooling load of the engine 10. The primary closed loop oil system 2000 includes an oil reservoir 2002 adapted to contain a volume of oil.In some implementations, gases are removed from the oil within the oil reservoir 2002 by a deaerator. A feed pump 2004 is configured to pump oil from the oil reservoir 2000 to the main fuel-oil heat exchanger 1006. The average temperature of the oil entering the main fuel-oil heat exchanger 1006, under cruising conditions, is higher than the average temperature of the fuel entering the main fuel-oil heat exchanger 1006. In the main fuel-oil heat exchanger 1006, thermal energy is transferred from the oil flow to the fuel flow.In this manner, the average temperature of the oil flow leaving the main fuel-oil heat exchanger 1006 is lower than the average temperature of the oil flow entering the main fuel-oil heat exchanger 1006, so that it is cooled before reuse as a lubricant and / or coolant, allowing the cooled oil to remove more heat from the system to be lubricated and / or cooled. Also in this manner, the average temperature of the fuel leaving the main fuel-oil heat exchanger 1006 is higher than the average temperature of the fuel entering the main fuel-oil heat exchanger 1006.

[0393] In a standard heat exchanger 1006, the fundamental limiting factor on the level of cooling provided is the temperature of the coolant (in this case, the fuel) - the temperature of the oil leaving the heat exchanger 1006 cannot be lowered below the temperature of the fuel entering the heat exchanger 1006 (and the fuel temperature also cannot be raised above that of the oil entering the heat exchanger), even if an effectively infinite flow of fuel or oil were used. Temperature equilibrium is the limit, as defined in the second law of thermodynamics. In some implementations, as illustrated in [Fig. 7], a refrigeration cycle apparatus 1007 is therefore provided. The refrigeration cycle apparatus 1007 is intended to provide thermal elevation by transferring more heat from the oil to the fuel, such that the fuel temperature is raised more than it would be by simply passing through the heat exchanger, and in some cases, it is above the oil temperature.The refrigeration cycle apparatus 1007 may take the form of an additional closed-loop circuit with a refrigerant fluid, with an evaporator between the oil and the refrigerant (to transfer heat from the oil to the refrigerant) and a condenser between the fuel and the refrigerant (to transfer heat from the refrigerant to the fuel). The refrigeration cycle apparatus 1007 may additionally include a pump, a compressor and / or an expansion valve / meter, and may be implemented according to any suitable design known in the art. It will be appreciated that the refrigeration cycle apparatus 1007 may also be referred to as a heat pump - it pumps heat from the oil to the fuel, thereby cooling the oil and heating the fuel.

[0394] [Fig. 18] shows in more detail an exemplary refrigeration cycle apparatus 1007. The illustrated refrigeration cycle apparatus 1007 contains a refrigerant, which is circulated through the apparatus 1007 during use. In the implementation described herein, the flow rate of the refrigerant can be adjusted to vary the amount of heat transferred, and the refrigeration cycle apparatus 1007 can be turned off by interrupting the flow of refrigerant. In other implementations, the refrigeration cycle apparatus 1007 may have a single operating speed, and regulating it may simply involve turning the refrigeration cycle apparatus 1007 on and off. The refrigerant may be 1,1,1,2-tetrafluoroethane (known commercially as refrigerant R134a), although other refrigerants may be used.

[0395] During operation, heat from the oil in the oil loop system 2000 is transferred to the refrigerant in an evaporator 1007a - the evaporator 1007a evaporates the liquid in the refrigerant (the refrigerant may be a liquid-gas mixture or may be a pure liquid at this point in the refrigeration cycle) to form a vapor, and in particular a saturated vapor. The evaporator 1007a is located on the oil loop system 2000, at or near where the oil is hottest - generally immediately after the last component of engine to be cooled by oil in a single pass through the oil loop system 2000 (this may be the reducer 30). One or more heat-conducting plates may be provided between the oil and the refrigerant (generally as part of the evaporator 1007a) to increase heat transfer; optionally, internal passages for the flow of oil and / or refrigerant may be provided within a heat-conducting plate or other structure to enhance heat transfer.

[0396] From the evaporator 1007a, the saturated vapor is then delivered to a refrigerant compressor 1007b. The refrigerant compressor 1007b may also function as a pump, circulating the refrigerant through the refrigeration cycle apparatus 1007. Control of the compressor 1007b may therefore be used to reduce, increase, or stop the flow of refrigerant through the apparatus 1007. Advantageously, the refrigeration cycle apparatus 1007 may therefore respond to a varying demand for oil cooling / fuel heating due to the ability to modulate the cooling flow rate using the refrigerant compressor 1007b. A separate / additional pump for the refrigeration cycle apparatus 1007 may be provided in some implementations, but using the compressor 1007b to circulate the refrigerant may reduce the number of components required, and thus the size and weight of the apparatus 1007.

[0397] Compressor 1007b compresses the saturated vapor into a superheated vapor and sends it to a condenser 1007c, which functions as a refrigerant-to-fuel heat exchanger 1007c. Condenser 1007c transfers heat from the compressed vapor to the fuel (in fuel management system 1500), condensing at least a portion of the superheated vapor into a liquid. In general, condenser 1007c may convert the compressed vapor of the refrigerant into a saturated liquid. The condenser 1007c is located on the fuel path 1500 through the engine 10, downstream of one or all of the fuel-oil heat exchangers 1004, 1006 - generally near the combustion chamber 16. The condenser 1007c may be the last engine component through which fuel flows before reaching the combustion chamber 16 (not including any fuel pipes / flow paths).The condenser 1007c may be the most downstream component of the heat exchange system 3000 relative to the fuel flow.

[0398] The refrigeration cycle apparatus 1007 of the implementation illustrated in [Fig. 18] further includes an expansion valve 1007d located between the condenser 1007c and the evaporator 1007a. The expansion valve 1007d is intended to allow the refrigerant fluid to expand, thereby cooling the refrigerant. For example, the action of the expansion valve 1007d may convert the saturated liquid into a mixture of liquid and vapor at reduced temperature. The expanded / cooled refrigerant is then returned to evaporator 1007a, ready to receive more heat from the oil.

[0399] The refrigeration cycle apparatus 1007 may also include a refrigerant reservoir (not shown) in some implementations. Such a refrigerant reservoir may serve to compensate for any volumetric variation of the refrigerant due to temperature and density changes, such that the refrigerant flow can perform its cooling / heat pumping function reliably and consistently under all engine operating conditions.

[0400] In the implementations illustrated in the figures, the motor 10 includes a reduction gear 30 which receives an input from the core shaft 26 and provides a drive to the blower 23 so as to drive the blower at a lower rotational speed than that of the core shaft, and the oil loop system 2000 is intended to supply oil to the reduction gear 30. However, it will be appreciated that the implementation shown in [Fig. 18] does not require that the oil loop system 2000 include a reduction gear 30, and that the same refrigeration cycle apparatus 1007, and the control thereof, can be implemented in a direct drive motor (without a reduction gear for the main shaft).

[0401] [Fig. 18] illustrates the connection of the refrigeration cycle apparatus 1007 to both the oil system 2000 and the fuel system 1500 within the engine 10. The refrigerant acts as an intermediate heat transfer fluid, pumping heat from the oil to the fuel. Heat is transferred from the oil system 2000 to the refrigerant in the evaporator 1007a, and this heat is then transferred from the refrigerant to the fuel system 1500 in the condenser 1007c. [Fig. 18] also schematically indicates two temperature sensors - an oil temperature sensor 2009 for providing an indication of the maximum oil temperature within the oil loop system 2000, and a fuel temperature sensor 1009 for providing an indication of the fuel temperature downstream of the refrigeration cycle apparatus 1007 / approaching the combustion chamber 16.Data from these sensors 1009, 2009 may be used to control the refrigeration cycle apparatus 1007 - for example, to control one or more of refrigerant flow, oil flow, and fuel flow through the relevant portion(s) of the refrigeration cycle apparatus 1007.

[0402] An oil valve may regulate the amount of oil that flows into the refrigeration cycle apparatus 1007 (and in particular, through the evaporator 1007a), and a fuel valve may regulate the amount of fuel that flows through the refrigeration cycle apparatus 1007 (and in particular, through the condenser 1007c). The refrigeration cycle apparatus 1007 is powered, generally electrically or mechanically (e.g., via a connection to a rotating shaft), to ensure this additional, forced heat transfer. The refrigeration cycle apparatus 1007 is shown in association with the main (primary) fuel-oil heat exchanger 1006 in the illustrated implementation, thereby providing a fuel temperature boost after the fuel has passed through both fuel-oil heat exchangers 1004, 1006, and further cooling the oil in the primary oil system 2000 before it is returned to the reducer 30. A bypass or recirculation pipe for the oil and / or fuel may be provided around the refrigeration cycle apparatus 1007 in various implementations.

[0403] The oil flow in the primary oil loop system 2000 is then routed to a power reducer 30, which may also be described as the main reducer 30 of the gas turbine engine 10. The power reducer 30 is for receiving input from the core shaft 26 and for providing drive to the fan 23 via the fan shaft 42 and includes gears 28, 32, 38 and bearings (e.g., journal bearings) that may be lubricated and cooled by oil. The engine 10 may also include one or more additional bearings for supporting the shafts 26, 42, which may be journal bearings.The oil may further be used to lubricate and / or cool the journal bearings, and typically increases in temperature significantly when used under cruising conditions, so as to help cool the bearings and reduction gear 30 as the oil flow carries heat away from the bearings and reduction gear 30. The oil may also be used to lubricate one or more other engine components 33, e.g., an auxiliary reduction gear (AGB) and / or one or more bearing chambers. The AGB 33, also known as an accessory drive, when present, is a reduction gear that is part of the gas turbine engine 10, although it is not part of the engine core 11 and does not drive the fan 23. The AGB instead drives engine accessories, e.g., fuel pumps, and typically handles large loads. A relatively large amount of heat can therefore be discharged into the oil from the AGB.One or more bearing chambers may be lubricated by the same oil, and may similarly dump heat into the oil. Per unit of oil flowing through them, the AGB and bearing chambers may add more heat to the oil than the main gear 30 in most implementations. The oil flow may be split into two or more parallel flows, e.g., one flow through the main gear 30 and one flow through the other engine components, or multiple parallel flows through the main gear 30 (e.g., through different gear components) and separate flows through the AGB and the or each bearing chamber 33.

[0404] From the power reducer 30 (and / or other engine components 33 such as the AGB, if applicable), oil is collected in a sump 2008. A scavenge pump 2010 is adapted to pump oil from the sump 2008 back into the oil reservoir 2002, ready for reuse.

[0405] [Fig.8] illustrates the secondary oil loop system 2000', which is another closed loop oil system 2000'. The secondary closed loop oil system 2000' includes a secondary oil reservoir 2002' adapted to contain a volume of oil. In some implementations, gases are removed from the oil within the oil reservoir 2002' by a deaerator. A secondary feed pump 2004' is configured to pump oil from the secondary oil reservoir 2002' to the secondary fuel-oil heat exchanger 1004, which in the illustrated implementation is the IDG fuel-oil heat exchanger 1004. The average temperature of the oil entering the IDG fuel-oil heat exchanger 1004, under cruising conditions, is higher than the average temperature of fuel entering the IDG fuel-oil heat exchanger 1004.In the IDG fuel-oil heat exchanger 1004, thermal energy is transferred from the oil flow to the fuel flow. In this manner, the average temperature of the oil flow leaving the IDG fuel-oil heat exchanger 1004 is lower than the average temperature of the oil flow entering the IDG fuel-oil heat exchanger 1004. In this manner also, the average temperature of the fuel leaving the IDG fuel-oil heat exchanger 1004 is higher than the average temperature of the fuel entering the IDG fuel-oil heat exchanger 1004. The oil flow is then routed to / back to an integrated drive generator 2006, where it lubricates and / or cools moving components and is heated in the process.In some implementations, the oil may be used primarily as a coolant for the IDG 2006, and may perform minimal or no lubrication. From the integrated drive generator 2006, the oil is collected in a secondary sump 2008'. A secondary scavenge pump 2010' is configured to pump oil from the secondary sump 2008' back into the secondary oil reservoir 2002' ready for reuse. In some implementations, a refrigeration cycle apparatus may also be provided on the secondary oil loop system 2000'.

[0406] Figures 7 and 8 each illustrate a serial oil flow path, with the entire oil flow passing sequentially through each component (although it will be appreciated that one or more bypass or recirculation pipes not shown may be provided for the oil). In other implementations, the oil flow may be divided into two or more parallel flows, for example, flow through the main fuel-oil heat exchanger 1006 and flow through an air-oil heat exchanger 2020 (as described below). It will be appreciated that while a bypass pipe technically provides a parallel, alternating flow path arrangement, a branch path as described herein with reference to parallel flows is different in that each branch route of the parallel arrangement includes a heat exchanger, whereas the bypass pipe is simply a pipe with no (significant) heat exchange.

[0407] [Fig.9] schematically represents an exemplary alternative section of the primary closed-loop oil system 2000 shown in [Fig.7]. In this section an oil flow is pumped by the feed pump 2004 through a valve 2016. The valve 2016' is operable to divide the oil flow between the primary fuel-oil heat exchanger 1006 and a first air-oil heat exchanger 2020, the first air-oil heat exchanger 2020 being arranged in parallel with the primary fuel-oil heat exchanger 1006.The oil flow path can be described as a branch, with the main fuel-oil heat exchanger 1006 on one branch and the first air-oil heat exchanger 2020 on the other branch, in a parallel configuration such that oil can flow through one branch or the other, but the same portion of oil cannot pass through both on the same cycle - the flow splits. Valve 2016 modulates the flow through both heat exchangers 1006, 2020, and can therefore be described as a modulating valve 2016. The oil flows are then recombined and routed to the power reducer 30 and other engine components 33.

[0408] Any suitable percentage of oil may flow through each of the first air-oil heat exchanger 2020 and the primary fuel-oil heat exchanger 1006. In some examples, the valve 2016 is operable to vary the flow of oil to the primary fuel-oil heat exchanger 1006 and the first air-oil heat exchanger 2020 upon request. In various examples, an oil-oil heat exchanger 2030 (not shown in [Fig. 7] but present in [Fig. 1 1]) may be provided, for example being arranged in series with the first air-oil heat exchanger 2020 on this branch of the parallel split. The oil-oil heat exchanger 2030 may provide heat exchange between the primary and secondary closed-loop oil systems 2000, 2000'.

[0409] [Fig.9] also illustrates details of the air-oil heat exchanger 2020 that are not shown in the other figures for clarity, including indicating the air flow by dotted arrows. The air-oil heat exchanger 2020 has an air inlet 2020a and an air outlet 2020b. The air inlet, or intake, 2020a may be intended to capture air in the bypass duct 22 of the engine, downstream of an outlet guide vane of the fan 23. The air / exhaust outlet 2020b of the heat exchanger may be intended to return the air to the bypass duct 22, downstream of the inlet 2020a, or directly to the outside atmosphere (e.g., if it is provided with a dedicated outlet nozzle).

[0410] A valve 2022, referred to as an air valve, is used to regulate the flow of air through the air-to-oil heat exchanger 2020. In the illustrated example, the air valve 2022 is located at or near the inlet 2020a of the heat exchanger 2020, and may thus be referred to as the air inlet valve 2022. In other implementations, the air valve 2022 may be located at or near the outlet 2020b of the heat exchanger 2020, and may thus be referred to as the air outlet valve. Any suitable position of the valve 2022 may be selected, provided that the valve 2022 can be adjusted to control the airflow through the air-oil heat exchanger 2020. In the example described, the air valve 2022 is continuously adjustable between a fully closed position (no airflow through the heat exchanger) and a fully open position (maximum airflow through the heat exchanger).In other examples, the air valve 2022 may be adjustable between a plurality of distinct positions rather than being continuously adjustable - e.g., six, five, four, three, or two different positions. In implementations with only two positions for the air valve 2022, these positions may be "open" and "closed" - the valve 2022 may be repeatedly opened and closed to provide a flow of forced air when an intermediate level of cooling is desired in some of these implementations.

[0411] [Fig. 10] schematically shows an exemplary arrangement and interaction of the first closed-loop oil system 2000, the second closed-loop oil system 2000', and the fuel system 1000, with fuel flow shown in thick black lines and oil flow shown in thinner black lines. The striated thick black line indicates a recirculation path 6011 taken by only a portion of the fuel. The combination of portions of the fuel 1000 and oil systems 2000, 2000' together forms the heat exchange system 3000. The primary closed-loop oil system 2000 of this exemplary arrangement is arranged as shown in [Fig.7]. The secondary closed-loop oil system 2000' of this exemplary arrangement is arranged as shown in [Fig.8]. The fuel system 1000 of this exemplary arrangement is arranged as shown in [Fig.5], but with an additional recirculation valve 6010 and pipe 6011 as described above. One or more bypass pipes 1005, 2005 may also be present, but are not shown for clarity.

[0412] In use, fuel is pumped from the fuel tank 50 by the low pressure fuel pump 1002. The fuel then flows through the secondary fuel-oil heat exchanger 1004. The secondary closed-loop oil system 2000' is designed such that its oil recirculation flow also flows through the secondary fuel-oil heat exchanger 1004. In standard operation of the engine 10, both at cruise and idle conditions, the average temperature of the oil flow entering the secondary fuel-oil heat exchanger 1004 is higher than the average temperature of the fuel flow entering the secondary fuel-oil heat exchanger 1004. The secondary fuel-oil heat exchanger 1004 is designed such that heat is transferred from the oil flow to the fuel flow.In this way, the average temperature of the oil flow at the outlet of the secondary fuel-oil heat exchanger 1004 is lower than the average temperature of the oil flow at the inlet of the secondary fuel-oil heat exchanger 1004. In the same way, the average temperature of the fuel flow at the outlet of the secondary fuel-oil heat exchanger 1004 is higher than the average temperature of the fuel flow at the inlet to the secondary fuel-oil heat exchanger 1004.

[0413] The fuel then flows through the primary fuel-oil heat exchanger 1006, and additionally through the refrigeration cycle apparatus 1007. The primary closed-loop oil system 2000 is designed such that its oil recirculation flow also flows through the primary fuel-oil heat exchanger 1006 and the refrigeration cycle apparatus 1007. In standard operation of the engine 10, both at cruise and idle conditions, the average temperature of the oil flow entering the primary fuel-oil heat exchanger 1006 is higher than the average temperature of the fuel flow entering the primary fuel-oil heat exchanger 1006. The primary fuel-oil heat exchanger 1006 is designed such that heat is transferred from the oil flow to the fuel flow.In this way, the average temperature of the oil flow at the outlet of the main fuel-oil heat exchanger 1006 is lower than the average temperature of the oil flow at the inlet of the main fuel-oil heat exchanger 1006. In the same way, the average temperature of the fuel flow at the outlet of the main fuel-oil heat exchanger 1006 is higher than the average temperature at the inlet of the main fuel-oil heat exchanger 1006.

[0414] The refrigeration cycle apparatus 1007, when active (i.e., when powered / started and used to actively move heat from the oil to the fuel), may provide an additional fuel temperature boost / additional oil temperature drop, possibly by raising the fuel above the oil temperature. After flowing through the primary fuel-oil heat exchanger 1006, the fuel flows to the engine fuel pump 1003, which is located downstream of the primary and secondary fuel-oil heat exchangers 1006, 1004 in the examples shown and is for delivering fuel to the combustion chamber 16 of the gas turbine engine 10.

[0415] Under cruising conditions, the average temperature of the oil flow flowing through the secondary fuel-oil heat exchanger 1004 may be lower than the average temperature of the oil flow flowing through the primary fuel-oil heat exchanger 1006. In this manner, the fuel passes through the heat exchanger 1004 having a lower average oil flow temperature first, before passing through the heat exchanger 1006 having a higher average oil flow temperature.

[0416] In addition to the branched oil flows 2000, 2000', the heat exchange system 3000 may further include branched fuel return paths 6020, such that fuel is returned to the main fuel path from the fuel inlet in the gas turbine engine 10 to the combustion chamber 16 in at least two different locations - for example upstream of, or downstream of, the prime mover pump 1003 as shown in [Fig. 6] with a branched path 6020 branching off from the main fuel flow path at the outlet 1006b from the primary heat exchanger 1006 and rejoining the main fuel flow path downstream of the pump 1003. At least one valve (not shown) may be provided to regulate a split of the fuel flow from the heat exchanger 1006 back to the main fuel path through engine 10.The valve may be controlled based on fuel temperature - for example, to send less fuel through the pump 1003 / more fuel to a position downstream of the pump if the fuel temperature is relatively high and more likely to degrade the pump seals or other components. The regulation of fuel flow in the branched return paths may be based on a fuel temperature measurement (for example, using a temperature sensor at a location downstream of the heat exchanger 1006 and the fuel, possibly also using a temperature measurement upstream of the fuel-oil heat exchanger). A return of the fuel caused to recirculate upstream of the fuel-oil heat exchanger 1006 may reduce the transfer of . heat from the oil to the fuel, thereby damping a transient overshoot that might otherwise occur at the start of the downshift, for example when the same amount of heat is generated within the oil system, but fuel flow is reduced, typically resulting in a temperature spike.

[0417] [Fig. 11] schematically shows an exemplary configuration of the primary closed loop oil system 2000 and the secondary closed loop oil system 2000' where the two independent oil recirculation flows are brought into a heat exchange relationship through an oil-oil heat exchanger 2030. The two oil loop systems 2000, 2000' have a branched arrangement of parallel pipes / heat exchangers in this example.

[0418] In the example illustrated in [Fig. 11], the primary closed-loop oil system 2000 is designed such that the oil recirculation flow is pumped by the feed pump 2004 through a valve 2016, which may be referred to as a modulating valve. The valve 2016 is operable to split the oil flow such that a portion of the oil flow passes to each of the primary fuel-oil heat exchanger 1006 and a first air-oil heat exchanger 2020. The valve 2016 may be controllable to adjust the proportion of oil sent through each heat exchanger. In the illustrated implementation, the first air-oil heat exchanger 2020 is in series with the oil-oil heat exchanger 2030, and the arrangement of the air-oil heat exchanger 2020 and oil-oil heat exchanger 2030 is in parallel with the main fuel-oil heat exchanger 1006.The modulating valve 2016 determines what proportion of the oil flows through each branch of the parallel arrangement. In various implementations, any appropriate portion of the oil flow may be diverted between the primary fuel-oil heat exchanger 1006 and the first air-oil heat exchanger 2020. In some examples, the valve 2016 may be operated to divert a fixed portion of the oil flow to each of the primary fuel-oil heat exchanger 1006 and the first air-oil heat exchanger 2020.In other examples, the valve 2016 may operate to divert a variable portion of the oil flow to each of the primary fuel-oil heat exchanger 1006 and the first air-oil heat exchanger 2020, for example using data from a temperature sensor for feedback to regulate the variable portion, and / or based on one or more fuel characteristics, as described above. The valve 2016 may be adjustable between a distinct number of fixed positions or continuously adjustable. In some implementations, for example when the fuel temperature at the inlet of the combustion chamber 16 is relatively low compared to the maximum operating temperature of . fuel (e.g., based on knowledge of fuel type or thermal stability), no oil can be sent to the air-oil heat exchanger 2020, and all oil can be sent through the fuel-oil heat exchanger 1006.

[0419] After flowing through the heat exchangers 1006, 2020, 2030, the oil flow in the primary closed loop system 2000 is then recombined and routed to the power reducer 30 (and / or other engine components 33 such as the AGB) and then to the crankcase 2008. The scavenge pump 2010 then pumps the oil from the crankcase 2008 to the oil reservoir 2002, for reuse.

[0420] The oil flow within the secondary closed-loop oil system 2000' is intended to be brought into a heat exchange relationship with the separate oil flow within the primary closed-loop oil system 2000 through the oil-to-oil heat exchanger 2030. In the oil-to-oil heat exchanger 2030 the oil flow within the primary closed-loop oil system 2000 does not mix with the oil flow within the secondary closed-loop oil system 2000'. The oil-to-oil heat exchanger 2030 is designed such that heat transfer can take place between the two separate oil flows. In this way, heat from a warmer oil flow can be transferred to the cooler oil flow within the oil-to-oil heat exchanger 2030.No air-oil heat exchanger is shown in the illustrated secondary closed-loop oil system 2000', but an air-oil heat exchanger may be provided - e.g., in series with the oil-oil heat exchanger 2030 or on a third parallel branch - in other examples.

[0421] In the implementation shown in [Fig. 11], the secondary closed-loop oil system 2000' is designed such that the oil recirculation flow is pumped by the secondary feed pump 2004' through a valve 2016', which may be referred to as a secondary modulating valve. The valve 2016' is operable to divert at least a portion of the oil flow between the IDG fuel-oil heat exchanger 1004 and the oil-oil heat exchanger 2030, where the oil-oil heat exchanger 2030 is arranged in parallel with the IDG fuel-oil heat exchanger 1004. In other implementations, the secondary closed-loop oil system 2000' may not have a branched configuration. For example, the IDG fuel-to-oil heat exchanger 1004 and the oil-to-oil heat exchanger 2030 may be arranged in series such that at least a majority of the oil passing through one also passes through the other in a given cycle..

[0422] In other implementations, an air-to-oil heat exchanger 2020 may be present on each closed-loop system, or only on the secondary closed-loop system, and / or no oil-to-oil heat exchanger may be present.

[0423] In examples, any suitable portion of the oil flow may be diverted between the IDG fuel-oil heat exchanger 1004 and the oil-oil heat exchanger 2030. In examples, the valve 2016' may operate to divert a fixed portion of the oil flow to each of the IDG fuel-oil heat exchanger 1004 and the oil-oil heat exchanger 2030. In examples, the valve 2016' is operable to divert a variable portion of the oil flow to each of the IDG fuel-oil heat exchanger 1004 and the oil-oil heat exchanger 2030. The valve 2016' may be adjustable between a distinct number of fixed positions, or continuously adjustable. After flowing through the heat exchangers 1004, 2030, the oil flow is then routed to the integrated drive generator 2006 and then to the secondary housing 2008'.The secondary recovery pump 2010' then pumps oil from the secondary sump 2008' to the secondary oil reservoir 2002', for reuse.

[0424] One or more temperature sensors 1009 may be provided, for example, for sensing fuel temperature as it approaches the fuel pump 1003 or the combustion chamber 16. Data from the one or more sensors, possibly in combination with other data (e.g., fuel characteristics as described above) may be used by a controller 58 of the heat exchange system 3000 to manage fuel and / or oil flows through and around the heat exchangers 1004, 1006, 2020, 2030, and / or to manage air flow through the air-oil heat exchanger 2020. The controller 58 may be part of, or provided by, an EEC, or may be a separate unit. The control may be automated, for example, by an EEC.

[0425] [Fig. 16] illustrates a recirculating oil system comprising two loops 2000, 2000' but without heat exchangers arranged on parallel branches of either oil system, as illustrated in [Fig. 11]. Instead, each oil loop 2000, 2000' provides a series of main oil flow paths through all the heat exchangers in that loop. To adjust the oil flow through the heat exchangers, one or more bypass pipes 2005, 2005', 2005a are provided in place of the bypass paths to different heat exchangers. It will be borne in mind that a combination of parallel arrangements of heat exchangers and branch pipes may be used in some implementations, and that Figures 11 (both loops have parallel branches, no branch pipes) and [Fig.16] (all heat exchangers in parallel, multiple branch pipes) can be seen as illustrating two different ends of a design spectrum.

[0426] In the example illustrated in [Fig. 16], the fuel flow is represented by a thick black line to provide context for the interaction between the fuel and oil systems. The primary oil loop system 2000 illustrated in [Fig. 16] provides oil flow paths in series from a reservoir 2002, through an oil pump 2004, then through the primary fuel-to-oil heat exchanger 1006 and then the air-to-oil heat exchanger 2020, before entering the reducer 30 (and possibly other components to be cooled and lubricated), then being collected in a sump 2008 and then pumped back into the reservoir 2002 by the oil pump 2010. The two heat exchangers 1006, 2020 are thus in a series arrangement.The order of the two heat exchangers 1006, 2020 may be reversed such that the fuel-oil heat exchanger 1006 is located after the air-oil heat exchanger 2020 in alternative implementations.

[0427] The primary oil loop system 2000 illustrated in [Fig. 16] includes two oil bypass pipes 2005, 2005a. The first bypass pipe 2005 is for allowing a portion of the oil to bypass the fuel-oil heat exchanger 1006 and is controlled by a first bypass valve 2007. The first bypass pipe 2005 takes oil upstream of the inlet of the primary heat exchanger 1006 and returns it to the main oil flow path before the air-oil heat exchanger 2020. The second bypass pipe 2005a is for allowing a portion of the oil to bypass the air-oil heat exchanger 2020 and is controlled by a second bypass valve 2007a.The second bypass pipe 2005a takes oil upstream of the inlet of the air-to-oil heat exchanger 2020 and returns it to the main oil flow path before the path reaches the gear reducer 30 (and possibly other components to be cooled and lubricated). In implementations of the primary oil loop system 2000 with a single oil bypass pipe 2005a, the position chosen may be that of the second bypass pipe 2005a, such that there is a bypass for the air-to-oil heat exchanger 2020 and not for the fuel-to-oil heat exchanger 1006. This may facilitate transferring the greatest amount of heat safely from the oil to the fuel, and maintaining high engine thermal efficiency by reducing heat loss to the environment.In implementations with the air-oil heat exchanger 2020 before the fuel-oil heat exchanger 1006, the oil flow rate through the air-oil heat exchanger 2020 may be adjusted to provide an appropriate oil temperature for the fuel-oil heat exchanger 1006. The presence of a bypass pipe 2005 on the fuel-oil heat exchanger 1006 may facilitate rapid adjustment of the oil flow ratio if there is a risk of it dropping lower than desired (e.g., based on characteristics . determined fuel values, a lower limit for the ratio significantly above zero can be set).

[0428] The temperature limits set may be strict for some fuels (and even for some oils), depending on their characteristics, so rapid adjustment of flow rates may be necessary to maintain temperatures within desired limits. The presence of a bypass pipe 2005, 2005a on the two heat exchangers 1006, 2020 may prevent the oil from becoming too cold under certain conditions, to prevent the oil from freezing excessively - it will be kept in mind that this may be more of a concern at start-up or ground idle in cold weather than at cruising speed.

[0429] The secondary oil loop system 2000' illustrated in [Fig. 16] includes a single heat exchanger 1004, which is the secondary fuel-oil heat exchanger, and a single bypass pipe 2005' for allowing a portion of the oil to bypass that heat exchanger 1004. In other implementations, no bypass pipe may be provided on the secondary oil loop system 2000', or more than one heat exchanger may be present (e.g., an air-oil and / or oil-oil heat exchanger in addition to the secondary fuel-oil heat exchanger 1004) and more than one bypass pipe may be provided, possibly one for each heat exchanger. A bypass valve 2007' is still provided to regulate the flow of oil through the bypass pipe 2005'.

[0430] The inventors were aware that the use of fuels different from traditional kerosene-based jet fuels, such as sustainable aviation fuels, may lead to different fuel properties, and that decisions on a desired heat transfer - and, accordingly, on fluid flow rates set by one or more heat exchangers - in operation may be adjusted to utilize the different fuel properties. In particular, the use of one or more controllable valves to adjust the flow of oil and / or air, and the implementation of careful control of these valves, may allow a method providing improved oil cooling (since the fuel can absorb more heat) and may also improve the overall thermal efficiency of the engine, with less heat being lost to the environment, while ensuring safe operation.The controllable heat exchange system 3000, and in particular one or more controllable valves 2016, 2022, 2007, 2007', 2007a, have a key role to play in managing fluid flow, and thereby heat transfer in such engines 10.

[0431] Furthermore, although cruise conditions generally represent a much larger proportion of an aircraft engine's operating time, the inventors were aware that idling operation is also important - since the fuel mass flow rate is much lower at idle than at cruise, even a relatively low heat load on the fuel can result in a large temperature increase - the use of non-traditional fuels can therefore have an even greater effect on optimal approaches to heat management at idle conditions. The methods 100, 200 of Figures 12 and 13 address these two aircraft operating scenarios. [Fig. 12] illustrates a method 100 implementing these considerations at cruise conditions, and [Fig. 13] illustrates a method 200 implementing these considerations at idle. This method 200 can be carried out when the aircraft 1 is on the ground, i.e., at ground idle, e.g.when starting the aircraft, when operating it while stationary during boarding and taxiing (to a runway or hangar, or between other ground locations), or when idling in flight, e.g. at the start of descent.

[0432] Each method 100, 200 is intended to be carried out in a geared gas turbine engine 10 comprising an engine core 11 comprising a turbine 19, a compressor 14, a combustion chamber 16 for burning fuel, and a core shaft 26 connecting the turbine to the compressor; a fan 23 located upstream of the engine core; a gear 30 which receives an input from the core shaft and delivers a drive to the fan so as to drive the fan at a lower rotational speed than the core shaft; an oil loop system (2000) for supplying oil to the gear; and a heat exchange system 3000.The heat exchange system 3000 includes an air-oil heat exchanger 2020 through which the oil in the oil loop system flows; a fuel-oil heat exchanger 1006 through which the oil in the oil loop system and the fuel flow such that heat is transferred between the oil and the fuel; and at least one valve 2016, 2007, 2007a (which may be referred to as an oil valve) for allowing a proportion of oil sent through at least one of the heat exchangers 1006, 2020 to be varied.

[0433] The at least one oil valve 2016, 2007, 2007a is adjustable / controllable, and optionally controllable by a control device 58 which may be part of / provided by the EEC of the aircraft. A control device 58 may therefore be provided to implement this control. The oil valve 2016, 2007, 2007a may be adjustable between a distinct number of fixed positions, or may be continuously adjustable.

[0434] The two heat exchangers 1006, 2020 may be arranged in parallel or in series in the oil loop system 2000. Furthermore, it is possible to provide more than two heat exchangers in certain implementations - for example with the oil loop system 2000 comprising multiple heat exchangers fuel-oil 1006, multiple air-oil heat exchangers 2020, and / or one or more additional heat exchangers, such as one or more oil-oil heat exchangers for exchanging heat between separate oil loop systems 2000, 2000'.

[0435] In parallel arrangements, the oil loop system 2000 may branch such that a proportion of the oil may flow along each branch, and the air-to-oil 2020 and fuel-to-oil 1006 heat exchangers may be arranged in a parallel configuration on different branches of the oil loop system, as shown in Figures 9 and 11. In such implementations, the at least one valve for allowing the proportion of oil sent through at least one of the heat exchangers 1006, 2020 to be varied may be or include a modulating valve 2016 for allowing the proportion of oil sent through each branch to be varied.

[0436] In series arrangements, such as those illustrated in [Fig. 16], the oil loop system 2000 includes one or more bypass pipes 2005, 2005a, each for allowing a portion of the oil to bypass one or more heat exchangers. In such implementations, the at least one valve for allowing variation in the proportion of oil sent through at least one of the heat exchangers 1006, 2020 may be or include one or more bypass valves 2007, 2007a. It will be appreciated that the bypass pipes 2005 and the bypass valves 2007 may also be provided in parallel arrangements - in such implementations, the at least one valve for allowing the proportion of oil sent through at least one of the heat exchangers 1006, 2020 to be varied may be or comprise a modulating valve 2016 and a bypass valve 2007, 2007a.The method 100, 200 may therefore comprise controlling 102, 202 multiple valves 2016, 2007, 2007a - for example, one bypass valve 2007a and one modulating valve 2016, or two bypass valves 2007, 2007a, or two bypass valves 2007, 2007a and one modulating valve 2016. Furthermore, the heat exchange system 3000 may comprise multiple separate oil loops 2000, 2000', and the method 100, 200 may also comprise controlling oil valves for additional heat exchangers 1004, 2030 of the second oil loop 2000', or in the case of an oil-oil heat exchanger 2030, of both loops. .

[0437] In implementations where a bypass pipe 2005a is provided across the air-oil heat exchanger 2020, the controlled bypass valve may be the valve 2007a for the bypass pipe 2005a across the air-oil heat exchanger 2020. In some implementations, this bypass pipe 2007a may be the only oil bypass pipe in the primary oil loop system 2000 - none Bypass pipe cannot be supplied for the 1006 fuel-oil heat exchanger.

[0438] In alternative implementations such as that illustrated in [Fig. 16], the heat exchange system 3000 includes at least two oil bypass pipes, and possibly three or more, each bypass pipe 2005, 2005', 2005a being configured to allow oil to bypass one of the heat exchangers 1006, 1004, 2020. The method 100, 200 may include modulating the amount of oil sent through each bypass pipe 2005, 2005', 2005a. In implementations with multiple bypass pipes in the same closed-loop oil system (e.g., with two bypass pipes in the primary oil loop system 2000, as illustrated in [Fig.16]), the same bypass valve 2007 - which may be a three-way valve - may be used to regulate the flow in both bypass pipes 2005, 2005a, or a different bypass valve 2007, 2007a may be provided for each of them, as shown in [Fig. 16].

[0439] In some implementations, the heat exchange system 3000 further includes a refrigeration cycle apparatus 1007 for providing thermal lift by transferring additional heat from the oil to the fuel over and above that transferred by the fuel-to-oil heat exchanger 1006. The flow of oil through the refrigeration cycle apparatus 1007 may also be adjusted as part of the method 100, 200. In order to transfer heat from the oil to the fuel—and thus exploit the increased heat capacity of many newer aviation fuels—a positive thermal gradient is normally required between the oil and the fuel (i.e., the oil is hotter than the fuel).With increased fuel thermal stability, new fuels may potentially have suitable operating temperatures higher than the temperature reached by the oil at the outlet of the reducer 30 or other components, thus thermal lift may be desirable to further increase the fuel temperature. Thermal lift is the ability to transfer heat from a cooler fluid to a warmer fluid (usually through a refrigeration cycle).

[0440] In some implementations, the heat exchange system 3000 further includes branched fuel return paths 6020 and at least one valve controlling a split of the fuel flow, as described above. The valve may be controlled based on feedback from one or more temperature sensors and / or based on one or more fuel characteristics. The method 100, 200 may include adjusting the fuel flow along each branch based on the oil flow ratio and / or the fuel temperature at the outlet of the fuel-oil heat exchanger 1006. The engine 10 may therefore have the ability to send heat (in the form of heated fuel) into the fuel system 1500 at different locations (e.g., upstream or downstream of the fuel pump 1003 or other component) - it will be appreciated that as fuel temperature increases, the heat capacity / heat resistance of fuel system components may become a limiting factor, and that returning hot fuel to a primary fuel flow path after a pump 1003, rather than before the pump 1003, may facilitate exploitation of the increased thermal stability / higher useful operating temperature of new fuels.

[0441] With particular regard to the method 100 to be carried out in cruising, the method 100 comprises the control 102 of the at least one valve 2016, 2007, 2007a such that, under cruising conditions, an oil flow ratio of:

[0442] oil flow rate in the air-oil heat exchanger (m5s') oil flow in 1 fuel-oil heat exchanger {m3»'1)

[0443] is in the range of 0 to 0.59, and optionally 0 to 0.50, 0 to 0.40, 0 to 0.30, 0 to 0.20, 0 to 0.10, 0 to 0.05, or optionally 0 to 0.01. Optionally, the oil flow ratio may be in the range of 0.05 to 0.55. The oil valve 2016, 2007, 2007a may be controlled such that the oil flow ratio is zero - e.g., preventing any oil flow into the air-oil heat exchanger 2020, in some implementations.

[0444] It will be borne in mind that even for a specific engine 10 operating with a fixed fuel, it is usual to have a range of values ​​for this ratio in cruise mode due to varying conditions, e.g., fuel temperature, atmospheric temperature, thrust demand, etc. - for example, an upper limit of the range may apply in cold weather (ISA-30 conditions) at low altitude (cruise: 35,000 feet), with low heat generation in the oil system, and a lower limit may apply in hot weather (ISA+40 conditions) at high altitude (cruise: 39,000 feet), with high heat generation in the oil system.

[0445] The method 100 may also include receiving 104 data to enable the oil flow ratio to be calculated or derived, e.g., pump speed data, fuel flow rate data, and / or oil flow rate data. Such data may be received 104 by a controller 58 and used 106 to adjust control 102 of the cruise oil valve(s) to maintain the oil flow ratio at a desired level or within desired limits. This checking and adjustment / correction 106 may be performed at regular intervals or in response to predetermined stimuli. (e.g., a change in temperature or flow rate of fuel or oil, or a change in engine operation - e.g., a demand for thrust - or altitude). These steps 104, 106 may alternatively be considered as part of the control 102 of the oil valve(s) 2007, 2007a, 2016. The method 100 may also be intended to use other information, e.g., temperature data (of oil, fuel, and / or the ambient temperature of an environment around the aircraft 1), flow rate data (of oil and / or fuel), and / or one or more fuel characteristics, in order to determine 106 the control actions to be taken.

[0446] The step 102 of controlling the at least one valve 2016, 2007, 2007a so as to adjust the oil flow ratio includes decreasing the amount of oil sent through the at least one air-oil heat exchanger 2020 when the oil flow ratio is too high in many implementations; possibly, closing the valve / completely closing one way of a three-way valve in some implementations such that the oil flow rate in the air-oil heat exchanger 2020 drops to zero. In some implementations, a minimal but non-zero oil flow rate through the air-oil heat exchanger 2020 may be maintained throughout operation to prevent oil from congealing within the air-oil heat exchanger 2020.

[0447] In some implementations, one or more temperature sensors 1009 may be provided, and data relating to fuel temperature—possibly at the inlet to the combustion chamber 16—may be used to refine control decisions.The method 100 may comprise controlling 102 the one or more oil valves 2007, 2007a, 2016 under cruising conditions such that the oil flow ratio is in the range of 0 to 0.35, and optionally 0 to 0.25 provided that the fuel temperature at the inlet of the combustion chamber 16 is at least 140°C; and / or such that the oil flow ratio is in the range of 0 to 0.20, and optionally 0 to 0.15, provided that the fuel temperature at the inlet of the combustion chamber 16 is at least 160°C; and / or such that the oil flow ratio is in the range from 0 to 0.1, and optionally from 0 to 0.075, provided that the fuel temperature at the inlet of the combustion chamber 16 is at least 180°C.

[0448] In some implementations, one or more fuel characteristics may be considered when determining how to control 102 the at least one oil valve 2007, 2007a, 2016 (the fuel characteristics may be determined using any of the approaches described above). For example, the method 100 may include controlling 102 the at least one valve 2007, 2007a, 2016 oil flow in cruise conditions such that the oil flow ratio is in the range from 0 to 0.20, and optionally from 0 to 0.15, provided that the fuel is at least 70% sustainable aviation fuel; and / or such that the oil flow ratio is in the range from 0 to 0.1, provided that the fuel is at least 80% sustainable aviation fuel.

[0449] With particular regard to the method 200 to be carried out at idle, the method 200 comprises controlling 202 the at least one oil valve 2016, 2007, 2007a such that, under idle conditions, an oil flow ratio of:

[0450] oil flow in the air-oil heat exchanger (m's 0 oil flow rate in the fuel-oil heat exchanger (m-V1)

[0451] is in the range of 0.62 to 5.29, and optionally 0.62 to 5.00, 0.62 to 4.50, 0.62 to 4.00, 0.62 to 3.50, 0.62 to 3.50, or 0.62 to 2.50. Optionally, the oil flow ratio may be in the range of 0.67 to 4.67. The method 200 may include controlling 202 the at least one oil valve such that, under idle conditions, the oil flow ratio is greater than 1.0, and optionally above 1.5 or 2.0. A control device 58 - which may be either a stand-alone unit or part of an EEC - may be provided to implement this control 202.

[0452] With respect to the method 100 performed at cruise, the method 200 performed at idle may further comprise receiving 204 data to enable the oil flow ratio to be calculated or derived, e.g., pump speed data, fuel flow rate data, and / or oil flow rate data. Such data may be received 204 by a controller 58 and used 206 to adjust the control 202 of the oil valve(s) 2007, 2007a, 2016 at idle, so as to maintain the oil flow ratio at a desired level or within desired limits. This checking and adjustment / correction 206 may be performed at regular intervals or in response to predetermined stimuli (e.g., a change in temperature or fuel or oil flow rate, or a change in engine operation, e.g., the start of taxiing).These steps 204, 206 may also be considered as part of the control 202 of the at least one oil valve 2007, 2007a, 2016. The method 200 may also be intended to use other information, e.g. temperature data (of oil, fuel and / or the ambient temperature of an environment around the aircraft 1), flow rate data (of oil and / or fuel) and / or one or more fuel characteristics, in order to determine 206 the control measures to be taken.

[0453] Similarly, the step 202 of controlling the at least one valve 2016, 2007, 2007a so as to adjust the oil flow ratio may comprise decreasing the amount of oil sent through the at least one air-oil heat exchanger 2020 when the oil flow ratio is too high; possibly, closing the valve / completely closing one way of a three-way valve in some implementations such that the oil flow rate through the air-oil heat exchanger 2020 drops to zero. In some implementations, a minimal but non-zero oil flow rate through the air-oil heat exchanger 2020 may be maintained throughout idle operation to prevent oil from congealing within the air-oil heat exchanger 2020, although it is kept in mind that at ground idle, ambient temperatures are generally warmer than at cruising altitude, so this may not be as necessary, if at all, at ground idle.However, in cold weather, for airports or runways in cold climates, it may be desirable to maintain the flow of oil to prevent it from freezing. The external / ambient temperature can therefore be used as an input in the oil flow control, and one or more temperature sensors can be provided accordingly.

[0454] In some implementations, one or more temperature sensors 1009 may be provided, and data relating to the fuel temperature - possibly at the inlet of the combustion chamber 16 - may be used to refine control decisions. The method 200 may include controlling 202 the one or more oil valves 2007, 2007a, 2016 under idle conditions such that the oil flow ratio is in the range of 0.62 to 4.0, or 0.62 to 3.0 provided that the fuel temperature at the inlet of the combustion chamber 16 is at least 160°C; and / or such that the oil flow ratio is in the range of 0.62 to 3.0, or 0.62 to 2.0, provided that the fuel temperature at the inlet of the combustion chamber 16 is at least 180°C.

[0455] In some implementations, one or more fuel characteristics may be considered when determining how to control 202 the at least one oil valve 2007, 2007a, 2016 (the fuel characteristics may be determined using any of the approaches described above). For example, the method 200 may include controlling 202 the at least one oil valve 2007, 2007a, 2016 under idle conditions such that the oil flow ratio is in the range of 0.62 to 3.67, and optionally 0.62 to 3.67, provided that the fuel is at least 70% sustainable aviation fuel; and / or such that the oil flow ratio is in the range of 0.62 to 2.67, or 0.62 to 1.67, provided that the fuel is at least 80% sustainable aviation fuel.

[0456] While the methods 100, 200 described with respect to Figures 12 and 13 optionally use knowledge of one or more fuel characteristics to adapt the specific control 102, 202, other methods such as the method 300 described with respect to [Fig. 14] are more focused on determining and using fuel characteristics to improve the performance of the engine 10, taking advantage of properties that vary between aviation fuels.

[0457] The method 300 illustrated in [Fig. 14] is carried out in a gas turbine engine 10 essentially as described for the preceding methods 100, 200, but the controllable oil valve(s) 2007, 2007a, 2016 are replaced by, or provided in addition to, a controllable air valve 2022 for controlling an air flow rate through the air-oil heat exchanger 2020.

[0458] The method 300 includes determining 302 at least one fuel characteristic of the fuel to be combusted by the combustion chamber 16. The at least one fuel characteristic may be or include any of the examples provided above - e.g., aromatics content (e.g., xylene, toluene, benzene, and / or phenol), paraffins content (e.g., iso-paraffins, n-paraffins, cyclo-paraffins), heteroatomic species concentration (e.g., sulfur-based compounds / sulfur content and / or % nitrogen-based compounds such as anilines or indoles), %SAF, and hydrogen content (generally given as an H / C molar ratio). The determination 302 may be performed by any one or more of the approaches described above, e.g.by chemical and / or physical detection and determination, by analyzing engine performance during fuel use, and / or by receiving fuel data (e.g. electronically or by manual data entry), possibly using a fuel composition determination module 57.

[0459] The method 300 then comprises controlling 304 the air valve 2022 based on the at least one fuel characteristic so as to adjust the air flow rate through the air-oil heat exchanger 2020. The air valve 2022 may be adjustable between a distinct number of fixed positions, or may be continuously adjustable, in various implementations. The control 304 may therefore comprise a distinct or continuous adjustment. A controller 58 - which may be a controller dedicated to the valve 2022 and / or the heat exchange system 3000 more generally, or part of a more general EEC - may be provided to implement this control 302. The method 300 may also be intended to use other information, e.g.temperature data (oil, fuel and / or ambient temperature of an environment around the aircraft 1), and / or flow rate data (oil and / or fuel), in order to determine the control actions to . take. The controller 58, or another processing module, may perform this determination of the appropriate control measure or measures.

[0460] As indicated by the dotted line in [Fig. 14], this method 300 may optionally be repeated. In some implementations, for example in implementations with a single fuel on board the aircraft 1, the method 300 may only be performed once during a flight cycle - e.g., upon refueling or upon reaching cruising altitude. In other implementations, however, for example in implementations with multiple fuel sources, the fuel supplied to the combustion chamber 16 may vary over time during a flight. The determination 302 may therefore be performed more than once - e.g.:

[0461] (i) once for each fuel source 50, 53 when refueling / at the start of flight in implementations with multiple fuel sources of which only one or the other is used at any given time (and knowledge of where the fuel source is drawn may result in selection of the appropriate stored fuel characteristic); or

[0462] (ii) frequently during a flight, e.g., in response to a change in the fuel source(s) (noting that the fuel supplied to the combustion chamber 16 may be a mixture of fuels from different sources, in some implementations), or at regular intervals.

[0463] The air valve 2022 may therefore be intended to remain in a fixed position during operation of the engine 10, the fixed position being determined at start-up of the engine 10 based on the at least one determined fuel characteristic (option (i) above). Alternatively, the air valve 2022 may be intended to have this position set during operation, so as to modify the airflow to the air-oil heat exchanger 2020 during operation of the engine 10 during a single flight (option (ii) above). The air valve 2022 may therefore be actively controlled to vary the airflow through the heat exchanger 2020, particularly in implementations where the aircraft 1 carries several different fuels in different tanks and may change the fuel (or fuel mixture) used during flight.Active control of the air valve 2022 may be automated and implemented by the control device 58.

[0464] It will be appreciated that a fixed position of the air valve 2022 does not necessarily mean a fixed airflow rate, as the air speed relative to the aircraft 1 may vary. In ground operation (e.g., at ground idle), any variation may be due solely to wind speed. In flight, aircraft speed may also influence the airflow rate in the air-oil heat exchanger 2020. In some implementations, the controller 58 may take into account wind speed and / or relative movement between the aircraft 1 and the ambient air to determine a appropriate air valve position. The air valve position can therefore be changed to keep the airflow more constant in certain scenarios.

[0465] The method 300 can be carried out at any time during the operation of the aircraft 1, for example at ground idle, in-flight idle and / or in cruise.

[0466] The air valve 2022 may be controlled 304 to effect relatively large reductions in airflow compared to the valve 2022 being fully open, provided that the determining step 302 indicates that the fuel is suitable for receiving the additional heat which is then no longer lost to the environment. For example, depending on the suitability of the at least one determined fuel characteristic, the method 300 may include controlling 304 the air valve 2022 such that the airflow into the air-oil heat exchanger 2020 is reduced to less than 60%, 50%, or 40% of what the flow rate would be with the valve fully open when the engine 10 is operating at idle conditions.Even greater reductions can be made at cruise - for example, the airflow through the air-oil heat exchanger 2020 can be reduced to less than 20% of what the flow would be with the valve fully open when the engine 10 is operating at cruise conditions, provided the fuel is able to absorb the additional heat, and the airflow can even be reduced to zero (closing valve 2022) when the engine is operating at cruise conditions. Viewed from another angle, if the engine 10 is then operated using fuel less able to absorb the additional heat, correspondingly large changes can be made when the valve 2022 is reopened.It will be borne in mind that airflow rates are generally much lower at ground idle than at flight idle or cruise, and that the same percentage change in airflow may therefore correspond to a much greater difference in airflow when aircraft 1 is in flight than when it is on the ground. Data on the current aircraft operating mode (e.g., aircraft altitude and / or speed) and, possibly, on the weather (e.g., wind speed) may therefore also be taken into account in the valve control.

[0467] In an exemplary geared engine 10 in which the air-to-oil heat exchanger 2020 is located upstream of the fuel-to-oil heat exchanger 1006, with flow modulation only on the air side of the air-to-oil heat exchanger 2020 (i.e., no oil flow modulation), the following percentages of air flow modulations may be typical where “% Air Flow Modulation” is the percentage of the total airflow that would pass through the heat exchanger 2020 if the valve 2022 were fully open (or even if there were no oil flow modulation): no valve present), simply based on the pressure ratio at the inlet 2020a and outlet 2020b of the heat exchanger: • In cruise, for fuel with a limit temperature of 120°C, the airflow modulation % can be 20% or 17%. It can then be decreased linearly with the limit temperature down to 0% (i.e. no airflow / valve closed for at least part of the time spent in cruise) for fuel with a limit temperature of 170°C. For fuel with a limit temperature equal to or higher than 260°C, the airflow modulation % can be 0% throughout the cruise / valve 2022 can be completely closed throughout the cruise. • At idle, and especially at ground idle, for fuel with a limit temperature of 120°C, the airflow modulation percentage can be 58%. It can then be decreased linearly with increasing temperature limit to 38% when the fuel temperature limit reaches 250°C, and then decreased linearly with increasing temperature limit to 28% when the fuel temperature limit reaches 315°C

[0468] The percentages may depend on the fuel temperature limit. The fuel temperature limit is the highest temperature deemed safe for aircraft operation for that fuel and engine 10, and may depend on fuel characteristics, such as thermal stability, as well as the heat tolerance of engine components.

[0469] In some implementations, the at least one fuel characteristic may be or include thermal stability and, in cruise, the air valve 2022 may be adjusted to reduce the air flow to less than 15% of what the flow rate would be with the valve 2022 fully open provided the fuel is stable in operation at temperatures above 160°C, and / or to less than 5% of what the flow rate would be with the valve 2022 fully open provided the fuel is stable in operation at temperatures above 180°C. In some implementations where the valve 2022 is continuously adjustable, the air flow rate may be continuously varied proportional to the thermal stability (possibly linearly with the thermal stability).

[0470] In some implementations, the at least one fuel characteristic may be or include the aromatic hydrocarbon content of the fuel, and the air valve 2022 may be adjusted to reduce the cruise airflow to less than 5% of what the flow rate would be with the valve fully open, provided that the fuel has a molar percentage of aromatic hydrocarbons less than 12%, and possibly less than 10% or less than 5%.

[0471] In some implementations, the at least one fuel characteristic may be or include the percentage of sustainable aviation fuel (SAF) in the fuel, and the air valve 2022 may be adjusted to reduce the cruise airflow to less than 5% of what the flow rate would be with the valve fully open if the fuel has a SAF content greater than 60%, and / or to less than 2% of what the flow rate would be with the valve fully open if the fuel has a SAF content greater than 80%.

[0472] Autoxidation of conventional fossil aviation fuels - which can lead to sge varnish or coking of combustion chamber nozzles and other fuel passageways - generally begins to increase exponentially when the fuel temperature is in the range of 100°C to 150°C (depending on fuel specifics). The inventors were aware that SAF-based fuels could offer the possibility of reaching fuel temperatures above 150°C, and possibly 200°C, or even 250°C or higher, without significant autoxidation. The maximum operating temperature of fuel depends on its composition; hence the importance of determining 302 at least one fuel characteristic.Fuel blends comprising SAF and conventional fossil fuels will again exhibit different thermal stabilities, and therefore different temperature limits, depending on the particular characteristics of the blend. In testing SAF blends with conventional Jet-A aviation fuel, it was found that increasing the % of SAF blended with Jet-A increased thermal stability, but in a non-linear manner. Therefore, it is preferable to opt for predetermined and distinct levels rather than continually changing the airflow rate based on the SAF content.

[0473] In some implementations, the at least one fuel characteristic may be or include the heating value of the fuel, and the air valve 2022 may be adjusted to reduce the cruise airflow to less than 4% of what the flow rate would be with the valve fully open, provided that the fuel has a heating value of at least 43.5 MJ / kg, possibly subject to the suitability of another fuel characteristic (e.g., thermal stability).

[0474] In some implementations, multiple fuel characteristics may be examined jointly, for example with a lower threshold for one fuel characteristic being used when another fuel characteristic is within a specific range or above / below a specific threshold.

[0475] Typically, more air may be required, and valve 2022 may be commanded 304 to be more open, as fuel thermal stability decreases, the fuel sulfur concentration increases (an example of heteroatomic species concentration, which is generally related to thermal stability in that an increased concentration generally reduces thermal stability), or if the heating value of the fuel exceeds a threshold (for high heating value fuels, the flow rate is generally reduced to achieve the same thrust level without wasting fuel - the fuel in the fuel-oil heat exchanger 1006 may therefore experience an increased temperature rise due to its reduced flow rate. However, new fuels (e.g.SAFs), with higher thermal stabilities, also have higher calorific values ​​in many cases - the temperature increase due to lower flow rate may therefore be acceptable in some scenarios; this demonstrates the usefulness of considering multiple fuel characteristics together instead of a single fuel characteristic).

[0476] The inventors were aware that allowing unsuitable fuels to reach these higher temperatures could be detrimental to engine performance, or even dangerous - excessive thermal decomposition of a fuel / deposit can pose safety concerns by blocking pathways and causing component malfunction (e.g., valve jamming and obstruction of nozzles, ports, valves, etc.) which can result in loss of thrust control. The method 400 illustrated in [Fig. 15] is therefore provided as a safety precaution.

[0477] The method 400 illustrated in [Fig. 15] may be implemented in a gas turbine engine 10 substantially as described for the preceding methods 100, 200 of controlling oil flow ratio but the at least one controllable oil valve 2007, 2007a, 2016 for varying a proportion of the oil sent through at least one of the air-oil heat exchanger 2020 and the fuel-oil heat exchanger 1006 is replaced by at least one controllable air or oil valve 2016, 2007, 2007a, 2022 for varying at least one of an oil flow rate and an air flow rate through at least one of the air-oil heat exchanger 2020 and the fuel-oil heat exchanger 1006. Air flow or flow control of oil, or both, can therefore be provided.In implementations where a controllable air valve 2022 is provided (possibly in addition to a controllable oil valve 2016, 2007, 2007a), the same engine 10 as used for the airflow control method 300 may be used. The engine 10 for use with this method 400 also requires a temperature sensor 1009 for providing an indication of fuel temperature—possibly at the inlet to the combustion chamber 16. More generally, the sensor 1009 may be located anywhere. downstream of the fuel-oil heat exchanger 1006 on the fuel side. This temperature sensor 1009 is generally located at or near an inlet of the combustion chamber 16, and more specifically near a nozzle inlet of the combustion chamber 16 so as to sense the fuel temperature at the inlet of the combustion chamber directly, although it will be kept in mind that variations in fuel temperature between the outlet 1006b of the fuel-oil heat exchanger 1006 and the combustion chamber 16 are likely to be small, and, if not deemed negligible, calculable from knowledge of the engine 10 if the fuel temperature at a location different from the sensor 1009 is desired.In some implementations where the fuel temperature at the inlet of the combustion chamber 16 is used, the sensor 1009 may be placed differently and the fuel temperature at the inlet of the combustion chamber may be calculated based on the temperature output and knowledge of the engine 10.

[0478] The method 400 comprises determining 402 whether the fuel temperature has increased above a set threshold under cruising conditions, based on an output from the temperature sensor 1009. For example, a direct comparison may be made between the data received from the temperature sensor 1009 and one or more temperature thresholds stored in the memory. This determination 402 may therefore be made automatically, possibly by a controller 58 (which may be or include a dedicated processing module, or may be provided by a more general EEC).

[0479] The method 400 further comprises, in response to determining that the fuel temperature has increased above the set threshold under cruising conditions, controlling 404 the at least one valve 2016, 2007, 2007a, 2022 so as to modify the at least one flow rate (air flow rate or oil flow rate) through the at least one heat exchanger 1006, 2020 as appropriate to reduce the fuel temperature. For example, adjusting an oil valve 2016, 2007 so as to send less oil through the fuel-oil heat exchanger 1006.For example, oil may be diverted from a first branch of an oil loop system 2000 including the fuel-oil heat exchanger 1006 and sent through a parallel branch including an air-oil heat exchanger 2020 instead of parallel configurations (as shown in Figures 9 and 11), using a modulating valve 2016, or may simply be sent through a bypass pipe 2005 to pass the fuel-oil heat exchanger 1006 and recombine with the remainder of the oil that passed through the fuel-oil heat exchanger 1006 at or downstream of the oil outlet of that heat exchanger 1006, using a bypass valve 2007, as shown in [Fig. 16]. In a parallel configuration as shown in [Fig. 11], the flow reduction. of oil in the fuel-oil heat exchanger 1006 by control of the modulating valve 2016 can automatically increase the flow of oil into the air-oil heat exchanger 2020 (unless a bypass pipe around the air-oil heat exchanger 2020 is present and used to compensate). The bypass valve 2007 and modulating valve 2016 may be used together in certain implementations.

[0480] As indicated by the dotted line in [Fig. 15], this process 400 may be repeated - frequent checks may be preferred for safety reasons, possibly at regular intervals, and / or when triggered by a change in engine operation (e.g., a change in altitude or thrust demand). The oil valve 2016, 2007, 2007a or the air valve 2022 may therefore be actively controlled 404 to vary the flow rate of oil or air through the respective heat exchanger 1006, 2020. Active control of valve 2016, 2007, 2007a, 2022 may be automated and implemented by controller 58. For example, active control of modulating valve 2016 may be automated and implemented by a controller 58 of heat exchange system 3000, which may be a dedicated controller or part of a more general EEC.In various implementations, control of the modulating valve 2016 may be closed-loop or open-loop, depending on the availability of online measurements - in particular, a closed-loop approach to control may be preferred when feedback data (e.g., from oil flow sensors) is available.

[0481] In implementations with both an air valve 2022 and an oil valve 2016, 2007a, airflow through the air-oil heat exchanger 2020 may be increased as more oil is sent through the air-oil heat exchanger 2020; the increase may be linear with increasing oil flow rate. In terms of % airflow, the increase may be significant, for example, from the air valve 2022 being fully closed to fully open, or from a first position with 10% of the maximum air flow rate to 90% or 100% of the maximum air flow rate when the fuel temperature is deemed too high.The method 400 may include, in response to determining 402 that the fuel temperature has increased above a set threshold under cruise conditions, controlling 404a the air valve 2022 to send more air through the air-to-oil heat exchanger 2020 and controlling 404b the oil valve 2016, 2007 to send less oil through the fuel-to-oil heat exchanger 1006. The controlling / adjusting 404a of the air valve 2022 may be performed simultaneously with the controlling / adjusting 404b of the oil valve 2016, 2007.

[0482] In some implementations, the airflow through the air-to-oil heat exchanger 2020 may not be actively regulated. In such implementations, the airflow is likely to change naturally under different operating conditions based on the pressure ratio between the air intake 2020a of the heat exchanger and the air exhaust / outlet 2020b of the heat exchanger. In this configuration, air continues to flow through the air passages of the air-to-oil heat exchanger 2020 even when the air-to-oil heat exchanger 2020 is completely bypassed on the oil side / when the oil flow rate through the heat exchanger 2020 is zero. In other implementations, active control of airflow through the air-oil heat exchanger 2020 is provided, for example as described for the process 300 illustrated in [Fig. 14].For example, the air-oil heat exchanger 2020 and the fuel-oil heat exchanger 1006 may be in series, without a bypass on the oil side of the air-oil heat exchanger 2020, but with an air valve 2022 on the air side that regulates the air flow (an oil bypass valve 2007' and an air flow valve 2022 may be provided in other implementations). In such a series arrangement, the air-oil heat exchanger 2020 may be upstream or downstream of the fuel-oil heat exchanger 1006 with respect to the oil flow.In implementations with such a series arrangement, the fact that the air-to-oil heat exchanger 2020 is upstream of the fuel-to-oil heat exchanger 1006 with respect to the oil flow can help to avoid overheating the fuel - the oil can be cooled as much as desired by increasing the air flow (and possibly recirculating the oil through the air-to-oil heat exchanger 2020) before it reaches the fuel-to-oil heat exchanger 1006.

[0483] In some implementations, an oil bypass pipe 2005a and corresponding control valve 2007a may be implemented only for the air-oil heat exchanger 2020. Optionally, only the oil flow rate through the air-oil heat exchanger 2020 may be actively controlled in the method 400 being described.

[0484] In various implementations: • A bypass pipe 2005a is provided on the oil side of the air-oil heat exchanger 2020 only (no oil bypass from the fuel-oil heat exchanger and no active air flow control); • A bypass pipe 2005a, 2005 may be provided for both the air-oil heat exchanger 2020 and the fuel-oil heat exchanger 1006 (no active air flow control); or • The air flow in the 2020 air-oil heat exchanger may be adjustable and there may be no oil bypass pipes.

[0485] In general, controlling at least one flow through the air-oil heat exchanger 2020 may be considered more important than controlling either flow through the fuel-oil heat exchanger 1006. An oil bypass pipe 2005 and / or a fuel bypass pipe 1005 (and corresponding control valve) may, however, be provided for the fuel-oil heat exchanger(s) 1006 in some implementations, possibly in addition to one or more other controllable valves.

[0486] A combination of air flow control and oil flow control may be implemented both in parallel and in series, but it will be appreciated that while all of the various control options may be used together, the increased complexity and number of components, and thus the increased weight of the overall heat exchange system 3000, may not be desirable. A selection of a subset of control options may therefore be preferred in many implementations - for example, either air flow control or oil flow control may be used to adjust the heat transfer in the air-oil heat exchanger 2020, thus the control of both for a given heat exchanger 2020 may be considered unnecessary.Similarly, in the implementation where the oil flows through the air-to-oil heat exchanger 2020 before the fuel-to-oil heat exchanger 1006, adjusting the temperature of the oil in the air-to-oil heat exchanger 2020 (by regulating the flow of air and / or oil in that heat exchanger) may be used as an alternative to adjusting oil flow through the fuel-to-oil heat exchanger 1006 to change the amount of heat transferred to the fuel in the fuel-to-oil heat exchanger 1006.

[0487] Additionally or alternatively, in implementations where the heat exchange system 3000 includes a refrigeration cycle apparatus 1007 for transferring additional heat from the oil to the fuel beyond that transferred by the fuel-oil heat exchanger 1006, the method 400 may also include controlling the refrigeration cycle apparatus 1007 to reduce the amount of additional heat transferred to the fuel in response to determining 402 that the fuel temperature has increased above the set threshold under cruise conditions. This control may be accomplished by reducing the flow of oil through the refrigeration cycle apparatus 1007, or by reducing the power of the refrigeration cycle apparatus 1007 (e.g., by reducing the flow rate of the refrigerant), or by disabling the refrigeration cycle apparatus 1007.The deactivation threshold of the refrigeration cycle apparatus 1007 may be lower than the threshold set for changing an oil and / or air flow rate through a heat exchanger 1006, 2020. .

[0488] Additionally or alternatively, in implementations where the heat exchange system 3000 includes a secondary oil loop system 2000' also including a fuel-oil heat exchanger 1004, the method may also include reducing oil flow through the secondary fuel-oil heat exchanger 1004 when the fuel temperature has increased above a set threshold (which may be different from the set threshold at which oil flow through the primary fuel-oil heat exchanger 1006 is reduced).

[0489] The threshold set for the control step 404 may be in the range of 140°C and 300°C, and possibly 200°C to 300°C, and possibly 250°C to 300°C. For example, the threshold set may be 140°C, 150°C, 160°C, 170°C, 180°C, 190°C, 200°C, 210°C, 220°C, 230°C, 240°C, 250°C, 260°C, 270°C, 280°C, 290°C, or 300°C.

[0490] The method 400 of some implementations further comprises determining 401 the set threshold, to which the temperature sensor output is compared. This determination 401 may be performed on the wing, and possibly in flight. The determination 401 may be or comprise calculating a threshold value, or identifying an appropriate threshold value from a set of threshold values ​​pre-stored in the memory.

[0491] The inventors were aware that knowledge of one or more fuel characteristics can be used in determination 401 to ensure safety while taking advantage of properties that vary between aviation fuels. Determination 401 may therefore be based on at least one fuel characteristic of the fuel. The at least one fuel characteristic may be or include one of the aforementioned examples, e.g., thermal stability of the fuel, nitrogen content of the fuel, sulfur content of the fuel, and / or sustainable aviation fuel (SAF) content of the fuel (%SAF). The at least one fuel characteristic may be determined by one or more of the aforementioned approaches.

[0492] The step 401 of determining the set threshold may comprise increasing the set threshold, possibly linearly, with increasing thermal stability of the fuel. The set threshold used to regulate the heat modulation may therefore be increased linearly with the thermal stability of the fuel. Thermal stability is the temperature at which the fuel begins to degrade and form deposits that can cause blockages and malfunctions of components - it will be kept in mind that this is generally not an instantaneous degradation when a given temperature is reached; aviation fuels include various components that are susceptible to degradation at different temperatures, and the time spent at high temperature is also a factor in fuel degradation. The threshold value may be set based on a comparison between a maximum allowable deposition rate due to fuel degradation and the thermal stability of the fuel used. A fuel may be considered stable at a given temperature if its degradation rate is below a certain threshold at that temperature. The method 400 may also include the use of a clock or timer, or a fuel flow sensor - a higher fuel temperature may be acceptable at a higher pumping rate, for example, since faster-moving fuel will spend less time at that temperature before being combusted.

[0493] Alternatively or in addition, the step 401 of determining the set threshold may comprise increasing the set threshold with increasing SAF content of the fuel (possibly linearly), for fuels with a SAF content greater than 70%. As mentioned above, it has been found that an increase in % SAF blended into a fossil-based aviation fuel increases thermal stability, but in a non-linear manner. Therefore, separate, predetermined thresholds based on % SAF may be preferred rather than continually changing the set threshold proportionally to the SAF content, or establishing a more complex (non-linear) but continuous relationship.

[0494] Alternatively or in addition, the step 401 of determining the set threshold may comprise decreasing the set threshold with increasing heteroatomic species content of the fuel (possibly linearly). For example, the thermal stability of Jet-A has been found to decrease with increasing nitrogen content (nitrogen content being a measure of the amount of nitrogen-containing species present). It is known that the interaction between sulfur-containing and nitrogen-containing species in the fuel can contribute significantly to fuel decomposition rates, so consideration of the composition of the fuel relative to the multiple heteroatomic species may be implemented to account for these interactions.

[0495] In some implementations, particularly in implementations where a direct measure of fuel thermal stability is not available, multiple fuel characteristics may be determined and used in decisions about valve control. The determined fuel characteristic(s) may in effect be translated into a measure of thermal stability.

[0496] The inventors were aware that in addition to the careful regulation of heat exchangers 1004, 1006, 2020, 2030 as described above for managing heat transfer in an engine 10, the incorporation of a refrigeration cycle 1007 into the heat exchange system 3000 may allow even more benefits to be obtained from newer fuels. In particular, the thermal rise provided by a refrigeration cycle apparatus 1007 may be used to raise the fuel to a temperature above that of the oil, thereby improving oil cooling and overall engine performance. It will be appreciated that a power input is required to move heat from a cooler fluid to a warmer fluid - an active evaporation and condensation process with powered components is therefore used in the described implementation. In particular, the oil loop system 2000 may be designed to transfer heat from the oil to the refrigerant in an evaporator 1007a which evaporates the liquid in the refrigerant (which may be entirely liquid or a liquid-gas mixture at this point in the cycle) to form a saturated vapor.

[0497] A compressor 1007b is then provided to compress the saturated vapor to form a superheated vapor. This hot vapor is then sent to a refrigerant-fuel heat exchanger 1007c; the heat exchanger is a condenser 1007c and the heat transfer from the compressed vapor to the fuel at least partially converts the superheated vapor back to liquid (and possibly converts it to a saturated liquid). The cooler refrigerant is then returned to evaporator 2007a, possibly passing through an expansion valve 2007d (between the condenser and the evaporator / oil interface) to convert the saturated liquid to a reduced temperature liquid and vapor mixture before it returns to evaporator 1007a.

[0498] Control of such a refrigeration cycle apparatus 1007 may allow the level of thermal rise provided to be adjusted appropriately for fuel and engine operating conditions.

[0499] A method 500 of controlling a gas turbine engine 10 to take advantage of thermal lift in managing engine performance is shown in [Fig. 17]. The method 500 is performed in an engine 10 substantially as described herein (noting that the same principles may be applied to a direct drive engine as well as a geared engine) and having a refrigeration cycle apparatus 1007. The method 500 includes controlling 502 the refrigeration cycle apparatus 1007 such that the fuel temperature at the inlet to the combustor 16 is higher than a maximum oil temperature in the oil loop system 2000.

[0500] Control 502 may be performed at regular intervals - correcting heat transfer to the fuel as needed - or may be applied continuously to maintain the fuel temperature above the highest oil temperature. Control 502 may be applied at any stage of engine operation - for example, at idle and / or cruising - and possibly throughout operation.

[0501] Control 502 may include transferring 502a heat from the oil to the refrigerant using evaporator 1007a. The flow rate of oil and / or refrigerant through evaporator 1007a may be adjusted as desired, typically using one or more valves. A bypass and / or recirculation valve around evaporator 1007a may be provided for the oil such that the flow rate of oil through evaporator 1007a may be adjusted without altering the flow rate of oil through other components. Alternatively or in addition, evaporator 1007a may be located on a separate parallel branch of oil system 2000 and modulating valve 2016 may be used to adjust the proportion of oil sent through evaporator 1007a. The pump speed of the 2004 oil pump can be adjusted to change the oil flow in some implementations.The flow rate of refrigerant through the evaporator 1007a may be controlled by regulating a pumping rate of refrigerant around the refrigerant cycle apparatus 1007.

[0502] The control 502 may then comprise the compression 502b of the saturated refrigerant vapor generated in the evaporator 1007a to form a superheated vapor using the refrigerant compressor 1007b. Again, the flow rate of the refrigerant may be controlled by regulating a pumping rate of the refrigerant around the refrigeration cycle apparatus 1007 - the compressor 1007b fulfills the role of the pump in the implementation illustrated in [Fig. 18], but a separate refrigerant pump may be provided in addition to the compressor 1007b in other implementations. The superheated vapor formed in the compressor 1007b is then sent to the condenser 1007c.

[0503] The control 502 may then comprise the transfer 502c of the heat from the superheated vapor to the fuel using the condenser 1007c. Under the effect of this heat transfer, the vapor of the refrigerant transforms at least partially into liquid (i.e., it condenses). A saturated liquid may form in the condenser 1007c.

[0504] In the implementation illustrated in [Fig. 18], the refrigeration cycle apparatus 1007 further comprises an expansion valve 1007d located downstream of the condenser 1007c (from a refrigerant flow perspective) and upstream of the evaporator 1007a. In such implementations, the method 500 comprises using 502d the expansion valve 1007d to convert the refrigerant from the condenser 1007c into a reduced temperature liquid and vapor mixture which is then returned to the evaporator 1007a. The expansion valve 1007d may be actively controlled 502d as part of the control step 502.

[0505] In the implementation illustrated in [Fig. 18], the engine 10 includes a temperature sensor 1009 for sensing the fuel temperature downstream of the refrigeration cycle apparatus 1007 (i.e., as it approaches the combustion chamber 16), and a temperature sensor 2009 for providing an indication of the maximum oil temperature in the oil loop system 2000 (e.g., located immediately downstream of the most downstream oil-cooled engine component). The method 500 may further comprise comparing 501 the oil and fuel temperatures and adjusting the control 502 of the refrigeration cycle apparatus 1007 based on the comparison. For example, if the fuel temperature is lower than the maximum oil temperature, the pump speed of the refrigeration cycle apparatus 1007 (and thus the refrigerant flow rate) may be increased and / or the proportion of oil sent through the evaporator 1007a may be increased (e.g., by adjusting an oil valve).

[0506] The method 500 may also include controlling 504 one or more other components of the heat exchange system 3000 - for example, the fluid flow through the fuel-oil heat exchanger 1006 may be adjusted 504 to increase heat transfer to the fuel by increasing the oil flow rate through the fuel-oil heat exchanger 1006 (e.g., using a bypass or modulating valve). Additionally or alternatively, the fluid flow through the fuel-oil heat exchanger 1006 may be adjusted 504 to increase heat transfer to the fuel by recirculating fuel through the fuel-oil heat exchanger 1006 (e.g., using a recirculation valve 6010).Additionally or alternatively, the heat transfer to the fuel in the fuel-oil heat exchanger 1006 may be increased 504 by reducing the air cooling of the oil before the oil enters the fuel-oil heat exchanger 1006 (e.g., using an air valve 2022). It will be appreciated that the reverse adjustments 504 may be made if the fuel temperature is too high. Although the step 504 of controlling components other than the refrigeration cycle apparatus 1007 is shown after the step 502 of controlling the refrigeration cycle apparatus 1007 in [Fig. 17], it will be appreciated that these steps 502, 504 may be performed in either order, or simultaneously. Further, only the refrigeration cycle apparatus 1007 may be controlled 502 in response to the comparison 501 of the fuel and oil temperatures in some implementations.

[0507] The temperature comparison step 501 may be performed at regular intervals throughout operation, or in response to certain stimuli - e.g., a change in engine operating mode, a change in altitude, a change in thrust demand, a change in fuel, or a change in fuel or oil temperature.

[0508] In various implementations, the method 500 includes controlling 502 the refrigeration cycle apparatus 1007 such that the fuel temperature at the inlet to the combustion chamber 16 is higher than the maximum oil temperature within the oil loop system 2000 by at least 2°C, and optionally of at least 5°C, 10°C, 15°C, 20°C or 25°C. The refrigeration cycle apparatus 1007 may be controlled 502 such that the fuel temperature at the inlet of the combustion chamber 16 is higher than the maximum oil temperature within the oil loop system 2000 by between 2°C and 50°C.

[0509] The method 500 may include controlling 502, 504 the engine 10 to operate with a fuel temperature at the inlet of the combustion chamber 16 higher than the maximum oil temperature for at least 10%, 20%, 30%, or 50% of the time in cruise. In some implementations, the fuel temperature at the inlet of the combustion chamber 16 may be maintained at a level higher than the maximum oil temperature throughout the cruise.

[0510] In some implementations, the control 502 is based on one or more fuel characteristics (e.g., any of the fuel characteristics described above, which may be determined by any of the methods described above).

[0511] The method 500 may include controlling 502 the refrigeration cycle apparatus 1007 (and optionally controlling 504 one or more other components of the heat exchange system 3000) such that the fuel temperature at the inlet of the combustion chamber 16 is higher than the maximum oil temperature in the oil loop system 2000 by an amount determined based on one or more characteristics of the fuel. For example, a greater increase in fuel temperature above the oil temperature may be provided when the thermal stability and / or heating value of the fuel are higher. Two or more fuel characteristics may be evaluated jointly in some implementations.The method 500 may therefore comprise obtaining one or more fuel characteristics and adjusting the control 502, 504 based on these characteristics, possibly in addition to other data - e.g., operating mode, altitude, ambient temperature, oil type, oil and / or fuel flow rates, refrigerant type, etc.

[0512] The method 500 is independent of the engine architecture and can be applied to both geared gas turbine engines 10 and direct-drive gas turbine engines. The oil loop system is intended to supply oil to cool, and optionally lubricate, one or more engine components - in geared engines 10, the cooled engine component(s) may be or include the power reducer 30. In geared or direct-drive engines, the cooled engine component(s) may be or include an auxiliary reducer 33, one or more bearings (e.g., for a shaft), power electronics, one or more components of an integrated drive generator, and / or any other part of the engine that needs to be cooled.

[0513] It will be understood that the invention is not limited to the embodiments described above and that various modifications and improvements may be made without departing from the concepts described herein. Except where mutually exclusive, any feature may be employed separately or in combination with other features and the description extends to and includes all combinations and subcombinations of one or more features described herein.

Claims

Claims

1. A method (200) of operating a gas turbine engine (10) for an aircraft (1), the gas turbine engine (10) comprising: an engine core (11) comprising a turbine (19), a compressor (14), a combustion chamber (16) for burning fuel, and a core shaft (26) connecting the turbine to the compressor; a fan (23) located upstream of the engine core (11); a reduction gear (30) which receives an input from the core shaft (26) and provides a drive to the fan (23) so as to drive the fan at a rotational speed lower than that of the core shaft; an oil loop system (2000) for supplying oil to the reduction gear (30); and a heat exchange system (3000) comprising: an air-oil heat exchanger (2020) through which the oil in the oil loop system flows;a fuel-oil heat exchanger (1006) through which oil in the oil loop system and fuel flow such that heat is transferred between the oil and the fuel; and at least one valve (2016, 2007, 2007a) for allowing a proportion of the oil sent through at least one of the heat exchangers (1006, 2020) to be varied, the method (200) comprising controlling (202) the at least one valve (2016, 2007, 2007a) such that, under idle conditions, an oil flow ratio of: oil flow rate in the air-oil heat exchanger (nvs'O oil flow rate in the fuel-oil heat exchanger (m3s4) is in the range of 0.62 to 5.29.;

2. The method (200) of claim 1, wherein the oil loop system (2000) branches such that a proportion of the oil can flow along each branch and the air-oil and fuel-oil heat exchangers (2020, 1006) are arranged in a parallel configuration on different branches of the oil loop system, and wherein the valve (2016) for allowing the proportion of the oil sent by the through at least one of the heat exchangers (1006, 2020) is a modulating valve (2016) intended to allow the proportion of oil sent through each branch to be varied.

3. The method (200) of claim 1, wherein the oil loop system (2000) further comprises at least one bypass pipe (2005, 2005a) for allowing a proportion of the oil to bypass at least one of the air-to-oil heat exchanger (2020) and the fuel-to-oil heat exchanger (1006), and wherein the valve (2007, 2007a) for allowing the proportion of the oil sent through at least one of the heat exchangers (1006, 2020) to be varied is a bypass valve (2007, 2007a) for allowing a proportion of the oil to bypass the at least one heat exchanger (2020, 1006).

4. The method (200) of claim 3, wherein the air-oil and fuel-oil heat exchangers (2020, 1006) are arranged in series in the oil loop system (2000).

5. The method (200) of claim 3 wherein the air-oil and fuel-oil heat exchangers (2020, 1006) are arranged in parallel, on different branches of the oil loop system (2000), and wherein the method (200) comprises controlling both the bypass valve (2007, 2007a) and a modulating valve (2016) to allow the proportion of oil sent through each branch to be varied.

6. The method (200) of claim 1, comprising controlling (202) the at least one valve (2016, 2007, 2007a) such that, under idle conditions, the oil flow ratio is below 5.50; optionally such that, under idle conditions, the oil flow ratio is below 5.0; optionally further such that, under idle conditions, the oil flow ratio is below 4.

5.

7. The method (200) of claim 1, wherein controlling (202) the at least one valve (2016, 2007, 2007a) to adjust the oil flow ratio comprises decreasing the amount of oil sent through the at least one air-oil heat exchanger (2020) when the oil flow ratio is too high.

8. The method (200) of claim 1, wherein the method (200) comprises controlling (202) the at least one valve (2016, 2007, 2007a) under idle conditions such that the heat transfer ratio is in the range of 0.62 to 3.00 when the fuel temperature at the inlet of the combustion chamber (16) is above 160°C; and / or such that the heat transfer ratio is in the range of 0.62 to 2.00 when the fuel temperature at the inlet of the combustion chamber (16) is above 180°C.

9. The method (200) of claim 1, wherein, under idle conditions, the method (200) comprises controlling (202) the at least one valve (2016, 2007, 2007a) such that the heat transfer ratio is in the range of 0.62 to 3.67 provided that the fuel is comprised of at least 70% sustainable aviation fuel.

10. The method (200) of claim 1, wherein the heat exchange system (3000) further comprises a refrigeration cycle apparatus (1007) for providing thermal elevation by transferring more heat from the oil to the fuel beyond that transferred by the fuel-oil heat exchanger (1006), and wherein the method (200) further comprises controlling the refrigeration cycle apparatus (1007) to adjust the amount of additional heat transferred to the fuel.

11. The method (200) of claim 1, wherein the heat exchange system (3000) comprises multiple bypass pipes (2005, 2005a, 2005') each for allowing oil to bypass a heat exchanger (1006, 1004, 2020), and wherein the step of controlling (202) the at least one valve (2016, 2007, 2007a) comprises controlling (202) at least two bypass valves.

12. A gas turbine engine (10) for an aircraft (1) comprising: an engine core (11) comprising a turbine (19), a compressor (14) and a core shaft (26) connecting the turbine to the compressor; a fan (23) located upstream of the engine core; and a reduction gear (30) which receives an input from the core shaft and provides a drive to the fan so as to drive the fan at a lower rotational speed than the core shaft; an oil loop system (2000) for supplying oil to the reducer (30); and a heat exchange system (3000) comprising: an air-oil heat exchanger (2020) through which the oil in the oil loop system flows; a fuel-oil heat exchanger (1006) through which the oil in the oil loop system and the fuel flow such that heat is transferred between the oil and the fuel; and at least one valve (2016, 2007, 2007a) for varying a proportion of the oil sent through at least one of the heat exchangers (1006, 2020), and wherein the at least one valve (2016, 2007, 2007a) is intended to be controlled such that, under idle conditions, an oil flow ratio of: oil flow rate in air-oil heat exchanger (m-'s'1) oil flow rate in fuel-oil heat exchanger is in the range of 0.62 to 5.

29.

13. The gas turbine engine (10) of claim 12, wherein the heat exchange system (3000) further comprises a refrigeration cycle apparatus (1007) for providing thermal elevation by transferring more heat from the oil to the fuel, such that the fuel temperature is raised above the oil temperature.

14. A gas turbine engine (10) according to claim 12, wherein: the turbine (19) is a first turbine, the compressor (14) is a first compressor, and the core shaft (26) is a first core shaft; the engine core further comprises a second turbine (17), a second compressor (15), and a second core shaft (27) connecting the second turbine to the second compressor; and the second turbine, the second compressor, and the second core shaft are adapted to rotate at a higher rotational speed than that of the first core shaft.

15. A gas turbine engine (10) according to claim 12, wherein the heat exchange system (3000) further comprises branched fuel return paths and at least one valve controlling a fuel flow division, the paths branched being adapted to return fuel from the heat exchange system to at least two different locations along a main fuel path from where the fuel enters the gas turbine engine (10) to the combustion chamber (16).

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