Gas turbine variable oil flow

Abstract

A method of operating a gas turbine engine for an aircraft is disclosed, the gas turbine engine comprising: an engine core comprising a turbine, a compressor, a combustor arranged to combust a fuel, and a mandrel connecting the turbine to the compressor; a fan located upstream of the engine core; a gearbox receiving an input from the mandrel and outputting a drive to the fan; an oil circuit system arranged to supply oil to the gearbox; and a heat exchange system, the 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 arranged to allow a change in the proportion of oil delivered via the at least one heat exchanger, the method comprising controlling the valve such that, under idle conditions, the oil flow ratio: # imgabs0 # is in the range of 0.62 to 5.29.

Description

[0001] Cross - reference to related applications

[0002] This specification claims the benefit of the priority of United Kingdom Patent Application No. 2319143.0, filed on December 14, 2023, the entire content of which is incorporated herein by reference. Background art Technical field

[0003] The present disclosure relates to aircraft propulsion systems and to methods of operating an aircraft that include the management of different fluids and heat transfer between them, and more particularly to the management of oil flow and / or air flow within an aircraft engine.

[0004] Description of Related Technologies

[0005] In the aviation industry, there is a trend towards using fuels different from the traditional kerosene - based jet fuels commonly used currently. These fuels may have different fuel properties relative to petroleum - based hydrocarbon fuels. Therefore, it is necessary to consider the fuel properties of these new fuels and adjust the gas turbine engine itself and the methods of operating the gas turbine engine. Summary of the invention

[0006] According to a first aspect, there is provided a method of operating a gas turbine engine of an aircraft, the gas turbine engine comprising:

[0007] An engine core including a turbine, a compressor, a burner arranged to burn fuel, and a shaft connecting the turbine to the compressor;

[0008] A fan located upstream of the engine core;

[0009] A gearbox that receives an input from the shaft and outputs drive to the fan so as to drive the fan at a rotational speed lower than that of the shaft;

[0010] An oil circuit system arranged to supply oil to the gearbox; and

[0011] A heat exchange system including:

[0012] An air - oil heat exchanger through which oil in the oil circuit system flows;

[0013] A fuel - oil heat exchanger through which oil and fuel in the oil circuit system flow such that heat is transferred between the oil and the fuel; and

[0014] At least one valve arranged to allow changing the proportion of oil conveyed through at least one of the heat exchangers.

[0015] The method includes controlling at least one valve such that, under cruise conditions, the oil flow ratio:

[0016]

[0017] is in the range from 0 to 0.59.

[0018] The inventors have recognized that using a fuel different from traditional kerosene-based jet fuel, such as sustainable aviation fuel, can result in different fuel properties and that parameters under cruise conditions can be adjusted to take advantage of the different fuel properties. In particular, some fuels can be heated to a higher temperature in one or more fuel-oil heat exchangers than traditional fuels without significantly increasing the deposition of fuel decomposition products, such as by coking or varnishing. This can enable a method of providing improved oil cooling (since the fuel is able to absorb more heat), and can also improve the overall thermal / thermodynamic efficiency of the engine, with less heat loss to the surroundings and potentially also more power recovered in the thermodynamic cycle. Being able to control the oil flow within the engine plays a key role in managing heat transfer. The inventors have realized that these principles can be applied to engines with branched oil circuit paths that have different heat exchangers on different branches (in a parallel arrangement), and that these principles can be applied to engines with heat exchangers in a substantially linear series arrangement, and where one or more bypass pipes can be used as an alternative to the branched main path. Introducing one or more controllable oil valves, and / or improving the control of existing oil valves, can thus enable a method of providing improved oil cooling (since the fuel used may carry away more heat than traditional fuels), and can also improve the overall thermal efficiency of the engine. One or more controllable oil valves allow the oil flow ratio to be adjusted appropriately according to a given fuel.

[0019] Although the oil flow ratio is dimensionless, units (cubic meters per second or kilograms per second) are shown above as examples. The numerator and denominator must have the same units to provide a dimensionless ratio. Thus, the flow rate used can be either a volumetric flow rate or a weight flow rate (i.e., a mass flow rate) if the numerator and denominator are consistent.

[0020] The oil circuit system can branch such that a certain proportion of the oil can flow along each branch. Air-oil heat exchangers and fuel-oil heat exchangers can be arranged in a parallel configuration on different branches of the oil circuit system. At least one valve arranged to allow changing the proportion of oil delivered via at least one heat exchanger can be or include a modulating valve arranged to allow changing the proportion of oil delivered via each branch.

[0021] The oil circuit system may include at least one bypass pipe arranged to allow a proportion of the oil to bypass at least one of the air-oil heat exchanger and the fuel-oil heat exchanger. At least one valve arranged to allow changing the proportion of the oil conveyed through at least one of the heat exchangers may be or include a bypass valve arranged to allow a proportion of the oil to bypass the at least one heat exchanger. The heat exchange system may include a plurality of bypass pipes, each bypass pipe arranged to allow the oil to bypass one heat exchanger - in such embodiments, the step of controlling at least one valve may be or include controlling at least two bypass valves. In particular, the oil circuit system may include at least one bypass pipe arranged to allow a proportion of the oil to bypass the air-oil heat exchanger.

[0022] The air-oil heat exchanger and the fuel-oil heat exchanger in embodiments having one or more bypass pipes may be arranged in series in the oil circuit system (such that the main oil flow path passes through both one after another), or in parallel in the oil circuit system (such that the main oil flow path divides and one branch passes through each). In embodiments where the air-oil heat exchanger and the fuel-oil heat exchanger are arranged in parallel on different branches of the oil circuit system, the method steps of controlling at least one valve may include controlling one or more bypass valves and a modulating valve arranged to allow changing the proportion of the oil conveyed through each branch.

[0023] The method may include controlling at least one valve such that, under cruise conditions, the oil flow ratio is in the range of 0 to 0.50, optionally in the range of 0 to 0.40, and further optionally in the ranges of 0 to 0.30, 0 to 0.20, or optionally in the range of 0 to 0.10.

[0024] The step of controlling at least one valve to adjust the oil flow ratio may include reducing the amount of oil conveyed through at least one air-oil heat exchanger when the oil flow ratio is too high. Additionally or alternatively, when the oil flow ratio is too high, the amount of oil conveyed through at least one fuel-oil heat exchanger may be increased.

[0025] The method may include determining the fuel temperature downstream of the fuel-oil heat exchanger (and optionally at the burner inlet) and adjusting the control of the oil flow ratio based on the fuel temperature (if necessary). The method may include controlling at least one valve under cruise conditions such that:

[0026] (i) if the fuel temperature downstream of the fuel-oil heat exchanger is at least 140 °C, the oil flow ratio is in the range of 0 to 0.4, optionally in the range of 0 to 0.3, and further optionally in the range of 0 to 0.25;

[0027] (ii) If the fuel temperature downstream of the fuel-oil heat exchanger is at least 160 °C, the oil flow ratio is in the range of 0 to 0.3, optionally in the range of 0 to 0.2, and further optionally in the range of 0 to 0.15; and / or

[0028] (iii) If the fuel temperature downstream of the fuel-oil heat exchanger is at least 180 °C, the oil flow ratio is in the range of 0 to 0.2, optionally in the range of 0 to 0.1, and further optionally in the range of 0 to 0.075;

[0029] The method may include determining one or more fuel properties and controlling (if necessary) the oil flow ratio based on the one or more determined fuel properties. For example, the method may include controlling at least one valve such that if the fuel is at least 70% sustainable aviation fuel (SAF), the oil flow ratio is in the range of 0 to 0.3, optionally in the range of 0 to 0.2, and further optionally in the range of 0 to 0.15. The SAF proportion (X%) may be by volume.

[0030] The heat exchange system may include a refrigeration cycle device arranged to provide a heat boost by transferring additional heat from the oil to the fuel in addition to the heat transferred by the fuel-oil heat exchanger. The method may include controlling the refrigeration cycle device so as to adjust the amount of additional heat transferred to the fuel. In such embodiments, the fuel temperature may be raised above the oil temperature.

[0031] According to a second aspect, there is provided a gas turbine engine for an aircraft, the gas turbine engine comprising:

[0032] An engine core including a turbine, a compressor, and a shaft connecting the turbine to the compressor

[0033] ;

[0034] A fan located upstream of the engine core; and

[0035] A gearbox that receives an input from the shaft and drives an output to the fan so as to

[0036] drive the fan at a rotational speed lower than that of the shaft;

[0037] An oil circuit system arranged to supply oil to the gearbox; and

[0038] A heat exchange system including:

[0039] An air-oil heat exchanger through which the oil in the oil circuit system flows;

[0040] A fuel-oil heat exchanger through which oil and fuel in an oil circuit system flow such that heat is transferred between the oil and the fuel; and

[0041] At least one valve arranged to allow a change in the proportion of oil delivered via at least one in the heat exchanger.

[0042] At least one valve is arranged to be controlled such that under cruise conditions, the oil flow ratio:

[0043]

[0044] Is in the range of 0 to 0.59.

[0045] The gas turbine engine may further include a controller arranged to control the valve. The gas turbine engine may further include one or more oil flow rate sensors. The controller may be arranged to receive the outputs from the one or more oil flow rate sensors and make control decisions based on these outputs. The oil flow rate may be sensed directly or may be inferred from one or more other measurements, such as using a pressure drop measurement at an orifice.

[0046] The heat exchange system may further include a refrigeration cycle device arranged to provide a heat boost by transferring more heat from the oil to the fuel, optionally raising the fuel temperature to be higher than the oil temperature.

[0047] The heat exchange system may further include a branched fuel return passage and at least one valve controlling the fuel flow split, the branched passages being arranged to return fuel from the heat exchange system to at least two different positions along a main fuel path from which the fuel enters the gas turbine engine to reach the burner.

[0048] The turbine may be a first turbine, the compressor may be a first compressor, and the shaft may be a first shaft. The engine core may further include a second turbine, a second compressor, and a second shaft connecting the second turbine to the second compressor. The second turbine, second compressor, and second shaft may be arranged to rotate at a higher rotational speed than the first shaft.

[0049] The engine of the second aspect may be arranged to perform the method of the first aspect and may have any of the features described in relation to the first aspect.

[0050] According to a third aspect, there is provided a method of operating a gas turbine engine for an aircraft, the gas turbine engine comprising:

[0051] An engine core including a turbine, a compressor, a burner arranged to burn fuel, and a shaft connecting the turbine to the compressor;

[0052] A fan located upstream of the engine core;

[0053] A gearbox that receives an input from a core shaft and drives an output to a fan to drive the fan at a rotational speed lower than that of the core shaft;

[0054] An oil circuit system that is arranged to supply oil to the gearbox; and

[0055] A heat exchange system that includes:

[0056] An air-oil heat exchanger through which the oil in the oil circuit system flows;

[0057] A fuel-oil heat exchanger through which the oil and fuel in the oil circuit system flow such that heat is transferred between the oil and the fuel; and

[0058] At least one valve that is arranged to allow changing the proportion of the oil conveyed through at least one of the heat exchangers.

[0059] The method includes controlling at least one valve such that, under idle conditions, the oil flow rate ratio:

[0060]

[0061] Is in the range of 0.62 to 5.29.

[0062] As discussed in relation to the first aspect, the inventors have recognized that using a fuel different from traditional kerosene-based jet fuel (such as sustainable aviation fuel) can result in different fuel properties, and the operating parameters can be adjusted to take advantage of the different fuel properties. In particular, some fuels can be heated to a higher temperature in one or more fuel-oil heat exchangers than traditional fuels without significantly increasing the deposition of fuel decomposition products, such as by coking or varnishing. This can enable a method of providing improved oil cooling (since the fuel can absorb more heat), and can also improve the overall thermal efficiency of the engine while less heat is lost to the surrounding environment. One or more controllable oil valves play a key role in managing the oil flow rate ratio. Additionally, the inventors have recognized that while cruise conditions typically account for most of the flight time of an aircraft engine, idle operation is also important. Since the fuel mass flow rate at idle is much lower than that during cruise, even a relatively small heat load on the fuel can result in a high degree of temperature rise - thus, using non-traditional fuels may have a greater impact on the optimal thermal management method under idle conditions - for example, during ground idle when the aircraft is starting up, when stationary during boarding, and when taxiing (towards the runway or hangar, or between other ground-based positions), or during flight idle, such as when starting a descent. Since the operating conditions between cruise and idle are very different, especially in terms of the desired thrust output of the engine, different oil flow controls are appropriate.

[0063] For the first and second aspects, although the oil flow rate ratio is dimensionless, the units (cubic meters per second or kilograms per second) are shown by way of example above, and it is demonstrated that the numerator and denominator must have the same units. The idle operation of the aircraft during ground operation can be referred to as "ground idle". The idle operation of the aircraft while in the air can be referred to as "flight idle". Unless otherwise stated, all options described below can be assumed to relate to ground idle conditions. The thrust of flight idle is generally slightly higher than ground idle. In some embodiments, only a relatively limited range may apply to the flight idle of a particular engine. In other embodiments, all options described below for this aspect also apply to flight idle conditions.

[0064] The oil circuit system can branch such that a certain proportion of the oil can flow along each branch. The air-oil heat exchanger and the fuel-oil heat exchanger can be arranged in a parallel configuration on different branches of the oil circuit system. At least one valve arranged to allow changing the proportion of oil delivered via at least one heat exchanger can be or include a modulating valve arranged to allow changing the proportion of oil delivered via each branch.

[0065] The oil circuit system can include at least one bypass pipe arranged to allow a certain proportion of the oil to bypass at least one of the air-oil heat exchanger and the fuel-oil heat exchanger. At least one valve arranged to allow changing the proportion of oil delivered via at least one of the heat exchangers can be or include a bypass valve arranged to allow a certain proportion of the oil to bypass the at least one heat exchanger. The heat exchange system can include a plurality of bypass pipes, each bypass pipe being arranged to allow the oil to bypass one heat exchanger - in such embodiments, the step of controlling at least one valve can be or include controlling at least two bypass valves.

[0066] The air-oil heat exchanger and the fuel-oil heat exchanger in embodiments having one or more bypass pipes can be arranged in series in the oil circuit system (such that the main oil flow path passes through both one after another), or in parallel in the oil circuit system (such that the main oil flow path divides and one branch passes through each). In embodiments where the air-oil heat exchanger and the fuel-oil heat exchanger are arranged in parallel on different branches of the oil circuit system, the method step of controlling at least one valve can include controlling one or more bypass valves and a modulating valve arranged to allow changing the proportion of oil delivered via each branch.

[0067] The method can include controlling at least one valve such that, under idle conditions, the oil flow rate ratio is less than 5.50, optionally less than 5.0, optionally less than 4.5, and further optionally less than 4.0.

[0068] The step of controlling at least one valve to adjust the oil flow ratio may include reducing the amount of oil delivered through at least one air-oil heat exchanger when the oil flow ratio is too high.

[0069] The method may include determining the fuel temperature downstream of the fuel-oil heat exchanger and optionally the fuel temperature at the burner inlet, and if necessary, adjusting the control of the oil flow ratio based on the fuel temperature. The method may include controlling at least one valve under idle conditions such that:

[0070] (i) When the fuel temperature downstream of the fuel-oil heat exchanger is higher than 140 °C, the oil flow ratio is in the range of 0.62 to 4.00;

[0071] (ii) When the fuel temperature downstream of the fuel-oil heat exchanger is higher than 160 °C, the oil flow ratio is in the range of 0.62 to 3.00; and / or

[0072] (iii) When the fuel temperature downstream of the fuel-oil heat exchanger is higher than 180 °C, the oil flow ratio is in the range of 0.62 to 2.00.

[0073] The method may include determining one or more fuel properties and adjusting the control of the oil flow ratio based on the one or more determined fuel properties (if necessary). For example, the method may include, under idle conditions, controlling at least one valve such that if the fuel is at least 70% sustainable aviation fuel (SAF), the oil flow ratio is in the range of 0.62 to 3.67.

[0074] The heat exchange system may include a refrigeration cycle device arranged to provide a heat boost by transferring additional heat from the oil to the fuel in addition to the heat transferred by the fuel-oil heat exchanger. The method may include controlling the refrigeration cycle device to adjust the amount of additional heat transferred to the fuel. In such embodiments, the fuel temperature may be raised above the oil temperature.

[0075] The methods of the first and third aspects may be complementary and may be performed together in various embodiments. The method of the third aspect may be performed using the engine of the second aspect.

[0076] According to a fourth aspect, there is provided a gas turbine engine for an aircraft, the gas turbine engine comprising:

[0077] An engine core including a turbine, a compressor, and a shaft connecting the turbine to the compressor;

[0078] A fan located upstream of the engine core; and

[0079] A gearbox that receives an input from the shaft and drives an output to the fan to

[0080] The fan is driven at a rotational speed lower than that of the spool;

[0081] An oil circuit system arranged to supply oil to the gearbox; and

[0082] A heat exchange system comprising:

[0083] An air - oil heat exchanger through which the oil in the oil circuit system flows;

[0084] A fuel - oil heat exchanger through which the oil and fuel in the oil circuit system flow such that heat is transferred between the oil and the fuel; and

[0085] At least one valve arranged to allow a change in the proportion of oil delivered through at least one of the heat exchangers.

[0086] The at least one valve is arranged to be controlled such that under idle conditions, the oil flow ratio:

[0087]

[0088] Is in the range of 0.62 to 5.29.

[0089] The heat exchange system may further include a refrigeration cycle device arranged to provide a heat boost by transferring more heat from the oil to the fuel, optionally causing the fuel temperature to rise above the oil temperature.

[0090] The turbine may be a first turbine, the compressor may be a first compressor, and the spool may be a first spool. The engine core may further include a second turbine, a second compressor, and a second spool connecting the second turbine to the second compressor. The second turbine, second compressor, and second spool may be arranged to rotate at a rotational speed higher than that of the first spool.

[0091] The heat exchange system may further include a branched fuel return passage and at least one valve controlling the fuel flow split, the branched passages being arranged to return fuel from the heat exchange system to at least two different positions along the main fuel path through which fuel enters the gas turbine engine to reach the burner.

[0092] The engine of the fourth aspect may be arranged to perform the methods of the first aspect and / or the third aspect and may have any one of the features described with respect to the first aspect, the second aspect, or the third aspect.

[0093] According to a fifth aspect, there is provided a method of operating a gas turbine engine of an aircraft, the gas turbine engine comprising:

[0094] An engine core, the engine core including a turbine, a compressor, a burner arranged to burn fuel, and a shaft connecting the turbine to the compressor;

[0095] A fan, the fan being located upstream of the engine core;

[0096] A gearbox, the gearbox receiving an input from the shaft and providing a drive output to the fan so as to drive the fan at a rotational speed lower than that of the shaft;

[0097] An oil circuit system, the oil circuit system being arranged to supply oil to the gearbox; and

[0098] A heat exchange system, the heat exchange system including:

[0099] An air-oil heat exchanger, oil in the oil circuit system flowing through the air-oil heat exchanger;

[0100] A fuel-oil heat exchanger, oil and fuel in the oil circuit system flowing through the fuel-oil heat exchanger such that heat is transferred between the oil and the fuel; and

[0101] An air valve, the air valve being arranged to control the flow rate of air passing through the air-oil heat exchanger.

[0102] The method includes:

[0103] Determining at least one fuel characteristic of fuel arranged to be burned by the burner; and

[0104] Controlling the air valve based on the at least one fuel characteristic so as to adjust the flow rate of air passing through the air-oil heat exchanger.

[0105] The inventors have recognized that using fuels different from traditional kerosene-based jet fuels, such as sustainable aviation fuels, can result in different fuel properties and that operating parameters can be adjusted to take advantage of the different fuel properties. In particular, some fuels can be heated to higher temperatures than traditional fuels in one or more fuel-oil heat exchangers without significantly increasing the deposition of fuel decomposition products, such as by coking or varnishing, thereby allowing the fuel to absorb more heat and thus reducing the need for air cooling. This can improve the combustion efficiency of the fuel and also improve the overall efficiency of the engine while less heat (via the air-oil heat exchanger) is lost to the environment. Thus, where fuel characteristics permit, a higher fuel temperature at the burner inlet can allow for a method of providing improved oil cooling. For example, the thermal stability of the fuel (a fuel characteristic) affects how much heat the fuel can accept / what temperature the fuel can be raised to without forming deposits within the pipes, burners, and / or hydraulic mechanical units or other engine components. Thus, when the thermal stability of the fuel is higher, taking the thermal stability of the fuel into account and reducing the air flow rate through the air-oil heat exchanger (optionally to zero) can provide a more thermally efficient engine while avoiding coking or excessive varnishing, thereby improving aircraft performance. Thus, using the fuel to extract more heat from the oil rather than relying on heat transfer from the oil to the environment / surrounding air (in the air-oil heat exchanger) can provide a more thermodynamically efficient engine and improve the cooling of the oil before it returns to the rest of the turbofan engine.

[0106] During a transition period, over the life of a gas turbine engine, the available aviation fuel changes over time and depends on the geographical location of the fuel supply point (among other variables), and it is important to determine the relevant fuel characteristics based on the specific fuel being used and perform control of the air flow rate. Thus, the gas turbine operation can be adjusted to make the most of the various fuels. A controllable air flow rate valve arranged to regulate the air flow rate through the air-oil heat exchanger plays a key role in such engine performance tuning.

[0107] The method can include, depending on the suitability of at least one determined fuel characteristic, controlling the air valve under idle conditions such that:

[0108] (i) when the engine is operating under idle conditions, the air flow rate into the air-oil heat exchanger is reduced to less than 60% of the flow rate when the valve is fully open; and / or

[0109] (ii) when the engine is operating under idle conditions, the air flow rate into the air-oil heat exchanger is reduced to less than 40% of the flow rate when the valve is fully open.

[0110] The method can include, depending on the suitability of at least one determined fuel characteristic, controlling the air valve under cruise conditions such that:

[0111] (i) When the engine is operating under cruise conditions, the air flow rate into the air-oil heat exchanger is reduced to less than 20%, and optionally less than 17%, of the flow rate when the valve is fully open; and / or

[0112] or

[0113] (ii) When the engine is operating under cruise conditions, the air flow rate into the air-oil heat exchanger is reduced to at least substantially zero.

[0114] The suitability of at least one determined fuel property can be determined by comparing one of the plurality of fuel properties with a threshold value set for the corresponding fuel property. For example, a SAF content (%SAF) exceeding 60%, 65%, 70%, 75% or 80% compared to conventional aviation fuel can be considered suitable for reducing the air flow rate. In some embodiments, multiple fuel properties can be checked simultaneously - for example, when one fuel property is within a specific range, a lower threshold value of another fuel property is used. In other embodiments, the fuel properties can be considered independently / isolatedly.

[0115] The methods of the first, third and fifth aspects can be complementary, and any two or more of them can be performed together in various embodiments. For example, at cruise and / or idle, the oil flow rate and the air flow rate can be controlled. The method of the fifth aspect can be performed using the engine of the second or fourth aspect.

[0116] At least one determined fuel property can be or include thermal stability. In such embodiments, if the fuel operates stably at a temperature above 160°C, then at cruise, the air flow rate into the air-oil heat exchanger can be reduced to less than 15% of the flow rate when the valve is fully open. Alternatively or additionally, if the fuel operates stably at a temperature above 180°C, then at cruise, the air flow rate into the air-oil heat exchanger can be reduced to less than 5% of the flow rate when the valve is fully open. In some embodiments, the air flow rate can be reduced as the thermal stability increases, optionally linearly.

[0117] At least one determined fuel property can be or include the aromatic content in the fuel. In such embodiments, at cruise, if the fuel has an aromatic molar percentage lower than 12% and optionally lower than 10% or lower than 5%, then the air flow rate into the air-oil heat exchanger can be reduced to less than 5% of the flow rate when the valve is fully open.

[0118] At least one determined fuel characteristic can be or include the percentage of sustainable aviation fuel (SAF) in the fuel (% SAF). In such embodiments, if the fuel has an SAF content higher than 60%, during cruise, the air flow entering the air-oil heat exchanger can be reduced to less than 5% of the flow rate when the valve is fully open. Alternatively or additionally, if the fuel has an SAF content higher than 80%, during cruise, the air flow entering the air-oil heat exchanger can be reduced to less than 2% of the flow rate when the valve is fully open. In some embodiments, once the fuel exceeds 60%, 65%, 70%, 75% or 80% SAF, the air flow can decrease as the % SAF increases, optionally linearly.

[0119] At least one determined fuel characteristic can be or include the calorific value of the fuel. In such embodiments, if the fuel has a calorific value of at least 43.5 MJ / kg, during cruise, the air flow entering the air-oil heat exchanger can be reduced to less than 4% of the flow rate when the valve is fully open. At least one determined fuel characteristic can be or include the thermal stability of the fuel.

[0120] Options for fuel characteristics and their determination are provided below. These options are provided by way of example only and are not intended to be limiting.

[0121] According to a sixth aspect, there is provided a gas turbine engine for an aircraft, the gas turbine engine comprising:

[0122] An engine core including a turbine, a compressor, and a spool connecting the turbine to the compressor;

[0123] A fan located upstream of the engine core;

[0124] A gearbox receiving an input from the spool and providing a drive output to the fan so as to drive the fan at a rotational speed lower than that of the spool;

[0125] An oil circuit system arranged to supply oil to the gearbox; and

[0126] A heat exchange system including:

[0127] An air-oil heat exchanger through which the oil in the oil circuit system flows;

[0128] A fuel-oil heat exchanger through which the oil and fuel in the oil circuit system flow such that heat is transferred between the oil and the fuel; and

[0129] An air valve arranged to control the flow of air through the air-oil heat exchanger; and

[0130] A fuel composition determination module, which is arranged to determine at least one fuel property of a fuel arranged to be burned by a burner.

[0131] An air valve is arranged to be controlled based on at least one fuel property so as to adjust the flow rate of air passing through the air-oil heat exchanger.

[0132] A controller may be provided to control the valve.

[0133] The turbine may be a first turbine, the compressor may be a first compressor, and the spool may be a first spool. The engine core may further include a second turbine, a second compressor, and a second spool connecting the second turbine to the second compressor. The second turbine, the second compressor, and the second spool may be arranged to rotate at a higher rotational speed than the first spool.

[0134] The heat exchange system may further include a branched fuel return passage and at least one valve controlling the diversion of fuel flow, and these branched passages are arranged to return fuel from the heat exchange system to at least two different positions along the main fuel path, from which the fuel enters the gas turbine engine to reach the burner.

[0135] The engine according to the sixth aspect may be arranged to perform the methods of the first aspect, the third aspect, and / or the fifth aspect, and may have any one of the features described in any of the foregoing aspects.

[0136] According to a seventh aspect, there is provided a method of operating a gas turbine engine of an aircraft, the gas turbine engine comprising:

[0137] An engine core including a turbine, a compressor, a burner arranged to burn fuel, and a spool connecting the turbine to the compressor;

[0138] A fan located upstream of the engine core;

[0139] A gearbox receiving an input from the spool and providing a drive output to the fan so as to drive the fan at a rotational speed lower than that of the spool;

[0140] An oil circuit system arranged to supply oil to the gearbox;

[0141] A heat exchange system including:

[0142] An air-oil heat exchanger through which the oil in the oil circuit system flows;

[0143] A fuel-oil heat exchanger through which the oil and fuel in the oil circuit system flow such that heat is transferred between the oil and the fuel; and

[0144] At least one valve, the at least one valve being arranged to allow changing 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;

[0145] And

[0146] A temperature sensor, the temperature sensor being arranged to provide an indication of the fuel temperature downstream of the fuel-oil heat exchanger (optionally at the burner inlet),

[0147] The method includes:

[0148] Based on the output from the temperature sensor, determining whether the fuel temperature has increased above a set threshold under cruise conditions; and

[0149] In response to determining that the fuel temperature has increased above a set threshold under cruise conditions, controlling the at least one valve to change at least one flow rate through at least one heat exchanger.

[0150] For example, the at least one valve may be or include a valve arranged to allow changing the proportion of oil conveyed via the 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 convey less oil through the fuel-oil heat exchanger.

[0151] The inventors have recognized that it is important for the oil temperature to be maintained within a desired range and for the fuel temperature not to exceed a limit, and thus in some embodiments, the focus of the flow rate regulation may be on the air-oil heat exchanger. Thus, the at least one valve may be or include a valve arranged to allow changing the proportion of oil conveyed via 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 cruise conditions, controlling the at least one valve to convey more oil through the air-oil heat exchanger.

[0152] In some cases, the lack of an oil bypass on the air-oil heat exchanger may result in overcooling of the oil (e.g., condensation, or, in less extreme cases, overcooling may result in inefficient operation of the gearbox, e.g., with more heat loss, and generally there is an efficiency loss for the entire engine cycle). Thus, bypass pipes may be provided for both heat exchangers. In some embodiments, at least a portion of the oil may bypass both heat exchangers.

[0153] The at least one valve may be or include a valve arranged to control the air flow rate through the air-oil heat exchanger. The method may include conveying more air through the air-oil heat exchanger in response to determining that the fuel temperature has increased above a set threshold under cruise conditions.

[0154] In some embodiments, the method may include controlling a plurality of valves, such as:

[0155] ● Controlling the oil flow through each of a fuel-oil heat exchanger and an air-oil heat exchanger separately; and / or

[0156] ● Controlling the air flow and the oil flow.

[0157] As described in the foregoing aspect, the inventors have recognized that using a fuel different from traditional kerosene-based jet fuel, such as sustainable aviation fuel, can result in different fuel properties, and parameters can be adjusted to take advantage of the different fuel properties. In particular, some fuels can be heated to a higher temperature in one or more fuel-oil heat exchangers than traditional fuels without significantly increasing the deposition of fuel decomposition products, such as by coking or varnishing, thereby allowing the fuel to absorb more heat and thus reducing the need for air cooling of the oil and improving engine efficiency. However, allowing an inappropriate fuel to reach these higher temperatures can be detrimental to engine performance and may even cause blockage of the fuel nozzles (or other components, such as small screen filters, orifices, and any small channels, such as those typically present in a hydraulic mechanical unit and / or a fuel heat exchanger) in some cases. Therefore, checks and balances are needed to ensure that engine performance is optimal for a given fuel. The method of this seventh aspect includes detecting the fuel temperature to check for any excessive fuel temperature and taking measures to reduce the heating of the fuel when appropriate. During cruise conditions, the fuel temperature downstream of the fuel-oil heat exchanger (e.g., at the inlet of the burner) may be defined as the average value over at least 1 minute, 2 minutes, 3 minutes, 4 minutes, or 5 minutes, and optionally over 10 minutes, 20 minutes, or 30 minutes, under steady-state cruise conditions. These average temperatures do not include transient spikes in temperature, which can be defined as fluctuations in the fuel temperature during operation, typically an increase in temperature. Therefore, an instantaneous peak reaching a higher temperature (e.g., a peak lasting only a few seconds or minutes) may not be sufficient to trigger a change in the oil flow.

[0158] During a transition period, over the life of a gas turbine engine, the available aviation fuel changes over time and depends on the geographical location of the fuel supply point (among other variables). It is important to determine the level of fuel heating based on the specific fuel being used. The less oil passed through the fuel-oil heat exchanger, the less heat is transferred to the fuel, and thus the lower the fuel temperature provided when approaching the burner. Therefore, the gas turbine operation can be adjusted to make the most of the various fuels. A controllable oil flow valve arranged to regulate the oil flow through the fuel-oil heat exchanger plays a key role in the adjustment of engine performance.

[0159] The inventors recognized that these principles can be applied to engines with branched oil circuit passages that have different heat exchangers (in parallel arrangement) on different branches, and that these principles can be applied to engines with heat exchangers in a substantially linear series arrangement, and where one or more bypass pipes can be used as an alternative to the branched main passage.

[0160] The oil circuit system can branch such that a certain proportion of the oil can flow along each branch, and the air-oil heat exchanger and the fuel-oil heat exchanger can be arranged in a parallel configuration on different branches of the oil circuit system. In such embodiments, at least one valve arranged to allow changing the proportion of oil delivered via the fuel-oil heat exchanger can be or include a modulating valve arranged to allow changing the proportion of oil delivered via each branch.

[0161] The oil circuit system can include at least one bypass pipe arranged to allow a certain proportion of the oil to bypass at least one of the fuel-oil heat exchanger and the air-oil heat exchanger. In such embodiments, at least one valve can be or include at least one bypass valve arranged to allow a certain proportion of the oil to bypass the corresponding heat exchanger / control the proportion of oil delivered via the bypass pipe. In such embodiments, the air-oil heat exchanger and the fuel-oil heat exchanger can be arranged in series or in parallel in the oil circuit system. In embodiments where the air-oil heat exchanger and the fuel-oil heat exchanger are arranged in parallel on different branches of the oil circuit system and there is additionally at least one bypass pipe, the method can include controlling both the bypass valve and the modulating valve, the modulating valve being arranged to allow changing the proportion of oil delivered via each branch.

[0162] In embodiments where the method is arranged to regulate the flow of oil through the fuel-oil heat exchanger, at least some of the oil transferred from the fuel-oil heat exchanger can be delivered to the air-oil heat exchanger. In some such embodiments, when more oil is delivered via the air-oil heat exchanger, the air flow through the air-oil heat exchanger can be increased.

[0163] In embodiments where the method is arranged to regulate the flow of oil through the fuel-oil heat exchanger or the air-oil heat exchanger, at least some of the oil transferred from the corresponding heat exchanger can be delivered via a bypass pipe around the corresponding heat exchanger.

[0164] The method can further include determining a set threshold based on at least one fuel characteristic of the fuel. In such embodiments, at least one fuel characteristic of the fuel can be or include at least one of the following: the thermal stability of the fuel, the nitrogen content of the fuel, the sulfur content of the fuel, and the sustainable aviation fuel (SAF) content of the fuel.

[0165] The step of determining the set threshold may include optionally linearly increasing the set threshold as the fuel thermal stability increases.

[0166] For fuels with a SAF content higher than 70%, the step of determining the set threshold may include optionally linearly increasing the set threshold as the SAF content of the fuel increases.

[0167] The method may further include determining at least one fuel property of the fuel, optionally by any one of the following methods.

[0168] The heat exchange system may include an air valve arranged to control the air flow entering the air-oil heat exchanger. The method may further include, in response to determining that the fuel temperature has increased above the set threshold under cruise conditions, controlling the air valve to deliver more air through the air-oil heat exchanger.

[0169] The heat exchange system may include a refrigeration cycle device arranged to provide a heat boost by transferring additional heat from the oil to the fuel in addition to the heat transferred by the fuel-oil heat exchanger. The method may further include controlling the refrigeration cycle device to reduce the additional heat transferred to the fuel in response to determining that the fuel temperature has increased above the set threshold under cruise conditions. For example, the refrigeration cycle device may be shut down / deactivated.

[0170] The heat exchange system may include at least one bypass pipe, and at least one valve may be or include a bypass valve arranged to control the flow through the bypass pipe. The heat exchange system may include a plurality of bypass pipes, each bypass pipe being arranged to allow the oil to bypass a heat exchanger (e.g., the fuel-oil heat exchanger or air-oil heat exchanger described above, or a secondary fuel-oil heat exchanger, an oil-oil heat exchanger arranged to transfer heat between two independent oil circuits of the heat exchange system, or any other suitable heat exchanger). The method may include controlling at least two bypass valves - for example, when the bypass valve of the fuel-oil heat exchanger is adjusted to deliver less oil to the fuel-oil heat exchanger, the bypass valve of the air-oil heat exchanger may be adjusted to deliver more oil to the air-oil heat exchanger. In some embodiments, the same valve (e.g., a three-way valve) may regulate the oil flow to two heat exchangers.

[0171] The set threshold may be in the range of 140°C to 300°C, and optionally in the range of 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.

[0172] The determining step may be performed at fixed time intervals during the cruise operation of the aircraft.

[0173] The methods of the first, third, fifth, and seventh aspects can be complementary, and any two or more of them can be performed together in various embodiments. The method of the seventh aspect can be performed using the engines of the second, fourth, or sixth aspects.

[0174] According to an eighth aspect, there is provided a gas turbine engine for an aircraft, the gas turbine engine comprising:

[0175] An engine core including a turbine, a compressor, and a spool connecting the turbine to the compressor;

[0176] A fan located upstream of the engine core;

[0177] A gearbox that receives an input from the spool and drives an output to the fan to drive the fan at a rotational speed lower than that of the spool;

[0178] An oil circuit system arranged to supply oil to the gearbox;

[0179] A heat exchange system including:

[0180] An air-oil heat exchanger through which the oil in the oil circuit system flows;

[0181] A fuel-oil heat exchanger through which the oil and fuel in the oil circuit system flow such that heat is transferred between the oil and the fuel; and

[0182] At least one valve arranged to allow changing 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;

[0183] A temperature sensor arranged to provide an indication of the fuel temperature downstream of the fuel-oil heat exchanger (optionally at the inlet of the burner); and

[0184] A controller arranged to receive an output from the temperature sensor under cruise conditions, determine whether the fuel temperature has risen above a set threshold based on the output, and

[0185] In response to determining that the fuel temperature has risen above the set threshold under cruise conditions, control at least one valve to change at least one flow rate through at least one heat exchanger.

[0186] For example, at least one valve can be arranged to allow changing the proportion of oil delivered via the fuel-oil heat exchanger; and the controller can be arranged to:

[0187] Receive the output from the temperature sensor under cruise conditions;

[0188] Based on the output, determine whether the fuel temperature has increased above a set threshold; and

[0189] In response to determining that the fuel temperature has increased above the set threshold under cruise conditions, control at least one valve so as to deliver less oil through the fuel - oil heat exchanger.

[0190] The controller or another processing module may be arranged to determine the set threshold based on one or more fuel properties of the fuel.

[0191] The heat exchange system may include a refrigeration cycle device arranged to provide a heat boost by transferring more heat from the oil to the fuel than is transferred by the fuel - oil heat exchanger. Optionally, the refrigeration device may allow the fuel temperature to rise above the oil temperature. The controller may be arranged to deactivate the refrigeration cycle device in response to determining that the fuel temperature has increased above the set threshold under cruise conditions.

[0192] The turbine may be a first turbine, the compressor may be a first compressor, and the spool may be a first spool. The engine core may further include a second turbine, a second compressor, and a second spool connecting the second turbine to the second compressor. The second turbine, second compressor, and second spool may be arranged to rotate at a higher rotational speed than the first spool.

[0193] The heat exchange system may further include a branched fuel return passage and at least one valve controlling the fuel flow diversion, the branched passages being arranged to return fuel from the heat exchange system to at least two different locations along the main fuel path, from which the fuel enters the gas turbine engine to reach the burner.

[0194] The engine of the eighth aspect may be arranged to perform the methods of the first, third, fifth, and / or seventh aspects and may have any of the features described in any of the foregoing aspects.

[0195] According to a ninth aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:

[0196] An engine core including a turbine, a compressor, a burner arranged to burn fuel, and a spool connecting the turbine to the compressor;

[0197] A fan located upstream of the engine core;

[0198] An oil circuit system arranged to supply oil to cool at least one engine component;

[0199] A fuel - oil heat exchanger through which oil and fuel in an oil circuit system flow such that heat is transferred from the oil to the fuel; and

[0200] A refrigeration cycle device through which oil and fuel in an oil circuit system flow,

[0201] The refrigeration cycle device being arranged to transfer more heat from the oil to the fuel.

[0202] The method includes controlling the refrigeration cycle device such that the fuel temperature at the burner inlet is higher than the highest oil temperature within the oil circuit system.

[0203] The present inventors have recognized that using fuels different from traditional kerosene - based jet fuels, such as sustainable aviation fuels, can result in different fuel properties and that parameters under cruise conditions can be adjusted to take advantage of the different fuel properties. In particular, some fuels can be heated to higher temperatures than traditional fuels without significantly increasing the deposition of fuel decomposition products, such as by coking or varnishing, and in some cases, a thermal lift can be utilized to raise the fuel to a temperature higher than that of the oil to fully utilize the fuel's performance. This can additionally achieve a method of providing improved oil cooling (since using a refrigeration cycle device allows heat to be removed from the oil even as the oil is reduced to the fuel temperature), and can also improve the overall thermal efficiency of the engine by burning hotter fuel. The control of the refrigeration cycle device allows for the provision of a thermal lift and, optionally, also allows for the appropriate adjustment of the level of the provided thermal lift according to fuel and engine operating conditions.

[0204] As used herein, the highest oil temperature within the oil circuit system represents the highest oil temperature at any location within the oil circuit system at a particular moment (i.e., at the moment of comparison with the fuel temperature). It should be understood that the oil temperature generally increases as it passes through one or more engine components to be oil - cooled and then decreases as it passes through at least one fuel - oil heat exchanger (and through any other heat exchanger arranged to remove heat from the oil, such as an air - oil heat exchanger). Thus, a temperature sensor arranged to provide an indication of the highest oil temperature within the oil circuit system can be located downstream of the engine component to be cooled and upstream of the fuel - oil heat exchanger (and any other heat exchanger arranged to remove heat from the oil). Both the fuel temperature and the oil temperature can be determined as described above; in particular, transient peaks are ignored. An average can be taken over a period of time, such as one minute or several minutes.

[0205] The engine components to be cooled may include one or more of the following: a main power gearbox, which is a gearbox that receives input from a core shaft and outputs drive to a fan, an accessory gearbox, one or more bearings (such as bearings for the core shaft), one or more generators, electrical wires, and / or one or more pumps. It should be understood that any structure within the engine that requires cooling can be classified as such an engine component.

[0206] The engine may include a gearbox that receives input from a core shaft and outputs drive to a fan so as to drive the fan at a lower rotational speed than the core shaft, and an oil circuit system can supply oil to the gearbox. Thus, a refrigeration cycle device can be used to provide auxiliary cooling to the main power gearbox in a geared engine.

[0207] The refrigeration cycle device may contain a refrigerant fluid that is arranged to transfer heat from the oil to the fuel. The refrigerant fluid may be 1,1,1,2 - tetrafluoroethane.

[0208] The refrigeration cycle device may include an evaporator, a refrigerant compressor, and a condenser. The method may include:

[0209] Using the evaporator to transfer heat from the oil to the refrigerant fluid so as to vaporize the liquid in the refrigerant fluid, thereby forming a saturated vapor;

[0210] Using the refrigerant compressor to compress the saturated vapor to form a superheated vapor and conveying the superheated vapor to the condenser; and

[0211] Using the condenser to transfer heat from the superheated vapor to the fuel, in the process at least partially converting the superheated vapor into a liquid.

[0212] The refrigerant compressor can also function as a pump to circulate the refrigerant fluid around the refrigeration cycle device. In some embodiments, a separate refrigerant pump may be provided.

[0213] The method may include converting the superheated vapor into a saturated liquid in the condenser (in the context of a thermodynamic / refrigeration cycle, a saturated liquid is a liquid that contains as much thermal energy as possible without boiling). In some embodiments, the condenser can form a near - saturated liquid.

[0214] The refrigeration cycle device may further 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 mixture of a liquid and a vapor with a reduced temperature (i.e., a mixture of a liquid and a gas with a lower temperature than the liquid output from the condenser). The refrigerant can then return to the evaporator, and the cycle can start again.

[0215] The engine may further include a temperature sensor arranged to sense the fuel temperature downstream of the refrigeration cycle device. The fuel temperature sensor may be located at or near the inlet of the burner. The engine may further include a temperature sensor arranged to provide an indication of the maximum temperature of the oil within the oil circuit system. The oil temperature sensor may be located at or near the inlet of the fuel-oil heat exchanger, or at or near the outlet of one or more engine components to be cooled. The method may further include comparing the oil temperature and the fuel temperature and adjusting the control of the refrigeration cycle device based on the comparison.

[0216] The method may include controlling one or more additional components of the engine's heat exchange system and controlling the refrigeration cycle device - multiple components may work together to raise the fuel temperature to a desired level.

[0217] For example, the method may include controlling the fluid flow through the fuel-oil heat exchanger so as to increase the heat transfer to the fuel by:

[0218] (i) increasing the oil flow through the fuel-oil heat exchanger, such as by controlling a bypass valve modulating valve or a recirculation valve as described elsewhere herein, or by controlling the pump speed of the oil pump in the oil circuit system; and / or

[0219] (ii) recirculating the fuel through the fuel-oil heat exchanger, such as by using a recirculation valve as described elsewhere herein.

[0220] In some embodiments, the engine further includes an air-oil heat exchanger located upstream of the fuel-oil heat exchanger with respect to the oil flow. In such embodiments, the method may include increasing the heat transfer from the oil to the fuel in the fuel-oil heat exchanger by reducing the air cooling of the oil before it enters the fuel-oil heat exchanger.

[0221] The method may be performed during cruise. The engine may be controlled to operate at a fuel temperature at the burner inlet that is higher than the maximum oil temperature for at least 10%, 20%, or 30% of the time during cruise.

[0222] The method may include controlling the refrigeration cycle device such that the fuel temperature at the burner inlet is at least 2°C, 5°C, 10°C, 15°C, 20°C, or 25°C higher than the maximum oil temperature within the oil circuit system. The method may include controlling the refrigeration cycle device such that the fuel temperature at the burner inlet is higher than the maximum oil temperature within the oil circuit system by between 2°C and 50°C.

[0223] The method may include controlling the refrigeration cycle device such that the fuel temperature at the burner inlet is higher than the maximum oil temperature within the oil circuit system by an amount determined based on at least one fuel property of the fuel.

[0224] The methods of the first, third, fifth, seventh, and ninth aspects may be complementary, and any two or more of them may be performed together in various embodiments. The method of the ninth aspect may be performed using the engines of the second, fourth, sixth, or eighth aspects.

[0225] According to a tenth aspect, there is provided a gas turbine engine for an aircraft, the gas turbine engine comprising:

[0226] An engine core including a turbine, a compressor, a burner arranged to combust fuel, and a shaft connecting the turbine to the compressor;

[0227] A fan located upstream of the engine core;

[0228] An oil circuit system arranged to supply oil to cool at least one engine component;

[0229] A fuel-oil heat exchanger through which oil and fuel in the oil circuit system flow, such that heat is transferred from the oil to the fuel; and

[0230] A refrigeration cycle device through which oil and fuel in the oil circuit system flow,

[0231] The refrigeration cycle device being arranged to transfer more heat from the oil to the fuel.

[0232] The fuel-oil heat exchanger and the refrigeration cycle device are arranged to transfer heat to the fuel such that the temperature of the fuel at the burner inlet is higher than the highest oil temperature within the oil circuit system.

[0233] The gas turbine engine may further include a gearbox that receives an input from the shaft and outputs a drive to the fan to drive the fan at a rotational speed lower than that of the shaft. Thus, the gas turbine engine may be a geared gas turbine engine. The oil circuit system may be arranged to supply oil to the gearbox.

[0234] The turbine may be a first turbine, the compressor may be a first compressor, and the shaft may be a first shaft. The engine core may further include a second turbine, a second compressor, and a second shaft connecting the second turbine to the second compressor. The second turbine, the second compressor, and the second shaft may be arranged to rotate at a rotational speed higher than that of the first shaft.

[0235] The gas turbine engine may further include a branched fuel return passage and at least one valve that controls the diversion of the fuel flow along the branched fuel return passage. The branched passage may be arranged to return fuel from the fuel-oil heat exchanger to at least two different positions along a main fuel path through which fuel enters the gas turbine engine to the burner.

[0236] The engine of the tenth aspect can be arranged to perform the methods of the first, third, fifth, seventh, and / or ninth aspects and can have any one of the features described in any of the foregoing aspects.

[0237] In any of the above aspects:

[0238] In embodiments using one or more fuel properties of the fuel, at least one fuel property of the fuel can include at least one of the following:

[0239] i. The percentage of sustainable aviation fuel (% SAF) in the fuel;

[0240] ii. The heteroatom species concentration of the fuel;

[0241] iii. The aromatic content of the fuel;

[0242] iv. The polyaromatic content of the fuel;

[0243] v. The percentage of nitrogenous substances in the fuel;

[0244] vi. The presence or percentage of tracer substances or trace elements in the fuel;

[0245] vii. The hydrogen-to-carbon ratio of the fuel;

[0246] viii. The hydrocarbon distribution of the fuel;

[0247] ix. The level of non-volatile particulate matter emissions during combustion;

[0248] x. The naphthalene content of the fuel;

[0249] xi. The sulfur content of the fuel;

[0250] xii. The cycloalkane content of the fuel;

[0251] xiii. The oxygen content of the fuel;

[0252] xiv. The thermal stability of the fuel (e.g., the thermal decomposition temperature; a numerical value can be assigned to the thermal stability at any temperature by taking the reciprocal of the deposition rate of the fuel decomposition products at a given temperature);

[0253] xv. The coking level of the fuel, or more generally, the deposition level of the fuel decomposition products;

[0254] xvi. An indication that the fuel is a fossil fuel;

[0255] xvii. At least one of density, viscosity, calorific value, and heat capacity.

[0256] The method may also include chemically or physically detecting one or more parameters associated with the fuel in the fuel tank after fuel replenishment. The detected parameter may be a fuel property, or may be used to calculate or infer a fuel property - for example, the detected parameter may be shaft speed and the mass flow rate of the fuel, from which the calorific value (a fuel property) may be determined, or the detected parameter may be fuel density and / or the presence of a tracer, both of which are fuel properties in themselves. Determining at least one fuel property may include obtaining stored fuel property data. Chemically and / or physically determining one or more parameters of the fuel in the fuel tank may be performed by extracting a fuel sample from the fuel tank for off-wing testing.

[0257] Determining at least one fuel property of the fuel may include obtaining at least one fuel property of any fuel already present in the fuel tank before fuel replenishment; determining at least one fuel property of the fuel added to the fuel tank during fuel replenishment; and calculating at least one fuel property of the resulting fuel in the fuel tank after fuel replenishment (based on this information).

[0258] Determining at least one fuel property may be performed based on the detection of at least one fuel property. A fuel property may be a fuel characteristic, or may be used to calculate or otherwise determine (e.g., by retrieving from a look-up table) a fuel property. The detection may be performed on-wing.

[0259] Determining at least one fuel property may be performed based on received fuel composition data. The fuel composition data may be provided to the aircraft during fuel replenishment. The fuel composition data may be manually input.

[0260] During at least one of taxiing, takeoff, and climb of the aircraft, at least one fuel property may be inferred from the performance of the gas turbine engine.

[0261] It should be understood that the features described with respect to one aspect may be used in combination with any other aspect, with necessary modifications.

[0262] As described elsewhere herein, the present disclosure may be applied to any relevant configuration of a gas turbine engine. Such a gas turbine engine may be, for example, a turbofan gas turbine engine, an open rotor gas turbine engine (where the propeller is not enclosed by a nacelle), a turboprop engine, or a turbojet engine. Any such engine may or may not be provided with an afterburner. Such gas turbine engines may be configured, for example, for land- or sea-based power generation applications.

[0263] A gas turbine engine according to any aspect of the present disclosure may include an engine core that includes a turbine, a combustor, a compressor, and a spool that connects the turbine to the compressor. Such a gas turbine engine may include a fan (with fan blades). Such a fan may be located upstream of the engine core. Alternatively, in some examples, a gas turbine engine may include a fan located downstream of the engine core, such as 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 propulsor).

[0264] In the case where the gas turbine engine is an open rotor or turboprop engine, the gas turbine engine may include two contra-rotating propeller stages attached to and driven by a free power turbine via a shaft. The propulsors may rotate in opposite directions such that one propulsor rotates clockwise about the axis of rotation of the engine and the other propulsor rotates counterclockwise about the axis of rotation of the engine. Alternatively, the gas turbine engine may include a propulsor stage and a stator vane stage constructed downstream of the propulsor stage. The stator vane stage may have variable pitch. Thus, the high-pressure, intermediate-pressure, and free power turbines may drive the high-pressure and intermediate-pressure compressors and the propulsor via suitable interconnecting shafts, respectively. Thus, the propulsor may provide most of the propulsive thrust.

[0265] In the case where the gas turbine engine is an open rotor or turboprop engine, one or more propulsor stages may be driven by a gearbox. The gearbox may be of the type described herein.

[0266] An engine according to the present disclosure may be a turbofan engine. Such an engine may be a direct drive turbofan engine in which the fan is directly connected to a fan drive turbine via a spool, for example without a gearbox. In such a direct drive turbofan engine, it can be said that the fan rotates at the same rotational speed as the fan drive turbine. By way of example only, the fan drive turbine may be a first turbine, the spool may be a first spool, and the gas turbine engine may further include a second turbine and a second spool that connects the second turbine to the compressor. The second turbine, compressor, and second spool may be arranged to rotate at a higher rotational speed than the first spool. In such an arrangement, the second turbine may be axially located upstream of the first turbine.

[0267] An engine according to the present disclosure may be a geared turbofan engine. In such an arrangement, the engine has a fan driven via a gearbox. Thus, such a gas turbine engine may include a gearbox that receives an input from the spool and outputs a drive to the fan in order to drive the fan at a lower rotational speed than the spool. The input to the gearbox may come directly from the spool or indirectly from the spool, such as via a spur shaft and / or gears. The spool may rigidly connect the turbine and the compressor such that the turbine and the compressor rotate at the same speed (wherein the fan rotates at a lower speed).

[0268] A 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 connecting the turbines and compressors, such as one shaft, two shafts, 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 also include a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor. The second turbine, second compressor, and second core shaft may be arranged to rotate at a higher rotational speed than the first core shaft.

[0269] In such an arrangement, the second compressor may be axially located downstream of the first compressor. The second compressor may be arranged to receive flow (e.g., directly receive, e.g., via a substantially annular duct) from the first compressor.

[0270] The gearbox may be arranged to be driven by a core shaft (such as the first core shaft in the above example) configured (e.g., in use) to rotate at the lowest rotational speed. For example, the gearbox may be arranged to be driven only by a core shaft configured (e.g., in use) to rotate at the lowest rotational speed (e.g., in the above example, only by the first core shaft and not the second core shaft). Alternatively, the gearbox may be arranged to be driven by any one or more shafts, such as the first shaft and / or the second shaft in the above example.

[0271] The gearbox may be a reduction gearbox (since the output to the fan has a lower rotational rate than the input from the core shaft). Any type of gearbox may be used. For example, the gearbox may be a "planetary" or "stellar" gearbox, as described in more detail elsewhere herein. Such a gearbox may be single stage. Alternatively, such a gearbox may be a compound gearbox, such as a compound planetary gearbox (which may have an input on the sun gear and an output on the ring gear and is thus referred to as a "compound star" gearbox), for example having two stages of reduction.

[0272] The gearbox can have any desired reduction ratio (defined as the rotational speed of the input shaft divided by the rotational speed of the output shaft), e.g., greater than 2.5, e.g., in the range of 3 to 4.2, or 3.2 to 3.8, e.g., approximately 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. For example, the gear ratio can be between any two values in the previous sentence. By way of example only, the gearbox can be a "star" gearbox having a reduction ratio in the range of 3.1 or 3.2 to 3.8. By way of a further example only, the gearbox can be a "star" gearbox having a reduction ratio in the range of 3.0 to 3.1. By way of a further example only, the gearbox can be a "planetary" gearbox having a reduction ratio in the range of 3.6 to 4.2. In some arrangements, the gear ratio can be outside these ranges.

[0273] In any gas turbine engine as described and / or claimed herein, fuel of a given composition or blend is provided to a combustor which can be disposed downstream (e.g., axially downstream) of the fan and compressor with respect to the flow path. For example, in the case where a second compressor is provided, the combustor can be located directly downstream of the second compressor (e.g., at its outlet). By way of another example, in the case where a second turbine is provided, the flow at the outlet of the combustor can be provided to the inlet of the second turbine. The combustor can be disposed upstream of one or more turbines.

[0274] The compressor or each compressor (e.g., the first compressor and the second compressor as described above) can include any number of stages, e.g., a plurality of stages. Each stage can include a row of rotor blades and a row of stator vanes, and the row of stator vanes can be variable stator vanes (since the angle of incidence of the row of stator vanes can be variable). The row of rotor blades and the row of stator vanes can be axially offset from each other. For example, the gas turbine engine can be a direct drive turbofan gas turbine engine including 13 or 14 compressor stages (excluding the fan). Such an engine can include, for example, 3 stages in the first (or "low pressure") compressor and 10 or 11 stages in the second (or "high pressure") compressor. By way of a further example, the gas turbine engine can be a "geared" gas turbine engine including 11, 12 or 13 compressor stages (excluding the fan) (wherein the fan is driven by a first shaft via a reduction gearbox). Such an engine can include 3 or 4 stages in the first (or "low pressure") compressor and 8 or 9 stages in the second (or "high pressure") compressor. By way of a further example, the gas turbine engine can 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.

[0275] The turbine or each turbine (e.g., the first and second turbines as described above) may include any number of stages, such as a plurality of stages. Optionally, each stage may include a row of rotor blades and a row of stator vanes, or vice versa. The corresponding rows of rotor blades and stator vanes may be axially offset from each other. The second (or “high pressure”) turbine may include 2 stages in any arrangement (e.g., regardless of whether it is a geared engine or a direct drive engine). The gas turbine engine may be a direct drive gas turbine engine including a first (or “low pressure”) turbine having 5, 6 or 7 stages. Alternatively, the gas turbine engine may be a “geared” gas turbine engine including a first (or “low pressure”) turbine having 3 or 4 stages.

[0276] Each fan blade may be defined as having a radial span that extends from a root (or hub) at a radially inner gas washing position or 0% span position to a tip at a 100% span position. The ratio of the radius of the fan blade at the hub to the radius of the fan blade at the tip may be less than (or approximately) any of the following: 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 radius of the fan blade at the hub to the radius of the fan blade at the tip may be within an inclusive range defined by any two of the values in the previous sentence (i.e., these values may form an upper or lower limit), e.g., in the range from 0.28 to 0.32 or from 0.29 to 0.30. These ratios are generally referred to as hub-tip ratios. The radius at the hub and the radius at the tip may both be measured at the leading (or axially foremost) part of the blade. Of course, the hub-tip ratio refers to the gas washing portion of the fan blade, i.e., the portion radially outside any platform.

[0277] The radius of the fan can be measured between the centerline of the engine and the tip at the leading edge of the fan blade. The fan diameter (which may simply be twice the fan radius) can be greater than (or approximately) any of the following: 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 (about 150 inches), 390 cm (about 155 inches), 400 cm, 410 cm (about 160 inches), or 420 cm (about 165 inches). The fan diameter can be within the inclusive range defined by any two of the values in the previous sentence (i.e., these values can form an upper or lower limit), such as in the range of 210 cm to 240 cm, or 250 cm to 280 cm, or 320 cm to 380 cm. By way of non-limiting example only, the fan diameter can 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.

[0278] The rotational speed of the fan can vary during use. Generally speaking, for fans with a larger diameter, the rotational speed is lower. By way of non-limiting examples only, the rotational speed of the fan under cruise conditions can be less than 3500 rpm, such as less than 2600 rpm, or less than 2500 rpm, or less than 2300 rpm. By way of additional non-limiting examples only, for a "geared" gas turbine engine with a fan diameter in the range of 200 cm to 210 cm, the rotational speed of the fan under cruise conditions can be in the range of 2750 to 2900 rpm. By way of additional non-limiting examples only, for a "geared" gas turbine engine with a fan diameter in the range of 210 cm to 230 cm, the rotational speed of the fan under cruise conditions can be in the range of 2500 to 2800 rpm. By way of additional non-limiting examples only, for a "geared" gas turbine engine with a fan diameter in the range of 340 cm to 360 cm, the rotational speed of the fan under cruise conditions can be in the range of 1500 to 1800 rpm. By way of additional non-limiting examples only, for a direct drive engine with a fan diameter in the range of 190 cm to 200 cm, the rotational speed of the fan under cruise conditions can be in the range of 3600 to 3900 rpm. By way of additional non-limiting examples only, for a direct drive engine with a fan diameter in the range of 300 cm to 340 cm, the rotational speed of the fan under cruise conditions can be in the range of 2000 to 2800 rpm.

[0279] When using a gas turbine engine, the fan (with associated fan blades) rotates about an axis of rotation. This rotation causes the tips of the fan blades to move at a speed U 尖端 The work done by the fan blades on the flow results in an enthalpy rise dH of the flow. The fan tip loading can be defined as dH / U 尖端 2 , where dH is the enthalpy rise across the fan (e.g., 1-D mean enthalpy rise), and U 尖端 is the (translational) speed 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 fan tip loading under cruise conditions can be greater than (or approximately) any of the following: 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 are dimensionless). The fan tip loading can be within an inclusive range defined by any two of the values in the previous sentence (i.e., these values can form an upper or lower limit), e.g., in the range of 0.28 to 0.31 or 0.29 to 0.3 (e.g., for a geared gas turbine engine).

[0280] A gas turbine engine according to 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 bypass ratio at cruise conditions may be greater than (or approximately) 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 bypass ratio at cruise conditions may be within the inclusive range defined by any two of the values in the previous sentence (i.e., these values may form an upper or lower limit), such as in the range of 12 to 16, or 13 to 15, or 13 to 14. By way of non-limiting example only, the bypass ratio at cruise conditions of a direct drive gas turbine engine according to the present disclosure may be in the range of 9:1 to 11:1. By way of another non-limiting example only, the bypass ratio at cruise conditions 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 at least substantially annular. The bypass duct may be located radially outward of the core engine. The radially outer surface of the bypass duct may be defined by a nacelle and / or a fan casing.

[0281] 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 entering 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 at cruise conditions may be greater than (or approximately) any of the following: 35, 40, 45, 50, 55, 60, 65, 70, 75. The overall pressure ratio may be within the inclusive range defined by any two of the values in the previous sentence (i.e., these values may form an upper or lower limit), such as in the range of 50 to 70. By way of non-limiting example only, 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. By way of non-limiting example only, 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. By way of non-limiting example only, 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. By way of non-limiting example only, 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.

[0282] The specific thrust of an engine can be defined as the net thrust of the engine divided by the total mass flow rate through the engine. In some examples, for a given thrust condition, the specific thrust can depend on the specific composition of the fuel provided to the combustor. Under cruise conditions, the specific thrust of the engines described and / or claimed herein can be less than (or approximately) any of the following: 110 N / kg -1 s, 105 N / kg -1 s, 100 N / kg -1 s, 95 N / kg -1 s, 90 N / kg -1 s, 85 N / kg -1 s or 80 N / kg -1 s. The specific thrust can be within an inclusive range defined by any two of the values in the previous sentence (i.e., these values can form an upper or lower limit), for example between 80 N / kg -1 s and 100 N / kg -1 s, or between 85 N / kg -1 s and 95 N / kg -1 s. Compared to conventional gas turbine engines, such engines can be particularly efficient. By way of non-limiting example only, the specific thrust of a geared gas turbine engine having a fan diameter in the range of 200 cm to 210 cm can be between 90 N / kg -1 s and 95 N / kg -1 s. By way of non-limiting example only, the specific thrust of a geared gas turbine engine having a fan diameter in the range of 210 cm to 230 cm can be between 80 N / kg -1 s and 90 N / kg -1 s. By way of non-limiting example only, the specific thrust of a geared gas turbine engine having a fan diameter in the range of 340 cm to 360 cm can be between 70 N / kg -1 s and 90 N / kg -1 s. By way of non-limiting example only, the specific thrust of a direct drive gas turbine engine having a fan diameter in the range of 300 cm to 340 cm can be between 90 N / kg -1 s and 120 N / kg -1 s.

[0283] The gas turbine engines as described herein and / or claimed may have any desired maximum thrust. By way of non-limiting example only, the gas turbines as described herein and / or claimed may produce a maximum thrust of at least (or approximately) any one 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 within an inclusive range defined by any two of the values in the previous sentence (i.e., these values may form an upper or lower limit). By way of non-limiting example only, the gas turbines as described herein and / or claimed may be capable of producing a maximum thrust within the range of 155 kN to 170 kN, 330 kN to 420 kN, or 350 kN to 400 kN. By way of non-limiting example only, the maximum thrust of a geared gas turbine engine having a fan diameter within the range of 200 cm to 210 cm may be within the range of 140 kN to 160 kN. By way of non-limiting example only, the maximum thrust of a geared gas turbine engine having a fan diameter within the range of 210 cm to 230 cm may be within the range of 150 kN to 200 kN. By way of non-limiting example only, the maximum thrust of a geared gas turbine engine having a fan diameter within the range of 340 cm to 360 cm may be within the range of 370 kN to 500 kN. By way of non-limiting example only, the maximum thrust of a direct drive gas turbine engine having a fan diameter within the range of 300 cm to 340 cm may be within the range of 370 kN to 500 kN. The thrust mentioned above may be the maximum net thrust at standard atmospheric conditions, at sea level, plus 15 °C (ambient pressure 101.3 kPa, temperature 30 °C), with the engine stationary.

[0284] In use, the temperature of the flow at the inlet of the high-pressure turbine can be particularly high. This temperature, which may be referred to as TET, can be measured at the outlet of the combustor, for example, just upstream of the first turbine blade, which itself may be referred to as the nozzle guide vane. In some examples, for a given thrust condition, the TET can depend on the specific composition of the fuel provided to the combustor. Under cruise conditions, the TET can be at least (or approximately) any of the following: 1400K, 1450K, 1500K, 1520K, 1530K, 1540K, 1550K, 1600K or 1650K. Thus, by way of non-limiting example only, a geared gas turbine engine having a fan diameter in the range of 200 cm to 210 cm can have a TET in the range of 1540K to 1600K under cruise conditions. By way of non-limiting example only, a geared gas turbine engine having a fan diameter in the range of 210 cm to 230 cm can have a TET in the range of 1590K to 1650K under cruise conditions. By way of non-limiting example only, a geared gas turbine engine having a fan diameter in the range of 340 cm to 360 cm can have a TET in the range of 1600K to 1660K under cruise conditions. By way of non-limiting example only, a direct drive gas turbine engine having a fan diameter in the range of 300 cm to 340 cm can have a TET in the range of 1590K to 1650K under cruise conditions. By way of non-limiting example only, a direct drive gas turbine engine having a fan diameter in the range of 300 cm to 340 cm can have a TET in the range of 1570K to 1630K under cruise conditions.

[0285] The TET under cruise conditions can be within an inclusive range defined by any two of the values in the previous sentence (i.e., these values can form an upper or lower limit), for example, from 1530K to 1600K. The maximum TET during engine use can be, for example, at least (or approximately) any of the following: 1700K, 1750K, 1800K, 1850K, 1900K, 1950K, 2000K, 2050K, or 2100K. Thus, by way of non-limiting example only, the maximum TET of a geared gas turbine engine having a fan diameter in the range of 200 cm to 210 cm can be in the range of 1890K to 1960K. By way of non-limiting example only, the maximum TET of a geared gas turbine engine having a fan diameter in the range of 210 cm to 230 cm can be in the range of 1890K to 1960K. By way of non-limiting example only, the maximum TET of a geared gas turbine engine having a fan diameter in the range of 340 cm to 360 cm can be in the range of 1890K to 1960K. By way of non-limiting example only, the maximum TET of a direct drive gas turbine engine having a fan diameter in the range of 300 cm to 340 cm can be in the range of 1935K to 1995K. By way of non-limiting example only, the maximum TET of a direct drive gas turbine engine having a fan diameter in the range of 300 cm to 340 cm can be in the range of 1890K to 1950K. The maximum TET can be within an inclusive range defined by any two of the values in the previous sentence (i.e., these values can form an upper or lower limit), for example, in the range of 1800K to 1950K or 1900K to 2000K. The maximum TET can occur, for example, under high thrust conditions, such as under maximum takeoff (MTO) conditions.

[0286] The fan blades and / or the airfoil portions of the fan blades described and / or claimed herein can be manufactured from any suitable material or combination of materials. For example, at least a portion of the fan blades and / or airfoils can be at least partially manufactured from a composite material, such as a metal matrix composite and / or an organic matrix composite, such as a carbon fiber composite. By way of further example, at least a portion of the fan blades and / or airfoils can be at least partially manufactured from a metal, such as a titanium-based metal or an aluminum-based material (such as an aluminum-lithium alloy) or a steel-based material. The fan blades can include at least two regions manufactured using different materials. For example, the fan blades can have a protective leading edge that can be manufactured using a material that better resists impacts (e.g., from birds, ice, or other materials) than the rest of the blade. Such leading edges can be manufactured, for example, using titanium or a titanium-based alloy. Thus, by way of example only, the fan blade can have a carbon fiber or an aluminum-based body (such as an aluminum-lithium alloy) with a titanium leading edge.

[0287] The fan as described and / or claimed herein may include a central portion from which fan blades may extend, such as radially. The fan blades may be attached to the central portion in any desired manner. For example, each fan blade may include a fixture that may engage a corresponding slot in a hub (or disc). By way of example only, such a fixture may be in the form of a dovetail that may be inserted into and / or engage a corresponding slot in the hub / disc to secure the fan blade to the hub / disc. By way of a further example, the fan blades may be integrally formed with the 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 blade may be machined from a block, and / or at least part of the fan blade may be attached to the hub / disc by welding (such as linear friction welding).

[0288] The gas turbine engine 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 exit area of the bypass duct to vary in use. The general principles of the present disclosure may be applied to engines with or without a VAN.

[0289] The fan of a gas turbine as described and / or claimed herein may have any desired number of fan blades, such as 14, 16, 18, 20, 22, 24 or 26 fan blades. In the case where the fan blades have a carbon fiber composite body, there may be 16 or 18 fan blades. In the case where the fan blades have a metal body (such as an aluminum-lithium or titanium alloy), there may be 18, 20 or 22 fan blades.

[0290] As used herein, the terms idle, taxi, takeoff, climb, cruise, descent, approach and landing (or one or more parts thereof) have their conventional meanings and will be readily understood by the person skilled in the art. Thus, for a given gas turbine engine for an aircraft, the person skilled in the art will immediately recognize that each term is used to refer to the whole or one or more parts of the operating phase of the engine of the aircraft to which the gas turbine engine is designed to be attached within a given mission.

[0291] In this regard, ground idle may refer to an engine operating phase in which the aircraft is stationary and in contact with the ground, but in which there are requirements for the engine to be operated. During idle, the engine may produce available thrust between 3% and 9% of the engine. In additional non-limiting examples, the engine may produce available thrust between 5% and 8% of the engine. In additional non-limiting examples, the engine may produce available thrust between 6% and 7% of the engine. Taxiing may refer to an engine operating phase in which the aircraft is propelled along the ground by thrust produced by the engine. During taxiing, the engine may produce available thrust between 5% and 15% of the engine. In additional non-limiting examples, the engine may produce available thrust between 6% and 12% of the engine. In additional non-limiting examples, the engine may produce available thrust between 7% and 10% of the engine. Takeoff may refer to an engine operating phase in which the aircraft is propelled by thrust produced by the engine. During the initial phase of the takeoff phase, the aircraft may be propelled while in contact with the ground. During a later phase of the takeoff phase, the aircraft may be propelled while not in contact with the ground. During takeoff, the engine may produce available thrust between 90% and 100% of the engine. In additional non-limiting examples, the engine may produce available thrust between 95% and 100% of the engine. In additional non-limiting examples, the engine may produce 100% of the available thrust.

[0292] Climb may refer to an engine operating phase in which the aircraft is propelled by thrust produced by the engine. During climb, the engine may produce available thrust between 75% and 100% of the engine. In additional non-limiting examples, the engine may produce available thrust between 80% and 95% of the engine. In additional non-limiting examples, the engine may produce available thrust between 85% and 90% of the engine. In this regard, climb may refer to the operating phase between takeoff and reaching cruise conditions in the aircraft flight cycle, reaching cruise conditions thus defining the start of the cruise phase or a part thereof of the aircraft flight. Additionally or alternatively, climb may refer to one or more nominal periods at a nominal point or during the aircraft flight cycle between takeoff and landing, in which a relative increase in altitude is required, which may require additional thrust requirements of the engine.

[0293] As used herein, the cruise conditions that may define the cruise phase (or a portion thereof) of an aircraft flight have their ordinary meaning and will be readily understood by a person skilled in the art. In some examples, for a given gas turbine engine of an aircraft, the cruise conditions may refer to the operating point at which the engine cruises in the middle of a given mission (referred to in the industry as an "economic mission"), and the gas turbine engine is designed to be attached to the aircraft. In this regard, the mid-cruise can be regarded as a key point in the aircraft flight cycle, at which point 50% of the total fuel burned between the highest point of ascent and the start of descent has been burned (which can be approximated in terms of time and / or distance to the midpoint between the highest point of ascent and the start of descent). Thus, the cruise conditions can define the operating point, phase, or a portion thereof of the flight, which, taking into account the number of engines provided to the aircraft to which the gas turbine engine is designed to be attached, provides the thrust that will ensure the 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 substantially a constant altitude and / or at least substantially a constant Mach number) of the aircraft. For example, if the engine is designed to be attached to an aircraft having two engines of the same type, then under cruise conditions, the engine can provide half of the total thrust required for the steady-state operation or at least substantially steady-state operation of the aircraft during mid-cruise.

[0294] In other words, for a given gas turbine engine of an aircraft, the cruise conditions can be defined as the operating point of the engine that provides a specified thrust under mid-cruise atmospheric conditions (defined by the International Standard Atmosphere according to ISO 2533 at the mid-cruise altitude), which is required to provide the steady-state operation, or at least substantially steady-state operation, of the aircraft to which the gas turbine engine is designed to be attached, in combination with any other engines on the aircraft, at a given mid-cruise Mach number. For any given gas turbine engine of an aircraft, the mid-cruise thrust, atmospheric conditions, and Mach number are known, and thus the operating point of the engine can be clearly defined under cruise conditions.

[0295] By way of example only, the forward speed under cruise conditions can be any point within the range from 0.7 Mach to 0.9 Mach, such as from 0.75 to 0.85, such as from 0.76 to 0.84, such as from 0.77 to 0.83, such as from 0.78 to 0.82, such as from 0.79 to 0.81, such as approximately 0.8 Mach, approximately 0.85 Mach, or within the range from 0.8 to 0.85. Any single speed within these ranges can be part of the cruise conditions. For some aircraft, the cruise conditions may be outside these ranges, such as below 0.7 Mach or above 0.9 Mach.

[0296] By way of example only, the cruise conditions may correspond to standard atmospheric conditions (according to the International Standard Atmosphere ISA) at altitudes within the following ranges: 10000m to 15000m, for example within the range of 10000m to 12000m, for example within the range of 10400m to 11600m (about 38000 feet), for example within the range of 10500m to 11500m, for example within the range of 10600m to 11400m, for example within the range of 10700m (about 35000 feet) to 11300m, for example within the range of 10800m to 11200m, for example within the range of 10900m to 11100m, for example approximately 11000m. The cruise conditions may correspond to the standard atmospheric conditions at any given altitude within these ranges.

[0297] By way of example only, 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 35000ft (10668m). Under such cruise conditions, the engine may provide a known required net thrust level. Of course, the known required net thrust level depends on the engine and its intended application, and may be a value within the range of, for example, 20kN to 40kN.

[0298] By way of further example only, the cruise 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 38000ft (11582m). Under such cruise conditions, the engine may provide a known required net thrust level. The known required net thrust level of course depends on the engine and its intended application, and may be a value within the range of, for example, 35kN to 65kN.

[0299] In use, the gas turbine engine described and / or claimed herein may operate under the cruise conditions defined elsewhere herein. Such cruise conditions may be determined by the cruise conditions of an aircraft, for example, an intermediate cruise condition, on which at least one (e.g., 2 or 4) gas turbine engines may be mounted to provide propulsion thrust.

[0300] In addition, those skilled in the art will immediately recognize that either or both of landing and approach refer to an operating phase within the flight cycle of an aircraft between cruise and landing, where approach in particular forms part of the landing and take-off (LTO) phase. During either or both of landing and approach, the engine may produce between 0% and 50% of the available thrust. In additional non-limiting examples, the engine may produce between 25% and 40% of the available thrust. In additional non-limiting examples, the engine may produce between 30% and 35% of the available thrust. Additionally or alternatively, landing may refer to a nominal point within the flight cycle of an aircraft between take-off and landing, where a relative reduction in altitude is required, and this may require a reduced thrust demand from the engine.

[0301] According to one aspect, there is provided an aircraft that includes a gas turbine engine as described herein and / or claimed. The aircraft according to this aspect is an aircraft to which the gas turbine engine has been designed to be attached. Thus, the cruise conditions according to this aspect can correspond to the operating point, phase, or a part thereof of the aircraft flight, as defined elsewhere herein.

[0302] According to one aspect, there is provided a method of operating a gas turbine engine as described herein and / or claimed. The operation can be carried out under any cruise conditions (e.g., in terms of thrust, atmospheric conditions, and Mach number) as can be defined elsewhere herein.

[0303] According to one aspect, there is provided a method of operating an aircraft that includes a gas turbine engine as described herein and / or claimed. The operation according to this aspect can include (or can be) an operation under any suitable conditions (e.g., at the intermediate cruise of the aircraft), as defined elsewhere herein.

[0304] The skilled person will understand that features or parameters described in relation to any one of the above aspects can be applied to any other aspect, unless mutually exclusive. Furthermore, any feature or parameter included or described herein can be applied to any aspect and / or combined with any other feature or parameter included or described herein, unless mutually exclusive.

[0305] Unless mutually exclusive, any parameter or value included or described herein can be applied to and / or combined with any one or more additional parameters and / or values included or described herein. For example, a first parameter or value (e.g., parameter A) included or described herein can be applied to and / or combined with any one or more additional parameters and / or values (e.g., parameter B; parameter C; and parameter D, etc.) included or described herein to express the product of their relationships. For example, the skilled person in the art will understand that in the case where parameter A and parameter B are disclosed separately, the product of their relationships can be expressed, as required, for example, as A / B, B / A, B*A, or any such other application, combination, or function of parameter A relative to parameter B. Description of the Drawings

[0306] Embodiments will now be described, by way of example only, with reference to the drawings, in which:

[0307] Figure 1 is a cross-sectional side view of a gas turbine engine;

[0308] Figure 2 is a close-up cross-sectional side view of an upstream portion of a geared gas turbine engine;

[0309] Figure 3 is a partial cross-sectional view of a gearbox for a gas turbine engine;

[0310] Figure 4 is a schematic view of an aircraft having a propulsion system that includes two gas turbine engines;

[0311] Figure 5 is a schematic view of an exemplary fuel system;

[0312] Figure 6 is a schematic view of an alternative exemplary fuel system;

[0313] Figure 7 is a schematic view of a part of an exemplary recirculation oil system (primary oil circuit system);

[0314] Figure 8 is a schematic view of another part of an exemplary recirculation oil system (secondary oil circuit system);

[0315] Figure 9 is a schematic view of a part of an alternative exemplary recirculation oil system, also showing the features of an air - oil heat exchanger;

[0316] Figure 10 is Figure 5 of an exemplary fuel system and Figure 7 and Figure 8 of a part of an exemplary recirculation oil system;

[0317] Figure 11 is a schematic view of another alternative exemplary recirculation oil system, showing all the main components of the heat exchange system;

[0318] Figure 12 shows an exemplary method of operating a gas turbine engine;

[0319] Figure 13 shows another exemplary method of operating a gas turbine engine;

[0320] Figure 14 shows another exemplary method of operating a gas turbine engine;

[0321] Figure 15 shows another exemplary method of operating a gas turbine engine;

[0322] Figure 16 shows another example of a recirculation oil system for a gas turbine engine;

[0323] Figure 17 shows another exemplary method of operating a gas turbine engine; and

[0324] Figure 18 Shows a refrigeration cycle device incorporated into a gas turbine engine. Detailed implementation

[0325] Figure 1 Shows a gas turbine engine 10 having a main rotational axis 9. The engine 10 includes an air intake 12 and a propulsive fan 23 which generates two airflows: a core airflow A and a bypass airflow B. The gas turbine engine 10 includes a core 11 which 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 fan 23 is attached to and driven by the low - pressure turbine 19 via a shaft 26 and an epicyclic gearbox 30.

[0326] In use, the core airflow A is accelerated and compressed by the low - pressure compressor 14 and is directed into the high - pressure compressor 15 for further compression. The compressed air discharged from the high - pressure compressor 15 is directed into the combustion equipment 16 where the compressed air is mixed with fuel F and the mixture is combusted. The combustion equipment 16 may be referred to as a burner 16, where the terms "combustion equipment 16" and "burner 16" may be used interchangeably herein. Then, the resulting hot combustion products expand through the high - pressure turbine and the low - pressure turbines 17, 19 before being discharged through the nozzle 20, thereby driving the high - pressure turbine and the low - pressure turbines to provide some propulsive thrust. The high - pressure turbine 17 drives the high - pressure compressor 15 via a suitable interconnecting shaft 27. The fan 23 is generally used to apply an increased pressure to the bypass airflow B flowing through the bypass duct 22 such that the bypass airflow B is discharged through the bypass exhaust nozzle 18 to generally provide most of the propulsive thrust. The epicyclic gearbox 30 is a reduction gearbox.

[0327] Figure 2 Shows an exemplary arrangement of a geared - fan gas turbine engine 10. The low - pressure turbine 19 (see Figure 1 ) drives a shaft 26 which is coupled to a sun gear or sunwheel 28 of an epicyclic gear arrangement 30. Radially outward of the sun gear 28 and meshing therewith are a plurality of planet gears 32 which are joined together by a planet carrier 34. The planet carrier 34 constrains the planet gears 32 to precess synchronously about the sun gear 28 while each planet gear 32 rotates about its own axis. The planet carrier 34 is coupled to the fan 23 via a link 36 so as to drive the fan to rotate about the engine axis 9. Radially outward of the planet gears 32 and meshing therewith is a ring gear or annulus 38 which is coupled to a fixed support structure 24 via a link 40.

[0328] Note that the terms "low pressure turbine" and "low pressure compressor" as used herein may refer to the lowest pressure turbine stage and the lowest pressure compressor stage respectively (i.e., excluding the fan 23), and / or the turbine stage and the compressor stage connected together by an interconnecting shaft 26 having the lowest rotational speed in the engine (i.e., excluding the gearbox output shaft driving the fan 23). In some literature, the "low pressure turbine" and "low pressure compressor" referred to herein may alternatively be referred to as "intermediate pressure turbine" and "intermediate pressure compressor". In the case of using such alternative nomenclature, the fan 23 may be referred to as the first or lowest pressure compression stage.

[0329] In Figure 3 the epicyclic gearbox 30 is shown in more detail by way of example. Each of the sun gear 28, the planet gears 32, and the ring gear 38 includes teeth around its periphery for meshing with other gears. However, for clarity, Figure 3 only an exemplary portion of the teeth is shown. Four planet gears 32 are shown, but it will be apparent to those skilled in the art that more or fewer planet gears 32 may be provided within the scope of the claimed invention. Practical applications of the planetary epicyclic gearbox 30 typically include at least three planet gears 32.

[0330] In Figure 2 and Figure 3 the epicyclic gearbox 30 shown by way of example is planetary, where the planet carrier 34 is coupled to the output shaft via a link 36 and the ring gear 38 is fixed. However, any other suitable type of epicyclic gearbox 30 may be used. By way of a further example, the epicyclic gearbox 30 may be a stellar arrangement, where the planet carrier 34 remains fixed, allowing the ring gear (or annulus) 38 to rotate. In such an arrangement, the fan 23 is driven by the ring gear 38. By way of another alternative example, the gearbox 30 may be a differential gearbox, where both the ring gear 38 and the planet carrier 34 are allowed to rotate.

[0331] It should be understood that Figure 2 and Figure 3 the arrangements shown in Figure 2The connecting rods 36, 40) in the example can have any desired degree of stiffness or flexibility. By way of further example, any suitable arrangement of bearings between the rotating and stationary components of the engine (e.g., between the input and output shafts from the gearbox and a stationary structure such as the gearbox housing) can be used, and the present disclosure is not limited to Figure 2 the exemplary arrangement of. For example, in the case where the gearbox 30 has a stellar arrangement (as described above), those skilled in the art will readily understand that the arrangement of the output connecting rod, the support connecting rod, and the bearing positions will generally be different from Figure 2 the arrangement shown by way of example in

[0332] Accordingly, the present disclosure extends to gas turbine engines having any arrangement in the gearbox type (e.g., stellar or planetary gears), support structure, input and output shaft arrangement, and bearing positions.

[0333] Optionally, the gearbox can drive additional and / or alternative components (e.g., an intermediate pressure compressor and / or a booster compressor).

[0334] Other gas turbine engines to which the present disclosure can be applied can have alternative configurations. For example, such engines can have an alternative number of compressors and / or turbines and / or an alternative number of interconnected shafts. By way of further example, Figure 1 the gas turbine engine shown in

[0335] has split nozzles 18, 20, which means that the flow through the bypass duct 22 has its own nozzle 18, which is separate from and radially outside the core engine nozzle 20. However, this is not limiting, and any aspect of the present disclosure can also be applied to an engine in which the flow through the bypass duct 22 and the flow through the core 11 mix or combine before (or upstream of) a single nozzle that can be referred to as a mixed flow nozzle. One or both nozzles (whether mixed or split) can have a fixed or variable area.

[0336] Other gas turbine engines to which the present disclosure can be applied can have alternative configurations. By way of example, such engines can have an alternative number of interconnected shafts (e.g., two) and / or an alternative number of compressors and / or turbines. Additionally, the engine can include a gearbox disposed in the drive train from the turbine to the compressor and / or fan.

[0337] Whilst the example described relates to a turbofan engine, the present disclosure may be applied to, for example, any type of gas turbine engine such as an open rotor (where the fan stage is not surrounded by a nacelle) or, for example, a turboprop engine. In some arrangements, the gas turbine engine 10 may not include a gearbox 30.

[0338] The geometry of the gas turbine engine 10 and its components is defined by a conventional axis system including an axial direction (aligned with the rotational axis 9), a radial direction (the direction from bottom to top in Figure 1 ), and a circumferential direction (perpendicular to the Figure 1 plane of the view). The axial, radial, and circumferential directions are perpendicular to each other.

[0339] The fuel F supplied to the combustion equipment 16 may include a fossil-based hydrocarbon fuel such as kerosene. Thus, the fuel F may include molecules from one or more of the chemical families of normal paraffins, isoparaffins, naphthenes, and aromatics. Additionally or alternatively, when blended with, mixed with, or replaced by an alternative fuel, the fuel F may include renewable hydrocarbons produced from biological or non-biological resources, also known as sustainable aviation fuel (SAF). In each provided embodiment, the fuel F may include one or more trace elements including, for example, sulfur, nitrogen, oxygen, inorganics, and metals.

[0340] Those skilled in the art understand SAF to refer to, for example, biofuels, renewable aviation fuels, renewable jet fuels, alternative fuels, or biojet fuels produced from biological or non-biological resources. SAF is understood to generally be synthesized from carbon-containing gases extracted from the atmosphere and / or captured from industrial processes; or from a wide range of sustainable feedstocks such as waste oils and fats; municipal solid waste; cellulosic waste (such as corn stover); cover crops such as camelina, carinata, and pennycress; abiogenic alternative fuels; jatropha; halophytes, and algae, rather than from fossil-based hydrocarbons. SAF is understood not to include fossil fuels.

[0341] The functional performance of a given fuel composition or fuel F blend for a given mission can be defined at least in part by the capabilities of the Brayton cycle of a fuel-serviced gas turbine engine 10. Parameters defining functional performance can include, for example, specific energy; energy density; thermal stability; and emissions including gases and / or particulate matter. In this regard, particulate emissions can include soot particles produced by the combustion of the fuel F, also known as non-volatile particulate matter (nvPM). Any reference herein to soot or smoke equally applies to other types of particulate emissions known in the art. Gaseous emissions can include one or more of the following: 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 (VOC) produced by the combustion of the fuel F. Any reference herein to gaseous emissions equally applies to other types of gaseous emissions known in the art.

[0342] A relatively high specific energy, expressed in MJ / kg (i.e., energy per unit mass), can at least in part reduce the takeoff weight and thus potentially provide a relative improvement in fuel efficiency. A relatively high energy density, expressed in MJ / L (i.e., energy per unit volume), can at least in part reduce the takeoff fuel volume, which can be particularly important for volume-constrained missions or military operations involving fuel replenishment. A relatively high thermal stability (i.e., inhibiting fuel degradation or coking under thermal stress) can allow the fuel to maintain elevated temperatures in the engine and fuel injectors and thus potentially provide a relative improvement in combustion efficiency. Reduced emissions, including particulate matter, can allow reduced contrail formation while reducing the environmental impact of a given mission.

[0343] Other properties of the fuel can also be critical to functional performance. For example, a relatively low freezing point (°C) can allow for long-duration missions to optimize the flight profile; a minimum aromatic concentration (%) can ensure sufficient swelling of certain materials used to construct O-rings and seals that have previously been exposed to fuels with high aromatic content; and a maximum surface tension (mN / m) can ensure sufficient spray breakup and atomization of the fuel.

[0344] The ratio of the number of hydrogen atoms to the number of carbon atoms in a molecule can affect the specific energy of a given composition or fuel blend. Fuels with a higher ratio of hydrogen atoms to carbon atoms can have a higher specific energy in the absence of bond strain. For example, fossil-based hydrocarbon fuels can include molecules having from about 7 to 18 carbons, with a significant portion of a given composition being derived from molecules having 9 to 15 carbons, with an average of 12 carbons.

[0345] Multiple sustainable aviation fuel blends have been approved for use. For example, some approved blends contain blend ratios of up to 10% sustainable aviation fuel, while other approved blends contain blend ratios of 10% to 50% sustainable aviation fuel (the remainder containing one or more fossil-based hydrocarbon fuels such as kerosene), with additional compositions awaiting approval. However, sustainable aviation fuel blends that are expected to include up to (and including) 100% sustainable aviation fuel (SAF) in the aviation industry will ultimately be approved for use.

[0346] Sustainable aviation fuel can include one or more of n-alkanes, isoalkanes, cycloalkanes, and aromatics, and can be produced, for example, from one or more of the following: syngas; lipids (e.g., fats, oils, and greases); sugars; and alcohols. Thus, sustainable aviation fuel can include either or both of lower aromatics and sulfur content (relative to fossil-based hydrocarbon fuels). Additionally or alternatively, sustainable aviation fuel can include either or both of higher isoalkane and cycloalkane content (relative to fossil-based hydrocarbon fuels). In some examples, sustainable aviation fuel can include either or both of a density between 90% and 98% of the density of kerosene and a calorific value between 101% and 105% of the calorific value of kerosene.

[0347] In some examples, the aromatic and / or other non-alkane content of the sustainable aviation fuel, or blend, provided to the combustion equipment 16 is relatively lower than that of kerosene. The sustainable aviation fuel can include an aromatic content, 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%. Depending on one or more of preference, fuel feedstock or supplier, and variations in the ingredients therein, the aromatic content of the sustainable aviation fuel can be within a numerical value or range including the end values defined by or defined within any two of the values in the previous sentence (i.e., these values can form an upper or lower limit), for example, 13.5%, 8.5%, 2.5%, 0.35%, 0.15%, 0.05%, 0.005% or 0%; or 0% to 0.75%, 0% to 0.5%, or 0.1% to 0.25%; or 0.15% to 0.65%, 0.35% to 0.55%, or 0.035% to 0.055%.

[0348] At least in part due to the molecular structure of sustainable aviation fuel, sustainable aviation fuel can provide benefits including, for example, one or more of the following: higher specific energy (although, 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, relative to fossil-based hydrocarbon fuels (e.g., when burned in combustion equipment 16). Thus, relative to fossil-based hydrocarbon fuels such as kerosene, sustainable aviation fuel can result in either or both a relative reduction in fuel consumption and a relative reduction in maintenance costs.

[0349] As Figure 4 depicted in, the aircraft 1 can include a plurality of fuel tanks 50, 53; for example, a larger primary fuel tank 50 located in the aircraft fuselage and smaller fuel tanks 53a, 53b located in each wing. In other examples, the aircraft 1 can have only a single fuel tank 50, and / or the wing fuel tanks 53 can be larger than the central fuel tank 50, or the central fuel tank can be omitted (all fuel instead being stored in the wings of the aircraft) - it should be understood that many different tank layouts are envisioned, and the illustrated examples are provided for ease of description and are not intended to be limiting.

[0350] Figure 4 An aircraft 1 is shown having a propulsion system 2 including two gas turbine engines 10. Fuel is supplied from a fuel supply system on the aircraft 1 to the gas turbine engines 10. The fuel supply system 1000 of the illustrated example includes a single fuel source. For the purposes of this application, the term "fuel source" means: 1) a single fuel tank; or 2) a plurality of fluidly interconnected fuel tanks. Each fuel source is arranged to provide a separate fuel source, i.e., the first fuel source can contain a first fuel having one or more characteristics different from the second fuel contained in the second fuel source. Thus, the first fuel source and the second fuel source are not fluidly coupled to each other in order to separate different fuels (at least under normal operating conditions). The use of multiple fuel sources allows the aircraft 1 to carry multiple different fuels and change the fuel used during operation and optionally even during cruise or when changing between different operating phases in flight.

[0351] In this example, the first (and only in these examples) fuel source includes a central fuel tank 50 located primarily in the fuselage of the aircraft 1 and a plurality of wing fuel tanks 53a, 53b, with at least one wing fuel tank located in the left wing (port wing) and at least one wing fuel tank located in the right wing (starboard wing) for balance. In the illustrated example, all tanks 50, 53 are fluidly interconnected, thereby forming a single fuel source. Each of the central fuel tank 50 and the wing fuel tanks 53 can include a plurality of fluidly interconnected fuel tanks.

[0352] In another example, the wing fuel tanks 53a, 53b may not be fluidly connected to the central fuel tank 50, thereby forming a separate second fuel source. For balance purposes, one or more fuel tanks in the left wing may be fluidly connected to one or more fuel tanks in the right wing. This may be via the central fuel tank (if the tank does not form part of another fuel source), or bypassing the central fuel tank, or both (for maximum flexibility and safety). In another example, the first fuel source includes the wing fuel tanks 53 and the central fuel tank 50, while the second fuel source includes another separate central fuel tank.

[0353] A fluid interconnection between the wing fuel tank and the central fuel tank of the first fuel source may be provided for the balance of the aircraft 1. In an aircraft 1 having multiple fuel sources, two or more of the fuel sources may thus contain fuels that are different from each other, such that the aircraft 1 can change fuels during flight. Therefore, being able to determine which fuel is being supplied to the burner 16 may be more complex than simply recording the single identity of the fuel on the aircraft 1 or checking it once at startup.

[0354] In some examples, the distribution of the fuel tanks 50, 53 available on the aircraft 1 may be constrained such that the first fuel source and the second fuel source are each substantially symmetric with respect to the aircraft centerline. In cases where asymmetric fuel tank distribution is allowed, suitable fuel transfer mechanisms are typically provided between the fuel tanks of the first fuel source and / or between the fuel tanks of the second fuel source such that the center of mass position of the aircraft can be maintained within acceptable lateral limits throughout the flight.

[0355] An aircraft is typically refueled at multiple different airports, for example at the start and end of a long - haul flight. Although there are standards that all aviation fuels must meet, as described above, different aviation fuels have different compositions, which depend, for example, on their source (e.g., different petroleum sources, biofuels or other synthetic aviation fuels (commonly referred to as sustainable aviation fuel SAF), and / or mixtures of petroleum - based fuels, and other fuels) and any additives contained (e.g., such as antioxidants and metal deactivators, biocides, static dissipators, icing inhibitors, corrosion inhibitors) and any impurities. Even for a given airport or fuel supplier, the fuel composition of the available aviation fuel may vary between batches. In addition, the fuel tanks 50, 53 of the aircraft 1 are typically not emptied before being filled for a subsequent flight, resulting in a mixture of different fuels within the tanks - in effect, fuels with different compositions resulting from the mixture.

[0356] The inventors recognize that since different fuels can have different properties while still meeting standards, knowledge of the fuel available for the aircraft 1 can allow for more effective and customized control of the aircraft 1, and more specifically of the propulsion system 2 of the aircraft (i.e., one or more gas turbine engines 10 of the aircraft 1 and associated controls and components). Thus, knowledge of the fuel can be used as a tool to improve aircraft performance, and therefore determining or monitoring the fuel composition can provide benefits. In particular, determining one or more fuel characteristics of the fuel to be supplied to the burner 16 (whether fuel from a single fuel source or a mixture of one or more fuels from different fuel sources) is thus important in determining engine operation. A key feature of engine operation is thermal management - engine thermal management is mainly performed by utilizing the heat transfer of oil and fuel into and out of the engine, and thus control of the heat exchange system 3000 - such as by controlling the oil flow, controlling the fuel flow, and / or otherwise controlling one or more heat exchangers 1004, 1006, 2020, 2030 or other heat exchange components - can allow the performance of the engine 10 to be optimized for a fuel with specific fuel characteristics.

[0357] As used herein, the term "fuel characteristic" refers to an inherent or intrinsic fuel property (such as fuel composition), rather than a variable property (such as volume or temperature). One or more fuel characteristics can be determined, and this data is used to adjust the control of the engine 10, particularly the control of the thermal management system 3000. Examples of fuel characteristics include one or more of the following:

[0358] 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 an indication that the fuel is a pure SAF fuel;

[0359] ii. Parameters of the hydrocarbon distribution of the fuel, such as:

[0360] · The aromatic content of the fuel, and optionally / alternatively the polyaromatic content of the fuel;

[0361] · The hydrogen-to-carbon ratio (H / C) of the fuel;

[0362] · Percentage composition information of some or all of the hydrocarbons present;

[0363] iii. The presence or percentage of specific elements or species, such as:

[0364] · The nitrogen content of the fuel / percentage of nitrogenous substances in the fuel;

[0365] · The presence or percentage of tracer substances or trace elements in the fuel;

[0366] · The naphthalene content of the fuel;

[0367] · The sulfur content of the fuel / the percentage of sulfur-containing substances in the fuel;

[0368] · The naphthene content of the fuel;

[0369] · The oxygen content of the fuel;

[0370] iv. One or more properties of the fuel used in the gas turbine engine 10, such as:

[0371] · The level of non-volatile particulate matter (nvPM) emissions or CO2 emissions during combustion;

[0372] · The fuel coking or varnishing level (or another measure of the deposition of fuel decomposition products);

[0373] v. One or more properties of the fuel itself independent of the use or combustion of the engine 10, such as:

[0374] · The thermal stability of the fuel (e.g., the thermal decomposition temperature); and

[0375] · One or more physical properties, such as density, viscosity, calorific value, freezing temperature, and /

[0376] or heat capacity.

[0377] The fuel characteristics to be determined can be selected based on which properties of the fuel are most relevant to the changes that can be made to the thermal management system 3000. The determination of the fuel characteristics can include obtaining the fuel characteristics of any fuel already present in the fuel tanks 50, 53 and an indication of the amount of remaining fuel before refueling, and then combining this information with information about the new fuel added to the fuel tanks 50, 53 during refueling.

[0378] Obtaining the fuel characteristics of any fuel already present in the fuel tanks 50, 53 before refueling, and / or obtaining the fuel characteristics of the fuel provided during refueling, can include one or more of the following:

[0379] (i) Physically and / or chemically detecting one or more characteristics or parameters of the fuel composition (which can allow direct detection of the fuel characteristics, and / or can allow the use of the detection results to determine the fuel characteristics), and / or detecting one or more tracer elements or compounds added to the fuel for ease of its identification (e.g., dyes);

[0380] (ii) Retrieving fuel characteristic information from on-vehicle memory / data repositories; and / or

[0381] (iii) Receiving data, for example, from an input provided at the user interface, or receiving data transmitted to the aircraft 1.

[0382] In some examples, one or more fuel properties can be determined during operation of the gas turbine engine 10, such as by inferring fuel properties of fuel provided to the combustor 16 from engine performance metrics, or by performing in-wing detection.

[0383] In some examples, a variety of different methods can be performed to obtain fuel properties. For example, different methods can be used for different properties, and / or different methods can be used for the same property as a check. For example, stored or otherwise provided fuel property data can be compared with the results of chemically or physically detecting one or more parameters of the fuel. If there is a mismatch between the stored fuel property and the corresponding detected parameter, an alert can be provided.

[0384] Fuel properties can be determined by physically and / or chemically detecting one or more characteristics of the fuel composition (e.g., in a test unit outside the wing, or when the fuel is transported to the fuel tank on the wing, or actually when used in the gas turbine engine 10), thereby allowing direct detection of fuel properties or providing data from which fuel properties can be determined as described above, and / or detecting one or more tracer elements or compounds added to the fuel for ease of its identification (e.g., dyes); or by receiving data from an input provided by a user interface, or by receiving data transmitted to the aircraft by scanning a barcode associated with fuel delivery, for example.

[0385] When using physical and / or chemical determination, fuel properties can be detected in various ways, including direct ways (e.g., from sensor data corresponding to the fuel property in question) and indirect ways (e.g., by inference or calculation from other properties or measurements, or by reference to data on specific detected tracers in the fuel). These properties can be determined as relative values compared to another fuel, or as absolute values. For example, one or more of the following detection methods can be used:

[0386] ● The aromatic or naphthene content in the fuel can be determined based on the measurement of the expansion of a sensor component made of a sealing material (such as a nitrile sealing material).

[0387] ● Trace substances or species naturally present in the fuel or added as tracers can be used to determine fuel properties, such as the percentage of sustainable aviation fuel in the fuel or whether the fuel is kerosene.

[0388] ● Measurement of the vibration mode of a piezoelectric crystal exposed to the fuel can be used as a basis for determining various fuel properties, including the aromatic content of the fuel, the oxygen content of the fuel, and the thermal stability or coking / varnishing level of the fuel, for example by measuring the accumulation of deposits on the upper surface of the piezoelectric crystal, which will cause a change in the vibration mode.

[0389] ● Each fuel characteristic can be determined by collecting performance parameters of the gas turbine engine 10 during a first operation (e.g., during takeoff) and optionally during a second operation (e.g., during cruise), and comparing these collected parameters with expected values if a fuel of known properties is used.

[0390] ● Each fuel characteristic, including the aromatic content of the fuel, can be determined based on sensor measurements of the presence, absence, or degree of formation of a trajectory during the operation of the gas turbine 10.

[0391] ● The fuel characteristic including the aromatic content can be determined based on ultraviolet-visible spectroscopic measurements performed on the fuel.

[0392] ● Each fuel characteristic, including sulfur content, naphthalene content, aromatic hydrogen content, and hydrogen-carbon ratio, can be determined by measuring substances present in the exhaust gas emitted by the gas turbine engine 10 during its use.

[0393] ● The calorific value of the fuel can be determined during the operation of the aircraft 1 based on measurements made during fuel combustion, e.g., using fuel flow rate and shaft speed or temperature changes at the burner 16.

[0394] ● Each fuel characteristic can be determined by making an operational change that is arranged to affect the operation of the gas turbine engine 10, sensing the response to the operational change; and determining one or more fuel characteristics of the fuel based on the response to the operational change.

[0395] ● By changing the fuel supplied to the gas turbine engine 10 from a first fuel to a second fuel and determining one or more fuel characteristics of the second fuel based on a change in the relationship between T30 and one of T40 and T41 (the relationship indicating a temperature rise at the burner 16), each fuel characteristic related to the fuel characteristics of the first fuel can be determined. These characteristics can be determined as relative values compared to the first fuel or as absolute values, e.g., by reference to known values of the first fuel.

[0396] As used herein, T30, T40, and T41, and any other numbered pressures and temperatures are defined using the station numbers listed in the standard SAE AS755, specifically:

[0397] · T30 = High-pressure compressor (HPC) outlet total temperature;

[0398] · T40 = Combustion outlet total temperature;

[0399] · T41 = High-pressure turbine (HPT) rotor inlet total temperature.

[0400] Any suitable method known in the art can be used, and the determination of fuel characteristics is not discussed further herein.

[0401] Accordingly, the aircraft 1 may include a fuel composition determination module 57 that is arranged to determine at least one fuel property of the fuel. This determination may be performed by obtaining values from a data repository (e.g., data provided to the aircraft 1 during fuel 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 Figure 4 the example shown, a fuel composition determination module 57 is provided as part of each engine 10 where fuel enters the engine 10. In other embodiments, the module 57 may be located differently within the engine 10 or elsewhere on the aircraft 1, such as in the fuel tanks 50, 53, on or near the fuel tanks. In embodiments having a fuel composition determination module 57, the module 57 may be arranged to provide an output to an electronic engine controller (EEC), or the module 57 may be provided as part of the EEC. Accordingly, one or more fuel properties determined by the fuel composition determination module 57 may be used to affect the control of the engine 10. A dedicated controller 58 may be provided to process the output of the fuel composition determination module 57 and provide control instructions to controllable engine components based on the fuel property information. In other embodiments, the control function 58 may be provided by the EEC, and a separate unit or module may not be provided.

[0402] Figure 5 An exemplary fuel system 1000 for a geared gas turbine engine 10 is shown, the fuel system 1000 including a fuel flow path from a fuel tank 50 to a burner 16 of the gas turbine engine 10 of the aircraft 1. The fuel system 1000 includes both a fuel supply system 50, 1002 of the aircraft 1 that supplies fuel to the engine 10 and a fuel management system 1500 that operates within the engine 10. The fuel management system 1500 has a role in managing the fuel temperature as well as the fuel flow rate, directing the fuel through one or more heat exchangers 1004, 1006 of the engine's heat exchange system 3000. The heat exchange system 3000 includes components of the fuel management system 1500 as well as a recirculating oil system 2000, 2000' (which will be described in more detail below). The heat exchange system 3000 is a general term for the systems and components for transferring heat between fluids (specifically oil and fuel) within the engine 10 and includes heat exchangers, valves, connecting pipes, and associated components such as pumps, refrigeration devices, etc.

[0403] Returning to the fuel system 1000, fuel is pumped from the fuel tank 50 to the gas turbine engine 10 by the low-pressure fuel supply pump 1002. The fuel then flows through the secondary fuel-oil heat exchanger 1004 and the primary fuel-oil heat exchanger 1006. The primary fuel-oil heat exchanger 1006 can be described as the main fuel-oil heat exchanger because the oil flowing through it can be used to cool and lubricate the main gearbox 30 of the engine 10. The gas turbine engine 10 of the described embodiment also includes a generator (specifically, an integrated drive generator) and a secondary oil circuit system arranged to supply oil to the generator.

[0404] The secondary fuel-oil heat exchanger 1004 can be described as an integrated drive generator fuel-oil heat exchanger because the oil flowing through it can be used to cool and / or lubricate one or more components of the integrated drive generator (IDG) of the engine 10. In other embodiments, different types of generators can be used in place of the IDG, such as a variable frequency generator (VFG) or a variable frequency starting generator (VFSG). The fuel system 1000 of such embodiments can be equivalent in other respects.

[0405] Thus, the engine 10 of the described example includes two fuel-oil heat exchangers 1004, 1006. In other embodiments, more or fewer fuel-oil heat exchangers can be provided. The illustrated fuel management system 1500 is arranged such that the fuel reaches the secondary fuel-oil heat exchanger 1004 before reaching the primary fuel-oil heat exchanger 1006. After leaving the primary fuel-oil heat exchanger 1006, the fuel then passes through the engine fuel pump 1003 and to the burner 16. The engine fuel pump 1003 can be described as the main fuel pump. In other embodiments, the engine fuel pump 1003 can be upstream of one or more of the heat exchangers 1004, 1006.

[0406] In Figure 5In the example shown, the fuel system 1000 also includes a temperature sensor 1009 that is arranged to sense the temperature of the fuel proximate to or reaching the burner 16. In the illustrated example, the temperature sensor 1009 is shown adjacent to the inlet of the burner 16, but in various examples, the sensor 1009 can be located anywhere downstream of the primary fuel - oil heat exchanger 1006 (or any fuel - oil heat exchanger that is the most downstream in other embodiments) on the fuel side to provide a measure of the fuel temperature. In the gas turbine engine 10 as described herein, the fuel passes through fuel nozzles before entering the combustion chamber itself, and these nozzles are very susceptible to fuel thermal decomposition, which can cause blockage of relatively narrow passages (such as metering slots); thus, in some embodiments, it is advisable to place the sensor 1009 as reasonably close as possible to the inlet of the nozzle. In some embodiments, multiple temperature sensors 1009 can be used. One or more temperature sensors 1009 are arranged to provide an output (temperature data) to the controller 58.

[0407] The primary fuel - oil heat exchanger 1006 and the secondary fuel - oil heat exchanger 1004 are configured such that in addition to the fuel flow passing through them, an oil flow is also conveyed through each heat exchanger. 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 the fuel passing through them. In the standard operation of the engine 10, such as under cruise 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 way, the primary fuel - oil heat exchanger 1006 and the secondary fuel - oil heat exchanger 1004 are each configured to transfer thermal energy from the oil flow to the fuel flow passing through them during operation.

[0408] These two oil flows (the oil flow passing through the primary heat exchanger and the oil flow passing through the secondary heat exchanger) can be separate - physically separate, and optionally also chemically different oils and / or have different flow rates. Thus, the oil passing through the primary fuel - oil heat exchanger 1006 can be different from the oil passing through the secondary fuel - oil heat exchanger 1004. Each heat exchanger 1004, 1006 can be on separate closed - loop oil systems 2000, 2000'.

[0409] Generally speaking, at least most of the fuel passing through the secondary fuel-oil heat exchanger 1004 also passes through the primary fuel-oil heat exchanger 1006. The two heat exchangers 1004, 1006 can thus be described as being in series with each other with respect to the fuel flow and along the main fuel flow path from the tank 50 to the burner 16. However, either or each of the heat exchangers 1004, 1006 can 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 duct 1005 as shown in Figure 5 . A valve (not shown) can determine what proportion of the fuel passes through the heat exchanger 1004 and what proportion of the fuel passes through the bypass duct 1005. In various embodiments, a bypass duct can be provided for each of the heat exchangers 1004, 1006, thereby allowing a portion of the fuel to bypass either or both of the heat exchangers. Additionally or alternatively, one or more bypass ducts 2005, 2005' for the oil can be provided for either or both of the fuel-oil heat exchangers 1004, 1006, thereby allowing a portion of the oil to bypass one or more of the heat exchangers. A valve arranged to control the fluid flow through the bypass duct 1005 can be referred to as a bypass valve.

[0410] Thus, the secondary (IDG) fuel-oil heat exchanger 1004 and the primary fuel-oil heat exchanger 1006 are configured such that, in addition to the fuel flow, an oil flow is also conveyed through each fuel-oil heat exchanger - in the described embodiment, the oil flowing through one fuel-oil heat exchanger is different from the oil flowing through the other fuel-oil heat exchanger, but it should be understood that in other embodiments, the same oil can flow through one fuel-oil heat exchanger and then through the other fuel-oil heat exchanger.

[0411] Thus, in the described embodiment, the two heat exchangers 1004, 1006 are in separate closed-loop systems 2000, 2000' ( Figure 7 , 8 ) with respect to the oil flow, i.e., the oil flowing through the primary fuel-oil heat exchanger and the secondary fuel-oil heat exchanger is fluidly separated and can be chemically different from each other. The two oil loop systems 2000, 2000' are used to circulate the oil through their respective fuel-oil heat exchangers 1006, 1004 and optionally also through one or more additional heat exchangers, such as air-oil or oil-oil heat exchangers, as described below. The two oil loop systems 2000, 2000' together can be described as providing a recirculation oil system for the engine 10.

[0412] Figure 6Shows an alternative exemplary fuel system 6000 including a fuel supply system and a fuel management system 6500, the alternative exemplary fuel system including a fuel flow path from a fuel tank 50 to a burner 16 of a gas turbine engine 10 of an aircraft 1. As with the fuel system 1000, fuel is pumped from the fuel tank 50 by a low-pressure fuel supply pump 1002. The fuel then flows through a secondary fuel-oil heat exchanger 1004 before reaching an engine fuel pump 1003 and then through a primary fuel-oil heat exchanger 1006, which pumps the fuel along its flow path to the burner 16. The fuel system 6000 differs from the fuel system 1000 in that the fuel system 6000 includes a recirculation valve 6010 that is located downstream of the primary fuel-oil heat exchanger 1006 and is arranged to recirculate at least a portion of the fuel that has left the primary fuel-oil heat exchanger back to an inlet 1006a of the primary fuel-oil heat exchanger 1006, thereby allowing further heat transfer between the oil and the fuel in the primary loop system. A fuel temperature sensor 1009 is located downstream of a branch point of the recirculation valve 6010 and is preferably adjacent to the burner 16 in order to provide a more accurate indication of the temperature at the inlet of the burner 16 (in some embodiments, another temperature sensor may be provided between the heat exchanger outlet and the branch point of the recirculation valve 6010). The recirculation valve 6010 can determine the proportion of fuel recirculated via a recirculation pipe 6011 and the proportion that continues more directly to the burner 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. In some embodiments where the recirculation valve 6010 is positioned upstream of the engine fuel pump 1003, an additional recirculation pump (not shown) may be provided to provide a positive pressure gradient. In some embodiments, there may be a fuel return tank (FRTT) pump, and it also contributes to recirculation. The recirculation valve 6010 is arranged to allow a controlled amount of fuel to return to the inlet 1006a of the primary heat exchanger 1006, and thus to flow through the primary heat exchanger 1006 multiple times before reaching the pump 1003 and the burner 16. It is contemplated that in an alternative embodiment, the recirculation valve may be positioned downstream of the engine fuel pump 1003, such as as Figure 10 shown. In such embodiments, the recirculation valve 6010 will be arranged to allow a controlled amount of fuel to return to the inlet 1006a of the primary heat exchanger 1006, and thus to flow through the primary heat exchanger 1006 and the pump 1003 multiple times before reaching the burner 16. This recirculation provides a mechanism for controlling the fuel flow within the fuel management system 6500 and within the thermal management system 3000 without changing the fuel flow from the fuel tank 50 to the engine 10.

[0413] A recirculation duct 6011 may thus be provided, which may be referred to as a recirculation duct as 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 again pass through portions between the flow paths, and the duct leads from a recirculation valve to a point on the flow path upstream of the inlet 1006a of the primary heat exchanger 1006. In some embodiments, a recirculation duct 6011 and a bypass duct 1005 may be provided for any given heat exchanger 1004, 1006. In some embodiments, the same ducts 1005, 6011 may be used as recirculation ducts and bypass ducts, and one or more valves may be used to control the direction of fluid flow therethrough.

[0414] In Figure 10 the illustrative example of Figure 6 unlike the illustrative example of

[0415] Controlling the fuel flow in the fuel system 6000 - by recirculating and / or bypassing one or more heat exchangers - can contribute to thermal management (e.g., affecting the fuel temperature at the burner 16 or the inlet of the pump 1003, or the heat transferred to the fuel). Thus, part of the control of the heat exchange system 3000 can be the control of one or more valves that control the oil and / or fuel flow through one or more recirculation and / or bypass ducts 6011, 1005.

[0416] The gas turbine engine 10 of the described aircraft 1 includes a recirculation oil system that is arranged to supply oil to lubricate a plurality of components and remove heat from a plurality of components. In the described embodiments, the recirculation oil system includes a primary oil circuit system 2000 and a secondary oil circuit system 2000', each of which is a closed - loop oil system. In Figure 7FIG. schematically shows an example of a primary closed-loop oil system 2000 - for a heat exchanger, this oil circuit system is referred to as "primary" because it is responsible for lubricating and cooling the main gearbox 30 and generally for the main / primary cooling load of the engine 10. The primary closed-loop oil system 2000 includes an oil tank 2002 adapted to hold a volume of oil. In some embodiments, gas is removed from the oil within the oil tank 2002 by a degasser. A feed pump 2004 is configured to pump oil from the oil tank 2000 to the main fuel-oil heat exchanger 1006. Under cruise conditions, the average temperature of the oil entering the main fuel-oil heat exchanger 1006 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 way, 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 the oil is cooled before being reused 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 way, 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.

[0417] In a standard heat exchanger 1006, the basic limiting factor for the level of cooling provided is the temperature of the coolant (in this case the fuel) - even with an effectively infinite flow rate of fuel or oil used, the temperature of the oil leaving the heat exchanger 1006 cannot be lower than the temperature of the fuel entering the heat exchanger 1006 (and the temperature of the fuel cannot be raised to be higher than the temperature of the oil entering the heat exchanger). By definition of the second law of thermodynamics, temperature equilibrium is the limit. In some embodiments, as Figure 7 shown, a refrigeration cycle device 1007 is thus provided. The refrigeration cycle device 1007 is arranged to provide a heat lift by transferring more heat from the oil to the fuel, such that the fuel temperature rises more than it would simply by passing through the heat exchanger and, in some cases, rises to be higher than the oil temperature. The refrigeration cycle device 1007 can take the form of an additional closed loop with a refrigerant fluid, having 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 device 1007 may additionally include pumps, compressors, and / or expansion valves / instruments and can be made in any suitable design known in the art. It should be understood that the refrigeration cycle device 1007 can also be referred to as a heat pump - it pumps heat from the oil to the fuel, thus cooling the oil and heating the fuel.

[0418] Figure 18FIG. 1007 shows an exemplary refrigeration cycle apparatus in more detail. The refrigeration cycle apparatus 1007 shown contains a refrigerant fluid that circulates through the apparatus 1007 in use. In the described embodiment, the flow rate of the refrigerant can be adjusted to vary the amount of heat transferred, and the refrigeration cycle apparatus 1007 can be shut down by stopping the refrigerant flow. In other embodiments, the refrigeration cycle apparatus 1007 can have a single operating speed, and controlling it can simply involve turning the refrigeration cycle apparatus 1007 on and off. The refrigerant fluid can be 1,1,1,2-tetrafluoroethane (commercially known as refrigerant R134a), but other refrigerant fluids can also be used.

[0419] During operation, heat from the oil in the oil circuit system 2000 is transferred to the refrigerant fluid in the evaporator 1007a, which causes the liquid in the refrigerant fluid (at this point in the refrigeration cycle, the refrigerant fluid can be a liquid-gas mixture or can be pure liquid) to evaporate to form a vapor, particularly a saturated vapor. The evaporator 1007a is located on the oil circuit system 2000, at or near where the oil is hottest, typically immediately after the last engine component that is cooled by the oil in a single pass around the oil circuit system 2000 (which can be a gearbox 30). One or more heat conducting plates can be provided between the oil and the refrigerant (usually as part of the evaporator 1007a) to increase heat transfer; optionally, internal channels for the flow of oil and / or refrigerant can be provided within the heat conducting plates or other structures to improve heat transfer.

[0420] The saturated vapor then passes from the evaporator 1007a to the refrigerant compressor 1007b. The refrigerant compressor 1007b can also act as a pump to circulate the refrigerant around the refrigeration cycle apparatus 1007. Thus, the control of the compressor 1007b can be used to reduce, increase, or stop the flow of refrigerant around the apparatus 1007. Advantageously, the refrigeration cycle apparatus 1007 can thus meet variable oil cooling / fuel heating requirements because the cooling flow rate can be modulated using the refrigerant compressor 1007b. In some embodiments, a separate / additional pump can be provided for the refrigeration cycle apparatus 1007, but using the compressor 1007b to circulate the refrigerant can reduce the number of components required and thus reduce the size and weight of the apparatus 1007.

[0421] Compressor 1007b compresses the saturated vapor into superheated vapor and delivers it to condenser 1007c, which serves as refrigerant-fuel heat exchanger 1007c. Condenser 1007c transfers heat from the compressed vapor to the fuel (in fuel management system 1500), condensing at least some of the superheated vapor into a liquid. Generally, condenser 1007c can convert the compressed vapor refrigerant into saturated liquid. Condenser 1007c is located on fuel passage 1500 through engine 10, downstream of any or all of fuel-oil heat exchangers 1004, 1006, and is typically near burner 16. Condenser 1007c can be the last engine component (excluding pipes / fuel flow passages) that the fuel flows through before reaching burner 16. Relative to the fuel flow, condenser 1007c can be the most downstream component of heat exchange system 3000.

[0422] Figure 18 The refrigeration cycle device 1007 of the illustrated embodiment further includes expansion valve 1007d located between condenser 1007c and evaporator 1007a. Expansion valve 1007d is arranged to allow the refrigerant to expand, thereby cooling the refrigerant. For example, the function of expansion valve 1007d can convert the saturated liquid into a mixture of liquid and vapor with a reduced temperature. The expanded / cooled refrigerant then returns to evaporator 1007a, ready to receive more heat from the oil.

[0423] In some embodiments, the refrigeration cycle device 1007 may further include a refrigerant storage tank (not shown). This refrigerant storage tank can be used to compensate for any volume changes of the refrigerant due to temperature and density variations, such that the refrigerant flow can reliably and consistently perform its cooling / heat pump function under all engine operating conditions.

[0424] In the embodiment shown in the figure, engine 10 includes gearbox 30, which receives an input from spindle 26 and outputs driving force to fan 23, thereby driving the fan at a lower speed than the spindle, and oil circuit system 2000 is arranged to supply oil to gearbox 30. However, it should be understood that Figure 18 the illustrated embodiment does not require oil circuit system 2000 to include gearbox 30, and the same refrigeration cycle device 1007 and its control can be implemented in a direct drive engine (main shaft without gearbox).

[0425] Figure 18 The connection of refrigeration cycle device 1007 to both oil system 2000 and fuel system 1500 within engine 10 is shown. The refrigerant acts as an intermediate heat transfer fluid, pumping heat to the fuel. Heat is transferred from oil system 2000 to the refrigerant in evaporator 1007a, and then heat is transferred from the refrigerant to fuel system 1500 in condenser 1007c. Figure 18Also schematically shown are two temperature sensors: an oil temperature sensor 2009, which is arranged to provide an indication of the highest temperature of the oil within the oil circuit system 2000; and a fuel temperature sensor 1009, which is arranged to provide an indication of the temperature of the fuel downstream of the refrigeration cycle device 1007 / adjacent to the burner 16. Data from these sensors 1009, 2009 can be used to control the refrigeration cycle device 1007, such as controlling one or more of the refrigerant flow rate, oil flow rate, and fuel flow rate through relevant portions of the refrigeration cycle device 1007.

[0426] The oil valve can control the amount of oil flowing through the refrigeration cycle device 1007 (specifically through the evaporator 1007a), and the fuel valve can control the amount of fuel flowing through the refrigeration cycle device 1007 (specifically through the condenser 1007c). The refrigeration cycle device 1007 is typically electric or mechanical (e.g., via a linkage to a rotating shaft) to provide this further forced heat transfer. The refrigeration cycle device 1007 is shown in the illustrated embodiment as being associated with the main (primary) fuel - oil heat exchanger 1006, thus providing a boost in fuel temperature after the fuel has passed through the fuel - oil heat exchangers 1004, 1006 and further cooling of the oil in the primary oil system 2000 before returning to the gearbox 30. In various embodiments, bypass pipes or recirculation pipes for the oil and / or fuel can be provided around the refrigeration cycle device 1007.

[0427] The oil flow in the primary oil circuit system 2000 is then delivered to the power gearbox 30, which can also be described as the main gearbox 30 of the gas turbine engine 10. The power gearbox 30 is arranged to receive an input from the spool 26 and deliver a drive output to the fan 23 via the fan shaft 42, and includes gears 28, 32, 38 and bearings (e.g., journal bearings) that can be lubricated and cooled by oil. The engine 10 may also include one or more additional bearings to support the shafts 26, 42, and these bearings can be journal bearings. The oil can additionally be used to lubricate and / or cool the journal bearings and, when used under cruise conditions, typically experiences a significant temperature rise, thus helping to cool the bearings and the gearbox 30 as the oil flow carries heat away from the bearings and the gearbox 30. The oil can also be used to lubricate one or more other engine components 33, such as an accessory gearbox (AGB) and / or one or more bearing housings. The AGB 33 (also referred to as the accessory drive when present) is a gearbox that forms part of the gas turbine engine 10 but is not part of the engine core 11 and does not drive the fan 23. Instead, the AGB drives engine accessories (e.g., fuel pumps) and typically handles large loads. Thus, a relatively large amount of heat can be discharged from the AGB into the oil. One or more bearing housings can be lubricated by the same oil and can similarly discharge heat into the oil. The AGB and the bearing housings can add more heat to the oil than the main gearbox 30 in many embodiments for each unit of oil flowing through them. The oil flow can be split into two or more parallel flows, such as one flow through the main gearbox 30 and one flow through the other engine components, or multiple parallel flows (e.g., via different components of the gearbox) through the main gearbox 30 and separate flows through the AGB and the bearing housing 33 or each bearing housing.

[0428] Oil is collected in the oil sump 2008 from the power gearbox 30 (and / or other engine components 33, such as the AGB when present). The scavenge pump 2010 is configured to pump the oil from the oil sump 2008 back to the oil tank 2002 for reuse.

[0429] Figure 8Shows a secondary oil circuit system 2000’, which is another closed-loop oil system 2000’. The secondary closed-loop oil system 2000’ includes a secondary oil tank 2002’ adapted to hold a volume of oil. In some embodiments, gas is removed from the oil within the tank 2002’ by a degasser. A secondary feed pump 2004’ is configured to pump oil from the secondary oil tank 2002’ to a secondary fuel-oil heat exchanger 1004, which in the illustrated embodiment is an IDG fuel-oil heat exchanger 1004. Under cruise conditions, the average temperature of the oil entering the IDG fuel-oil heat exchanger 1004 is higher than the average temperature of the fuel entering the IDG fuel-oil heat exchanger 1004. In the IDG fuel-oil heat exchanger 1004, heat energy is transferred from the oil flow to the fuel flow. In this way, 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. Also in this way, 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 delivered to / returned to an integrated drive generator 2006, where the oil flow lubricates and / or cools moving components and is heated in the process. In some embodiments, the oil can be used primarily as a coolant for the IDG 2006 and can provide minimal or no lubrication. The oil is collected from the integrated drive generator 2006 in a secondary oil sump 2008’. A secondary return pump 2010’ is configured to pump the oil from the secondary oil sump 2008’ back into the secondary oil tank 2002’ for reuse. In some embodiments, a refrigeration cycle device can also be provided on the secondary oil circuit system 2000’.

[0430] Figure 7 and Figure 8 Each schematically shows a series flow path of the oil, with all oil flows sequentially passing through each component (it should be understood that one or more bypass pipes or recirculation pipes not shown may be provided for the oil). In other embodiments, the oil flow can be split into two or more parallel flows, for example, one flow through the main fuel-oil heat exchanger 1006 and another flow through an air-oil heat exchanger 2020 (described below). It should be understood that although the bypass pipes technically provide parallel, alternative flow paths, the branched paths described here with reference to parallel flows differ in that each branched route in the parallel arrangement includes a heat exchanger, while the bypass pipes are merely pipes without (significant) heat exchange.

[0431] Figure 9 Schematically shows Figure 7An alternative exemplary segment of the primary closed-loop oil system 2000 shown. In this segment, the oil flow is pumped by the feed pump 2004 through the valve 2016. The valve 2016 is operable to divert the oil flow between the main fuel-oil heat exchanger 1006 and the first air-oil heat exchanger 2020, where the first air-oil heat exchanger 2020 is arranged in parallel with the main fuel-oil heat exchanger 1006. The oil flow path can be described as branched, with the main fuel-oil heat exchanger 1006 on one branch and the first air-oil heat exchanger 2020 on the other branch, and these branches are in a parallel configuration such that the oil can flow through one branch or the other, but the same portion of the oil cannot cross both branches on the same cycle - flow diversion. The valve 2016 modulates the flow through the two heat exchangers 1006, 2020 and can thus be described as a modulating valve 2016. The oil flow is then recombined and delivered to the power gearbox 30 and / or other engine components 33.

[0432] Any suitable percentage of the oil can flow through each of the first air-oil heat exchanger 2020 and the main fuel-oil heat exchanger 1006. In some examples, the valve 2016 is operable to change the oil flow to the main fuel-oil heat exchanger 1006 and the first air-oil heat exchanger 2020 as needed. In various examples, an oil-oil heat exchanger 2030 ( Figure 7 not shown in Figure 11 but present in

[0433] Figure 9 is provided, for example, arranged in series with the first air-oil heat exchanger 2020 on this branch of the parallel split. The oil-oil heat exchanger 2030 can allow heat exchange between the primary closed-loop oil system 2000 and the secondary closed-loop oil system 2000'.

[0434] A valve 2022, referred to as an air valve, is used to control the flow of air through the air - oil heat exchanger 2020. In the example shown, the air valve 2022 is disposed at or near the inlet 2020a of the heat exchanger 2020 and can thus be referred to as an air inlet valve 2022. In other embodiments, the air valve 2022 can be disposed at or near the outlet 2020b of the heat exchanger 2020 and can thus be referred to as an air outlet valve. If the valve 2022 can be adjusted to control the air flow through the air - oil heat exchanger 2020, any suitable location of the valve 2022 can be selected. In the example described, the air valve 2022 can be continuously adjusted between a fully closed position (no air flow through the heat exchanger) and a fully open position (maximum air flow through the heat exchanger). In other examples, the air valve 2022 can be adjusted between multiple discrete positions rather than continuously, for example, six, five, four, three, or two different positions. In embodiments where the air valve 2022 has only two valve positions, these positions can be "open" and "closed" - in some such embodiments, when a medium level of cooling is required, the valve 2022 can be repeatedly opened and closed to provide a pulsating air flow.

[0435] Figure 10 An exemplary arrangement and interaction of a first closed - loop oil system 2000, a second closed - loop oil system 2000', and a fuel system 1000 are shown, where the fuel flow is shown as a thick black line and the oil flow is shown as a thin black line. The striped thick black line represents a recirculation path 6011 taken by only a portion of the fuel. The combination of the components of the fuel system 1000 and the oil systems 2000, 2000' together form a heat exchange system 3000. The primary closed - loop oil system 2000 of this exemplary arrangement is arranged as Figure 7 shown. The secondary closed - loop oil system 2000' of this exemplary arrangement is arranged as Figure 8 shown. The fuel system 1000 of this exemplary arrangement is arranged as Figure 5 shown, but with the 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.

[0436] In use, fuel is pumped from fuel tank 50 by low pressure fuel pump 1002. The fuel then flows through secondary fuel-oil heat exchanger 1004. Secondary closed loop oil system 2000’ is configured such that its recirculating oil flow also flows through secondary fuel-oil heat exchanger 1004. In standard operation of engine 10, at cruise conditions and at idle, the average temperature of the oil flow entering secondary fuel-oil heat exchanger 1004 is higher than the average temperature of the fuel flow entering secondary fuel-oil heat exchanger 1004. Secondary fuel-oil heat exchanger 1004 is configured 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 secondary fuel-oil heat exchanger 1004 is lower than the average temperature of the oil flow at the inlet of secondary fuel-oil heat exchanger 1004. In the same way, the average temperature of the fuel flow at the outlet of secondary fuel-oil heat exchanger 1004 is higher than the average temperature of the fuel flow at the inlet of secondary fuel-oil heat exchanger 1004.

[0437] The fuel then flows through primary fuel-oil heat exchanger 1006 and additionally through refrigeration cycle device 1007. Primary closed loop oil system 2000 is configured such that its recirculating oil flow also flows through primary fuel-oil heat exchanger 1006 and refrigeration cycle device 1007. In standard operation of engine 10, at cruise conditions and at idle, the average temperature of the oil flow entering primary fuel-oil heat exchanger 1006 is higher than the average temperature of the fuel flow entering primary fuel-oil heat exchanger 1006. Primary fuel-oil heat exchanger 1006 is configured 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 primary fuel-oil heat exchanger 1006 is lower than the average temperature of the oil flow at the inlet of primary fuel-oil heat exchanger 1006. In the same way, the average temperature of the fuel flow at the outlet of primary fuel-oil heat exchanger 1006 is higher than the average temperature of the fuel flow at the inlet of primary fuel-oil heat exchanger 1006.

[0438] Refrigeration cycle device 1007, when active (i.e., when powered on / opened and used to actively transfer heat from the oil to the fuel), can further increase the fuel temperature / further decrease the oil temperature, optionally raising the fuel to a temperature higher than the oil temperature. After flowing through primary fuel-oil heat exchanger 1006, the fuel flows to engine fuel pump 1003, which in the example shown is located downstream of primary fuel-oil heat exchanger 1006 and secondary fuel-oil heat exchanger 1004 and is arranged to deliver fuel to burner 16 of gas turbine engine 10.

[0439] Under cruise conditions, the average temperature of the oil flow through the secondary fuel-oil heat exchanger 1004 can be lower than the average temperature of the oil flow through the primary fuel-oil heat exchanger 1006. In this way, the fuel first passes through the heat exchanger 1004 with a lower average oil flow temperature before passing through the heat exchanger 1006 with a higher average oil flow temperature.

[0440] In addition to the branched oil flows 2000, 2000', the heat exchange system 3000 may also include a branched fuel return passage 6020 such that fuel returns to the main fuel path from which fuel enters the gas turbine engine 10 to the burner 16 at at least two different locations (e.g., upstream or downstream of the main engine pump 1003), as Figure 6 shown, the branched path 6020 diverges from the outlet 1006b of the primary heat exchanger 1006 from the main fuel flow path and rejoins the main fuel flow path downstream of the pump 1003. At least one valve (not shown) may be provided to control the diversion of the fuel flow returning from the heat exchanger 1006 through the engine 10 to the main fuel path. This valve can be controlled based on the fuel temperature, e.g., if the fuel temperature is relatively high and more likely to degrade the pump seal or other components, less fuel / more fuel is delivered to a location downstream of the pump via the pump 1003. The control of the fuel flow through the branched fuel return passage can be based on fuel temperature measurements (e.g., using temperature sensors at the heat exchanger 1006 and at a location downstream of the fuel, and potentially also using temperature measurements upstream of the fuel-oil heat exchanger). The return of the recirculated fuel upstream of the fuel-oil heat exchanger 1006 can allow for a reduction in the heat transfer from the oil to the fuel, thereby suppressing an instantaneous overshoot that may occur at the start of the landing phase. For example, at the start of the landing phase, the same amount of heat is generated within the oil system, but the fuel flow decreases, thus typically resulting in a temperature peak.

[0441] Figure 11 An exemplary configuration of the primary closed-loop oil system 2000 and the secondary closed-loop oil system 2000' is schematically shown, in which two separate recirculating oil flows form a heat exchange relationship through the oil-oil heat exchanger 2030. In this example, both of the oil loop systems 2000, 2000' have a branched arrangement of parallel pipes / heat exchangers.

[0442] At Figure 11In the example shown, the primary closed-loop oil system 2000 is configured such that the recirculating oil 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 divert the oil flow such that a portion of the oil flow goes to each of the main fuel-oil heat exchanger 1006 and the first air-oil heat exchanger 2020. The valve 2016 is controllable to regulate the proportion of oil delivered via each heat exchanger. In the illustrated embodiment, the first air-oil heat exchanger 2020 is in series with the oil-oil heat exchanger 2030, and the air-oil heat exchanger 2020 and the oil-oil heat exchanger 2030 are arranged in parallel with the main fuel-oil heat exchanger 1006. The modulating valve 2016 determines what proportion of the oil travels through each branch of the parallel arrangement. In various embodiments, any suitable portion of the oil flow can be diverted between the main fuel-oil heat exchanger 1006 and the first air-oil heat exchanger 2020. In some examples, the valve 2016' is operable to divert a fixed portion of the oil flow to each of the main fuel-oil heat exchanger 1006 and the first air-oil heat exchanger 2020. In other examples, the valve 2016 is operable to divert a variable portion of the oil flow to each of the main fuel-oil heat exchanger 1006 and the first air-oil heat exchanger 2020, for example using feedback from a temperature sensor to control the variable portion, and / or based on one or more fuel characteristics, as described below. The valve 2016 can be adjustable between a discrete number of set positions, or continuously adjustable. In some embodiments, for example, when the fuel temperature at the inlet of the burner 16 is relatively low compared to the maximum operating temperature of the fuel (e.g., based on knowledge of the fuel type or thermal stability), no oil can be delivered to the air-oil heat exchanger 2020, and all of the oil can be delivered via the fuel-oil heat exchanger 1006.

[0443] After flowing through the heat exchangers 1006, 2020, 2030, the oil flow in the primary closed-loop system 2000 is then recombined and delivered to the power gearbox 30 (and / or other engine components 33 such as the AGB) and then to the oil sump 2008. The scavenge pump 2010 then pumps the oil from the oil sump 2008 to the fuel tank 2002 for reuse.

[0444] The oil flow within the secondary closed-loop oil system 2000’ is arranged to form a heat exchange relationship with a separate oil flow within the primary closed-loop oil system 2000 through an oil-oil heat exchanger 2030. In the oil-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-oil heat exchanger 2030 is configured such that heat transfer can occur between the two separate oil flows. In this way, heat from the hotter oil flow can be transferred to the cooler oil flow within the oil-oil heat exchanger 2030. An air-oil heat exchanger is not shown in the illustrated secondary closed-loop oil system 2000’, but in other examples, an air-oil heat exchanger may be provided, such as in series with the oil-oil heat exchanger 2030 or on a third parallel branch.

[0445] In Figure 11 the illustrated embodiment, the secondary closed-loop oil system 2000’ is configured such that a recirculating oil flow is pumped by a 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 an 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 embodiments, the secondary closed-loop oil system 2000’ may not have a branched configuration. For example, the IDG fuel-oil heat exchanger 1004 and the oil-oil heat exchanger 2030 may be arranged in series such that at least a majority of the oil passing through one heat exchanger in a given cycle also passes through the other heat exchanger.

[0446] In other embodiments, an air-oil heat exchanger 2020 may be present on each closed-loop system, or only on the secondary closed-loop system, and / or an oil-oil heat exchanger may not be present.

[0447] In an example, any suitable portion of the oil flow can be diverted between the IDG fuel-oil heat exchanger 1004 and the oil-oil heat exchanger 2030. In an example, the valve 2016’ is operable 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 an example, 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 discrete number of set positions or continuously adjustable. After flowing through the heat exchangers 1004, 2030, the oil flow is then delivered to an integrated drive generator 2006 and then to a secondary oil sump 2008’. The secondary return oil pump 2010’ then pumps the oil from the secondary oil sump 2008’ to a secondary oil tank 2002’ for reuse.

[0448] One or more temperature sensors 1009 may be provided, such as being arranged to sense the fuel temperature at the inlet of the fuel pump 1003 or the burner 16. The controller 58 of the heat exchange system 3000 may use the data from one or more sensors, optionally in combination with other data (such as the fuel characteristics as described above), to manage the fuel and / or oil flow through and around the heat exchangers 1004, 1006, 2020, 2030 and / or to manage the air flow through the air-oil heat exchanger 2020. The controller 58 may be part of or provided by the EEC, or may be a separate unit. The control may be automatic, for example via the EEC.

[0449] Figure 16 A recirculation oil system is shown, which includes two loops 2000, 2000', but does not have a heat exchanger arranged on the parallel branches of either oil system as Figure 11 shown. Instead, each oil loop 2000, 2000' provides a main oil flow path in series through all the heat exchangers in that loop. To regulate the oil flow through the heat exchangers, one or more bypass pipes 2005, 2005', 2005a are provided in place of branch paths leading to different heat exchangers. It should be understood that in some embodiments, a combination of parallel arrangements of heat exchangers and bypass pipes may be used, and Figure 11 (both loops having parallel branches and no bypass pipes) and Figure 16 (all heat exchangers in parallel and having multiple bypass pipes) may be considered to show two different ends of the design range.

[0450] In Figure 16 the example shown, the fuel flow is represented by thick black lines to provide context for how the fuel and oil systems interact. Figure 16 The primary oil loop system 2000 shown in provides a series oil flow path that starts from the tank 2002, passes through the oil pump 2004, forward through the primary fuel-oil heat exchanger 1006, then through the air-oil heat exchanger 2020, then into the gearbox 30 (and optionally other components to be cooled and lubricated), then is collected in the oil sump 2008, and then is pumped back to the tank 2002 by the oil pump 2010. Thus, the two heat exchangers 1006, 2020 are arranged in series. In an alternative embodiment, the order of the two heat exchangers 1006, 2020 may be reversed such that the fuel-oil heat exchanger 1006 is after the air-oil heat exchanger 2020.

[0451] Figure 16The primary oil circuit system 2000 shown includes two oil bypass pipes 2005, 2005a. The first bypass pipe 2005 is arranged to allow 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 arranged to allow 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-oil heat exchanger 2020 and returns it to the main oil flow path before the path reaches the gearbox 30 (and optionally other components to be cooled and lubricated). In an embodiment of the primary oil circuit system 2000 having only one oil bypass pipe 2005a, the selected location may be the location of the second bypass pipe 2005a such that there is a bypass for the air-oil heat exchanger 2020 and no bypass for the fuel-oil heat exchanger 1006. This helps to absorb the heat in the oil into the fuel as safely as possible and maintain the high thermal efficiency of the engine by reducing the heat loss to the environment. In an embodiment having the air-oil heat exchanger 2020 before the fuel-oil heat exchanger 1006, the oil flow rate through the air-oil heat exchanger 2020 can be adjusted to provide a suitable oil temperature for the fuel-oil heat exchanger 1006. Having the bypass pipe 2005 on the fuel-oil heat exchanger 1006 can help to quickly adjust the oil flow rate ratio if there is a risk that the drop is lower than desired (e.g., depending on the determined fuel characteristics, a lower limit of a ratio significantly higher than zero can be set).

[0452] Depending on the fuel characteristics, the temperature limits set for some fuels (and indeed for some oils) may be strict, so it may be necessary to quickly adjust the flow rate to keep the temperature within the desired range. Having bypass pipes 2005, 2005a on both heat exchangers 1006, 2020 can prevent the oil from becoming too cold under certain conditions to avoid the risk of excessive oil condensation - it should be understood that this may be of more concern during cold start or ground idle than during cruising.

[0453] Figure 16The secondary oil circuit system 2000’ shown includes a single heat exchanger 1004 and a single bypass conduit 2005’, the heat exchanger being a secondary fuel-oil heat exchanger, and the bypass conduit being arranged to allow a portion of the oil to bypass the heat exchanger 1004. In other embodiments, the bypass conduit may not be provided on the secondary oil circuit system 2000’, or there may be more than one heat exchanger (e.g., an air-oil heat exchanger and / or an oil-oil heat exchanger in addition to the secondary fuel-oil heat exchanger 1004), and more than one bypass conduit may be provided, optionally one bypass conduit for each heat exchanger. A bypass valve 2007’ is again provided to control the oil flow through the bypass conduit 2005’.

[0454] The inventors recognize that using fuels different from traditional kerosene-based jet fuels, such as sustainable aviation fuels, may result in different fuel properties and can adjust decisions regarding desired heat transfer in operation, and accordingly decisions regarding set fluid flows through one or more heat exchangers, to take advantage of the different fuel properties. In particular, using one or more controllable valves to regulate oil flow and / or air flow and exercising careful control over these valves can achieve a method of providing improved oil cooling (since the fuel can absorb more heat), and can also improve the overall thermal efficiency of the engine, resulting in less heat loss to the environment while still ensuring safe operation. The controllable heat exchange system 3000, and in particular one or more controllable valves 2016, 2022, 2007, 2007’, 2007a, play a key role in managing fluid flow in such engines 10 and thus heat transfer.

[0455] In addition, the inventors recognize that while cruise conditions typically account for most of the flight time of an aircraft engine, idle operation is also important because the fuel mass flow rate during idle is much lower than during cruise, and even though the heat load of the fuel is relatively small, it can still cause temperature increases - thus, the use of non-traditional fuels may have a greater impact on the optimal heat management method under idle conditions. Figure 12 and Figure 13 Methods 100, 200 address these two scenarios of aircraft operation. Figure 12 Method 100 is shown for achieving these considerations under cruise conditions, and Figure 13 Method 200 is shown for achieving these considerations under idle conditions. Method 200 can be executed when the aircraft 1 is on the ground, i.e., during ground idle, such as when the aircraft is starting up, operating stationary during boarding, and taxiing (towards the runway or hangar, or between other ground-based positions), or during flight idle, such as when starting descent.

[0456] Each method 100, 200 is arranged to be performed in a geared gas turbine engine 10, which includes an engine core 11 that includes a turbine 19, a compressor 14, a burner 16 configured to combust fuel, and a shaft 26 that connects the turbine to the compressor; a fan 23 located upstream of the engine core; a gearbox 30 that receives an input from the shaft and outputs drive to the fan to drive the fan at a rotational speed lower than that of the shaft; an oil circuit system 2000 arranged to supply oil to the gearbox; and a heat exchange system 3000. The heat exchange system 3000 includes an air-oil heat exchanger 2020 through which oil in the oil circuit system flows; a fuel-oil heat exchanger 1006 through which oil and fuel in the oil circuit system 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) arranged to allow changing the proportion of oil conveyed through at least one of the heat exchangers 1006, 2020.

[0457] At least one of the oil valves 2016, 2007, 2007a is adjustable / controllable and is optionally controlled by a controller 58, which may form part of / is provided by the aircraft EEC. Thus, a controller 58 can be provided to effect such control. The oil valves 2016, 2007, 2007a can be adjusted between a discrete number of set positions or can be adjusted continuously.

[0458] The two heat exchangers 1006, 2020 can be arranged in the oil circuit system 2000 in a parallel arrangement or a series arrangement. Additionally, in some embodiments, more than two heat exchangers can be provided, such as the oil circuit system 2000 includes a plurality of fuel-oil heat exchangers 1006, a plurality of air-oil heat exchangers 2020, and / or one or more additional heat exchangers, such as one or more oil-oil heat exchangers, to exchange heat between separate oil circuit systems 2000, 2000'.

[0459] In a parallel arrangement, the oil circuit system 2000 can branch such that a proportion of the oil can flow along each branch, and the air-oil heat exchanger 2020 and the fuel-oil heat exchanger 1006 can be arranged in a parallel configuration on different branches of the oil circuit system, as Figure 9 and Figure 11 shown. In such embodiments, at least one valve arranged to allow changing the proportion of oil conveyed through at least one of the heat exchangers 1006, 2020 can be or include a modulating valve 2016 arranged to allow changing the proportion of oil conveyed through each branch.

[0460] In as Figure 16In the series arrangement shown, the oil circuit system 2000 includes one or more bypass pipes 2005, 2005a, each bypass pipe being arranged to allow a proportion of the oil to bypass one or more heat exchangers. In such embodiments, at least one valve arranged to allow variation of the proportion of oil conveyed via at least one of the heat exchangers 1006, 2020 may be or include one or more bypass valves 2007, 2007a. It should be understood that the bypass pipes 2005 and the bypass valves 2007 may also be arranged in parallel, and in such embodiments, at least one valve arranged to allow variation of the proportion of oil conveyed via at least one of the heat exchangers 1006, 2020 may be or include a modulating valve 2016 and bypass valves 2007, 2007a. Methods 100, 200 may thus include controlling 102, 202 a plurality of valves 2016, 2007, 2007a - such as 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. Additionally, the heat exchange system 3000 may include a plurality of separate oil circuits 2000, 2000', and methods 100, 200 may further include controlling the oil valves for additional heat exchangers 1004, 2030 of the second oil circuit 2000', or in the case of an oil - oil heat exchanger 2030, controlling the oil valves for both circuits.

[0461] In embodiments where the bypass pipe 2005a is arranged to pass through the air - oil heat exchanger 2020, the controlled bypass valve may be the valve 2007a for the bypass pipe 2005a passing through the air - oil heat exchanger 2020. In some such embodiments, the bypass pipe 2007a may be the only oil bypass pipe in the primary oil circuit system 2000, and no bypass pipe may be provided to the fuel - oil heat exchanger 1006.

[0462] In an alternative embodiment such as Figure 16 shown, the heat exchange system 3000 includes at least two oil bypass pipes, and optionally includes three or more oil bypass pipes, each bypass pipe 2005, 2005', 2005a being arranged to allow oil to bypass one of the heat exchangers 1006, 1004, 2020. Methods 100, 200 may include modulating the amount of oil conveyed via each bypass pipe 2005, 2005', 2005a. In embodiments having a plurality of bypass pipes in the same closed - loop oil system (e.g., as Figure 16 shown, having two bypass pipes in the primary oil circuit system 2000), the same bypass valve 2007 (which may be a three - way valve) may be used to control the flow through the two bypass pipes 2005, 2005a, or different bypass valves 2007, 2007a may be provided for each bypass pipe, as Figure 16as shown

[0463] In some embodiments, the heat exchange system 3000 further includes a refrigeration cycle device 1007 that is arranged to provide a heat boost by transferring additional heat from the oil to the fuel in addition to the heat transferred by the fuel-oil heat exchanger 1006. As part of methods 100, 200, the oil flow through the refrigeration cycle device 1007 can also be adjusted. In order to transfer heat from the oil to the fuel and thus take advantage of the increased heat capacity of many newer aviation fuels, a positive heat gradient (i.e., the oil is hotter than the fuel) is typically required between the oil and the fuel. As the thermal stability of the fuel increases, newer fuels may have a suitable operating temperature that is higher than the temperature that the oil reaches when leaving the gearbox 30 or other components, so a heat boost may be desirable to further increase the fuel temperature. A heat boost is the ability to transfer heat from a colder fluid to a hotter fluid (usually via a refrigeration cycle).

[0464] In some embodiments, the heat exchange system 3000 further includes a branched fuel return passage 6020 and at least one valve that controls the diversion of the fuel flow, as described above. The valve can be controlled based on feedback from one or more temperature sensors and / or based on one or more fuel characteristics. Methods 100, 200 can include adjusting the fuel flow along each branch based on the oil flow ratio and / or the fuel temperature when leaving the fuel-oil heat exchanger 1006. Thus, the engine 10 can have the ability to deliver heat (in the form of heated fuel) to the fuel system 1500 at different locations (e.g., upstream or downstream of the fuel pump 1003 or another component) - it should be understood that as the fuel temperature increases, the thermal capacity / heat resistance of the fuel system components may become a limiting factor, and returning the hot fuel to the main fuel flow path after rather than before the pump 1003 can help take advantage of the increased thermal stability / higher useful operating temperature of the new fuel.

[0465] Turning to method 100 that is specifically performed during cruise, method 100 includes controlling 102 at least one of the valves 2016, 2007, 2007a such that, under cruise conditions, the oil flow ratio:

[0466]

[0467] is in the range of 0 to 0.59, and optionally in the range of 0 to 0.50, 0 to 0.40, 0 to 0.30, 0 to 0.20, 0 to 0.10, or 0 to 0.05, and optionally in the range of 0 to 0.01. Optionally, the oil flow ratio can be in the range of 0.05 to 0.55. In some embodiments, the oil valves 2016, 2007, 2007a can be controlled such that the oil flow ratio is equal to zero, for example, by preventing any oil from flowing into the air-oil heat exchanger 2020.

[0468] It should be understood that even for a specific engine 10 operating with a set fuel, due to varying conditions (such as fuel temperature, atmospheric temperature, thrust demand, etc.), there is typically a range of values for this ratio during cruise. For example, the upper limit of this range may apply to cold days (ISA - 30 conditions) at low altitude (cruise: 35,000 ft), where the oil system heat generation is low, and the lower limit may apply to hot days (ISA + 40 conditions) at high altitude (cruise: 39,000 ft), where the oil system heat generation is high.

[0469] Method 100 may also include receiving 104 data to allow calculation or inference of the oil flow ratio, such as pump speed data, fuel flow data, and / or oil flow data. Such data may be received 104 by the controller 58 and used 106 to adjust the control 102 of one or more oil valves during cruise so as to maintain the oil flow ratio within a desired level or desired bounds. Such checking and adjusting / correcting 106 may be performed at fixed time intervals or in response to a predetermined stimulus (such as a change in the temperature or flow rate of the fuel or oil, or a change in engine operation (such as thrust demand) or altitude). These steps 104, 106 are alternatively considered to be part of the control 102 of one or more oil valves 2007, 2007a, 2016. Method 100 may also be arranged to utilize other information when determining 106 what control action to take, such as temperature data (oil, fuel, and / or ambient temperature of the environment around the aircraft 1), flow data (oil and / or fuel), and / or one or more fuel characteristics.

[0470] The step 102 of controlling at least one valve 2016, 2007, 2007a to adjust the oil flow ratio includes: in many embodiments, when the oil flow ratio is too high, reducing the amount of oil delivered via at least one air - oil heat exchanger 2020; optionally, in some embodiments, closing the valve / fully closing one passage of a three - way valve such that the oil flow rate into the air - oil heat exchanger 2020 drops to zero. In some embodiments, a minimum but non - zero oil flow through the air - oil heat exchanger 2020 may be maintained throughout operation to prevent oil from condensing within the air - oil heat exchanger 2020.

[0471] In some embodiments, one or more temperature sensors 1009 may be provided, and data related to the fuel temperature - optionally at the inlet of the burner 16 - may be used to fine-tune control decisions. Method 100 may include controlling 102 one or more fuel valves 2007, 2007a, 2016 under cruise conditions such that if the fuel temperature at the inlet of the burner 16 is at least 140 °C, the fuel flow ratio is in the range of 0 to 0.35, and optionally in the range of 0 to 0.25; and / or such that if the fuel temperature at the inlet of the burner 16 is at least 160 °C, the fuel flow ratio is in the range of 0 to 0.20, and optionally in the range of 0 to 0.15; and / or such that if the fuel temperature at the inlet of the burner 16 is at least 180 °C, the fuel flow ratio is in the range of 0 to 0.1, and optionally in the range of 0 to 0.075;

[0472] In some embodiments, when determining how to control 102 one or more fuel valves 2007, 2007a, 2016 (any of the above methods may be used to determine the fuel characteristics), one or more fuel characteristics may be considered. For example, method 100 may include controlling 102 at least one fuel valve 2007, 2007a, 2016 under cruise conditions such that if the fuel is at least 70% sustainable aviation fuel, the fuel flow ratio is in the range of 0 to 0.20, and optionally in the range of 0 to 0.15; and / or such that if the fuel is at least 80% sustainable aviation fuel, the fuel flow ratio is in the range of 0 to 0.1.

[0473] Turning to method 200 specifically performed at idle, method 200 includes controlling 202 at least one fuel valve 2016, 2007, 2007a such that under idle conditions, the fuel flow ratio:

[0474]

[0475] is in the range of 0.62 to 5.29, and optionally in the range of 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 fuel flow ratio may be in the range of 0.67 to 4.67. Method 200 may include controlling 202 at least one fuel valve such that under idle conditions, the fuel flow ratio is higher than 1.0, and optionally higher than 1.5 or 2.0. A controller 58 may be provided to implement such control 202, which is optionally an independent unit or part of an EEC.

[0476] As for method 100 performed during cruise, method 200 performed at idle may also include receiving 204 data to allow calculation or inference of the oil flow ratio, such as pump speed data, fuel flow data, and / or oil flow data. Such data may be received 204 by controller 58 and used 206 to adjust the control 202 of one or more oil valves 2007, 2007a, 2016 at idle so as to maintain the oil flow ratio within a desired level or desired bounds. This checking and adjustment / correction 206 may be performed at fixed time intervals or in response to a predetermined stimulus (e.g., a change in the temperature or flow of fuel or oil, or a change in engine operation (e.g., starting to taxi)). These steps 204, 206 may alternatively be considered part of the control 202 of at least one oil valve 2007, 2007a, 2016. Method 200 may also be arranged to utilize other information when determining 206 what control action to take, such as temperature data (the ambient temperature of the oil, fuel, and / or the environment around the aircraft 1), flow data (oil and / or fuel), and / or one or more fuel characteristics.

[0477] Similarly, the step of controlling 202 at least one valve 2016, 2007, 2007a to adjust the oil flow ratio may include reducing the amount of oil delivered via at least one air-oil heat exchanger 2020 when the oil flow ratio is too high; optionally, in some embodiments, closing the valve / fully closing one passage of a three-way valve such that the oil flow rate into the air-oil heat exchanger 2020 drops to zero. In some embodiments, a minimum but non-zero oil flow through the air-oil heat exchanger 2020 may be maintained throughout the idle operation to prevent oil from condensing within the air-oil heat exchanger 2020, but it should be understood that at ground idle, the ambient temperature is typically higher than at cruise altitude, so this may not be as necessary if at ground idle. Nevertheless, in cold weather / for airports or runways in cold climate regions, it may be desirable to keep the oil flowing to reduce the chance of condensation. Accordingly, the external / ambient temperature may be used as an input for oil flow rate control, and one or more temperature sensors may be provided.

[0478] In some embodiments, one or more temperature sensors 1009 may be provided, and data related to the fuel temperature (optionally data related to the fuel temperature at the inlet of the burner 16) may be used to fine-tune control decisions. Method 200 may include controlling 202 one or more oil valves 2007, 2007a, 2016 under idle conditions such that if the fuel temperature at the inlet of the burner 16 is at least 160 °C, the oil flow ratio is in the range of 0.62 to 4.0 or 0.62 to 3.0; and / or such that if the fuel temperature at the inlet of the burner 16 is at least 180 °C, the oil flow ratio is in the range of 0.62 to 3.0 or 0.62 to 2.0.

[0479] In some embodiments, one or more fuel properties may be considered when determining how to control 202 one or more fuel valves 2007, 2007a, 2016 (the fuel properties may be determined using any of the methods described above). For example, the method 200 may include controlling 202 at least one fuel valve 2007, 2007a, 2016 at idle conditions such that the oil flow ratio is within the range of 0.62 to 3.67, and optionally within the range of 0.62 to 3.67 if the fuel is at least 70% sustainable aviation fuel; and / or such that the oil flow ratio is within the range of 0.62 to 2.67 or 0.62 to 1.67 if the fuel is at least 80% sustainable aviation fuel.

[0480] Although about Figure 12 and Figure 13 The described methods 100, 200 optionally utilize knowledge of one or more fuel characteristics to adjust specific controls 102, 202, but such as with respect to Figure 14 Other aspects of the method 300 described are more focused on the determination and use of fuel properties to improve engine 10 performance by taking advantage of varying properties between aviation fuels.

[0481] Figure 14 The illustrated method 300 is performed in a gas turbine engine 10, essentially as described in the aforementioned methods 100, 200, but one or more controllable oil valves 2007, 2007a, 2016 are replaced by a controllable air valve 2022, or a controllable air valve is provided in addition to the controllable air valve, which is arranged to control the flow of air through the air-oil heat exchanger 2020.

[0482] The method 300 includes determining 302 at least one fuel property of a fuel arranged to be combusted by the combustor 16. The at least one fuel property may be or include any of the examples provided above, such as aromatic content (e.g., xylene, toluene, benzene, and / or phenol content), paraffin content (e.g., iso-paraffins, normal-paraffins, cyclo-paraffins), heteroatom species concentration (e.g., sulfur-based compounds / sulfur content and / or % nitrogen-based compounds, such as aniline or indole), % SAF, and hydrogen content (typically given as a H / C molar ratio). The determination 302 may optionally be performed using the fuel composition determination module 57 by any one or more of the methods described above, such as by chemical and / or physical detection and determination, by analyzing engine performance when using the fuel, and / or by receiving fuel data (e.g., electronically or by manual data entry).

[0483] Method 300 then includes controlling 304 air valve 2022 based on at least one fuel characteristic so as to adjust the flow rate of air through air-oil heat exchanger 2020. In various embodiments, air valve 2022 may be adjusted between a discrete number of set positions or may be adjusted continuously. Accordingly, controller 304 may include discrete or continuous adjustment. A controller 58 may be provided to implement this control 302, which may be a dedicated controller for valve 2022 and / or more generally for heat exchange system 3000, or may be part of a more general EEC. Method 300 may also be arranged to utilize other information (such as temperature data (oil, fuel, and / or ambient temperature of the surroundings of aircraft 1) and / or flow rate data (air, oil, and / or fuel)) to determine what control action to take. Controller 58 or another processing module may perform the determination of the appropriate control action.

[0484] As Figure 14 shown by the dashed line in, method 300 may optionally be repeated. In some embodiments, such as embodiments where there is only one fuel on aircraft 1, method 300 may be performed only once in a flight cycle, such as during fuel replenishment or upon reaching cruise altitude. However, in other embodiments, such as embodiments having multiple fuel sources, the fuel supplied to burner 16 may vary over time during flight. Accordingly, determination 302 may be performed more than once - for example:

[0485] (i) In embodiments having multiple fuel sources, where only one or the other fuel source is used at any given time (and knowledge of which fuel source the fuel is drawn from may prompt selection of appropriate stored fuel characteristics), perform the determination once for each fuel source 50, 53 at fuel replenishment / at the start of flight; or

[0486] (ii) Perform the determination frequently during flight, such as in response to a change in which (which) fuel source the fuel is drawn from (note that in some embodiments, the fuel supplied to burner 16 may be a mixture of fuels from different sources), or at fixed time intervals.

[0487] Accordingly, the air valve 2022 can be arranged to remain in a fixed position during operation of the engine 10, the fixed position being determined based on at least one determined fuel characteristic upon startup of the engine 10 (option (i) above). Alternatively, the air valve 2022 can be arranged to adjust its position during operation so as to vary the air flow to the air-oil heat exchanger 2020 during operation of the engine 10 during a single flight (option (ii) above). Accordingly, the air valve 2022 can be actively controlled to vary the air flow rate through the air-oil heat exchanger 2020, particularly in embodiments where the aircraft 1 carries multiple different fuels in different fuel tanks and the fuel (or fuel mixture) used during flight can be varied. The active control of the air valve 2022 can be automatic and implemented by the controller 58.

[0488] It should be understood that a fixed position of the air valve 2022 does not necessarily mean a fixed air flow rate, as the airspeed relative to the aircraft 1 can vary. During ground operation (e.g., ground idle), any variations may be caused solely by wind speed. During flight, the aircraft speed may also affect the air flow rate into the air-oil heat exchanger 2020. In some embodiments, the controller 58 can take into account the wind speed and / or the relative movement between the aircraft 1 and the surrounding air when determining the appropriate air valve position. Accordingly, in some cases, the position of the air valve can be varied to maintain a more constant air flow rate.

[0489] The method 300 can be performed at any point during operation of the aircraft 1, such as during ground idle, flight idle, and / or cruise.

[0490] If it is determined that step 302 indicates that the fuel is suitable for receiving additional heat (which additional heat is then no longer lost to the environment), the air valve 2022 can be controlled 304 so as to relatively greatly reduce the air flow rate compared to when the valve 2022 is fully open. For example, depending on the suitability of at least one determined fuel characteristic, method 300 can include controlling 304 the air valve 2022 such that the air flow rate into the air-oil heat exchanger 2020 is reduced to less than 60%, 50%, or 40% of the flow rate when the valve is fully open while the engine 10 is operating under idle conditions. A greater reduction can be made during cruising. For example, if the fuel is suitable for absorbing additional heat, when the engine 10 is operating under cruising conditions, the air flow rate into the air-oil heat exchanger 2020 can be reduced to less than 20% of the flow rate when the valve is fully open, and when operating under cruising conditions, the air flow rate can even be reduced to zero (closing the valve 2022). From another perspective, if the engine 10 subsequently operates with a fuel that is less capable of absorbing additional heat, a correspondingly large change will be made when the valve 2022 is reopened. It should be understood that the air flow rate during ground idle is typically much lower than that during cruising or flight idle. Therefore, the same percentage change in air flow rate may correspond to a much larger difference in air flow rate for the aircraft 1 during flight compared to when on the ground. Accordingly, data regarding the current aircraft operating mode (e.g., altitude and / or aircraft speed) and optionally data regarding the weather (e.g., wind speed) can also be considered in valve control.

[0491] In the exemplary geared engine 10, where the air-oil heat exchanger 2020 is located upstream of the fuel-oil heat exchanger 1006, flow modulation is performed only on the air side of the air-oil heat exchanger 2020 (i.e., there is no oil flow modulation). The following percentages of air flow modulation may be typical, where the "percentage of air flow modulation" is the percentage of the total air flow rate that would pass through the heat exchanger 2020 with the valve 2022 fully open (or effectively no valve present), based solely on the pressure ratio at the heat exchanger inlet 2020a and outlet 2020b.

[0492] ● During cruising, for a fuel with a temperature limit of 120°C, the percentage of air flow modulation can be 20% or 17%. For a fuel with a temperature limit of 170°C, this can linearly decrease with the temperature limit increasing to 0% (i.e., there is no air flow / valve closed at least for a part of the cruising). For a fuel with a temperature limit of 260°C or higher, throughout the cruise, the percentage of air flow modulation can be 0%, or the valve 2022 can be fully closed throughout the cruise.

[0493] ● At idle, especially at ground idle, for a fuel with a 120 °C temperature limit, the percentage of air flow modulation can be 58%. Once the fuel temperature limit reaches 250 °C, this can linearly decrease to 38% as the temperature limit increases, and once the fuel temperature limit reaches 315 °C, this can further linearly decrease to 28% as the temperature limit increases.

[0494] The percentage may depend on the fuel temperature limit. The fuel temperature limit is the maximum fuel temperature considered safe for the operation of the aircraft with that fuel and that engine 10, and may depend on fuel properties such as thermal stability and the heat resistance of engine components.

[0495] In some embodiments, at least one fuel property may be or include thermal stability, and during cruise, the air valve 2022 can be adjusted such that if the fuel is stable at a temperature above 160 °C, the air flow is reduced to less than 15% of the flow when the valve 2022 is fully open, and / or if the fuel is stable at a temperature above 180 °C, the air flow is reduced to less than 5% of the flow when the valve 2022 is fully open. In some embodiments where the valve 2022 is continuously adjustable, the air flow can vary continuously (optionally linearly) in proportion to the thermal stability.

[0496] In some embodiments, at least one fuel property may be or include the aromatic content in the fuel, and if the molar percentage of aromatics in the fuel is below 12%, and optionally below 10% or below 5%, the air valve 2022 can be adjusted to reduce the air flow during cruise to less than 5% of the flow when the valve is fully open.

[0497] In some embodiments, at least one fuel property may be or include the percentage of sustainable aviation fuel (SAF) in the fuel, and the air valve 2022 can be adjusted such that if the fuel has an SAF content above 60%, the air flow during cruise is reduced to less than 5% of the flow when the valve is fully open, and / or if the fuel has an SAF content above 80%, the air flow during cruise is reduced to less than 2% of the flow when the valve is fully open.

[0498] The auto-oxidation of conventional fossil fuel aviation fuels (which can lead to the varnishing or coking of burner nozzles and other fuel passages) typically begins to increase exponentially when the fuel temperature is in a range between 100 °C and 150 °C (depending on fuel characteristics). The inventors have recognized that SAF-based fuels can offer the opportunity to reach fuel temperatures above 150 °C, and optionally up to 200 °C, or even 250 °C or higher, without significant auto-oxidation. The maximum fuel operating temperature will depend on the fuel composition; thus, it is important to determine302 at least one fuel characteristic. Fuel blends containing SAF and conventional fossil fuels will again have different thermal stabilities and thus different temperature limits, depending on the specific characteristics of the blend. In tests of blends of SAF and conventional aviation fuel Jet A, it was found that an increase in the percentage of SAF mixed with Jet A increased the thermal stability, but in a non-linear manner. Thus, preset, discrete levels may be advantageous rather than continuously varying the air flow proportionally to the SAF content.

[0499] In some embodiments, at least one fuel characteristic may be or include the calorific value of the fuel, and if the fuel has a calorific value of at least 43.5 MJ / kg, the air valve 2022 can be adjusted to reduce the air flow during cruise to less than 4% of the flow when the valve is fully open, optionally depending on the suitability of another fuel characteristic (such as thermal stability).

[0500] In some embodiments, multiple fuel characteristics can be examined together, such as using a lower threshold of another fuel characteristic when one fuel characteristic is within a specific range or above / below a specific lower threshold.

[0501] Generally, more air may be required and the valve 2022 can be controlled304 to open more when the fuel thermal stability decreases, the fuel sulfur concentration increases (an example of a heteroatom species concentration, which is typically related to thermal stability as an increase in concentration generally decreases thermal stability), or if the fuel calorific value exceeds a threshold (for higher calorific value fuels), generally reducing the flow to achieve the same level of thrust without wasting fuel - the fuel in the fuel-oil heat exchanger 1006 can thus experience an increased temperature rise due to its reduced flow. However, in many cases, newer fuels with higher thermal stability (such as SAF) also have higher calorific values - thus, in some cases, the increased temperature rise due to the lower flow rate is acceptable; this demonstrates the utility of considering multiple fuel characteristics rather than a single fuel characteristic.

[0502] However, the inventors have recognized that allowing inappropriate fuel to reach these higher temperatures can be detrimental to engine performance and even potentially dangerous - excessive thermal decomposition of the fuel / deposits can cause safety problems by blocking passages and causing component failures (e.g., valve sticking and blocked nozzles, orifices, valves, etc.), which can lead to a loss of thrust control. Thus, Figure 15 The method 400 shown is provided as a safety precaution.

[0503] Figure 15 The method 400 shown in can be carried out in the gas turbine engine 10, substantially as described in the aforementioned oil flow ratio control methods 100, 200, but with one or more controllable oil valves 2007, 2007a, 2016 arranged to allow changing the proportion of oil delivered via at least one of the air - oil heat exchanger 2020 and the fuel - oil heat exchanger 1006 replaced by at least one controllable oil valve or air valve 2016, 2007, 2007a, 2022, which is arranged to allow changing at least one of the oil flow and the air flow through at least one of the air - oil heat exchanger 2020 and the fuel - oil heat exchanger 1006. Thus, air flow control or oil flow control, or both, can be provided. In embodiments where a controllable air valve 2022 is provided (optionally in addition to the controllable oil valves 2016, 2007, 2007a), the same engine 10 as used in the method 300 for air flow control can be used. The engine 10 used with this method 400 also requires a temperature sensor 1009, which is arranged to provide an indication of the fuel temperature (optionally at the inlet of the burner 16). More generally, the sensor 1009 can be located anywhere downstream of the fuel side of the fuel - oil heat exchanger 1006. The temperature sensor 1009 is typically located at or near the inlet of the burner 16, and more specifically near the nozzle inlet of the burner 16, so as to directly sense the fuel temperature at the burner inlet, but it should be understood that the change in fuel temperature between the outlet 1006b of the fuel - oil heat exchanger 1006 and the burner 16 may be small, and if this change is not considered insignificant, the fuel temperature at a location different from the sensor 1009 can be calculated based on knowledge of the engine 10. In some embodiments where the fuel temperature at the inlet of the burner 16 is used, the sensor 1009 can be located at a different position, and the fuel temperature at the burner inlet can be calculated based on the temperature output and knowledge of the engine 10.

[0504] Method 400 includes determining 402 whether the fuel temperature has increased above a set threshold under cruise conditions based on the output of temperature sensor 1009. For example, a direct comparison can be made between the data received from temperature sensor 1009 and one or more temperature thresholds stored in memory. Thus, this determination 402 can be performed automatically, optionally by controller 58 (which can be or include a dedicated processing module, or can be provided by a more general EEC).

[0505] Method 400 further includes, in response to determining that the fuel temperature has increased above a set threshold under cruise conditions, controlling 404 at least one of valves 2016, 2007, 2007a, 2022 so as to appropriately change at least one flow rate (air flow rate or oil flow rate) through at least one of heat exchangers 1006, 2020 to reduce the fuel temperature. For example, oil valves 2016, 2007 are adjusted so as to deliver less oil through fuel-oil heat exchanger 1006. For example, the oil can be diverted from a first branch of oil circuit system 2000 including fuel-oil heat exchanger 1006 and delivered via a parallel branch including air-oil heat exchanger 2020 using modulating valve 2016, rather than being delivered in a parallel configuration (as Figure 9 and Figure 11 shown), or it can be delivered only via bypass conduit 2005 to pass through fuel-oil heat exchanger 1006 and recombined with the remaining oil flowing through fuel-oil heat exchanger 1006 at or downstream of the oil outlet of heat exchanger 1006 using bypass valve 2007, as Figure 16 shown. In a parallel configuration as Figure 11 shown, reducing the oil flow through fuel-oil heat exchanger 1006 by controlling modulating valve 2016 can automatically increase the oil flow through air-oil heat exchanger 2020 (unless there is a bypass conduit around air-oil heat exchanger 2020 and it is used to compensate). In some embodiments, bypass valve 2007 and modulating valve 2016 can be used in combination.

[0506] As Figure 15As shown by the dashed lines in [the figure], the method 400 can be repeated - for safety reasons, optionally according to a regular schedule, and / or when triggered by changes in the operation of the engine (e.g., changes in altitude or thrust requirements), frequent checks may be beneficial. Thus, the oil valves 2016, 2007, 2007a or the air valve 2022 can be actively controlled 404 to vary the oil or air flow through the respective heat exchangers 1006, 2020. The active control of the valves 2016, 2007, 2007a, 2022 can be automatic and implemented by the controller 58. For example, the active control of modulating valve 2016 can be automatic and implemented by the controller 58 of the heat exchange system 3000, which can be a dedicated controller or part of a more general EEC. In various embodiments, based on the availability of online measurements, the control of modulating valve 2016 can be closed-loop or open-loop - in particular, when feedback data (e.g., from an oil flow rate sensor) is available, a closed-loop control method may be preferred.

[0507] In an embodiment having an air valve 2022 and oil valves 2016, 2007a, when more oil is conveyed via the air-oil heat exchanger 2020, the air flow through the air-oil heat exchanger 2020 can be increased; as the oil flow rate increases, this increase can be linear. In terms of the percentage of air flow, the increase can be significant - for example, from the air valve 2022 being fully closed to fully open, or when the fuel temperature is considered too high, from a first position of 10% of the maximum air flow to 90% or 100% of the maximum air flow. The method 400 can 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 convey more air through the air-oil heat exchanger 2020, and controlling 404b the oil valves 2016, 2007 to convey less oil via the fuel-oil heat exchanger 1006. The control / regulation 404a of the air valve 2022 can be performed simultaneously with the control / regulation 404b of the oil valves 2016, 2007.

[0508] In some embodiments, the air flow through the air-oil heat exchanger 2020 may not be actively controlled. In such embodiments, based on the pressure ratio between the heat exchanger inlet 2020a and the heat exchanger exhaust / outlet 2020b, the air flow may vary naturally under different operating conditions. In this configuration, even when the air-oil heat exchanger 2020 is completely bypassed on the oil side / when the oil flow rate through the heat exchanger 2020 is zero, air will still flow through the air channels of the air-oil heat exchanger 2020. In an alternative embodiment, active control of the air flow through the air-oil heat exchanger 2020 is provided, such as Figure 14as described in method 300 shown. For example, the air-oil heat exchanger 2020 and the fuel-oil heat exchanger 1006 can be in series, with no bypass on the oil side of the air-oil heat exchanger 2020, but an air valve 2022 for controlling the air flow on the air side (an oil bypass valve 2007' and an air flow valve 2022 can be provided in other embodiments). In such a series arrangement, the air-oil heat exchanger 2020 can be upstream or downstream of the fuel-oil heat exchanger 1006 with respect to the oil flow. In embodiments having such a series arrangement, having the air-oil heat exchanger 2020 upstream of the fuel-oil heat exchanger 1006 with respect to the oil flow can help prevent overheating of the fuel - the oil can be cooled as much as possible by increasing the air flow (and optionally also recirculating the oil through the air-oil heat exchanger 2020) before the oil reaches the fuel-oil heat exchanger 1006.

[0509] In some embodiments, the oil bypass pipe 2005a and the corresponding control valve 2007a can be implemented only for the air-oil heat exchanger 2020. Optionally, in the described method 400, only the oil flow through the air-oil heat exchanger 2020 can be actively controlled.

[0510] In various embodiments:

[0511] ● The bypass pipe 2005a is provided only on the oil side of the air-oil heat exchanger 2020 (no bypass of the oil of the fuel-oil heat exchanger and no active air flow control);

[0512] ● Bypass pipes 2005a, 2005 can be provided for both the air-oil heat exchanger 2020 and the fuel-oil heat exchanger 1006 (no active air flow control); or

[0513] ● The air flow through the air-oil heat exchanger 2020 can be controllable and there can be no oil bypass pipe.

[0514] Generally speaking, controlling at least one of the flows through the air-oil heat exchanger 2020 can be considered more important than controlling either of the flows through the fuel-oil heat exchanger 1006. However, in some embodiments, an oil bypass pipe 2005 and / or a fuel bypass pipe 1005 (and the corresponding control valves) can be provided for the fuel-oil heat exchanger 1006, optionally in addition to one or more other controllable valves.

[0515] Combinations of air flow control and oil flow control can be achieved in both parallel and series arrangements, but it should be understood that while all the various control options can be used together, the increased complexity and number of components and thus the increased weight of the entire heat exchange system 3000 may be undesirable. Thus, in many embodiments, it may be preferable to select a subset of control options - for example, controlling the air flow or controlling the oil flow can be used to regulate heat transfer within the air-oil heat exchanger 2020, and thus using both controls for a given heat exchanger 2020 may be considered redundant. Similarly, in embodiments where the oil flows through the air-oil heat exchanger 2020 before the fuel-oil heat exchanger 1006, regulation of the oil temperature in the air-oil heat exchanger 2020 (by controlling the air flow and / or oil flow in that heat exchanger) can be used as an alternative to regulating the oil flow through the fuel-oil heat exchanger 1006 to vary the heat transferred to the fuel in the fuel-oil heat exchanger 1006.

[0516] Additionally or alternatively, in embodiments where the heat exchange system 3000 includes a refrigeration cycle device 1007, which is arranged to transfer additional heat from the oil to the fuel in addition to the heat transferred by the fuel-oil heat exchanger 1006, method 400 may further include controlling the refrigeration cycle device 1007 to reduce the additional heat transferred to the fuel in response to determining 402 that the fuel temperature has increased above a set threshold during cruise conditions. This control can be performed by reducing the oil flow through the refrigeration cycle device 1007, or by reducing the power of the refrigeration cycle device 1007 (e.g., by reducing the flow rate of the refrigerant fluid), or by deactivating the refrigeration cycle device 1007. The set threshold for deactivation of the refrigeration cycle device 1007 can be lower than the set threshold for changing the flow rate of the oil and / or air through the heat exchangers 1006, 2020.

[0517] Additionally or alternatively, in embodiments where the heat exchange system 3000 includes a secondary oil circuit system 2000' that also includes a fuel-oil heat exchanger 1004, the method may further include reducing the 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 for reducing the oil flow through the primary fuel-oil heat exchanger 1006).

[0518] The set threshold for controlling step 404 can be in the range of 140°C to 300°C, and optionally in the range of 200°C to 300°C, and further optionally in the range of 250°C to 300°C. For example, the set threshold can 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.

[0519] The method 400 of some embodiments further includes determining 401 a set threshold, and comparing the output of a temperature sensor with the set threshold. This determination 401 can be performed on-wing, and optionally in-flight. The determination 401 can be or include calculating a threshold, or identifying a suitable threshold from a pre-stored set of thresholds stored in a memory.

[0520] The inventors recognize that knowledge of one or more fuel properties can be used in the determination 401 to ensure safety while still taking advantage of the varying nature between aviation fuels. Thus, the determination 401 can be based on at least one fuel property of the fuel. The at least one fuel property can be or include any of the examples above, such as the thermal stability of the fuel, the nitrogen content of the fuel, the sulfur content of the fuel, and / or the sustainable aviation fuel (SAF) content (% SAF) of the fuel. The at least one fuel property can be determined by any one or more of the methods above.

[0521] The step 401 of determining the set threshold can include optionally linearly increasing the set threshold as the fuel thermal stability increases. Thus, the set threshold for controlling the thermal modulation can linearly increase with the fuel thermal stability. Thermal stability refers to the temperature at which the fuel begins to decompose and form deposits that can cause component blockages and failures - it should be understood that this is generally not an instantaneous change at a specific temperature; aviation fuels include various components that are prone to decomposition at different temperatures, and the time spent at high temperatures is also a factor in fuel decomposition. The threshold can be set based on a comparison of the maximum allowable deposition rate of fuel decomposition with the thermal stability of the fuel in use. If, at a given temperature, the decomposition rate of the fuel is below a certain threshold, the fuel can be classified as stable at that temperature. The method 400 can also include using a clock or timer, or a fuel flow sensor - for example, at higher pumping rates, a higher fuel temperature is acceptable because the faster-moving fuel spends less time at that temperature before combustion.

[0522] Alternatively or additionally, for fuels with a SAF content higher than 70%, the step of determining the 401 set threshold may include increasing the set threshold (optionally linearly) as the SAF content of the fuel increases. As described above, it has been found that an increase in % SAF blended with fossil-based aviation fuel increases thermal stability, but in a non-linear manner. Therefore, a pre-set, discrete threshold based on % SAF may be advantageous rather than continuously varying the set threshold proportionally to the SAF content, or a more complex (non-linear) but continuous relationship may be established.

[0523] Alternatively or additionally, the step of determining the 401 set threshold may include decreasing the set threshold (optionally linearly) as the content of heteroatom species in the fuel increases. For example, it has been found that the thermal stability of Jet A decreases as the nitrogen content increases (the nitrogen content being a measure of the amount of nitrogenous substances present). It is well known that the interaction between sulfurous and nitrogenous substances in fuels can be an important contributor to the fuel decomposition rate, and thus considerations regarding the fuel composition of multiple heteroatom species can be implemented to account for these interactions.

[0524] In some embodiments, particularly those in which a direct measure of the thermal stability of the fuel is not available, multiple fuel properties may be determined and used in the decision-making for valve control. One or more of the determined fuel properties can be effectively translated into a measure of thermal stability.

[0525] The inventors recognize that in addition to carefully controlling heat exchangers 1004, 1006, 2020, 2030 as described above to manage heat transfer within engine 10, incorporating a refrigeration cycle 1007 into the heat exchange system 3000 can allow for more benefits to be obtained from the newer fuels. In particular, the heat lift provided by the refrigeration cycle device 1007 can be used to raise the fuel to a temperature higher than that of the oil, thereby improving the cooling of the oil and overall engine performance. It should be understood that power input is required to transfer heat from a cooler fluid to a hotter fluid - and thus an active evaporation and condensation process with a power component is used in the described embodiments. In particular, the oil circuit system 2000 can be configured to transfer heat from the oil to the refrigerant fluid in evaporator 1007a, which evaporates the liquid (which can be all liquid, or a liquid-gas mixture at this stage of the cycle) in the refrigerant fluid to form a saturated vapor.

[0526] A compressor 1007b is then provided to compress the saturated vapor to form superheated vapor. This hot vapor is then delivered to a refrigerant-fuel heat exchanger 1007c; this heat exchanger is a condenser 1007c, and the heat transfer from the compressed vapor to the fuel converts the superheated vapor at least partially back to a liquid (and optionally converts it to a saturated liquid). The cooler refrigerant then returns to the evaporator 2007a, optionally passing through an expansion valve 2007d (between the condenser and the evaporator / interface with the oil) to convert the saturated liquid into a mixture of a liquid and vapor at a reduced temperature, and then returns to the evaporator 1007a.

[0527] Control of this refrigeration cycle device 1007 can allow adjustment of the provided heat lift level according to fuel and engine operating conditions.

[0528] Figure 17 A method 500 for controlling a gas turbine engine 10 to utilize heat lift to manage engine performance is shown. The method 500 is performed in an engine 10 substantially as described herein (note that the same principles can apply to direct drive engines as well as geared engines), and has a refrigeration cycle device 1007. The method 500 includes controlling 502 the refrigeration cycle device 1007 such that the fuel temperature at the inlet of the burner 16 is higher than the highest oil temperature within the oil circuit system 2000.

[0529] The control 502 can be performed at fixed time intervals (correcting the heat transfer to the fuel as needed), or can be applied continuously to keep the fuel temperature higher than the highest oil temperature. The control 502 can be applied to any stage of engine operation, such as idle and / or cruise, and optionally throughout the entire operation.

[0530] The control 502 can include using the evaporator 1007a to transfer 502 heat from the oil to the refrigerant fluid. The flow rate of the oil and / or refrigerant through the evaporator 1007a can be adjusted as needed, typically using one or more valves. A bypass pipe and / or a recirculation pipe can be provided for the oil around the evaporator 1007a such that the oil flow rate through the evaporator 1007a can be adjusted without changing the oil flow rate through other components. Alternatively or additionally, the evaporator 1007a can be located on a separate parallel branch of the oil system 2000, and a modulating valve 2016 can be used to adjust the proportion of the oil delivered through the evaporator 1007a. In some embodiments, the pump speed of the oil pump 2004 can be adjusted to change the oil flow rate. The flow rate of the refrigerant through the evaporator 1007a can be controlled by controlling the pumping rate of the refrigerant around the refrigerant cycle device 1007.

[0531] Control 502 may then include compressing 502b the saturated refrigerant vapor generated in evaporator 1007a using refrigerant compressor 1007b to form superheated vapor. Also, the refrigerant flow rate may be controlled by controlling the pumping rate of the refrigerant around refrigerant cycle device 1007 - in Figure 18 the illustrated embodiment, compressor 1007b serves as a pump, but in other embodiments, a separate refrigerant pump may be provided in addition to compressor 1007b. The superheated vapor formed in compressor 1007b is then conveyed to condenser 1007c.

[0532] Control 502 may then include transferring 502c heat from the superheated vapor to the fuel using condenser 1007c. Due to this heat transfer, the refrigerant vapor is at least partially converted to a liquid (i.e., it condenses). Saturated liquid may be formed in condenser 1007c.

[0533] In Figure 18 the illustrated embodiment, refrigerant cycle device 1007 further includes expansion valve 1007d, which is located downstream of condenser 1007c (from the perspective of refrigerant flow) and upstream of evaporator 1007a. In such embodiments, method 500 includes using 502d expansion valve 1007d to convert the refrigerant from condenser 1007c into a temperature - reduced liquid and vapor mixture, which then returns to evaporator 1007a. As part of control step 502, expansion valve 1007d may be actively controlled 502d.

[0534] In Figure 18 the illustrated embodiment, engine 10 includes: temperature sensor 1009, which is arranged to sense the fuel temperature downstream of refrigerant cycle device 1007 (i.e., at its location near burner 16); and temperature sensor 2009, which is arranged to provide an indication of the maximum temperature of the oil within oil circuit system 2000 (e.g., directly downstream of the most downstream engine component cooled by oil). Method 500 may also include comparing 501 the oil temperature and the fuel temperature, and adjusting control 502 of refrigerant cycle device 1007 based on this comparison. For example, if the fuel temperature is lower than the maximum oil temperature, the pump speed of refrigerant cycle device 1007 (and thus the refrigerant flow rate) may be increased and / or the proportion of oil conveyed via evaporator 1007a may be increased (e.g., by adjusting an oil valve).

[0535] 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 valve or modulating valve). Additionally or alternatively, the fluid flow rate through the fuel - oil heat exchanger 1006 may be adjusted 504 to increase heat transfer to the fuel by recirculating the fuel through the fuel - oil heat exchanger 1006 (e.g., using a recirculation valve 6010). Additionally or alternatively, before the oil enters the fuel - oil heat exchanger 1006, by reducing the air cooling of the oil (e.g., using an air valve 2022), the heat transfer to the fuel in the fuel - oil heat exchanger 1006 can be increased 504. It should be understood that if the fuel temperature is too high, the opposite adjustment 504 can be made. Although in Figure 17 the control step 504 for components other than the refrigeration cycle device 1007 is shown after the control step 502 for the refrigeration cycle device 1007, it should be understood that these steps 502, 504 can be performed in any order or simultaneously. Additionally, in some embodiments, only the refrigeration cycle device 1007 may be controlled 502 in response to a comparison 501 of the fuel temperature and the oil temperature.

[0536] The temperature comparison step 501 can be performed at fixed time intervals throughout the operation or in response to certain stimuli, such as a change in the engine operation mode, a change in altitude, a change in thrust demand, a change in fuel, or a change in the fuel or oil temperature.

[0537] In various embodiments, method 500 includes controlling 502 the refrigeration cycle device 1007 such that the fuel temperature at the inlet of the burner 16 is at least 2 °C higher than the highest oil temperature within the oil circuit system 2000, and optionally at least 5 °C, 10 °C, 15 °C, 20 °C, or 25 °C higher. The refrigeration cycle device 1007 can be controlled 502 such that the fuel temperature at the inlet of the burner 16 is between 2 °C and 50 °C higher than the highest oil temperature within the oil circuit system 2000.

[0538] Method 500 may include controlling 502, 504 the engine 10 to operate at a fuel temperature at the inlet of the burner 16 that is higher than the highest oil temperature for at least 10%, 20%, 30%, or 50% of the time during cruise. In some embodiments, throughout the cruise, the fuel temperature at the inlet of the burner 16 may be maintained higher than the highest oil temperature.

[0539] In some embodiments, the control 502 is based on one or more fuel properties of the fuel (e.g., any of the above - exemplary fuel properties, which may be determined by any of the above methods).

[0540] Method 500 may include controlling 502 the refrigeration cycle device 1007 (and optionally also controlling 504 one or more other components of the heat exchange system 3000) such that the fuel temperature at the inlet of the burner 16 is higher than the highest oil temperature within the oil circuit system 2000 by an amount determined based on one or more fuel properties of the fuel. For example, when the thermal stability and / or calorific value of the fuel is relatively high, a greater increase in fuel temperature above the oil temperature may be provided. In some embodiments, two or more fuel properties may be evaluated in combination. Thus, method 500 may include obtaining one or more fuel properties and adjusting the controls 502, 504 based on these properties, optionally along with other data such as operating mode, altitude, ambient temperature, oil type, oil and / or fuel flow rate, refrigerant type, etc.

[0541] Method 500 is independent of the engine architecture and can be applied to geared gas turbine engines 10 and direct drive gas turbine engines. The oil circuit system is arranged to supply oil to cool and optionally also lubricate one or more engine components - in a geared engine 10, the one or more engine components to be cooled may be or include the power gearbox 30. In a geared drive engine or a direct drive engine, the one or more engine components to be cooled may be or include the accessory gearbox 33, one or more bearings (such as bearings for a shaft), power electronics, one or more components of an integrated drive generator, and / or any other component of the engine to be cooled.

[0542] It should be understood that the present invention is not limited to the above-described embodiments and various modifications and improvements can be made without departing from the concepts described herein. Unless mutually exclusive, any feature can be used alone or in combination with any other feature, and the present disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.

Claims

1. A method of operating a gas turbine engine for an aircraft, the gas turbine engine comprising: an engine core comprising a turbine, a compressor, a combustor arranged to combust a fuel, and a spindle connecting the turbine to the compressor; a fan located upstream of the engine core; a gearbox receiving input from the spindle and outputting drive to the fan so as to drive the fan at a slower rotational speed than the spindle; an oil circuit system arranged to supply oil to the gearbox; as well as A heat exchange system, the heat exchange system comprising: an air-oil heat exchanger, through which the oil in the oil circuit system flows; a fuel-oil heat exchanger through which the oil and the fuel in the oil circuit system flow so that heat is transferred between the oil and the fuel; as well as at least one valve arranged to allow varying the proportion of said oil delivered via at least one of said heat exchangers, The method comprises controlling the at least one valve so that, under idle conditions, the oil flow ratio is: In the range of 0.62 to 5.

29.

2. The method according to claim 1, wherein: The oil circuit system is branched so that a certain proportion of the oil can flow along each branch, and the air-oil heat exchanger and the fuel-oil heat exchanger are arranged in a parallel configuration on different branches of the oil circuit system, and wherein the valve arranged to allow changing the proportion of the oil delivered via at least one of the heat exchangers is a modulating valve arranged to allow changing the proportion of the oil delivered via each branch.

3. The method according to claim 1, wherein: The oil circuit system further comprises at least one bypass conduit arranged to allow a proportion of the oil to bypass at least one of the air-oil heat exchanger and the fuel-oil heat exchanger, and wherein the valve arranged to allow variation of the proportion of the oil delivered via at least one of the heat exchangers is a bypass valve arranged to allow a proportion of the oil to bypass the at least one heat exchanger.

4. The method according to claim 3, wherein: The air-oil heat exchanger and the fuel-oil heat exchanger are arranged in series in the oil circuit system.

5. The method according to claim 3, wherein: The air-oil heat exchanger and the fuel-oil heat exchanger are arranged in parallel on different branches of the oil circuit system, and wherein the method comprises controlling both the bypass valve and a modulation valve, the modulation valve being arranged to allow varying the proportion of the oil delivered via each branch.

6. The method of claim 1, comprising controlling the at least one valve such that under idle conditions the oil flow ratio is below 5.

50.

7. The method of claim 1, comprising controlling the at least one valve such that under idle conditions the oil flow ratio is below 5.

0.

8. The method of claim 1, comprising controlling the at least one valve such that under idle conditions the oil flow ratio is below 4.

5.

9. The method according to claim 1, wherein: Controlling the at least one valve to adjust the oil flow ratio includes reducing an amount of oil delivered via at least one air-to-oil heat exchanger when the oil flow ratio is too high.

10. The method according to claim 1, wherein: The method comprises controlling the at least one valve 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 burner is above 160°C.

11. The method according to claim 1, wherein: The method comprises controlling at least one valve under idle conditions 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 burner is above 180°C.

12. The method according to claim 1, wherein: At idle conditions, the method includes controlling the at least one valve such that if the fuel is at least 70% sustainable aviation fuel, the heat transfer ratio is in the range of 0.62 to 3.

67.

13. The method according to claim 1, wherein: The heat exchange system further comprises a refrigeration cycle device arranged to provide a heat lift by transferring additional heat from the oil to the fuel in addition to the heat transferred by the fuel-oil heat exchanger, and wherein the method further comprises controlling the refrigeration cycle device so as to adjust the amount of additional heat transferred to the fuel.

14. The method according to claim 1, wherein: The heat exchange system comprises a plurality of bypass conduits, each bypass conduit being arranged to allow oil to bypass a heat exchanger, and wherein the step of controlling the at least one valve comprises controlling at least two bypass valves.

15. A gas turbine engine for an aircraft, the gas turbine engine comprising: an engine core, the engine core comprising a turbine, a compressor, and a spindle connecting the turbine to the compressor; a fan located upstream of the engine core; as well as a gearbox receiving input from the spindle and outputting drive to the fan so as to drive the fan at a slower rotational speed than the spindle; an oil circuit system arranged to supply oil to the gearbox; as well as A heat exchange system, the heat exchange system comprising: an air-oil heat exchanger, through which the oil in the oil circuit system flows; a fuel-oil heat exchanger through which the oil and the fuel in the oil circuit system flow such that heat is transferred between the oil and the fuel; and at least one valve arranged to allow varying the proportion of said oil delivered via at least one of said heat exchangers, And wherein the at least one valve is arranged to be controlled such that, under idle conditions, the oil flow ratio is: In the range of 0.62 to 5.

29.

16. The gas turbine engine according to claim 15, wherein: The heat exchange system further comprises a refrigeration cycle device arranged to provide a heat boost by transferring more heat from the oil to the fuel such that the fuel temperature is raised above the oil temperature.

17. The gas turbine engine of claim 15, wherein: The turbine is a first turbine, the compressor is a first compressor, and the spindle is a first spindle; The engine core also includes a second turbine, a second compressor, and a second spindle connecting the second turbine to the second compressor; and The second turbine, the second compressor and the second spindle are arranged to rotate at a higher rotational speed than the first spindle.

18. The gas turbine engine of claim 15, wherein: The heat exchange system also includes a branch fuel return passage and at least one valve for controlling the diversion of the fuel flow, wherein the branch passage is arranged to return fuel from the heat exchange system to at least two different locations along a main fuel path, from which the fuel enters the gas turbine engine to the combustor.

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