Leakage detection in gas-fuelled power plants

Abstract

A power system for a vehicle comprising a gas-fuelled power plant (2), one or more fuel delivery devices (26) for delivering fuel to the power plant, a fuel tank (34) configured to provide fuel to the one or more fuel delivery devices; a shut-off valve (38) positioned between the fuel tank and the one or more fuel delivery devices thereby defining a fuel volume between the shut-off valve and the one or more fuel delivery devices. A controller (11) is configured to: detect a transition of the power system to a coasting state; operate the shut-off valve to isolate gas in the fuel volume; determine the pressure of gas within the fuel volume at a first sample time; determine the pressure of gas within the fuel volume at a second sample time; determine a change in gas quantity and / or the rate of change in gas quantity within the fuel volume between the first sample time and the second sample time based on the determined gas pressure at the first and second sample times; and trigger a response action if the change and / or the rate of change of gas quantity exceeds a predetermined threshold. An advantage of the invention is that it provides a more accurate approach to determine possible leakage situations associated with the common rail.

Description

[0001] LEAKAGE DETECTION IN GAS-FUELLED POWER PLANTS

[0002] Technical Field

[0003] The examples of the invention relate to systems, method and approaches for detecting leaks and emissions from gas-fuelled power plants, and particularly internal combustion engines fuelled at least partially with hydrogen.

[0004] Background

[0005] Gaseous fuels such as hydrogen are promising alternative fuels to gasoline and diesel due to their potential for low or zero carbon emissions. However gaseous fuels present some challenges relating to their containment and handling.

[0006] Hydrogen, for example, has a very low density which makes it challenging to contain, transport and use in the context of a fuel system for an internal combustion engine. In general, following deactivation of a hydrogen-fueled engine, it is desirable to prevent any leakage of hydrogen as it presents a safety risk due to its combustibility and it is also a greenhouse gas.

[0007] It is with these issues in mind that the embodiments of the invention have been devised.

[0008] Summary of the Invention

[0009] Against this background, examples of the invention provide a power system for a vehicle comprising a gas-fuelled power plant, one or more fuel delivery devices for delivering fuel to the power plant, a fuel tank configured to provide fuel to the one or more fuel delivery devices; a shut-off valve positioned between the fuel tank and the one or more fuel delivery devices thereby defining a fuel volume between the shut-off valve and the one or more fuel delivery devices. The power system further comprising a controller configured to perform the following steps when the power system is in a power-producing state: detect a transition of the power system to a coasting state; operate the shut-off valve to isolate gas in the fuel volume; determine the pressure of gas within the fuel volume at a first sample time; determine the pressure of gas within the fuel volume at a second sample time; determine a change in gas quantity and / or the rate of change in gas quantity within the fuel volume between the first sample time and the second sample time based on the determined gas pressure at the first and second sample times; and trigger a response action if the change and / or the rate of change of gas quantity exceeds a predetermined threshold.

[0010] An advantage of the invention is that it provides a more accurate approach to determine possible leakage situations associated with the common rail which is less susceptible to temperature changes. Moreover, leakages may be at a wide range of rates, to measuring the rate of change of gas quantity in the isolated fuel volume is more effective at detecting low leakage rates.

[0011] In determining the quantity of gas in the isolated fuel volume, the controller may be configured to detect gas pressure within the fuel volume and base the determination of gas quantity on the detected gas pressure and one or more further parameters. The one or more further parameters may include a determined temperature level, which may be determined using a temperature sensor attached to the common rail. Other temperature sensing means may be provided.

[0012] In an example, in determining the gas pressure at the first and second sample times, the controller may further be configured to determine the quantity of gas in the fuel volume at the first sample time based on the gas pressure determined at the first sample time; and determine the quantity of gas in the fuel volume at the second sample time based on the gas pressure determined at the second sample time.

[0013] It is envisaged that determining the gas quantity in the fuel volume at each of the first and second sample times before determining the change in gas quantity and / or rate of change thereof may be beneficial because it can guard against the effects of temperature changes since temperature data is taken into account when the calculations are performed.

[0014] In another aspect, examples of the invention reside in a gas-fuelled engine system comprising: a gas-fuelled engine, a fuel tank and one or more fuel injectors; and a shut-off valve positioned between the fuel tank and the one or more fuel injectors so as to define a fuel volume between the shut-off valve and the one or more fuel injectors. The method comprises detecting a transition of the engine system from a power-producing state to a coasting state and, in response, to: activate one or more spark plugs associated with the gas-fuelled engine system; monitor the occurrence of combustion associated with the gas-fuelled engine system; and trigger a response action if combustion is detected.

[0015] The detection of a combustion event may be based on receiving a signal indicative of engine vibration such as from an engine knock sensor or a signal indicating that engine torque is being produced, such as from a crank speed sensor.

[0016] It will be appreciated that in this aspect of the invention, the controller is configured to monitor for the occurrence of combustion rather than monitor the quantity of gas in the fuel volume, as is the case in the previously described example. However, in both aspects, the controller is solving a technical problem of how to determine gas leaking in the engine system in response to a power on request for the engine system. Both aspects of the invention therefore provide alternative solutions.

[0017] The invention extends to and therefore embraces control methods carried out by the controller as defined above.

[0018] Preferred and / or optional features of the first aspect of the invention may be combined with other aspects of the invention as appropriate.

[0019] Further optional and advantageous features are referenced in the detailed description and the appended claims.

[0020] BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Examples of the invention will now be described with reference to the following figures:

[0022] Figure 1 is a schematic view of a gas-fuelled internal combustion engine, being an example of a power plant to which the examples of the invention apply;

[0023] Figure 2 is a flow chart illustrating an example algorithm that may be implemented by a controller of the gas-fuelled internal combustion engine for detecting leakages. Figure 3 is a chart depicting the quantity of gas in an isolated fuel volume of the internal combustion engine of Figure 1 and how that quantity may vary over time during a leak.

[0024] Detailed

[0025] In general, the examples of the invention provide approaches for detecting the leakage of gaseous fuel in a power plant, particularly an internal combustion engine, before that power plant has been activated to produce power. In this way, leakage of gas can be detected at an early stage in operation of the power plant and measures can be taken to put the power plant into a safe state as soon as possible.

[0026] T o put the examples of the invention into technical context, a discussion of an internal combustion engine which is fuelled with gaseous hydrogen will now be described with reference to Figure 1 .

[0027] In overview, an internal combustion engine system 2 comprises an engine block 4, an air inlet system 6, a fuel delivery system 8 and an exhaust system 10. The engine system 2 further comprises a controller or control unit 11 which is adapted to receive data input 11.1 to sense operational parameters of the engine and to provide suitable control output signals 11 .2 to the engine system 2 to control its operation based on driver demands, as is conventional. It will be appreciated that the internal combustion engine system 2 may form part of an automotive vehicle (not shown).

[0028] The engine block 4 of the illustrated example comprises four combustion chambers 12, or cylinders, in an ‘inline’ configuration. However, it should be noted that this is for illustrative purposes only and the engine block may comprise any suitable number of combustion chambers in any suitable configuration, as would be well understood by the skilled person. Common engine configurations are single cylinder engines, twin cylinders, triples, in-line sixes or V-sixes, and V8 engines. Herein, the term ‘combustion chamber’ will be considered synonymous with ‘cylinder’.

[0029] The air inlet system 6 comprises an air inlet 14 which feeds fresh air into a network of air pipes 16 through an air filter 17. An air mass flow sensor 18 is provided to provide data to the control unit 11 about the airflow entering the engine system 2. The network of air pipes 16 feeds incoming air through a compressor 19 and, subsequently, to an intercooler 20. The functionality of the compressor 19 and the intercooler 20 are known in the art so a further discussion will not be provided. The network of pipes 16 leads from the intercooler 20 through a throttle valve 22 to an air inlet duct or ‘manifold’ 24. As is known, the air inlet manifold 24 directs fresh air to each of the combustion chambers 12 of the engine block 4 via separate air channels.

[0030] The fuel delivery system 8 comprises a set of one or more fuel injectors 26 (only one of which is labelled) that are arranged to inject combustible fuel, in this case hydrogen gas, into the fresh air flowing into the cylinders 12.

[0031] In the illustrated example, there are a plurality of fuel injectors 26, the number of which corresponds to the number of combustion chambers 12. Each of the fuel injectors 26 is arranged to inject fuel into the air inlet manifold 24 at a dedicated channel which leads to a respective one of the cylinders 12.

[0032] Other arrangements are possible. For example, a single fuel injector may be arranged at a relatively upstream position. However, such an arrangement is generally less desirable as it affords less control over the quantify of fuel that is injected into the combustion chamber associated with each fuel injector 26. Furthermore, in some other examples the fuel injectors 26 may be arranged to inject fuel directly into a respective one of the combustion chambers 12.

[0033] The fuel injectors 26 are each connected to a fuel accumulator or ‘common rail’ 28. As is known, the common rail 28 provides a relatively large volume of fuel which is maintained at a predetermined, and controllable, pressure level which means that the fuel injectors 26 are connected to a source of fuel having a pressure level that is in essence static and is not affected by their operation. It should be noted, however, that the fuel pressure within the common rail 28 can be modified in use due to various requirements that are beyond the scope of this discussion.

[0034] The pressure of fuel within the common rail 28 is determined by the control unit 11 by means of a fuel pressure sensor 30. The fuel pressure sensor 30 is shown as being connected to the end of the common rail 28 which has an elongated shape, in this example. However, the shape of the common rail 28 and the relative position of the pressure sensor 30 are configurational aspects that are not central to the invention.

[0035] Fuel and air mixture in the cylinders 12 is ignited by respective spark plugs 31 , in the usual manner.

[0036] The common rail 28 is supplied with fuel by a fuel supply system 32. The fuel supply system 32 includes a pressurised fuel source or reservoir 34, a pressure regulating device 36, a shut-off valve 38 and a gas supply line 40 which connects the shut-off valve 38 to the common rail 28. In some examples, the shut-off valve 38 may be connected directly to the common rail 28 although it is usual for a length of gas supply line 40 to be present so that a desired separation distance may be achieved between the engine system 2 and the fuel supply system 32. The pressurised fuel source 34 or ‘fuel tank’ may suitably be configured to store hydrogen gas at an appropriate pressure level, which typically is between 350 and 700 bar, whereas the pressure regulating device 36 is configured to reduce the gas pressure in the fuel tank 34 to a pressure suitable for injection, which may be between 5 bar and 10 bar but could be much higher for some systems, for example direct injection systems. It should be noted that the configuration of the fuel supply system 32 is simplified for the purposes of this discussion and more components would be present in a practical system. However, the components shown here are the principal components relevant to the examples of the invention. Furthermore, the pressurised fuel source 34 may comprise more than one fuel tank.

[0037] Further sensing means may be provided for the control unit 11 in order for it to operate the engine system 2 effectively. In the illustrated example, the engine block 4 is equipped with a knock sensor 41. As is known in the art, a knock sensor provides a means to detect high frequency vibration of the engine block 4 from which a determination can be made about whether combustion has occurred within the combustion chamber 12. Knock sensors, or at least software associated with such sensors, are able to discriminate between combustion occurring in different ones of the combustion chambers 12. Optionally, individual knock sensors may be associated with each cylinder of the engine block 4. A knock sensor is conventional technology and so further discussion will be omitted. The engine system 2 further includes an air pressure sensor 42 which is configured to provide the control unit 11 with data relating to the pressure of air within the air inlet manifold 24.

[0038] The engine system 2 further includes a crank position sensor 44 which is configured to provide the control unit 11 with data relating to the position and rotational speed of the crankshaft. It should be noted that the crankshaft, pistons, intake valves and exhaust valves are not shown on Figure 1 , but their presence is implied. Data from the crankshaft position sensor 44 may be used by the control unit 11 to control fuel injection and ignition timing. Common mounting positions for the crankshaft position sensor 44 include on the engine flywheel (not shown), the camshaft (not shown) or the main crankshaft pulley (not shown). The crankshaft position sensor 44 is shown as being associated with the engine block 4 in Figure 4, for ease of illustration.

[0039] A temperature sensor 45 may also be provided on or associated with the common rail 28 to provide the functionality of providing a measurement of the temperature of the gas within the common rail 28 as a data input 11.1 to the control unit 11. The temperature sensor 45 is shown here connected to the common rail 28 but other positions would be acceptable, for example attached to the fuel supply line 40 or the shut-off valve 38. The functionality of the temperature sensor 45 and the functionality of the pressure sensor 30 may also be combined into a single unit or package. Such temperature sensing functionality may also be determined by a suitable temperature sensing algorithm that predicts the gas temperature based on ambient temperature, engine loading, tank temperature and other suitable factors.

[0040] Note that the fuel pressure sensor 30, the knock sensor 41 , the air pressure sensor 42 and the crank position sensor 44 may communicate with the control unit 11 in a conventional manner to provide it suitable data input 11.1. This may be achieved by suitable wired connections, or through the connection of a CAN-bus (Controller Area Network) which is conventional in automotive technology.

[0041] The engine system 2 further comprises a starter motor 46 which is configured to turn the crankshaft (not shown) in order to initiate self-sustaining power-producing operation of the engine system 2. Turning now to the exhaust system 10, combustion gases from the combustion chamber 12 feed into an exhaust duct or ‘manifold’ 50 which combines the gas out flow into a single pipe which leads to a turbine 52. As is known, the turbine 52 is connected to the compressor 19 and, together, the turbine 52 and the compressor 19 constitute a turbocharger of the engine system 2. Turbochargers provide a means to increase the density of the charge of air delivered to the cylinders 12 thereby providing more efficient and powerful combustion. However, their use is not essential to operation. Turbochargers are known in automotive technology so a full discussion will not be provided here for the sake of brevity.

[0042] It should be noted that in the above discussion, the fuel delivery system 8 is configured into a ‘port injection’ arrangement which means that the fuel injectors 26 are arranged to inject fuel into the air inlet manifold 24 so that the injected fuel, in this hydrogen gas, is mixed with fresh air in the inlet manifold before entering the cylinders 12 of the engine block 4.

[0043] The skilled person would appreciate that the engine system 2 that is the focus of the above discussion has been simplified for present purposes and that in practice an engine system would be more complex. However, the illustrated engine system 2 is intended to demonstrate the principal components and subsystems that are relevant to the examples of the invention.

[0044] As has been discussed above, the control unit 11 is operable to perform various engine monitoring and control objectives to manage the performance of the vehicle into which it is installed. The general operation of the control unit 11 would be well known to the skilled person and is outside of the scope of this discussion.

[0045] One challenge associated with hydrogen-fuelled engines is the potential for leaks to occur.

[0046] Figure 2 illustrates an example algorithm or method that can be implemented by the control unit 11 in order to detect whether leaks of hydrogen gas have occurred from the engine system 2. More specifically, the method depicted in Figure 2 is suitable to be performed whilst the engine system 2 is running in the sense of producing output power, albeit the engine system 2 may on occasion enter into an ‘overrun state’ during which zero or negative torque (i.e. engine braking) is output from the engine system 2. An overrun state may be known by various terminology, such as a ‘cruising state’ or ‘coasting state’, such terms being used here synonymously.

[0047] At this point, it should be appreciated that the control unit 11 may be any suitable control environment provided by the engine system 2. The control unit 11 may be the “engine ECU” of the engine system or it may be another control unit which is configured to carry out other performance and monitoring tasks within the engine system 2 of the broader vehicle. In particular, the control unit 11 may be a control environment provided specifically for the purposes of performing the method.

[0048] Irrespective of the functionality of the control unit 11 , it will be appreciated that the control unit has the necessary memory, processing environment and communications interface to be integrated into the engine system and the broader system of the associated vehicle. For example, the engine control unit 11 may include a memory component 13. The memory component 13 may store data such as self-learnt control parameters and operating history data which can be retrieved by the ECU even after a power down cycle. These specific parts of the control unit 11 are not shown in the Figure, but their presence is implied.

[0049] In accordance with an example of the invention, a leak detection method 100 begins at an initial condition indicated by step 102 at which point the engine system 2 is in a power-producing state. In a power-producing state, the engine system 2 is considered to be operational in the sense that the fuel injectors 26 are injecting fuel into the manifold 24 such that the cylinders 12 are receiving a suitable air-fuel mix and the spark plugs 31 are operating so as to combust the air-fuel mix therefore generating mechanical torque for driving the associated vehicle.

[0050] The conditions at step 102 may exist for a significant period as the vehicle is being driven. However, as is the case in a typical driving pattern, from time to time the engine system 2 may transition to a coasting state. This may be for various reasons; for example, when the user of the vehicle releases their foot from the accelerator pedal, e.g. when going downhill or allowing the vehicle to slow down naturally, or when the vehicle has come to a standstill and is idling. In off-road applications, such a transition may occur in the context of an electrical generator driven by a gas-powered engine system which is initially under load but then is required to change its power output, for example by reducing output voltage and / or frequency. In such an event, the engine system of the generator would cease fuelling in response to change in torque demand from a non-zero level to a zero-torque demand. The engine system 2 would therefore enter a coasting state for a period of time.

[0051] During the time the engine system 2 is in a power-producing state, the control unit 11 is adapted to detect or sense a transition to a non-power-producing state, as discussed above. This is represented by method step 104.

[0052] The detection of a transition to a non-power producing state acts as an initial condition for triggering a set of diagnostic subroutines, as indicated at method steps 106, 108 and 110.

[0053] As shown in Figure 2, method steps 106,108 and 110 are run concurrently as this approach provides an efficient way of determining whether a leak is present promptly so that appropriate action can be taken. Each of the method steps 106,108,110 may provide individual techniques to determine that a leak of gas fuel is present in the engine system 2 and can in principle be processed individually. However, in the illustrated example the subroutines are run at the same time to provide a more reliable approach to determine a leak. It is envisaged that the method 100 may be configurable by a vehicle manufacturer so that one or more of the subroutines may be triggered selectably for various reasons.

[0054] Referring firstly to method step 106, at this step the control unit 11 is configured to monitor the quantity of gas within the fuel volume isolated in the common rail 28 and the fuel supply line 40, herein after referred to simply as the ‘isolated fuel volume’. To do so, the control unit 11 is configured to determine the quantity of gas in the isolated fuel volume at two or more sample times that are spaced apart temporally. To cater for high leakages rates, a relatively fast sample time separation is envisaged to be most suitable, for example between 5ms and 15ms, for example 10ms.

[0055] At this point in the method 100, the control unit 11 also configures the shut-off valve 38 into a closed position, thereby isolating the gas within the common rail 28 and the fuel supply line 40. This is indicated at method step 105. Moreover, the fuel injectors 26 are configured into a disabled state and the spark plugs 31 are activated, since the engine system 2 is in a non-power-producing state. Note that the control unit 11 may control the shut-off valve 38 directly, or indirectly, by way of a different control unit. The functionality of step 106 is also illustrated in more detail in the chart of Figure 3. In Figure 3, the gas quantity in the isolated fuel volume is shown on the Y-axis and time is represented on the X-axis.

[0056] As can be seen, prior to time T 1 the gas quantity in the common rail 28 is at a high level as the engine system 2 is running in a power-producing state. At time T1 , the control unit 11 detects the transition to a non-power producing state and the control unit 11 commands the shut-off valve 38 to close, thereby isolating gas in the fuel volume, as has been discussed above.

[0057] Once the shut-off valve 38 has been closed, at T1 , the control unit 11 determines the quantity of gas in the fuel volume at a first data sample time T2, as indicated at Q1 . For clarity of Figure 2, time T2 is shown spaced temporally from time T 1 . The quantity of gas that is determined is based on a measured pressure at the sample time T1 by way of rail pressure sensor 30. The exact timing of T1 may vary. For example, in Figure 3 the timing of T2 is shown temporally spaced from T1 , which may provide time for the gas pressure in the isolated fuel volume to settle. In other examples, however, T2 may be very close to or may coincide with T 1 .

[0058] Following a preset time interval, shown here as T_delta, the control unit 11 makes a further determination of gas quantity in the fuel volume at second sample time T3, as indicated at Q2, based on a corresponding pressure measurement using rail pressure sensor 30. The time interval T_delta is selected in order to provide a suitable time period to detect an appreciable drop in gas quantity but to avoid false positives and increasing the length of the analysis time. It is envisaged that a suitable time period for T_delta is between 5ms and 15ms, as mentioned above.

[0059] It should be noted that the quantity of gas in the fuel volume may be determined by use of the ideal gas law, as mentioned above, because the volume of the common rail 28 and the fuel supply line 40 are known, and the temperature of the common rail 28 can also be determined using the temperature sensor 45. This approach may be particularly suited to when the pressure is relatively low e.g. around 1300kPa and temperature is relatively high (e.g. above critical point for hydrogen). In another approach, a density lookup table may be used which may be suitable for any pressure and temperature combinations. A suitable density table may be stored in the control unit 11 with pressure and temperature as the two axes. The control unit may then determine the density of the gas from pressure and temperature measurements with standard linear interpolation. Density data can be sourced from experimental work or published sources. Gas quantity can thus be determined from the density data because volume is a known quantity.

[0060] As the skilled person will appreciate, by application of the ideal gas law, it is possible to calculate the quantity or ‘amount’ of gas using the relationship PV = nRT, where P is pressure (e.g. in Pa), V is volume (e.g. in m3), R is the gas constant (e.g. 8.31JK'1mol-1), and T is temperature (in Kelvin), and ‘n’ is the quantity of gas in moles. Thus, the term ‘n’ represents the amount of gas in moles, but may also be expressed as the total mass of gas (in kg) divided by the molar mass (in kgs per mole). So, in this discussion, the term ‘quantity’ of gas is used to refer to the determination of the amount of gas, in moles or a comparable metric. This molar quantity may also be expressed as a mass value derived from the quantity value, in moles, divided by the molar mass, as would be understood by a skilled person. As would also be understood by a skilled person the quantity of gas may also be expressed as a volume of gas when considered at the standard conditions of temperature and pressure for comparative purposes. It is known that standard conditions zero degrees Celsius and 1 bar pressure (105Pa). Thus, the change of quantity of gas and flow rate may be expressed in terms of standard cubic centimetres per minute (SCCM) which equates to 1cm3sTp / min, where ‘STP’ relates to the standard temperature and pressure i.e standard conditions. As would be understood SCCM is a measure of the mass flow rate of a fluid, i.e. volumetric flow rate. So, from the ideal gas law equation as reference above, it is possible to convert the volume of gas into the standard condition, so that the calculated volume of gas at the standard conditions can be derived as: where Vscc is the calculated volume of standard cubic centimetres (at STP), P0 is 1 bar, TO is 273.15K, P is measured pressure and V is the known volume of the system (i.e. the isolated fuel volume as has been discussed). So, it should be noted that the calculated volume at STP is the equivalent to mass or mole number because the density is known at the defined standard conditions. At this point, the control unit 11 is configured to carry out suitable calculations so that it can be determined whether a leak is present.

[0061] As a first determination, the control unit 11 is configured to evaluate the change in gas quantity in the isolated fuel volume. This is illustrated as Q_delta in Figure 3. If the change is gas quantity between T2 and T3 exceeds a predetermined leakage gas threshold, then the control unit 11 can infer that a leak is present. The threshold may be set to take into account a ‘dead band’ to avoid false positives due to measurement noise on the signal.

[0062] As a second determination, the control unit 11 is configured to determine the rate of change of gas quantity in the isolated fuel volume between T2 and T3. This can be achieved by dividing the change in gas quantity Q_delta by the time interval T_delta. The rate of change of gas quantity can provide an indication of the severity of the leak from the isolated fuel volume. If the rate of change of gas quantity exceeds a predetermined leakage rate threshold, then the control unit 11 can infer that a leakage is present.

[0063] The leakage rate of gas may be determined in units of standard cubic centimetres per minute, ‘seem’ which is also expressed as cm3STP / min which defines the volume of gas corrected to a standard pressure and temperature.

[0064] The first and second determinations may be performed as alternatives or may be performed together. It is believed that the second determination can provide a more accurate determination that a leakage is present and, possibly, provides the option of determining where the leak is located based on the severity of the rate of change of gas quantity.

[0065] It will be noted that since the shut off valve 38 is closed, the fuel pressure sensor 30 is measuring the quantity of gas not just in the common rail 28 but also in the fuel supply line 40. Therefore, it is the entire volume of fuel isolated between the shut-off valve 38 and the injectors 26 that is being measured. Since this volume may be considerable, and since the fuel supply line 40 may extend over a significant length (e.g. from the fuel tank 34 to the common rail 28 which may be a different ends of the vehicle), and due to the additional factor of variable temperature, determining the quantity of gas present rather than simply the gas pressure is believed to provide a more accurate determination. In this connection, it should be noted that the leakage from the common rail 28 and associated component may be at a wide range of rates, in which case a slow leakage rate may mean that checking just the pressure of gas in the fuel volume might not indicate a problem. Checking the rate of change of gas quantity is considered to be more effective at identifying slow leakage rates.

[0066] In the above discussion, it will be noted that the difference in gas quantity at time T2 and time T3 is negative, in the sense that the quantity of gas reduces. However, it is possible that the quantity of gas may increase. This may be the case where, for example, the fuel injectors are in good working order and therefore do not leak, but the shut-off valve 38 may itself exhibit some leakage. In such a case, then the quantity of gas in the isolated fuel volume will increase between T2 and T3. The control unit 11 is therefore adapted to detect reductions in gas quantity and also increases in gas quantity. This information can be used to identify the component that is likely to be leaking.

[0067] If, as a result of the above determinations, the control unit 11 determines that a leak is present, at step 112, then the method proceeds to step 114 where the engine system 2 is put into a safe state. This may involve the control unit 11 putting the engine into a limp home mode at a reduced power level, or providing an alert to the driver that that the vehicle must be stopped immediately. In stationary power applications, the engine system 2 may simply be deactivated. It should be noted that the response to the determination that a leak is present may be configurable in the system. Moreover, the response that the control unit 11 implements after detecting a leak may be dependent on the severity of the leak. For example, is the determined leak is relatively small, for example based on detection of a low leakage rate or a relatively low intensity combustion, then the control unit 11 may issue a suitable warning to an associated vehicle HMI but still allow the vehicle to be driven. Still further, the performance of the vehicle may be reduced, or an immediate shut down of the supply system may be necessary.

[0068] As a further measure, the control unit 11 may be configured to control suitable purging equipment to purge the gas within the isolated fuel volume so that the system can be considered as in a safe state. Such purging processes and hardware are outside the scope of this discussion. However, the process of purging the gas from the fuel volume may involve feeding the purged gas through a reactor to reduce the concentration of hydrogen in the air, or to combust the hydrogen.

[0069] In the above calculations, it has been described that gas quantity is determined at sample times T2 and T3 sequentially, based on pressure readings, and then the change in gas quantity is calculated. However, alternatively the pressure readings may be taken at sample times T2 and T3 and then, subsequently, the gas quantity at those sample times may be determined. It should also be noted that sample time T2 and T3 are examples of two sample times and that many more sample times may be generated to provide continuous monitoring over a longer time period. In principle, it is also acceptable to determine the difference in gas pressure between the sample times T2 and T3 and to determine the leakage rate of gas quantity from the gas pressure readings. In this sense, data samples may be taken for as long as the engine system is in a non-power-producing state, which in many circumstances is expected to last only momentarily when the user lifts their foot off the throttle for example.

[0070] For example, the time period between T2 and T3, or T_delta, may vary appreciably, from the order of milliseconds to a few seconds. It is envisaged, however, that the samples T3 and T4 as discussed above will constitute two samples of a continuous string of samples during which the gas quantify is calculated continuously over a predetermined time period in order to monitor for a leak. Note that the time for a leak to cause a noticeable pressure drop can vary significantly, for example from a few cubic centimetres per minute to several litres per minute. Correspondingly, the time it takes for a leak to cause a noticeable gas quantity drop can vary a lot depending on leakage rate. Therefore, it is believed that a relatively high data sampling rate (e.g. around 10ms, as mentioned above) extending over a period of, say 1 to 3 seconds, provides a useful approach for detecting a wide range of leakages. However, the concept may also be performed adequately by two data samples taken a predetermined duration apart, e.g. 1 second or 2 seconds apart.

[0071] In the above discussion it should be appreciated that the control unit 11 is monitoring the state of the engine system 2 for a transition back to a power-producing state. For example, the control unit 11 may monitor the drive torque demand signal. This is illustrated by the parallel thread indicated as step 118. If torque demand is detected at any point, then the process will terminate, the control unit 11 will open the shut off valve 38, as indicated at step 120, and the process will return to step 102 with the engine system 2 being in a power-producing state, thereby ending the non-power- producing state. The parallel monitoring at step 118 runs throughout the monitoring of gas quantity, but also during monitoring of combustion, which will be discussed further below.

[0072] If a leak is determined not to be present, then the process terminates at step 122. The process would resume when the engine system 2 returns to the power-producing state at step 102. In alternative examples, it is envisaged that the method 102 may continue to loop through to step 106 to continue to monitor gas quantity.

[0073] Turning now to method steps 108 and 110, as mentioned above, each of these method steps provides an alternative way to detect combustion occurring in one of the cylinders 12 of the engine. Since the fuel injectors 26 are not injecting fuel into the cylinders 12, any vibration from the engine block 4 that is consistent with a combustion event whilst the engine system 2 is in a coasting state can be attributed to a fuel leakage from the common rail 28 into the cylinders 12. This may be due to faulty injector components, for example.

[0074] At method step 108, the control unit 11 activates the spark plugs 26 and monitors the output from the one or more knock sensors 41 associated with the engine block 4. As is known, the one or more knock sensors 41 monitor for structure- borne vibrations from the engine block 4 and convert these to electrical voltage signals which are suitably filtered and evaluated by the control unit 11. The control unit 11 is configured to apply a predetermined combustion threshold that is indicative of combustion occurring within the combustion chambers 12. The combustion threshold may be a suitable level of acceleration that is detectable by the one or more knock sensors 41.

[0075] At method step 110, the control unit 11 activates the spark plugs 26 and monitors the output from the crank position sensor 44. As is known, the crank position sensor 44 provides a signal indicative of the crank position, for example, by use of an optical encoder sensing techniques, inductive sensing techniques and Hall effect sensing techniques. The crank position signal is then processed to provide crank speed / acceleration. The control unit 11 monitors the crank data and applies a suitable speed / acceleration threshold that is indicative of combustion occurring within the combustion chambers 12, thereby providing an engine torque indicator signal. The monitoring that is implemented during method steps 108 and 110 may be applied for a set time period, for example 1 to 2 seconds, the precise length of which would be a calibratable value.

[0076] The control unit 11 applies suitable thresholds at decision step 124 to identify the occurrence of combustion following method steps 108 and 110.

[0077] If combustion is detected, then the method flows to step 114 as discussed above at which the control unit 11 configured the engine system 2 into a safe state, as has been discussed above.

[0078] If combustion is not detected over the monitoring period of method steps 108 and 110, then the method terminates at step 126.

[0079] The skilled person would understand that various modifications may be made to the specific examples of the invention discussed above without departing from the scope of the invention as defined by the claims. Some variants have been discussed above. Others will now be discussed below.

[0080] In the above discussion, the monitoring of gas quantity at step 106 is shown as being preceded by closing of the shut-off valve 38, whereas method steps 108 and 110 are not preceded by closing of the shut-off valve 38. The position of the shut-off valve 38 is therefore not important for the correct functioning of method steps 108 and 110 and it may be in open or closed positions. If method steps 108 and 110 are carried out concurrently with step 106 then they may be performed with the shut off valve 38 in the closed position. It is envisaged that in other examples, the control unit 11 may be configured such that the method steps 106, 108 and 110 may be performed as selected by settings that are configurable by the manufacturer, for example. It will also be appreciated that in the illustrated example the monitoring of gas quantity at step 106 has been discussed as occurring concurrently with combustion monitoring steps 108 and 110 during a monitoring period. However, this is not essential to the invention and it is also envisaged that the gas quantity monitoring (step 106) may take place independently from method steps 108 and 110, in which case the engine cranking step 104 need not take place. It will therefore be understood that the determination of a leak via monitoring of gas quantity as covered by method steps 106 and 112 may be run independently from the determination of a leak via monitoring of engine combustion as covered by method steps 108, 110 and 124. In the above discussion, the operation of the shut-off valve 38 between open and closed positions does not mean that there is a physical disconnection of the fuel supply line 40, but that the flow of gas between the fuel tank and the fuel supply line 40 / common rail 28 is severed or blocked such that there is no exchange of fuel.

[0081] In the above discussion, the engine system 2 is an example of a power plant that uses a gaseous fuel such as hydrogen. However, other power plants that use gaseous fuels, such as hydrogen fuel cells, also apply to examples of the invention as the same leakage issues are relevant. Therefore, the internal combustion engine system 2 and a hydrogen fuel cell are two examples of gas-fuelled power plants to which examples of the invention apply. In the case of a hydrogen fuel cell, the function of the injectors 26 as generic fuel delivery devices will be taken by equivalent components such as valves, injectors or other flow control devices which control the flow of gas to the fuel cell from a fuel reservoir at a known pressure, to which they are attached either directly or indirectly.

Claims

CLAIMS:1 . A power system for a vehicle comprising: a gas-fuelled power plant (2), one or more fuel delivery devices (26) for delivering fuel to the power plant, a fuel tank (34) configured to provide fuel to the one or more fuel delivery devices; a shut-off valve (38) positioned between the fuel tank and the one or more fuel delivery devices thereby defining a fuel volume between the shut-off valve and the one or more fuel delivery devices, the power system further comprising a controller (11) configured to perform the following steps when the power system is in a powerproducing state: detect (104) a transition of the power system to a coasting state; operate (105) the shut-off valve to isolate gas in the fuel volume; determine (106) the pressure of gas within the fuel volume at a first sample time; determine (106) the pressure of gas within the fuel volume at a second sample time; determine (106,112) the change in gas quantity and / or the rate of change in gas quantity within the fuel volume between the first sample time and the second sample time based on the determined gas pressure at the first and second sample times; and trigger (114) a response action if the change and / or the rate of change of gas quantity exceeds a predetermined threshold.

2. The power system of Claim 1 , wherein the step of detecting the transition of the power system to a coasting state comprises detecting a zero-torque demand of the power system.

3. The power system of Claims 1 or 2, wherein following the power system transitioning to a coasting state, the controller is configured to monitor for the power system transitioning back to a power-producing state.

4. The power system of Claim 3, wherein in the event that the controller detects that the power system has transitioned back to the power-producing state, the controller is configured to operate the shut-off valve to permit gas to flow into the fuel volume.

5. The power system of any one of the preceding claims, wherein, in performing the response action, the controller is adapted to configure the power plant into a safe state.

6. The power system of Claim 5, wherein, in configuring the power plant into the safe state, the controller is adapted to purge the gas contained in the fuel volume.

7. The power system of any one of the preceding claims, wherein, when the power system is in a coasting state, the one or more injectors are deactivated.

8. The power system of any one of the preceding claims, wherein the controller is further configured to: determine the quantity of gas in the fuel volume at the first sample time based on the gas pressure determined at the first sample time; and determine the quantity of gas in the fuel volume at the second sample time based on the gas pressure determined at the second sample time.

9. The power system of any one of the preceding claims, wherein the first sample time and the second sample time are two of a plurality of sample times extending over a monitoring period, and wherein the controller is configured to monitor the change in gas quantity and / or the rate of change in gas quantity over themonitoring period and to trigger the response action if the change in gas quantity and / or the rate of change of gas quantity exceeds the predetermined threshold within the monitoring period.

10. A gas-fuelled engine system comprising: a gas-fuelled engine (2), a fuel tank (34) and one or more fuel injectors (26); and a shut-off valve (38) positioned between the fuel tank and the one or more fuel injectors so as to define a fuel volume between the shut-off valve and the one or more fuel injectors, the gas-fuelled engine system further comprising a controller (11) configured to perform the following steps when the engine system is in a power-producing state: detect (104) a transition of the engine system to a coasting state and, in response, to: activate (108,110) one or more spark plugs associated with the gas-fuelled engine system; monitor (108,110) the occurrence of combustion associated with the gas- fuelled engine system; trigger (114) a response action if combustion is detected.

11. The engine system of Claim 10, wherein, to monitor the occurrence of combustion, the controller is configured to: receive a signal from an engine knock indicator (41).

12. The engine system of Claim 10, wherein, to monitor the occurrence of combustion, the controller is configured to: receive a signal from an engine torque indicator (44).

13. The power system of any one of Claims 10 to 12, wherein the step of detecting the transition of the power system to a coasting state comprises detecting a zero-torque demand of the power system.

14. The power system of any one of Claims 10 to 13, wherein, when the power system is in a coasting state, the controller is operable to deactivate the one or more injectors.

15. The power system of any one of Claims 10 to 14, wherein following the power system transitioning to a coasting state, the controller is configured to monitor (118) for the power system transitioning back to a power-producing state.

16. The power system of any one of Claims 10 to 15, wherein, in performing the response action, the controller is adapted to configure the power plant into a safe state (114).

17. The power system of Claim 16, wherein, in configuring the power plant into the safe state, the controller is adapted to purge the gas contained in the fuel volume.

18. A method of controlling a power system for a vehicle, the power system comprising a gas-fuelled power plant (2), one or more fuel delivery devices (26) for delivering fuel to the power plant, a fuel tank (34) configured to provide fuel to the one or more fuel delivery devices; a shut-off valve (38) positioned between the fuel tank and the one or more fuel delivery devices thereby defining a fuel volume between the shut-off valve and the one or more fuel delivery devices, the method comprising: detecting (104) a transition of the power system from a power-producing state to a coasting state; operating (105) the shut-off valve to isolate gas in the fuel volume; determining (106) the pressure of gas within the fuel volume at a first sample time; determining (106) the pressure of gas within the fuel volume at a second sample time; determining (112) the change in gas quantity and / or the rate of change in gas quantity within the fuel volume between the first sample time and the second sample time based on the determined gas pressure at the first and second sample times;triggering (114) a response action if the change and / or the rate of change of gas quantity exceeds a predetermined threshold.

19. A method of controlling a gas-fuelled engine system, the system comprising: a gas-fuelled engine (2), a fuel tank (34) and one or more fuel injectors (26); and a shut-off valve (38) positioned between the fuel tank and the one or more fuel injectors so as to define a fuel volume between the shut-off valve and the one or more fuel injectors, the method comprising: detecting (104) a transition of the engine system from a power-producing state to a coasting state and, in response, activating (108,110) one or more spark plugs associated with the gas-fuelled engine system; monitoring (108,110) the occurrence of combustion associated with the gas- fuelled engine system; and triggering (114) a response action if combustion is detected.

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