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, fuel injectors 26, for delivering fuel to the power plant; a fuel supply 34 configured to provid

Description

5 Technical Field The examples of the invention relate to systems, method and approaches for detecting leaks and emissions from gas-fuelled power plants, and particularly internal 10 combustion engines fuelled at least partially with hydrogen. Background Gaseous fuels such as hydrogen are promising alternative fuels to gasoline and diesel 15 due to their potential for low or zero emissions. However gaseous fuels present some challenges relating to their containment and handling. 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 20 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. It is with these issues in mind that the embodiments of the invention have been 25 devised. Summary of the Invention Against this background, examples of the invention provides a power system for a 30 vehicle comprising a gas-fuelled power plant according to Claim 1. The power system may also include a gas regulator for regulating the supply of gas from the fuel tank to a rail forming part of the fuel volume. The shut off valve is typically located between the gas regulator and the rail. 35 When the invention is employed in an engine of a vehicle, typically there may be more than one fuel tank configured to provide fuel to the one or more fuel delivery devices. 02 10 25 The invention provides the benefit that gas leakage within a gas-fuelled power plant, such as a vehicle engine, can be determined accurately when the engine is not operational, for example when parked overnight or when temporarily powered off. As the periods for which the engine is powered-off are often relatively long (compared to 5 engine running), the method ensures that even relatively small leaks can be detected accurately. The controller may be configured to determine a first gas quantity of fuel within the fuel volume at the first time; determine a second gas quantity of fuel within the fuel 10 volume at the second time; and determine the gas leak rate during the time interval based on the first and second gas quantities and the time interval. The controller may be configured to compare the determined gas leak rate with a predetermined gas leak rate and, if the determined gas leak rate is more than the 15 predetermined gas leak rate, provide an output to indicate that there is a leak fault in the power system. The controller may be configured to provide an output to place the power system in a safe state if there is a determination that there is a leak fault. 20 By way of example, the controller may be configured to maintain the shut-off valve closed to isolate gas in the fuel volume if there is a determination that a leak fault is present. 25 Features of the invention further provide the advantage that the rail is not emptied to such an extent, during the off period, that the leak detection calculation cannot be performed, by ‘waking up’ the power system before a user-commanded power on request may occur. In other embodiments, or in other operating configurations, the controller may be 30 configured to detect a power-on request from a user command to define the second time. The controller may be configured to compare the gas pressure at the first time with the gas pressure at the second time and, if the gas pressure at the second time is 02 10 25 higher than the gas pressure at the first time, provide a determination that there is a leak fault in the power system. In embodiments of the invention, the fuel volume is a volume of a fuel rail in addition 5 to a volume of pipework between the shut off valve and the fuel rail and / or a volume of pipework between the fuel rail and the fuel delivery devices. The invention may also be expressed as a method of detecting a gas leak in a gas-fuelled power plant for a vehicle, as detailed int eh appended claims. 10 The method is implemented in a controller of a power system for a vehicle. It will be appreciated that preferred and / or optional features of any aspect of the invention may be incorporated in other aspects of the invention also, alone or in appropriate combination. 15 Further optional and advantageous features are referenced in the detailed description and the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS 20 Examples of the invention will now be described with reference to the following figures: Figure 1 is a schematic view of a gas-fuelled internal combustion engine, being 25 an example of a power plant to which the examples of the invention apply; and 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. 30 Detailed Description In general, the examples of the invention provide a power system for a vehicle in which a controller is configured to detect a leak of gas from the system when the 35 engine in powered off i.e. between key off and key on. The method is beneficial 02 10 25 especially because it allows detection of leaks even when the quantities are very small, because the method is performed when the engine is shut down (typically for longer periods e.g. overnight or when parked). For very small leakage rates, a longer time is typically required to determine that there has been a leak. The method relies 5 on a measurement of gas leak rate between ‘key off’ and ‘key on, or another power on event, to identify when there is a leak condition. To put the examples of the invention into technical context, a discussion of a typical internal combustion engine which is fuelled with gaseous hydrogen will now be 10 described with reference to Figure 1. 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 control unit 11, referred to as the engine control unit 15 (ECU), which is adapted to receive data input 11.1 to sense operational parameters of the engine to provide suitable control output signals 11.2 to the engine system 2 to control its operation based on driver demands and sensor measurements, as is conventional. For example, a power-on or power-off request for the engine may be generated by a vehicle system in response to a user of the vehicle turning the ignition 20 key to the ‘engine’ on or off position, respectively, or by pressing an ‘engine stop / start’ button on a vehicle screen interface or physical button, for example. This is often referred to as ‘key on / off’. The engine control unit 11 receives the key on / off signal in the data input 11.1. 25 The engine control unit 11 includes a non-volatile memory component (NVM) 13. The NVM 13 stores data such as self-learnt control parameters and operating history data which can be retrieved by the ECU even after a power down cycle. The ECU 11 also includes a low power counter 15 which is used to determine a time interval and which relies on a permanent power supply and is operable regardless of the key on / off state. 30 The engine block 4 of the illustrated example comprises four combustion chambers 12, or cylinders, in an ‘in-line’ 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 35 by the skilled person. Common engine configurations are single cylinder engines, twin 02 10 25 cylinders, triples, in-line sixes or V-sixes, and V8 engines. Herein, the term ‘combustion chamber’ will be considered synonymous with ‘engine cylinder’. The air inlet system 6 comprises an air inlet 14 which feeds fresh air into a network of 5 air pipes 16 through an air filter 17. An air mass flow sensor 18 is provided to provide data to the control unit 11 (signals not shown) about the airflow entering the engine system 2. The network of air pipes 16 feeds incoming air through a compressor 19 and, 10 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 engine cylinders 12 of the engine block 4 via separate air channels (not 15 identified). 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 engine cylinders 12. 20 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 engine cylinders 12. 25 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 30 other examples the fuel injectors 26 may be arranged to inject fuel directly into a respective one of the engine cylinders 12. 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 35 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 02 10 25 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. 5 The pressure of fuel within the common rail 28 is determined by the control unit 11 and is monitored 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 10 the invention. The fuel pressure sensor 30 provides an output signal (not shown), indicative of the fuel pressure in the common rail 28, to the control unit 11. Fuel and air mixture in the engine cylinders12 is ignited by respective spark plugs 31, in the usual manner. 15 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 20 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 may be between 350 and 700 bar, whereas the pressure 25 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 higher for some systems, for example direct injection systems. Together the gas supply line 40, the rail 28 and the injectors 26, and the various connections between these components, may be considered as the ‘low pressure circuit’ LPC of the system. 30 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. 35 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 02 10 25 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 a particular combustion chamber 12 using associated software. The knock sensors and 5 the associated software are able to discriminate between combustion occurring in different ones of the combustion chambers 12. 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 10 to provide the control unit 11 with data relating to the pressure of air within the air inlet manifold 24. 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 15 the crankshaft. It should be noted that the crankshaft, pistons, intake and exhaust valves, and spark plugs 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 20 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. A temperature sensor 45 may also be provided on or associated with the common rail 25 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 30 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 any other appropriate factors, as is known in the art. 02 10 25 Note that the fuel pressure sensor 30, the knock sensor 41, the air pressure sensor 42, the temperature sensor 45 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, through the connection of a 5 CAN-bus (Controller Area Network) (not shown) which is conventional in automotive technology, or through SENT communications. 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 10 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 15 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 combustion chambers 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 20 discussion will not be provided here for the sake of brevity. 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 25 gas, is mixed with fresh air in the inlet manifold before entering the combustion chambers 12 of the engine block. 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 30 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. As has been discussed above, the control unit 11 is operable to perform various 35 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 02 10 25 known to the skilled person and is outside of the scope of this discussion. 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 5 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. Irrespective of the functionality of the control unit 11, it will be appreciated that the 10 control unit has the necessary memory (not shown), processing environment (not shown) and communications interface (not shown) to be integrated into the engine system and the broader system of the associated vehicle. These specific parts of the control unit 11 are not shown in the Figure, but their presence is implied. 15 One challenge associated with hydrogen-fuelled engines is the potential for leaks to occur. Figure 2 illustrates an example algorithm or method that can be implemented by the control unit 11 in order to detect whether leaks have occurred from the engine system 20 2. More specifically, the method depicted in Figure 2 is suitable to be performed before the engine is in a power off state, and before a power on state is initiated, to mitigate the risk of a hazardous situation, such as engine damage, if gas is introduced into the fuel rail when a fault is present. 25 Once the engine has stopped as a result of the power-off request (key off), the shut off valve 38 is closed so that no further fuel is supplied to the fuel rail 28 from the pressurised fuel source 34. In ideal operating conditions, where there is no fault, the residual pressure of hydrogen gas will be isolated in the fuel rail unless there is a leak from anywhere in the low pressure circuit, including the rail 28, the injectors 26, the 30 pipes (e.g. 40), the sensors (e.g. 44, 45) and / or the various connections between the components. In an ideal system, therefore, the quantity of hydrogen in the rail 28 should remain the same for the period of time between power-off and power-on, indicating that the system is leak-free. In a realistic system, however, there will be a small amount of static leakage. 02 10 25 In accordance with an example of the invention, a leak detection method 90 begins after a power-off request is received by the control unit 11. The method is focussed on gas quantity decay during the period for which the control unit 11 is off. The power-off request may be generated by a vehicle system in response to a user of the vehicle 5 turning the ignition key to the ‘engine’ stop or off position, or pressing an ‘engine start / stop’ button on a screen interface or a physical button, for example. Other power off commands are possible, as will be discussed further below. Once there has been a power-off request (step 100), a measurement is taken of the 10 rail pressure (P0), as determined by the rail pressure sensor 30, and of the temperature (TO) of fuel in the rail, as determined by a temperature sensor 45, at step 101. The values for P0 and TO are stored in the NVM 13 of the control unit 11 at an assigned time, TO. The storage of the pressure P0 and temperature TO measurements occurs during a delay period between the power off request being generated and the 15 controller 11 being placed in sleep mode. This is often referred to as the ‘power latch phase’. In other words, the control unit 11 being placed in sleep mode does not occur instantaneously with the power off request being received, and may, for example, occur between several seconds and up to 10 minutes after the power off request. The delay period will depend, for example, on the operating conditions (such as the 20 temperature) and the house keeping procedures which the control unit 11 has to run in the power latch phase. This means that the step of saving the rail pressure measurement P0 and the temperature measurement TO does not occur at the instant that the power off request is received at the control unit 11. 25 Our co-pending patent application, with the same priority date as the present application, describes a method of gas leak detection within the power latch phase. In addition, although at power-off (step 100) the control unit 11 is placed into a sleep mode, the low power counter module 15 is able to work through a permanent power 30 supply line from the vehicle battery to calculate a time interval T until the next power-on event 102 at time T1. The low power counter module 15 decrements a counter by 1 every second after power off at step 100 until the control unit 11 is powered on by the subsequent power-35 on event at key-on. In this way the low power counter 15 determines a time interval between power off and power on. 02 10 25 At the subsequent power-on request at step 102 (time T1), the method proceeds to step 104 where a valid check is made of the count on the low power counter 15 and a valid check is made on the NVM 13. The valid checks determine, for example, 5 whether there has been a power cut to the low power counter 15 as a result of a battery disconnection, and / or whether there is a data corruption in the NVM 13 caused by an interrupted data saving process. If the check is valid at step 104, the method continues to step 106. If the check is not valid, other leak detection methods are implemented at step 107 before the method continues. 10 At step 106 the rail pressure (P0) and the temperature (TO) at time TO are recalled from the NVM 13 together with the counter value from the low power counter 15 (time interval). 15 At step 108 the rail pressure (P1) and the temperature (T1) at time T1 are recorded. In one embodiment, this may be in response to a power on request from the user, for example. At step 110 the average leak rate is calculated for the time that the engine is off, based 20 on the low power counter measurement and the comparison between P0 / P1 and T0 / T1. The average leakage rate is determined by considering the quantity of fuel that has leaked from the system, based on the ideal gas law. If the volume of the rail 28 is known, the quantity of fuel can be determined based on the pressure and temperature measurements. It will be noted that since the shut-off valve 38 is closed at power-off, 25 the fuel pressure sensor 30 is measuring the pressure of gas not just in the common rail 28 but also in the fuel supply line 40. Therefore, it is the pressure of 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 30 which may be different ends of the vehicle), determining the quantity of gas, and leak rate, rather than simply measuring gas pressure, provides a more accurate assessment of the leakage in the system. The volume of the fuel pipes forming part of the low pressure system needs to be known to factor into the determination of the average leak rate. 02 10 25 At step 112 the average leak rate is compared to a predetermined threshold leak rate which is stored in the NVM 13 of the control unit 11. If the average leak rate is greater than the predetermined, acceptable leak rate, a fault is identified and appropriate action can be taken to record and act on the fault (step 114). 5 If the average leak rate is below the predetermined leak rate, then it is determined that the leak is not significant and the process continues to the subsequent steps of normal engine operation. At step 116, assuming there is no leak indicated at step 112 and on the assumption that no other leak detection routine indicates a leak (as 10 represented at step 107), normal engine operation commences. A check is made at step 120 about whether a further power off command has been received. If not, normal engine operation continues through its cycle via step 116. 15 If a power-off command is received at step 120, the shut-off valve 38 is activated to shut off the supply of high pressure fuel to the rail 28 and the rail pressure (P0) and the temperature (TO) of fuel in the rail is recorded again (at time TO), as before, and the process starts again. 20 In the event that a leak is detected, the control unit 11 initiates a response action (represented as 11.3 in Figure 1) to place the power system in a safe state and / or may issue a warning signal to the driver and / or another command signal. For example, the engine start may be aborted by cutting power to the starter motor 46 to cease engine cranking and by cutting power to the ignition coils to the spark plugs for 25 the injectors 26. As a further measure, the control unit 11 may be configured to provide a command signal 11.3 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 and normal engine operation can resume. Such purging processes and hardware are outside the scope of this discussion. However, the process of purging the gas from 30 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. In this connection, the engine cranking may continue momentarily as part of the purging strategy. It is noted that a scenario may arise where the control unit 11 off time is such a long period that the rail has become empty as a result of a permitted static leak. In other 35 words, such a low permitted static leak can empty the rail if the off period is long 02 10 25 enough. In this case it is not possible to conclude whether there is a leak or not and a further detection method would be required to check for leaks before the engine is allowed to start again. In a pre-calibration phase, a control-unit-off time threshold beyond which a leak cannot be determined with certainty can be estimated using the 5 residual gas quantity before the power off occurs, and the known system static leak rate. In this scenario the method is aborted if the time interval after power off exceeds the determined time threshold. In an alternative approach, the low power counter 15 can be configured in such a way that when a certain period has passed it wakes up the control unit 10 to perform the 10 leak calculation. In this scenario the calculation of leak rate can commence before the power off time extends over too long a period and renders the method inconclusive. Such a time threshold for the low power counter 15 to wake up the control unit 11 can be determined as per the system requirements, for example, by estimating an appropriate ‘off time’ using the maximum allowed leak rate for a particular application 15 (which could be higher than the static leak) and the residual rail gas quantity. Such a threshold can then be programmed into the low power counter 15. The appropriate time threshold will also depend on the quantity of gaseous fuel remaining, based on the pressure and temperature measurements (P0, TO) at the previous power-off. The higher the quantity of gaseous fuel that is present at the point of the previous power-20 down, the higher the time threshold before it is necessary to wake up the control unit 10 to perform the leak calculation. After the leak calculation has occurred, the control unit 11 can be put into sleep mode again until the subsequent (normal) power on (key on) event, or another low power counter 15 wake-up event, occurs. 25 In any embodiment of the invention, the method of the invention uses rail pressure and temperature readings at power off, and at the subsequent power on, to determine the gas leak rate. The power on may be initiated by a user demand, for example key on, or may be initiated by the low power counter 15 ‘waking up’ the control unit 11 after a pre-determined time period following power off. The smaller the leak rate, the 30 longer the time interval required to detect a discernible amount of gas leakage. As the typical engine off time is not insignificant, and often amounts to hours (e.g. when an engine is turned off overnight), the method allows small leakages to be detected which it is not possible to detect with other methods. 02 10 25 The leak rates measured for each power off phase, or for selected ones of the power off phases, may be stored in the NVM 13 to gather field data relating to system leak rates. This may provide useful data offline for servicing and maintenance, for example. 5 In a further modification to the invention (not shown), it is possible to incorporate an additional step to compare whether the rail pressure at power-on (T1) is higher than the rail pressure at the previous power-off (TO). In this case, it can be ascertained that the shut-off valve 38 is leaking because there is an indication that fuel continues to 10 be supplied to the rail 28, even after the shut-off valve 38 is closed. This is referred to as the shut-off valve leak test. As before, it will be noted that since the shut-off valve 38 is closed, the fuel pressure sensor 30 is measuring the pressure of gas not just in the common rail 28 but also in 15 the fuel supply line 40. Therefore, it is the pressure of the entire volume of fuel isolated between the shut-off valve 38 and the injectors 26 that is being measured. To this end, and as described previously, the control unit 11 is configured to use the ideal gas law to determine the quantity of gas, since 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 20 be determined using the temperature sensing means 45. When the quantity of gas in the fuel volume is determined at power-on, a determination is made as to whether the gas quantity is different to the gas quantity that was determined at the point the engine system 2 was previously powered down. If the current gas quantity at power on (time T1) has not changed (taking into account a suitable tolerance), then it can be inferred 25 that there is no leak in the shut-off valve 38. If, however, it is determined that the quantity of gas in the rail 28 and pipes 40 has increased above the predetermined acceptable threshold, then it is determined that the shut-off valve 38 has a leak. This check on the shut-off valve may be run in addition to the leakage method. 30 Typically the shut-off valve leak test is performed when the rail pressure is lower than the supply pressure in the fuel tank at the end of a rail purge event during which the rail pressure is lowered by actuating the fuel delivery devices to remove any residual fuel, while the shut-off valve is closed. If the shut-off valve leak test indicates a fault with the shut-off valve 38 then a fault may be output to the user and / or stored in the 35 NVM 13 of the control unit 11 to be read later when the engine is in servicing or undergoing maintenance. In practice it is not necessary or desirable to remove the residual fuel completely during purging, for various reasons, including but not limited to the requirement for there to be a limited amount of gaseous fuel remaining for leak detection purposes. 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. Other will now be discussed below. 10 15 CXI In this 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 the 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 the examples of the invention apply. CXI 02 10 25

Claims

1. A power system for a vehicle comprising:5 a gas-fuelled power plant (2);one or more fuel delivery devices (26) for delivering fuel to the power plant (2);a fuel supply (34) configured to provide fuel to the one or more fuel delivery 10 devices (26);a shut-off valve (38) positioned between the fuel supply (34) and the one or more fuel delivery devices (26) so as to define a fuel volume (28, 40) between the shut-off valve (38) and the one or more fuel delivery devices (26),15the power system further comprising a controller (11) configured to:detect a power-off request for the power system at a first time (TO) and, in response to the power-off request, activate the shut-off valve (38) to close to isolate 20 gas in the fuel volume;determine a gas leak rate during a time interval between the first time and a second, later time (T1) at which a power-on request is generated for the power system; and25compare the determined gas leak rate with a predetermined gas leak rate for a power system operating with a gas leak to provide a determination of whether or not there is a leak fault in the power system,30 wherein the controller is configured to;compare the time interval with a predetermined maximum time interval fordetermining the leak rate based on a permitted residual static leak for the power plant and, if the time interval exceeds the predetermined maximum time interval,02 10 25generate a power-on request within the controller (11) to define the second time (T1),and wherein the controller (11) is further configured to set a maximum permitted 5 duration for the time interval based on the quantity of gaseous fuel present at the first time (TO), determined from the measured pressure at that time, such that a greater stored fuel quantity corresponds to a longer maximum permitted duration for the time interval before the leak calculation is performed..10 2. The power system as claimed in claim 1, the controller being configured to:determine a first gas quantity of fuel within the fuel volume (28, 40) at the first time (TO);15 determine a second gas quantity of fuel within the fuel volume (28, 40) at thesecond time (T1); anddetermine the gas leak rate during the time interval based on the first and second gas quantities and the time interval.

203. The power system as claimed in claim 1 or claim 2, wherein the controller (11) is configured to compare the determined gas leak rate with a predetermined gas leak rate and, if the determined gas leak rate is more than the predetermined gas leak rate, provide an output to indicate that there is a leak fault in the power system.

254. The power system as claimed in any of claims 1 to 3, wherein the controller (11) is configured to provide an output to place the power system in a safe state if there is a determination that there is a leak fault.30 5. The power system as claimed in any of claims 1 to 4, wherein the controller(11) is configured to maintain the shut-off valve (38) closed to isolate gas in the fuel volume (28, 40) if there is a determination that a leak fault is present.02 10 256. The power system as claimed in any of claims 1 to 5, wherein the controller is configured to detect a power-on request from a user command to define the second time (T1).5 7. The power system as claimed in any of claims 1 to 6, wherein the controller(11) is configured to;compare the gas pressure at the first time (TO) with the gas pressure at the second time (T1) and,10if the gas pressure at the second time (T1) is higher than the gas pressure at the first time (TO), provide a determination that there is a leak fault in the power system.15 8. The power system as claimed in any of claims 1 to 7, wherein the fuel volume(28, 40) is a volume of a fuel rail (28) in addition to a volume of pipework (40) between the shut off valve (38) and the fuel rail and / or a volume of pipework between the fuel rail (28) and the fuel delivery devices (26).20 9. A method of detecting a gas leak in a gas-fuelled power plant for a vehicleincluding 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 (26); a shut-off valve (38) positioned between the fuel tank (34) and the one or more fuel delivery devices (26) so as to define a fuel volume (28, 40) between the shut-off25 valve (38) and the one or more fuel delivery devices (26), the method comprising:detecting a power-off request for the power system at a first time (TO) and, in response to the power-off request, activate the shut-off valve (38) to isolate gas in the fuel volume;30determining a gas leak rate during a time interval between the first time (TO) and a second, later time (T1) at which a power-on request is generated for the power system; andcomparing the determined gas leak rate with a predetermined gas leak rate for a power system operating with a gas leak to provide a determination of whether there is a leak fault in the power system,5 wherein the method further comprises:comparing the time interval with a predetermined maximum time interval for determining the leak rate based on a permitted residual static leak for the power plant and, if the time interval exceeds the predetermined maximum time interval, generating10 a power-on request within the controller (11) to define the second time (T1),15wherein the method further comprises setting a maximum permitted duration for the time interval based on the quantity of gaseous fuel present at the first time (TO), determined from the measured pressure at that time, such that a greater stored fuel quantity corresponds to a longer maximum permitted duration for the time interval before the leak calculation is performed.

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