Leakage detection in gas-fuelled power plants

Abstract

A power system for a vehicle comprising: a gas-fuelled power plant, one or more fuel delivery devices 26 for delivering fuel to the power plant, a fuel tank 34 configured to provide gaseous fuel to th

Description

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. Background 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. 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. It is with these issues in mind that the embodiments of the invention have been devised. Summary of the Invention 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 shutoff valve and the one or more fuel delivery devices. The power system further comprises a controller configured to: detect a power-on request for the power system; operate the shut-off valve isolate gas in the fuel volume; determine the pressure of gas within the fuel volume at a first sample time; and determine the pressure of gas within the fuel volume at a second sample time, and determine, 30 09 25 using the gas pressure determined at the first and second sample times together with a known fuel volume and a determined temperature level, the quantity of gas in the fuel volume at each of the first and second sample times. The controller is further configured to determine the change in gas quantity, and / or the rate of change 5 in gas quantity, in the fuel volume between the first sample time and the second sample time, based on the gas quantities determined at the first and second sample times, and to trigger a response action if the change and / or the rate of change of gas quantity exceeds a predetermined threshold. 10 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 considered to be more effective at detecting low leakage rates. 15 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, 20 which may be determined using a temperature sensor attached to the common rail. Other temperature sensing means may be provided. Optionally, the controller may further be configured to determine gas pressure and / or gas quantity within the fuel volume at a third sample time which it at a point prior to 25 the step of operating the shut-off valve to provide gas to the fuel volume. In this way, the controller is configured to carry out a ‘pre-check’ on the common rail prior to operating the shut off valve. In some instances, therefore, it may not be necessary to charge the common rail with gas prior to performing the leakage checks. 30 As a further optional measure, the controller may be configured to disable the one or more fuel delivery devices and activate a starter motor associated with the power plant. In such a case, therefore, the controller initiates cranking of the power plant in order to check whether any leakage of gas from the fuel volume will cause combustion, thereby providing a further means to discriminate the presence of 35 leakages. In such a scenario, the controller may firstly monitor for the power plant to come up to a minimum cranking speed before isolating gas in the fuel volume. 30 09 25 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 5 of change thereof may be beneficial because it can guard against the effects of temperature changes. The invention extends to and therefore embraces control methods carried out by the controller as defined above. 10 Preferred and / or optional features of the first aspect of the invention may be combined with other aspects of the invention as appropriate. Further optional and advantageous features are referenced in the detailed description 15 and the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS Examples of the invention will now be described with reference to the following 20 figures: 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; 25 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 30 the internal combustion engine of Figure 1 and how that quantity may vary over time during a leak. Detailed Description 35 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, 30 09 25 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. 5 To 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. In overview, an internal combustion engine system 2 comprises an engine block 4, 10 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. The terms ‘controller’ and ‘control unit’ may be 15 used herein synonymously. 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 20 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’. 25 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, 30 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. 30 09 25 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 combustion chambers 12. 5 In the illustrated example, there area 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. 10 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 15 respective one of the 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 maintained at a predetermined, and controllable, pressure level which means that the 20 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. 25 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. 30 Fuel and air mixture in the cylinders 12 is ignited by respective spark plugs 31, in the usual manner. The common rail 28 is supplied with fuel by a fuel supply system 32. The fuel supply 35 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 30 09 25 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 5 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 10 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. 15 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 20 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 cylinders 12. Optionally, individual knock sensors may be associated with each cylinder of the engine block 4. A knock sensor is conventional technology and so 25 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. 30 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 35 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 30 09 25 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. 5 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 10 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. 15 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 20 Network) which is conventional in automotive technology. 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. 25 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 30 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. 30 09 25 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 5 engine block 4. 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 10 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 engine monitoring and control objectives to manage the performance of the vehicle 15 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. One challenge associated with hydrogen-fuelled engines is the potential for leaks to occur. 20 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 2. More specifically, the method depicted in Figure 2 is suitable to be performed before the engine system 2 is running in the sense of producing output power. 25 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 30 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 control unit has the necessary memory, processing environment and communications 35 interface to be integrated into the engine system and the broader system of the 30 09 25 associated vehicle. These specific parts of the control unit 11 are not shown in the Figure, but their presence is implied. In accordance with an example of the invention, a leak detection method 100 begins 5 at step 102 when a power-on request is received by the control unit 11. The power-on request may be generated by a vehicle system in response to a user of the vehicle turning the ignition key to the ‘engine’ start position or pressing an ‘engine start’ button on a screen interface or a physical button, for example. 10 Following receipt of the power-on request at step 102, the method proceeds to step 104 where it activates the starter motor 46 in order to rotate the crankshaft. At this point in the method, the control unit 11 also operates the shut-off valve isolate gas within the common rail 28 and the fuel supply line 40. In this case, this means commanding the shut-off valve 38 into a closed position. In the event that the shut-off 15 valve 38 is already closed at this point, the term ‘operating’ covers checking the position status of the shut-off valve 38 and leaving it in the closed position. Moreover, the fuel injectors 26 are configured into a disabled state and the spark plugs 31 are activated. Ordinarily, when the starter motor 46 operates, the fuel 20 injectors 26 are configured to inject fuel into the cylinders and the spark plugs are configured to provide sparks at suitable timing to initiate combustion. However, in the context of the method of the invention, an objective is to monitor how the engine responds without initiating combustion by injecting fuel using the injectors 26. 25 At step 106, the control unit 11 performs an initial check on the pressure of gas within the common rail 28 by using the fuel pressure sensor 30. The reason for this check is to determine whether there is a likelihood of a leak being present such that a further checking strategy may need to be implemented. In one example, the control unit 11 reads the current pressure of gas in the common rail 28 and determined whether the 30 gas pressure is above a predetermined threshold. In this case, the threshold may be set as the ambient barometric pressure, taking into account a suitable tolerance. The barometric pressure may be given by the air pressure sensor 42 in the manifold, for example. If the gas pressure is above atmospheric pressure, then it can be inferred that a leak is not likely to be present and so no further analysis is required. Therefore, 35 a positive determination at step 106 means that the method passes to step 108 at which point the engine system 2 is cleared to activate, for example by opening the 30 09 25 shut-off valve 38 therefore supplying as to the common rail 28 and enabling the fuel injectors 26, at which point the engine system is started at step 110. As an alternative, the quantity of gas may be determined, rather than simply the 5 pressure of gas. 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 10 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 15 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. 20 To this end, 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 is known, and the temperature of the common rail 28 can also be determined using a suitable temperature sensing means e.g. temperature sensor 45. When the quantity of gas in the fuel volume is determined, it can be checked whether the gas quantity is 25 different to the gas quantity that was determined at the point the engine system 2 was previously powered down, since all data is stored in memory of the control unit 11 for comparison purposes. If the current gas quantity has not changed, taking into account a suitable tolerance, then it can be inferred that there is no leak and the method can proceed to step 108, as has been discussed above. 30 If, at method step 106, it is determined that there is the potential for a leak to have occurred, the method proceeds to steps 112 and 114, which take place sequentially. An alternative at this point is for the control unit 11 to check at step 106 whether there 35 is a sufficient quantity / pressure of gas in the isolated fuel volume in which case the subsequent method steps 112 and 114, as described below, are not necessary to ‘top 30 09 25 up’ the gas in the common rail 28 and, instead, the process flow may move directly to steps 116,118,120. The effect of method steps 112 and 114 is to charge the common rail 28, and more 5 broadly the fuel volume including the common rail 28 and fuel supply line 40. More specifically, at step 112, the control unit 11 commands the shut-off valve 38 to open to admit fuel from the fuel tank 34 to the common rail 28 via the pressure regulating device 36. Note that the control unit 11 may control the shut-off valve 38 directly, or indirectly by way of a different control unit. 10 The shut-off valve 38 is held open until the pressure of fuel within the common rail 28 reaches a predetermined threshold. The control unit 11 is able to monitor the pressure of fuel within the common rail 28 using the fuel pressure sensor 30. Once the gas pressure reaches that threshold, the control unit 11 commands the shut-off valve 38 15 to close, at step 114, therefore isolating fuel in the common rail 28 and the fuel supply line 40. Note that in some examples, the control unit 11 makes the determination to close the shut-off valve 38 based on a calculation of the quantity of gas in the fuel volume rather 20 than based simply on pressure, as mentioned above. At this point, the control unit 11 has isolated a defined quantity (or pressure, as applicable) of gas within the fuel volume defined by the common rail 28 and the fuel supply line 40. At the same time, the starter motor 46 is turning the engine and the 25 spark plugs are activated. The control unit 11 has therefore established a set of initial conditions from which it is able to run a set of diagnostic subroutines, as indicated at method steps 116, 118 and 120. As shown in Figure 2, method steps 116,118 and 120 are run concurrently as this 30 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 116,118,120 may provide individual techniques to infer 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 ran together to provide a more reliable approach to 35 determine a leak. 30 09 25 Referring firstly to method step 116, 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 5 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. This is illustrated in more detail in the chart of Figure 3. In Figure 3, the gas quantity 10 in the isolated fuel volume is shown on the Y-axis and time is represented on the X-axis. As can be seen, prior to time T1 there is a residual gas quantity in the common rail 28 and the shut-off valve 38 is in the closed position. At time T1, the control unit 11 15 operates the shut-off valve to deliver gas to the common rail, that is, the control unit 11 commands the shut-off valve 38 to open which increases gas quantity in the common rail. At time T2, the control unit 11 operates the shut-off valve 38 to isolate gas in the fuel 20 volume, as has been discussed above. In this example, this means that the control unit 11 commands the shut-off valve 38 to close. It will be noted that the gas pressure peaks and stabilises before T2, which is because it reaches the predetermined delivery pressure as set by the pressure regulating device 36. In this example, therefore, the time interval between T1 and T2 is a set time interval which may be 25 determined to be in excess of the time required for the quantity of gas in the isolated fuel to increase so that the pressure is at the predetermined delivery pressure. In other examples, the time T2 may be set based on a closed loop process by which the control unit 11 monitors gas pressure in the common rail 28 and closes the shut-off valve 38 once the predetermined delivery pressure has been reached. 30 Once the shut-off valve 38 has been closed, at T2, the control unit 11 determines the quantity of gas in the fuel volume at a first data sample time T3, shown as Q1 in Figure 3. The quantity of gas is determined is based on a measured pressure at the sample time T3. The exact timing of T3 may vary. For example, in Figure 3 the timing of T3 is 35 shown temporally spaced from T2, which may provide time for the gas pressure in the 30 09 25 fuel volume to settle. In other examples, however, T3 may be very close to or may coincide with T2. Following a preset time interval, shown here as T_delta, the control unit 11 makes a 5 further determination of gas quantity in the fuel volume at second sample time T4, based on a corresponding pressure measurement. This is shown as Q2 in Figure 3. 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 10 between 5ms and 15ms, as mentioned above. 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 15 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 20 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. 25 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. As a first determination, the control unit 11 is configured to evaluate the change in gas 30 quantity in the isolated fuel volume. This is illustrated as Q_delta in Figure 3. If the change is gas quantity between T3 and T4 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. 30 09 25 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 T3 and T4. 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 5 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. The leakage rate of gas may be determined in units of standard cubic centimetres per 10 minute, ‘seem’ which is also expressed as cm3STP / min which defines the volume of gas corrected to a standard pressure and temperature. 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 15 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. If, as a result of the above determinations, the control unit 11 determines that a leak 20 is present, at step 122, then the method proceeds to step 124 where the engine starting is aborted. The engine start may be aborted by cutting power to the start motor 46 to cease engine cranking and by cutting power to the spark plugs. 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 25 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. In this connection, the engine cranking may continue momentarily as part of the purging strategy. 30 In the above calculations, it has been described that gas quantity is determined at sample times T3 and T4 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 T3 and T4 and then, subsequently, the gas quantity at 35 those sample times may be determined. It should also be noted that sample time T3 and T4 are examples of two sample times and that many more sample times may be 30 09 25 generated to provide continuous monitoring over a longer time period. In principle, it is also acceptable to determine the different in gas pressure between the sample times T3 and T4 and to determine the leakage rate of gas quantity from the gas pressure readings. For example, the time period between T3 and T4, or T_delta, may 5 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 10 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 15 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. Furthermore, in the above discussion it will be noted that the difference in gas quantity at time T3 and at time T4 is negative, in the sense that the quantity of gas reduces. 20 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 T3 and T4. The control unit 11 is therefore adapted to detect reductions in gas quantity and also 25 increases in gas quantity. This information can be used to identify the component that is likely to be leaking. Turning to method steps 118 and 120, each of these method steps provides an alternative way to detect combustion occurring in one of the cylinders 12 of the engine. 30 Since the fuel injectors 26 are not injecting fuel into the combustion chambers 12, any vibration from the engine block 4 that is consistent with a combustion event can be attributed to a fuel leakage from the common rail 28 into the combustion chambers 12. This may be due to faulty injector components, for example. 35 At method step 118, the control unit 11 monitors the output from the one or more knock sensors 41 associated with the engine block 4. As is known, the one or more 30 09 25 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 indictive of combustion occurring within the combustion 5 chambers 12. At method step 120, the control unit 11 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, 10 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. The monitoring that is implemented during 15 method steps 118 and 120 may be applied for a set time period, for example 1 to 2 seconds, the precise length of which would be a calibratable value. The control unit 11 applies suitable thresholds at decision step 126 to identify the occurrence of combustion following method steps 118 and 120. If combustion is 20 detected, then the method flows to the abort start step 124 as discussed above. If combustion is not detected over the monitoring period of method steps 118 and 120, then the method flows to step 108 at which point the shut-off valve 38 is opened to admit gas flow into the common rail 28 and injector operation is enabled so as to 25 enable engine start, at step 110. 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. 30 Other will now be discussed below. In the above discussion the power-on request at step 102 may also be generated when the vehicle is put into standby mode, for example if the ignition key is turned to the ‘ignition on’ position or when the systems of the vehicle are powered-up but no 35 command has been triggered to start the engine system 2. 30 09 25 In this respect, it should also be noted that in the illustrated example, the monitoring of gas quantity at step 116 has been discussed as occurring when the engine is cranking, as step 116 is implemented concurrently with combustion monitoring steps 118 and 120 during a monitoring period. However, this is not essential to the invention 5 and it is also envisaged that the gas quantity monitoring may take place whilst the engine is not cranking. Thus, method step 116 may take place independently from method steps 118 and 120, 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 116 and 122 may be run independently from 10 the determination of a leak via monitoring of engine combustion as covered by method steps 118, 120 and 126. 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 15 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. 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 20 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 25 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. In the above discussion, method step 106 has been described as being performed by 30 the control unit 11 after the control unit 11 has activated the starter motor 46 at step 104. However, it should be noted that the order in which steps 104 and 106 are performed may be reversed. 30 09 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,a fuel tank (34) configured to provide fuel to the one or more fuel delivery devices;10a 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:15detect (102) a power-on request for the power system;operate (114) the shut-off valve to isolate gas in the fuel volume;determine (116) the pressure of gas within the fuel volume at a first sample time;20 determine (116) the pressure of gas within the fuel volume at a secondsample time;determine, using the gas pressure determined at the first and second sample times together with a determined volume of the fuel volume and a determined temperature level, the quantity of gas in the fuel volume at each25 of the first and second sample times,determine (116,122) the change in gas quantity, and / or the rate of change in gas quantity, in the fuel volume between the first sample time and the second sample time based on the gas quantities determined at the first and second sample times; and30 trigger (124) a response action if the change and / or the rate of change of gasquantity exceeds a predetermined threshold.

2. The power system of Claim 1, wherein the controller is further configured to:30 09 25determine (106) gas pressure and / or gas quantity within the fuel volume at a third sample time which is at a point prior to the step of operating the shut-off valve to isolate gas in the fuel volume.5 3. The power system of Claims 1 or 2, wherein the controller is furtherconfigured to:operate (112) the shut-off valve to deliver gas to the fuel volume prior to isolating gas in the fuel volume, and then to perform the subsequent steps, if 10 the determined gas pressure and / or gas quantity detected at the thirdsample time is less than a predetermined threshold.

4. The power system of any preceding claims, wherein the gas-fuelled power plant is an internal-combustion engine and wherein the controller is configured to15 perform the following steps following the detection of a power-on request, and prior to the step of operating the shut-off valve to isolate gas in the fuel volume:disable the one or more fuel delivery devices and activate a starter motor associated with the internal combustion engine.

205. The power system of Claim 4, wherein the controller is configured to:perform the step of operating the shut-off valve to isolate gas in the fuel volume, after the controller detects that the internal combustion engine has25 exceeded a minimum cranking speed.

6. 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.

307. The power system of Claim 6, wherein, in configuring the gas-fuelled powerplant into the safe state, the controller is adapted to purge the gas contained in the fuel volume.30 09 258. A method of determining gas leakage in 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; and a shut-off valve (38)5 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, wherein the method comprises:detecting (102) a power-on request for the power system;10 operating (104,114) the shut-off valve to isolate gas in the fuel volume;determining (116) the pressure of gas within the fuel volume at a first sample time;determining (116) the pressure of gas within the fuel volume at a second sample time;15determine, using the gas pressure determined at the first and second sample times together with a determined volume of the fuel volume and a determined temperature level, the quantity of gas in the fuel volume at each of the first and second sample times,20determining (116,122) the change in gas quantity, and / or the rate of change in gas quantity, in the fuel volume between the first sample time and the second sample time based on the determined gas quantities at the first and second sample times; and25triggering (124) a response action if the change and / or the rate of change of gas quantity exceeds a predetermined threshold.30

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