Leakage detection in gas-fuelled power plants

Abstract

Disclosed is 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 fuel t

Description

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 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 shut-off valve and the one or more fuel delivery devices. The power system further comprises a controller configured to perform the following steps when the power system is in a power-producing state: detect a power-off indication for the power system; 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 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, 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 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. 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 deterred 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. 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. 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. The invention extends to and therefore embraces control methods carried out by the controller as defined above. 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 and the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS 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 an example of a power plant to which the examples of the invention apply; 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. Figures 3a to 3c are a series of charts depicting the quantity of gas in an isolated fuel volume of the internal combustion engine of Figure 1 and how that quantity may vary overtime during a leak. Detailed Description 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. 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, 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 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 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’. 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. 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. 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 combustion chambers 12. 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. 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. 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. Fuel and air mixture in the combustion chambers 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 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. It should be noted that the pressurised fuel source 34 may comprise more than one fuel tank. 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 sensing arrangements generally 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. 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. 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. 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. The control unit 11 may receive some data input 11.1 via a connection to a CAN-bus (Controller Area 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. 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 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 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 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 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 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. One challenge associated with hydrogen-fuelled engines is the potential for leaks to occur. Leaks may occur from the engine when the engine is in a non-power producing state, or otherwise. Typically, such a leak would occur due to an injector being in a stuck open position which may result in a fast leakage of gas, reducing pressure in the common rail 28 quickly. Other leaks would be slower and more challenging to detect, for example worn or contaminated injector needles, and can occur before, during and after, operation of the engine in a power-producing state. Figure 2 illustrates an example algorithm or method that can be implemented by the control unit 11 to detect whether leaks have occurred from the engine system 2. More specifically, the method depicted in Figure 2 is suitable to be performed after an ‘ignition off’ state has occurred. It should be noted that an ignition off state may occur for various reasons, for example at a point when the user of the vehicle has turned the ignition key to the off position, or the vehicle ‘start button’ has been pressed to halt operation of the engine system, or when the engine has stalled. More generically, these states apply to the engine system 2 transitioning from a power-producing state to a non-power-producing state, either by a user command, from an internal signal that has been generating to cease operation of the vehicle, or due to an accidental event due to the vehicle being caused to stall or being in an accident. 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. 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. 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 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. The engine system 2 then transitions to a non-power-producing state or ‘power off’ state, which the control unit 11 is adapted to detect, at step 104. The engine system 2 may transition to a non-power-producing state for various reasons. Most typically, an engine system 2 will transition from a power-producing state to a non-power-producing state in response to a user of that vehicle turning off ‘the ignition’, for example by turning the ignition key to the ‘engine off’ position or pressing an ‘engine start / stop’ button on a screen interface or a physical button, for example. In this situation, a power-off command will be generated by another vehicle control subsystem to command the engine system 2 to stop operating. For example, the engine may be put into this condition as part of an automated stop / start system. The power-off command is therefore an indicator that the engine system 2 is, or is to be, in a non-power-producing state. However, an engine system 2 may also transition from a power-producing state to a non-power-producing state because the engine has encountered a power-stopping event, for example the vehicle has been controlled in such a way that it has been caused to stall, or it has been in an accident that has caused the engine to stall. In either case, the engine system 2 is in a state that the fuel injectors 26 have stopped injecting fuel and the spark plugs 31 are non-operational, and one or more indicators will be generated by vehicle sub-systems and / or the engine system that the engine system 2 has entered a non-power-producing state. In view of the above discussion, the control unit 11 is adapted to detect when the engine system 2 is commanded to or has already been placed into a non-power producing state by identifying one or more suitable indicators that may be represented by internal data signals or by monitored operating state parameters of the engine system 2. Once the control unit 11 has detected a suitable power-off indicator, the engine system 2 does not require fuel to be delivered and so the control unit 11 is configured to isolate the common rail 28 from the fuel tank 34 by operating the shut-off valve 38 to isolate fuel in the common rail 28 and the fuel supply line 40 from the fuel tank 34. This is indicated at method step 106. In this case, this means commanding the shutoff valve 38 into a closed position. In the event that the shut-off 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. Following step 106, at method step 108 the control unit 11 controls the fuel injectors 26 to configure them into a disabled state, so no fuel is injected into the manifold 24. Moreover, the spark plugs 31 are deactivated. Once the shut-off valve 38 is closed, fuel injection is ceased, and the spark plugs are deactivated, the method can then proceed to monitor the gas within the isolated fuel volume represented by the common rail 28 and the gas supply line 40. At this point in the method, it should be appreciated that the pressure of gas, and also the quantity of gas, within the isolated fuel volume is, or can be assumed to be, the same as the pressure / quantity of gas within the isolated fuel volume at the point that the shut-off valve was closed, at method step 106. From this point, therefore, the control unit 11 is configured to monitor for leaks within the isolated fuel volume before shutting down the engine system 2 fully. In this way, the control unit 11 is able to determine whether there is a leakage due to a faulty injector, for example, that would mean that it would be unsafe to allow the engine system 2 to be restarted at a later date. In such a circumstance, a suitable alert may be provided to the user, for example through a visual interface or attention-getter on the vehicle dashboard which would indicate that attention is required and that the engine system 2 is unable to be re-started. However, an optional step is shown at method step 110, which represents a partial purge of the gas within the isolated fuel volume. A gas purge may be performed in order to reduce the quantity of gas in the isolated fuel volume as a safety measure whilst still leaving a suitable quantity of gas in the isolated fuel volume at a suitable pressure so that leak testing can be performed. 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 chamber to reduce the concentration of hydrogen in the air, or to combust the hydrogen. Purging of some of the gas within the isolated fuel volume may also be achieved through leaving the engine system 2 running (fuel injectors operating and spark plugs activated) for a period of time whilst the shut off valve 38 is closed. This is shown in Figure 2 by the order of method step 110 which is before method step 108. Therefore, the intention is that the engine system 2 continues to operate for a period of time with the shut-off valve 38 in a closed position to reduce the pressure of gas in the common rail 28. For further illustration, reference will be made also to Figures 3a-3c which illustrates the quantity of gas within the isolated fuel volume during a period covered by the method of the invention. In Figure 3c, the gas quantity in the isolated fuel volume is shown on the Y-axis and time is represented on the X-axis. In Figures 3a-3c, it should be appreciated that the time prior to T1 corresponds to normal operation of the engine system 2, as is represented by step 102 in Figure 2. At T1, therefore, the operational state of the engine transitions from a powerproducing state to a non-power-producing state, which may be caused by a user of the vehicle turning the ignition off, either via a key or by an engine start / stop button, as discussed above. Figures 3a and 3b show, respectively, the engine system 2 transitioning between a power-on status and a power-off status (Figure 3a) and the shut-off valve 38 transitioning from an open position to a closed position. Method step 110 corresponds to the time interval between T1 to T2, during which it can be observed that the quantity of gas within the isolated fuel volume is reduced from a high level Q0, to a second level, Q1. Q1 is a configurable threshold or target level which may be based on pressure or gas quantity. 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. The control unit 11 has therefore established initial conditions from which it is able to run a diagnostic subroutine, as indicated at method steps 112, and 114. Referring firstly to method step 112, at this step the control unit 11 is configured to monitor the quantity of gas within the fuel volume 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 this end, the control unit 11 uses data obtained by the rail pressure sensor 30 at first and second sample times, which are illustrated at T2 and T3 on Figure 3c, and as illustrated in Figure 2 in the inset panel at steps 112.1 and 112.2. 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. 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 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 determined. 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), determining the quantity of gas present rather than simply the gas pressure is believed to provide a more accurate determination. To this end, the control unit 11 is configured to use the ideal gas law to determine the quantity of gas based on the measured gas pressure, 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 sensor 45. 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. The time interval between T2 and T3, shown here as T_delta, between the two data samples Q1 and Q2, 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. 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 quantity in the isolated fuel volume. This is illustrated as Q_delta in Figure 3c 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. 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. 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. 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. In this connection, it should be noted that the leakage from the common rail 28 and associated components 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. It is believed that monitoring the rate of change of gas quantity 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. If, as a result of the above determinations, the control unit 11 determines that a leak is present, at step 114, then the method proceeds to step 116 where the determination of a leak is logged in memory within the control unit 11 and an appropriate action is taken to alert the user of the vehicle. Appropriate action can include simply alerting the driver to the presence of a leak or, more significantly, configuring the control unit 11 such that the engine system 2 is unable to be restarted until appropriate maintenance actions are taken to investigate the issue and resolve it as appropriate. Thus, the control unit 11 is adapted in these circumstances to place the engine system 2 into a safe state. In the above discussion, it will be noted that the difference in gas quantity Q1 at time T2 and Q2 at 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. 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. For example, 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 and / or rate of change of 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 defined time period. In principle, it is also acceptable to determine the different in gas pressure between the sample times T2 and T3 and to determine the leakage rate of gas quantity from the gas pressure readings. 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. 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. 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 5 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. 10

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 power-off indication for the power system;operate (106) the shut-off valve to isolate gas in the fuel volume;determine (112) the pressure of gas within the fuel volume at a first sample time;determine (112) the pressure of gas within the fuel volume at a second sample time;determine (112,114) 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,trigger (116) 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:following the detection of a power-off indication for the power system, the controller is adapted to purge (110) gas contained in the fuel volume to reduce the gas pressure to a predetermined target level.

3. The power system of Claim 1, wherein to purge gas contained in the fuel volume, the controller is adapted to continue to operate the one or more fuel delivery devices to reduce gas pressure in the fuel volume before determining the pressure of gas within the fuel volume at the first sample time.

4. 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.

5. 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 the monitoring 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.

6. 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.

7. A method of determining a leak of gas in a power system for a vehicle comprising, 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, wherein the method (100) comprises:detecting (104) a power-off indication for the power system when the power system is in a power-producing state;operating (106) the shut-off valve to isolate gas in the fuel volume;determining (112) the pressure of gas within the fuel volume at a first sample time;determining (112) the pressure of gas within the fuel volume at a second sample time;determining (112,114) 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 (116) a response action if the change and / or the rate of change of gas quantity exceeds a predetermined threshold.

8. The method of Claim 8, further comprising, purging (110) gas contained in the fuel volume to reduce the gas pressure to a predetermined target level, following the detection of a power-off indication for the power system.

9. The method of Claim 7 or 8, 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 the monitoring 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. The method of any one of Claims 7 to 9, further comprising determining 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 determining the quantity of gas in the fuel volume at the second sample time based on the gas pressure determined at the second sample time.

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