Fault detection in gas-fuelled engine systems

Abstract

Disclosed is an engine system 2 for a vehicle, the engine system having a cranking phase during which the engine system is cranking and a pre-cranking phase prior to engine cranking. The engine system

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 or another gaseous fuel delivered by port fuel injectors. Background Gaseous fuels such as hydrogen are promising alternative fuels to gasoline and diesel 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 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. The injector becoming stuck open at the end of an injection event can result in unwanted leakage of fuel into the engine. In some injectors, for example injectors for gasoline or diesel fuel, conventional methods for detecting stuck open injectors can rely on a glitch detection where a voltage is measured as the valve needle hits the conductive valve seat at the end of injection. Analysing the voltage can determine whether the valve needle has seated properly. However, in a gaseous fuel injector the valve seat is not metallic and instead the injector is provided with a synthetic rubber seat, such as a neoprene seat. Such glitch detection methods cannot therefore be used for gaseous fuel injectors. 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, in a first aspect, an engine system for a vehicle, the engine having a cranking phase during which the engine is cranking and a pre-cranking phase prior to engine cranking, the engine system comprising a rail volume for receiving fuel via a shut off valve; an air inlet manifold; a throttle valve which is operable in response to a user-demand to change the throttle valve position to control the supply of air into the air inlet manifold; a fuel injector for receiving fuel from the rail volume and for delivering fuel to the air inlet manifold; a rail sensorfor measuring a rail characteristic of the rail volume. The engine system further comprises a controller configured to, during the cranking phase, activate the shut off valve to close fuel delivery to the rail volume; control the throttle valve to close off the air supply to the air inlet manifold, regardless of the userdemand, thereby to define a throttle override phase during the cranking phase, and receive the rail characteristic for the throttle-override phase from the rail sensor; and compare the rail characteristic with a system characteristic determined for one of the pre-cranking phase or the cranking phase to determine a leak state of the injector. It is a benefit of the invention that an accurate determination of the leak state of the injector can be determined, before gaseous fuel is introduced to the rail (i.e. when there is no fuel supply to the injectors). Thus, a leak state determination can be made before a hazardous situation may arise. The system characteristic may be a pre-calibrated or pre-determined characteristic, such as the rail pressure in the pre-cranking phase, or may be a system characteristic such as the rate of change of air inlet manifold pressure during the throttle-override phase. A comparison of the leak rate of air from the rail volume in the throttle-override phase, with a pre-determined or pre-calibrated leak rate of air in a non-leaky injector, also enables a determination of the extent of the leak in the injector. In embodiments of the invention, the rail sensor is a rail pressure sensor and the rail characteristic is a pressure characteristic of fuel within the rail volume during the throttle-override phase, as measured by the rail pressure sensor. The pressure characteristic may be an average rail pressure within the rail volume during the throttle-override phase. In other embodiments, the pressure characteristic output is a rate of change in rail pressure within the rail volume during the throttle-override phase. The system may further comprise an air pressure sensor for determining air pressure within the air inlet manifold, the system characteristic being a characteristic of the pressure of air within the air inlet manifold measured by the air pressure sensor during the throttle-override phase. For example, the system characteristic may be a rate of change of air pressure within the air inlet manifold during the throttle-override phase, the controller being further configured to compare the rate of change in rail pressure within the rail volume during the throttle-override phase with the rate of change of air pressure within the air inlet manifold during the throttle-override phase, to determine the leak state of the injector. In embodiments, the rail sensor determines the system characteristic, which is an average rail pressure measured in the pre-cranking phase. In other embodiments, the controller may be configured to estimate the quantity of air within the rail volume in the pre-cranking phase, based on the rail characteristic measured during the pre-cranking phase; estimate the quantity of fuel within the rail volume in the throttle-override phase, based on the rail characteristic measured during the throttle-override phase; in a first comparison step, compare the estimate of the quantity of air within the rail volume in the pre-cranking phase with the estimate of the quantity of air within the rail volume in the throttle-override phase and determine a rate of leakage of air from the rail volume based on a comparison; and in a second comparison step, compare the rate of leakage of air from the rail volume with a predetermined leak rate indicative of a non-leaky injector to determine a leak state of the injector. By way of example, the pre-cranking phase may be at the start of a cranking phase of a crankshaft of the engine system. According to a second aspect of the invention, there is provided a method of detecting a leak in an engine system having a cranking phase during which the engine is cranking and a pre-cranking phase prior to engine cranking, the engine system comprising a rail volume for receiving fuel via a shut off valve; an air inlet manifold; a throttle valve which is operable in response to a user-demand to control the throttle valve position to control the supply of air into the air inlet manifold; and a fuel injector for receiving fuel from the rail volume and for delivering fuel to the air inlet manifold. The method may comprise measuring a system characteristic for one of the precranking phase or the cranking phase; measuring a rail characteristic of the rail volume, at least during the cranking phase; during the cranking phase, activating the shut off valve to close fuel delivery to the rail volume; controlling the throttle valve to close off the air supply to the air inlet manifold, regardless of the user-demand, to define a throttle override phase during the cranking phase, receiving the rail characteristic for the throttle-override phase from the rail sensor; and comparing the rail characteristic with the system characteristic for one of the pre-cranking phase or the cranking phase to determine a leak state of the injector. For example, the system characteristic may be a characteristic of the pressure of air within the air inlet manifold measured by the air pressure sensor during the throttleoverride phase. It will be appreciated that preferred and / or optional features of the first aspect of the invention may be incorporated alone or in appropriate combination within the second aspect of the invention also. 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 an engine system 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 engine system of Figure 1; and Figure 3 is a graph to illustrate rail pressure as a function of time compared with the air inlet manifold pressure as a function of time in an engine system in accordance with the invention, when detecting an injector leak. 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, particularly a fuel injector leak. The method is applicable to a port fuel injection system in which each of the fuel injectors of the power system is arranged to inject fuel into an air inlet manifold at a dedicated channel which leads to a respective one of the engine cylinders. The method is performed while closing a shut off valve between a fuel tank and a common rail for storing fuel, and placing the system in a throttle-override condition, in which the throttle demand from the user (e.g. vehicle driver) is overridden and the throttle is closed. 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 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 (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 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. 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. 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 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 ‘engine 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 (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, 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 identified). The fuel delivery system 8 comprises a set of one or more fuel delivery devices in the form of 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. In the illustrated example, there area plurality of fuel injectors 26, the number of which corresponds to the number of combustion chambers 12. The system is a port injection engine and so 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. 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. 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 and is monitored by means of a sensor in the form of a fuel or rail pressure sensor 30. The rail 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 rail pressure sensor 30 are configurational aspects that are not central to the invention. The rail pressure sensor 30 provides an output signal (not shown), which is representative 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. 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 may be 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 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. 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. 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 a particular combustion chamber 12 using associated software. The knock sensors and 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 a system sensor in the form of an air pressure sensor (or air inlet manifold 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 pressure sensor 42 provides a pressure sensor output to the control unit 11 which is representative of the air pressure in 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 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 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 rail 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. Note that the rail 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, or through the connection of a CAN-bus (Controller Area Network) (not shown) which is conventional in automotive technology. Other sensors may be included in the system, such as a temperature sensor (not shown) for measuring the temperature of the fuel in the rail 28. 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. 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 11 has the necessary memory (e.g. NVM 13), processing environment (not shown) and communications interface (not shown) to be integrated into the engine system and the broader system of the associated vehicle. One challenge associated with hydrogen-fuelled engines is the potential for leaks to occur. In the present invention, a method is presented in which a stuck open fuel injector can be detected and action taken to prevent further leaks. This may arise, for example, due to mechanical tolerances or as a result of debris or another foreign object being embedded in the valve seat. In more sophisticated methods of the invention, the leak rate can be determined so that the extent of damage to the injector can be ascertained. An example algorithm or method can be implemented by the control unit 11 in order to determine when the valve needle of an injector 26 has become stuck open and there is a gas leak. More specifically, the method is suitable to be performed on the port injection system described previously in which the fuel injectors deliver fuel to the air inlet manifold before delivery to the engine, as shown in Figure 1. The method is based on the principle that, if the injector valve needle is stuck open, there is a communication path through the stuck open injector between the air inlet manifold and the common rail 28 and this can be used to provide a determination of the leak state of the injector. Rail pressure measurements are sampled to determine the leak state of the injector. In practice, the rail pressure measurements may be sampled continuously throughout engine operation, for a variety of other reasons, until the subsequent sleep mode for the control unit 11, but in the method of the invention the rail pressure measurements are analysed under a particular set of conditions, or over a specified period, to determine whether there is an injector leak. Importantly, the rail pressure measurements are made when there is no supply of fuel to the common rail 28. Air inlet manifold pressure measurements may also be made to add an enhancement to the method. As an initial step of the method, at key on, a measurement is made of the pressure P0 within the common rail 28 and this is recorded in the RAM of the control unit 11. This provides a pre-cranking system characteristic in the form of a pressure measurement (P0) for the common rail. The control unit 11 may include a Non Volatile Memory (NVM) 13. As the engine starts to crank, the shut off valve 38 is closed. The measurement of P0 may be taken at the start of cranking, or before cranking, which is a period referred to as the pre-cranking phase of the engine. With the shut-off valve closed, no fuel is delivered to the common rail 28. The cranking speed starts to increase and, once a cranking speed threshold is exceeded, the control unit 11 overrides the driver-demanded throttle position and closes the throttle valve 22. In practice it may be beneficial to close the throttle valve 22 as soon as the engine starts to crank and so the cranking speed threshold may be set to zero (this may be a calibratable threshold value). This is referred to as the “throttle-override phase” during which, as the engine continues to crank, there is a depression in the air inlet manifold 24 as the intake stroke of the cylinder piston sucks air from the air inlet manifold 24. The signal from the rail pressure sensor 30 is sampled and recorded at fixed intervals through the throttle-override phase to monitor the air pressure in the rail through this phase. The pressure in the air inlet manifold 24 is also monitored in this phase. With the engine cranking and the throttle valve 22 closed, as a first check on the leak state of the injector a rail characteristic measurement is made, in the form of a rail pressure measurement P1. The rail pressure measurement P1 may be made at a predetermined time or predetermined engine crank angle. The rail pressure measurement P1 is compared with the rail pressure measurement P0. The difference in rail pressure between the before-cranking condition and the after-cranking condition (i.e. P1-P0) is compared with an expected, acceptable and pre-calibrated pressure difference for a ‘healthy’ engine. If the difference (P1-P0) is below a predetermined pressure difference representative of a healthy engine, or is not within a predetermined tolerance of the predetermined pressure difference, it may be determined that there may be a leak in the injector. In practice the rail pressure difference (P1-P0) may be monitored for a continuous period of time (a defined period) to check whether any pressure drop from the initial rail pressure measurement P0 is maintained fora period of time. If there is a leak, a continuous pressure difference is seen between P1 and P0. In contrast, in a healthy system where there is no leak, this would not be expected. In this case the rail pressure P1 would be independent of the air inlet manifold variation. To avoid any false detection of a leak, a filtering stage may be applied to the rail pressure measurements before the comparison is made between P0 and P1. In one embodiment of the invention, the method may cease at this point and a fault signal may be provided to the driver to indicate that there is a leak and / or the engine may be shut down. Whilst the comparison of the rail pressure before cranking and after cranking (with the throttle valve closed) can provide an indication of whether there is a leak, in order to improve on the detection strategy, further steps may be implemented to add additional accuracy to the method. In other embodiments of the invention, therefore, subsequent steps may be performed (in addition or as an alternative) to determine with increased accuracy whether there is a leak. Figure 2 is a state diagram to illustrate the methodology of measuring air inlet manifold pressure and rail pressure and looking at the relationship between the two to determine whether there is a leak. The control unit 11 overrides the user-demanded throttle position and closes the throttle valve 22 so that, as the engine continues to crank, there is a depression in the air inlet manifold 24 as the intake stroke of the cylinder piston sucks air from the air inlet manifold 24. The signal from the rail pressure sensor 28 is sampled and recorded through the throttle-override phase, together with the signal from the air inlet manifold sensor 42. The expected, average steady-state pressure in the air inlet manifold 24 will drop below the ambient pressure prior to cranking. The decrease in pressure will depend on factors such as engine characteristics and health status (e.g. the valves, rings, and head gasket) and the position of the throttle valve 22. With a vacuum drawn in the air inlet manifold 24, and with the throttle valve 22 closed, gas will be extracted from the common rail 28 through a leaky injector so that the rail pressure will follow the trend of the air inlet manifold pressure, but with a time lag. Depending on the leak rate and the length of the throttle-closed phase, the rail pressure may or may not reach the level of the air inlet manifold pressure. Therefore, by comparing the rail pressure characteristic (e.g. rail pressure as a function of time) with the air inlet manifold pressure characteristic (e.g. air inlet manifold pressure as a function of time), the extent of any leak can be determined. The rail pressure characteristic and the air inlet manifold characteristic are stored in the NVW 13 and the control unit 11 carries out the analysis to compare the characteristics to ascertain the extent of the leak. This is illustrated in Figure 3 where the first trace A shows the rail pressure as a function of time compared to the air inlet manifold pressure over the same time period. It can be seen that the general characteristic of the rail pressure signal as a function of time follows the air inlet manifold pressure as a function of time, with both decreasing after time TO when cranking starts. Eventually the air inlet manifold pressure reaches a steady state before detection ends at time T1. The rail pressure signal is monitored in the throttle-closed phase and if a decrease is seen, together with a corresponding decrease in the air inlet manifold pressure, then this may be suggestive of a leak. By way of example, the traces A and B shown in Figure 3 would indicate a leaky injector as the decrease in rail pressure (trace A) follows the decrease in air inlet manifold pressure (trace B). In other words, seeing both a decrease in rail pressure (which is not consistent with a healthy injector), and a corresponding decrease in air inlet manifold pressure, implies a leak because it implies there is a communication path open between the two volumes (i.e. through the leaky injector). The rate at which pressure in the rail decreases, relative to the rate of decrease of pressure in the air inlet manifold, will vary dependent on the leak rate through the injector so a comparison of the rail pressure signal with the air inlet manifold pressure signal can also be used to estimate the leak rate. For example, the higher the rate of decrease in rail pressure for a given change in pressure across the injector (the difference between rail pressure P0 and the air inlet manifold pressure at steady state) the greater the extent of the leak. If the pressure comparisons suggest that the injector is leaky (partially or fully), then the engine may be shut down and / or a fault signal may be generated. If the pressure comparisons suggest that the injector is not leaky (partially or fully), then the injectors are enabled, the throttle is controlled according to the user commands and engine operation continues normally. In an alternative embodiment of the invention, the following steps are performed immediately after key on and without necessarily performing the previous mentioned rail pressure and / or air inlet manifold pressure measurements. Prior to cranking an estimate is made of the quantity (Q1) of air in the common rail 28. This is based on the temperature measurement from the temperature sensor 45 and the pressure measurement from the rail pressure sensor 30. The quantity of air in the common rail 28 prior to cranking Q1 may be referred to as “the standard condition”. The value of the air quantity Q1 is stored in the NVM 13. In response to an engine start request (e.g. at key on), the control unit 11 activates the starter motor 46. The shut off valve 38 is closed and the spark and injectors 26 are disabled. The air quantity Q2 is estimated based on the temperature measurement from the temperature sensor 45 and the pressure measurement from the rail pressure sensor 30 during the cranking phase. As before, it may be that a cranking speed threshold is set to zero so that the shut off valve is closed only after the cranking speed reaches a predetermined threshold. By comparing the air quantity before and after cranking (Q1 compared to Q2), a determination of the change in air quantity in the common rail 28 can be made. As the time period over which the change in air quantity (Q1 -Q2) is measured is also known, the leak rate of air through the injector 26 can be estimated to give a determination of whether or not there is a significant leak of air through the injector 26. As mentioned before, the leak rate is a function of the change in pressure across the injector (the difference between the rail pressure and the air inlet manifold pressure). In this calculation, the value (Q1-Q2) is the average leak rate at the average change in pressure across the injector. The average leak rate may be used to estimate or predict the leak rate at certain operating conditions (e.g. nominal rail pressure) which may be useful. The leak rate can be compared to a predetermined leak rate threshold stored in the NVM 13, above which it can be ascertained that there is a leak of concern through the injector. The predetermined leak rate threshold is calculated prior to engine running in a calibration phase of operation of the engine. With the throttle valve 22 closed, and the engine cranking at a speed greater than the threshold cranking speed, the air inlet manifold pressure will be decreasing from ambient pressure until it reaches a steady-state pressure (as seen in trace A of Figure 5). The pressure signal will have some perturbations, as seen in Figure 5, corresponding to the engine phases. The perturbations can be eliminated before the air inlet manifold pressure characteristic is analysed using suitable filtering techniques. In this phase or operation, the average pressure in the air inlet manifold 24 will be below the ambient (pre-cranking) pressure (as seen in Figure 4). If the leak rate is not within an acceptable limit (which may be zero leak rate), the method proceeds to abort the engine start and a fault signal may be provided to the driver and / or stored in the NVM 13. If the leak rate is within an acceptable threshold of the threshold level, normal operation continues so that the injectors 26 and the spark are enabled and the shut off valve 38 is opened. This continues until the next key off / key on cycle when the method is repeated to gain check for a leaky injector. The rail pressure measurements may be sampled at a convenient sampling interval which is selected according to the cranking speed of the engine through the throttleoverride phase. As the cranking speed is relatively low in this phase (typically around 200 rpm), the sampling frequency need not be a high rate and may typically be once every 10 ms. In practice, measurements may be sampled at a higher rate and used for other purposes also, as well as leak detection. 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.

Claims

1. An engine system (2) for a vehicle, the engine system (2) having a cranking phase during which the engine system (2) is cranking and a pre-cranking phase prior to engine cranking, the engine system (2) comprising;a rail volume (28) for receiving fuel via a shut off valve (38);an air inlet manifold (24);a throttle valve (22) which is operable in response to a user-demand to change the throttle valve position to control the supply of air into the air inlet manifold (24);a fuel injector (26) for receiving fuel from the rail volume (28) and for delivering fuel to the air inlet manifold (24);a rail sensor (30) for measuring a rail characteristic of the rail volume (28); anda controller configured to, during the cranking phase:activate the shut off valve (38) to close fuel delivery to the rail volume (28);control the throttle valve (22) to close off the air supply to the air inlet manifold (24), regardless of the user-demand, thereby to define a throttle override phase during the cranking phase, andreceive the rail characteristic for the throttle-override phase from the rail sensor (30); andcompare the rail characteristic with a system characteristic determined for one of the pre-cranking phase or the cranking phase to determine a leak state of the injector (26).

2. The engine system (2) as claimed in claim 1, wherein the rail sensor is a rail pressure sensor (30) and the rail characteristic is a pressure characteristic of fuel within the rail volume (28) during the throttle-override phase, as measured by the rail pressure sensor (30).

3. The engine system (2) as claimed in claim 2, wherein the pressure characteristic is an average rail pressure within the rail volume (28) during the throttle-override phase.

4. The engine system (2) as claimed in claim 2, wherein the pressure characteristic is a rate of change in rail pressure within the rail volume (28) during the throttle-override phase.

5. The engine system (2) as claimed in any of claims 1 to 4, further comprising an air pressure sensor (42) for determining air pressure within the air inlet manifold (24), the system characteristic being a characteristic of the pressure of air within the air inlet manifold (24) measured by the air pressure sensor (42) during the throttle-override phase.

6. The engine system (2) as claimed in claim 5 when dependent on claim 4, wherein the system characteristic is a rate of change of air pressure within the air inlet manifold (24) during the throttle-override phase, the controller being further configured to compare the rate of change in rail pressure within the rail volume (28) during the throttle-override phase with the rate of change of air pressure within the air inlet manifold (24) during the throttle-override phase, to determine the leak state of the injector (26).

7. The engine system (2) as claimed in any of claims 1 to 3, wherein the rail sensor (30) determines the system characteristic, which is an average rail pressure (P0) measured in the pre-cranking phase.

8. The engine system (2) as claimed in claim 1, wherein the controller is configured to;estimate the quantity of air within the rail volume (28) in the pre-cranking phase, based on the rail characteristic measured during the pre-cranking phase;estimate the quantity of fuel within the rail volume (28) in the throttle-override phase, based on the rail characteristic measured during the throttle-override phase;in a first comparison step, compare the estimate of the quantity of air within the rail volume (28) in the pre-cranking phase with the estimate of the quantity of air within the rail volume (28) in the throttle-override phase and determine a rate of leakage of air from the rail volume (28) based on a comparison; andin a second comparison step, compare the rate of leakage of air from the rail volume (28) with a pre-determined leak rate indicative of a non-leaky injector to determine a leak state of the injector.

9. The engine system (2) as claimed in any of claims 1 to 8, wherein the precranking phase is at the start of a cranking phase of a crankshaft of the engine system (2).

10. A method of detecting a leak in an engine system (2) having a cranking phase during which the engine is cranking and a pre-cranking phase prior to engine cranking, the engine system (2) comprising a rail volume (28) for receiving fuel via a shut off valve (38); an air inlet manifold (24); a throttle valve (22) which is operable in response to a user-demand to control the throttle valve position to control the supply of air into the air inlet manifold (24); and a fuel injector (26) for receiving fuel from the rail volume (28) and for delivering fuel to the air inlet manifold (24); the method comprising:measuring a system characteristic for one of the pre-cranking phase or the cranking phase;measuring a rail characteristic of the rail volume, at least during the cranking phase;during the cranking phase, activating the shut off valve (38) to close fuel delivery to the rail volume (28);controlling the throttle valve (22) to close off the air supply to the air inlet manifold (24), regardless of the user-demand, to define a throttle override phase during the cranking phase,receiving the rail characteristic for the throttle-override phase from the rail sensor (30); andcomparing the rail characteristic with the system characteristic for one of the pre-cranking phase or the cranking phase to determine a leak state of the injector (26).21

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