Fault detection in gas-fuelled engine systems

Abstract

Disclosed is n engine system 2 for a vehicle, the engine system comprising a fuel injector 26 for delivering fuel directly into an associated engine cylinder 12, the associated cylinder having a cylin

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 or another gaseous fuel delivered by direct injection 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 an injector for gasoline or diesel, for example, conventional methods for detecting stuck open injectors rely on a glitch detection where a voltage is measured as the valve needle hits the 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 system comprising a fuel injector for delivering fuel directly into an associated engine cylinder, the associated cylinder having a cylinder piston which is driven to perform a piston cycle upon rotation of an engine crankshaft; and a rail volume for receiving fuel via a shut off valve (and delivering fuel to the injector, the shut off valve having a shut off valve closed phase when the shut off valve is closed to prevent fuel delivery to the rail volume. The engine system comprises means for determining a cylinder characteristic in the shut off valve closed phase; a rail pressure sensor for measuring rail pressure as a function of crankshaft angular position in the shut off valve closed phase to define a rail pressure characteristic for the rail volume and a controller configured to activate the shut off valve to close fuel delivery to the rail volume in the shut off valve closed phase; disable the fuel injector in the shut off valve closed phase; receive the rail characteristic from the rail pressure sensor; and perform a comparison by comparing the rail pressure characteristic with the cylinder characteristic to determine a leak state of the injector. The invention is applicable to a direct injection engine in which the fuel injectors inject fuel directly to the engine cylinders, as opposed to a port injection system where the injectors deliver fuel to the air inlet manifold. One benefit of the invention is that the leak state of the injector can be determined without the requirement to supply gaseous fuel to the injector as the shut off valve is closed for the test. The controller therefore implements a method that is hazard free and does not risk gaseous fuel being supplied to a leaky injector. The invention allows the leak state of the injectors to be determined by looking at the rail pressure characteristic and the cylinder characteristic (for example a measured cylinder pressure characteristic or a pre-determined cylinder pressure characteristic), and checking whether the rail pressure characteristic tracks or follows the cylinder pressure characteristic. If the rail pressure characteristic shows a variation in the rail pressure, as a function of the crankshaft angular position, in the same manner as the cylinder pressure characteristic, this is indicative of a leak. In embodiments of the invention the means for determining a cylinder characteristic may comprise a sensor for measuring cylinder pressure as a function of crankshaft angular position, to define a cylinder pressure characteristic. In other embodiments, the means for determining the cylinder characteristic comprises a memory of the controller which stores a predetermined cylinder pressure characteristic relating the cylinder pressure to the crankshaft angular position, thus defining the cylinder characteristic. By way of example, the controller may be configured, for the comparison, to determine whether the rail pressure characteristic has a pressure peak which corresponds to a peak in the cylinder pressure characteristic and, if it is determined that there is a correspondence, provide an output to indicate that there is a leak state of the injector. In other embodiments, looking for a peak in the rail pressure corresponding to a position of top-dead-centre (TDC) for a cylinder piston can also be used to provide an indication of a leak. In other words, the cylinder characteristic may be a position of top-dead-centre for an engine cylinder, the controller being configured to perform the comparison by comparing a position of a peak in the rail pressure with the position of top-dead-centre for an engine cylinder to determine the leak state of the injector associated with said engine cylinder. For example, the controller may be configured to determine a peak rail pressure from the rail pressure characteristic at a selected crankshaft angular position; determine a peak cylinder pressure from the cylinder pressure characteristic at the selected crankshaft angular position; and compare the peak rail pressure with the peak cylinder pressure and, as a result of the comparison, determine the extent of the leak state of the fuel injector. The engine system may comprise a plurality of fuel injectors, each for delivering fuel to an associated engine cylinder, wherein the controller is configured to determine, based on the rail pressure characteristic having a plurality of peaks in rail pressure as a function of crankshaft angular position, a frequency of the peaks, and compare the frequency of the peaks with a crankshaft frequency of rotation and provide a comparison output, on the basis of the comparison output, determine which of the plurality of fuel injectors is leaking. In any embodiment, the controller may be configured to output a fault detection signal in the event that a leak state is determined in the or each injector, or may be configured to provide any other appropriate response in the event that a leak is detected (for example, shutting down the engine system in its entirety). In a second aspect of the invention, there is provided a method of determining a leak state of a fuel injector for delivering fuel directly into an associated engine cylinder in an engine system, the engine system having a rail volume for receiving fuel via a shut off valve and for delivering fuel to the injector and the associated engine cylinder having a piston which is driven to perform a piston cycle upon rotation of an engine crankshaft. The method comprises closing of supply of fuel to the rail volume in a shut off valve closed phase; disabling the fuel injector in the shut off valve closed phase; determining a cylinder characteristic in the shut off valve closed phase whilst the crankshaft is rotating; measuring rail pressure as a function of crankshaft angular position in the shut off valve closed phase to define a rail pressure characteristic for the rail volume; and performing a comparison by comparing the rail pressure characteristic with the cylinder characteristic to determine a leak state of the injector. 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 diagram illustrating the initial steps of a method implemented in an example algorithm that may be implemented by a controller of the engine system of Figure 1; Figure 3 is a graph to show the theoretical cylinder pressure characteristic compared with the rail pressure characteristic which may be used to implement a leak detection method implemented in an algorithm that may be implemented by a controller of the engine system of Figure 1; Figure 4 is an example graph of cylinder pressure as a function of crankshaft angular position (crank angle) for the engine system in Figure 1; and Figure 5 is a flow diagram illustrating further steps of a method implemented in an example algorithm that may be implemented by a controller of the engine system of Figure 1. Detailed Description In general, the examples of the invention provide a power or engine 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 direct fuel injection system in which each of the fuel injectors of the engine system is arranged to inject fuel directly into 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, so that the fuel supply to the common rail is turned off during the leak detection method. 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 ECU includes the appropriate memory to store values of the calculations and measurements, code instructions and calibration data, including non volatile permanent memory (Flash / ROM) and temporary memory (RAM). For example, the ECU may store the values of calculations and measurements in RAM, and may store code instructions and calibration data in the Flash / ROM. Therefore, the permanent memory 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. A low power counter is may also be included 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, directly into the engine cylinders 12. This type of engine is referred to as a direct injection fuel system. In the illustrated example, there area plurality of fuel injectors 26, the number of which corresponds to the number of combustion chambers 12. In the example of a direct injection fuel system the fuel injectors 26 are arranged to inject fuel directly into a respective one of the engine cylinders 12. The fuel injectors 26 are each connected to a fuel accumulator or ‘common rail’ 28. As is known, the common rail 28 provides a relatively large volume of fuel which is 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. A barometric sensor may be provided (not shown) to measure atmospheric pressure. Fuel and air mixture in the engine 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 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 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. 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 1, 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 cylinder 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. 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. 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 and 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 direct injection system described previously in which the fuel injectors deliver fuel directly to the engine cylinder, as shown in Figure 1. The method is based on the principle that, if the injector valve needle is stuck open in the injector 26, there is a relationship between the pressure in the engine cylinder 12 and the pressure in the rail 28 because of the communication path between the two, through the leaky injector 26. By comparing the pressure variation in the cylinder 12 with the pressure variation in the rail 28, the leak state of the injector 26 can therefore be determined. If the rail pressure is constant, or substantially constant, and does not track the cylinder pressure variation, it can be ascertained that there is no leak. In a more fundamental implementation of the invention, when there is no injection occurring and the shut off valve 38 is closed, looking for a peak in rail pressure at a point corresponding to TDC can be taken as an indication that there is a leak. 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 pressure measurements are analysed under a particular set of conditions, or over a specified period, to determine whether there is an injector leak. Referring to Figure 2, as an initial step of the method, at key on, a measurement may be 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 28. This provides a reference value for the rail pressure. The measurement of P0 may be taken at the very start of cranking, or sometime prior to cranking but after key on, and this is a period referred to as the pre-cranking phase of the engine. The starter motor 46 is activated and the engine starts to crank with the shut off valve 38 closed. The injectors 26 are disabled (i.e. not injecting). The crankshaft of the engine is rotated at a speed of rotation referred to as the engine rpm. The cranking speed starts to increase and, once a cranking speed threshold is exceeded, the control unit 11 enters the next steps of the leak detection method. With the engine crankshaft rotating, and with the injectors 26 disabled so that there is no injection by the injectors (no combustion), each cylinder piston moves between bottom-dead-centre (BDC) and top-dead-centre (TDC) and the pressure in the associated engine cylinder fluctuates between ambient (atmospheric) pressure at BDC and peak pressure at TDC. The peak pressure will depend on the engine compression ratio and the status, but for a healthy engine (non-leaky injector(s)), the peak pressure should be at least 10 bar. If there is a leaky injector 26, air will be pushed through the leaky injector 26 into the common rail 28 as the cylinder piston cycles, causing the pressure in the common rail 28 to cycle also. Depending on the size of the leak (i.e. the restriction), the pressure in the common rail 28 will cycle with cylinder pressure, but with rail pressure unlikely to reach peak pressure. In an initial embodiment, the rail pressure may be analysed to look for a peak in rail pressure, and a comparison may be made against a cylinder characteristic for the cylinders 12, for example comparing when a peak in rail pressure occurs relative to when TDC occurs. If a peak in rail pressure is seen at the point of TDC for a particular cylinder 12 (in other words if the rail pressure is above a certain pre-determined threshold background level at the point of TDC), this indicates that there is a leak in the injector 26 associated with that cylinder 12. In this case, no knowledge of the cylinder pressure is required at all: it is only required to know the cylinder characteristic of when TDC occurs. Figure 3 shows the cylinder pressure peaks for a four-cylinder engine which is firing in the usual sequence of cylinder 1-cylinder 3-cylinder 4-cylinder 2. If a rail pressure peak is seen near a TDC position, the corresponding cylinder 12 is likely to have a leaky injector 12. In this example, there is a leak with the injector 26 for cylinder 3 as a peak in the rail pressure is seen corresponding to TDC for cylinder 3. If multiple injectors 26 are leaking, the rail 28 will have multiple communication channels to the outside environment through the leaky injectors 26. At any giving point (around TDC) air may be compressed into the rail 28 through one of the leaky injectors 26, but at the same time, once rail pressure increases, air will escape from other leaky injectors. This means that multiple peaks in rail pressure are seen if there are multiple leaky injectors 26. However, the change in pressure across a leaky injector 26 is different between different injectors 26 depending on the point in the cycle: the injector 26 for the cylinder 12 at the compression TDC will have higher change in pressure across the injector 26 than the rest. That means that, if injectors 26 are equally leaky, the volume of air entering the rail 28 is greater than the volume of air leaving the rail 28. This results in fluctuations in the rail pressure measured by the control unit 11 having a lower amplitude. The method of the invention is more sensitive in detecting a single leaky injector 26, or if the leakage in one injector 26 is significantly worse than the leakage in the other injectors 26 if multiple injectors 26 are leaking, which in practice it should be the most common scenario. In an advancement of the previous embodiment, the cylinder characteristic to be compared with the rail pressure may take the form of the cylinder pressure characteristic. A comparison of the rail pressure as a function of the crankshaft angular position (referred to as a rail pressure characteristic), relative to the cylinder pressure as a function of crankshaft angular position (referred to as a cylinder pressure characteristic), can therefore be used to determine the leak state of the injector. For a non-leaky injector 26 there should be no variation in rail pressure as a function of crankshaft angular position, regardless of the pressure cycle in the cylinder 12. In other words, the rail pressure should remain substantially constant if the shut-off valve 38 is closed and the injectors 26 are disabled / closed. Moreover, the rail pressure should be independent of the cycle of cylinder pressure because in theory the two volumes are isolated from one another with the injectors 26 closed. For a leaky injector 26, however, the rail pressure is a function of the crankshaft angular position and follows the characteristic of cylinder pressure shown in the graph in Figure 4. Over several cranking cycles the cylinder pressure varies between minimum and maximum pressures, due to the operation of the inlet and exhaust valves of the engine and the reciprocating movement of the cylinder piston, to reduce and expand the volume of the engine cylinder 12. If there is a leaky injector, the rail pressure (not shown in Figure 4) follows the cyclic variation of cylinder pressure through the cycle of engine rotation. In general, the extent of the variation in peak rail pressure, relative to peak cylinder pressure, indicates the extent to which an injector 26 is leaking, depending on how high a pressure is reached in the common rail 28. The variation in the peak rail pressure can therefore be monitored to determine or quantify the extent (severity) of the injector leak. In the unlikely event that multiple injectors 26 are leaking at the same time, this method may not be decisive in quantifying the leak, as will be discussed further below. In practice the comparison of the rail pressure with the cylinder pressure may be performed by comparing the rail pressure and the cylinder pressure at a particular crankshaft position, or by measuring the rail pressure over a fixed interval and using an averaging method to determine an average rail pressure value which is then compared to the average cylinder pressure over the same time interval. The time interval for measurement would be selected to be a time period within the compression stroke for the cylinder 12. The further steps of the leak detection method are illustrated in the flow diagram of Figure 5. As the next step in the method, and with the shut-off valve 38 still closed and the engine cranking above the predetermined cranking speed threshold, the rail pressure is measured as a function of crankshaft angular position. This pressure is then compared with the cylinder pressure to determine the injector leak state. If a leak state is determined as a result of the common rail pressure varying with crankshaft angular position, the leak detection 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. If there is substantially no variation of common rail pressure with crank angle, it can be assumed that the injectors 26 are healthy (i.e. no leak) and normal engine operation follows. Prior to the control unit 11 comparing the rail pressure variation as a function of crankshaft angular position with the cylinder pressure variation with crankshaft angular position, the signal from the sensors 30, 42 indicative of the pressure variations may be filtered to remove perturbations on the signal due to noise. The rail pressure measurements may be sampled at a convenient sampling interval which is selected according to the cranking speed of the engine. The sampling frequency may typically be once every 1 ms. In practice, rail pressure measurements may be sampled and used for other purposes also, as well as leak detection. In other embodiments, the rail pressure may be sampled at a fixed crankshaft position, rather than a fixed sampling interval, as mentioned previously. It is one possibility that at any one time, more than one injector 26 may be leaking. If only one injector 26 is leaking, the frequency of the peaks in the rail pressure characteristic will be equivalent to 0.5 engine rpm (i.e. the peaks will occur once for every half revolution of the crankshaft). This is because each cylinder 12 has a compression stroke every two engine revolutions. If two injectors 26 are leaking, the frequency of the rail pressure variation will be double this (i.e. the frequency of the peaks in the rail pressure characteristic will be equivalent to one engine rpm), and if three injectors 26 are leaking, the frequency of the peaks in the rail pressure characteristic will be equivalent to 1.5 engine rpm. If all injectors 26 are leaking together then the frequency of the peaks in the rail pressure characteristic would be expected to be equivalent to 2 engine rpm. Therefore, whereas a variation in the rail pressure measurement as a function of crank angle indicates a leak in an injector 26 (or more than one injector), and the amplitude of the peaks in the rail pressure characteristic indicates the extent of the leak, a measurement of the frequency of the peaks in the rail pressure characteristic, relative to the engine rpm, provides an indication of the number of injectors 26 which have a leak. Further analysis may be performed to determine which of the injectors 26 are leaky. In other embodiments, as an alternative to using a cylinder pressure sensor to measure the cylinder pressure characteristic which is to be compared to the rail pressure characteristic, a cylinder pressure characteristic for comparison purposes may be determined on the basis of calibration data relating to the expected cylinder pressure. This calibration data relating to the cylinder pressure characteristic is obtained offline, prior to installation in the engine or when the engine is new, and is stored in the permanent memory of the control unit 11. The method follows the same steps as for the previous embodiment where the cylinder pressure sensor measurements are used for comparison with rail pressure, but here the rail pressure is compared with the pre-determined cylinder pressure characteristic for the appropriate engine operating conditions. 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 embodiments have been discussed above.

Claims

1. An engine system (2) for a vehicle, the engine system (2) comprising a fuel injector (26) for delivering fuel directly into an associated engine cylinder (12), the associated cylinder (12) having a cylinder piston which is driven to perform a piston cycle upon rotation of an engine crankshaft;a rail volume (28) for receiving fuel via a shut off valve (38) and delivering fuel to the injector (26) the shut off valve (38) having a shut off valve closed phase when the shut off valve (38) is closed to prevent fuel delivery to the rail volume (28);means (42) for determining a cylinder characteristic in the shut off valve closed phase;a rail pressure sensor (30) for measuring rail pressure as a function of crankshaft angular position in the shut off valve closed phase to define a rail pressure characteristic for the rail volume (28); anda controller (11) configured to:activate the shut off valve (38) to close fuel delivery to the rail volume (28) in the shut off valve closed phase;disable the fuel injector (26) in the shut off valve closed phase;receive the rail characteristic from the rail pressure sensor (30); andperform a comparison by comparing the rail pressure characteristic with the cylinder characteristic to determine a leak state of the injector (26).

2. The engine system (2) as claimed in claim 1, wherein the cylinder characteristic is a position of top-dead-centre (TDC) for the engine cylinder (12); and wherein the controller (11) is configured to perform the comparison by comparing a position of a peak in rail pressure with the position of top-dead-centre for the engine cylinder (12) to determine the leak state of the injector (26) associated with said engine cylinder (12).

3. The engine system (2) as claimed in claim 1, wherein the means (42) for determining a cylinder characteristic is a sensor (42) for measuring cylinder pressure as a function of crankshaft angular position, to define the cylinder characteristic.

4. The engine system (2) as claimed in claim 1, wherein the means (42) for determining the cylinder characteristic comprises a memory of the controller (11) which stores a predetermined cylinder pressure characteristic relating the cylinder pressure to the crankshaft angular position, to define the cylinder characteristic.

5. The engine system (2) as claimed in claim 3 or claim 4, wherein the controller (11) is configured, for the comparison, to determine whether the rail pressure characteristic has a pressure peak which corresponds to a peak in the cylinder pressure characteristic and, if it is determined that there is a correspondence, provide an output to indicate that there is a leak state of the injector (26).

6. The engine system (2) as claimed in any of claims 1 to 5, wherein the controller (11) is configured to;determine a peak rail pressure from the rail pressure characteristic at a selected crankshaft angular position;determine a peak cylinder pressure from the cylinder pressure characteristic at the selected crankshaft angular position; andcompare the peak rail pressure with the peak cylinder pressure and, as a result of the comparison, determine the extent of the leak state of the fuel injector (26).

7. The engine system (2) as claimed in any of claims 1 to 6, comprising a plurality of fuel injectors (26), each for delivering fuel to an associated engine cylinder (12), wherein the controller (11) is configured to;determine, based on the rail pressure characteristic having a plurality of peaks in rail pressure as a function of crankshaft angular position, a frequency of the peaks, and;compare the frequency of the peaks with a crankshaft frequency of rotation and provide a comparison output,on the basis of the comparison output, determine which of the plurality of fuel injectors (26) is leaking.

8. The engine system (2) as claimed in any of claims 1 to 7, wherein the controller (11) is configured to output a fault detection signal in the event that a leak state is determined in the or each injector (26).

9. A method of determining a leak state of a fuel injector (26) for delivering fuel directly into an associated engine cylinder (12) in an engine system (2), the engine system (2) having a rail volume (28) for receiving fuel via a shut off valve (38) and for delivering fuel to the injector (26) and the associated engine cylinder (12) having a piston which is driven to perform a piston cycle upon rotation of an engine crankshaft, the method comprising;closing of supply of fuel to the rail volume (28) in a shut off valve closed phase;disabling the fuel injector (26) in the shut off valve closed phase;determining a cylinder characteristic in the shut off valve closed phase whilst the crankshaft is rotating;measuring rail pressure as a function of crankshaft angular position in the5 shut off valve closed phase to define a rail pressure characteristic for therail volume (28); andperforming a comparison by comparing the rail pressure characteristic with the cylinder characteristic to determine a leak state of the injector (26).10

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