Fault detection in gas-fuelled engine systems

Abstract

An engine system (2) for a vehicle, comprising a rail volume (28) for receiving gaseous fuel; a shut off valve (38) for controlling the supply of fuel to the rail volume (28); an injector assembly including a plurality of fuel injectors (26), each one of the injectors (26) for receiving fuel from the rail volume (28) and for delivering fuel to an associated engine cylinder in an injection event; and a rail sensor (30) for measuring a rail characteristic of the rail volume (28) during an engine running phase. A controller is configured to receive a pre-determined rail pressure characteristic for a normal-functioning engine system (2), the pre-determined rail pressure characteristic including a pre-determined rail pressure gradient for an injection event corresponding to at least one of the fuel injectors (26) when functioning normally in the engine running phase; and to activate the shut off valve (38) to close fuel delivery to the rail volume (28). The rail characteristic is received from the rail sensor (30) for the engine running phase, the rail pressure characteristic including a rail pressure gradient for an injection event corresponding to at least one of the fuel injectors (26) when in the engine running phase; and the rail pressure characteristic is analysed by comparing the rail pressure gradient for the injection event with the pre-determined rail pressure gradient to determine a fault state of the engine system (2).

Description

[0001] FAULT DETECTION IN GAS-FUELLED ENGINE SYSTEMS

[0002] Technical Field

[0003] 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.

[0004] Background

[0005] 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.

[0006] 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.

[0007] 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.

[0008] It is with these issues in mind that the embodiments of the invention have been devised. Summary of the Invention

[0009] Against this background, examples of the invention provide, in a first aspect, an engine system comprising a rail volume for receiving gaseous fuel; a shut off valve for controlling the supply of fuel to the rail volume; and an injector assembly including a plurality of fuel injectors, each one of the injectors for receiving fuel from the rail volume and for delivering fuel to an associated engine cylinder in an injection event. The engine system further includes a rail sensor for measuring a rail characteristic of the rail volume during an engine running phase; and a controller configured to receive a pre-determined rail pressure characteristic for a normal-functioning engine system, the pre-determined rail pressure characteristic including a pre-determined rail pressure gradient for an injection event corresponding to at least one of the fuel injectors when functioning normally in the engine running phase; and activate the shut off valve to close fuel delivery to the rail volume. The controller is further configured to receive the rail characteristic from the rail sensor for the engine running phase, the rail pressure characteristic including a rail pressure gradient for an injection event corresponding to at least one of the fuel injectors when in the engine running phase; and analyse the rail pressure characteristic by comparing the rail pressure gradient for the injection event with the pre-determined rail pressure gradient to determine a fault state of the engine system.

[0010] The invention is applicable to both port injection fuel systems in which the fuel injectors deliver fuel to the engine cylinders via the air inlet manifold, or to direct injection systems in which the fuel injectors deliver fuel to the engine cylinders directly.

[0011] The invention allows a means for determining whether there is a leak state of the injector, and how the injector is faulty (e.g. stuck open, stuck closed or partially stuck open). The invention also allows leaks downstream of the shut off valve to be determined, for example in the pipework or interfaces of the downstream parts of the system.

[0012] The controller may be configured to provide an output to indicate that there is a fault state if the rail pressure gradient for the injection event for the or each of the other injectors is greater than the predetermined rail pressure gradient. This provides a means to determine when an injector is stuck partially open (not responsive), or is not sealing, or there is a leak downstream of the shut off valve, while the engine is running.

[0013] Further steps of the method can determine which scenario has given rise to the leak.

[0014] For example, the controller may be configured to provide an output to indicate that there is a stuck-open injector in a fault state if, in addition to the previous step, there is no change in the rail pressure gradient of the rail pressure characteristic for one of the injectors (the stuck open injector) upon initiation of the injection event. The extent gradient difference between the rail pressure characteristic for the stuck open injector and the predetermined rail pressure characteristic provides a means to determine the extent of the leak.

[0015] Each of the injectors may be engageable with an injector valve seat to terminate injection, and the controller may be configured to provide an output to indicate that an injector is not sealing at the injector valve seat when commanded to close against the injector valve seat if the rail pressure gradient is equivalent to the predetermined rail pressure gradient, in addition to the rail pressure gradient for the or each of the other injectors being greater than the predetermined rail pressure gradient.

[0016] In addition, or alternatively, the controller may be configured to provide an output to indicate that there is a fault downstream of the shut off valve if the rail pressure gradient for the injection event for all of the plurality of injectors is greater than the predetermined rail pressure gradient for the corresponding one of the injectors.

[0017] In addition, or alternatively, the controller may be configured to provide an output to indicate that there is a stuck-closed injector (i.e. the injector cannot be lifted from its valve seat) if the rail pressure gradient for an injector does not change upon initiation of an injection event for said injector, in addition to the rail pressure gradient for the or each other of the injectors being the same as the predetermined rail pressure gradient for the corresponding one of the injectors.

[0018] The controller may be configured to provide the output to generate at least one of (a) an indication to a user of the engine system that there is a fault; (b) a shut-down of the engine system; a shut-down of the injector in the fault state. According to a second aspect of the invention, there is provided a method of determining a leak state in an engine system for a vehicle, the engine system comprising a rail volume for receiving gaseous fuel; a shut off valve for controlling the supply of fuel to the rail volume; and an injector assembly including a plurality of fuel injectors, each one of the injectors for receiving fuel from the rail volume and for delivering fuel to an associated engine cylinder in an injection even. The method comprises measuring a rail characteristic of the rail volume from a rail sensor during an engine running phase; receiving a pre-determined rail pressure characteristic for a normal-functioning engine system, the pre-determined rail pressure characteristic including a pre-determined rail pressure gradient for an injection event corresponding to at least one of the fuel injectors when functioning normally in the engine running phase; activating the shut off valve to close fuel delivery to the rail volume; receiving the rail characteristic from the rail sensor for the engine running phase, the rail pressure characteristic including a rail pressure gradient for an injection event corresponding to at least one of the fuel injectors when in the engine running phase; and analysing the rail pressure characteristic by comparing the rail pressure gradient for the injection event with the pre-determined rail pressure gradient to determine a fault state of the engine system.

[0019] 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.

[0020] Further optional and advantageous features are referenced in the detailed description and the appended claims.

[0021] BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Examples of the invention will now be described with reference to the following figures:

[0023] 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 to illustrate the steps of a method which may be implemented by a controller of the engine system in Figure 1 , to detect the presence of a faulty injector;

[0024] Figures 3 to 7 illustrate graphs to show as a function of time (a) state of rail gas supply, (b) rail pressure and (c) injector drive pulse for the engine system of Figure 1 , in which: an injector of the engine system is partially stuck open (Figure 3); an injector of the engine system is partially open and not sealing but opens fully when commanded (Figure 4); there is a leak downstream of the shut off valve of the engine system (Figure 5); an injector of the engine system is stuck fully open (Figure 6); and an injector of the engine system is stuck fully closed (Figure 7).

[0025] Detailed Description

[0026] In general, the examples of the invention provide an engine system for a vehicle in which a common rail delivers fuel to a plurality of injectors and in which a controller is configured to detect a leak of gas from the system, particularly a fuel injector leak where the shut off valve controls a supply of gaseous fuel to the common rail. The method is applicable to a port fuel injection system in which each of the fuel injectors of the engine 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 also applicable to a direct injection system where the fuel is delivered directly to the cylinders of the engine. As well as providing a method of detecting a leak in the fuel injectors, the invention also provides a method of detecting a leak in the shut off valve itself, and / or anywhere in the fuel system downstream of the shut off valve (for example at the interface between the injectors and the common rail, as well as between in line between the common rail and the injectors). The method is performed while the engine is running and by closing the shut off valve between a fuel tank and the common rail, but in such a way as to minimise intrusion to the driver (i.e. the carrying out of the method should be imperceptible).

[0027] 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 . For the purpose of this description, and the main implementation of the invention, an engine system including a port fuel injection system will be described, but the invention is equally applicable to a direct fuel injection system.

[0028] 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.

[0029] 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.

[0030] 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’.

[0031] 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.

[0032] 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).

[0033] The fuel delivery system 8 comprises a set of one or more fuel delivery devices in the form of an injector assembly including a plurality 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.

[0034] In the illustrated example, there are a 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. The invention is also applicable to a direct fuel injection system in which the fuel injectors deliver fuel directly to the engine cylinder, and not via the air inlet manifold 24 (the direct injection system is not shown in the figures).

[0035] 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.

[0036] Fuel and air mixture in the engine cylinders12 is ignited by respective spark plugs 31 , in the usual manner.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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

[0044] 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.

[0045] 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 invention, however, is equally applicable to a direct injection system.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] The method is also able to detect leaks in other parts of the fuel injection system, downstream of the shut off valve 38.

[0050] 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.

[0051] The method is based on the principle that, if the injector valve needle is properly closed and not stuck open (i.e. if there is no leak at the valve needle seat), then there is no communication path through the injector between the air inlet manifold 24 and the common rail 28 (or between the common rail the engine cylinder in a direct injection system). Therefore, for a period of engine running when the shut off valve 38 is closed and the injector is not commanded to inject, there should be no reduction in gas volume / pressure in the common rail 28. If a reduction in gas volume / pressure is detected during this phase then it can be determined that a leak is occurring somewhere.

[0052] In practice, the rail pressure measurements may be sampled continuously throughout engine operation, for a variety of other reasons, but in the method of the invention the rail pressure measurements are analysed under a particular set of conditions to determine whether there is an injector leak. Importantly, the rail pressure measurements are carried out periodically during engine running by closing the shut off valve 38 of the engine system (off state) briefly and monitoring the fuel pressure in the rail for the period for which the shut off valve is closed.

[0053] During normal engine operation the injectors of the engine system are open and closed in accordance with the driver commands to deliver the required fuel quantity to the engine cylinder during a plurality of injection events. An algorithm of the controller 11 performs, intermittently, a leak detection method in accordance with one aspect of the invention. The leak test is performed only during certain engine operating conditions to ensure that leak detection is both robust and safe. For example, the method may be performed when running under steady state conditions or under low speed / low load conditions such as engine idle. Typically the conditions are selected so that the test is robust, safe and unintrusive to the driver when carried out. For example, it would not be desirable to run the test so that there was less engine torque than the drive intended, and so it is not likely that the method would be implemented when the driver is using the full torque of the engine.

[0054] Referring to Figure 2, as a first step 100 in the process the shut off valve 38 is closed so that the fuel supply to the common rail 28 is shut off. Closure of the shut off valve defines the start of a test phase for the system during which the rail pressure is measured periodically. Rail pressure measurements are then monitored and sampled continuously 102, through the test phase. These measurements are sampled through an injection sequence 104 including each injector being actuated to inject a delivery of fuel. The injection sequence includes a series of injection events (when the injector is injecting fuel) separated by between-injection periods (when the injector is not injecting fuel). When a specific injector is injecting, a valve needle of that injector is lifted away from a valve seat, and when that injector is not injecting the valve needle of the injector is seated against the valve seat.

[0055] The rail pressure measurements are analysed (steps 106, 108) to determine whether there is a fault or leak state of one of the injectors, as will be described in further detail below. With the shut off valve closed, and with no leaks at any of the injector 26 or within the low pressure system downstream of the shut off valve 38, it would be expected that the rail pressure in the common rail 28 stays constant. In contrast, during injection when there is a communication path between the air inlet manifold and the common rail through the injector, it would be expected that the rail pressure decreases according to the change in pressure within the air inlet manifold and the duration of the demanded injection pulse (in practice the duration of the injection pulse has the largest effect on the reduction of pressure in the rail). If a leak state is detected, appropriate action is taken at step 110 to output a fault signal, to terminate injection or to perform further diagnostic steps.

[0056] Figure 3 is a graph to illustrate the rail pressure characteristic which is measured using the leak detection method for an injector assembly for which the valve needle for one of the injectors (I NJ 1) is stuck open, thereby creating a leak path from the injector into the air inlet manifold (or in the case of the direct injection system, into the engine cylinder). The upper line graph shows the state of the gas supply to the common rail 28. This is initially ON and is then switched OFF at time T 1 when the shut off valve 38 is closed to define the start of the test phase. The shut off valve 38 remains closed until the end of the test phase at time T2. The lower line graph shows the injector drive pulses which initiate the injection sequence for injectors INJ1-INJ4.

[0057] The injector drive pulses are applied at regular intervals to injectors I NJ1 , INJ2, INJ3 and INJ4. Each injector drive pulse initiates an injection event in the associated injector whereby the valve needle of the injector is moved away from its valve seat to commence injection and, after a predetermined time interval, closes again to terminate injection when the drive pulse is removed. In the illustration shown in Figure 3, all four injectors INJ1 to INJ 4 inject once through the test phase between T1 and T2.

[0058] The middle line graph shows the rail pressure characteristic for an injector assembly including a partially stuck open injector (INJ1), identified as line B, and the rail pressure characteristic for an injector assembly in which all injectors are functioning normally, identified as Line A.

[0059] The steps of the leak detection method, as illustrated in Figure 2, will now be described in further detail.

[0060] For a forthcoming injection sequence, the shut off valve 38 is closed immediately at the end of an injection event (identified at X) for the previous injection sequence to commence the test phase at time T 1 .

[0061] Once all injectors have carried out an injection event in the test phase (four in the example shown), the shut off valve 38 is opened at time T2 to terminate the test phase, allowing fuel to once again fill the common rail 28 and normal operation to continue (assuming no leak is detected) until the next test phase.

[0062] For each injector drive pulse, the rail pressure is sampled and recorded continuously through the test phase between T 1 and T2, both during injection events and between injection events when no injector is injecting. The rail pressure is sampled at least at the rising edge of each injector drive pulse, several times between adjacent injector drive pulses, and at the falling edge of each injector drive pulse, at least. This ensures that the required gradient measures for the rail pressure are obtained to determine any fault or leak state, as described further below.

[0063] It is important to note that once each injector is actuated to close, by removing the drive signal, to terminate injection by that injector, there is a small delay before the injector actually closes. This is referred to as the mechanical closing delay. The actual sampling point for the rail pressure after an injector drive pulse has been removed must therefore take place after the falling edge of the injector drive pulse, and the closing delay time.

[0064] The timing between injections, and the frequency of sampling the rail pressure measurements, will be dependent on the engine speed and the number of injectors / cylinders.

[0065] In the case of a normal functioning injector, where these is no injector leak, the rail pressure characteristic through the test phase would follow Line A, where the rail pressure starts at an initial rail pressure level RO and decreases after each injection event to a new, lower level between adjacent injection events. Between injection events the rail pressure remains stable at a constant level. After the final injector INJ4 has delivered an injection of fuel, the shut off valve is opened again at T2 and so the rail pressure increases back to the initial rail pressure level RO.

[0066] If a leak path exists through the first injector INJ1 then, before the next injection is initiated, there will be a continuous decay in the rail pressure characteristic between injection events, rather than a stable constant rail pressure value. This is because there is a constant leak through INJ1 into the air inlet manifold, even if the injector is nominally closed. As a result, the pressure in the common rail is reduced to a much lower level at the end of the test phase than would otherwise have been the case when there is no leak.

[0067] In addition, looking at the rail pressure characteristic for I N J 1 , there is no change in the gradient of the rail pressure characteristic between the end of injection for INJ1 , at the start of the test phase, and the end of the injection. This is because the injector INJ1 is not responsive with the valve needle stuck partially open, so that opening and closing the leaky injector has no effect on the rate of decrease of rail pressure through this period, including for the period of the injection event for INJ1 where the gradient remains constant at G1 .

[0068] Furthermore, and as can be seen in Line B of Figure 3, the gradient of decreasing rail pressure is maintained until the injector INJ2 injects because, even once the injector I NJ 1 is nominally closed, there remains a leak path out of the common rail 28, through the injector INJ1.

[0069] For the subsequent injection events (corresponding to INJ2, INJ3 and INJ4), the gradient of the rail pressure characteristic is greater than the gradient for a normal functioning injector in which there is no leak, as identified at G2, G3 and G4, respectively. This is because when the other injectors INJ2, INJ3 and INJ4 are injecting, this is accompanied by an additional leak through INJ1 (as it is always stuck open). The rail pressure characteristic through the test phase (with the shut off valve 38 closed) can therefore be analysed to determine whether there is a partially stuck open and non-responsive injector present in the injector assembly.

[0070] The defective injector (I NJ1) can in this case be identified by looking for a constant rail pressure gradient (decreasing rail pressure) for an injection event by the injector, instead of a change in rail pressure gradient, as would be expected for a normal functioning injector. The measurements of the rail pressure gradient when others of the injectors are injecting (e.g. INJ2, INJ3, INJ4) can be compared to normal / expected operation for a non-leaky injector (e.g. based on flow rate, rail pressure RO and injection pulse length) to determine the extent of the leak from injector IN J 1.

[0071] In the algorithm, the gradient of the rail pressure characteristic for each injection event, for all four injectors, is analysed to check whether there is a leak in any one (or more) of the injectors.

[0072] In the event that a leak is identified, an output fault signal can be generated to indicate a fault and / or engine operation can be ceased altogether (e.g. by shutting down the gas supply to the engine).

[0073] Referring to Figure 4, a different rail pressure characteristic will be measured if the leaky injector is responsive but is not sealing properly when it is closed. In this case the rail pressure gradient G1 through the injection by INJ1 is different to the rail pressure gradient immediately before the injection for INJ 1 , and immediately after the injection for INJ1 , because through the injection event the injector is responsive and there is a change compared to the non-injecting state.

[0074] In this case the rail pressure gradient for the injection by INJ 1 will be equivalent to the expected rail pressure gradient for a normal functioning injector, but the subsequent injection events for INJ2, INJ3 and INJ4 will show differences in gradient, G2, G3, G4 respectively, because, as well as the decrease in rail pressure due to their injections, there is also a contribution to the rail pressure decrease as a result of INJ 1 not sealing properly and the continual leak through INJ1. In other words, in order to detect a leaky injector which does not seal correctly (as opposed to an injector which is partially stuck open), it is necessary to look at the gradient for the rail pressure characteristic for an injection event for at least one other of the injectors. The measurement of the rail pressure gradient for one of the injectors alone may indicate that there is a leak (e.g. the rail pressure gradient of INJ2 compared to the expected / normal rail pressure gradient), but rail pressure measurements are required for at least one further injector to determine which of the injectors is not sealing properly.

[0075] In the event that a leak is identified, an output fault signal can be generated to indicate a fault and / or engine operation can be ceased altogether.

[0076] Referring to Figure 5, another scenario is shown in which there is a leak somewhere downstream of the shut off valve 38, but not necessarily with one of the injectors. In this case, the rail pressure gradient G1 , G2, G3, G4 for each injection for INJ 1 , INJ2, INJ3, INJ4, respectively, is greater than the expected rail pressure gradient (based on flow rate, rail pressure RO and injection pulse length). In this case Trace B in Figure 5 differs from Trace B in Figure 4 because, for INJ1 , the rail pressure gradient is greater than the expected gradient (as indicated at G1 in Figure 5), whereas for the injector which does not seal correctly (as in Figure 4) the rail pressure gradient for INJ1 is the same as the expected gradient. In Figure 5, therefore, the rail pressure gradient for all four injectors is greater than the normal expected gradient, indicating that the leak is non-injector specific and occurs somewhere downstream of the shut off valve 38.

[0077] Referring to Figure 6, another scenario is where the injector INJ1 is stuck fully open. Here it can be seen in Line B that, compared to Figure 3 for a partially stuck open injector, the rate of decrease of rail pressure is much greater than the normal expected rate of decrease of rail pressure as seen in Line A. There is a relatively large continuous pressure decay through the test phase when the shut off valve 38 is open. The rail pressure characteristic has a gradient G2 for the injection for INJ1 that is the same as the gradient immediately before and after the injection, but for the other injectors, INJ2, INJ3, INJ4, the rate of decrease in rail pressure is greater than for a normal expected rail pressure characteristic (as shown at G2, G3, G4). This is because for the injectors INJ2, INJ3 and INJ4, there is a contribution to the fuel leak out of the injector assembly due to the injecting injector, as well as leakage through the fully stuck open injector IN J 1.

[0078] In the algorithm, the gradient of the rail pressure characteristic for each injection event, for all four injectors INJI-INJ4, will be analysed to check whether there is a leak in any one (or more) of the injectors. The measurements of the rail pressure gradient for injectors INJ2, INJ3 and INJ4 can be compared to normal / expected operation for a non-leaky injector (e.g. based on flow rate, rail pressure RO and injection pulse length) to determine the extent of the leak from injector IN J1.

[0079] Referring to Figure 7, another scenario is shown in which one of the injectors INJ1 is stuck in a fully closed position and so will not open when commanded. In this case it can be seen from Line A that rail pressure level for the first injector INJ1 does not decrease because the first injector INJ1 fails to open. The initial rail pressure RO is therefore maintained in the common rail 28 until the point at which the second injection event is initiated by the injector INJ2, at which point the rail pressure decreases as a result of the injection (at G2). The rate of decrease in pressure for the second injection event is determined by the rate of injection through the second injector INJ2.

[0080] It can be seen in Figure 7 that the rail pressure characteristic for the scenario of an injector assembly where one of the injectors (I N J 1 ) is stuck closed follows the normal expected rail pressure characteristic once the normally functioning injectors are initiated, but with a higher starting value for the rail pressure level because of the lack of an injection event from the first injector I N J 1.

[0081] Analysing the rail pressure characteristic by comparing this to a normal, expected rail pressure characteristic, by comparing the rail pressure level between injection events with the rail pressure level for a normal, expected rail pressure level, and / or comparing the rail pressure gradient during the injection events with the rail pressure gradient for a normal, expected rail pressure gradient, can be used to determined whether one of the injectors is fully stuck open.

[0082] In embodiments of the invention, the controller may generate an output to indicate a fault state within the engine system, for example in one of the injectors or with the shut off valve 38, and this output may be used to terminate operation of the engine system at the point at which a fault is detected (e.g. a partially stuck open injector, a fully stuck open injector or a stuck closed injector), provide a fault signal to the driver to indicate that there is a leak or a fault and / or shut down the faulty injector.

[0083] By performing the test under low speed / low load conditions, the test sensitivity is increased because the quantity of any leak is more prominent relative to the quality of fuel being injected through the test phase. This makes it easier to detect even a relatively small leak. Also, performing the test under low speed and load conditions (e.g. engine idle) allows the test phase duration to be prolonged before there is a requirement to recharge the common rail 28 by opening the shut off valve 38. It also ensures that the test is unobtrusive and the possibility for the driver to detect the test is minimised.

[0084] It is important that, depending on which type of fault is being looked for, the rail pressure measurements are sampled frequently enough to be able the required gradient measurements to be determined, for comparison with the ideal scenario.

[0085] 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

CLAIMS:1 . An engine system (2) for a vehicle, the engine system (2) comprising; a rail volume (28) for receiving gaseous fuel; a shut off valve (38) for controlling the supply of fuel to the rail volume (28); an injector assembly including a plurality of fuel injectors (26), each one of the injectors (26) for receiving fuel from the rail volume (28) and for delivering fuel to an associated engine cylinder in an injection event; a rail sensor (30) for measuring a rail characteristic of the rail volume (28) during an engine running phase; and a controller configured to: receive a pre-determined rail pressure characteristic for a normal-functioning engine system (2), the pre-determined rail pressure characteristic including a predetermined rail pressure gradient for an injection event corresponding to at least one of the fuel injectors (26) when functioning normally in the engine running phase; activate the shut off valve (38) to close fuel delivery to the rail volume (28); receive the rail characteristic from the rail sensor (30) for the engine running phase, the rail pressure characteristic including a rail pressure gradient for an injection event corresponding to at least one of the fuel injectors (26) when in the engine running phase; and analyse the rail pressure characteristic by comparing the rail pressure gradient for the injection event with the pre-determined rail pressure gradient to determine a fault state of the engine system (2).

2. The engine system (2) as claimed in claim 1 , wherein the controller is configured to;provide an output to indicate that there is a fault state if the rail pressure gradient for the injection event of at least one of the injectors is greater than the predetermined rail pressure gradient.

3. The engine system (2) as claimed in claim 2, wherein the controller is configured to; provide an output to indicate that there is a stuck-open injector in a fault state if there is no change in the rail pressure gradient of the rail pressure characteristic for an injector upon initiation of the injection event.

4. The engine system (12) as claimed in claim 2 or claim 3, wherein each of the injectors is engageable with an injector valve seat to terminate injection, and wherein the controller is further configured to: provide an output to indicate that an injector is not sealing at the injector valve seat when commanded to close against the injector valve seat if the rail pressure gradient is equivalent to the predetermined rail pressure gradient, in addition to the rail pressure gradient for the or each of the other injectors being greater than the predetermined rail pressure gradient.

5. The engine system (2) as claimed in any of claims 2 to 4, wherein the controller is further configured to: provide an output to indicate that there is a fault with the shut off valve (38) if the rail pressure gradient for the injection event for all of the plurality of injectors is greater than the predetermined rail pressure gradient for the corresponding one of the injectors.

6. The engine system (2) as claimed in any of claims 1 to 5, wherein the controller is further configured to: provide an output to indicate that there is a stuck-closed injector if the rail pressure gradient for an injector does not change upon initiation of an injection event for said injector, in addition to the rail pressure gradient for the or eachof the other injectors being the same as the predetermined rail pressure gradient for the corresponding one of the injectors.

7. The engine system (2) as claimed in any of claims 1 to 6, wherein the controller is configured to provide the output to generate at least one of (a) an indication to a user of the engine system (2) that there is a fault; (b) a shut-down of the engine system (2); a shut-down of the injector in the fault state.

8. A method of determining a leak state in an engine system (2) for a vehicle, the engine system (2) comprising a rail volume (28) for receiving gaseous fuel; a shut off valve (38) for controlling the supply of fuel to the rail volume (28); and an injector assembly including a plurality of fuel injectors (26), each one of the injectors (26) for receiving fuel from the rail volume (28) and for delivering fuel to an associated engine cylinder in an injection event; the method comprising: measuring a rail characteristic of the rail volume (28) from a rail sensor (30) during an engine running phase; receiving a pre-determined rail pressure characteristic for a normal-functioning engine system (2), the pre-determined rail pressure characteristic including a pre-determined rail pressure gradient for an injection event corresponding to at least one of the fuel injectors (26) when functioning normally in the engine running phase; activating the shut off valve (38) to close fuel delivery to the rail volume (28); receiving the rail characteristic from the rail sensor (30) for the engine running phase, the rail pressure characteristic including a rail pressure gradient for an injection event corresponding to at least one of the fuel injectors (26) when in the engine running phase; and analysing the rail pressure characteristic by comparing the rail pressure gradient for the injection event with the pre-determined rail pressure gradient to determine a fault state of the engine system (2).

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