GB2642322A - Diagnostic system and method - Google Patents

Abstract

A diagnostic system for an engine system comprising a plurality of engine cylinders 30, and comprising a plurality of fuel injectors 46, each for delivering fuel directly to an associated one of the engine cylinders 30, wherein each injector 46 has an injector-open state when the injector 46 is delivering fuel to the associated engine cylinder 30 and an injector-closed state when the injector 46 is not delivering fuel to the associated engine cylinder 30, and a controller 18 configured to: (i) identify the occurrence of a pre-ignition event in one of the engine cylinders 30, (ii) determine if the pre-ignition event occurred while the injector 46 was in the injector-open state, and (iii) determine if an injector damage criterion has been satisfied and to send a control signal to display a warning signal if so. The invention aims to prevent injector damage or injector drift in hydrogen fuelled engines.

Description

[0001] DIAGNOSTIC SYSTEM AND METHOD

[0002] TECHNICAL FIELD

[0003] The examples of the invention relate to a diagnostic system for an injector. In particular, the examples relate to systems, methods and approaches for predicting and/or warning of injector leakage in internal combustion engines fuelled at least partially with hydrogen or another gaseous fuel delivered by direct injection 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, and leakages must be avoided -or at least detected and eliminated -wherever possible. One potential problem with hydrogen injectors is the occurrence of a pre-ignition event where the hydrogen mixture ignites before the intended spark is generated. This can arise for several reasons, including elevated inlet air temperatures or hot spots occurring in the combustion chamber. Inlet air temperatures can arise due to an ineffective intercooler, turbocharger control issues or general engine bay temperatures. If a pre-ignition event occurs when an injector is open, the injector can be damaged which can cause to injector leakage or injector drift (where the demanded fuel delivery does not correspond to the delivered fuel delivery).

[0006] It is with these issues in mind that the embodiments of the invention have been devised.

[0007] SUMMARY OF THE INVENTION

[0008] Against this background, examples of the invention provide, in a first aspect, a diagnostic system for an engine system comprising a plurality of engine cylinders.

[0009] The diagnostic system comprises: a plurality of fuel injectors, each for delivering fuel directly to an associated one of the engine cylinders, wherein each of the plurality of injectors has an injector-open state when the injector is delivering fuel to the associated engine cylinder and an injector-closed state when the injector is not delivering fuel to the associated engine cylinder; and a controller. The controller is configured to: (i) identify the occurrence of a pre-ignition event in one of the engine cylinders; (ii) determine if the pre-ignition event occurred while the injector associated with said one of the engine cylinders was in the injector-open state; wherein if the pre-ignition event occurred while said injector was in the injector-open state, the controller is additionally configured to: (iii) determine if an injector damage criterion has been satisfied; and (iv) to communicate a warning signal in the event that the injector damage criterion is satisfied.

[0010] By determining if the pre-ignition event occurred while the injector associated with the cylinder in which the pre-ignition event occurred was in the injector-open state, it is possible to more effectively monitor damage to the injector and subsequently determine if the injector damage criterion has been satisfied.

[0011] The controller may be configured to identify the occurrence of a pre-ignition event in dependence on an output signal received from a knock sensor of the engine system.

[0012] In embodiments, the controller may be configured to determine if an injector damage criterion has been satisfied by incrementing an injector-open pre-ignition counter value for one of the engine cylinders when the pre-ignition event occurs while the injector associated with said one of the engine cylinders is in the injector-open state; and determining if the injector open pre-ignition counter value exceeds an injector-open pre-ignition count threshold value.

[0013] In embodiments, the controller may be configured to determine if an injector damage criterion has been satisfied by determining a leakage rate for the injector; and determining if the leakage rate exceeds a leakage rate threshold value.

[0014] By way of example, the controller may be configured to determine if an injector damage criterion has been satisfied by determining a first leakage rate for the injector before the pre-ignition event; determining a second leakage rate for the injector after the pre-ignition event; determining an increase from the first leakage rate to the second leakage rate; and determining if the increase exceeds a leak rate increase threshold value.

[0015] According to a further aspect of the invention, there is provided a method of diagnosing damage to an injector of a direct-injection engine system, the method comprising: identifying the occurrence of a pre-ignition event in an engine cylinder; determining if the pre-ignition event occurred while an injector associated with the cylinder was in an injector-open state; and, if the pre-ignition event occurred while the injector was in the injector open state, determining if an injector damage criterion has been satisfied; and communicating a warning signal in the event that the injector damage criterion is satisfied.

[0016] The method may comprise identifying the occurrence of a pre-ignition event in dependence on an output signal received from a knock sensor of the engine system. 20 The step of determining if an injector damage criterion has been satisfied may, for example, comprise incrementing an injector-open pre-ignition count value for one of the engine cylinders when the pre-ignition event occurs while the injector associated with said one of the engine cylinders is in the injector-open state; and determining if the injector-open pre-ignition count exceeds an injector open pre-ignition count threshold.

[0017] The step of determining if an injector damage criterion has been satisfied may comprise determining a leakage rate for the injector; and determining if the leakage rate exceeds a leakage rate threshold value.

[0018] The step of determining if an injector damage criterion has been satisfied may comprise determining a first leakage rate for the injector before the pre-ignition event; determining a second leakage rate for the injector after the pre-ignition event; determining an increase from the first leakage rate to the second leakage rate; and determining if the increase exceeds a leak rate increase threshold value.

[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 further 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: 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 graph to show, relative to engine position (a) the stroke of the engine cycle; (b) where the injection occurs in the compression stroke (open to closed injector); (c) cylinder pressure signal; and (d) the processed knock sensor signal; Figure 3 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; and Figure 4 is a flow diagram to illustrate the steps of an additional, or alternative, method which may be implemented by a controller of the engine system in Figure 1 to detect the presence of a faulty injector.

[0023] DETAILED DESCRIPTION

[0024] In general, the examples of the invention provide a diagnostic system for use in an engine system, where the diagnostic system is used to generate pre-emptive warning signals in the event that an injector leak problem may be developing or that damage to an injector may have occurred. The diagnostic system is applicable to a direct 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.

[0025] To put the examples of the invention into technical context, a discussion of a direct injection internal combustion engine which is fuelled with gaseous hydrogen will now be described with reference to Figure 1.

[0026] In overview, an internal combustion engine system comprises an engine block 10, an air inlet system 12, a fuel delivery system 14 and an exhaust system 16. The engine system further comprises a control unit, referred to as the engine control unit (ECU) or the controller 18, which is adapted to receive data input 22, from sensors 20 which sense operational parameters of the engine system, and to provide suitable control output signals 24 to the functions 26 of the engine system to control its operation based on driver demands and the sensor data, as is conventional.

[0027] The ECU 18 includes a non-volatile memory component (NVM) (not shown). The NVM stores data such as self-learnt control parameters, operating history data and comparison thresholds which can be retrieved by the ECU even after a power down cycle. The ECU relies on a permanent power supply and is operable regardless of the key on/off state of the engine.

[0028] The engine block 10 of the illustrated example comprises four combustion chambers 30, or cylinders, in an 'in-line' configuration. However, it should be noted that this is for illustrative purposes only and the engine block 10 may comprise any suitable number of combustion chambers 30 in any suitable configuration, as would be well understood by the skilled person. Herein, the term 'combustion chamber' will be considered synonymous with 'engine cylinder', or simply 'cylinder'.

[0029] The air inlet system 12 comprises an air inlet 32 which feeds fresh air into a network of air pipes through an air filter 34. An air mass flow sensor 36 is provided to provide data to the control unit (signals not shown) about the airflow entering the engine 30 system.

[0030] The network of air pipes feeds incoming air through a compressor 38 and, subsequently, to an intercooler 40. The functionality of the compressor 38 and the intercooler 40 are known in the art so a further discussion will not be provided. The network of pipes leads from the intercooler 40 through a throttle valve 42 to an air inlet duct or 'manifold' 44. As is known, the air inlet manifold 44 directs fresh air to each of the engine cylinders 30 of the engine block 10 via separate air channels (not identified).

[0031] The fuel delivery system comprises a set of one or more fuel delivery devices in the form of an injector assembly including a plurality of fuel injectors (only one of which is labelled as 46) that are arranged to inject combustible fuel, in this case hydrogen gas, directly into the engine cylinders 30. This type of engine is referred to as a direct injection fuel system.

[0032] In the illustrated example, there are a plurality of fuel injectors 46, the number of which corresponds to the number of engine cylinders 30. The system is a direct injection engine and so each of the fuel injectors 46 is arranged to inject fuel directly into a respective one of the engine cylinders 30. The injectors 46 are each configured to be movable between two states: an injector open state (or simply open state) where the injector 46 is capable of injecting fuel into the associated cylinder 30; and an injector closed state (or simply closed state) where the injector 46 is prevented from injecting fuel into its associated cylinder 30.

[0033] The fuel injectors 46 are each connected to a fuel accumulator or 'common rail' 48. As is known, the common rail 48 provides a relatively large volume of fuel which is maintained at a predetermined, and controllable, pressure level which means that the fuel injectors 46 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 48 can be modified in use due to various requirements that are beyond the scope of this discussion. A pressure sensor 50 determines fuel pressure within the common rail.

[0034] Fuel and air mixture in the engine cylinders 30 is ignited by respective spark plugs 47, in the usual manner.

[0035] The common rail 48 is supplied with fuel by a fuel supply system 52. The fuel supply system 52 includes a pressurised fuel source or reservoir 54, a pressure regulating device 56, a shut-off valve 58 and a gas supply line 59 which connects the shut-off valve 58 to the common rail. The pressurised fuel source or 'fuel tank' 54 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 56 is configured to reduce the gas pressure in the fuel tank 54 to a pressure suitable for injection, which may be between 5 bar and 10 bar but could be higher for some systems. Together the gas supply line 59, the common rail 48 and the injectors 46, and the various connections between these components, may be considered as the low pressure circuit' [PC of the system.

[0036] Further sensing means may be provided for the control unit in order for it to operate the engine system effectively. In the illustrated example, the engine block 10 is equipped with a knock sensor 60. As is known in the art, a knock sensor 60 provides a means to detect high frequency vibration of the engine block 10 from which a determination can be made about whether combustion has occurred within a particular combustion chamber using associated software. The knock sensor 60 and the associated software are able to discriminate between combustion occurring in different ones of the combustion chambers 30. A knock sensor 60 is conventional technology and so further discussion will be omitted.

[0037] The engine system further includes a system sensor in the form of an air pressure sensor (or air inlet manifold pressure sensor) 51 which is configured to provide the control unit with data relating to the pressure of air within the air inlet manifold 44. The pressure sensor 51 provides a pressure sensor output to the control unit which is representative of the air pressure in the air inlet manifold 44.

[0038] The engine system further includes a crank position sensor 45 which is configured to provide the control unit with data relating to the position and rotational speed of the crankshaft. It should be noted that the crankshaft, pistons, and intake and exhaust valves are not shown on Figure 1, but their presence is implied. Data from the crankshaft position sensor 45 may be used by the controller 18 to control fuel injection and ignition timing. Common mounting positions for the crankshaft position sensor 45 include on the engine flywheel (not shown), the camshaft (not shown) or the main crankshaft pulley (not shown).

[0039] A temperature sensor 43 may also be provided on or associated with the common rail 48 to provide the functionality of providing a measurement of the temperature of the gas within the common rail 48. The temperature sensor 43 is shown here connected to the common rail 48 but other positions would be acceptable, for example attached to the gas supply line 59 or the shut-off valve 58. The functionality of the temperature sensor 43 and the functionality of the rail pressure sensor 50 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.

[0040] Note that the rail pressure sensor 50, the knock sensor 60, the air pressure sensor 51, the temperature sensor 43 and the crank position sensor 45 may communicate with the controller 18 in a conventional manner to provide it suitable data input. The engine system also includes a cylinder pressure sensor (not shown) for measuring cylinder pressure. Communication of the signals from the various sensors to the controller 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.

[0041] The engine system further comprises a starter motor 62 which is configured to turn the crankshaft (not shown) in order to initiate self-sustaining power-producing operation of the engine system.

[0042] For the exhaust system, combustion gases from the combustion chambers 30 feed into an exhaust duct or 'manifold' 72 which combines the gas out flow into a single pipe which leads to a turbine 74. As is known, the turbine 74 is connected to the compressor 38 and, together, the turbine and the compressor constitute a turbocharger of the engine system.

[0043] It should be noted that in the above discussion, the fuel delivery system is configured into a direct injection arrangement which means that the fuel injectors 48 are arranged to inject fuel, in this hydrogen gas, directly into the combustion chambers 30 of the engine block 10.

[0044] As has been discussed above, the controller 18 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 controller 18 would be well known to the skilled person and is outside of the scope of this discussion. It should be appreciated that the controller 18 may be any suitable control environment provided by the engine system. The controller 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 of the broader vehicle. In particular, the controller 18 may be a control environment provided specifically for the purposes of performing the method.

[0045] One of the roles of the controller 18 is to monitor signals from the various sensors in the engine system to determine if a so-called pre-ignition event has occurred. In a pre-ignition event, the fuel/air mixture present in the engine is ignited before the intended spark event. As already referenced, there are many possible causes for a pre-ignition event, such as elevated inlet temperatures or hot spots in the combustion chamber 30. Pre-ignition events may not necessarily be damaging to the engine or the components thereof, but in the case of direct-injection engines, damage to the injectors 46 can occur if a pre-ignition event happens while an injector 46 is open. To this end, it is important to correlate the timings of any pre-ignition event with periods in which the injectors 46 are open, and to subsequently assess if any damage has occurred so that a warning alert can be generated and communicated to the user to indicate that the injectors 46 may need servicing, or replacing.

[0046] Figure 2 shows a stroke of the engine cycle for a single cylinder 30, with the different parts of the stroke denoted in the first row (a). The stroke moves from the intake phase, in which air is drawn into the cylinder via the air inlet manifold 44 as the piston in the cylinder 30 moves from the top dead centre position to the bottom dead centre position, to the compression phase, where this air is compressed, together with fuel injected by the injector 46 associated with this cylinder 30, by the action of the piston returning to top dead centre. Figure 2(b) shows that the injector 46 is open for most of the compression phase, as marked by the window Q, except for short periods at the start and end of the compression phase. The duration of the window Q will depend on the necessary fuel quantity based upon the torque demand. In this Figure 2(b) it is open for most of the period. However, at idle the window is likely to be a much shorter duration.

[0047] In an ideal scenario, ignition of the fuel/air mixture by the spark plug 47 occurs when the injector 46 is closed, just before the end of the compression phase, as indicated by the vertical dotted line X in Figure 2. This causes an expansion of the gas inside the cylinder 30, generating power which is transferred eventually to forward motion of the vehicle, as is conventional. The piston is forced back towards bottom dead centre, before reaching top dead centre in the exhaust phase to expel the combustion products from the cylinder.

[0048] Figure 2(c) shows the cylinder pressure output from the cylinder when a pre-ignition event occurs just before the injector has closed. The fluctuation in cylinder pressure (indicated at Y) just before the injector is closed corresponds to a spike in the processed knock sensor output signal (row (d) of Figure 2 at Z). This indicates that the processed knock sensor signal can be used to provide an indication that pre-ignition has occurred. By considering the output of the knock sensor 60 relative to when the various injectors are open and closed, and determining that a spike occurs when an injector is still open (and before a demanded spark pulse), it is therefore possible for the controller to determine if a pre-injection event has occurred in a certain cylinder 30, and if so whether this occurred while the injector 46 associated with that cylinder 30 was open. The output from the knock sensors enables a determination that there is a peak in cylinder pressure.

[0049] In practice the knock sensor signal is processed before it is used to make a determination on pre-ignition. This processing step involves amplifying the knock sensor signal, filtering the signal with a variety of filters (e.g. to remove unwanted environmental noise, such as noise from the valve train of the engine), and integrating the signal to determine the intensity. The determined intensity of the processed signal can then be used to make a determination on whether a threshold has been met to indicate a pre-ignition event.

[0050] In other embodiments, the engine may have an associated cylinder pressure transducer. However, some engines may not have such a transducer, in which case a detection of the knock sensor occurring before the demanded spark pulse provides an indication of pre-ignition.

[0051] While Figure 2 has focussed on the identification of a pre-ignition event based on the output received by the controller 18 from the knock sensor 60, the skilled person will be aware of other ways in which the occurrence of a pre-ignition event can be identified using other sensors present in the engine system. For example, in other embodiments, a pre-ignition event may also be detected by looking at the instantaneous speed of the crankshaft via the crankshaft position sensor 45. The combustion occurring as the piston is travelling upwards towards TDC causes an unexpected deceleration in the crankshaft speed which can be measured to identify a pre-ignition event. Either method of detection of a pre-ignition event is suitable for use as a step in the method of the invention.

[0052] Having identified that a pre-ignition event has occurred in a cylinder 30 while that cylinder's injector 46 was open, a determination must be made by the controller 18 regarding whether or not to cause an alert to be displayed to let the driver or user of the vehicle know that the injector 46 is damaged, or requires replacement or servicing.

[0053] To do this, the controller 18 makes a determination about whether an injector damage criterion has been satisfied. If the injector damage criterion is satisfied, an injector fault or warning signal is output by the controller to alert the vehicle user of the need for repair, servicing or replacement of the injector 46. The pressure signal is monitored constantly to determine whether a pre-ignition event has occurred and if a pre-ignition event is detected a determination is made about whether this occurred when an injector was open.

[0054] In one embodiment, the injector damage criterion may relate to the number of pre-ignition events that have occurred while the associated injector 46 is in the open state.

[0055] For the injector damage criterion to be satisfied, the number of pre-ignition events would need to exceed an injector-open pre-ignition count threshold value. The injector-open pre-ignition count threshold value is the number of pre-ignition events which can be experienced by the injector, with the injector open, before it is deemed that the damage which may have been caused to the injector is too high, and action needs to be taken.

[0056] Figure 3 shows a flow chart that illustrates how this injector damage criterion may be applied by the controller 18. At step 400, the controller 18 detects that a pre-ignition event has occurred. This may be through analysis of the output signal from the knock sensor 60, as described above, or by other conventional means of detecting a pre-ignition event, such as using the crankshaft position sensor. The method progresses to step 402, where the controller 18 determines if the injector 46 was open during the pre-ignition event. This occurs by comparing the timing of the injector 46 opening and closing (i.e. window Q in Figure 2) with the time at which the pre-ignition event is determined to have occurred (e.g. knock sensor signal Z in Figure 2). If the controller 18 determines that the injector 46 was not open when pre-ignition occurred, the method progresses to step 404, with no further action taken before the next pre-ignition event is recorded and the method starts again. If, on the other hand, the controller 18 determines that the injector 46 was open during the pre-ignition event, the method moves to step 406 and a counter (referred to as Counter A in Figure 3) is incremented.

[0057] The count recorded in Counter A therefore represents the number of pre-ignition events that have occurred since last servicing of the injector (e.g. since last rebuild of the injector). Having incremented the counter to account for the pre-ignition event that has occurred, the controller 18 determines if the recorded counter value exceeds the injector open pre-ignition count threshold value at step 408 (the injector-open pre-ignition count threshold value represents a pre-determined value at which the injector 46 is considered to be damaged and either in need of servicing or replacement). If the injector-open pre-ignition count threshold value is not exceeded, the controller moves to step 410, where no action is taken and the method is terminated before the next pre-ignition event is identified and the method starts again. However, if the recorded counter value exceeds the injector-open pre-ignition count threshold value, the method moves to step 412 and the controller outputs a signal to provide an alert that the injector 46 must be serviced or replaced.

[0058] One or more other injector damage criteria may also be applied either instead of or, preferably, in combination with the injector damage criterion described above. These additional injector damage criteria may relate to the leakage experienced by an injector 46. In general, the controller 18 will monitor various sensors in the engine system in order to determine a leakage rate from each injector 46. The skilled person will be aware that there are a number of ways for this to occur in practice. For example, previous application GB 2315408.1 belonging to the Applicant describe a way in which the leakage rate from an injector 46 can be determined by monitoring the relationship between pressure in the common rail 48 and the pressure in the cylinder 30.

[0059] Having detected a pre-ignition event, and that the injector 46 associated with the cylinder 30 in which the pre-ignition event occurred was open during the pre-ignition event, one injector damage criterion that the controller 18 may apply is to determine if the pre-ignition event is accompanied by the measured leakage rate exceeding a leakage rate threshold value. Alternatively, the controller 18 may compare the leakage rate from before the pre-ignition event to the leakage rate after the pre-ignition event, determine a difference between the two leakage rates, and then, if the leakage rate has increased, determine if the increase in the leakage rate exceeds a leakage rate increase threshold value.

[0060] It is important to understand that the controller 18 may also operate injector damage criteria that are wholly independent of the occurrence of any pre-ignition events, whether injectors 46 are open during these pre-ignition events, or otherwise. For example, a critical leakage rate threshold, separate to the leakage rate threshold, may be applied in cases where the leakage rate from the injector 46 is so high that it is clear that the injector 46 must be replaced or serviced, regardless of the detection of any pre-ignition event(s). If the controller 18 determines that the leakage rate from an injector 46 exceeds a critical leakage rate threshold value, it sends a control signal to cause an alert to be communicated to let the driver or user of the vehicle know that the injector 46 is damaged and needs servicing or replacement.

[0061] The controller may operate one or all of these injector damage criteria and Figure 4 shows a flow chart of how multiple injector damage criteria may be applied in one single process. At step 500, the controller commences a leak monitoring method by monitoring the leakage rate from an injector 46. The leak monitoring method may be commenced in response to a specific trigger, or may be a continuous process that occurs throughout the life of the engine system. At step 502, the controller determines the current leakage rate and compares it to the critical leakage rate threshold value. If the leakage rate exceeds the critical leakage rate threshold value, this indicates serious damage to the injector and so the method passes to step 504, where the controller 18 sends a control signal to cause an alert to be communicated indicating that the injector 46 requires servicing or replacement. The alert may specifically reference that the critical leakage rate threshold value has been exceeded and is not dependent on any pre-ignition event being detected for the alert to be generated.

[0062] Should the critical leakage rate threshold value not be exceeded, the method passes to step 506, where the controller 18 determines if the leakage rate has increased compared to the previous leakage rate measurement. If the leakage rate has not increased compared to the previous leakage rate measurement, the current value of the counter of the number of pre-ignition events where the injector 46 was open is stored at step 508. If the leakage rate has increased compared to the previous leakage rate measurement, the method moves to step 510, where the controller 18 determines if the counter value has been incremented since the previous measurement of the leakage rate (i.e., has there been a pre-ignition event where the injector 46 was open since the previous leakage rate measurement). If not, the controller takes no action as per step 512 and the method starts again.

[0063] If the counter value has been incremented since the previous leakage rate measurement, the method moves to step 514, where the current value of the counter is stored. Then, at step 516, the controller 18 compares the leakage rate to the leakage rate threshold value (referred to as 'the alert rate' in Figure 5). The leakage rate threshold value of the method of Figure 5 is set at a lower value compared to the critical leakage threshold value, because even if the critical leakage threshold value is not exceeded, to cause the immediate output of an alert signal (step 504), it may be that the leakage rate is still at an unacceptable level when accompanied by a pre-ignition event occurring when the injector is open.

[0064] If the leakage rate exceeds the leakage rate threshold, the method moves to step 518, where the controller 18 sends a control signal to communicate an alert that the injector 46 is damaged and is in need of servicing or replacement.

[0065] If the leakage rate does not exceed the leakage rate threshold, the method moves to step 520, where the controller assesses the increase in the leakage rate from the previous measurement to the current measurement and compares this increase to the leakage rate increase threshold value. If the increase exceeds the leakage rate increase threshold value, the method moves to step 522 and the controller 18 sends a control signal to communicate an alert that the injector 46 is damaged and is in need of servicing or replacement. If, at step 522 it is determined that the increase in the leakage rate is less than the leakage rate increase threshold value, it can be determined that the leakage rate is not increasing by an unacceptable amount, no action needs to be taken (step 524) and the method starts again.

[0066] The method of Figure 4 therefore works through a number of different checks and tests to see whether three different injector damage criteria are satisfied; (i) an initial check against the critical leakage rate threshold; (ii) a check to see whether the leakage rate is above an acceptable leakage rate threshold value, which is lower than the critical leakage rate threshold value, and (iii) a check to see if an increase in the leakage rate is above an acceptable leakage rate increase threshold value. If steps (I) and 00 are not satisfied positively, so that there is no need to generate an alert after steps (i) and 00, it may be that the third stage of the test still causes an alert to be generated. The additional checks at steps 506 and 510 effectively help to determine whether or not there is any point in applying checks at steps (ii) and (iii): if either the leakage rate has not increased, or a pre-ignition event has not occurred while an injector 46 was open, the injector damage criteria of steps (ii) and (iii) will not be satisfied.

[0067] 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 appended claims.

Claims (10)

1. CLAIMS1. A diagnostic system for an engine system comprising a plurality of engine cylinders (30), the diagnostic system comprising: a plurality of fuel injectors (46), each for delivering fuel directly to an associated one of the engine cylinders (30), wherein each of the plurality of injectors (46) has an injector-open state when the injector (46) is delivering fuel to the associated engine cylinder (30) and an injector-closed state when the injector (46) is not delivering fuel to the associated engine cylinder (30); and a controller (18) configured to: identify the occurrence of a pre-ignition event in one of the engine cylinders (30), determine if the pre-ignition event occurred while the injector (46) associated with said one of the engine cylinders was in the injector-open state; wherein if the pre-ignition event occurred while said injector (46) was in the injector-open state, the controller is additionally configured to: determine if an injector damage criterion has been satisfied; and to communicate a warning signal in the event that the injector damage criterion is satisfied.

2. The diagnostic system of Claim 1, wherein the controller (18) is configured to identify the occurrence of a pre-ignition event in dependence on an output signal received from a knock sensor (60) of the engine system.

3. The diagnostic system of Claim 1 or Claim 2, wherein the controller is configured to determine if an injector damage criterion has been satisfied by: incrementing an injector-open pre-ignition counter value for one of the engine cylinders when the pre-ignition event occurs while the injector (46) associated with said one of the engine cylinders is in the injector-open state; and determining if the injector open pre-ignition counter value exceeds an injector-open pre-ignition count threshold value.

4. The diagnostic system of any preceding claim, wherein the controller (18) is configured to determine if an injector damage criterion has been satisfied by: determining a leakage rate for the injector (46); and determining if the leakage rate exceeds a leakage rate threshold value.

5. The diagnostic system of any preceding claim, wherein the controller (18) is configured to determine if an injector damage criterion has been satisfied by: determining a first leakage rate for the injector (46) before the pre-ignition event; determining a second leakage rate for the injector (46) after the pre-ignition determining an increase from the first leakage rate to the second leakage rate; determining if the increase exceeds a leak rate increase threshold value.

6. A method of diagnosing damage to an injector of a direct-injection engine system, the method comprising: identifying the occurrence of a pre-ignition event in an engine cylinder (30); event; and determining if the pre-ignition event occurred while an injector (46) associated with the cylinder (30) was in an injector-open state; and, if the pre-ignition event occurred while the injector was in the injector open state, determining if an injector damage criterion has been satisfied; andScommunicating a warning signal in the event that the injector damage criterion is satisfied.

7. The method of Claim 6, comprising identifying the occurrence of a pre-ignition event in dependence on an output signal received from a knock sensor (60) of the engine system.

8. The method of Claim 6 or Claim 7, wherein the step of determining if an injector damage criterion has been satisfied comprises: incrementing an injector-open pre-ignition count value for one of the engine cylinders (30) when the pre-ignition event occurs while the injector (46) associated with said one of the engine cylinders (30) is in the injector-open state; and determining if the injector-open pre-ignition count exceeds an injector open pre-ignition count threshold.

9. The method of any of Claims 6 to 8, wherein the step of determining if an injector damage criterion has been satisfied comprises: determining a leakage rate for the injector (46) and determining if the leakage rate exceeds a leakage rate threshold value.

10. The diagnostic system of any of Claims 6 to 9, wherein the step of determining if an injector damage criterion has been satisfied comprises: determining a first leakage rate for the injector (46) before the pre-ignition event; event; 19 and determining a second leakage rate for the injector (46) after the pre-ignition determining an increase from the first leakage rate to the second leakage rate; determining if the increase exceeds a leak rate increase threshold value.