Diagnostic system and method

Abstract

A diagnostic system is provided for an engine system comprising a plurality of engine cylinders 30 and an air intake manifold 44. The system includes a plurality of fuel injectors 46 for delivering fu

Description

Technical Field The examples of the invention relate to a diagnostic system for an engine system. In particular, the examples relate to systems, methods and approaches for predicting and / or warning of leakage in internal combustion engines fuelled at least partially with hydrogen or another gaseous fuel delivered by port fuel injectors. Background Gaseous fuels such as hydrogen are promising alternative fuels to gasoline and diesel due to their potential for low or zero emissions. However gaseous fuels present some challenges relating to their containment and handling, and leakages must be avoided - or at least detected and eliminated - wherever possible. There are known methods of monitoring injector leakage or injector drift (including individual engine cylinder Lambda correction and leakage detection at key on). However, these detection methods are not ideal as they must be set at a level to prevent false positives, which are highly detrimental, and so they cannot detect the smallest levels of leakage or injector drift. Once a leak is detected, because the detected leaks are significant, action is usually taken to shut down the engine altogether which can cause inconveniences for the user. There remains a need in gaseous fuel systems to provide a reliable means for detecting low levels of leakage so that preventative action can be taken before the need for entire engine shut down. It is with this issue 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 a diagnostic system for an engine system comprising a plurality of engine cylinders and an air intake manifold for receiving an intake of air during an intake phase of an engine cycle, the diagnostic system comprising a plurality of fuel injectors, each for delivering fuel to an associated one of the engine cylinders via the air intake manifold, wherein each of the plurality of injectors has an injector-open state when the injector is delivering fuel during the air intake phase of the associated engine cylinder and an injector-closed state when the injector is not delivering fuel. A sensor is configured to measure the air pressure in the air intake manifold. The diagnostic system further comprises a controller configured to determine an occurrence of a backfire event in an associated one of the engine cylinders on the basis of the measured air pressure; measure the number of backfire events that have occurred within each engine cylinder; compare the number of backfire events with a predetermined count threshold, and, if the measured number of backfire events is greater than the predetermined count threshold, output a warning signal. In hydrogen port injected engines, for example, where hydrogen is injected into an air intake manifold ahead of the combustion cylinder, backfire events can occur (also referred to as “pop-back”) where combustion can occur in the air intake manifold rather than the cylinder. This can occur when hydrogen leaks from the injector into the air intake manifold. The occurrence of a backfire event can lead to injector drift or leakage to further increase the risk of backfire, creating a positive feedback mechanism. Injector drift is the scenario where the quantity of fuel injected is not equivalent to the demanded quantity of fuel. The present invention implements the detection of a back-fire event as part of an early-warning system that an injector leak has occurred The invention allows a more accurate determination of a leak event, so that there can be a more considered response to the determination rather than full engine shut down. In embodiments of the invention, the controller is configured to determine whether the backfire event occurred within the associated engine cylinder when the associated injector is in the injector-open state or the injector-closed state; determine an openstate backfire count being the number of times a backfire event occurs when the associated injector is in the open state and; determine a closed-state backfire count being the number of times a backfire event occurs when the associated injector is in the ciosed state. The controller may be further configured to, in a backfire event comparison step, compare at ieast one of the open-state backfire count with an open-state threshold and the closed-state backfire count with a closed-state threshold; and if at least one of the open-state backfire count is greater than the open-state threshold and the closed-state backfire count is greater than the closed-state threshold, output a warning signal. The controller may be configured to determine the occurrence of a backfire event directly from the measured air pressure in the air intake manifold as a function of engine position. The controller may be configured to determine the occurrence of the backfire event from the gradient of the measured air pressure in the air intake manifold as a function of engine position. In embodiments, the controller may be further configured to determine an injector leak rate for a first time period, the injector leak rate being indicative of the rate of leakage of fuel from the injector during the first time period; and in a first leak rate comparison step, compare the determined leak rate for the first time period with an initial leak rate threshold; and, if the determined leak rate for the first time period is greater than the initial leak rate threshold, output a warning signal for the injector. The controller may, for example, be further configured to determine an injector leak rate for a reference time period prior to the first time period, the leak rate being indicative of the rate of leakage of fuel from the injector in the reference time period; determine the open-state backfire count in the first time period and determine the open-state backfire count in the reference time period; compare the determined leak rate for the first time period with the determined leak rate for the reference time period; and, if the measured leak rate for the first time period is greater than the measured leak rate for the reference time period, compare the open-state backfire count in the first time period with the open-state backfire count in the reference time period. By way of example, the controller may be further configured to, if the open-state backfire count in the first time period is greater than the open-state backfire count in the reference time period, then, in a second leak comparison step, compare the determined leak rate for the first time period with a predetermined second leak rate threshold, wherein the predetermined second leak rate threshold is less than the predetermined first leak rate threshold. The controller may be further configured to, if the determined leak rate for the first time period is greater than the predetermined second leak rate threshold, output a warning signal In embodiments, the controller may be further configured to compare the closed-state backfire count in the first time period with the closed-state backfire count in the reference time period and, if the measured leak rate for the first time period is greater than the measured leak rate for the reference time period, compare the closed-state backfire count in the first time period with the closed-state backfire count in the reference time period. The controller may be further configured to, if the closed-state backfire count in the first time period is greater than the closed-state backfire count in the reference time period, then, in a third leak comparison step, compare the determined leak rate for the first time period with a predetermined second leak rate threshold, wherein the predetermined second leak rate threshold is less than the predetermined first leak rate threshold. By way of example, the controller may be further configured to, if the determined leak rate for the first time period is greater than the predetermined second leak rate threshold, output a warning signal. According to a second aspect of the invention, there is provided a diagnostic method for an engine system comprising a plurality of engine cylinders and an air intake manifold for receiving an intake of air during an intake phase of an engine cycle, a plurality of fuel injectors, each for delivering fuel to an associated one of the engine cylinders via the air intake manifold, and a sensor configured to measure the air pressure in the air intake manifold; the method comprising; determining an occurrence of a backfire event in an associated one of the engine cylinders on the basis of the measured air pressure; measuring the number of backfire events that have occurred within each engine cylinder; comparing the number of backfire events with a predetermined count threshold, and, if the measured number of backfire events is greater than the predetermined count threshold, outputting a warning signal. 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 graph to illustrate (a) air intake manifold pressure as a function of engine position; (b) air intake manifold gradient as a function of engine position; and (c) a counter signal to illustrate the counter value for a respective cylinder, to indicate that a backfire event has occurred in that cylinder; Figure 3 is a graph to show (a) the injection window; (b) the intake valve open window; and (c) backfire event windows for closed and open injectors, as a function of engine position; Figure 4 is a flow diagram to illustrate the steps of a first embodiment of the method which may be implemented by a controller of the engine system in Figure 1; and Figure 5 is a flow diagram to illustrate the steps of a second embodiment of the method which may be implemented by a controller of the engine system in Figure 1. Detailed Description 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. The diagnostic system 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. To put the examples of the invention into technical context, a discussion of a port-injected 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 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 controi 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. 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 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. 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 system. 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). 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, into the fresh air flowing into the engine cylinders 30. In the illustrated example, there are a plurality of fuel injectors 46, the number of which corresponds to the number of combustion chambers 30. The system is a port injection engine and so each of the fuel injectors 46 is arranged to inject fuel into the air inlet manifold 44 at a dedicated channel which leads to a respective one of the engine cylinders. 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. Fuel and air mixture in the engine cylinders 30 is ignited by respective spark plugs, in the usual manner. The common rail 48 is supplied with fuel by a fuel supply system 52. The fuel supply system 52 includes a pressurised fuei source or reservoir 54, a pressure regulating device 56, a shut-off valve 58 and a gas supply line 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, the rail and the injectors, and the various connections between these components, may be considered as the low pressure circuit’ LPC of the system. 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 an optional component. 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 sensors 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. The engine system further includes a system sensor in the form of an air pressure sensor 61 (or air inlet manifold pressure sensor) which is configured to provide the control unit with data relating to the pressure of air within the air inlet manifold 44. The air pressure sensor 61 provides a pressure sensor output to the control unit which is representative of the air pressure in the air inlet manifold 44. 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, intake and exhaust valves, and spark piugs 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 indude on the engine flywheel (not shown), the camshaft (not shown) or the main crankshaft pulley (not shown). Note that the rail pressure sensor 50, the knock sensor 60, the air pressure sensor 61, and the crank position sensor 45 may communicate with the controller 18 in a conventional manner to provide suitable data input. This may be achieved by suitable wired connections, or through SENT protocol, which is conventional in automotive technology. 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. 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. It should be noted that in the above discussion, the fuel delivery system is configured into a port injection arrangement which means that the fuel injectors 48 are arranged to inject fuel into the air inlet manifold 44 so that the injected fuel, in this hydrogen gas, is mixed with fresh air in the inlet manifold 44 before entering the combustion chambers 30 of the engine block 10. 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 may be a control environment provided specifically for the purposes of performing the method. in hydrogen port-injected engines of the type described previously, where hydrogen is injected into an air intake manifold 44 ahead of the combustion cylinders 30, backfire events can occur where combustion occurs in the air intake manifold 44 rather than the cylinders 30. This can occur, for example, when hydrogen leaks from an injector 46 (referred to as INJ1, INJ2, INJ3, INJ4) into the air intake manifold 44. Other causes can also initiate a backfire event. The result of backfire can be damage to the injectors 46, which in turn further exacerbates leakage and injector drift, if a backfire event occurs when an injector 46 is open, the event can lead to a combustion event in the injector itself 46 or in the fuel rail 48. This can lead to internal damage and / or further leakage problems. If a backfire event occurs when an injector 46 is closed, this can cause damage to the injector actuator, which may result in a nonfunctioning injector. In either case, it is desirable to monitor backfire events and to take action to prevent such resultant damage if the events are occurring. In overview, the method of the invention is based on the principle that the detection of a backfire event provides a means of early warning detection for a potential leak problem, and / or a check on whether a measured leak rate is cause for concern. When a backfire event is detected, a correlation is made with the firing engine cylinder. A further correlation is made about whether the backfire event for the associated injector occurred when the injector was open (injector-open condition) or when the injector was closed (injector-closed). A count is made of the total number of backfire events occurring in the injector-closed condition and the total number of backfire events occurring in the injector-open condition, and the counts are compared with a respective threshold. If the threshold is exceeded in either case, an alert or warning signal is made to service the injector. As an initial step in this method, it is therefore necessary to determine a backfire event and to assign the backfire event to a particular injector. Figures 2(a) to (c) shows a graph to indicate the presence of a backfire event in an engine cylinder by monitoring the manifold inlet pressure, as determined by the manifold air pressure sensor, and how this can be used to provide an early warning on injector leakage. Figure 2(b) shows a plot 80 of the gradient of the manifold air pressure signal from the manifold air pressure sensor as a function of engine position and Figure 2(a) shows a plot 82 of the manifold air pressure measurement as a function of engine position. The peak in the manifold air pressure signal can be seen in Figure 2(a), at X. The peak in the gradient of the manifold air pressure signal can be seen in Figure 2(b), at Y. The controller 18 may be configured to derive an indication of the backfire event directly from the manifold air pressure signal, by identifying the peak X at engine position X in Figure 2(a). In an alternative embodiment, the controller 18 may be configured to derive an indication of the backfire event from the gradient of the manifold air pressure signal as a function of engine position, by identifying the peak Y in the gradient signal of Figure 2(b). Either a peak in the manifold air pressure signal or a peak in the gradient of the manifold air pressure signal therefore result in a backfire event being identified by the controller. Using the gradient of the manifold air pressure signal to identify a backfire event may be a more accurate method because it allows smaller ‘peaks’ in the manifold air pressure to be detected. A backfire event can cause a very rapid increase in the manifold air pressure, which is easier to identify in the gradient signal of Figure 2(a). In a still further embodiment, an accelerometer fitted to the engine may be used to generate an output signal from which it can be determined when a back-fire event has occurred, using a conventional method for back-fire detection. A backfire event is followed by a misfire on the cylinder for which the corresponding injector has leaked, because the gaseous fuel that was intended to be combusted has instead caused the backfire event, leaving a misfiring cylinder. By comparing the peak in the manifold air pressure signal at X, or a peak in the gradient of the manifold air pressure signal at Z, with the misfiring cylinder enables the leaking injector to be identified. The mis-firing cylinder (corresponding to one of the injectors) is identified from Figure 2(c) which shows the counter value for the misfiring cylinder increasing by one, corresponding to the peak in the air manifold pressure and the gradient of the air manifold pressure. The controller therefore assigns the backfire event to the relevant engine cylinder depending on the corresponding engine position Z for the peak, X or Y, and the injector which is in the air intake phase when the backfire event occurred. In this example, the count for the backfire events is increased by one, as shown in Figure 2(c), for the third injector, INJ3. Figures 3(a) to (c) illustrate graphs to show the status of the cylinder associated with the leaking injector INJ3 which has caused the backfire event (Figure 3(a)), the status of the intake valve for the injector INJ3 (Figure 3(b)) and the timing windows for the backfire event (Figure 3(c)), all as a function of engine position. The four strokes of the corresponding engine cylinder are represented in Figure 3(a) by the four stroke regions in the graph; Power, Exhaust, Intake, Compression. Injection occurs during a period of the intake stroke for which the cylinder intake valve is open, as indicate in Figure 3(a). Two timing windows are set in software, as shown in Figure 3(c), corresponding to a period of the intake stroke for which the injector is open and injecting (period A) and a period of the intake stroke for which the injector is closed and not injecting (period B). Depending on when the backfire event occurs will affect the likely damage to the injector, so it is helpful to determine the number of backfire events which occur for an injector when it is open and the number of backfire events which occur when it is closed. If the backfire event occurs within the timing window A, this is referred to as an “open injector backfire event” and if the backfire event occurs within the timing window B, this is referred to a “closed injector backfire event”. Figure 4 shows the method of the invention as implemented in the controller 18. in a first step 100, the backfire event is identified for a specific injector using the method steps described previously, by monitoring the manifold air pressure signal and identifying a backfire event (pop-back backfire) based on the peak in the manifold air pressure signal or a peak in the manifold air pressure signal gradient, and knowing which cylinder is in the intake stroke when the backfire peak occurs. At step 102, using the timing windows A, B in Figure 3(c), a determination is made as to whether the backfire event is an injector-open backfire event or an injector-closed backfire event. If there is an injector-open backfire event, a counter is incremented at step 104 to increase the injector- open backfire total count by 1. This value may be referred to as the injector-open backfire count. If there is an injector-closed backfire event, a counter is incremented at step 106 to increase the injector-closed backfire count by 1. This value may be referred to as the injector-closed backfire count. The count for injector-open backfire events and injector-closed backfire events are stored in two counters, A and B respectively, forming part of the controller 18. At step 108 the injector-open backfire count is compared with an injector-open threshold count. The injector-open threshold count is a predetermined threshold value stored in the NVM of the controller 18. If the injector-open backfire count is greater than the injector-open threshold count, a warning signal is issued at step 110 to notify the driver of the vehicle that there is a problem with the injector and the injector may need servicing. Likewise, at step 112 the injector-closed backfire count is compared with an injector-closed threshold count. The injector-closed threshold count is a predetermined threshold value stored in the NVM of the controller 18. If the injector-closed backfire count is greater than the injector-closed threshold count, a warning or alert signal is issued at step 110 to notify the driver that there is a problem with the injector and the injector needs servicing. If the count values for injector-open and injector-closed backfire events do not exceed the predetermined count thresholds (i.e. the injector-open backfire events and the injector-closed backfire events are considered to be at a safe level), no action is taken (step 114). The injector-closed threshold and the injector-open threshold may be the same value or may take different values. For example, suitable thresholds to be set in software may be determined in a pre-calibration phase by inducing back-fire events. Figure 5 iliustrates a method of a second embodiment of the invention. In this method the detection of backfire events, and in particular the total count of injector-open backfire events or injector-closed backfire events, is correlated with a measured injector leak rate. The method of detecting and counting backfire events, as described previously, is run in parallel with the injector leak detection method. The output from the backfire detection method is used together with the leak detection method to reduce the risk of false positives from the leak detection method, but to ensure that indications that the injector performance is degrading - which may not be detected using a leak detection method alone - do not go unnoticed. Injector leak detection tests are known in the art. For example, our co-pending UK applications describe leak detection methods which allow an injector leak event to be identified and the leak rate to be measured: GB2315401.6, GB2315402.4, ,GB2315404.0, GB2315407.3, GB2315408.1. In step 200, the injector leakage detection method (leak detection monitor advice) is implemented in the controller 18, a leak detection event is identified and the associated injector leak rate is measured. An output is generated by the controller 18 to indicate the measured leak rate. At step 202, the measured leak rate is compared with an initial, predetermined high leak rate threshold above which the leak rate is deemed to be unacceptably high. If the leak detection method indicates that the measured leak rate is above the initiai high leak rate threshold, a ‘gross leakage’ is indicated and a warning signal is generated at step 204 that service action is required (severe action / alert). This alert at step 204 occurs even if the counters far the injectoropen and the injector-closed backfire events are below their respective count thresholds. At step 206, the leak rate measurement at step 200 is compared with the prior leak rate detection event (i.e. the measured leak rate for a previous leak event), and a comparison is made of whether the measured leak rate is increasing. The prior leak detection event therefore provides a reference event for the current measured leak rate. In other words, if the leak measurement at step 200 occurs in a first time period, the comparison is made with a leak rate which is measured at a prior time period, which may be referred to as the reference time period. The counter values for the injector-closed backfire event and the injector-open backfire event are stored in the memory as reference counts, A and B (step 208). If the leak rate has increased since the previous leak event, a comparison is made at step 210 to see whether the counter value for the injector-closed backfire event, count A, or the counter value for the injector-open backfire event, count B, has increased since the last detected leak. If there is no increase in the counter values then no action is taken (step 212) and the method exits and returns to step 200. In these circumstances it can be assumed that even if the leak rate has increased since the reference time period, without a correlation with a backfire event, it need not be considered of serious consequence and can be monitored but does not require a warning signal. If there is an increase in the counter value for the injector-closed backfire event, count A, or in the counter value for the injector-open backfire event, count B, then the new counter values are stored in the memory at step 214, and these become the reference count values (step 210) for the next iteration of the method, starting at step 200. At step 216 the measured teak rate is compared with a moderate teak rate threshold stored in the memory of the controller and if the measured leak rate is greater than the moderate leak rate threshold a warning signal is output at step 218 to service the injectors soon. The moderate leak rate threshold used at step 216 is set below the level of the initial, high teak rate threshold in step 202. At step 220, the measured leak rate for the current time period is compared to the leak rate for the reference time period and a difference value is calculated. The difference value is compared with a predetermined difference threshold. If the difference value is above the difference threshold then a warning or alert signal is generated by the controller that service action is required soon (step 222). If the determined difference threshold is below the difference threshold then no action is taken (step 224) and the method returns to the start at step 200. Step 220 of the method therefore provides a check that the measured leak rate is not increasing at an unacceptably high rate which is indicative of a serious problem which requires service action. This determination that the measured leak rate is increasing at an unacceptably high level may occur even if the initial high leak rater threshold at step 202 is not satisfied. In summary, the method of the embodiment checks, in sequence, (i) whether the measured teak rate is unacceptably high (above the high leak rate threshold), by performing a first comparison step, and if so a warning signal is generated; (ii) if the leak rate has increased, even if only by a small amount, a check is made, in second and third comparison steps, on the backfire event status for open-state and closed-state backfire events respectively, so that any increase in the leak rate which is accompanied by a backfire event is not ignored and a warning signal is generated: and (ill) in a further comparison step, if the leak rate is above a moderate leak rate threshold, after checks (i) and (li) are made, then a warning signal is generated anyway. The backfire status is therefore used in stage (ii) as a check to verify that the teak rate measurements are serious enough to warrant a warning. The method can avoid false positives by setting the initial test (step (I)) against the leak rate threshold at a high level, but the test against a moderate leak rate threshold (step (iii)) does not miss small teaks developing which can be identified through the correlation with the backfire detection steps in step (ii). It will be appreciated that the nature of the alert or warning signal at steps 204, 218 and 222 may be different. For example, at step 204, when the leak rate is above the high leak rate threshold, the alert or warning signal may be a serious warning signal to indicate that immediate action is required. For the alerts or warning signals at steps 5 218 and 222, the notification may be of an advisory nature, for example to suggest that servicing may be necessary soon because, even if the measured leak rate is below the high leak rate threshold, the leak is accompanied by a backfire event which may suggest a problem. 10 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.

Claims

1. A diagnostic system for an engine system comprising a plurality of engine cylinders (30) and an air intake manifold (44) for receiving an intake of air during an intake phase of an engine cycle, the diagnostic system comprising;a plurality of fuel injectors (46), each for delivering fuel to an associated one of the engine cylinders (30) via the air intake manifold (44), wherein each of the plurality of injectors (46) has an injector-open state when the injector (46) is delivering fuel during the air intake phase of the associated engine cylinder (30) and an injector-closed state when the injector (46) is not delivering fuel;a sensor (61) configured to measure the air pressure in the air intake manifold; anda controller (18) configured to:determine an occurrence of a backfire event in an associated one of the engine cylinders (30) on the basis of the measured air pressure;measure the number of backfire events that have occurred within each engine cylinder (30);- compare the number of backfire events with a predetermined count threshold, andif the measured number of backfire events is greater than the predetermined count threshold, output a warning signal.

2. The diagnostic system as claimed in claim 1, wherein the controller (18) is configured to;- determine whether the backfire event occurred within the associated engine cylinder (30) when the associated injector (46) is in the injectoropen state or the injector-closed state;- determine an open-state backfire count being the number of times a backfire event occurs when the associated injector (46) is in the open state and;- determine a closed-state backfire count being the number of times a backfire event occurs when the associated injector (46) is in the closed state.

3. The diagnostic system as claimed in ciaim 2, wherein the controller (18) is further configured to, in a backfire event comparison step, compare at least one of the open-state backfire count with an open-state threshold and the closed-state backfire count with a closed-state threshold; and if at least one of the open-state backfire count is greater than the open-state threshold and the closed-state backfire count is greater than the closed-state threshold, output a warning signal.

4. The diagnostic system as claimed in claim 3, wherein the controller is configured to determine the occurrence of a backfire event directly from the measured air pressure in the air intake manifold (44) as a function of engine position.

5. The diagnostic system as claimed in claim 3, wherein the controller (18) is configured to determine the occurrence of the backfire event from the gradient of the measured air pressure in the air intake manifold (44) as a function of engine position.

6. The diagnostic system as claimed in any of claims 1 to 5, wherein the controller (18) is further configured to;determine an injector leak rate for a first time period, the injector leak rate being indicative of the rate of leakage of fuel from the injector (46) during the first time period; andin a first leak rate comparison step, compare the determined leak rate for the first time period with an initial leak rate threshold; and,if the determined leak rate for the first time period is greater than the initial leak rate threshold, output a warning signal for the injector (46).

7. The diagnostic system as claimed in claim 6, wherein the controller (18) is further configured to;determine an injector teak rate for a reference time period prior to the first time period, the leak rate being indicative of the rate of leakage of fuel from the injector in the reference time period;determine the open-state backfire count in the first time period and determine the open-state backfire count in the reference time period;compare the determined teak rate for the first time period with the determined leak rate for the reference time period; andif the measured leak rate for the first time period is greater than the measured leak rate for the reference time period, compare the open-state backfire count in the first time period with the open-state backfire count in the reference time period.

8. The diagnostic system as claimed in claim 7, wherein the controller (18) is further configured to:if the open-state backfire count in the first time period is greater than the openstate backfire count in the reference time period, then, in a second teak comparison step, compare the determined leak rate for the first time period with a predetermined second teak rate threshold, wherein the predetermined second leak rate threshold is less than the predetermined first teak rate threshold.

9. The diagnostic system as claimed in claim 8, wherein the controller (18) is further configured to:if the determined leak rate for the first time period is greater than the predetermined second leak rate threshold, output a warning signal.

10. The diagnostic system as claimed in any of claims 7 to 9, wherein the controller (18) is further configured to compare the closed-state backfire count in the first time period with the closed-state backfire count in the reference time period and, if the measured leak rate for the first time period is greater than the measured leak rate for the reference time period, compare the closed-state backfire count in the first time period with the closed-state backfire count in the reference time period.

11. The diagnostic system as claimed in claim 10, wherein the controller (18) is further configured to:if the closed-state backfire count in the first time period is greater than the closed-state backfire count in the reference time period, then, in a third leak comparison step, compare the determined leak rate for the first time period with a predetermined second leak rate threshold, wherein the predetermined second leak rate threshold is less than the predetermined first leak rate threshold.

12. The diagnostic system as claimed in claim 11, wherein the controller (18) is further configured to:if the determined leak rate for the first time period is greater than the predetermined second leak rate threshold, output a warning signal.

13. A diagnostic method for an engine system comprising a plurality of engine cylinders (30) and an air intake manifold (44) for receiving an intake of air during an intake phase of an engine cycle, a plurality of fuel injectors (46), each for delivering fuel to an associated one of the engine cylinders (30) via the air intake manifold (44), and a sensor configured to measure the air pressure in the air intake manifold; the diagnostic method comprising;- determining an occurrence of a backfire event in an associated one of the engine cylinders (30) on the basis of the measured air pressure;measuring the number of backfire events that have occurred within each engine cylinder (30);- comparing the number of backfire events with a predetermined count threshold, and- if the measured number of backfire events is greater than the predetermined count threshold, outputting a warning signal.21

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