Improvements in injector control in gaseous fuel injection systems

Abstract

A common rail fuel assembly for a gaseous fuel engine system, the common rail fuel assembly comprising a rail housing defining a rail volume 12 for storing pressurised gaseous fuel, and a plurality of

Description

Technical Field The examples of the invention relate to improvements in control of fuel injection, and particularly fuel injection 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. Hydrogen, for example, has a very low density which makes it challenging to contain, transport and use in the context of a fuel system for an internal combustion engine. In general, following deactivation of a hydrogen-fueled engine, it is desirable to prevent any leakage of hydrogen as it presents a safety risk due to its combustibility and it is also a greenhouse gas. The injectors for hydrogen and other gaseous often have similarities with those use for diesel or gasoline injection with regard to the injector valve needle and the actuation mechanism for the valve needle. Typically, the valve needle of the injector is controlled to move towards and away from a valve seat to control the delivery of gaseous fuel into an associated engine cylinder. However, another problem with gaseous fuel is that traditional methods used in gasoline or diesel injectors, for determining when the valve needle in an injector has actually seated or opened, cannot be used in gaseous fuel injectors. In an injector for gasoline or diesel fuel, 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. Designers for gaseous fuel injectors cannot therefore rely on conventional diesel or gasoline glitch detection methods to accurately control valve needle opening and closing movement. It is with these issues in mind that the embodiments of the invention have been devised. Summary of the Invention Against this background, according to a first aspect of the invention, there is provided a common rail fuel assembly for a port fuel injection system of an engine system, for example including an air inlet manifold; the common rail fuel assembly comprising; a rail housing defining a rail volume for storing pressurised gaseous fuel; and a plurality of injectors mounted to the rail housing, the rail volume being configured to deliver gaseous fuel to the plurality of the injectors and each of the plurality of injectors being configured to deliver gaseous fuel to an associated engine combustion chamber, for example via the air inlet manifold, each of the plurality of injectors having a valve needle which is movable towards and away from a valve seat to control fuel injection into the engine combustion chamber via the air inlet manifold. The common rail fuel assembly further comprises an accelerometer mounted to the rail housing to provide an accelerometer output to indicate, for at least one of the plurality of injectors, a time corresponding to opening of the valve needle of the injector away from the associated valve seat and / or a time corresponding to closing of the valve needle of the injector against the associated valve seat. It is a benefit of the invention that the accelerometer output signal can be used as a feedback signal for controlling the timing of a subsequent injection event, to improve the accuracy of control of the timing of injection in a gaseous fuel system such as one used for hydrogen injection. The accelerometer output signal may be used to provide an accurate indication of the exact point in time at which the valve needle lifts at the start of injection, and then seats again at the end of injection. The accelerometer output signal can also be used to prove an indication of the exact point in time at which the valve needle is seated at the end of injection to terminate an injection event. The method can therefore can also be used to indicate precisely the actual duration of an injection event. This data can be used to adjust the drive signals for subsequent injection events so that there is a greater degree of accuracy over when precisely injection is started / stopped. The invention is particularly applicable to a port injection fuel system in which the injectors deliver fuel into the engine manifold, so the mounting point for the injectors is close to the point of injection. Further example features of the invention to achieve one or more of these functions are explained below. For example, the common rail fuel assembly may include a controller configured to apply an opening drive signal to the injector at a drive opening time to initiate lift of the valve needle away from the valve seat to commence injection at an actual opening time; to receive the accelerometer output from the accelerometer which is indicative of the actual opening time; and to compare the drive opening signal with the accelerometer output to determine an opening response characteristic of the injector. In addition, or alternatively, the controller may be configured to apply a closing drive signal to the injector at a drive closing time to initiate closure of the valve needle against the valve seat to terminate injection at an actual closing time; to receive the accelerometer output from the accelerometer which is indicative of the actual closing time; and to compare the closing drive signal with the accelerometer output to determine a closing response characteristic of the injector. By way of example, the controller may be configured to, for a current injection event, determine, from the accelerometer output, an actual opening time representative of the valve needle lifting away from the valve seat as a result of the drive opening signal; perform a comparison between the drive opening time with the actual opening time; and determine, on the basis of the comparison, the opening response characteristic in the form of an opening delay between application of the drive opening signal and actual opening of the valve needle. The controller may also be configured to, for a subsequent injection event to the current injection event, adjust the opening drive signal for initiation of lift of the valve needle in response to the opening delay for the current injection event. By way of further example, the controller may be configured to, for a current injection event, determine, from the accelerometer output, an actual closing time at which the valve needle actually closes against the valve seat as a result of the drive signal; perform a comparison between the drive closing time with the actual closing time; and determine, on the basis of the comparison, a closing response characteristic in the form of a closing delay between initiation of the drive closing signal and actual closing of the valve needle. In embodiments of the invention, the controller may be configured to, fora subsequent injection event; adjust the closing drive signal for the closing of the valve needle in response to the closing delay for the current injection event. The controller may be configured to determine, from the accelerometer output for a current injection event, an actual injection pulse duration for which the injector is injecting; performing a comparison between the actual injection pulse duration and a pre-determined injection pulse duration; and adjusting, for a subsequent injection event to the current injection event, the demanded injection pulse duration on the basis of the comparison to ensure a required quantity of fuel is injected. There may be one accelerometer in the common rail fuel assembly which is preferably mounted equidistant between two of the injectors. In a four injector system, for example the accelerometer is preferably mounted equidistant between two of the injectors and equidistant between the other two injectors, the injectors being regularly spaced along the rail housing. In other embodiments, the common rail fuel assembly may include a plurality of accelerometers, one for each of the plurality of injectors. The controller may be further configured to determine a corresponding time period within which a selected one of the plurality of injectors is commanded to inject fuel; determine which of said one of the plurality of injectors has given rise to the accelerometer output by performing a comparison between the corresponding time period and the time corresponding to opening of the injector; and assign the accelerometer output to the selected one of the injectors based on the comparison. According to another aspect of the invention, there is provided a method of controlling the injection duration in a fuel injector of a common rail fuel assembly including a rail volume for delivering fuel to a plurality of injectors mounted to a rail housing, each of the plurality of injectors being configured to deliver gaseous fuel to an associated engine combustion chamber and having a valve needle which is movable towards and away from a valve seat to control fuel injection into the engine combustion chamber; the common rail fuel assembly further comprising an accelerometer mounted to the rail housing to provide an accelerometer output; the method comprising determining, on the basis of the accelerometer output for a current injection event, an actual injection pulse duration for which one of the injectors is injecting; performing a comparison between the actual injection pulse duration and a pre-determined injection pulse duration; and adjusting, for a subsequent injection event to the current injection event, the subsequent injection pulse duration for a subsequent injection event to the current injection event on the basis of the comparison. It will be appreciated that any of the preferred or optional features of the first aspect of the invention may be incorporated alone or in appropriate combination in 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 side view of a fuel rail assembly of an embodiment of the invention, the common rail being provided with an accelerometer and a plurality of fuel injectors; Figure 2 is a graph to illustrate an accelerometer signal which may be processed by the controller in Figure 1, to detect a leak in one of the fuel injectors. Figure 3 is a flow chart illustrating an example algorithm that may be implemented to detect a leak in one of the fuel injectors in Figure 1; and Figure 4 is a flow chart illustrating an alternative example algorithm that may be implemented to detect a leak in one of the fuel injectors in Figure 1. Detailed Description In general, the examples of the invention provide a common rail fuel assembly for an engine, and including a controller which is configured to detect a leak of gas from the system, particularly a fuel injector leak. The invention also relates to the controller. The method is applicable to a port fuel injection system in which each of the fuel injectors of the engine is arranged to inject fuel into an air inlet manifold at a dedicated channel which leads to a respective one of the engine cylinders. In overview, and referring to Figure 1, the common rail fuel assembly 10 includes a common rail, referred to generally as 12, which, in the illustration show, takes the form of an elongate rail housing defining a rail volume. The common rail 12 has a rail inlet (not shown) located at one end of the rail, which receives gaseous fuel from a pressurised fuel source (also not shown). The common rail 12 is also provided with a pressure / temperature sensor 14. The common rail 12 provides a relatively large volume of gaseous fuel which is maintained at a predetermined, and controllable, pressure level. The common rail 12 delivers fuel to a plurality of fuel injectors, each of which is associated with a different cylinder or combustion chamber of the engine. The injectors are labelled as INJ1, INJ2, INJ3, INJ4. The engine typically comprises four combustion chambers, or cylinders, in an ‘in-line’ configuration and, hence, each of the injectors INJ1, INJ2, INJ3, INJ4 delivers fuel to a respective one of the cylinders. Herein, the term ‘combustion chamber’ will be considered synonymous with ‘engine cylinder’. In the illustrated example, there are a plurality of fuel injectors, the number of which corresponds to the number of combustion chambers. Each combustion chamber has an inlet valve and an exhaust valve in a conventional manner to control, respectively, the airflow into and the exhaust flow out of the combustion chamber. A network of air pipes feeds incoming fuel to an air inlet duct or ‘manifold’. As is known in the art, the air inlet manifold directs fresh air to each of the engine cylinders via separate air channels. The fuel injectors I NJ 1 -1N J4 are arranged to inject gaseous fuel, in this case hydrogen gas, into the fresh air flowing into the engine cylinders. The system therefore takes the form of a port-injection fuel system. The common rail 12 is provided with an accelerometer 16 which is mounted to the common rail in a mid-way position along its length. Preferably the accelerometer 16 is mounted equi-distant between two of the four injectors (i.e. INJ2, INJ3), and equidistant between the other two of the four injectors (INJ1, INJ4). For example, in the illustration shown the accelerometer 16 is mounted equi-distant between INJ2 and INJ3 and equidistant between INJ1 and INJ4. The injectors IN J1 -I N J4 are mounted directly to the common rail 12. In other words, an injector housing of each injector is mounted on and attached to the common rail 12, without the need for separate pipework between the common rail 12 and an inlet of each injector. The system is a port injection engine and so each of the fuel injectors INJ1, INJ2, INJ3, INJ4 is arranged to inject fuel into the air inlet manifold ata dedicated channel which leads to a respective one of the engine cylinders. The engine system further comprises a control unit 20, referred to as the engine control unit (ECU), which is adapted to receive data input to sense operational parameters of the engine to provide suitable control output signals to the fuel injectors and other aspects of the engine system to control its operation based on driver demands and sensor measurements, as is conventional. Typically, the injectors may include electromagnetic actuators to which a drive current is applied, in the form of an injector drive signal or drive pulse, to actuate the associated injector to switch between an injecting state, in which the injector delivers fuel to the associated cylinder, and a closing state, in which no injection occurs. The common rail 12 is supplied with fuel by a fuel supply system (not shown) including a pressurised fuel source or reservoir of gaseous fuel. The pressurised fuel source or ‘fuel tank’ may suitably be configured to store hydrogen gas at an appropriate pressure level, which may be between 350 and 700 bar. A pressure regulating device (not shown) is configured to reduce the gas pressure in the fuel tank to a pressure suitable for injection, which may be between 5 bar and 10 bar but could be higher for some systems. Other sensors may be included in the system, such as a rail pressure sensor and a temperature sensor. The control unit 20 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 20 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 may be any suitable control environment provided by the engine system 2. The control unit 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 control unit 20 may be a control environment provided specifically for the purposes of performing the method. The control unit 20 is configured to control operation of the fuel injectors I NJ 1 -1 NJ 4 through a plurality of injector drive signals, in accordance with engine demand and other sensor outputs. The accelerometer 16 is arranged to provide an accelerometer output signal to the control unit 20 which them influences the control of the injectors, as will be discussed in further detail below. More specifically, the method is suitable to be performed on the port injection system in which the fuel injectors deliver fuel to the air inlet manifold before delivery to the engine cylinders. As part of the process of controlling the injectors INJ1-INJ4 accurately, an example algorithm or method can be implemented by the control unit 20 to determine when the valve needle of an injector IN J1 -INJ4 has opened or closed, and also the time period for which the valve needle is held open, and to control subsequent injection events to take account of these real-time measurements. It is important to have accurate control of the opening and closing times, and the duration of valve needle opening, as it allows precise control of the quantity of fuel which is delivered in an injection event. In practice, the demanded opening and closing time may not be fulfilled due to a variety of factors, including, for example, injector to injector tolerances, wear of the valve needle seat and temperature variations. As an example, a seat wear of around 20 pm can give rise to a difference in fuel injection quantity of around 10%, which is not insignificant. Figure 2 is a graph to illustrate the accelerometer response as a function of time, together with the injector drive signal which is applied to deliver an injection event. The injector drive signal is denoted as Line A and the accelerometer output signal is denoted as Une B. Initially, the injector drive signal is applied to the injector at time T1 and the valve needle of the injector starts to lift away from the valve seat. As can be seen from Une B, at time T2, the valve reaches its fully open position and hits an internal lift stop of the injector. Time T2 is taken to represent a timing of opening of the injector valve needle. This is seen as an opening peak X in the accelerometer output signal B which arises as a result of the valve needle hitting the stop. This opening peak X equates to the full lift position of the valve needle. As can be seen by comparing Une A and Line B, a time delay (opening time delay DT1) exists between the onset of the injector drive signal at T1 and the full lift position of the valve needle at T2. The opening time delay defines an opening response characteristic for the injector. After an appropriate time period, the injector drive signal is reduced to a steady value where the valve needle is held against its lift stop and the injector is injecting. After a predefined injection period, during which the required quantity of fuel is injected, the drive signal is removed at time T3 (referred to as the closing drive signal), to end injection. As the valve needle closes and hits the valve seat to terminate injection, a peak closing peak Y is seen in the accelerometer output signal at time T4. A time delay (closing time delay DT2) exists between the removal of the injector drive signal at time T3 and the actual closing peak in the accelerometer output signal at time T4. The closing time delay defines a closing response characteristic for the injector. The time period between the opening peak at time T2 and the closing peak at time T4 represents the actual duration of the injection event (Dlnj). By monitoring the opening time delay DT1, the closing time delay DT2 and the duration of the injection event Dlnj, as seen in Figure 2, the accuracy of injector control can be improved using a feedback system to control subsequent injections. Figure 3 illustrates the steps of the method which may be implemented to control the opening of the injector more accurately, based on the accelerometer output signal. As an initial step (step 100), the accelerometer signal is received at the control unit 20 within a pre-defined crank angle range for the engine. In other words, as the engine crankshaft rotates through 360 degrees, an angular crank shaft range is selected over which to record the accelerometer output signal in Figure 2. Typically, the angular range may be between 390 and 220 degrees BTDC (before top-dead-centre). At step 102, the accelerometer signal is passed through a band pass filter to remove any noise perturbations from the signal before they are processed further. At step 104 the opening peak X in the accelerometer signal is determined and recorded in the control unit 20. At step 106, the opening time delay DT1 is determined based on the time T1 for the initiation of the injector event and the measured time T2 for the accelerometer opening peak X. The control unit 20 is programmed with or stores pre-determined calibration data for the injector, which is obtained for a new injector ahead of engine running when installed in a vehicle. The calibration data includes, for a range of different engine operating conditions for a new injector, a calibration value for the opening time delay DT1, a calibration value for the closing time delay DT2, and a calibration value for the injector duration Dlnj. This calibration data forms the basis of the control strategy for the injector so that, for any engine operating conditions, the required injector drive signals can be applied to achieve the desired start and end times of an injection event. At step 108, the measured opening time delay DT1 is compared to the calibration value for the opening delay under those conditions. At step 110, if it is determined that the opening time delay DT1 is within a predetermined threshold value for the calibration value of the opening time delay, the injector drive signal for a subsequent injection is adjusted to allow for the shift in the opening delay, so that the actual onset of injection is more accurately controlled. In addition, at step 112, if it is determined that the opening time delay DT1 is above the predetermined threshold value, this can be taken as an indication that there is a fault and, at step 114, the appropriate diagnostic response is applied (for example the associated cylinder is deactivated or a limp home strategy is employed). The method therefore allows an adjustment to be made to the injector drive signal to ensure that the opening delay which occurs following a demanded start to injection, and which may vary through the injector service life, is adjusted to ensure more accurate control of the timing of an injection. Figure 4 illustrates further steps of the method to control the closing of the injector more accurately, based on the accelerometer output signal. As an initial step (step 200), the accelerometer signal is received at the control unit 20 within a pre-defined crank angle range for the engine. In other words, as the engine crankshaft rotates through 360 degrees, an angular crank shaft range is selected over which to record the accelerometer output signal in Figure 2. Typically the angular range may be between 250 and 180 BTDC, although in any scenario the ideal range will be dependent on engine speed. At step 202, the accelerometer signal is passed through a band pass filter to remove any noise perturbations from the signal before they are processed further. At step 204 the closing peak Y in the accelerometer signal is determined and recorded in the control unit 20. At step 206, the closing time delay DT2 is determined for a current injection event based on the time T3 for the termination of the injector event and the time T4 for the accelerometer closing peak Y. At step 208, the closing time delay DT2 is compared to the calibration value for the closing time delay. At step 210, if it is determined that the closing time delay DT2 is within a predetermined threshold value for the opening time delay (as obtained during the calibration), the injector drive signal for a subsequent injection is adjusted to allow for the shift in the expected closing delay, so that the actual termination point for injection is more accurately controlled. In addition, at step 212, if it is determined that the closing time delay DT2 is above the predetermined threshold value, this can be taken as an indication that there is a fault and, at step 214, the appropriate diagnostic response is applied (for example the associated cylinder is deactivated or a limp home strategy is employed). The method therefore allows an adjustment to be made to the injector drive signal to ensure that the closing delay which occurs following a demanded end of injection, and which may vary through the injector service life, is controlled accurately to ensure the correct timing of the termination of injection. As a further refinement to the method, the duration of the injection event may also be monitored in the same way by comparing the measured injection event duration for a current injection event with a calibrated injection event duration, and applying an adjustment for a subsequent injection event to control the injection duration more accurately. This ensures the quantify of fuel injected during an injection event is more accurately controlled. As before, if the injection event duration is found to be beyond a predetermined threshold value for the injection event duration, an appropriate diagnostic response may be applied. The injection event duration may be adjusted by adjusting the injector opening time, the injector closing time, or a combination of both. It will be appreciated that any one or more of the injector opening delay, the injector closing delay or the injection event duration may be measured and controlled in subsequent injections, as described previously. For example, the duration of injection may be monitored based on the monitoring of both the actual opening and closing times, T2, T4 respectively, and any appropriate adjustment compared to the calibration data is made accordingly to ensure the correct volume of fuel is delivered, as required, during the injection. However, there are situations to be avoided when controlling the closing delay. It is important that adjustments are not made so that the end of injection (referred to as EOI) occurs after the inlet valve to the cylinder has closed (referred to as “inlet valve closure” IVC). It would not be desirable to inject gaseous fuel after IVC because then fuel is injected into the ait inlet manifold, and not into the cylinder. Therefore, if the adjustment to the injector closing delay is monitored and the required adjustment would place the end of injection after IVC, an adjustment needs to be made to the injector opening time to ensure EOI is before IVC. Alternatively, if the adjustment to the closing delay DT2 does not place the EOI after IVC, there may be no need to adjust the injector opening time also and the closing delay only may be monitored and adjusted. The injector closing delay DT2 may be measured in isolation for the purpose of ensuring that the injector is functioning correctly, and to provide a fault diagnostic (e.g. to avoid hydrogen leaks on injector closing). In order to be able to identify which injector of the assembly has caused the opening and closing peaks in the accelerometer signal, a time filter is used to assign different peaks in the accelerometer signal to different ones of the injectors. For example, if INJ1 is known to inject within a particular time period, it can be taken that the accelerometer signal in that time period corresponds to the injection event from INJ1, and likewise for injector INJ2, and so on. For this purpose the control unit 20 determines the time period for which a particular injector is demanded to inject and analyses the accelerometer signal within that time period to determine the opening and / or closing response characteristic for that particular injector. In an alternative embodiment, one accelerometer may be provided for each injector on the rail, although this adds to the cost of the system with additional parts. Monitoring of the accelerometer signal may be performed continuously or may be enabled only after the engine is rotating above a certain speed. The invention is more accurate when applied to an injector assembly which is directly mounted to the rail, as in a port injection system, rather than one for which the injectors connect to the rail via separate pipework. It will be appreciated that other variants of the invention are envisaged without departing from the scope of the invention as set out in the accompanying claims.

Claims

1. A common rail fuel assembly for a gaseous fuel engine system; the common rail fuel assembly comprising;a rail housing defining a rail volume (12) for storing pressurised gaseous fuel; anda plurality of injectors (INJ1-1NJ4) mounted to the rail housing, the rail volume (12) being configured to deliver gaseous fuel to the plurality of the injectors and each of the plurality of injectors (INJ1-INJ4) being configured to deliver gaseous fuel to an associated engine combustion chamber, each of the plurality of injectors (INJ1-INJ4) having a valve needle which is movable towards and away from a valve seat to control fuel injection into the engine combustion chamber via the air inlet manifold;the common rail fuel assembly further comprising an accelerometer (16) mounted to the rail housing to provide an accelerometer output to indicate, for at least one of the plurality of injectors (INJ1-INJ4), a time corresponding to opening of the valve needle of the injector away from the associated valve seat and a time corresponding to closing of the valve needle of the injector against the associated valve seat.

2. The common rail fuel assembly of claim 1, further comprising a controller (20) configured to;apply an opening drive signal to the injector (INJ1-INJ4) at a drive opening time (T1) to initiate lift of the valve needle away from the valve seat to commence injection at an actual opening time (T2); andreceive the accelerometer output from the accelerometer (16) which is indicative of the actual opening time (T2); andcompare the drive opening signal with the accelerometer output to determine an opening response characteristic (T2-T1; Dlnj) of the injector.

3. The common rail fuel assembly of claim 1 or claim 2, wherein the controller (20) is configured to;apply a closing drive signal to the injector at a drive closing time (T3) to initiate closure of the valve needle against the valve seat to terminate injection at an actual closing time (T4);receive the accelerometer output from the accelerometer (16) which is indicative of the actual closing time (T4); andcompare the closing drive signal with the accelerometer output to determine a closing response characteristic (T4-T3) of the injector.

4. The common rail fuel assembly of claim 2 or 3 when dependent on claim 2, wherein the controller (20) is configured to, for a current injection event;determine, from the accelerometer output, an actual opening time (T2) representative of the valve needle lifting away from the valve seat as a result of the drive opening signal;perform a comparison between the drive opening time (T1) with the actual opening time (T2); anddetermine, on the basis of the comparison, the opening response characteristic in the form of an opening delay (T2-T1) between application of the drive opening signal and actual opening of the valve needle.

5. The common rail fuel assembly as claimed in claim 4, wherein the controller (20) is configured to, for a subsequent injection event;adjust the opening drive signal for initiation of lift of the valve needle in response to the opening delay (T2-T1) for the current injection event.

6. The common rail fuel assembly of any of claims 3 to 5 when dependent on claim 3, wherein the controller (20) is configured to, for a current injection event;determine, from the accelerometer output, an actual closing time (T4) at which the valve needle actually closes against the valve seat as a result of the drive signal;perform a comparison between the drive closing time (T3) with the actual closing time (T4); anddetermine, on the basis of the comparison, a closing response characteristic in the form of a closing delay (T4-T3) between initiation of the drive closing signal and actual closing of the valve needle.

7. The common rail fuel assembly as claimed in claim 6, wherein the controller is configured to, for a subsequent injection event:adjust the closing drive signal for the closing of the valve needle in response to the closing delay (T4-T3) for the current injection event.

8. The common rail fuel assembly as claimed in any of claims 3 to 7, wherein the controller is configured to:determine, from the accelerometer output for a current injection event, an actual injection pulse duration for which the injector is injecting;performing a comparison between the actual injection pulse duration and a pre-determined injection pulse duration; andadjusting, for a subsequent injection event to the current injection event, the demanded injection pulse duration on the basis of the comparison to ensure a required quantity of fuel is injected.

9. The common rail fuel assembly as claimed in any of claims 1 to 8, wherein the accelerometer is mounted equidistant between two of the injectors.

10. The common rail fuel assembly as claimed in any of claims 1 to 8, wherein the common rail fuel assembly includes a plurality of accelerometers, one for each of the plurality of injectors.

11. The common rail fuel assembly as claimed in claim 10 when dependent on any of claims 2 to 8, the controller being configured to:determine a corresponding time period within which a selected one of the plurality of injectors is commanded to inject fuel;determine which of said one of the plurality of injectors has given rise to the accelerometer output by performing a comparison between the corresponding time period and the time corresponding to opening of the injector; andassign the accelerometer output to the selected one of the injectors based on the comparison.

12. A method of controlling the injection duration in a fuel injector of a common rail fuel assembly including a rail volume (12) for delivering fuel to a plurality of injectors (INJ1-INJ4) mounted to a rail housing, each of the plurality of injectors (INJ1-INJ4) being configured to deliver gaseous fuel to an associated engine combustion chamber and having a valve needle which is movable towards and away from a valve seat to control fuel injection into the engine combustion chamber; the common rail fuel assembly further comprising an accelerometer (16) mounted to the rail housing to provide an accelerometer output; the method comprisingdetermining, on the basis of the accelerometer output for a current injection event, an actual injection pulse duration for which one of the injectors (INJ1-INJ4) is injecting;performing a comparison between the actual injection pulse duration and a pre-determined injection pulse duration; andadjusting, for a subsequent injection event to the current injection event, the subsequent injection pulse duration on the basis of the comparison.19

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