EP2666996A1

EP2666996A1 - Fuel Monitoring System

Info

Publication number: EP2666996A1
Application number: EP12169284.2A
Authority: EP
Inventors: Andreas Nilsson
Original assignee: Delphi Technologies Holding SARL
Current assignee: Delphi Technologies Operations Luxembourg SARL
Priority date: 2012-05-24
Filing date: 2012-05-24
Publication date: 2013-11-27

Abstract

A method of compensating for fuel injector performance variations in a fuel injection system (1) of an engine, the engine comprising a fuel pump (3,4) arranged to supply pressurised fuel to a common accumulator volume (5) that supplies the pressurised fuel in turn to a plurality of fuel injectors (8); and the method comprising: measuring (202) a first pressure within the accumulator volume; sending (204) a drive pulse signal to initiate an injection event in a first injector; measuring (206) a second pressure within the accumulator volume; calculating (208) a pressure difference between the first and second pressure measurements; using (212) the calculated pressure difference to compare actual fuel injection system performance with expected fuel injection system performance; compensating (214) for any deviations between actual fuel injection system performance and expected fuel injection system performance.

Description

Field of the Invention

The present invention relates to a fuel monitoring system. In particular, the present invention relates to a system and method for monitoring the fuel amounts injected in a fuel injection system and compensating for variations in the injected fuel amounts due to component wear.

Background to the Invention

In conventional common rail fuel systems for diesel engines, it is known to supply a drive current to a fuel injector in order to cause movement of a valve needle away from a valve seating to initiate fuel injection from the fuel injection system into the engine.
In conventional fuel systems data relating to engine operation may be received at an engine control unit. The received data may comprise driver input data (e.g. from the position of the accelerator pedal), data relating to the engine speed and load and also data relating to the particular combustion mode the engine system is operating in. Having received the engine operation data the engine control unit (ECU) calculates, via an appropriate control algorithm, injection parameters required to operate the engine in accordance with the received data. These injection parameters may include multiple injection timings, fuel quantities etc depending on the engine system in question. Having calculated the required injection parameters the control unit may output a suitable drive pulse or drive pulses to the injection system. Fuel injection may therefore be initiated at the desired stage of the injection cycle.
It is noted that the required drive pulse(s) may be derived from a look up table or calculated within an engine model. The model or look up tables may be calibrated on test equipment.
Mechanical wear of components within the engine may alter the injection characteristics of the engine and may require suitable compensation to ensure that engine performance is not compromised and to ensure that the engine meets NOx & Particulate emissions regulations throughout the life of the engine.
Current methods of compensating for changes in injector performance within the fuel injection equipment (FIE) system determine fuel amounts being injected into the cylinders of the engine system by means of an oxygen sensor in the exhaust system together with estimated air flow into the cylinders. In this manner variations between the demanded fuel amount and the injected fuel amount can be compensated. However, such techniques require the use of data from specialist engine sensors.
It is noted that within a multi-injector fuel injection system the different injectors may wear at different rates such that the injectors become unbalanced. Traditionally, injector balancing is done by detecting fluctuations in crank tooth wheel speed caused by combustion. It is therefore noted that the processes of injector wear determination and injector balancing require the use of different sensors and the data from such sensors to be input into a suitable compensation controller.
It is therefore an object of the present invention to provide a method and system of compensating for changes in injector performance that substantially overcomes or mitigates the above mentioned problems.

Summary of the Invention

According to a first aspect of the present invention, there is provided a method of compensating for fuel injector performance variations in a fuel injection system of an engine, the engine comprising a fuel pump arranged to supply pressurised fuel to a common accumulator volume that supplies the pressurised fuel in turn to a plurality of fuel injectors; and the method comprising: measuring a first pressure within the accumulator volume; sending a drive pulse signal to initiate an injection event in a first injector; measuring a second pressure within the accumulator volume; calculating a pressure difference between the first and second pressure measurements; using the calculated pressure difference to compare actual fuel injection system performance with expected fuel injection system performance; compensating for any deviations between actual fuel injection system performance and expected fuel injection system performance.
In the present invention data relating to engine operation is received at an engine control unit. The received data may comprise driver input data (e.g. from the position of the accelerator pedal), data relating to the engine speed and load and also data relating to the particular combustion mode the engine system is operating in. Having received the engine operation data the engine control unit (ECU) calculates injection parameters required to operate the engine in accordance with the received data. These injection parameters may include multiple injection timings, fuel quantities etc depending on the engine system in question. Having calculated the required injection parameters the control unit outputs a suitable drive pulse or drive pulses to the injection system.
Preferably, the first and second measurement steps and sending step may be performed when the engine is in a steady driving state.
It is noted that the fuel pump within the engine may pump fuel, in a fuel pumping event, into the accumulator volume in advance of each injection of fuel into the engine. There may however be more than one fuel pump and the fuel pumps may, between themselves pump fuel in a fuel pumping event into the accumulator volume in advance of each injection of fuel into the engine. Regardless of the number of fuel pumps within the engine system, each injection event from an injector in the fuel system may be associated with a fuel pumping event in which the fuel pump(s) is arranged to pump fuel into the accumulator volume, and the method may further comprise suspending the fuel injection event corresponding to the first injector immediately prior to the sending step.
The using step may comprise calculating, from the calculated pressure difference, the injected amount of fuel injected into the engine during the injection event of the first injector and comparing to a demanded fuel amount. The relationship between pressure drop and fuel amount may be determined in advance and stored in a suitable format (e.g. a function map within the ECU or associated storage volume) and the method may comprise looking up the injected fuel amount from the stored relationship.
Preferably, the compensating step may comprise adjusting a fuel injection parameter to compensate for any deviations between actual fuel injection system performance and expected (demanded) fuel injection performance.
Preferably, the fuel injection characteristics of all the injectors within the engine may be assessed by repeating the first measuring, sending, second measuring and calculating steps for each of the plurality of fuel injectors in turn. A pressure difference (pressure drop) for each fuel injector may then be calculated and in the event that the injectors are injecting different amounts of fuel the compensation step may comprise balancing the injectors within the injector system. [It is noted that the pressure drop may be calculated for a given injector before pressure measurements are taken in respect of the next injector within the engine. Alternatively pressure measurement data may be collected for all of the injectors in turn prior to performing the pressure drop calculation for each injector.]
The balancing step may comprise adjusting the amount of fuel injected by a particular injector while keeping the total fuel amount injected by the plurality of injectors into the engine during a given engine cycle at a constant value. In other words the balancing may comprise ensuring that any fuel value corrections sum to zero.
It is noted that each fuel injector may be balanced by applying a fuelling offset to the injector to alter the timing of the drive pulse sent to the each injector (Each drive pulse may be associated with a pumping event from the fuel pump and the fuelling offset may comprise offsetting the timing between a given drive pulse and the associated pumping event).
Alternatively, each fuel injector may be balanced by applying an injection duration offset to alter the amount of injected fuel supplied to the engine system.
In the event that all the fuel injectors within the injector system are experiencing reduced performance the compensation step may comprise applying a bulk offset to the plurality of injectors in order to adjust a total fuel amount supplied to the engine by the fuel injection system in an engine cycle.
In the event that the pressure measurements are received from a pressure sensor that has limited resolution in its pressure measurements then the method may conveniently further comprise measuring first and second pressure values for an injector over a plurality of engine cycles and, for each fuel injector, averaging the calculated pressure differences and using the averaged calculated pressure difference for each injector to compare actual injector system performance with expected fuel injection system performance.
An engine may be expected to experience pressure losses due to natural decay and/or switching losses and in such an event the using step may conveniently comprise adjusting the calculated pressure difference in order to adjust for pressure losses due to natural decay and/or switching losses. Any such adjustment may preferably be performed in respect of each fuel injector.
The method may further comprise sampling multiple pressure readings during each injection event cycle in order to determine turn on/turn off delays associated with the injecting injector (i.e. to compensate for any delays caused by a solenoid injector charging or discharging).
The method may also further comprise outputting a faulty injector signal in the event that the calculating step determines that there has been no or substantially no pressure drop during the injection event (associated with the fuel injector being assessed).
According to a second aspect of the present invention there is provided a compensation module for compensating for fuel injector performance variations in a fuel injection system of an engine, the engine comprising a fuel pump arranged to supply pressurised fuel to a common accumulator volume that supplies the pressurised fuel in turn to a plurality of fuel injectors; and the module comprising: inputs to receive first and second pressure measurements within the accumulator volume; an output arranged, between the reception of the first and second pressure measurements, to send a drive pulse signal to initiate injection in a first injector; and a processor arranged to: calculate a pressure difference between the first and second pressure measurements; use the calculated pressure difference to compare actual fuel injection system performance with expected fuel injection system performance; and to compensate for any deviations between actual fuel injection system performance and expected fuel injection system performance.
The engine system may comprise an electronic control unit (ECU) in which case the ECU may perform the functions of the compensation module above. Alternatively a dedicated compensation module may be carry out the above functions. Such a dedicated module could be integrated within the ECU or could be a standalone component.
According to a further aspect of the present invention there is provided a method of compensating for fuel injector performance variations in a fuel injection system of an engine, the engine comprising a fuel pump arranged to supply pressurised fuel to a common accumulator volume that supplies the pressurised fuel in turn to a plurality of fuel injectors; and the method comprising: measuring a first pressure within the accumulator volume; sending a drive pulse signal to initiate an injection event in a first injector; measuring a second pressure within the accumulator volume; repeating the measuring and sending steps for each of the plurality of injectors and calculating a pressure difference between the first and second pressure measurements for each injector; using the calculated pressure differences to compare actual fuel injection system performance with expected fuel injection system performance; compensating for any deviations between actual fuel injection system performance and expected fuel injection system performance.
The invention extends to a carrier medium for carrying a computer readable code for controlling an electronic control unit to carry out the method of the first aspect of the invention.
Preferred features of the first aspect of the present invention extend to the second and further aspects of the present invention.

Brief Description of the Drawings

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which like reference numerals are used for like parts, and in which:

Figure 1 shows a representation of a fuel injection system suitable for use with embodiments of the present invention;
Figure 2a shows a plot of rail pressure/pumping event/injection event versus time for an engine in a steady state driving condition;
Figure 2b shows a plot of rail pressure/pumping event/injection event versus time for an engine operating in accordance with embodiments of the present invention;
Figure 3 shows a pumping waveform/injection waveform and rail pressure across an entire engine cycle for an engine operating in accordance with embodiments of the present invention;
Figure 4 to 6 show pressure drop versus fuel quantity graphs for a variety of different engine operating conditions and fuel types;
Figure 7 shows a graph of pressure versus time that illustrates natural pressure leakage within an engine system;
Figure 8 is a flow chart of an embodiment of the present invention;
Figures 9 and 10 show graphs of pressure drop (relative to an average) versus fuel quantity for injectors within an engine.

Detailed Description

Figure 1 shows a representation of a fuel injection system 1 within an engine comprising a fuel tank 2, controllable high pressure fuel pumps (3, 4), a common rail (common accumulator volume/fuel reservoir) 5, a rail pressure sensor 6, a pressure limiter 7, a plurality of injectors 8 and an electronic control unit (ECU) 9.
In use the ECU 9 controls pumping of fuel from the tank 2 to the rail 5 by the pumps 3, 4. The ECU 9 also controls the operation of the injectors 8 and receives sensor data on the pressure within the rail 5 from the pressure sensor 6.
The ECU may store (or be in communication with via a data store) a plurality of data look up tables 10 relating engine data such as demanded fuel (e.g. from the position of the accelerator pedal), engine speed and load and also the particular combustion mode the engine system is operating in to one or more injection parameters. The ECU may use such look up tables to calculate injection parameters required to operate the engine in accordance with the received engine data. These injection parameters may include injection timings, fuel quantities etc and the ECU may output a drive pulse signal (comprising one or more drive pulses) to the injectors to initiate injection.
The fuel pumps may be driven by a cam arrangement (not shown) to produce a number of pumping events per engine cycle. In the arrangement depicted in Figure 1 there are six injectors and two pumps and so each pump may be driven by a three-lobed cam arrangement. It is noted however that the number of injectors and/or pumps may be varied within the scope of the claimed invention.
Figure 2a is a graph showing an engine operating in normal driving conditions (steady state driving conditions). In Figure 2a, a number of regularly spaced pumping events 20 are shown along with an injection event 30 for one of the injectors 8 within the engine system. The pressure within the common rail 5 is represented by the third trace line 40. The pressure within the rail 5 can be seen to be generally constant at a particular pressure value 50 (the "baseline" pressure value).
It can be seen that following each pumping event 20 there is an associated increase 60 in pressure within the common rail 5. Furthermore, after the injection event 30 the pressure returns to the particular pressure value 50 noted above.
(Note: for the purpose of clarity, only a single injection event is shown in Figure 2a. However, in reality there will be an injection event associated with each pumping event)
Figure 2b shows a similar graph to that of Figure 2a. However, in this case is can be seen that the pumping event 20' that immediately precedes the highlighted injection event 30 has been suppressed (e.g. turned off). In this case instead of a small pressure rise followed by a return to the baseline pressure level it can be seen that there is a pressure drop 70 following the injection event 30.
Figure 3 shows a representation of a pumping waveform 80, injection waveform 90 and rail pressure 40 across an entire engine cycle for an engine that is running a compensation method in accordance with embodiments of the present invention. It can be seen that there are six injection events corresponding to the six injectors 8 shown in Figure 1. However, there are only five pumping events with pumping event 3 being suppressed in order to allow the compensation method to be run during engine operation. As a consequence of the suppressed injection event it can be seen that there is a drop 70 in rail pressure 40 around the time of the third injection event.
It is noted that in between each injection/pumping event the pressure within the rail decays slightly due to natural leakage within the fuel injection system. It is further noted that the pressure drop after the third injection event comprises a number of components, namely: pressure drop due to injected fuel from the injector into the associated cylinder of the engine; pressure drop due to natural leakage; pressure drop due to switching losses.
Switching losses represent the total leakage of an injector which occurs when an injector changes from a non-actuated condition to a full needle lift injection condition. This leakage is a made up of a combination of the following:

Leakage during transition of a needle control valve within the injection from a non-actuated seating position to an actuated seating position. Such leakages are pressure dependent and a higher rail pressure results in higher leakage.
Volume reduction of a control chamber with the valve as the nozzle needle rises from its nozzle seat to full lift. (It is noted that the reduction in pressure of the remaining volume of the control chamber will involve pressure dependent leakage but this portion can be assumed negligible.)

For a solenoid actuated injector there will be periods at the beginning and end of an individual injection event where the solenoid is charging or discharging. There will therefore be lags in initiating and terminating injection, the "turn-on" and "turn-off" delays. The turn-on delay 100 relates to where the solenoid is initially responding to the start of the drive pulse and the turn-off delay 110 relates to where the solenoid is responding to the end of the drive pulse.
The natural leakage within the engine system may be measured in foot-off driving conditions (see also Figure 7 below) where no injections take place. In order to achieve such conditions and measure the leakage within the fuel system, the pumping of fuel into the rail may be suspended. The rate of decay of the rail pressure may then be measured. It is noted that the rate of decay should be measured at a number of different rail pressures as the rate of decay will be higher at high rail pressures compared to low rail pressures. The rate of decay may subsequently be stored in the ECU (or an associated data store or in a look up table) for use in conjunction with embodiments of the present invention.
Switching losses may also be measured during such foot-off periods by demanding an injection pulse that is too short for injection and measuring the pressure drop that results.
The present invention determines whether any adjustment to the injection parameters of the engine/to the control algorithm are required by turning off the pumping for an injector and measuring the pressure drop caused by an injection on that injector while running in steady state conditions. Conveniently it is noted that the pressure drop is substantially linear to the fuel quantity being injected (see Figures 4 to 6 below). Remaining pumping events may be used to restore the pressure within the rail.
Figure 4 shows a plot of pressure drop versus fuel quantity for the engine system of Figure 1 that is operating according to the pumping 80 and injection 90 profile of Figure 3. The pressure readings were taken at a baseline rail pressure of 2500 bar. The readings were also repeated at two different engine speeds (1000rpm and 1400rpm).
It can be seen that at both engine speeds there is a substantially linear relationship between the pressure drop during injection event 3 and the quantity of fuel injected from the injector into the engine system. There is a slight offset between the two linear relationships shown in Figure 4 and it is anticipated that the pressure drop will have a slight temperature dependency. It may therefore be necessary to store pressure drop vs. fuel quantity relationships for a variety of engine speeds at a variety of different temperatures.
Figure 5 shows a graph of pressure drop versus fuel quantity at a variety of different rail pressures at a fixed engine speed for an engine that is using a regular fuel mix (EN590). Figure 6 shows a corresponding graph of pressure drop versus fuel quantity for an engine running at the same fixed engine speed for a biofuel fuel mix (B1 OO(RME)).
From Figures 5 and 6 it can be seen that the pressure drop changes with different fuel properties. The ECU may therefore also store pressure drop vs. fuel quantity relationships for a variety of fuel types (or be in communication with a data store storing such information).
Figure 7 shows a pressure versus time plot for various biofuels that are operating in a foot off/no pumping mode. It can be seen that there is a pressure decay within the rail in dependence on the type of fuel. The ECU may therefore determine a natural leakage profile for the engine system from a given starting rail pressure in order to more accurately determine whether any compensation for injector wear is required using embodiments of the present invention.
Figure 8 shows a detection method in accordance with an embodiment of the present invention.
In Step 200, while the engine is in a constant driving condition (i.e. at a constant speed and load), the pumping event associated with a first injector (injector N) is suspended. In Step 202, a first pressure measurement is taken of the pressure in the accumulator volume/rail 5 (the pressure measurement comprises the pressure sensor 6 taking a pressure reading and supplying this reading to the ECU). In Step 204, a drive pulse is sent to the first injector to initiate injection and in Step 206 a further (second) pressure measurement is taken of the pressure in the accumulator volume/rail 5.
It is noted that the first and second pressure measurements are taken at predefined periods before and after the drive pulse.
In Step 208 a pressure drop within the fuel rail is calculated. The pressure drop may be adjusted to account for the pressure drop expected due to natural leakage and switching losses.
In Step 210, the process is repeated for all injectors within the engine. In Step 212 the measured engine performance may be compared to the predicted (/expected/demanded) engine performance according to the engine control algorithm. The comparison process may comprise converting the measured pressure drop data to an injected fuel amount (in milligrams) using a relationship as shown in Figures 4 to 6. The expected/demanded fuel amount may then be compared to the actual injected fuel amount and an appropriate correction to the injection parameters may be made (Step 214).
Figure 8 shows the pressure drop for a given injector (N) being calculated before the next injector is tested. It is to be appreciated however that pressure measurement data may be collected for all the injectors in turn (i.e. one by one) prior to performing any pressure drop calculations. In other words Steps 208 and 210 may be interchanged.
In the event of any deviation from the expected performance then a compensating adjustment may be made. For example, either a fuel timing offset or a fuel injection duration offset may be made.
The method according to Figure 8 may be repeated following the application of the compensation offset to ensure that the engine performance has returned within acceptable operational parameters.
In the event that the pressure sensor within the engine system comprises an analogue-to-digital converter (ADC), it is noted that it may be necessary to repeat the pressure drop measurements for each injector over a number of engine cycles due to the resolution of the pressure sensor.
For example, a 2850 bar rail pressure sensor feeding into a 12 bit ADC gives a resolution of 0.87 bar/bit. At 1500 bar the pressure drop per injected milligram of fuel is 0.37 bar. Therefore a one bit error before and after the injection (2 x 0.87 = 1.74 bar) would lead to an error of 1.74/0.37 = 4.7mg. Such an error would be larger for lower rail pressures. The pressure measurements may therefore be performed over a number of engine cycles to compensate for the sensor resolution.
In addition to the pressure drop calculation the compensation algorithm according to embodiments of the present invention may additionally receive inputs relating to the actual rail pressure level (e.g. the "baseline" pressure noted above), fuel temperature, coolant temperature, fuel mixture, boost temperature, boost pressure etc. (It is noted that the "boost" temperature and pressure relate to the inlet air temperature and pressure of air flowing into the cylinders of the engine. It is referred to as "boost" because it is the temperature/ pressure after the turbo in a turbo diesel).
The compensation adjustment made by the method according to embodiments of the present invention may comprise applying a timing offset to an injector, varying the fuel amount injected by an injector (the fuel injector duration offset), balancing all the injectors, applying a global offset to the fuel amount injected by all the injectors etc.
Figures 9 and 10 show the deviation in calculated pressure drop versus fuel quantity for injectors in an engine system at 1000rpm (Figure 9) and 1400rpm (Figure 10). In both instances it can be seen that injectors 1, 3 and 5 are injecting an above average amount of fuel compared to injectors 2, 4 and 6.
The compensation method according to embodiments of the present invention may apply an adjustment to the fuel injection parameters of each injector such that each injector injects the same quantity of fuel, i.e. the compensation method may balance the injectors by reducing the fuel injected by injectors 1, 3 and 5 and/or increasing the fuel injected by injectors 2, 4 and 6. In doing so it is noted that the total fuel injected per engine cycle may be maintained at the same value.
As an alternative, or additionally, the compensation method may apply a global adjustment to all injectors to increase the total amount of fuel injected into the engine per engine cycle. Such a global adjustment may be implemented where all injectors are showing a general decrease in performance since installation into the engine.

Table 1 below shows the results of pressure drop calculations (in bar) for injectors within an engine for various demanded fuel amounts. It is noted that the pressure values may be converted into fuel amounts (in milligrams) using a relationship similar to those shown in Figures 4 to 6.

Fuel Q	1(ΔP)	2(ΔP)	3(ΔP)	4 (ΔP)	5(ΔP)	6(ΔP)	Average (ΔP)
37.37	33.37264	34.19303	35.9209	33.19204	35.85821	33.54677	34.34726
58.95	44.45572	42.22338	43.75174	42.86468	45.14826	42.50398	43.49129
77.61	53.96269	51.86617	52.77363	54.22239	52.94527	53.58408	53.2257
98.17	60.81244	60.03433	61.66716	60.22289	60.48209	60.41542	60.60572
118.52	67.40697	69.53433	68.94328	69.1607	67.66418	69.55025	68.70995
138.66	77.13284	76.70547	77.27463	76.42388	76.66368	76.71791	76.81973
158.9	84.48557	85.32537	85.08756	84.89403	83.68806	84.07463	84.59254
179.17	93.76468	93.27114	93.15323	93.18905	93.51393	92.94279	93.3058
198.6	100.8423	100.5896	100.4552	100.2677	100.307	101.498	100.66
223.68	109.194	111.1801	109.2975	112.5776	109.6015	111.3995	110.5417
249.36	122.4244	118.2547	118.6318	118.3229	118.402	118.9915	119.1712

In the event that an entry in Table 1 has a zero value this indicates that the injector in question has a fault, e.g. needle stuck open/closed. The ECU may output a fault signal in the event that a zero value is detected.
Additional (rail) pressure measurements may be taken over the course of each injection event. This in turn may allow the ECU to additionally calculate the turn-on and turn-off delays for each injector and to adjust the injection parameters for the engine accordingly.
The embodiments of the present invention described above are in relation to the electronic control unit (ECU) within the engine system carrying out all the necessary analysis of the pressure data and determining whether any offsets are required to the injection parameters of the engine. It should be noted however that a dedicated compensation module may carry out all of the above described processes. Such a module could be integrated within the ECU or may be a standalone component that outputs compensation data to the ECU for further use in operating the engine.
Further variations and modifications not explicitly described above may also be contemplated without departing from the scope of the invention as defined in the appended claims.

Claims

A method of compensating for fuel injector performance variations in a fuel injection system (1) of an engine, the engine comprising a fuel pump (3,4) arranged to supply pressurised fuel to a common accumulator volume (5) that supplies the pressurised fuel in turn to a plurality of fuel injectors (8); and the method comprising:
measuring (202) a first pressure within the accumulator volume;

sending (204) a drive pulse signal to initiate an injection event in a first injector;

measuring (206) a second pressure within the accumulator volume;

calculating (208) a pressure difference between the first and second pressure measurements;

using (212) the calculated pressure difference to compare actual fuel injection system performance with expected fuel injection system performance;

compensating (214) for any deviations between actual fuel injection system performance and expected fuel injection system performance.
A method as claimed in Claim 1, wherein the first (202) and second (204) measuring steps and sending step (204) are performed when the engine is in a steady driving state.
A method as claimed in Claim 1 or Claim 2, wherein the fuel pump (3, 4) is arranged to pump fuel in a fuel pumping event into the accumulator volume (5) in advance of each injection of fuel into the engine and the method further comprises suspending the fuel injection event corresponding to the first injector immediately prior to the sending step.
A method as claimed in any preceding claim, wherein the using step (212) comprises calculating, from the calculated pressure difference, the injected amount of fuel injected into the engine during the injection event of the first injector and comparing to a demanded fuel amount.
A method as claimed in any preceding claim, wherein the compensating step (214) comprises adjusting a fuel injection parameter to compensate for any deviations between actual fuel injection system performance and expected fuel injection performance.
A method as claimed in any preceding claim, further comprising repeating (210) the first measuring, sending, second measuring and calculating steps for each of the plurality of fuel injectors.
A method as claimed in Claim 6, wherein a pressure difference is calculated, in the calculating step (208), for each injector within the fuel injection system and the compensation step (214) comprises balancing all the fuel injectors within the injector system.
A method as claimed in Claim 7, wherein the balancing step comprises keeping the total fuel amount injected by the plurality of injectors (8) into the engine during a given engine cycle at a constant value.
A method as claimed in Claim 7 or Claim 8, wherein each fuel injector (8) is balanced by applying a fuelling offset to the injector to alter the timing of the drive pulse sent to the each injector.
A method as claimed in Claim 7 or Claim 8, wherein each fuel injector (8) is balanced by applying an injection duration offset to alter the amount of injected fuel supplied to the engine system.
A method as claimed in either of Claims 6 or 7, wherein the compensation step (214) comprises applying a bulk offset to the plurality of injectors in order to adjust a total fuel amount supplied to the engine by the fuel injection system in an engine cycle.
A method as claimed in any one of Claims 6 to 11, further comprising measuring first and second pressure values for an injector over a plurality of engine cycles and, for each fuel injector, averaging the calculated pressure differences and using the averaged calculated pressure difference for each injector to compare actual injector system performance with expected fuel injection system performance.
A method as claimed in any preceding claim, wherein the using step comprises adjusting the calculated pressure difference in order to adjust for pressure losses due to natural decay and/or switching losses.
A compensation module for compensating for fuel injector performance variations in a fuel injection system (1 ) of an engine, the engine comprising a fuel pump (3,4) arranged to supply pressurised fuel to a common accumulator volume (5) that supplies the pressurised fuel in turn to a plurality of fuel injectors (8); and the module comprising:
inputs to receive first and second pressure measurements within the accumulator volume;

an output arranged, between the reception of the first and second pressure measurements, to send a drive pulse signal to initiate injection in a first injector; and

a processor arranged to:
calculate a pressure difference between the first and second pressure measurements;

use the calculated pressure difference to compare actual fuel injection system performance with expected fuel injection system performance; and to

compensate for any deviations between actual fuel injection system performance and expected fuel injection system performance.
A computer readable medium comprising a computer program arranged to configure a computer or an electronic control unit to implement the method to any of Claims 1 to 13.