GB2543260A

GB2543260A - Fuel injection control in an internal combustion engine

Info

Publication number: GB2543260A
Application number: GB1517500.3A
Authority: GB
Inventors: Vioulac Maxime; Francois Maxime; Ghirlanda Alessio; Hanus Pierre
Original assignee: Delphi International Operations Luxembourg SARL
Current assignee: Delphi International Operations Luxembourg SARL
Priority date: 2015-10-05
Filing date: 2015-10-05
Publication date: 2017-04-19
Also published as: GB201517500D0

Abstract

Disclosed is a method of fuel injection control in an internal combustion engine having at least one cylinder with an associated fuel injector for performing injector events of predetermined pulse widths. The fuel injector comprises a nozzle controlled by a control chamber, in which the pressure is selectively adjusted via a control valve operated by an actuator, the fuel injector having a fuel return passage connected to a fuel return line. The method is characterized by the steps of: monitoring the fuel pressure in the fuel return path in relation to an injector event in the fuel return line and determining at least one injector characteristic based on the variation of fuel pressure in said fuel return path during said monitoring period. The injector characteristic may be a control valve opening delay, a nozzle opening delay or a minimum delivery pulse. The information can be used to correct fuel injector timings to provide more accurate control of fuel injection quantities.

Description

FUEL INJECTION CONTROL IN AN INTERNAL COMBUSTION ENGINE

FIELD OF THE INVENTION

The present invention generally relates to the field of internal combustion engines and more specifically to the control of fuel injection in such engines.

BACKGROUND OF THE INVENTION

The contemporary design of internal combustion engines must cope with the increasingly stringent regulations on pollutant emissions. Accordingly, automotive engineers strive for designing engines with low fuel consumption and low emission of pollutants, which implies including electronic devices capable of monitoring the combustion performance and emissions in the exhaust gases.

In this connection, a proper operation of a fuel-injected engine requires that the fuel injectors and their controller allow for a timely, precise and reliable fuel injection.

In particular, to improve the accuracy of fuel injection, it is known to detect the time of opening of the injector nozzle, which is delayed compared to the injector drive signal, and therefore generally referred to as nozzle opening delay.

One known approach described in US 2013/0054120 is based on monitoring fuel pressure in the high pressure fuel channel inside the injector body, to determine an actual injection timing at which the fuel has been sprayed.

Accordingly, US 2013/0054120 discloses a fuel injector comprising a valve group having a needle slideably arranged in the injector body in order to control the spray orifices through its displacement. The needle displacement is controlled by a hydraulic pressure force in a control chamber, in which the pressure force can be selectively adjusted with the help of an actuator. The control chamber is in communication via an inlet orifice with the injector's high pressure fuel channel. A control valve is connected via a leakage orifice to the control chamber. Opening the control valve allows fuel to escape from the control chamber to a low pressure passage, thereby creating a drop of pressure in the control chamber that will allow the needle to lift and open the spray orifices. A pressure sensor unit is arranged in the injector head (opposite the spray orifices) to sense the pressure of the high pressure fuel channel in the injector body. For this purpose, a sensor channel is arranged in the body that branches off from the main high pressure channel and provides fluidlic communication with the sensor unit. The sensor unit comprises a diaphragm mounted at the end of the sensor channel that elastically deforms in response to high fuel pressure. As it is known, the injection of fuel at the spray tip causes a pressure drop in the high pressure fuel channel, which can thus be detected by the fuel pressure sensor. As soon as the injector stops spraying fuel, the sensor unit will detect the build-up of the pressure up to the nominal pressure. The sensor signal delivered by the sensor unit allows monitoring the pressure by the Engine Control Unit and determining operating parameters such as a fuel-injection-start time, a fuel-injection-end time and a fuel injection quantity.

The approach proposed in US 2013/0054120 requires modifying conventional injector design to provide the additional sensor channel and sensor unit, which adds on costs. Furthermore, the sensor unit is exposed to harsh operating conditions (high temperature and high fuel pressure of up to 2 000 bar and more), thus requiring appropriate design and complex calibration.

OBJECT OF THE INVENTION

Hence, there is a need for an alternative way of determining fuel injector characteristics values for improving injection control in internal combustion engines.

This object is achieved by a method and system according to claims 1 and 14, respectively.

SUMMARY OF THE INVENTION

The present invention relates to a method of fuel injection control in an internal combustion engine having at least one cylinder with an associated fuel injector for performing injector events of predetermined pulse widths. The method is typically designed for use with fuel injectors of the type comprising a nozzle controlled by a control chamber, in which the pressure is selectively adjusted via a control valve associated with an actuator, the fuel injector having a back leak passage downstream of the control valve connected to a fuel return line.

According to the present invention, the method comprises the steps of: a) monitoring, during a predetermined monitoring period, the fuel pressure in the fuel return path in relation to an injector event; b) determining at least one injector characteristic based on the evolution of fuel pressure in the fuel return path during the monitoring period.

As described below, the present method allows determining injector characteristics such as control valve opening delay, nozzle opening delay and minimum delivery pulse (MDP - minimum duration of drive signal required to cause an actual fuel injection).

Contrary to the approach proposed in US 2013/0054120, the present method uses pressure signals measured on the low pressure side (fuel return path) of the injection system, i.e. downstream of the control valve, and in particular in the piping portion that collects the injector leakage fuel exiting the fuel injectors. Hence, the pressure can be measured by a pressure sensor installed on the back leak collector piping to which the set of injectors are connected. This is a substantial advantage since only a single pressure sensor is required, which furthermore operates at low pressures and temperatures (as compared to the injector of US 2013/0054120).

As used in connection with pressure monitoring step a), the term "fuel return path" encompasses the fuel return passage in the injector body and the fuel return line to which the injector(s) is/are connected. Indeed, the pressure variation following an injector event can be observed in both in the fuel return passage and fuel return line. The monitoring of the fuel pressure by means of a single pressure sensor in the main collector portion of the fuel return line is efficient and economically advantageous. Nevertheless, the return fuel pressure can alternatively be measured by means of sensors arranged in the injector to measure the pressure in the fuel return passage, or in individual return lines connecting the injectors to the main collector of the return fuel line.

The injector characteristics of interest can be determined by analyzing the evolution of the pressure curves (resp. of the corresponding data) and recognizing predetermined patterns in the pressure evolution following the injector drive pulse.

In order to identify the timing of the control valve opening, the method comprises the step of determining an increase of the pressure in the fuel return path during said monitoring period and identifying the corresponding timing as said control valve opening timing. The increase is generally determined by comparison with an initial pressure value (absolute or relative) in the fuel return line. Before an injector event, the design pressure in fuel return line is normally substantially constant, and is also referred to as base pressure or static pressure.

The increase in the return fuel pressure may be determined as an increase of the absolute or relative return path pressure above a predetermined threshold. In particular, the start of the increase in the return path pressure may be taken as the control valve opening timing.

Proper opening of the control valve will trigger opening of the needle. This needle opening can be observed from the pressure evolution in the fuel return path as a particular pattern after the initial increase corresponding to the control valve opening: the pressure levels off and describes a plateau or passes through an inflexion point. Therefore, the method advantageously comprises the step of: determining when the pressure in said return, after a first increase, levels off and describes a plateau or passes through an inflexion point, preferably without pressure decrease during a given time period after said plateau, respectively inflexion point.

The determination of the inflection point may conveniently include the step of calculating the second time derivative of the fuel pressure in the return line and determining when the second time derivative passes through zero and changes sign. The corresponding timing is the needle opening timing. Depending on the injector design, when the pulse width is sufficient for injection to occur, the pressure will not describe an inflexion point but will level off to a plateau section, before increasing again. In such case, the end of the plateau is the timing of interest indicative of needle opening. A further injector specific parameter is the MDP. It can be determined under the present method by: - performing a plurality of injector events over an increasing range of pulse widths and determining for each injector event a pressure related parameter determined from pressure measurement in the return line to acquire a corresponding data set indicative of the pairs (pulse widths ; pressure parameter); - processing the data set to identify a first, flat line fitting low pulse widths, a second, growing line fitting higher pulse widths, and third line fitting data pairs intermediate the former; - identifying the intersection of the second line and third line as the injector minimum delivery pulse and/or identifying the intersection of the first line and third line as the control valve minimum delivery pulse.

In practice, The MDP may be determined in a learning routine, and the data are acquired by performing a sequence of injections of gradually/incrementally increasing pulse widths. But the data set could be acquired in a random manner. Preferably, the pressure related parameter is the maximum pressure reached in the fuel return path following the injector event and over the monitoring period.

According to another aspect, the invention concerns an internal combustion engine comprising a fuel injection system with at least one fuel injector associated with an engine cylinder, wherein: - the fuel injector comprises a nozzle controlled by a control chamber, in which the pressure is selectively adjusted via a control valve associated with an actuator, the fuel injector having a fuel return passage downstream of the control valve; - the fuel injection system comprises a high pressure line for supplying high pressure fuel to the fuel injector and a fuel return line to which the fuel injector fuel return passage is connected; - an engine control unit is configured to control the fuel injection system and in particular to perform injector events of predetermined pulse widths; - at least one pressure sensor is arranged to measure the pressure in the fuel return path; and the engine control unit is programmed to monitor the pressure variations in the fuel return path in response to an injector event to detect at least one injector characteristic based on the variation of fuel pressure in said fuel return path.

The ECU is advantageously configured to perform the above-described method.

In one embodiment, the engine comprises a plurality of injectors and a pressure sensor is arranged to measure the pressure in the fuel return line, the pressure sensor being positioned at equidistance from each injector.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1: comprises two graphs illustrating: a) injector current and injection rate vs. time and b) back leak pressure vs. time, and this for injector events of various durations;

Figure 2: is a graph of injector current, rail pressure and back leak pressure vs. crank shaft angle;

Figure 3: is a graph of back leak pressure and heat release vs. time for injector event of increasing duration;

Figure 4: is a graph of injector current, rail pressure and back leak pressure vs. crank shaft angle for 4 different pulse widths;

Figure 5: is a principle diagram of fuel injection system implementing the present method;

Figure 6: is a principle diagram of a conventional fuel injector.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to Fig.6, a fuel injector 10 of conventional design comprises a valve group having a needle 30 that is axially movable in order to open and close one or more flow orifices 32 through which fuel is sprayed into an engine combustion chamber. In modern fuel injectors, as used e.g. in common rail systems in Diesel engines, the opening and closing of the needle 30 is controlled by a hydraulic pressure force that can be selectively adjusted with the help of an actuator 34 of the electromagnetic (solenoid) or piezoelectric type. This hydraulic pressure is formed in a control chamber 36, which is typically arranged to be filled with high-pressure fuel (through an inlet orifice 38 (INO) branched off from the injector internal high pressure passage 40) so as to exert, at least indirectly, a pressure force on the needle 30 in its closing direction. An outlet path 42 (also known as spill orifice SPO) of the control chamber 36 is controlled by a control valve arrangement 44, which, when open, allows fuel to escape from the control chamber 36, thereby creating a pressure drop in the control chamber and causing the needle 30 to lift and open the injector flow orifices 32 in order to spray fuel into the engine combustion chamber. The valve member of the control valve arrangement 44 is operated by the actuator 34.

Such a fuel injector comprising a control chamber with control valve arrangement operated by means of a solenoid actuator is e.g. known from EP 2 647 826.

The fuel injector, respectively the actuator, is operated by a drive current that is applied during a predetermined duration, set by a logical signal. "Pulse Width" (PW) is the term used to designate the length of this logical signal, and is herein used to generally designate the length during which drive current is applied to an injector to cause an injector event.

Generally, to inject a fuel amount Q, a value of pulse width PW is read from a table and possibly corrected, and the fuel injector is operated, for a given injector event, so that the drive signal is applied during a time period corresponding to the PW, to influence a desired injection time and normally inject a given fuel amount. Hence, for any electromagnetically actuated fuel injection to be performed, a PW is generated to command a corresponding injector opening duration in order to deliver fuel.

It shall be kept in mind that when the control valve 44 opens, the fuel at high pressure escapes through the control valve into a back leak passage 46 in the injector body that forms the low pressure side of the injector. The back leak passage ends by a back leak port 48 by which it is connected to a fuel return line or piping, in which injector leakage fuel is collected to be returned to the fuel tank.

Fig.5 is a principle diagram illustrating a set of four fuel injectors 10 fluidly connected to a high pressure fuel rail 12 (common rail) of a Diesel engine, itself supplied with fuel from a high pressure fuel pump 14 connected to a fuel tank (not shown). Each of the injectors 10 is associated with a respective combustion chamber (not shown). The fuel injectors 10 correspond to the above described conventional design; i.e. they comprise a hydraulic control valve to control communication between the control chamber and their internal, low pressure fuel return path formed by the back leak passage ending with the back leak port. Reference sign 16 generally indicates the fuel return line/piping to which each injector is connected. In the shown embodiment, the back leak port of each injector is fluidly connected to an individual fuel return line 18, which opens at its opposite end into a collecting chamber or main collector 16i, e.g. in the form of a pipe/cylinder portion. This collecting chamber 16i is then connected by a duct (not shown) to the fuel tank for returning the fuel. Conventionally, the pressure inside collecting chamber 16i is kept at a positive back leak value, e.g. in the range of 5 to 10 bars, by means of e.g. a pressure regulator, a low pressure pump or the like (not shown).

It shall be appreciated that the present inventors have found that certain injector characteristics can be detected from the pressure variation in the fuel return piping 16 in relation to a corresponding injector event. The pressure monitoring is advantageously achieved by means of a pressure sensor 20 installed on the fuel return chamber 16 to measure the pressure therein.

More specifically, it has been found that by monitoring the pressure variations in the fuel return path in relation to respective injector events, it is possible to determine the following injector specific parameters: - control valve opening timing; - injector needle opening timing; - minimum delivery pulse (MDP).

Once these injector specific characteristic values have been determined, they can advantageously be used in the ECU for controlling injection. In particular they will allow a more accurate control of injection timing and quantities, thereby improving the injection control and reducing emissions. 1. Determining control valve opening and needle opening

Reference is now made to Fig. 1, where graph a) illustrates the current trace corresponding to a plurality of injector events of increasing pulse widths -from 150 ps to 1 ms. Conventionally, a pulse width PW is determined to inject a desired fuel quantity, and the current trace shown in fig. 1a) corresponds to the drive signal applied during the duration of the PW. The injection rate is also illustrated over the same time period (vertical axis on the right), this parameter being proportional to the injected fuel quantity.

Graph b) in Fig. 1 represents the pressure in the fuel return piping 16 measured over the same time period. As can be seen, the pressure varies depending on the drive current, respectively on the PW.

The graphs of Fig. 1 can be analyzed as follows, in the light of the injector design technology.

When the drive current duration is small, i.e. at PW known to be too small for injection to occur, the control valve does not move, neither the injector needle. There is no fuel flow to the injector back leak passage nor to the back leak piping 16. The pressure in the return piping 16 (also referred to hereinafter as back leak pressure or BL pressure) as measured by pressure sensor 20 is substantially constant: the pressure trace is flat. This is the case for PW=150 ps.

For large pulse widths (here 290, 500 & 1000 ps), the BL pressure increases from the base/static pressure and levels off on a first stage, from which it will follow an increasing trend.

In fact, for drive signals/PWs where injection actually occurs, the observed evolution of the BL pressure in the back leak piping 16 is in relation with the flow of fuel returned from the injector into the back leak piping. When the control valve opens, a small leak is first allowed from the control chamber to the back leak passage. This leak will increase as the control valve moves to its fully open position. Sufficient opening of the control valve will in turn cause the injector needle to lift from its seat and the spraying of fuel. As fuel is further fed from the common rail to injector high pressure channel, the fuel leakage through the control valve will increase further due to needle motion into the control chamber, increasing consequently the flow through the outlet path of the control chamber (and thus into the injector return fuel passage).

Monitoring and analysis of the pressure in the fuel return piping 16 (or more generally downstream of the control valve) allows determining the characteristic moments of the control valve opening and the needle opening.

It can be seen in Fig. 1 a) that from a certain drive pulse duration upwards (here from 290 ps), the shape of the first part of the pressure trace (noted Si) is similar: the rate of increase of the BL pressure from the base pressure to the first stage is the same.

It has been found that the beginning of the increasing pressure section Si to the first stage (or level) correlates well with the opening of the control valve.

It has further been found that the moment of opening of the injector needle correlates well with an inflexion point in the back leak pressure after the levelling off at the first stage. In fact, to ensure a more accurate detection of the needle opening, the pressure should not decrease about this inflexion point.

For intermediate pulse widths, here 160 and 180 ps, the applied drive current did not cause fuel injection, but only a brief movement of the control valve, which is indicated by the small increase in BL pressure.

Fig.2 shows these characteristic points of interest over a given engine cycle portion, for an injector event with a drive signal known to cause injection. At the beginning of the drive pulse (before -30° CA) the BL pressure is constant, i.e. the pressure trace is flat. Shortly after the end of the drive pulse, the pressure starts to increase (at about -26°) from the base pressure pb. This increase of BL pressure coincides with the opening of the control valve that will permit fuel to flow through the injector back leak passage to the back leak piping 16. The corresponding effect in the common rail is a drop of pressure.

The increasing BL pressure levels off to a first stage pi, from which the pressure trace then describes an inflexion point p2 and further increases. As indicated, this inflexion point p2 correlates well with the moment of the needle opening. Opening of the needle will allow fuel to be injected into the combustion chamber and a steep decrease of fuel pressure in the common rail is observed after the timing of p2.

Based on the above, it can be seen that the timing of the opening of the control valve can be determined by monitoring the fuel pressure in the fuel return line in relation to an injector event and detecting an increase in the BL pressure. In practice, the measured BL pressure data (or data representing the BL pressure in the back leak piping) are processed to identity a first increasing portion from a base pressure pb. The start of the increasing section, or the foot portion thereof, can be taken to identify the moment when the control valve starts opening (p0 in Fig.2).

Alternatively, the timing of control valve opening can e.g. simply be determined by comparing the measured BL pressure to a pressure threshold noted PTH_v· The timing at which the BL pressure reaches or exceeds P-mvis then the control valve opening time.

To detect the opening time of the injector needle, the data are processed to identify a level off at a first stage (pi) following the moment of control valve opening (p0) and an inflexion point (p2) following this first stage. The inflexion point can be determined by taking the second time derivative of the measured BL pressure and determining the timing at which the second time derivative passes through zero and changes sign. Preferably this point is retained as the needle opening timing only if the pressure does not decrease during a time period of e.g. about 300 to 500 ps following the inflexion point.

It may be noticed here that depending on the injector design, the first stage p1 may take the form of a plateau, from which the pressure will subsequently increase further. In such case, there is no inflexion point, but the point of interest, i.e. the moment indicating the needle opening, is the end of the plateau, just before the pressure starts increasing again. 2. Determining the minimum delivery pulse

Conventionally, the term "Minimum Delivery Pulse" is used with respect to the injector needle and designates the minimum duration of pulse width PW required to cause the smallest amount of fuel to flow through the injector spray orifices (as compared lower PW too small for injection to occur).

The concept of MDP can also be applied to the control valve, as designating the minimum PW required for a minimal fuel amount to flow through the control valve into the back leak passage.

The MDP is a parameter that is generally learned by progressively increasing the pulse width from a known no-flow pulse, and detecting for each PW value whether injection has occurred or not.

It has been found that the MDP for both the control valve and needle can be determined from the pressure variation in the fuel return piping/path.

In a preferred approach, injector events are performed for a plurality of PW values ranging from small, no-flow values to larger PWs. The pressure in the return piping 16 is monitored over a predetermined time window and, in the present embodiment, the maximum pressure PBL_max is determined. The maximum pressure can be an absolute or relative pressure (with respect to pb). In alternative embodiments, the method can be performed with a response pressure value other than the maximum pressure, or another value derived therefrom, more generally referred to as "pressure related parameter", which is thus determined from pressure measurement in the fuel return path.

By repeating these steps, a series of pairs (PW; PBL_max) is obtained. Fig.3 shows a plot of the obtained data (PW; PBL_max), which results from a test sequence with a plurality of injections of incrementally increasing pulse widths.

As can be seen, the curve representing the (PW; PBL_max) data set consists of 3 sections: - a flat section at low pulse widths (below 260 ps). No injection has occurred and the pressure in the return piping remains fairly constant. This curve section can be fitted with a first line noted L1. - a steadily growing section for pulse widths above 320 ps. This section corresponds to PW having a length that causes actual fuel injection. The BL pressure in the return piping increases as more and more fuel is injected by the injector due to the increasing PW. This curve section can be fitted with a second line noted L2. - a transition section at intermediate pulse widths, that rises more sharply and joins the flat section to the steadily growing section. Here, the PW is sufficient to actuate the control valve but not to lift the injector needle. This curve section can be fitted with a third line noted L3.

To illustrate these different situations, four exemplary points ΟΘΘΘ have been selected and explanatory graphs are shown in Fig.4.

Point O. The PW is too small to cause any movement of the control valve or needle, which is indicated by the rather constant BL pressure and negligible variations on the high pressure line shown in Fig.4 O.

Fig.4 Θ shows the pressure measurements corresponding to a larger PW at point Θ. A rising edge can now be observed in the BL pressure trace, in relation to a sensible pressure drop in the high pressure line. The PW was sufficient to lift the control valve member from its seat, but the needle did not move. Point Θ is thus on line L3.

Point © in Fig.3 corresponds to a large PW for which fuel injection has oc-cured. The BL pressure follows the expected evolution (discussed in fig.2) of a first plateau followed by an inflexion point and a further pressure increase. The pressure in the high-pressure line undergoes a first drop due to control valve and needle actuation, and a steeper drop as fuel is sprayed into the combus tion chamber. The closure of the needle is responsible for the pressure waves (water hammer) observed on the high pressure line about -20 to -15°CA.

Graph Θ describes the situation where the applied PW is sufficient to actuate the control valve but causes only a small displacement of the needle, leading to a minimal fuel injection. This can be seen from the pressure curves: - the BL pressure reaches the expected plateau with an inflexion point; - however in the high pressure line, the pressure drop is longer than that observed for point Θ but does not undergo the steep decrease observed for point ©. The presence of the pressure wave at -20°CA corresponds to the needle closing and thus means that the needle did open.

Point Θ corresponds to the minimum delivery pulse of the needle/injector MDPinj.

It shall be noted here that the MDPinj coincides with the intersection of lines L2 and L3.

It has further been found that the minimum delivery pulse of the control valve MDPV coincides with the intersection of lines L1 and L3.

In practice, the MDPs of the control valve and needle can thus be determined by performing a plurality of injector events over a range of pulse widths covering non-injecting pulse width to injecting pulse width and determining for each injector event the maximum pressure reached in the return piping 16 to acquire a corresponding data set indicative of the pairs (PW ; pBL_max)·

The obtained data set is then processed to by appropriate mathematic approaches to identify the three curve portions corresponding to lines L1, L2 and L3. The intersections of the 3 lines give the points of interest: - Intersection of L1 and L3: MDP valve - Intersection of L2 and L3: MDP needle 3. Remarks

As it will be understood, the embodiment shown in Fig.5 is of great advantage since a single pressure sensor allows determining the injector specific characteristics of the various injectors. Furthermore, the pressure sensor can be arranged in an engine area where a larger room is available.

For simplicity of implementation, the all of the intermediate lines 18 connecting the injectors 10 to the main fuel collector 16i have the same length. In fact, the propagation time of the pressure waves in the return piping mainly on the fluid type, temperature and pressure. Since the fluid is known and the temperature is normal and fairly constant, the propagation mainly depends on temperature.

Since the intermediate lines 18 are of same length, expected events will happen at same expected timings in all cylinders, whereby the same algorithm can be used to analyse the pressure data. It is thus of advantage when the pressure sensor is equidistant from each injector, as is the case in the configuration of Fig.5.

It may be further noticed that the fuel return line is not subject to pumping effects, contrary to the common rail. Pressure analysis is this simplified. Before an injection event in a combustion chamber the fuel pressure is normally at a nominal/design pressure fixed by the pressure regulator. Preferably, in the context of the invention the fuel return line is at a positive pressure. Accordingly, before an injector event the pressure in the fuel return line 16 is generally at a static/constant design pressure, from which it will increase in case the control valve opens.

Claims

1. A method of fuel injection control in an internal combustion engine having at least one cylinder with an associated fuel injector for performing injector events of predetermined pulse widths, said fuel injector comprising a nozzle controlled by a control chamber, in which the pressure is selectively adjusted via a control valve operated by an actuator, said fuel injector having a fuel return passage connected to a fuel return line, the method being characterized by a) monitoring, during a predetermined monitoring period, the fuel pressure in the fuel return path in relation to an injector event; b) determining at least one injector characteristic based on the variation of fuel pressure in said fuel return path during said monitoring period.

2. The method according to claim 1, wherein said at least one injector characteristic is a control valve opening timing, said step b) comprising determining an increase of the pressure in said fuel return path during said monitoring period and identifying the corresponding timing as said control valve opening timing.

3. The method according to claim 2, wherein said increase is determined as an increase of the absolute or relative return path pressure above a predetermined threshold.

4. The method according to claim 2 or 3, wherein the start of the increase in the return path pressure is taken as the control valve opening timing.

5. The method according to claim 2, 3 or 4, wherein said increase is determined from a static pressure in said fuel return path.

6. The method according to any one of the preceding claims, wherein said at least one injector characteristic is a needle opening timing, said step b) comprising determining when the pressure in said return, after a first increase, levels off at a plateau or passes through an inflexion point, prefera bly without pressure decrease during a given time period after said plateau, respectively inflexion point.

7. The method according to claim 6, wherein said determining when the fuel pressure in the return line passes through an inflection point includes the step of calculating the second time derivative of the fuel pressure in the return line and determining when the second time derivative passes through zero and changes sign, and taking the corresponding timing as the needle opening timing.

8. The method according to claim 6, wherein the timing at which said plateau ends is taken as the needle opening timing.

9. The method according to any one of the preceding claims, wherein said at least one injector characteristic is a minimum delivery pulse of the control valve, said method comprising: - performing a plurality of injector events over an increasing range of pulse widths and determining for each injector event a pressure related parameter determined from pressure measurement in the fuel return path to acquire a corresponding data set indicative of the pairs (pulse widths ; pressure parameter); - processing said data set to identify a first, flat line fitting low pulse widths, a second, growing line fitting higher pulse widths, and third line fitting data pairs intermediate the former; identifying the intersection of the second line and third line as the injector minimum delivery pulse and/or identifying the intersection of the first line and third line as the control valve minimum delivery pulse.

10. The method according to claim 9, wherein said pressure related parameter is the maximum pressure reached in the fuel return path following the injector event and over the monitoring period.

11. The method according to any one of claims 10, wherein the engine at least two fuel injectors and wherein a pressure sensor is arranged to measure the fuel pressure in said fuel return line, the pressure sensor being positioned equidistant from each injector.

12. The method according to any one of the preceding claims, wherein said monitoring period is a time window starting about the timing of the beginning of the applied drive pulse.

13. The method according to claim 12, wherein said monitoring period ends before the start of a subsequent injector event.

14. An internal combustion engine comprising a fuel injection system with at least one fuel injector associated with an engine cylinder, wherein said fuel injector comprises a nozzle controlled by a control chamber, in which the pressure is selectively adjusted via a control valve associated with an actuator, said fuel injector having a fuel return passage downstream of said control valve; the fuel injection system comprises a high pressure line for supplying high pressure fuel to the fuel injector and a fuel return line to which the fuel injector fuel return passage is connected; an engine control unit is configured to control said fuel injection system and in particular to perform injector events of predetermined pulse widths; characterized in that at least one pressure sensor is arranged to measure the pressure in the fuel return path; and said engine control unit is programmed to monitor the pressure variations in said fuel return path in response to an injector event to detect at least one injector characteristic based on the variation of fuel pressure in said fuel return path.

15. The internal combustion engine according to claim 14, wherein the ECU is configured to perform the method as claimed in any one of claims 1 to 13.

16. The internal combustion engine according to claim 14 or 15, wherein the engine comprises a plurality of injectors and a pressure sensor is arranged to measure the pressure in said fuel return line, the pressure sensor being positioned at equidistance from each injector.