WO2008111899A1

WO2008111899A1 - Method relating to diagnosis of a combustion engine

Info

Publication number: WO2008111899A1
Application number: PCT/SE2008/050239
Authority: WO
Inventors: Kim KYLSTRÖM; Klas Telborn
Original assignee: Scania Cv Ab (Publ)
Priority date: 2007-03-09
Filing date: 2008-03-04
Publication date: 2008-09-18
Also published as: DE112008000659T5; DE112008000659B4

Abstract

A method for controlling a common rail system using a signal provided from common rail pressure sensor as one input parameter to control the common rail system. When it is determined that the pressure sensor is malfunctioning, the pressure sensor input parameter is replaced with an input parameter based on the pressure sensor input parameter and a parameter representing an offset value corresponding to a difference between a theoretically correct value and the pressure sensor value. Methods for detecting a malfunctioning pressure sensor are also described: The first method by lowering the pressure until a needle no longer opens, recording the pressure output from the pressure sensor, comparing the recorded pressure value to a predetermined value and determining that a pressure sensor is malfunctioning if the recorded value and the theoretical value differs more than a predetermined threshold value. A second method by dumping fuel from at least one injector, monitoring the pressure reduction rate, recording the pressure value read when the pressure reduction rate starts to decrease and comparing the recorded pressure value to a theoretical value. A third method by increasing the pressure until a safety valve opens, recording the pressure value read in conjunction with the opening of the safety valve and comparing the recorded pressure valve to a theoretical value.

Description

METHOD RELATING TO DIAGNOSIS OF A COMBUSTION ENGINE

TECHNICAL FIELD The present invention relates to a method and an apparatus for performing diagnosis of a combustion engine. In particular the invention relates to a method and an apparatus for performing diagnosis of a combustion engine employing a common rail system.

BACKGROUND Common rail direct fuel injection is a modern variant of direct injection system for diesel engines. It features a high-pressure (1000+ bar) fuel rail feeding individual solenoid valves, as opposed to low-pressure fuel pump feeding pump nozzles or high-pressure fuel line to mechanical valves controlled by cams on the camshaft. Third generation common rail diesels now feature piezoelectric injectors for even greater accuracy, with fuel pressures up to 170 MPascal / 1700 bar or above.

In common rail systems, the distributor injection pump is eliminated. Instead an extremely high pressure pump stores a reservoir of fuel at high pressure — up to 1,800 bar (180 MPa) or even higher — in a "common rail", basically a tube which in turn branches off to computer-controlled injector valves, each of which contains a precision-machined nozzle and a plunger driven by a solenoid. Driven by a control unit, which also controls the amount of fuel to the pump, the valves, rather than pump timing, control the precise moment when the fuel injection into the cylinder occurs and also allow the pressure at which the fuel is injected into the cylinders to be increased. As a result, the fuel that is injected atomises easily and burns cleanly, reducing exhaust emissions and increasing efficiency.

In order for a common rail system to operate properly it is important for the control system of the engine to have knowledge about the current pressure of the common rail. This is important for a number of different reasons. For example if the control system is fed with a value lower than the true pressure, the engine will be run at a common rail pressure that is too high resulting in higher power than intended and emission values not being optimized. The common rail is therefore provided with a pressure sensor connected to the control unit of the control system of the engine.

However should the pressure sensor malfunction for some reason and give an incorrect output to the control system, the engine will be run at a pressure not being optimized, which could lead to damages on the engine and/or emission value limits being exceeded. It is therefore important to be able to detect a malfunction of the pressure sensor of the common rail so that proper action can be taken, such as replacing the pressure sensor.

Hence, there exist a need for a method and an apparatus that is capable of detecting a malfunctioning pressure sensor of a common rail.

SUMMARY

It is an object of the present invention to provide a method and a system that is capable of detecting a malfunctioning pressure sensor of a common rail.

It is another object of the present invention to provide a method and a system that is capable of determining an offset between a theoretically correct value and the value provided by a pressure sensor for a common rail system.

It is yet another object of the present invention to provide a method and a system that is capable of taking into account the difference between a theoretically correct value and the value provided by a pressure sensor for a common rail system for better controlling the common rail system.

These objects and others are obtained by the method, apparatus and computer program product as set out in the appended claims.

In accordance with one embodiment a common rail system is controlled using a signal provided from common rail pressure sensor as one input parameter to control the common rail system. When it is determined that the pressure sensor is malfunctioning, the pressure sensor input parameter is replaced with an input parameter based on the pressure sensor input parameter and a parameter representing an offset value corresponding to a difference between a theoretically correct value and the pressure sensor value. In accordance with one embodiment the common rail system comprises at least one injector, where each injector comprises a spring loaded needle movable between an open position and a closed position at a pre- determined spring load. The pressure of the common rail system is lowered until a needle of an injector no longer opens, and the pressure output from the pressure sensor of the common rail system is recorded at the time when the injector needle no longer opens. Then the recorded pressure value is compared to a theoretical value derived from a number of parameters including at least the spring load of the spring, and the pressure sensor is determined to malfunction if the recorded value and the theoretical value differs more than a predetermined threshold value. Hereby the system can be alerted that there is a problem with the pressure sensor of the common rail system and appropriate action can be taken such as replacement of the pressure sensor.

In accordance with one embodiment the injectors are determined to no longer open by using a flywheel sensor. In accordance with another embodiment the injectors are determined to no longer open by using a knocking sensor. This gives the designer a freedom to choose from different input signals.

In accordance with one embodiment of the invention fuel is dumped from the injector(s), and the pressure reduction rate in the common rail system is monitored. The pressure value read when the pressure reduction rate starts to decrease is then recorded and compared to a theoretical value derived from a number of parameters including at least the spring load of the spring, and finally the pressure sensor is determined to be malfunctioning if the recorded value and the theoretical value differs more than a predetermined threshold value. Hereby the system can be alerted that there is a problem with the pressure sensor of the common rail system and appropriate action can be taken such as replacement of the pressure sensor.

In accordance with one embodiment the pressure of the common rail system is increased until a safety valve of the common rail opens, recording the pressure value read in conjunction with the opening of the safety valve, and comparing the recorded pressure value to a theoretical value derived from the characteristics of the safety valve, it can be determined that the pressure sensor is malfunctioning if the recorded value and the theoretical value differs more than a predetermined threshold value. Hereby the system can be alerted that there is a problem with the pressure sensor of the common rail system and appropriate action can be taken such as replacement of the pressure sensor. In one embodiment, if the safety valve is of a type that opens at a first pressure and then closes to maintain a reduced second pressure, the pressure can be recorded initially when the safety valve opens and/or when the pressure reaches steady pressure at the reduced second pressure whereby the known characteristics of the pressure valve is taken into account to provide the theoretically correct pressure.

In another embodiment, if the safety valve is of a type that opens at a first pressure, the pressure is recorded when the safety valve opens whereby the known characteristics of the pressure valve is taken into account to provide the theoretically correct pressure.

Once it has been determined that the pressure sensor is malfunctioning, an error flag in the control system can be set so that the information can be transferred to a diagnostics tool connectable to the vehicle in which the control system for the common rail is located.

Also, once it has been determined that the pressure sensor is malfunctioning, an adjustment of the input signal representing the common rail pressure may be performed such that the control unit receives an optimized input signal, at least until the common rail pressure sensor is replaced.

The invention also extends to a system and a computer program adapted to perform the method in accordance with the above.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described in more detail by way of non-limiting examples and with reference to the accompanying drawings, in which:

- Fig. Ia is a general view of a common rail system,

- Fig. Ib is a partial cross-sectional view of an injector of a common rail system, - Fig. 2 is a flow chart illustrating detection of a malfunctioning pressure sensor in accordance with a first embodiment of the invention,

- Fig. 3 is a flow chart illustrating detection of a malfunctioning pressure sensor in accordance with a second embodiment of the invention,

- Fig. 4 is a flow chart illustrating detection of a malfunctioning pressure sensor in accordance with a third embodiment of the invention, - Fig 5 is a diagram illustrating the pressure in a common rail when dumping fuel, and

- Fig. 6 is a flow chart illustrating detection of a malfunctioning pressure sensor in accordance with a fourth embodiment of the invention.

DETAILED DESCRIPTION

In Fig. Ia a general view of a common rail system is shown. The common rail system depicted in Fig. Ia can for example be designed to be used in a diesel engine of a heavy vehicle such as a truck or a bus. The system shown in Fig Ia is fitted on a combustion engine generally denoted 1 in Fig. Ia. The common rail system comprises a fuel line 2 connecting a fuel tank 3 with cylinders of the engine for providing each cylinder with fuel. A fuel pump 4 is fitted on the fuel line 2 to pump fuel from the fuel tank 3 to the cylinders. The line 2 runs via the fuel pump 4 via a filter 5 and a high pressure pump 6 to a common- rail 7. The common rail 7 is adapted to maintain a high pressure enabling injection of fuel into the cylinders via injectors 8, one injector 8 being provided for each cylinder.

In order to control the injection of fuel into the cylinders the injectors 8 are adapted to operate in two states, one open state when fuel is allowed to enter the cylinder and one closed state when the fuel is prevented from entering the cylinder. The mechanism whereby this is obtained can typically be by a spring loaded needle movable between positions corresponding to the open and closed states.

A control unit 9 is provided to control the operation of the injectors 8 of the engine 1. The control unit 9 can typically be a computer in the form of an electronic control unit (ECU), which is also adapted to control the fuel pump 4, and the high pressure pump 6 in addition to controlling the injectors 8. Furthermore, a pressure sensor 7a is provided on the common- rail to provide the control unit 9 with the reading of the current pressure in the common-rail in order for the control unit to accurately control the injectors in response to the pressure of the common rail and to control the fuel pump 4 and the high pressure pump 6.

The common rail system is also provided with a safety valve 7b. The safety valve can be designed to operate in a number of different ways. For example the safety valve can be designed to close at a predetermined pressure, typically around 900 bar, and maintain this pressure until being reset. Another type of safety valve can be designed to only open when the pressure is above some predetermined pressure and be closed when the pressure does not exceed this predetermined pressure. To perform the tasks of controlling the opening and closing of the injectors and preprogrammed times and to control the pressure in the common-rail 7 in accordance with preprogrammed instructions, the control unit is provided with one or many program modules as indicated by program modules 9a and 9b. The program modules 9a and 9b are typically software modules stored on a computer program product in the form of a memory, such as flash memory or EEPROM having a computer program stored thereon. The control unit is supplied with a number of input signals to accurately control the common rail system to provide optimized performance. The input signals include signals indicative of for example the engine speed and engine load as well as the common rail pressure as read by the common rail pressure sensor 7a.

Because the control unit uses the reading from the pressure sensor 7a as an input parameter to control the fuel injection it is important that the value provided by the pressure sensor 7a is correct. Hence it is important to detect a malfunctioning common rail pressure sensor so that the malfunctioning pressure sensor can be replaced. Moreover it should preferably also be possible to determine the offset from a correct value if an incorrect pressure value is determined so that the control unit can compensate for the incorrect pressure value when controlling the common rail system.

In Fig. Ib a partial cross-sectional view of an injector of a common rail system such as the system shown in Fig. Ia is schematically shown. The injector comprises a chamber (14a, 14b, 14c) in which a needle 15 can move between an open and a closed position. To keep the needle in a closed position the needle is spring loaded by a spring 16 having known characteristics. In order to open the needle has to overcome the force exercised by the spring. This is achieved when the pressure in the part 14c of the chamber generates a force that overcomes the force generated by the pressure in the parts 14a and 14 b of the chamber and the force of the spring.

In Fig 2, a flow chart illustrating detection of a malfunctioning pressure sensor in accordance with a first embodiment of the invention is shown. Thus, first in a step 201 when the engine is running, the pressure of the common rail system is lowered. Next, in a step 203, a parameter indicating that the needle of the injector no longer opens is monitored. The monitoring of the needle can for example be performed by monitoring the speed of the flywheel or detecting knockings in the engine by means of a knocking sensor. If in step 203 the monitored parameter indicates that the needle of the injector opens properly, the procedure returns to step 201 and the pressure is decreased even more. If, on the other hand, the monitored parameter in step 203 reveals that the needle does not open any more the procedure proceeds to a step 205. In step 205 the current pressure of the common rail system is read and stored in a memory. Next in a step 207 the pressure value read in step 205 is compared with the theoretical value determined by the geometry of the injector and the force provided by the spring. If the value determined in step 205 deviates from the theoretical value by more than some pre-determined threshold value the pressure sensor is determined to be malfunctioning and a corresponding error flag is set in the control system in a step 209 else the procedure continues directly to a step 211 where the pressure is increased to a normal idling pressure in order to keep the engine running.

In Fig. 3, a flow chart illustrating detection of a malfunctioning pressure sensor in accordance with a second embodiment of the present invention is shown. Thus, first in a step 301 when the engine is not running, the injectors are activated to dump fuel into the cylinders. Next in a step 303 the pressure of the common rail system is monitored as a function of time. In particular the rate at which the pressure is lowered is monitored. The rate monitored in step 303 will have a knee at the pressure level where the fuel injection stops (typically at about 120 bar) and where the continuing pressure drop is a result of fuel exiting solely through the pilot valve. This is illustrated in Fig. 4 in which the knee corresponds to the point/region where the pressure drop rate is lower than a certain predetermined threshold value and or the pressure drop rate is determined to have changed more than a retardation value.. In step 305 the pressure corresponding to the knee in the rate monitored in step 303 of the common rail system is read and stored in a memory. Next, in a step 307, the pressure value read in step 405 is compared with the theoretical value determined by the geometry of the injector and the force provided by the spring. If the value determined in step 305 deviates from the theoretical value by more than a predetermined threshold value, the pressure sensor is determined to be malfunctioning and a corresponding error flag is set in the control system in a step 309 else the procedure continues directly to a step 311 where the procedure ends.

As an alternative or additional check the control system can be set to read the pressure of the common rail system at a time when the pressure of the common rail system should be equal or close to zero, i.e. after a relatively long time and check that the common rail pressure sensor gives a corresponding value. However, for some common rail systems the time required may be too long to be feasible to implement.

In Fig. 5 a flow chart illustrating detection of a malfunctioning pressure sensor in accordance with a third embodiment of the present invention is shown. Thus, first in a step 501 the pressure of the common rail system is increased to a level where the safety pressure valve of the common rail opens. This is typically around 3000 bar. The safety valve can be designed to close at a predetermined pressure, typically around 900 bar, and maintain this pressure until being reset. In such an embodiment, the pressure can be measured directly after raising the pressure to a level opening the valve. This is performed in a step 503. Next in a step 505 the pressure value read in step 503 is compared with the theoretical value determined by the characteristics of the valve. If the value determined in step 503 deviates from the theoretical value by more than a predetermined threshold value the pressure sensor is determined to be malfunctioning and a corresponding error flag is set in the control system in a step 507 else the procedure continues directly to a step 509 where the system is re-set to operate as usual.

In an alternative embodiment of the third embodiment of the present invention, when the safety valve is of a type that only opens at some predetermined pressure and not closes, the maximum pressure is advantageously measured and compared to the theoretical opening pressure of the safety valve. This approach can also be applied to a safety valve of a type closing at some pressure.

In order to provide a better accuracy in the determination of the fault for the pressure sensor of the common rail system, the different procedures can be run more than one time and the data recorded for a number of such determinations can be analyzed statistically.

The methods described above can each be used to determine the existence of a bias fault of the pressure sensor provided for a common rail system. The methods will also provide a numeric value for the bias fault. This is very useful if the aim of the diagnosis is to detect a malfunctioning pressure sensor. Moreover, in addition to setting an error flag in the control system a signal may be given to the driver of the vehicle to alert the driver of the faulty pressure sensor. However, if the control system also needs to be provided with information regarding a possible gain fault in the pressure sensor for the common rail system, data for more than one pressure has to be recorded. This is achieved by combining two or more of the alternative methods described above in conjunction with Figs 3, 4 and 5. A method for determining the gain fault of a pressure sensor for a common rail system is described below with reference to Fig. 6.

In Fig 6, a flow chart illustrating detection of a malfunctioning pressure sensor in accordance with a fourth embodiment of the invention is shown. First, in a step 601, data relating to the common rail pressure at some known pressures and the corresponding pressure sensor output values for those known pressure values are generated and recorded. This can preferably be performed using a combination of the methods described above in conjunction with Figs. 3 - 5. Next, in a step 603, the gain of the pressure sensor for the common rail system is determined using the data recorded in step 601 is calculated. Thereupon, in a step 605, the gain calculated in step 603 is compared to the of the pressure sensor of the common rail system to determine and record a deviation, if any. Thereupon the procedure ends in a step 607.

In addition to detect bias and gain fault of a pressure sensor for a common rail system the method and device as described above can be use to better control the common rail pressure. Hence, using the information about a fault in the output from a pressure sensor of a common rail system the control system controlling the pressure of the common rail can be set to take into account the fault and thereby provide correct pressure to the common rail system. In other words, if a malfunctioning pressure sensor is detected the control unit can be programmed to replace the reading of the pressure sensor with a reading taking into account the determined fault in the sensor reading such as an offset fault or an bias fault.

Claims

1. A method of controlling a common rail system using a signal provided from common rail pressure sensor as one input parameter to control the common rail system, characterized by the step of: - upon determining that the pressure sensor is malfunctioning, replacing the pressure sensor input parameter with an input parameter based on the pressure sensor input parameter and a parameter representing an offset value corresponding to a difference between a theoretically correct value and the pressure sensor value.

2. The method according to claim 1, characterized in that the offset value is determined by

- lowering the pressure of the common rail system until a needle of an injector no longer opens,

- recording the pressure output from the pressure sensor of the common rail system at the time when the injector needle no longer opens, - comparing the recorded pressure value to a theoretical value derived from a number of parameters including at least the spring load of the spring, and

- determining the offset to be equal to the difference between the theoretical value and the recorded value.

3. The method according to any of claims 1 or 2 characterized in that the offset value is determined by

- dumping fuel from the at least one injector of the common rail system, when the engine is not running,

- monitoring the pressure reduction rate in the common rail system, - recording the pressure value read when the pressure reduction rate starts to decrease,

- comparing the recorded pressure value to a theoretical value derived from a number of parameters including at least the spring load of a spring (16), and

- determining the offset to be equal to the difference between the recorded value and the theoretical value.

4. The method according to any of claims 1 - 3 characterized in that the offset value is determined by

- increasing the pressure of the common rail system until the safety valve opens,

- recording the pressure value read in conjunction with the opening of the safety valve, - comparing the recorded pressure value to a theoretical value derived from the characteristics of the safety valve, and

5. A method of detecting a malfunctioning pressure sensor (7a) of a common rail system for a combustion engine (1), the common rail system comprising at least one injector (8), each injector (8) comprising a spring loaded needle (15) movable between an open position and a closed position at a pre- determined spring load characterized by the steps of: - lowering (201) the pressure of the common rail system until a needle (15) of an injector (8) no longer opens, when the engine is running

- recording (205) the pressure output from the pressure sensor (7a) of the common rail system at the time when the injector needle (15) no longer opens,

- comparing (207) the recorded pressure value to a predetermined value derived from a number of parameters including at least the spring load of the spring (16), and

- determining (209) that the pressure sensor is malfunctioning if the recorded value and the theoretical value differs more than a predetermined threshold value.

6. The method according to claim 5, characterized in that the injectors are determined to no longer open by using a flywheel sensor.

7. The method according to claim 5, characterized in that the injectors are determined to no longer open by using a knocking sensor.

8. The method according to any of claims 5 - 7, characterized by the additional step of setting an error flag in the control system in response to determining that the pressure sensor is malfunctioning.

9. A system for detecting a malfunctioning pressure sensor (7a) of a common rail system for a combustion engine (1), the common rail system comprising at least one injector (8), each injector (8) comprising a spring loaded needle (15) movable between an open position and a closed position at a pre- determined spring load characterized by:

- means (9) for lowering the pressure of the common rail system until a needle of an injector no longer opens, when the engine is running - means (9) for recording the pressure output from the pressure sensor of the common rail system at the time when the injector needle no longer opens,

- means for comparing the recorded pressure value to a predetermined value derived from a number of parameters including at least the spring load of the spring, and - means (9) for determining that the pressure sensor is malfunctioning if the recorded value and the theoretical value differs more than a predetermined threshold value.

10. The system according to claim 9, characterized by a flywheel sensor adapted to determine that that, a needle of an injector no longer opens.

11. The system according to claim 9, characterized by a knocking sensor adapted to determine that that, a needle of an injector no longer opens.

12. The system according to any of claims 9 - 11, characterized by means for setting an error flag in the control system in response to a determination that the pressure sensor is malfunctioning.

13. A system for controlling a common rail system using a signal provided from common rail pressure sensor (7a) as one input parameter for a controller unit of the common rail system, characterized by:

- means (9) for replacing the pressure sensor input parameter with an input parameter corresponding to the pressure sensor input parameter and a parameter representing an offset value corresponding to a difference between a theoretically correct value and the pressure sensor value when it is determined that the pressure sensor is malfunctioning.

14. The system according to claim 13, characterized by

- means (9) for lowering the pressure of the common rail system until a needle (15) of an injector (8) no longer opens,

- means (9) for recording the pressure output from the pressure sensor of the common rail system at the time when the injector needle no longer opens,

- means (9) for comparing the recorded pressure value to a theoretical value derived from a number of parameters including at least the spring load of the spring (16), and

- means for determining the offset to be equal to the difference between the theoretical value and the recorded value.

15. The system according to claim 13 or 14, characterized by

- means for dumping fuel from the at least one injector of the common rail system, when the engine is not running, - means for monitoring the pressure reduction rate in the common rail system,

- means fir recording the pressure value read when the pressure reduction rate starts to decrease,

- means for comparing the recorded pressure value to a theoretical value derived from a number of parameters including at least the spring load of a spring (16), and - means for determining the offset to be equal to the difference between the recorded value and the theoretical value.

16. The system according to any of claim 13 or 15, characterized by:

- means for increasing the pressure of the common rail system until the safety valve opens, - means for recording the pressure value read in conjunction with the opening of the safety valve,

- means for comparing the recorded pressure value to a theoretical value derived from the characteristics of the safety valve, and

- means for determining the offset to be equal to the difference between the recorded value and the theoretical value.

17. A computer program product for detecting a malfunctioning pressure sensor (7a) of a common rail system for a running combustion engine (1), the computer program product comprising program segments that when executed on a computer for controlling the fuel injection causes the computer to perform the steps:

- lowering the pressure of the common rail system until a needle of an injector of the common rail system no longer opens,

- recording the pressure output from the pressure sensor of the common rail system at the time when the injector needle no longer opens,

- comparing the recorded pressure value to a theoretical value derived from a number of parameters including at least the spring load of the spring, and

- determining that the pressure sensor is malfunctioning if the recorded value and the theoretical value differs more than a predetermined threshold value.

18. The computer program product according to claim 17, characterized by computer program segments for setting an error flag in the control system in response to determining that the pressure sensor is malfunctioning.

19. A computer program product for controlling a common rail system, the computer program product comprising program segments that when executed on a computer uses a signal provided from common rail pressure sensor as one input parameter computer program and-upon determining that the pressure sensor is malfunctioning, replacing the pressure sensor input parameter with an input parameter corresponding to the pressure sensor input parameter and a parameter representing an offset value corresponding to a difference between a theoretically correct value and the pressure sensor value.

20. The computer program product according to claim 19, characterized by computer program segments for determining the offset value by: - lowering the pressure of the common rail system until a needle of an injector no longer opens,

21. The computer program product according to claim 19 or 20, characterized by computer program segments for determining the offset value by:

- monitoring the pressure reduction rate in the common rail system,

- recording the pressure value read when the pressure reduction rate starts to decrease, - comparing the recorded pressure value to a theoretical value derived from a number of parameters including at least the spring load of a spring (16), and

22. The computer program product according to any of claims 19 - 21, characterized by computer program segments for determining the offset value by:

23. A method of detecting a malfunctioning pressure sensor (7a) of a common rail system for a combustion engine (1), the common rail system comprising at least one injector (8), which comprises a spring loaded needle (15) movable between an open and a closed position at a pre-determined spring load characterized by the steps of:

- dumping (301) fuel from the at least one of the at least one injector, when the engine is not running,

- monitoring (303) the pressure reduction rate in the common rail system,

- recording (305) the pressure value read when the pressure reduction rate starts to decrease,

- comparing (307) the recorded pressure value to a theoretical value derived from a number of parameters including at least the spring load of the spring (16), and - determining (309) that the pressure sensor is malfunctioning if the recorded value and the theoretical value differs more than a predetermined threshold value.

24. The method according to claim 23, characterized by the additional step of setting an error flag in the control system in response to determining that the pressure sensor is malfunctioning.

25. The method according to any of claims 23 or 24, characterized by the step of:

- upon determining that the pressure sensor is malfunctioning by at least some predetermined offset value, replacing the pressure sensor input parameter with an input parameter corresponding to both the pressure sensor input parameter and a parameter representing said offset value.

26. The method according to any of claims 23 -25, characterized in that the pressure value is recorded when the pressure reduction rate is below a predetermined value.

27. A system for detecting a malfunctioning pressure sensor (7a) of a common rail system for a combustion engine (1), the common rail system comprising at least one injector (8), which comprises a spring loaded needle (15) movable between an open and a closed position at a pre-determined spring load characterized by: - means (9) for dumping fuel from at least one of the at least on injector(8) when the engine is not running,

- means (9) for monitoring the pressure reduction rate in the common rail system,

- means (9) for recording the pressure value read when the pressure reduction rate starts to decrease, - means (9) for comparing the recorded pressure value to a theoretical value derived from a number of parameters including at least the spring load of the spring (16) , and

- means (9) for determining that the pressure sensor is malfunctioning if the recorded value and the theoretical value differs more than a predetermined threshold value.

28. The system according to claim 27, characterized by means for setting an error flag in the control system in response to determining that the pressure sensor is malfunctioning.

29. The system according to any of claims 27 or 28, characterized by means for means for replacing the pressure sensor input parameter with an input parameter corresponding to both the pressure sensor input parameter and a parameter representing a offset value corresponding to the difference between the recorded value and the theoretical value when determining that the pressure sensor is malfunctioning.

30. The system according to any of claims 27 -29, characterized in that the means for recording the pressure value is adapted to record the pressure value when the pressure reduction rate is below a predetermined value.

31. A computer program product for detecting a malfunctioning pressure sensor (7a) of a common rail system having at least one injector (8), which comprises a spring loaded needle (15) movable between an open and a closed position at a pre-determined spring load, the computer program product comprising program segments that when executed on a computer for controlling fuel injection causes the computer to perform the steps of

- dumping fuel from at least one of the at least one injector(8),

- monitoring the pressure reduction rate in the common rail system, - recording the pressure value read when the pressure reduction rate starts to decrease, - comparing the recorded pressure value to a theoretical value derived from a number of parameters including at least the spring load of the spring, and

32. The computer program product according to claim 31, characterized by program segments for executing the additional step of setting an error flag in the control system in response to determining that the pressure sensor is malfunctioning.

33. The computer program product according to any of claims 31 or 32, characterized by program segments for executing the additional step of:

34. A method of detecting a malfunctioning pressure sensor (7a) of a common rail system of a combustion engine (1), the common rail system comprising at least one safety valve (7b) designed to open at a predetermined pressure characterized by the steps of: - increasing (501) the pressure of the common rail system until the safety valve opens,

- recording (503) the pressure value read in conjunction with the opening of the safety valve,

- comparing (505) the recorded pressure value to a theoretical value derived from the characteristics of the safety valve, and - determining (507) that the pressure sensor is malfunctioning if the recorded value and the theoretical value differs more than a predetermined threshold value.

35. The method according to claim 34, when the safety valve is of a type that opens at a first pressure and then closes to maintain a reduced second pressure, characterized in that the pressure is recorded initially when the safety valve opens and/or when the pressure reaches steady pressure at the reduced second pressure .

36. The method according to claim 34 when the safety valve is of a type that opens at a first pressure, characterized in that the pressure is recorded when the safety valve opens.

37. The method according to any of claims 34 - 36, characterized by the additional step of setting an error flag in the control system in response to determining that the pressure sensor is malfunctioning.

38. The method according to any of claims 34 - 37, characterized by the step of:

39. A system for detecting a malfunctioning pressure sensor (7a) of a common rail system combustion engine, the common rail system comprising at least one safety valve (7b) designed to open at a predetermined pressure characterized by:

- means (9) for increasing the pressure of the common rail system until the safety valve opens,

- means (9) for recording the pressure value read in conjunction with the opening of the safety valve,

- means (9) for comparing the recorded pressure value to a theoretical value derived from the characteristics of the safety valve, and - means (9) for determining that the pressure sensor is malfunctioning if the recorded value and the theoretical value differs more than a predetermined threshold value.

40. The system according to claim 39, when the safety valve is of a type that opens at a first pressure and then closes to maintain a reduced second pressure, characterized by means for recording the pressure when the safety valve opens and/or when the pressure reaches steady pressure at the reduced second pressure .

41. The system according to claim 39 when the safety valve is of a type that opens at a first pressure, characterized by means for recording the pressure when the safety valve opens.

42. The system according to any of claims 39 - 41, characterized by means for setting an error flag in the control system in response to determining that the pressure sensor is malfunctioning.

43. The system according to any of claims 39 - 42, characterized by means for replacing the pressure sensor input parameter with an input parameter corresponding to both the pressure sensor input parameter and a parameter representing a offset value corresponding to the difference between the recorded value and the theoretical value when determining that the pressure sensor is malfunctioning.

44. A computer program product for detecting a malfunctioning pressure sensor of a common rail system, the computer program product comprising program segments that when executed on a computer for controlling a common rail system causes the computer to perform the steps of

- increasing the pressure of a common rail system comprising a safety valve of the common rail until the safety valve opens,

- recording the pressure value read in conjunction with the opening of the safety valve,

- comparing the recorded pressure value to a theoretical value derived from the characteristics of the safety valve, and

45. The computer program product according to claim 44, characterized in that the pressure is recorded initially when the safety valve opens and/or when the pressure reaches steady pressure at the reduced second pressure .

46. The computer program product according to any of claims 44 or 45, characterized by program segments for executing the additional step of setting an error flag in the control system in response to determining that the pressure sensor is malfunctioning.

47. The computer program product according to any of claims 44 - 46, characterized by program segments for executing the additional step of:

- upon determining that the pressure sensor is malfunctioning by at least some pre- determined offset value, replacing the pressure sensor input parameter with an input parameter corresponding to both the pressure sensor input parameter and a parameter representing said offset value.

48. A digital storage medium having the computer program product according to any of claims 13 - 16, 31 - 33 or 44 - 47 stored thereon.