Abstract:
In order to locate errors of the rail pressure sensor ( 38 ) when starting problems occur, the following method steps are proposed: determination of whether starting problems occur in the engine and if this is the case, an engine state is induced, in which the engine controller is already active but the starting phase of the engine has not yet commenced; substitution of the measured rail pressure sensor value used by the engine controller with a substitute value; start attempt of the engine and determination whether the engine has achieved an independent operation; and identification of an error function of the rail pressure sensor if the starting problems only occur when the measured rail pressure sensor value is used.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Stage Application of International Application No. PCT/EP2009/054508 filed Apr. 16, 2009, which designates the United States of America, and claims priority to German Application No. 10 2008 024 955.6 filed May 23, 2008, the contents of which are hereby incorporated by reference in their entirety. 
     TECHNICAL FIELD 
     The invention relates to a method for identifying an error function and in particular a drift of a rail pressure sensor in a common rail injection system of an internal combustion engine. 
     BACKGROUND 
     Such a method is known example from DE 10 2007 015 876 A1. 
     Modern internal combustion engines are provided with a common rail injection system with which fuel is conveyed by a pump into a pressure reservoir (common rail) and is pressurized. The fuel is then injected from the common rail via controllable injectors into the combustion chambers of the internal combustion engine. 
     A common rail injection system is known for example from DE 198 34 660. The common rail is provided with a rail pressure sensor with which the pressure in the rail is measured, with a pressure valve and/or the pump of the injection system being controlled using open-loop and/or closed-loop control depending on the measured pressure. The (analog) pressure signal of the rail pressure sensor which is processed in the control device is thus the control variable for closed-loop control of the rail pressure. An error function of the rail pressure sensor or drift behavior during operation and over the lifetime of the rail pressure sensor respectively have a negative affect on the accuracy of the setpoint pressure to be set and thereby on the precision of the injection amount. 
     Component faults occurring in the common rail injection system frequently lead to undesired vehicle behavior in which the engine is only able to be started with difficulty or is no longer able to be started at all. On-board diagnosis systems only allow the precise cause of the error in the injection system to be determined to a restricted extent for starting problems, for example with an electrical short circuit, without any active intervention into the system. This also applies especially for a defective rail pressure sensor e.g. one exhibiting an offset, but free from electrical errors however. Typically it is then only possible to detect whether the rail pressure control is approaching a limit, without actually being able to fully distinguish whether a valve or the rail pressure sensor is now defective for example. 
     Because of this lack of knowledge of the precise cause of the error unnecessary components or too many components are frequently replaced. Thus the described undesired vehicle behavior can typically initially lead to the replacement of the high-pressure pump although the start problem is actually being caused by a drifted rail pressure sensor. 
     In order to undertake the corresponding repair in a targeted manner in the case of an error in the operation of an internal combustion engine, a method for the diagnosis of the component, especially a rail pressure sensor, of an internal combustion engine is proposed in DE 100 40 254 B4 in which a component which can be the direct cause of an error—combustion outliers are exclusively cited—is checked by the internal combustion engine being explicitly put into a checking state in which the component cannot be the indirect cause of the error which has occurred and a check is then made as to whether the same error is occurring. In detail the explicit switching-off of the rail pressure sensor contained in a closed-loop control path is discussed, with the pressure control valve being controlled after switching off such that the pressure in the rail assumes a so-called default pressure. As an alternative to introducing the test operating state by switching off the component, in the cited patent document it is also mentioned that the component, for example a sensor, can be explicitly replaced by a model valid in specific operating states. It is proposed in particular that the signal delivered by a sensor be computed from the signals of other sensors and that the test mode of the internal combustion engine be based on these computed variables. 
     SUMMARY 
     According to various embodiments, a method can be created which, for starting problems of the engine, makes possible a diagnosis of the rail pressure sensor, especially in respect of the presence of drift effects. 
     According to an embodiment, a method for detecting an error function and especially a drift of a rail pressure sensor in a common rail injection system of an internal combustion engine, may comprise the following steps:—Establishing whether starting difficulties of the engine are occurring and, if such difficulties are occurring,—Instigating an engine state, in which the engine controller is already active, but the start-up phase of the engine has not yet begun however,—Replacing the measured rail pressure sensor value used by the engine controller by a replacement value predetermined for detecting an error function of the rail pressure sensor,—Attempting to start the engine and subsequently establishing whether autonomous operation of the engine is achieved,—Detecting an error function of the rail pressure sensor if the starting difficulties only occur when the measured rail pressure sensor value is used. 
     According to a further embodiment, the measured rail pressure sensor value can be changed slowly and constantly enough to the replacement value for the engine controller not to diagnose any electrical error function of the rail pressure sensor. According to a further embodiment, a rail pressure sensor value valid for a model in specific operating states of the start phase of the engine can be determined and is prespecified as the replacement value. According to a further embodiment, a rail pressure setpoint value valid in specific operating states of the start phase of the engine in accordance with the control strategy implemented by the engine controller can be accepted or modified and can be predetermined as the replacement value. According to a further embodiment, the error function detection can be undertaken on-board while the vehicle is operating. According to a further embodiment, on detection of an error function of the rail pressure sensor, the engine can be started and can be operated by permanently replacing the measured rail pressure sensor value by a replacement value in an emergency mode with predetermined emergency reactions. According to a further embodiment, a computer program for executing the method on a computer can be used. According to a further embodiment, the computer program can be stored in a flash memory of a vehicle service facility external to the vehicle. According to a further embodiment, the computer program can be stored in a flash memory of the engine control. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a block diagram of an internal combustion engine with common rail injection system known from the prior art. 
         FIG. 2  illustrates the detection of a defective rail pressure sensor according to various embodiments. Plotted in the upper section of the diagram shown is the temporal curve of the engine state in the start phase and in the lower section of the diagram the temporal curve of the rail pressure sensor value. 
         FIG. 3  illustrates, using the same form of presentation as  FIG. 2 , the detection of a non-defective rail pressure sensor according to various embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The solution according to various embodiments of the above problem therefore comprises the following steps: Determining whether starting difficulties of the engine are occurring and, if they are, initiating an engine state in which the engine control is already active but the starting phase of the engine has however not yet begun, replacing the measured rail pressure sensor value used by the engine control by a predetermined replacement value for detecting an error function of the rail pressure sensor, attempting to start the engine and establishing whether autonomous operation of the engine is achieved and detecting an error function of the rail pressure sensor if the starting difficulties only occur when the measured rail pressure sensor value is used. 
     The various embodiments are based on the knowledge that adding a replacement value to the rail pressure sensor value allows conclusions to be drawn about relationships that exist or do not exist respectively between the start problems and the rail pressure sensor. This makes it possible for various embodiments to pinpoint the cause of the error in the injection system for starting problems. 
     According to an embodiment, the replacement value is implemented in a manner almost tricking the engine controller, with the measured rail pressure sensor value being changed so slowly and constantly to the replacement value that the engine controller does not diagnose any electrical malfunction of the rail pressure sensor. 
     According to a further embodiment of these forms of embodiment the replacement value can be specified in a simple manner by a rail pressure sensor value valid as a model in specific operating states of the start phase of the engine being determined and specified as the replacement value. As an alternative, a rail pressure setpoint value valid in specific operating states of the start phase of the engine in accordance with the control strategy implemented by the engine controller can be accepted or modified and predetermined as the replacement value. 
     The method according to various embodiments is especially suitable for execution in well-defined operating conditions which are present in particular in the workshop, but typically also for stationary vehicle operating conditions (e.g. vehicle at a standstill, driver attempting to start the vehicle). Error function detection can advantageously be carried out on board during operation of the vehicle. In accordance with a development of these forms of embodiment, when an error function of the rail pressure sensor is detected, the engine can be started and operated in an emergency mode by permanent replacement of the measured rail pressure sensor value by the replacement value with predetermined emergency mode reactions. 
     To execute the method a computer program on a computer can advantageously be used, which can be stored outside or inside the vehicle, for example in an engine controller or the transmission controller. 
       FIG. 1  shows a sketch of a basic diagram of an internal combustion engine with common rail injection system, with the internal combustion engine as a whole being labeled with the reference sign  10 . The engine  10  essentially comprises a combustion chamber  12  to which air is supplied via an induction pipe  14 . The combustion exhaust gases are removed through an exhaust pipe  16  with catalytic converter  18 . Fuel enters the combustion chamber  12  via high-pressure injection valves  22  to which fuel is supplied via a collective fuel line referred to as a rail  22 . This in its turn is connected to a fuel tank  24  and is pressurized by a high-pressure pump  26 . A pressure control valve  28  is connected on one side to the rail  22  and on the other side to a return flow line  30  back to the fuel tank  24 . 
     Also shown in  FIG. 1  in the combustion chamber  12  are spark plugs  32 , which are supplied by an ignition or glow system  34 , as well as a crankshaft  40 . 
     The internal combustion engine  10  also comprises an engine controller  36  which is connected on the output side to the glow system  34 , the high pressure injection valves  20  and the pressure control valve  28 . On the input side the engine control  36  receives signals from a rail pressure sensor  38  which detects the fuel pressure in the rail  22 . The rail pressure sensor  38 , the pressure control valve  28  and the engine control  36  form a closed-loop control circuit for controlling the pressure in the rail  22 . Depending on the pressure signal provided by the rail pressure sensor  38  as well as the output signals of further sensors, the engine controller  36  applies control signals to the injector  20  which control the dispensing of the fuel. The injector  20  then injects the fuel stored in the rail  22 . 
     The start condition for the method according to various embodiments is that the engine can no longer be started or it is only possible to start the engine with difficulty. In the upper part of  FIGS. 2 and 3  the abbreviation IGK designates an engine state in which only ignition key (if implemented) and engine controller  36  are activated. The abbreviation CRK means a state in which the engine is in the start-up phase. IS/PL finally means an engine state for example idling or load in which a stable operation of the engine is achieved. The sections of the engine state curves shown correspond in time terms to the sections shown in the lower parts of the respective figures of the signal curves of the rail pressure sensor value. 
     In section  1  of the engine state curve of  FIG. 2  and thereafter a (first) start-up of the engine is undertaken, which does not however lead to autonomous operation of the engine but only up to engine state CRK. The rail pressure sensor value “PFU” used by the engine control  36  in each case is shown in the lower part of  FIG. 2 . In section  2  the result is a minimal perceptible increase in pressure which is not sufficient for engine start however. In time section  3  there is therefore a switchover to the replacement value of the rail pressure sensor value, with the replacement pressure being connected in a delayed or constant manner respectively, cf. the rising curve in section  5 , to the—measured—rail pressure sensor value in section  2 . In section  4  there is a renewed attempt at starting which in this case leads to a stable engine state with the engine running, section  6 . In the case shown in  FIG. 2  an engine start is thus possible again after switching over to the replacement value, from which a defective rail pressure sensor can be deduced. 
     In the case depicted in  FIG. 3  there is also a switchover to the replacement value in time section  3 . Although the rail pressure sensor value used by the engine control  36  accordingly again rises to the replacement value, cf. Section  5 , the real pressure in rail  22  also indicated in the lower part of  FIG. 3 , cf. Section  7 , remains minimal, which corresponds to the case shown in the upper part of  FIG. 3  that the new start in section  4 —despite replacement value—has not led to an engine start (but only up to engine state CRK). In this case, even after switchover to the replacement value, an engine start is still not possible, from which it can be concluded that the real pressure sensor is not defective so that further error tracing is necessary. 
     The test intervention according to various embodiments is undertaken such that the measured rail pressure sensor value is replaced by a value able to be predetermined within the test routine. This can for example be a valid setpoint value from the engine controller valid in accordance with the closed-loop control strategy or a valid modeled value, for example a computed value. The engine controller uses or processes this replacement value in the same way as a measured rail pressure sensor value. Afterwards a renewed attempt is made to start the engine, compare the respective sections  4  in  FIGS. 2 and 3 . 
     The relationship between system reaction and diagnosis according to various embodiments is thus essentially produced as follows: 
     If, after replacement of the measured rail pressure sensor value by the replacement value autonomous operation of the engine, for example idling, is achieved, a defective rail pressure sensor, especially a rail pressure sensor affected by a sensor drift, is detected. If not, this is taken as an indication that the rail pressure sensor is operable. In this case further error tracing is necessary. 
     With on-board execution of the test routine according to various embodiments, for a detected sensor error an engine start and thus vehicle operation might still be possible again. Suitable emergency reactions are to be defined for this (e.g. MIL on, speed limiting etc.) in order to encourage the driver to take the vehicle for repair. 
     Storage of a corresponding error code gives the workshop the information for replacing the defective rail pressure sensor, or makes it possible in the event that there is no error (no sensor drift) to conduct a more targeted further error search.