Abstract:
The invention relates to an internal combustion engine comprising a fuel supplying device. Said fuel supplying device comprises a low-pressure circuit provided with a low-pressure pump and a high-pressure pump that is coupled to the low-pressure circuit on the input side and transports fuel into a fuel accumulator. A fuel transporting flow of the low-pressure pump is corrected according to an actual and a previously pre-determined nominal value of the fuel pressure in the fuel accumulator.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. National Stage of International Application No. PCT/EP2005/053901, filed Aug. 8, 2005 and claims the benefit thereof. The International application claims the benefits of German application No. 10 2004 045 738.7 filed Sep. 21, 2004, both of the applications are incorporated by reference herein in their entirety. 
     FIELD OF INVENTION 
     The invention relates to a method and an associated device for controlling an internal combustion engine with the aid of a fuel delivery device. Said fuel delivery device comprises a low-pressure circuit provided with a low-pressure pump and a high-pressure pump that is coupled to the low-pressure circuit on the input side and conveys fuel into a fuel accumulator. 
     BACKGROUND OF THE INVENTION 
     A fuel delivery device of the said kind is known from DE 101 62 989 C1. Further disclosed is a circuit arrangement for regulating an adjustable fuel pump for an injection system of an internal combustion engine, said arrangement being provided with a controller which compares a desired value of a fuel pressure with an actual value of the fuel pressure and determines an adjustment value for the delivery rate of the fuel pump as a function of the difference between the values. Furthermore a pilot control unit and an adder unit are provided. The adder unit determines a control signal from the adjustment value and a pilot control value for regulating the delivery rate of the fuel pump. The pilot control unit determines the pilot control value as a function of a desired delivery volume. 
     SUMMARY OF INVENTION 
     The object of the invention is to create a method and an associated device that can provide reliable control of an internal combustion engine in a simple manner. 
     This object is achieved by means of the features which will emerge from the independent claims. Further advantageous embodiments of the invention are characterized in the dependent claims. 
     The invention is characterized by a method and an associated device for controlling an internal combustion engine with the aid of a fuel delivery device. Said fuel delivery device comprises a low-pressure circuit provided with a low-pressure pump and a high-pressure pump that is coupled to the low-pressure circuit on the input side and delivers fuel into a fuel accumulator. A fuel delivery flow of the low-pressure pump is corrected as a function of a current and a preceding predetermined setpoint value of the fuel pressure in the fuel accumulator. 
     This has the advantage that the fuel delivery flow of the low-pressure pump can be controlled so as to take into account an additional quantity of fuel that is conveyed by the high-pressure pump from the low-pressure circuit into the fuel accumulator due to an increase in the predetermined setpoint value of the fuel pressure or a smaller quantity of fuel conveyed by the high-pressure pump from the low-pressure circuit into the fuel accumulator or drained off from the fuel accumulator into the low-pressure circuit due to a reduction in the predetermined setpoint value of the fuel pressure. An unwanted increase or reduction in the fuel pressure within the low-pressure circuit can be avoided in this way. 
     By taking into account the current and preceding predetermined setpoint values of the fuel pressure, the fuel delivery flow of the low-pressure pump can be corrected virtually without delay. In this way the components in the low-pressure circuit, such as the low-pressure pump or a pressure relief valve, can be very easily kept free of overload and thus protected from damage. This enables the fuel delivery device to be particularly reliable. 
     The current and the preceding predetermined setpoint values of the fuel pressure in the fuel accumulator are preferably determined as a function of operating variables or the operating mode of the internal combustion engine, for example as a function of an engine speed or a fuel mass that needs to be injected, or as a function of a homogeneous or layered operation. 
     The preceding predetermined setpoint value of the fuel pressure is a predetermined setpoint value of the fuel pressure that was determined at some time prior to the current predetermined setpoint value of the fuel pressure, and was determined for example in the last preceding stationary phase of the setpoint value of the fuel pressure. 
     The fuel pressure in the fuel accumulator is preferably adjusted by a control device as a function of the current predetermined setpoint value of the fuel pressure. 
     In an advantageous embodiment of the invention, correction of the fuel delivery flow of the low-pressure pump is activated as a function of the current and the preceding predetermined setpoint values of the fuel pressure in the fuel accumulator. This has the advantage that the fuel delivery flow of the low-pressure pump is corrected only when necessary. Preferably correction of the fuel delivery flow of the low-pressure pump is started if the predetermined setpoint value of the fuel pressure is changed by a large amount, that is to say, when for example the amount of the difference between the current and the preceding predetermined setpoint values of the fuel pressure is about 100 bar or the ratio between the current and the preceding predetermined setpoint value of the fuel pressure amounts to about 50 percent. 
     In a further advantageous embodiment of the invention, a first correction value is determined when correction of the fuel delivery flow of the low-pressure pump is activated. The first correction value is determined as a function of a current and a preceding quantity, said quantity being representative of a fuel delivery flow of the high-pressure pump, which fuel delivery flow is set in each case as a function of the current predetermined setpoint value of the fuel pressure in the fuel accumulator. The fuel delivery flow of the low-pressure pump is corrected as a function of the first correction value. 
     The invention utilizes the finding that the fuel delivery flow of the high-pressure pump is controlled or adjusted in each case as a function of the current predetermined setpoint value of the fuel pressure in the fuel accumulator, and that the current and preceding quantities then contain information about how the fuel delivery flow of the high-pressure pump changes following a change in the predetermined setpoint value of the fuel pressure. This information can be very easily put to use for the purpose of making an appropriate adjustment to the fuel delivery flow of the low-pressure pump. The quantity that is representative of a fuel delivery flow of the high-pressure pump may be a corrective signal for setting the fuel delivery flow of the high-pressure pump, or may equally be a measured value of a measurement variable captured by a sensor, or an estimated quantity. 
     In this connection it is advantageous if the first correction value is assigned a neutral value after a predetermined interval immediately following the last activation of correction to the fuel delivery flow of the low-pressure pump. This has the advantage that the correction to the fuel delivery flow of the low-pressure pump is limited in time and that otherwise there is no intervention in any control or adjustment that may be provided as necessary for the fuel pressure in the low-pressure circuit. 
     In this connection it is a further advantage to determine a current second correction value, equal to the first correction value, while correction of the fuel delivery flow of the low-pressure pump is activated. The current second correction value is further determined as a function of the difference between a previous second correction value and a reset value when correction of the fuel delivery flow of the low-pressure pump is not activated, until the current second correction value has a neutral value. The fuel delivery flow of the low-pressure pump is corrected as a function of the second correction value. This has the advantage that any control or adjustment means that may be provided as necessary for the fuel pressure in the low-pressure circuit is relieved of overloading by the avoidance of large, erratic changes in the fuel delivery flow from the low-pressure pump when correction of the fuel delivery flow from the low-pressure pump has been deactivated. 
     In a further advantageous embodiment of the invention, a third correction value is determined when correction of the fuel delivery flow of the low-pressure pump is activated. The third correction value is determined as a function of the current and the preceding predetermined setpoint value of the fuel pressure in the fuel accumulator. The fuel delivery flow of the low-pressure pump is corrected as a function of the third correction value. Correction of the fuel delivery flow of the low-pressure pump is therefore particularly simple. A correction of the said kind can be made even if there is no control element available for changing the fuel delivery flow of the high-pressure pump at constant engine speed. 
     In this connection it is advantageous if the third correction value is determined from an engine operating map. This has the advantage that determining the third correction value is very easy and the required computational overhead is small. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention are explained below with reference to the schematic drawings, in which: 
         FIG. 1  shows an internal combustion engine with a fuel delivery device, 
         FIG. 2  is the block diagram of a control device for adjusting the fuel pressure in a fuel accumulator, 
         FIGS. 3 ,  4  show a flowchart for a first embodiment of a program for determining the fuel delivery flow of the low-pressure pump, and 
         FIG. 5  shows a flowchart for a second embodiment of the program for determining the fuel delivery flow of the low-pressure pump. 
     
    
    
     Elements which have the same design or function are given the same reference characters in all the figures. 
     DETAILED DESCRIPTION OF INVENTION 
     An internal combustion engine ( FIG. 1 ) includes an intake duct  1 , an engine block  2 , a cylinder head  3  and an exhaust duct  4 . The engine block  2  includes a plurality of cylinders having pistons and connecting rods via which they are coupled to a crankshaft  21 . 
     The cylinder head  3  includes a valve train assembly having a gas inlet valve, a gas outlet valve and valve operating mechanisms. The cylinder head  3  further includes an injection valve  34  and a spark plug. 
     A fuel delivery device  5  is also provided. Said device has a fuel tank  50  which is connected via a first fuel line to a low-pressure pump  51 . The low-pressure pump  51  is effectively linked on the output side to an inlet  53  of a high-pressure pump  54 . Further the low-pressure pump  51  is provided on the output side with a pressure relief valve  52  which is connected on the output side to the fuel tank  50  via a further fuel line. The low-pressure pump  51 , the pressure relief valve  52 , the first fuel line, the further fuel line and the inlet  53  form a low-pressure circuit. 
     The low-pressure pump  51  is preferably designed so that when the internal combustion engine is operating, said pump always delivers a sufficient quantity of fuel to guarantee that the pressure does not fall below a predetermined minimum. 
     The inlet  53  feeds into the high-pressure pump  54  which on the output side conveys the fuel into a fuel accumulator  55 . As a rule the high-pressure pump  54  is driven by the camshaft. Thus when the crankshaft  21  is running at a constant speed, said pump delivers a constant volume of fuel to the fuel accumulator  55 . 
     The injection valves  34  are effectively connected to the fuel accumulator  55 . The fuel is thus supplied to the injection valves  34  via the fuel accumulator  55 . 
     Installed upstream of the high-pressure pump  54  is a volume flow control valve  56  which enables the volume flow supplied to the high-pressure pump  54  to be set. A setpoint value FUP_SP of the fuel pressure in the fuel accumulator  55  can be set by appropriately controlling the volume flow control valve  56 . The volume flow control valve  56  is a servo drive that controls a fuel delivery flow of the high-pressure pump  54 . The volume flow control valve  56  controls the fuel delivery flow of the high-pressure pump  54  as a function of a corrective signal PWM_HP of the high-pressure pump  54 . Said signal may be a pulse-width modulated electrical current and the fuel delivery flow of the high-pressure pump  54  is then a function of its pulse width. The corrective signal PWM_HP of the high-pressure pump  54  is thus a quantity representative of the fuel delivery flow of the high-pressure pump  54 . 
     As an alternative to the volume flow control valve  56  and the high-pressure pump  54 , it is instead possible, for example, for the fuel delivery flow of the high-pressure pump  54  to be dependent on a triggering angle. The triggering angle corresponds to a crankshaft angle at which the high-pressure pump  54  starts to convey fuel into the fuel accumulator  55  on every revolution of the crankshaft. Delivery of the fuel ends in each case when the crankshaft reaches a predetermined crankshaft angle. In this case the triggering angle is a quantity representative of the fuel delivery flow from the high-pressure pump  54  and the corrective signal PWM_HP of the high-pressure pump  54  is for example the triggering angle. 
     The quantity that is representative of the fuel delivery flow from the high-pressure pump  54  may also be an estimated quantity determined as a function of determined, captured or predetermined operating variables of the internal combustion engine. In the same way a sensor can be provided in which the measurement variable is the fuel delivery flow of the high-pressure pump  54 . The measured value of this measurement variable is then representative of the fuel delivery flow from the high-pressure pump  54 . 
     The fuel delivery device  5  can alternatively or additionally be provided with an electromechanical pressure regulator  57  which is arranged on the output side of the fuel accumulator  55  and provided with a return line into the low-pressure circuit. A setpoint value FUP_SP of the fuel pressure in the fuel accumulator  55  can be set by appropriately controlling the electromechanical pressure regulator  57 . If the fuel pressure in the fuel accumulator  55  is greater than the fuel pressure predetermined by appropriately controlling the electromechanical pressure regulator  57 , the electromechanical pressure regulator  57  opens and fuel is drained off from the fuel accumulator  55  into the low-pressure circuit. 
     The volume flow control valve  56  can also be integrated into the high-pressure pump  54 . A common servo drive can be assigned to the electromechanical pressure regulator  57  and the volume flow control valve  56 . 
     A fuel delivery flow of the low-pressure pump  51  is dependent on a corrective signal PWM_LP of the low-pressure pump  51 , which in the same way as the corrective signal PWM_HP of the high-pressure pump  54  may be a pulse-width modulated current and the fuel delivery flow of the low-pressure pump  51  is then a function of its pulse width. 
     The internal combustion engine is also provided with a control device  6 , and this in turn is provided with sensors which capture different measurement variables and determine the measured value of each measurement variable. Dependent on at least one of the measurement variables, the control device  6  determines control variables that are then converted into corresponding corrective signals for regulating control elements with the aid of corresponding servo drives. 
     Said sensors can be for example a pedal position indicator which captures the position of a foot pedal, a crankshaft angle sensor which captures the crankshaft angle and to which a speed of rotation is then assigned, a mass airflow sensor, a first fuel pressure sensor  58  which captures an actual value FUP_AV for the fuel pressure in the fuel accumulator  55 , and a second fuel pressure sensor  59  which captures an actual value for the fuel pressure in the low-pressure circuit. There may be a smaller or greater number of sensors, depending on the embodiment of the invention. 
     Control elements may for instance be in the form of gas inlet valves or gas outlet valves, injection valves  34 , spark plugs, throttle valves, low-pressure pump  51 , volume flow control valve  56  or electromechanical pressure regulator  57 . 
     The internal combustion engine preferably also has further cylinders to which corresponding control elements are then assigned. 
       FIG. 2  shows a block diagram of a control device which can be used to adjust the fuel pressure in the fuel accumulator  55  during a first operating mode of the fuel delivery device  5 . The fuel pressure in the fuel accumulator  55  is dependent on the set quantity of fuel conveyed by the high-pressure pump  54  from the low-pressure circuit into the fuel accumulator  55 . The quantity of fuel can be a fuel mass or a fuel volume. The quantity of fuel conveyed is dependent on the fuel delivery flow of the high-pressure pump  54 , said flow being set by the corrective signal PWM_HP of the high-pressure pump  54 . 
     If more fuel is conveyed into the fuel accumulator  55  than is injected into the combustion chambers of the internal combustion engine, the fuel pressure rises in the fuel accumulator  55 . If less fuel is conveyed into the fuel accumulator  55  than is injected into the combustion chambers of the internal combustion engine, the fuel pressure falls correspondingly in the fuel accumulator  55 . 
     In a second operating mode of the fuel delivery device  5  the volume flow control valve  56  is preferably closed. Only a very small flow seeps through the volume flow control valve  56  if the need arises. The second operating mode can also be used if no volume flow control valve  56  is available in the fuel delivery device and the high-pressure pump  54  conveys virtually the same quantity of fuel from the low-pressure circuit into the fuel accumulator  55  with each revolution of the crankshaft  21 . If the electromechanical pressure regulator  57  is closed and less fuel is injected into the combustion chambers of the internal combustion engine than is conveyed into the fuel accumulator  55 , fuel pressure rises in the fuel accumulator  55  until the electromechanical pressure regulator  57  opens and redirects fuel into the inlet  53 . This limits the fuel pressure in the fuel accumulator  55  to the setpoint value FUP_SP for fuel pressure. 
     A difference between the setpoint value FUP_SP of the fuel pressure and the actual value FUP_AV of the fuel pressure is used to determine a control difference FUP_DIF. The control difference FUP_DIF is supplied to a controller in block B 1 . This controller is preferably designed as a PI controller. In block B 1  a control value MFF_FB_CTRL is defined. The setpoint value FUP_SP of the fuel pressure and the actual value FUP_AV of the fuel pressure are used in a block B 2  to determine a precontrol value MFF_PRE. The precontrol value MFF_PRE, the control value MFF_FB_CTRL and a fuel mass MFF_INJ to be injected are summed together into a fuel mass MFF_REQ to be conveyed, preferably the fuel mass to be conveyed per cylinder segment. 
     The fuel mass MFF_REQ to be conveyed, a segment interval T_SEG_AV and correction variables COR are used in a block B 3  to determine the corrective signal PWM_HP of the high-pressure pump  54 . Preferably the fuel mass MFF_REQ to be conveyed is divided by the segment interval T_SEG_AV and multiplied by a correction factor determined from the correction variables COR, in particular the fuel density in the fuel accumulator  55 . The segment interval T_SEG_AV is equal to the duration needed for one revolution of the crankshaft  21  divided by half the number of cylinders in the internal combustion engine, since injection into the same cylinder occurs only every second revolution of the crankshaft  21 . The correction variables COR include for example the fuel density in the fuel accumulator  55  and/or a fuel temperature. 
     A block B 4  represents the fuel delivery device  5  shown in  FIG. 1 . The corrective signal PWM_HP of the high-pressure pump  54  is the input variable for the block B 4 . The output variable of the block B 4  is the actual value FUP_AV of the fuel pressure, captured for example by means of the fuel pressure sensor  58 . 
     A corresponding control device can also be provided for the second operating mode of the fuel delivery device  5 , in which a corrective signal for the electromechanical pressure regulator  57  is generated for the purpose of controlling the fuel pressure in the fuel accumulator  55 . 
     If the fuel pressure in the fuel accumulator  55  is reduced, some of the fuel mass additionally stored in the volume of the fuel accumulator  55  at the previously higher fuel pressure compared to the lower fuel pressure prevailing following the pressure reduction is freed up due to the compressibility of the fuel. Said fuel mass is dependent on the pressure difference between the fuel pressure in the fuel accumulator  55  before and after the pressure reduction, on the volume that is filled with fuel in the fuel accumulator  55 , on the fuel density and on the compressibility of the fuel. 
     The fuel pressure in the fuel accumulator  55  can be reduced to a predetermined fuel pressure by reducing the fuel delivery flow of the high-pressure pump  54 , compared to the fuel delivery flow immediately before the start of the pressure reduction, until enough fuel is directed away from the fuel accumulator  55  into the combustion chambers of the internal combustion engine by fuel injection processes. In this case less fuel may be taken from the low-pressure circuit than is conveyed by the low-pressure pump  51  into the inlet  53 . In the same way fuel in the low-pressure circuit can be directed away from the fuel accumulator  55  into the inlet  53  via the electromechanical pressure regulator  57 . In this case fuel is introduced into the low-pressure circuit in addition to the fuel conveyed by the low-pressure pump  51 . In both cases, therefore, fuel pressure in the low-pressure circuit can increase to more than the predetermined fuel pressure. This places an additional load on the components of the low-pressure circuit and can reduce their reliability and service life. 
       FIGS. 3 and 4  show a flowchart for a first embodiment of a program for determining the fuel delivery flow of the low-pressure pump  51 . The program is stored in the control device  6  and is run while the internal combustion engine is operating. The program starts at a step S 1  ( FIG. 3 ) in which necessary preparations are made, particularly when the program is executed for the first time. For example logical variables are assigned their predetermined values or counters are reset. 
     In a step S 2  the corrective signal PWM_HP of the high-pressure pump  54  and the setpoint value FUP_SP of the fuel pressure are determined at a current instant t_n. The corrective signal PWM_HP of the high-pressure pump  54  may for example be determined as shown in  FIG. 2 . In a step S 3  a check is made on whether a logical variable LV_LP_COR has been assigned a predetermined logical value, e.g. one. The logical variable LV_LP_COR represents the activation status of the fuel delivery flow correction for the low-pressure pump  51 . 
     If the condition in the step S 3  is not fulfilled, that is, if correction to the fuel delivery flow of the low-pressure pump  51  is not activated, then in a step S 4  a setpoint value difference in fuel pressure FUP_SP_DIF between the setpoint value FUP_SP of the fuel pressure at the current instant t_n and the setpoint value FUP_SP of the fuel pressure at a previous instant t_n−1 is determined. In the event that the setpoint value FUP_SP of the fuel pressure is reduced, the setpoint value difference FUP_SP_DIF of the fuel pressure is negative. 
     In a step S 5  the setpoint value difference FUP_SP_DIF determined for the fuel pressure is checked. If the setpoint value difference FUP_SP_DIF of the fuel pressure is less than or equal to a threshold value FUP_SP_DIF_THR of the setpoint value difference FUP_SP_DIF for the fuel pressure, then in a step S 6  correction of the fuel delivery flow is activated for the low-pressure pump by assigning the associated logical value, e.g. one, to the logical variable LV_LP_COR. The threshold value FUP_SP_DIF_THR of the setpoint value difference FUP_SP_DIF for the fuel pressure is preferably negative. 
     In a step S 7  the corrective signal PWM_HP of the high-pressure pump  54  at the previous instant t_n−1 is saved as a reference value PWM_HP_REF for the corrective signal PWM_HP of the high-pressure pump  54 . In a step S 8  a counter CTR is reset, for example to zero. 
     In a step S 9 , a first correction value PWM_LP_COR 1  is determined from the reference value PWM_HP_REF for the corrective signal PWM_HP of the high-pressure pump  54  and the corrective signal PWM_HP of the high-pressure pump  54  at the current instant t_n. In a step S 10  the value of the first correction value PWM_LP_COR 1  is assigned to a second correction value PWM_LP_COR 2  at the current instant t_n. In a step S 11  the counter CTR is incremented by for example one. In a step S 12  the counter CTR is checked. If the counter CTR is less than a predetermined threshold value CTR_THR for the counter CTR, the program is continued in a step S 13 . 
     In a step S 13  the corrective signal PWM_LP of the low-pressure pump  51  is determined as the difference between a corrective signal request PWM_LP_REQ for the low-pressure pump  51  and the second correction value PWM_LP_COR 2  at the current instant t_n. The corrective signal request PWM_LP_REQ for the low-pressure pump  51  is determined for example as a function of a setpoint value for the fuel pressure in the low-pressure circuit, a fuel temperature, and a setpoint value for the fuel delivery flow of the low-pressure pump  51 , as disclosed in DE 101 62 989 C1, which is incorporated herein by reference. 
     In a step S 14  the corrective signal PWM_HP of the high-pressure pump  54  at the current instant t_n is saved as a corrective signal PWM_HP for the high-pressure pump  54  at the previous instant t_n−1. The setpoint value FUP_SP of the fuel pressure at the current instant t_n is correspondingly stored as the setpoint value FUP_SP of the fuel pressure at the previous instant t_n−1, and the second correction value PWM_LP_COR 2  at the current instant t_n is stored as the second correction value PWM_LP_COR 2  at the previous instant t_n−1. 
     In a step S 15  the program run is concluded and then continued in the step S 1  after a waiting time T_W ( FIG. 3 ). The waiting time T_W can for example be equal to the segment interval T_SEG_AV and specifies the time interval in which the program is executed. The time interval between the current instant t_n and the previous instant t_n−1 is preferably equal to the waiting time T_W. The previous instant t_n−1 can however also be assigned to an instant at which an operating variable of the internal combustion engine was last stationary. Thus the setpoint value FUP_SP of the fuel pressure at the previous instant t_n−1 is preferably equal to the last stationary setpoint value FUP_SP of the fuel pressure in the fuel accumulator  55  and the setpoint value FUP_SP of the fuel pressure at the current instant t_n is the new stationary target value to which the fuel pressure in the fuel accumulator  55  needs to be set or adjusted. 
     If the condition in the step S 3  is fulfilled, that is, if correction to the fuel delivery flow of the low-pressure pump  51  is activated, the program is continued in the step S 9 . 
     If in the step S 12  the counter CTR is equal to or greater than the predetermined threshold value CTR_THR of the counter CTR, the activation status of the fuel delivery flow correction for the low-pressure pump  51  is reset in a step S 16  by assigning the associated logical value, e.g. zero, to the logical variable LV_LP_COR. The program is then continued in the step S 13 . 
     If the condition in the step S 5  is not fulfilled, that is, if the setpoint value difference FUP_SP_DIF of the fuel pressure is greater than the threshold value FUP_SP_DIF_THR for the setpoint value difference FUP_SP_DIF of the fuel pressure, the program is continued in the step S 17 . In the step S 17  the first correction value PWM_LP_COR 1  is assigned a neutral value, e.g. zero. 
     In a step S 18  a check is made on whether the amount of the second correction value PWM_LP_COR 2  at the current instant t_n is greater than the amount of a reset value LIM. If this condition is fulfilled, then in a step S 19  a difference between the second correction value PWM_LP_COR 2  at the previous instant t_n−1 and the reset value LIM is assigned to the second correction value PWM_LP_COR 2  at the current instant t_n. The program is then continued in the step S 13 . If however the condition in the step S 18  is not fulfilled, then in a step S 20  the second correction value PWM_LP_COR 2  at the current instant t_n is assigned a neutral value, e.g. zero. The program is then continued in the step S 13 . 
     Correction to the fuel delivery flow of the low-pressure pump  51  can likewise be activated if the setpoint value FUP_SP of the fuel pressure rises. In this case the setpoint value difference FUP_SP_DIF determined for the fuel pressure in the step S 4  is positive. The step S 5  is then replaced by a step S 21 , in which a check is made on whether the setpoint value difference FUP_SP_DIF of the fuel pressure is equal to or greater than the threshold value FUP_SP_DIF_THR for the setpoint value difference FUP_SP_DIF of the fuel pressure. The threshold value FUP_SP_DIF_THR is preferably positive. If the condition in the step S 21  is fulfilled, the program is continued in the step S 6 . Otherwise the program is continued in the step S 17 . 
     The threshold value CTR_THR of the counter CTR is preferably chosen so that correction to the fuel delivery flow of the low-pressure pump  51  is only activated for a duration in the order of magnitude of some few hundred milliseconds, e.g. for three hundred milliseconds, that is, the logical variable LV_LP_COR is reset in the step S 16  just a few hundred milliseconds after it was set in the step S 6 . During this duration the counter CTR counts the number of program runs until the condition in the step S 12  is fulfilled. 
     In the steps S 18  and S 19  the reset value LIM is chosen so that the amount of the second correction value PWM_LP_COR 2  at the current instant t_n decreases toward a neutral value, e.g. zero, at each time step, for example after every expiration of the waiting time T_W. The neutral value is preferably reached after a few hundred milliseconds, for example after three hundred milliseconds. 
       FIG. 5  shows a flowchart for a second embodiment of the program for determining the fuel delivery flow of the low-pressure pump  51 . The steps S 1 , S 3  to S 6 , S 8 , S 11 , S 12 , S 15 , S 16  and S 21  are executed in accordance with the first embodiment of the program. The step S 2  is replaced by a step S 22 , in which the setpoint value FUP_SP of the fuel pressure at the current instant t_n is determined. The program is continued in the step S 3 . The step S 7  is replaced by a step S 21 , in which the setpoint value difference FUP_SP_DIF of the fuel pressure is stored as a reference value FUP_SP_DIF_REF for the setpoint value difference FUP_SP_DIF of the fuel pressure. The program is then continued in the step S 8 . 
     After the step S 8 , or if the condition in the step S 3  is fulfilled, that is, correction to the fuel delivery flow of the low-pressure pump  51  is activated, then in a step S 24 , which replaces the step S 9 , a third correction value PWM_LP_COR 3  is determined as a function of the stored reference value FUP_SP_DIF_REF for the setpoint value difference FUP_SP_DIF of the fuel pressure and as a function of the counter CTR. This may be carried out for example by means of an engine operating map in which are stored suitable values that have preferably been determined in advance by trials on an engine test bench, by simulation or by road trials. Alternatively functions such as those based on physical models can also be used. Following the step S 24 , the program is continued in the step S 11 . 
     If the condition in the step S 5  is not fulfilled, that is, if the setpoint value difference FUP_SP_DIF of the fuel pressure is greater than the threshold value FUP_SP_DIF_THR for the setpoint value difference FUP_SP_DIF of the fuel pressure, then in a step S 25 , which replaces the steps S 17  to S 20 , the third correction value PWM_LP_COR 3  is assigned a neutral value, e.g. zero. The program is then continued in a step S 26 . 
     Similarly, following the step S 16  the program is continued in the step S 26 . In the step S 26  the corrective signal PWM_LP of the low-pressure pump  51  is determined as the difference between the corrective signal request PWM_LP_REQ for the low-pressure pump  51  and the third correction value PWM_LP_COR 3 . In a step S 27  the setpoint value FUP_SP of the fuel pressure at the current instant t_n is then stored as the setpoint value FUP_SP of the fuel pressure at the previous instant t_n−1; the program run is then concluded in the step S 15  and continued in the step S 1  after the waiting time T_W.