Controlling a pressure regulating valve of a fuel rail

Abstract

A method for operating an internal combustion engine having an injection system which has a high-pressure accumulator, wherein a high pressure in the high-pressure accumulator is regulated via a suction throttle on the low-pressure side as a first pressure control member in a first high-pressure control loop, wherein in a normal operation a high-pressure disturbance variable is produced via a pressure control valve on the high-pressure side as a second pressure control member, via which fuel is redirected from the high-pressure accumulator to a fuel reservoir. For this purpose, the high pressure in a safety operation is regulated by the pressure control valve via a second high-pressure control loop, or, in the safety operation, a maximum fuel volume flow is continuously redirected from the high-pressure accumulator to the fuel reservoir via the pressure control valve.

Claims

1. A method for operating an internal combustion engine having an injection system with a high-pressure accumulator, the method comprising the steps of: regulating a high pressure in the high-pressure accumulator using a low-pressure-side suction throttle as a first pressure setting element in a first high-pressure regulating loop; generating, in a normal operating mode, a high-pressure disturbance variable using of a high-pressure-side pressure regulating valve as a second pressure setting element by way of which fuel is discharged from the high-pressure accumulator into a fuel reservoir; upon failure of the first high-pressure regulating loop, setting a first operation type of a protective operating mode if the high pressure reaches or overshoots a first pressure threshold value, and regulating the high pressure using the pressure regulating valve by way of a second high-pressure regulating loop in the first operation type; and setting a second operation type of the protective operating mode if the high-pressure overshoots a second pressure threshold value, wherein in the second operation type the pressure regulating valve is permanently opened, wherein a setpoint volume flow in the normal operating mode and a setpoint flow in the protective operating mode are calculated differently.

2. The method according to claim 1, wherein, for the pressure regulating valve in the normal operating mode, and in the first operation type of the protective operating mode, a normal function is set in which the pressure regulating valve is actuated in a manner dependent on a setpoint volume flow, and, for the pressure regulating valve in the second operation type of the protective operating mode, a standstill function is set in which the pressure regulating valve is not actuated.

3. The method according to claim 1, including permanently opening the suction throttle in the second operation type and/or in the first operation type of the protective operating mode.

4. An injection system for an internal combustion engine, comprising: a high-pressure pump; at least one injector; a high-pressure accumulator that is fluidically connected at one side to the at least one injector and at another side via the high-pressure pump to a fuel reservoir; a suction throttle assigned to the high-pressure pump as the first pressure setting element; a pressure regulating valve that fluidically connects the high-pressure accumulator to the fuel reservoir; and a control unit operatively connected to the at least one injector, to the suction throttle and to the pressure regulating valve, wherein the control unit is operative to carry out a method according to claim 1.

5. The injection system according to claim 4, wherein the pressure regulating valve is open when deenergized.

6. The injection system according to claim 4, wherein the pressure regulating valve is closed when unpressurized and deenergized, wherein said pressure regulating valve is closed when subjected to a pressure up to an opening pressure value prevailing on an inlet side, wherein said pressure regulating valve opens when the pressure prevailing on an inlet side reaches or overshoots the opening pressure value in a deenergized state.

7. The injection system according to claim 4, wherein the injection system has no mechanical pressure relief valve.

8. An internal combustion engine comprising an injection system according to claim 4.

9. A method for operating an internal combustion engine having an injection system with a high-pressure accumulator, the method comprising the steps of: regulating a high pressure in the high-pressure accumulator using a low-pressure-side suction throttle as a first pressure setting element in a first high-pressure regulating loop; generating, in a normal operating mode, a high-pressure disturbance variable using of a high-pressure-side pressure regulating valve as a second pressure setting element by way of which fuel is discharged from the high-pressure accumulator into a fuel reservoir; upon failure of the first high-pressure regulating loop due to a fault or defect in the first high-pressure regulating loop, setting a first operation type of a protective operating mode if the high pressure reaches or overshoots a first pressure threshold value, and regulating the high pressure using the pressure regulating valve by way of a second high-pressure regulating loop in the first operation type; and setting a second operation type of the protective operating mode if the high-pressure overshoots a second pressure threshold value, wherein in the second operation type the pressure regulating valve is permanently opened, wherein the fault or defect in the first high-pressure regulating loop is a failure of the suction throttle as the first pressure setting element, wherein the failure of the suction throttle is one of the group consisting of: breakage of a cable to the suction throttle plug connector, disconnection of the suction throttle plug connector, jamming of the suction throttle, and an accumulation of dirt in the suction throttle.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 is a schematic illustration of an exemplary embodiment of an internal combustion engine having an injection system;

(2) FIG. 2 is a first schematic detail illustration of an embodiment of the method;

(3) FIG. 3 is a second schematic detail illustration of an embodiment of the method;

(4) FIG. 4 is a third schematic detail illustration of an embodiment of the method;

(5) FIG. 5 is a fourth schematic detail illustration of an embodiment of the method;

(6) FIG. 6 is a fifth schematic detail illustration of an embodiment of the method; and

(7) FIG. 7 is a sixth schematic detail illustration of an embodiment of the method.

DETAILED DESCRIPTION OF THE INVENTION

(8) FIG. 1 is a schematic illustration of an exemplary embodiment of an internal combustion engine 1 which has an injection system 3. The injection system 3 is preferably in the form of a common-rail injection system. Said injection system has a low-pressure pump 5 for the delivery of fuel from a fuel reservoir 7, an adjustable, low-pressure-side suction throttle 9 for influencing a fuel volume flow flowing through said low-pressure pump, a high-pressure pump 11 for delivering the fuel at elevated pressure into a high-pressure accumulator 13, the high-pressure accumulator 13 for storing the fuel, and a multiplicity of injectors 15 for injecting the fuel into combustion chambers 16 of the internal combustion engine 1. It is optionally possible for the injection system 3 to also be formed with individual accumulators, wherein then, it is for example the case that an individual accumulator 17 as an additional buffer volume is integrated in the injector 15. An in particular electrically actuable pressure regulating valve 19 is provided, by way of which the high-pressure accumulator 13 is fluidically connected to the fuel reservoir 7. By way of the position of the pressure regulating valve 19, a fuel volume flow which is discharged from the high-pressure accumulator 13 into the fuel reservoir 7 is defined. Said fuel volume flow is denoted in FIG. 1 and in the following text by VDRV, and represents a high-pressure disturbance variable of the injection system 3.

(9) The injection system 3 has no mechanical pressure relief valve, such as is commonly provided in the prior art so as to connect the high-pressure accumulator 13 to the fuel reservoir 7. According to the invention, the mechanical pressure relief valve can be dispensed with because its function is performed entirely by the pressure regulating valve 19.

(10) The operation of the internal combustion engine 1 is defined by an electronic control unit 21 which is preferably in the form of an engine control unit (ECU) of the internal combustion engine 1. The electronic control unit 21 comprises the conventional constituent parts of a microcomputer system, for example a microprocessor, I/O components, buffers and memory components (EEPROM, RAM). The operating data relevant for the operation of the internal combustion engine 1 are stored in the memory components in the form of characteristic maps/characteristic curves. Using these, the electronic control unit 21 calculates output variables from the input variables. In FIG. 1, the following input variables are illustrated by way of example: a measured, still-unfiltered high pressure p, which prevails in the high-pressure accumulator 13 and which is measured by way of a high-pressure sensor 23, a present engine speed n.sub.I, a signal FP relating to the power demanded by an operator of the internal combustion engine 1, and an input variable E. The input variable E preferably encompasses further sensor signals, for example a charge-air pressure of an exhaust-gas turbocharger. In the case of an injection system 3 with individual accumulators 17, an individual-accumulator pressure p.sub.E is preferably an additional input variable of the control unit 21.

(11) As output variables of the electronic control unit 21, FIG. 1 illustrates, by way of example, a signal PWMSD for the actuation of the suction throttle 9 as first pressure setting element, a signal ve for the actuation of the injectors 15, said signal predefining in particular a start of injection and/or an end of injection or else an injection duration, a signal PWMDRV for the actuation of the pressure regulating valve 19 as a second pressure setting element, and an output variable TA. By way of the preferably pulse-width-modulated signal PWMDRV, the position of the pressure regulating valve 19 and thus the high-pressure disturbance variable VDRV are defined. The output variable A represents further control signals for the control and/or regulation of the internal combustion engine 1, for example a control signal for the activation of a second exhaust-gas turbocharger in the case of a sequential supercharging arrangement.

(12) FIG. 2 is a first schematic illustration of an embodiment of the method. A first high-pressure regulating loop 25 is provided, by way of which, in a normal operating mode of the injection system 3, the high pressure in the high-pressure accumulator 13 is regulated by means of the suction throttle 9 as first pressure setting element. The first high-pressure regulating loop 25 will be discussed in more detail in conjunction with FIG. 7, where it is presented in detail. The first high-pressure regulating loop 25 has, as an input variable, a setpoint high pressure p.sub.S for the injection system 3. Said setpoint high pressure is preferably read out from a characteristic map in a manner dependent on an engine speed of the internal combustion engine 1, a load or torque demand on the internal combustion engine 1, and/or in a manner dependent on further variables, which serve in particular for correction purposes. Further input variables of the first high-pressure regulating loop 25 are in particular a measured engine speed in of the internal combustion engine 1 and a setpoint injection quantity Q.sub.S, which is in particular likewise read out from a characteristic map. As an output variable, the first high-pressure regulating loop 25 has, in particular, the high pressure p measured by the high-pressure sensor 23, said high pressure preferably being subjected to a first filtering with a relatively long time constant in order to determine the actual high pressure p.sub.I, wherein said high pressure is preferably simultaneously subjected to a second filtering with a relatively short time constant in order to calculate a dynamic rail pressure p.sub.dyn. Said two pressure values p.sub.I, p.sub.dyn constitute further output variables of the first high-pressure regulating loop 25.

(13) FIG. 2 illustrates the actuation of the pressure regulating valve 19. It is preferably the case that a first switching element 27 is provided by way of which a switchover between the normal operating mode and a first operation type of a protective operating mode can be performed in a manner dependent on a first logic signal SIG1. The switching element 27 is preferably realized entirely on an electronic or software level. Here, the functionality described below is preferably switched over in a manner dependent on the value of a variable corresponding to the first logic signal SIG1, which variable is in particular in the form of a so-called flag and can assume the values true or false. It is however self-evidently alternatively also possible for the switching element 27 to be in the form of a physical switch, for example a relay. Said switch can then be switched for example in a manner dependent on a level of an electrical signal. In the case of the specific embodiment illustrated here, the normal operating mode is set if the first logic signal SIG1 has the value false. By contrast, the first operation type of the protective operating mode is set if the first logic signal SIG1 has the value true.

(14) A second switching element 29 is provided which is set up for switching the actuation of the pressure regulating valve 19 from the normal function to the standstill function and back. Here, the second switching element 29 is controlled in a manner dependent on a second logic signal SIG2 or in a manner dependent on the value of a corresponding variable. The second switching element 29 may be in the form of a virtual, in particular software-based switching element which switches between the normal function and the standstill function in a manner dependent on the value of a variable which is in particular in the form of a flag. It is however alternatively also possible for the second switching element to be in the form of a physical switch, for example a relay, which switches in a manner dependent on a signal value of an electrical signal. In the specific embodiment illustrated here, the second logic signal SIG2 corresponds to a state variable which can assume the values 1 for a first state and 2 for a second state. Here, the normal function for the pressure regulating valve is set if the second logic signal SIG2 assumes the value 2, wherein the standstill function is set if the second logic signal SIG2 assumes the value 1. It is self-evidently possible for the second logic signal SIG2 to be defined differently, in particular such that a corresponding variable can assume the values 0 and 1.

(15) Firstly, a description will be given of the actuation of the pressure regulating valve 19 in the normal operating mode and in the case of the normal function having been set. A calculation element 31 is provided which outputs a calculated setpoint volume flow V.sub.S,ber as an output variable, wherein the present engine speed n.sub.I, the setpoint injection quantity Q.sub.S, the setpoint high pressure p.sub.S, the dynamic rail pressure p.sub.dyn and the actual high pressure p.sub.I are input as input variables into the calculation element 31. The functioning of the calculation element 31 is described in detail in the German patents DE 10 2009 031 528 B3 and DE 10 2009 031 527 B3. Here, it is shown in particular that, in a low-load range, for example during idle operation of the internal combustion engine 1, a positive value is calculated for a steady-state setpoint volume flow, whereas a steady-state setpoint volume flow of 0 is calculated in a normal operating range. The steady-state setpoint volume flow is preferably corrected by adding a dynamic setpoint volume flow, which in turn is calculated by way of a dynamic correction in a manner dependent on the setpoint high pressure p.sub.S, the actual high pressure p.sub.I and the dynamic rail pressure p.sub.dyn. The calculated setpoint volume flow V.sub.S,ber is finally the sum of the steady-state setpoint volume flow and the dynamic setpoint volume flow. The calculated setpoint volume flow V.sub.S,ber is thus a resultant setpoint volume flow.

(16) In the normal operating mode, when the first logic signal SIG1 has the value false, the calculated setpoint volume flow V.sub.S,ber is transmitted as setpoint volume flow V.sub.S to a pressure regulating valve characteristic map 33. Here, as described in the German patent DE 10 2009 031 528 B3, the pressure regulating valve characteristic map 33 replicates an inverse characteristic of the pressure regulating valve 19. An output variable of said characteristic map is a pressure regulating valve setpoint current I.sub.S; input variables are the setpoint volume flow V.sub.S to be discharged and also the actual high pressure p.sub.I.

(17) In an alternative embodiment of the method, it is also possible for the setpoint volume flow V.sub.S not to be calculated by way of the calculation element 31 but to be predefined as a constant in the normal operating mode.

(18) The pressure regulating valve setpoint current I.sub.S is fed to a current regulator 35 which has the task of regulating the current for the actuation of the pressure regulating valve 19. Further input variables of the current regulator 35 are for example a proportional coefficient kp.sub.I,DRV and an ohmic resistance R.sub.I,DRV of the pressure regulating valve 19. An output variable of the current regulator 35 is a setpoint voltage U.sub.S for the pressure regulating valve 19, which setpoint voltage is, in relation to an operating voltage U.sub.B, converted in conventional fashion into an activation duration for the pulse-width-modulated signal PWMDRV for the actuation of the pressure regulating valve 19, and is fed to said pressure regulating valve in the normal function, that is to say when the second logic signal SIG2 has the value 2. For the current regulation, the current at the pressure regulating valve 19 is measured as current variable I.sub.DRV, filtered in a current filter 37 and supplied as a filtered actual current Ii to the current regulator 35 again.

(19) As already indicated, the activation duration PWMDRV of the pulse-width-modulated signal is, for the actuation of the pressure regulating valve 19, calculated in a conventional manner from the setpoint voltage U.sub.S and the operating voltage U.sub.B in accordance with the following equation:
PWMDRV=(U.sub.S/U.sub.B)100.(1)

(20) In this way, in the normal operating mode, a high-pressure disturbance variable, specifically the discharged setpoint volume flow V.sub.S, is generated by way of the pressure regulating valve 19 as second pressure setting element.

(21) If the first logic signal SIG1 assumes the value true, the switching element 27 switches over from the normal operating mode to the first operation type of the protective operating mode. The conditions under which this is performed will be discussed in conjunction with FIG. 3. With regard to the actuation of the pressure regulating valve 19, there is no difference in the first operation type of the protective operating mode, because it is also the case here that the pressure regulating valve 19 is actuated with the setpoint volume flow V.sub.S, in any case for as long as the normal function is set by way of the switching element 29. In this respect, in FIG. 2, to the right of the switching element 27, there is no change in relation to the explanations given above. However, the setpoint volume flow V.sub.S is calculated differently in the first operation type of the protective operating mode than in the normal operating mode, specifically by way of a second high-pressure regulating loop 39.

(22) In this case, the setpoint volume flow V.sub.S is set to be identical to a limited output volume flow V.sub.R of a pressure regulating valve pressure regulator 41. This corresponds to the upper switch position of the switch element 27. The pressure regulating valve pressure regulator 41 has, as an input variable, a high-pressure regulating deviation e.sub.p which is calculated as the difference between the setpoint high pressure p.sub.S and the actual high pressure p.sub.I. Further input variables of the pressure regulating valve pressure regulator 41 are preferably a maximum volume flow V.sub.max for the pressure regulating valve 19, the setpoint volume flow V.sub.S,ber calculated in the calculation element 31, and/or a proportional coefficient kp.sub.DRV. The pressure regulating valve pressure regulator 41 is preferably implemented as a PI(DT.sub.1) algorithm which will be discussed in more detail in FIG. 6. Here, as will be discussed further, an integrating component (I component) is, at the time at which the switching element 27 is switched over from its lower switch position illustrated in FIG. 2 into its upper switch position, initialized with the calculated setpoint volume flow V.sub.S,ber. The I component of the pressure regulating valve pressure regulator 41 is upwardly limited to the maximum volume flow V.sub.max for the pressure regulating valve 19. Here, the maximum volume flow V.sub.max is preferably an output variable of a two-dimensional characteristic curve 43 which has the maximum volume flow passing through the pressure regulating valve 19 as a function of the high pressure, wherein the characteristic curve 43 receives the actual high pressure p.sub.I as input variable. An output variable of the pressure regulating valve pressure regulator 41 is an unlimited volume flow V.sub.U which is limited to the maximum volume flow V.sub.max in a limitation element 45. The limitation element 45 finally outputs, as output variable, the limited setpoint volume flow V.sub.R. Using this as setpoint volume flow V.sub.S, the pressure regulating valve 19 is then actuated by virtue of the setpoint volume flow V.sub.S being supplied, in the manner already described, to the pressure regulating valve characteristic map 33.

(23) FIG. 3 shows the conditions under which the first logic signal SIG1 assumes the values true and false. For as long as the dynamic rail pressure p.sub.dyn does not reach or overshoot a first pressure threshold value p.sub.G1, the output of a first comparator element 47 has the value false. Upon starting of the internal combustion engine 1, the value of the first logic signal SIG1 is initialized with false. In this way, the output of a first OR element 49 is also false for as long as the output of the first comparator element 47 has the value false. The output of the first OR element 49 is supplied to an input of a first AND element 51, to the other input of which the negative, indicated by a horizontal dash, of a variable MS is supplied, wherein the variable MS has the value true if the internal combustion engine 1 is at a standstill and has the value false when the internal combustion engine 1 is running. Accordingly, during the operation of the internal combustion engine, the value of the negative of the variable MS is true. Altogether, it is now the case that the output of the AND element 51 and thus the value of the first logic signal SIG1 is false for as long as the dynamic rail pressure p.sub.dyn does not reach or overshoot the first pressure threshold value p.sub.G1.

(24) If the dynamic rail pressure p.sub.dyn reaches or overshoots the first pressure threshold value p.sub.G1, the output of the first comparator element 47 changes from false to true. Thus, the output of the first OR element 49 also changes from false to true. When the internal combustion engine 1 is running, the output of the first AND element 51 also changes from false to true, such that the value of the first logic signal SIG1 becomes true. Said value is supplied to the first OR element 49 again, which however does not change the fact that the output thereof remains true. Even a drop of the dynamic rail pressure p.sub.dyn to below the first pressure threshold value p.sub.G1 can no longer change the logic value of the first logic signal SIG1. Said value rather remains true until the variable MS and thus also the negative thereof changes its logic value, specifically when the internal combustion engine 1 is no longer running.

(25) The following is thus the case: the normal operating mode is realized for as long as the dynamic rail pressure p.sub.dyn lies below the threshold value p.sub.G1. In this case, the setpoint volume flow V.sub.S is identical to the calculated setpoint volume flow V.sub.S,ber, because the first logic signal SIG1 assumes the value false, and thus the switching element 27 is arranged in its lower position in FIG. 2. If the dynamic rail pressure p.sub.dyn reaches or overshoots the threshold value p.sub.G1, the first logic signal SIG1 assumes the value true, and the switching element 27 assumes its upper switch position. Therefore, in this case, the setpoint volume flow V.sub.S is identical to the limited volume flow V.sub.R of the second high-pressure regulating loop 39. This means that, in the normal operating mode, a high-pressure disturbance variable is generated by way of the pressure regulating valve 19, wherein, in the first operation type of the protective operating mode, whenever the dynamic rail pressure p.sub.dyn reaches the first pressure threshold value p.sub.G1, the high pressure is subsequently regulated by the pressure regulating valve pressure regulator 41 until it is identified that the internal combustion engine 1 is at a standstill, because it is only in this case that the variable MS assumes the value true, the negative thereof thus assumes the value false and thus, ultimately, the first logic signal SIG1 assumes the value false again, whereby the switching element 27 is moved into its lower switch position again.

(26) It is after all the case that, in the first operation type of the protective operating mode, the pressure regulating valve 19 performs the regulation of the high pressure by way of the second high-pressure regulating loop 39.

(27) Returning to FIG. 2, the second operation type of the protective operating mode will be discussed below: a switch is made to the second operation type if, here, the second logic signal SIG2 assumes the value 1. In this case, the second switching element 29 is arranged in its upper switching position illustrated in FIG. 2, wherein, in this way, a standstill function for the pressure regulating valve 19 is set. In said standstill function, the pressure regulating valve 19 is not actuated, that is to say the signal PWMDRV is set to 0. Since a pressure regulating valve 19 which is open when deenergized is preferably used, said pressure regulating valve now constantly discharges a maximum fuel volume flow from the high-pressure accumulator 13 into the fuel reservoir 7.

(28) By contrast, if the second logic signal SIG2 has the value 2, it is the case, as already discussed, that the normal function for the pressure regulating valve 19 is set, and said pressure regulating valve is actuated by means of the setpoint volume flow V.sub.S and the signal PWMDRV calculated therefrom.

(29) FIG. 4 schematically shows a state change diagram for the pressure regulating valve 19 from the normal function into the standstill function and vice versa. Here, the pressure regulating valve 19 is preferably designed so as to be closed when unpressurized and deenergized, wherein said pressure regulating valve is furthermore designed so as to be closed when a pressure up to an opening pressure value prevails on the inlet side, wherein said pressure regulating valve opens if the pressure prevailing on the inlet side reaches or overshoots the opening pressure value in the deenergized state. The opening pressure value may for example be 850 bar.

(30) In FIG. 4, a first circle K1 symbolizes the standstill function, wherein, at the top right, a second circle K2 symbolizes the normal function. A first arrow P1 represents a transition between the standstill function and the normal function, wherein a second arrow P2 illustrates a transition between the normal function and the standstill function. A third arrow P3 indicates an initialization of the internal combustion engine 1 after starting, wherein the pressure regulating valve 19 is firstly initialized in the standstill function. Only when it is identified that the internal combustion engine 1 is running and, at the same time, the actual high pressure p.sub.I overshoots a starting value p.sub.St is the normal function set for the pressure regulating valve 19along the arrow P1and the standstill function reset. The normal function is reset, and the standstill function set along the arrow P2, if the dynamic rail pressure p.sub.dyn overshoots a second pressure threshold value p.sub.G2, or if a defect of a high-pressure sensorillustrated in this case by a logic variable HDSDis identified or if it is identified that the internal combustion engine 1 is at a standstill. In the standstill function, the pressure regulating valve 19 is not actuated, wherein, in the normal functionas discussed in conjunction with FIG. 2said pressure regulating valve is actuated by means of the setpoint volume flow V.sub.S.

(31) The following functionality is now realized: upon starting of the internal combustion engine 1, it is initially the case that high pressure does not prevail in the high-pressure accumulator 13, and the pressure regulating valve 19 is arranged in its standstill function, such that it is unpressurized and deenergized, that is to say closed. During the running-up of the internal combustion engine 1, it is thus possible for a high pressure to be rapidly built up in the high-pressure accumulator, which high pressure at some point exceeds the starting value p.sub.St. Said starting value is preferably lower than the opening pressure value of the pressure regulating valve 19, such that, for said pressure regulating valve, the normal function is firstly set before said pressure regulating valve opens. In this way, it is advantageously ensured that the pressure regulating valve 19 is actuated every time it first opens. Since said pressure regulating valve is closed when unpressurized, it remains closed even when actuated until the actual high pressure p.sub.I also overshoots the opening pressure value, wherein said pressure regulating valve then opens and is actuated in the normal function, specifically either in the normal operating mode or in the first operation type of the protective operating mode.

(32) However, if one of the above-described situations arises, it is in turn the case that the standstill function for the pressure regulating valve 19 is set.

(33) This is the case in particular if the dynamic rail pressure p.sub.dyn overshoots the second pressure threshold value p.sub.G2, wherein said second pressure threshold value is preferably selected to be higher than the first pressure threshold value p.sub.G1, and has in particular a value at which, in the case of a conventional embodiment of the injection system, a mechanical pressure relief valve would open. Since the pressure regulating valve 19 is open under the action of pressure and when deenergized, said pressure regulating valve in this case opens fully in the standstill function and thus safely and reliably ensures the function of a pressure relief valve.

(34) The transition from the normal function to the standstill function also takes place if a defect in the high-pressure sensor 23 is detected. If a defect is present here, it is no longer possible for the high pressure in the high-pressure accumulator 13 to be regulated. In order that the internal combustion engine 1 can nevertheless still be operated safely, the transition from the normal function to the standstill function is effected for the pressure regulating valve 19, such that said pressure regulating valve opens and thus prevents an inadmissible rise of the high pressure.

(35) Furthermore, the transition from the normal function into the standstill function is performed in a situation in which it is detected that the internal combustion engine 1 is at a standstill. This corresponds to a resetting of the pressure regulating valve 19, such that, upon a restart of the internal combustion engine 1, the cycle described here can begin again from the start.

(36) If, for the pressure regulating valve 19, under the action of pressure in the high-pressure accumulator 13, the standstill function is set, said pressure regulating valve is opened to the maximum extent and discharges a maximum volume flow from the high-pressure accumulator 13 into the fuel reservoir 7. This corresponds to a protective function for the internal combustion engine and the injection system 3, wherein said protective function can in particular replace the absence of a mechanical pressure relief valve.

(37) It is essential here that the pressure regulating valve 19 hasby contrast to the prior artonly two states, specifically the standstill function and the normal function, wherein said two states are entirely sufficient to replicate the entire relevant functionality of the pressure regulating valve 19 including the protective function for replacing a mechanical pressure relief valve.

(38) FIG. 5 is a schematic illustration of the pressure regulating valve pressure regulator 41, which in this case is in the form of a PI(DT.sub.1) pressure regulator. Here, it can be seen that the output variable V.sub.U of the pressure regulating valve pressure regulator 41 is composed of three added-together regulator components, specifically a proportional component A.sub.P, an integral component A.sub.I and a differential component A.sub.DTI. Said three components are added together at a summing junction 53 to form the unlimited volume flow V.sub.U. Here, the proportional component A.sub.P represents the product of the regulating deviation e.sub.p, multiplied at a multiplication junction 55 by the value 1, with the proportional coefficient kp.sub.DRV. The integrating component A.sub.I results from the sum of two summands. The first summand is in this case the present integral component A.sub.I delayed by a sampling step T.sub.a. The second summand is the product of a gain factor r2.sub.DRV and the sum of the present regulating deviation e.sub.p and of said regulating deviation delayed by one sampling stepagain multiplied at the multiplication junction 55 by the factor 1. The sum of the two summands is in this case limited upwardly to the maximum volume flow V.sub.max in a limitation element 57. The gain factor r2.sub.DRV is calculated in accordance with the following formula, in which tnD.sub.RV is a reset time:

(39) $\begin{array}{cc}r{2}_{\mathrm{DRV}}=\frac{64{\mathrm{kp}}_{\mathrm{DRV}}{T}_a}{{\mathrm{tn}}_{\mathrm{DRV}}}.& \left(2\right)\end{array}$

(40) The integrating component A.sub.I is dependent on whether the dynamic rail pressure p.sub.dyn has reached the first pressure threshold value p.sub.G1 for the first time after the starting of the internal combustion engine 1. If this is the case, the first logic signal SIG1 assumes the value true, and a switching element 59 illustrated in FIG. 5 switches into its lower switch position. In said switch position, the integrating component A.sub.I is identical to the output signal of the limitation element 57, that is to say the integrating component A.sub.I is limited to the maximum volume flow V.sub.max. If it is identified that the internal combustion engine 1 is at a standstill, it is the caseas already discussed in conjunction with FIG. 3that the first logic signal SIG1 assumes the value false, and the switching element 59 switches into its upper switch position. The integrating component A.sub.I is in this case set to the calculated volume flow V.sub.S,ber. Thus, the calculated setpoint volume flow V.sub.S,ber constitutes the initialization value of the integrating component A.sub.I for the situation in which the pressure regulating valve pressure regulator 41 is activated when the dynamic rail pressure p.sub.dyn overshoots the first pressure threshold value psi.

(41) The calculation of the differential component A.sub.DTI is illustrated in the lower part of FIG. 5. Said component is formed as the sum of two products. The first product results from a multiplication of the factor r4.sub.DRV with the differential fraction A.sub.DTI delayed by one sampling step. The second product is formed from the multiplication of the factor r3.sub.DRV with the difference between the regulating deviation e.sub.p multiplied by the factor 1 and the corresponding regulating deviation e.sub.p delayed by one sampling step and multiplied by the factor 1.

(42) Here, the factor r3.sub.DRV is calculated in accordance with the following equation, in which tv.sub.DRV is a lead time and t1.sub.DRV is a lag time:

(43) $\begin{array}{cc}r{3}_{\mathrm{DRV}}=\frac{2{\mathrm{kp}}_{\mathrm{DRV}}{\mathrm{tv}}_{\mathrm{DRV}}}{2t{1}_{\mathrm{DRV}}+T_a}.& \left(3\right)\end{array}$

(44) The factor r4.sub.DRV is calculated in accordance with the following equation:

(45) $\begin{array}{cc}r{4}_{\mathrm{DRV}}=\frac{2t{1}_{\mathrm{DRV}}-T_a}{2t{1}_{\mathrm{DRV}}+T_a}.& \left(4\right)\end{array}$

(46) It is thus evident that the gain factors r2.sub.DRV and r3.sub.DRV are dependent on the proportional coefficient kp.sub.DRV. The gain factor r2.sub.DRV is additionally dependent on the reset time tn.sub.DRV, the gain factor r3.sub.DRV is additionally dependent on the lead time tv.sub.DRV and on the lag time t1.sub.DRV. The gain factor r4.sub.DRV is likewise dependent on the lag time t1.sub.DRV.

(47) FIG. 6 is a schematic illustration of a logic arrangement for the calculation of the value of a third logic signal SIG3 which is used to ensure that, here, in the first and in the second operation types of the protective operating mode, the suction throttle 9 is actuated for permanently open operation. This approach will be discussed in more detail in conjunction with FIG. 7. The value of the third logic signal SIG3 results from a second AND element 61, into the first output of which it is again the case that the negative of the variable MS is input, wherein the result of a prior calculation that will be discussed in more detail below is input into the second input. The third logic signal SIG3 is, upon the starting of the internal combustion engine 1, firstly initialized with the value false. Into the first input of a second OR element 63 there is input the result of a second comparator element 65, in which it is checked whether the dynamic rail pressure p.sub.dyn is greater than or equal to the first pressure threshold value p.sub.G1. Into the second input of the second OR element 63 there is input the result of a comparison element 67 which checks whether the value of the logic variable HDSD, which indicates a sensor defect of the high-pressure sensor 23, is equal to 1, wherein, in this case, a sensor defect is present, and wherein no sensor defect is present if the value of the variable HDSD is equal to 0. It is thus evident that the output of the second OR element 63 assumes the value true if at least one of the outputs of the second comparator element 65 or of the comparison element 67 assumes the value true. Thus, in order for the output of the second OR element 63 to assume the value true, at least one of the following conditions must be met: the dynamic rail pressure p.sub.dyn must have reached or overshot the first pressure threshold value p.sub.G1, and/or a sensor defect in the high-pressure sensor 23 must have been detected, such that the variable HDSD assumes the value 1. If neither of said conditions is met, the output of the second OR element 63 has the value false.

(48) The output of the second OR element 63 is input into a first input of a third OR element 69, into the second input of which the value of the third logic signal SIG3 is input. Since said third logic signal is originally initialized with the value false, the output of the third OR element 69 has the value false until the output of the second OR element 63 assumes the value true. If this is the case, the output of the third OR element 69 also changes to the value true. In this case, the value of the second AND element 61 also changes to true if the internal combustion engine 1 is running, such that the value of the third logic signal SIG3 also changes to true. It is evident from FIG. 6 that the value of the third logic signal SIG3 remains true until it is identified that the internal combustion engine 1 is at a standstill, wherein, in this case, the variable MS assumes the value true, and thus the negative thereof assumes the value false.

(49) If, alternatively, it is sought for the suction throttle 9 to be permanently open only in the second operation type of the protective operating mode, this can be achieved by virtue of the second pressure threshold value p.sub.G2 instead of the first pressure threshold value p.sub.G1 being used in the second comparator element 65 and being compared with the dynamic rail pressure p.sub.dyn.

(50) FIG. 7 is a schematic illustration of the first high-pressure regulating loop 25 including a switching element 71 for realizing the permanently open operation of the suction throttle 9 in the first and second operation types of the protective operating mode, wherein the third logic signal SIG3, the calculation of which has been described in conjunction with FIG. 6, is input into the switching element 71 for the actuation thereof. It is possible for the switching element 71 to be in the form of a software switch, that is to say in the form of a purely virtual switch, as has already been described in conjunction with the switching elements 27, 29. Alternatively, it is self-evidently also possible for the switching element 71 to be in the form of a physical switch, for example a relay.

(51) As has already been discussed, an input variable of the high-pressure regulating loop 25 is the setpoint high pressure p.sub.S which, for the calculation of the regulating deviation e.sub.p, is compared with the actual high pressure p.sub.I. Said regulating deviation e.sub.p is an input variable of a high-pressure regulator 73, which is preferably implemented as a PI(DT.sub.1) algorithm. A further input variable of the high-pressure regulator 73 is preferably a proportional coefficient kp.sub.SD. An output variable of the high-pressure regulator 73 is a fuel volume flow V.sub.SD for the suction throttle 9, to which, at a summing junction 75, a fuel setpoint consumption V.sub.Q is added. Said fuel setpoint consumption V.sub.Q is calculated in a calculation element 77 in a manner dependent on the engine speed n.sub.I and the setpoint injection quantity Q.sub.S, and constitutes a disturbance variable of the first high-pressure regulating loop 25. A sum of the output variable V.sub.SD of the high-pressure regulator 73 and of the disturbance variable V.sub.Q yields an unlimited fuel setpoint volume flow V.sub.U,SD. This is, in a limitation element 79, limited in a manner dependent on the engine speed n.sub.I to a maximum volume flow V.sub.max,SD for the suction throttle 9. An output of the limitation element 79 is a limited fuel setpoint volume flow V.sub.S,SD for the suction throttle 9, this being input as an input variable into a pump characteristic curve 81. The latter converts the limited fuel setpoint volume flow V.sub.S,SD into a characteristic curve suction throttle current I.sub.KL,SD.

(52) If the switch element 71 is in the upper switching state illustrated in FIG. 7, which is the case if the third logic signal SIG3 has the value false, a suction throttle setpoint current I.sub.S,SD is set equal to the characteristic curve suction throttle current I.sub.KL,SD. Said suction throttle setpoint current I.sub.S,SD constitutes the input variable of a suction throttle current regulator 83 which has the task of regulating the suction throttle current through the suction throttle 9. A further input variable of the suction throttle current regulator 83 is, inter alia, an actual suction throttle current I.sub.I,SD. An output variable of the suction throttle current regulator 83 is a suction throttle setpoint voltage U.sub.S,SD which is finally, in a calculation element 85, converted in a manner known per se into an activation duration of a pulse-width-modulated signal PWMSD for the suction throttle 9. The suction throttle is actuated using said signal, wherein the signal thus acts overall on a regulating path 87 which has in particular the suction throttle 9, the high-pressure pump 11 and the high-pressure accumulator 13. The suction throttle current is measured, wherein the result is an unprocessed measurement value I.sub.R,SD which is filtered in a current filter 89. The current filter 89 is preferably in the form of a PT.sub.1 filter. An output variable of said filter is the actual suction throttle current I.sub.I,SD, which in turn is supplied to the suction throttle current regulator 83.

(53) The regulating variable of the first high-pressure regulating loop 25 is the high pressure in the high-pressure accumulator 13. Unprocessed values of said high pressure p are measured by way of the high-pressure sensor 23 and filtered by way of a first high-pressure filter element 91, which, as output variable, has the actual high pressure p.sub.I. Furthermore, the unprocessed values of the high pressure p are filtered by way of a second high-pressure filter element 93, the output variable of which is the dynamic rail pressure p.sub.dyn. Both filters are preferably implemented by way of a PT.sub.1 algorithm, wherein a time constant of the first high-pressure filter element 91 is greater than a time constant of the second high-pressure filter element 93. In particular, the second high-pressure filter element 93 is configured so as to be a faster filter than the first high-pressure filter element 91. The time constant of the second high-pressure filter element 93 may also be identical to the value zero, such that then, the dynamic rail pressure p.sub.dyn corresponds to, or is identical to, the measured unprocessed values of the high pressure p. Thus, with the dynamic rail pressure p.sub.dyn, a highly dynamic value for the high pressure is available, which is in particular required whenever a fast reaction to certain occurring events is necessary.

(54) Output variables of the first high-pressure regulating loop are thus, aside from the unfiltered high pressure p, the filtered high-pressure values p.sub.I, p.sub.dyn.

(55) If the third logic signal SIG3 assumes the value true, the switching element 71 switches into its lower switching position illustrated in FIG. 7. In this case, the suction throttle setpoint current I.sub.S,SD is no longer identical to the characteristic curve suction throttle current I.sub.KL,SD, but rather is set equal to a suction throttle emergency current I.sub.N,SS. The suction throttle emergency current I.sub.N,SD preferably has a predetermined constant value, for example 0 A, wherein then, the suction throttle 9, which is preferably open when deenergized, is opened to a maximum extent, or said suction throttle emergency current has a low current value in relation to a maximum closed position of the suction throttle 9, for example 0.5 A, such that the suction throttle 9 is opened not fully but substantially. Here, the suction throttle emergency current I.sub.N,SD and the associated opening of the suction throttle 9 reliably prevent the internal combustion engine 1 from coming to a standstill when it is operated in the second operation type of the protective operating mode with pressure regulating valve 19 opened to the maximum extent. Here, the opening of the suction throttle 9 has the effect that, even in a medium to low engine speed range, it is still possible for enough fuel to be delivered into the high-pressure accumulator 13 that operation of the internal combustion engine 1 is possible without stalling. In the first operation type, it is achieved in this way that twofold regulation of the high pressure both by way of the suction throttle and by way of the pressure regulating valve is prevented.

(56) Altogether, it is evident that, with the aid of the method, the injection system 3 and the internal combustion engine 1, it is possible for stable pressure regulation to be implemented even if the first high-pressure regulating loop 25 can no longer perform the pressure regulation, wherein it is alternatively or additionally possible to omit a mechanical pressure relief valve, because the functionality thereof is performed by the pressure regulating valve 19.