INJECTION SYSTEM FOR AN INTERNAL COMBUSTION ENGINE AND INTERNAL COMBUSTION ENGINE HAVING SUCH AN INJECTION SYSTEM

Abstract

An injection system for an internal combustion engine including at least one injector and a high-pressure accumulator, which has a fluid connection to the at least one injector on the one side and has a fluid connection to a fuel reservoir via a high-pressure pump on the other side, wherein a suction throttle is associated with the high-pressure pump as a first pressure-setting element. At least two pressure control valves are provided, via which the high-pressure accumulator can be brought into fluid connection with the fuel reservoir.

Claims

1-10. (canceled)

11. An injection system for an internal combustion engine, comprising: at least one injector; a high-pressure accumulator which is fluidically connected at one side to the at least one injector and at another side via a high-pressure pump to a fuel reservoir; a suction throttle assigned to the high-pressure pump as a pressure setting element; and at least two pressure regulating valves by which the high-pressure accumulator is fluidically connectable to the fuel reservoir.

12. The injection system according to claim 11, further comprising a control unit operatively connected to the suction throttle and to the at least two pressure regulating valves, wherein the injection system is configured to, a) in a normal operating mode, regulate a high pressure in the high-pressure accumulator by actuating the suction throttle as the pressure setting element, wherein, at least one first pressure regulating valve of the at least two pressure regulating valves is actuated in order to generate a high-pressure disturbance variable; b) in a first operation type of a protective operating mode, regulate the high pressure in the high-pressure accumulator by actuating the at least one first pressure regulating valve as pressure setting element; and c) in a second operation type of the protective operating mode, actuate at least one second pressure regulating valve of the at least two pressure regulating valves, which differs from the at least one first pressure regulating valve, in addition to the at least one first pressure regulating valve as pressure setting element to regulate the high pressure in the high-pressure accumulator.

13. The injection system according to claim 12, wherein the injection system is configured to, in a third operation type of the protective operating mode, permanently open the at least one first pressure regulating valve and the at least one second pressure regulating valve.

14. The injection system according to claim 13, wherein the injection system is configured to d) switch to the first operation type of the protective operating mode when the high pressure reaches or overshoots a first pressure threshold value or if a defect of the suction throttle is detected, and/or e) switch to the second operation type of the protective operating mode when the high pressure reaches or overshoots a second pressure threshold value, and/or f) switch to the third operation type of the protective operating mode when the high pressure reaches or overshoots a third pressure threshold value or if a defect of a high-pressure sensor is detected.

15. The injection system according to claim 12, wherein the injection system is configured to, in at least one operation type of the protective operating mode, actuate the suction throttle so that the suction throttle assumes a permanently open position.

16. The injection system according to claim 11, wherein at least one of the at least two pressure regulating valves is configured to be open when deenergized.

17. The injection system according to claim 12, wherein the injection system is configured to generate a first actuation signal and a second actuation signal and to actuate the at least one first pressure regulating valve and the at least one second pressure regulating valve alternately with the first actuation signal and the second actuation signal.

18. The injection system according to claim 11, wherein the injection system is free from a mechanical pressure relief valve.

19. An internal combustion engine comprising an injection system according to claim 11.

20. The internal combustion engine according to claim 19, wherein the internal combustion engine is a large engine.

Description

[0066] The invention will be discussed in more detail below on the basis of the drawing, in which:

[0067] FIG. 1 is a schematic illustration of an exemplary embodiment of an internal combustion engine having an injection system;

[0068] FIG. 2 is a first schematic detail illustration of an actuation of the injection system;

[0069] FIG. 3 is a second schematic detail illustration of an actuation of the injection system;

[0070] FIG. 4 is a third schematic detail illustration of an actuation of the injection system;

[0071] FIG. 5 is a fourth schematic detail illustration of an actuation of the injection system;

[0072] FIG. 6 is a fifth schematic detail illustration of an actuation of the injection system;

[0073] FIG. 7 is a sixth schematic detail illustration of an actuation of the injection system;

[0074] FIG. 8 is a seventh schematic detail illustration of an actuation of the injection system; and

[0075] FIG. 9 is an eighth schematic detail illustration of an actuation of the injection system.

[0076] FIG. 1 is a schematic illustration of an exemplary embodiment of an internal combustion engine 1 which has an injection system 3. Said injection system 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. A first, in particular electrically actuable pressure regulating valve 19 is provided, by means of which the high-pressure accumulator 13 is fluidically connected to the fuel reservoir 7. By means of the position of the first 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 VDRV1, and represents a high-pressure disturbance variable of the injection system 3.

[0077] The injection system 3 has a second, in particular electrically actuable pressure regulating valve 20, by means of which the high-pressure accumulator 13 is likewise fluidically connected to the fuel reservoir 7. The two pressure regulating valves 19, 20 are accordingly in particular arranged fluidically in parallel with respect to one another. By means of the second pressure regulating valve 20, too, a fuel volume flow can be defined which can be discharged from the high-pressure accumulator 13 into the fuel reservoir 7. Said fuel volume flow is denoted in FIG. 1 and in the following text by VDRV2.

[0078] The injection system 3 has no mechanical pressure relief valve, such as is commonly provided in the prior art so as to then 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 valves 19, 20.

[0079] It is possible for the injection system 3 to have more than two pressure regulating valves 19, 20. For the sake of a simpler illustration, however, the functioning of the injection system 1 according to the invention will be discussed below on the basis of the exemplary embodiment illustrated here, which has exactly two pressure regulating valves 19, 20.

[0080] 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 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 means of a high-pressure sensor 23, a present engine speed 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.

[0081] 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 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 first signal PWMDRV1 for the actuation of a first pressure regulating valve of the two pressure regulating valves 19, 20 and a second signal PWMDRV2 for the actuation of a second pressure regulating valve of the two pressure regulating valves 19, 20. As will be discussed in more detail below, the assignment, illustrated in FIG. 1, of the first signal PWMDRV1 to the first pressure regulating valve 19 and of the second signal PWMDRV2 to the second pressure regulating valve 20 is not fixed at all times, it rather being the case that the pressure regulating valves 19, 20 are preferably actuated with the signals PWMDRV1, PWMDRV2 alternately. The signals PWMDRV1, PWMDRV2 are preferably pulse-width-modulated signals by means of which the position of a pressure regulating valve 19, 20 and thus the volume flow VDRV1, VDRV2 respectively associated with the pressure regulating valve 19, 20 can be defined. Also illustrated in FIG. 1 is an output variable A, which 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.

[0082] FIG. 2 is a first schematic illustration of an embodiment of the method. Here, below, the functioning of the method will firstly be discussed for the actuation of only one of the pressure regulating valves 19, 20, wherein the functionality that is added through the addition of a further pressure regulating valve 20, 19 will then be discussed in a next step. A first high-pressure regulating loop 25 is provided, by means 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 pressure setting element. The first high-pressure regulating loop 25 will be discussed in more detail in conjunction with FIG. 9, 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 the engine speed n.sub.I 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 the engine speed n.sub.I 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 an 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.

[0083] FIG. 2 illustrates in particular the actuation of a first pressure regulating valve of the two pressure regulating valves 19, 20, for example the actuation of the first pressure regulating valve 19. It is preferably the case that a first switching element 27 is provided by means 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 a 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 operation type of the protective operating mode is set if the first logic signal SIG1 has the value true.

[0084] A second switching element 29 is provided which is set up for switching the first actuation signal PWMDRV1 between two modes, wherein in particular, a pressure regulating valve 19, 20 that is actuated with the first actuation signal PWMDRV1 can be switched from a normal function to a standstill function and back. Here, the second switching element 29 is controlled in a manner dependent on a second logic signal Z 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 Z 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 actuated pressure regulating valve 19, 20 is set if the second logic signal Z assumes the value 2, wherein the standstill function is set if the second logic signal Z assumes the value 1. It is self-evidently possible for the second logic signal Z to be defined differently, in particular such that a corresponding variable can assume the values 0 and 1.

[0085] Firstly, a description will be given of the actuation of a first pressure regulating valve 19, 20 in the normal operating mode and in the case of the normal function having been set. A first 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 first 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 means 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.

[0086] 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 a pressure regulating valve 19, 20 that is used. In a preferred embodiment, the injection system has identical pressure regulating valves 19, 20, such that the same pressure regulating valve characteristic map 33 can be used for each of the pressure regulating valves 19, 20. It is however alternatively also possible to use different pressure regulating valves 19, 20, wherein then, for each pressure regulating valve 19, 20, a pressure regulating valve characteristic map separately assigned thereto is used. An output variable of the pressure regulating valve characteristic map 33 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.

[0087] In an alternative embodiment of the method, it is also possible for the setpoint volume flow V.sub.S not to be calculated by means of the first calculation element 31 but to be predefined as a constant in the normal operating mode.

[0088] The pressure regulating valve setpoint current is fed to a first current regulator 35 which has the task of regulating the current for the actuation of the pressure regulating valve 19, 20. Further input variables of the first 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, 20. An output variable of the first current regulator 35 is a first setpoint voltage U.sub.S for the pressure regulating valve 19, 20, which setpoint voltage is, in relation to an operating voltage U.sub.B, converted in conventional fashion into an activation duration for the first, pulse-width-modulated signal PWMDRV1 for the actuation of the pressure regulating valve 19, 20, and is fed to said pressure regulating valve in the normal function, that is to say when the second logic signal Z has the value 2. For the current regulation, the current at the pressure regulating valve 19, 20 actuated with the first actuation signal PWMDRV1 is measured as first current variable I.sub.R, filtered in a first current filter 37 and supplied as a first filtered actual current I.sub.I to the current regulator 35 again.

[0089] As already indicated, the activation duration in the form of the first, pulse-width-modulated actuation signal PWMDRV1 is, for the actuation of a pressure regulating valve 19, 20, calculated in a conventional manner from the first setpoint voltage U.sub.S and the operating voltage U.sub.B in accordance with the following equation:

PWMDRV1=(U.sub.S/U.sub.B)100. (1)

[0090] 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 means of one of the pressure regulating valves 19, 20.

[0091] If the first logic signal SIG1 assumes the value true, the first switching element 27 switches over from the normal operating mode to 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, 20, there is no difference in the first and second operation type of the protective operating mode, because it is also the case here that the pressure regulating valve 19, 20 is actuated with the setpoint volume flow V.sub.S, in any case for as long as the normal function is set by means 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 and second operation type of the protective operating mode than in the normal operating mode, specifically by means of a second high-pressure regulating loop 39.

[0092] In this case, the setpoint volume flow V.sub.S is set to be identical to a limited output volume flow V.sub.R from a pressure regulating valve pressure regulator 41aside from a factor f.sub.DRV discussed in more detail below. This corresponds to the upper switching position of the first switching 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 dynamic rail pressure p.sub.dyn. 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, 20, the setpoint volume flow V.sub.S,ber calculated in the first 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. 7. Here, as will be discussed further, an integrating component (I component) is, at the time at which the first switching element 27 is switched over from its lower switching position illustrated in FIG. 2 into its upper switching 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, 20. Here, the maximum volume flow V.sub.max is preferablyaside from the factor f.sub.DRVan output variable of a two-dimensional characteristic curve 43 which has the maximum volume flow passing through the pressure regulating valve 19, 20 as a function of the high pressure, wherein the characteristic curve 43 receives the dynamic rail pressure p.sub.dyn as input variable. As already indicated, it is assumed in this exemplary embodiment that the pressure regulating valves 19, 20 are of identical form, such that an identical characteristic curve 43 can be used for both pressure regulating valves. It is however also possible for different pressure regulating valves 19, 20 to be used, wherein then, a separate characteristic curve 43 is used for each of the pressure regulating valves 19, 20. The direct 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. Then, using this, aside from the factor f.sub.DRV discussed in more detail below, the pressure regulating valve 19, 20 is actuated as setpoint volume flow V.sub.S 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.

[0093] Accordingly, in this way, in the first operation type of the protective operating mode, an actuation of a pressure regulating valve 19, 20 as pressure setting element in order to regulate the high pressure in the high-pressure accumulator 13 is performed by means of the second high-pressure regulating loop 39.

[0094] The functionality that is added through the addition of a second pressure regulating valve 20, 19 will now be discussed below.

[0095] As will also be discussed in more detail in conjunction with FIG. 3, the first logic signal SIG1 assumes the logic value true if the dynamic rail pressure P.sub.dyn reaches a first pressure threshold value p.sub.G1, for example as a result of a cable breakage of the suction throttle plug connector. As a result, the first switching element 27 switches into the upper switching position illustrated in FIG. 2, such that the high pressure is now regulated by means of the second high-pressure regulating loop 39 and one of the pressure regulating valves 19, 20. As will likewise be discussed in more detail in conjunction with FIG. 3, a third logic signal SIG2 has the value false if the dynamic rail pressure p.sub.dyn has not yet reached a second pressure threshold value p.sub.G2. A second pressure regulating valve setpoint current I.sub.S,2 for a second pressure regulating valve 20, 19 is then read out from a second pressure regulating valve characteristic map 49 which has the actual high pressure p.sub.I and the value zero for the setpoint volume flow as input variables. If the two pressure regulating valves 19, 20 are of identical design, the second pressure regulating valve characteristic map 49 is identical to the first pressure regulating valve characteristic map 33 and differs only with regard to the fact that the setpoint volume flow that is input is set to zero. If different pressure regulating valves 19, 20 are used, the two pressure regulating valve characteristic maps 33, 49 may differ. By virtue of the fact that the second pressure regulating valve characteristic map 49 has the value zero as an input setpoint volume flow, the pressure regulating valve 19, 20 actuated in this way is actuated so as to be fully closed, wherein said pressure regulating valve discharges no fuel into the fuel reservoir 7. The high pressure is thus regulated only by means of one pressure regulating valve 19, 20 until the dynamic rail pressure p.sub.dyn reaches the second pressure threshold value p.sub.G2.

[0096] A fourth switching element 44 is provided which determines the value of the factor f.sub.DRV already mentioned above. Said fourth switching element 44 is likewise controlled in a manner dependent on the third logic signal SIG2, and assumes its lower switching position illustrated in FIG. 2 if the third logic signal SIG2 has the value false. In this case, the output variable of the characteristic curve 43 is multiplied by the factor 1. Correspondingly, the limited setpoint volume flow V.sub.R resulting from the limitation element 45 is divided by the factor 1.

[0097] If the dynamic rail pressure p.sub.dyn rises and reaches or overshoots the second pressure threshold value p.sub.G2, the third logic signal SIG2 assumes the value true. This has the effect that the third switching element 47 and the fourth switching element 44 switch into their upper switching position in FIG. 2. Considering firstly the third switching element 47, it is evident that, as a result, in the specific exemplary embodiment illustrated here, the second pressure regulating valve setpoint current I.sub.S,2 is now identical to the first pressure regulating valve setpoint current I.sub.S, such that both pressure regulating valves 19, 20 are acted on with the same setpoint current as a result. This in turn assumes that the two pressure regulating valves 19, 20 are of identical form, which corresponds to a preferred embodiment. It is however self-evidently possible for said pressure regulating valves to be acted on with separate setpoint currents, resulting in particular from separate characteristic maps, if the pressure regulating valves 19, 20 differ.

[0098] Two identical pressure regulating valves 19, 20 can discharge a doubled fuel quantity in relation to a single pressure regulating valve 19, 20. For this reason, if one now considers the fourth switching element 44, the factor f.sub.DRV now assumes the value 2, whereby the maximum volume flow V.sub.max resulting from the characteristic curve 43 is doubled. By contrast, the limited volume flow V.sub.R resulting from the limitation element 45 is divided by the factor f.sub.DRV and thus in this case by two, because ultimately the resultant pressure regulating valve setpoint volume flow V.sub.S corresponds in each case to one pressure regulating valve 19, 20 and serves in each case for the actuation of one pressure regulating valve 19, 20. This approach, too, is adapted to the preferred embodiment in which the two pressure regulating valves 19, 20 that are used are of identical form. By contrast, if said pressure regulating valves are of different form, it is preferable for different characteristic curves 43, different second high-pressure regulating loops 39, and different pressure regulating valve characteristic maps 33, 49 to be used for the actuation of the various pressure regulating valves. By contrast, if more than two pressure regulating valves of identical form are provided, these may be actuated entirely analogously to the illustration in FIG. 2 by means of a multiplication of the actuating elements illustrated there for each pressure regulating valve 19, 20, wherein the number of pressure regulating valves used can be used as factor f.sub.DRV in the upper switching position of the fourth switching element 44.

[0099] The second pressure regulating valve setpoint current I.sub.S,2 is the input variable of a second current regulator 51, which is otherwise preferably of exactly the same design as the first current regulator 35. The actuation mechanism for generating the second actuation signal PWMDRV2 otherwise corresponds to that for generating the first actuation signal PWMDRV1, wherein here, a fifth switching element 53 is also provided for the switching between the normal function and the standstill function, and wherein, for the filtering of a second, measured current variable I.sub.R,2, a second current filter 55 is provided which has, as output variable, a second actual current I.sub.I,2 which is fed to the second current regulator 51. The regulator parameters of the second current regulator 51 are preferably set in the same way as the corresponding parameters of the first current regulator 35.

[0100] On the basis of the second switching element 29 and the fifth switching element 53, it can also be seen that the activation duration of the actuation signals PWMDRV1, PWMDRV2 is identical to 0% in the standstill function. By contrast, in the normal function, the respective actuation signal PWMDRV1, PWMDRV2 is generated by the actuation mechanism assigned thereto, as has already been discussed above.

[0101] The two actuation signals PWMDRV1, PWMDRV2 are fed to a switchover logic 57 which will be discussed in more detail below in conjunction with FIGS. 5 and 6, wherein the switchover logic 57 ensures that the pressure regulating valves 19, 20 are actuated with the actuation signals PWMDRV1, PWMDRV2 alternately. Likewise, the measured current variables I.sub.R, I.sub.R,2 are also taken from the switchover logic 57, wherein the latter ensures that said current variables are always measured at the respective pressure regulating valves 19, 20 correctly assigned to the actuation signals PWMDRV1, PWMDRV2, in order to ensure defined regulation of each of the pressure regulating valves 19, 20 by means of the current regulators 35, 51.

[0102] FIG. 3 shows the conditions under which the first logic signal SIG1 and the third logic signal SIG2 in each case assume the values true and false.

[0103] This will be discussed below firstly on the basis of FIG. 3a) for the first logic signal SIG1. 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 59 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 61 is also false for as long as the output of the first comparator element 59 has the value false. The output of the first OR element 61 is supplied to an input of a first AND element 63, 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 wherein it has the value false when the internal combustion engine 1 is running. Accordingly, during the operation of the internal combustion engine 1, the value of the negative of the variable MS is true. Altogether, it is now the case that the output of the AND element 63 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. 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 59 changes from false to true. Thus, the output of the first OR element 61 also changes from false to true. When the internal combustion engine 1 is running, the output of the first AND element 63 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 61 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 logical value of the first logic signal SIG1. Said value rather remains true until the variable MS and thus also the negative thereof changes its logical value, specifically when the internal combustion engine 1 is no longer running. 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 first pressure 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. If the dynamic rail pressure p.sub.dyn reaches or overshoots the first pressure threshold value p.sub.G1, the first logic signal SIG1 assumes the value true, and the first switching element 27 assumes its upper switching 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 39aside from the factor f.sub.DRV. This means that, in the normal operating mode, a high-pressure disturbance variable is generated by means of one of the pressure regulating valves 19, 20, 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. In the first operation type of the protective operating mode, at least one of the pressure regulating valves 19, 20 performs the regulation of the high pressure by means of the second high-pressure regulating loop 39.

[0104] FIG. 3b) illustrates the logic for the switching of the third logic signal SIG2. Here, it is evident that this logic corresponds entirely to the logic for the switching of the first logic signal SIG1, it merely being the case that the second pressure threshold value p.sub.G2 rather than the first pressure threshold value p.sub.G1 is used as input variable. The corresponding logic switching components are in this case provided with reference designations with an apostrophe suffix in relation to FIG. 3a). Owing to the entirely identical functioning, reference is made to the explanations relating to FIG. 3a). Analogously to the first logic signal SIG1, the following is the case for the second logic signal SIG2: said second logic signal is, initialized with the value false upon the commencement of operation of the internal combustion engine 1, wherein said second logic signal changes its logical value to true if the dynamic rail pressure p.sub.dyn reaches or overshoots the second pressure threshold value p.sub.G2. The logical value of the third logic signal SIG2 thereupon remains true until it is identified that the internal combustion engine 1 is at a standstill.

[0105] With reference to FIG. 2, it is evident that the second operation type of the protective operating mode is activated if the third logic signal SIG2 changes its logical value from false to true, wherein, in this case, the hitherto inactive pressure regulating valve 20, 19 is activated, such that the high pressure is regulated by both pressure regulating valves 19, 20.

[0106] Returning to FIG. 2, the third operation type of the protective operating mode will also be discussed below: a switch is made to said third operation type if the second logic signal Z assumes the value 1. In this case, the second switching element 29 and also the fifth switching element 53 are placed into their upper switching position illustrated in FIG. 2, wherein, in this way, the standstill function for both pressure regulating valves 19, 20 is set. In said standstill function, the pressure regulating valves 19, 20 are no longer actuated, that is to say the actuation signals PWMDRV1, PWMDRV2 are set to zero. Since pressure regulating valves 19, 20 which are open when deenergized, at least under the action of inlet pressure, are preferably used, said pressure regulating valves now constantly discharge a maximum fuel volume flow from the high-pressure accumulator 13 into the fuel reservoir 7.

[0107] By contrast, if the second logic signal Z has the value 2, it is the case, as already discussed, that the normal function for the pressure regulating valves 19, 20 is set, and said pressure regulating valves are actuated with their respective setpoint currents I.sub.S, I.sub.S,2 and the actuation signals PWMDRV1, PWMDRV2 calculated therefrom.

[0108] FIG. 4 schematically shows a state change diagram for the pressure regulating valves 19, 20 from the normal function into the standstill function and vice versa. Here, the pressure regulating valves 19, 20 are preferably designed so as to be closed when unpressurized and deenergized, wherein said pressure regulating valves are furthermore preferably designed so as to be closed when a pressure up to an opening pressure value prevails on the inlet side, wherein said pressure regulating valves open if the pressure prevailing on the inlet side reaches or overshoots the opening pressure value in the deenergized state. Said pressure regulating valves are then open when deenergized under the action of inlet pressure, and can be actuated toward the closed state by energization. The opening pressure value may for example be 850 bar.

[0109] 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 valves 19, 20 are 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 predetermined starting value p.sub.St is the normal function set for the pressure regulating valves 19, 20along the arrow P1and the standstill function reset, in particular by virtue of the second logic signal Z changing its value from 1 to 2. The normal function is reset, and the standstill function set along the arrow P2, if the dynamic rail pressure p.sub.dyn overshoots the third pressure threshold value p.sub.G3, 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, in which the second logic signal Z again assumes the value 1, the pressure regulating valves 19, 20 are not actuated, wherein, in the normal functionas already discussed in conjunction with FIG. 2said pressure regulating valves are actuated by means of the setpoint currents I.sub.S, I.sub.S,2 respectively assigned thereto.

[0110] 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 valves 19, 20 are arranged in their standstill function, such that they are 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 valves 19, 20, such that, for said pressure regulating valves, the normal function is firstly set before said pressure regulating valves open. In this way, it is advantageously ensured that the pressure regulating valves 19, 20 are actuated every time they first open. Since said pressure regulating valves are closed when unpressurized, they remain closed even when actuated until the actual high pressure p.sub.I also overshoots the opening pressure value, wherein said pressure regulating valves then open and are actuated in the normal function, specifically either in the normal operating mode or in the first operation type of the protective operating mode.

[0111] However, if one of the above-described situations arises, it is in turn the case that the standstill function for the pressure regulating valves 19, 20 is set.

[0112] This is the case in particular if the dynamic rail pressure p.sub.dyn overshoots the third pressure threshold value p.sub.G3, wherein said third pressure threshold value is preferably selected to be higher than the first pressure threshold value p.sub.G1 and the second pressure threshold value p.sub.G2, 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 valves 19, 20 are open under the action of pressure and when deenergized, said pressure regulating valves in this case open fully in the standstill function and thus safely and reliably ensure the function of a pressure relief valve.

[0113] 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 valves 19, 20, such that said pressure regulating valves open and thus prevent an inadmissible rise of the high pressure.

[0114] 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 valves 19, 20, such that, upon a restart of the internal combustion engine 1, the cycle described here can begin again from the start.

[0115] If, for the pressure regulating valves 19, 20, under the action of pressure in the high-pressure accumulator 13, the standstill function is set, said pressure regulating valves are opened to the maximum extent and discharge 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 1 and the injection system 3, wherein said protective function can in particular replace the absence of a mechanical pressure relief valve.

[0116] It is important here that the pressure regulating valves 19, 20 have only two functional states, specifically the standstill function and the normal function, wherein said two functional states are entirely sufficient to replicate the entire relevant functionality of the pressure regulating valves 19, 20 including the protective function for replacing a mechanical pressure relief valve.

[0117] It is evident that, even after an overshooting of the second pressure threshold value p.sub.G2, stable regulation of the high pressure by means of the pressure regulating valves remains possible, because the delivery capacity of the high-pressure pump 11 is dependent on engine speed. It is thus possible for engine operating values, in particular emissions values, to still be adhered to in this case. Only in the relatively high engine speed range must an overshooting of the third pressure threshold value p.sub.G3 be expected. In this case, the pressure regulating valves 19, 20 open fully, and an impairment of the engine operating values, in particular the emissions, must be expected. At least stable operation of the engine however then remains ensured.

[0118] Even in the event of a failure of the high-pressure sensor 23, stable engine operation remains possible, even if an impairment of the engine operating values, in particular the emissions values, possibly occurs in this case.

[0119] By virtue of the fact that the second pressure threshold value p.sub.G2 is higher than the first pressure threshold value p.sub.G1, a situation is avoided in which the two pressure regulating valves 19, 20 are simultaneously transferred from the closed state into an open state. In this way, large pressure gradients, which could have a damaging effect on the injection system 3, are avoided.

[0120] As already indicated, the pressure regulating valves 19, 20 are acted on alternately with the actuation signals PWMDRV1 and PWMDRV2. This means that one of the two pressure regulating valves 19, 20 is acted on with the first actuation signal PWMDRV1 during a predetermined time period, for example 5000 operating hours. At the same time, the other pressure regulating valve 20, 19 is acted on with the second actuation signal PWMDRV2. After the predetermined time period has elapsed, it is conversely the case that said one pressure regulating valve 19, 20 is acted on with the second actuation signal PWMDRV2 and the other pressure regulating valve 20, 19 is acted on with the first actuation signal PWMDRV1, in turn for the predetermined time period. This will now be discussed in more detail in conjunction with FIGS. 5 and 6.

[0121] FIG. 5 shows a schematic illustration of a logic for alternating actuation of the pressure regulating valves 19, 20 on the basis of various diagrams. Here, a first diagram 1) shows a time counter Z.sub.DRV plotted versus the time t. Curved brackets are used to illustrate in each case one predetermined time period t.sub.DRV. The time counter Z.sub.DRV has its maximum value, for example 5000 operating hours, at a first time point t.sub.1, after the predetermined time period t.sub.DRV has elapsed.

[0122] The second, middle diagram 2) shows the logic variable MS as a function of the time t, wherein said logic variable assumes the value 0 when the internal combustion engine 1 is running and the value 1 when the internal combustion engine 1 is at a standstill. Up to a second time point t.sub.2, the variable MS assumes the value 0, that is to say the internal combustion engine 1 is running. At the second time point t.sub.2, said variable assumes the value 1, that is to say it is identified that the internal combustion engine 1 is at a standstill.

[0123] On the basis of the first, upper diagram, it is evident that the time counter Z.sub.DRV is now reset to 0. Said time counter subsequently runs up to its maximum value again, which is then reached again at a third time point t.sub.3. Between the first time point ti and the second time point t.sub.2, no change in the time counter Z.sub.DRV occurs, because this has reached its maximum value, but it has not yet been identified that the internal combustion engine 1 is at a standstill. At the third time point t.sub.3, the time counter Z.sub.DRV is reset to the value 0 again because the second diagram indicates that the engine is at a standstill. Subsequently, the time counter Z.sub.DRV is incremented again until it finally reaches its maximum value again at a fourth time point t.sub.4. Since the second diagram indicates that the engine is at a standstill at a fifth time point t.sub.5, the time counter is, corresponding to the first diagram, reset to the value 0 at the fifth time point t.sub.5. Thereafter, the counter runs up to its maximum value again, which it reaches again at a sixth time point t.sub.6. The third, lower diagram 3) illustrates a fourth logic signal SIG4 plotted versus the time t. Said fourth logic signal SIG4 indicates when a change in the assignment of the actuation signals PWMDRV1, PWMDRV2 to the corresponding pressure regulating valves 19, 20 should be performed. Said fourth logic signal SIG4 has the value 0 at the time point 0. A change in the value of the fourth logic signal SIG4 occurs whenever the time counter Z.sub.DRV has reached its maximum value and, at the same time, the logic signal MS indicates that the internal combustion engine 1 is at a standstill. This means that the signal SIG4 changes from the value 0 to the value 1 at the second time point t.sub.2, from the value 1 to the value 0 at the third time point t.sub.3, and from the value 0 to the value 1 again at the fifth time point t.sub.5. Altogether, therefore, a change in the value of the fourth logic signal SIG4 and thus in the assignment of the actuation signals PWMDRV1, PWMDRV2 to the pressure regulating valves 19, 20 occur at said time points.

[0124] FIG. 6 shows a function of the switching logic 57 in a schematic illustration. Said switching logic has a sixth switching element 65 and a seventh switching element 67, which change their switching position in a manner dependent on the fourth logic signal SIG4. If the fourth logic signal SIG4 assumes the value 0, both switching elements 65, 67 are in their upper switching position illustrated in FIG. 6. Thus, the first actuation signal PWMDRV1 is assigned to the first pressure regulating valve 19, wherein, at the same time, the second actuation signal PWMDRV2 is assigned to the second pressure regulating valve 20. At the same timepossibly by means of additional physical switching elements, but discussed here together with the actuation signals for the sake of a simpler illustrationthe first measured current variable I.sub.R is measured at the first pressure regulating valve 19, wherein the second measured current variable I.sub.R,2 is measured at the second pressure regulating valve 20.

[0125] If the fourth logic signal SIG4 assumes the value 1, the switching elements 65, 67 switch to their lower switching position illustrated in FIG. 6. Thus, the first actuation signal PWMDRV1 is now assigned to the second pressure regulating valve 20, wherein the second actuation signal PWMDRV2 is assigned to the first pressure regulating valve 19. At the same time, the first measured current variable I.sub.R is measured at the second pressure regulating valve 20, wherein the second measured current variable I.sub.R,2 is measured at the first pressure regulating valve 19.

[0126] The switching logic 57 thus has the effect, in a manner dependent on the fourth logic signal SIG4, that the pressure regulating valves 19, 20 are actuated with the different actuation signals PWMDRV1, PWMDRV2 alternately, wherein it is at the same time ensured that the current regulators 35, 51 provided for this purpose are supplied the correct measured current variables I.sub.R, I.sub.R,2 in each case.

[0127] FIG. 7 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 is evident 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 69 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 71 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 71 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 73. The gain factor r2.sub.DRV is calculated in accordance with the following formula, in which tn.sub.DRV is a reset time:

[00001]$\begin{array}{cc}r.Math..Math.2_{\mathrm{DRV}}=\frac{64.Math.{\mathrm{kp}}_{\mathrm{DRV}}.Math.T_a}{{\mathrm{tn}}_{\mathrm{DRV}}}.& \left(2\right)\end{array}$

[0128] 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 an eighth switching element 75 illustrated in FIG. 7 switches into its lower switching position. In said switching position, the integrating component A.sub.I is identical to the output signal of the limitation element 73, 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 eighth switching element 75 switches into its upper switching 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 p.sub.G1.

[0129] The calculation of the differential component A.sub.DTI is illustrated in the lower part of FIG. 7. 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.

[0130] 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:

[00002]$\begin{array}{cc}r.Math..Math.3_{\mathrm{DRV}}=\frac{2.Math.{\mathrm{kp}}_{\mathrm{DRV}}.Math.{\mathrm{tv}}_{\mathrm{DRV}}}{2.Math..Math.t.Math..Math.1_{\mathrm{DRV}}+T_a}.& \left(3\right)\end{array}$

[0131] The factor r4.sub.DRV is calculated in accordance with the following equation:

[00003]$\begin{array}{cc}r.Math..Math.4_{\mathrm{DRV}}=\frac{2.Math..Math.t.Math..Math.1_{\mathrm{DRV}}-T_a}{2.Math..Math.t.Math..Math.1_{\mathrm{DRV}}+T_a}.& \left(4\right)\end{array}$

[0132] 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.

[0133] FIG. 8 is a schematic illustration of a logic arrangement for the calculation of the value of a fifth logic signal SIG5 which is used to ensure that, 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. 9. The value of the fifth logic signal SIG5 results from a third AND element 77, into the first input 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 fifth logic signal SIG5 is, upon the starting of the internal combustion engine 1, firstly initialized with the value false. Into a first input of a third OR element 79 there is input the result of a third comparator element 81, in which it is checked whether the dynamic rail pressure p.sub.dyn is greater than or equal to the third pressure threshold value p.sub.G3. Into the second input of the third OR element 79 there is input the result of a comparison element 83 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 third OR element 79 assumes the value true if at least one of the outputs of the third comparator element 81 or of the comparison element 83 assumes the value true. Thus, in order for the output of the third OR element 79 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 third pressure threshold value p.sub.G3, and/or a sensor defect of 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 third OR element 79 has the value false.

[0134] The output of the third OR element 79 is input into a first input of a fourth OR element 85, into the second input of which the value of the fifth logic signal SIG5 is input. Since said fifth logic signal is originally initialized with the value false, the output of the fourth OR element 85 has the value false until the output of the third OR element 79 assumes the value true. If this is the case, the output of the fourth OR element 85 also changes to the value true. In this case, the value of the third OR element 77 also changes to true if the internal combustion engine 1 is running, such that the value of the fifth logic signal SIG5 also changes to true. It is evident from FIG. 8 that the value of the fifth logic signal SIG5 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.

[0135] If it is sought for the suction throttle 9 to be permanently open also in the second and/or in the first operation type of the protective operating mode, in particular in order to prevent duplicate regulation of the high pressure by means of the suction throttle 9 and the pressure regulating valves 19, 20, this can be achieved by virtue of the second pressure threshold value p.sub.G2 or the first pressure threshold value p.sub.G1 instead of the third pressure threshold value p.sub.G3 being used in the third comparator element 81 and being compared with the dynamic rail pressure p.sub.dyn.

[0136] FIG. 9 is a schematic illustration of the first high-pressure regulating loop 25 including a ninth switching element 87 for realizing the permanently open operation of the suction throttle 9 in the first, second and/or third operation types of the protective operating mode, wherein the fifth logic signal SIG5, the calculation of which has been described in conjunction with FIG. 8, is input into the ninth switching element 87 for the actuation thereof. It is possible for the ninth switching element 87 to be in the form of a software switch, that is to say in the form of a purely virtual switch. Alternatively, it is self-evidently also possible for the ninth switching element 87 to be in the form of a physical switch, for example a relay.

[0137] As has already been discussed, an input variable of the first high-pressure regulating loop 25 is the setpoint high pressure p.sub.S which in this case, 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 89, which is preferably implemented as a PI(DT.sub.1) algorithm. A further input variable of the high-pressure regulator 89 is preferably a proportional coefficient kp.sub.SD. An output variable of the high-pressure regulator 89 is a fuel volume flow V.sub.SD for the suction throttle 9, to which, at a summing junction 91, a fuel setpoint consumption V.sub.Q is added. Said fuel setpoint consumption V.sub.Q is calculated in a calculation element 93 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 89 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 95, 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 95 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 97. The latter converts the limited fuel setpoint volume flow V.sub.S,SD into a characteristic curve suction throttle current I.sub.KL,SD.

[0138] If the ninth switching element 87 is in the upper switching state illustrated in FIG. 9, which is the case if the fifth logic signal SIG5 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 99 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 99 is, inter alia, an actual suction throttle current I.sub.I,SD. An output variable of the suction throttle current regulator 99 is a suction throttle setpoint voltage U.sub.S,SD which is finally, in a calculation element 101, 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 9 is actuated using said signal, wherein the signal thus acts overall on a regulating path 103 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 105. The current filter 105 is preferably in the form of a PT.sub.I 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 99.

[0139] 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 means of the high-pressure sensor 23 and filtered by means of a first high-pressure filter element 107, which, as output variable, has the actual high pressure p.sub.I. Furthermore, the unprocessed values of the high pressure p are filtered by means of a second high-pressure filter element 109, the output variable of which is the dynamic rail pressure p.sub.dyn. Both high-pressure filter elements are preferably implemented by means of a PT.sub.I algorithm, wherein a time constant of the first high-pressure filter element 107 is greater than a time constant of the second high-pressure filter element 109. In particular, the second high-pressure filter element 109 is configured so as to be a faster filter than the first high-pressure filter element 107. The time constant of the second high-pressure filter element 109 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.

[0140] Output variables of the first high-pressure regulating loop 25 are thus, aside from the unfiltered high pressure p, the filtered high-pressure values p.sub.I, p.sub.dyn.

[0141] If the fifth logic signal SIG5 assumes the value true, the ninth switching element 87 switches into its lower switching position illustrated in FIG. 9. 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,SD. 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 third operation type of the protective operating mode with pressure regulating valves 19, 20 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 and/or second operation type, it is achieved in this way that twofold regulation of the high pressure both by means of the suction throttle 9 and by means of the pressure regulating valves 19, 20 is prevented.

[0142] 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 valves 19, 20. It is furthermore evident that the injection system 3 can be readily scaled with regard to a size of an internal combustion engine 1 with which it is used, by virtue of the number of pressure regulating valves 19, 20 being adapted. It is thus possible in particular to use pressure regulating valves 19, 20 which can be produced inexpensively, such as are known for example from automobile series production. If, for example, a cable breakage of a suction throttle plug connector occurs in the lower engine speed range, then in said range, after the first or second pressure threshold value p.sub.G1, p.sub.G2 is reached or overshot, stable regulation of the high pressure remains possible by means of the pressure regulating valves 19, 20, because the delivery capacity of the high-pressure pump is dependent on engine speed. It is possible for predetermined engine operating values, in particular emissions values, to still be adhered to in this case. Only in relatively high engine speed ranges must an overshooting also of the third pressure threshold value p.sub.G3 be expected. In this case, the pressure regulating valves 19, 20 open fully, and an impairment of the engine operating values, in particular the emissions, must be expected. At least stable operation of the internal combustion engine 1 however then remains ensured. Even in the event of a failure of the high-pressure sensor 23, stable operation of the internal combustion engine 1 is possible, even if an impairment of the operating values possibly occurs in this case.

[0143] By virtue of the fact that the pressure regulating valves 19, 20 are not activated simultaneously, a situation is prevented in which the injection system 3 is damaged by excessively large high-pressure gradients. If more than two pressure regulating valves 19, 20 are provided, it is possible for separate pressure threshold values to be defined for an activation of each of said pressure regulating valves 19, 20 or for an activation of groups of said pressure regulating valves 19, 20, which pressure threshold values may in particular be staggered in terms of their magnitude.

[0144] The pressure regulating valves 19, 20 are utilized uniformly by way of alternate actuation.

[0145] Altogether, the following functionality for the internal combustion engine 1 and the injection system 3 is also evident:

[0146] Said internal combustion engine comprises at least two pressure regulating valves 19, 20 but no mechanical pressure relief valve. If the high pressure rises, for example as a result of a cable breakage of a suction throttle plug connector, and if the dynamic rail pressure p.sub.dyn in this case reaches the first pressure threshold value p.sub.G1, then the second high-pressure regulating loop 39 performs the regulation of the high pressure by actuating one of the pressure regulating valves 19, 20. Here, the other pressure regulating valve 20, 19 is actuated so as to remain closed.

[0147] If, while the internal combustion engine 1 is running, despite activation of one pressure regulating valve 19, 20, the dynamic rail pressure p.sub.dyn reaches or exceeds the second pressure threshold value p.sub.G2, which is preferably higher than the first pressure threshold value p.sub.G1, then the further pressure regulating valve 20, 19 is additionally activated in order to regulate the high pressure. It is preferable for both pressure regulating valves 19, 20 to be actuated with the same setpoint current I.sub.S, I.sub.S,2.

[0148] If the dynamic rail pressure p.sub.dyn reaches or exceeds the third pressure threshold value p.sub.G3, which is preferably higher than the first pressure threshold value p.sub.G1 and the second pressure threshold value p.sub.G2, or if the high-pressure sensor 23 fails, the pressure regulating valves 19, 20 are actuated such that they reliably, permanently and preferably fully open. In all cases, the suction throttle 9 is preferably simultaneously actuated so as to likewise be operated in the fully open state. The pressure regulating valves 19, 20 are actuated alternately at predefinable time intervals. Here, a change may be performed only when the internal combustion engine 1 is at a standstill.