METHOD FOR OPERATING AN INTERNAL COMBUSTION ENGINE HAVING AN INJECTION SYSTEM, AND INJECTION SYSTEM FOR CARRYING OUT SUCH A METHOD

Abstract

A method for operating an internal combustion engine having an injection system having a high-pressure accumulator, wherein an instantaneous high pressure in the high-pressure accumulator is monitored in a time-dependent manner by a high-pressure sensor. A first alarm stage is set, if a) a first predetermined high-pressure limit value is exceeded without interruption by the instantaneous high pressure for a predetermined limit time period, and/or b) if the first predetermined high pressure limit value is exceeded for the first time at a predetermined, first limit frequency by the instantaneous high pressure.

Claims

1-10. (canceled)

11. A method for operating an internal combustion engine with an injection system that has a high-pressure accumulator, comprising the steps of: monitoring an instantaneous high pressure in the high-pressure accumulator against time by a high pressure sensor; and, setting a first alarm stage when a) a first predetermined high pressure limit value of the instantaneous high pressure is continuously exceeded for a predetermined limit period, and/or when b) the first predetermined high pressure limit value is exceeded for a first time by the instantaneous high pressure with a predetermined, first limit frequency.

12. The method according to claim 11, further comprising starting a recording of a period of time of the instantaneous high pressure exceeding the first high pressure limit value when the instantaneous high pressure reaches or exceeds the first high pressure limit value from below the first high pressure limit value, and comparing the recorded period with the predetermined limit period.

13. The method according to claim 11, including incrementing a frequency value, which indicates a current frequency of exceeding the first high pressure limit value by the instantaneous high pressure, when the instantaneous high pressure exceeds the first high pressure limit value from below a second high pressure limit value, wherein the second high pressure limit value is lower than the first high pressure limit value, and wherein the frequency value is compared with the predetermined first limit frequency.

14. The method according to claim 12, further including setting the recorded period to zero when the instantaneous high pressure falls below the first high pressure limit value from above the first high pressure limit value.

15. The method according to claim 11, further including setting a second alarm stage when the first high pressure limit value is exceeded for the first time by the instantaneous high pressure with a predetermined, second limit frequency, wherein the second limit frequency is less than the first limit frequency.

16. The method according to claim 11, further including terminating an injection of fuel from the high-pressure accumulator into at least one combustion chamber of the internal combustion engine when the first alarm stage is set.

17. The method according to claim 16, including: a) continuing the injection of fuel with the first alarm stage set if the instantaneous high pressure falls below a third high pressure limit value from above the third high pressure limit value, wherein the third high pressure limit value is less than the first high pressure limit value, and b) stopping the continued injection with the first alarm stage set as soon as the instantaneous high pressure reaches or exceeds the first high pressure limit value.

18. The method according to claim 13, further including terminating an injection of fuel from the high-pressure accumulator into at least one combustion chamber of the internal combustion engine when the first alarm stage is set, including: a) continuing the injection of fuel with the first alarm stage set if the instantaneous high pressure falls below a third high pressure limit value from above the third high pressure limit value, wherein the third high pressure limit value is less than the first high pressure limit value, and b) stopping the continued injection with the first alarm stage set as soon as the instantaneous high pressure reaches or exceeds the first high pressure limit value.

19. The method according to claim 18, wherein the third high pressure limit value is equal to the second high pressure limit value.

20. The method according to claim 15, including canceling the first alarm stage and/or the second alarm stage when a standstill of the internal combustion engine is detected and simultaneously setting an alarm reset request.

21. The method according to claim 15, including selecting the predetermined limit period to be at least 2 s to not more than 3 s, preferably 2.5 s, and/or selecting the first, predetermined limit frequency to be at least 45 to not more than 55, preferably 50 or 51, and/or selecting the second predetermined limit frequency to be at least 25 to not more than 35, preferably 30 or 31.

22. The method according to claim 21, including selecting the predetermined time limit to be 2.5 s.

23. The method according to claim 21, including selecting the first, predetermined limit frequency to be 50 or 51.

24. The method according to claim 21, including selecting the second predetermined limit frequency to be 30 or 31.

25. The method according to claim 17, wherein the injection or the continued injection is terminated by: a) setting a target injection quantity to zero, and/or by b) setting an energization period for at least one injector to zero.

26. An injection system for an internal combustion engine, comprising: at least one injector; a high-pressure accumulator with a flow connection to the at least one injector; a high pressure sensor set up and arranged to record an instantaneous high pressure in the high-pressure accumulator against time; and a control unit connected to the high pressure sensor and configured to carry out the method according to claim 11.

Description

[0035] The invention is explained in more detail below on the basis of the drawing. In the figures:

[0036] FIG. 1 shows a schematic representation of an exemplary embodiment of an internal combustion engine with an exemplary embodiment of an injection system;

[0037] FIG. 2 shows a schematic representation of a high pressure control circuit for controlling a high pressure in a high-pressure accumulator of the injection system;

[0038] FIG. 3 shows a schematic representation of a revolution rate control circuit with a possibility of optionally performing or preventing an injection;

[0039] FIG. 4 shows a diagrammatic representation of a first embodiment of a method for operating an injection system;

[0040] FIG. 5 shows a schematic diagrammatic representation of a second embodiment of such a method, and

[0041] FIG. 6 shows a schematic representation of another embodiment of the method in the form of a flowchart.

[0042] FIG. 1 shows a schematic representation of an exemplary embodiment of an internal combustion engine 1 that has an injection system 3. The injection system 3 is preferably in the form of a common rail injection system. It has a low-pressure pump 5 for conveying fuel from a fuel reservoir 7, an adjustable, low-pressure suction choke 9 for influencing a volumetric fuel flow flowing through it, a high-pressure pump 11 for conveying the fuel under increased pressure into a high-pressure accumulator 13, the high-pressure accumulator 13 for storing the fuel, and a plurality of injectors 15 for injecting the fuel into combustion chambers 16 of the internal combustion engine 1. Optionally, it is possible that the injection system 3 is also implemented with individual accumulators, wherein then for example, an individual accumulator 17 is integrated within the injector 15 as an additional buffer volume. A particularly electrically controllable pressure control valve 19 is provided, via which the high-pressure accumulator 13 has a flow connection to the fuel reservoir 7. A volumetric fuel flow, which is discharged from the high-pressure accumulator 13 into the fuel reservoir 7, is defined by the position of the pressure control valve 19. This volumetric fuel flow is designated in FIG. 1 with VDRV and is a high pressure control variable of the injection system 3.

[0043] The injection system 3 preferably does not have a mechanical overpressure valve, which is conventionally provided and connects the high-pressure accumulator 13 to the fuel reservoir 7. Its function can be carried out by the pressure control valve 19.

[0044] The operating mode of the internal combustion engine 1 is determined by an electronic control unit 21, which is preferably designed as the engine control unit of the internal combustion engine 1, namely as a so-called Engine Control Unit (ECU). The electronic control unit 21 contains the usual components of a microcomputer system, for example a microprocessor, I/O modules, buffers and memory modules (EEPROM, RAM). The operating data relevant for the operation of the internal combustion engine 1 are applied in characteristic fields/characteristic curves in the memory modules. Using these, the electronic control unit 21 calculates output variables from input variables. In FIG. 1, the following input variables are shown as examples: a measured, still unfiltered high pressure p prevailing in the high-pressure accumulator 13 and measured by means of a high pressure sensor 23, a current engine speed n.sub.I, a signal FP for specifying the power by an operator of the internal combustion engine 1, and an input variable E. Further sensor signals are preferably summarized in the input variable E, for example a charge air pressure of an exhaust gas turbocharger. In the case of an injection system 3 with individual accumulators 17, a single accumulator pressure p.sub.E is preferably an additional input variable of the control unit 21.

[0045] By way of example, a signal PWMSD for controlling the suction choke 9 as the first pressure control element, a signal ve for controlling the injectors 15which in particular specifies an injection start and/or an injection end or even an injection period-, a signal PWMDRV for controlling the pressure control valve 19 as a second pressure control element, and an output variable A are shown in FIG. 1 as output variables of the electronic control unit 21. The position of the pressure control valve 19 and thus the high pressure interference parameter VDRV is defined by the preferably pulse-width modulated signal PWMDRV. The output variable A is representative of further control signals for the control and/or regulation of the internal combustion engine 1, for example for a control signal for activating a second exhaust gas turbocharger during charging of an accumulator.

[0046] FIG. 2 shows a schematic representation of a high pressure control circuit 25. Input variables of the high pressure control circuit 25 are a target high pressure p.sub.S for the injection system 3, which is preferably specified depending on the operating point by the control unit 21, in particular is read out from a characteristic field, and which is compared with an instantaneous high pressure p.sub.I for calculation of a control error e.sub.p. This control error e.sub.p is an input variable of a high pressure regulator 27, which is preferably implemented as a PI(DT.sub.1) algorithm. A further input variable of the high pressure regulator 27 is preferably a proportional coefficient kp.sub.SD. The output variable of the high-pressure regulator 27 is a volumetric fuel flow V.sub.SD for the suction choke 9, to which a target fuel consumption VQ is added in an addition point 29. This target fuel consumption VQ is calculated in a first calculation element 31 as a function of the current speed n.sub.I and a target injection quantity QS and represents an interference variable of the high pressure control circuit 25. The sum of the output variable V.sub.SD of the high pressure controller 27 and the interference variable VQ results in an unlimited target volumetric fuel flow V.sub.U, SD. This is limited in a limiting element 33 as a function of the speed n.sub.I to a maximum volumetric flow V.sub.max,SD for the suction choke 9. The output of the limiting element 33 is a limited target volumetric fuel flow V.sub.S, SD for the suction choke 9, which is input into a pump characteristic curve 35 as the input variable. This converts the limited target volumetric fuel flow V.sub.S, SD into a target suction choke current I.sub.S,SD.

[0047] The target suction choke current I.sub.S, SD represents an input variable of a suction choke current controller 37 that has the task of regulating the suction choke flow through the suction choke 9. A further input variable of the suction choke current controller 37 is, among other things, an actual suction choke current I.sub.I,SD. The output variable of the suction choke current controller 37 is a suction choke target voltage Us,s.sub.D, which is finally converted in a second calculation element 39 in a known way into a switch-on time of a pulse-width modulated signal PWMSD for the suction choke 9. The suction choke 9 is controlled with this pulse-width modulated signal PWMSD, wherein the signal thus acts overall on a control path 41, which in particular comprises the suction choke 9, the high-pressure pump 11 and the high-pressure accumulator 13. The suction choke current is measured, wherein a raw measured value I.sub.R,SD results, which is filtered in a current filter 43. The current filter 43 is preferably in the form of a PT.sub.1 filter. The output variable of this current filter 43 is the actual suction choke current I.sub.I,SD, which in turn is fed to the suction choke current controller 37.

[0048] The control variable of the first high pressure control circuit 25 is the high pressure in the high-pressure accumulator 13. Raw values of this high pressure p are measured by the high pressure sensor 23 and filtered by a high-pressure filter 45, which has the instantaneous high pressure p.sub.I as the output variable. The high pressure filter 45 is preferably implemented by a PT.sub.1 algorithm.

[0049] The output variable of the high pressure control circuit 25 is therefore, in addition to the unfiltered high pressure p, the filtered high pressure or the actual high pressure p.sub.I, which is also referred to in particular as the instantaneous high pressure.

[0050] FIG. 3 shows a speed control circuit 47, which is used for speed control. The current engine speed n.sub.I is subtracted from a target speed n.sub.S specified by the control unit 21, resulting in a speed control error e. This speed control error e is an input variable of a speed controller 49, in this case a PI(DT.sub.1) controller. The speed controller 49 has as a further input variable, among other things a proportional coefficient kp.sub.Drz and has a speed controller torque M.sub.S.sup.PI(DTI) as the output variable. This is added to a load signal torque M.sub.S.sup.L, wherein the load signal torque M.sub.S.sup.L is an interference variable. Due to the inclusion of this interference variable, a system signal can be used to improve the dynamics of the speed control circuit 47. The sum of the speed controller torque M.sub.S.sup.PI(DTI) and the load signal torque M.sub.S.sup.L is then limited in a torque limiter 51 downwards to a minimum target torque M.sub.S.sup.Min and upwards to a maximum target torque M.sub.S.sup.Max. A friction torque M.sub.S.sup.R is finally added to a target torque M.sub.S limited in this way, resulting in a corrected target torque M.sub.korr. This is an input variable of an engine controller 53 in addition to other variables such as the current engine speed n.sub.I. An output variable of the engine controller 53 is the target injection quantity Q.sub.S. This is injected into the combustion chambers 16 of the internal combustion engine 1. Raw values n.sub.r of the engine speed are recorded and converted into the current actual speed n.sub.I using a speed filter 55.

[0051] The target injection quantity Q.sub.S is taken from the high-pressure accumulator 13 and injected into the combustion chambers 16 by means of the injectors 15. If the high pressure in the high-pressure accumulator 13 exceeds a certain threshold for too long, or if the high pressure in the high-pressure accumulator 13 exceeds the predetermined threshold too often, the injectors 15 may be damaged.

[0052] In accordance with the method proposed here, it is therefore provided that the high pressure in the high-pressure accumulator 13 is monitored against time by means of the high pressure sensor 23, wherein a first alarm stage is set when a first predetermined high pressure limit value is continuously exceeded by the instantaneous high pressure for a predetermined limit time, and/or if the first predetermined high pressure limit value is exceeded by the instantaneous high pressure for the first time with a predetermined first limit frequency. In this way, an operator of the internal combustion engine 1 can be warned if damage to the injectors 15 is threatened or has already occurred, and preferably further operation of the internal combustion engine 1 can be at least temporarily stopped to prevent further damage or even complete destruction of the injectors 15.

[0053] When the first alarm stage is set, the injection of fuel from the high-pressure accumulator 13 into the combustion chambers 16 is preferably terminated. However, the injection is preferably continued with the first alarm stage set if the instantaneous high pressure falls below a third high pressure limit value from above the third high pressure limit value, wherein the instantaneous high pressure is below the third high pressure limit value. The continued injectionduring the set first alarm stageis again terminated as soon as the instantaneous high pressure reaches or exceeds the first high pressure limit value againfrom below. In this way, on the one hand the injectors 15 can be protected, and on the other hand the internal combustion engine 1 can continue to operate at least to a limited extent, for example in order to be able to reach a safe station, in particular a seaport or the like. This means that an emergency running function or limp home function is provided.

[0054] The injection or continued injection is preferably terminated by setting the injection quantity Q.sub.S to zero.

[0055] However, another method to end the injection or the continued injection is alternatively or even additionally possible, wherein this possibility is shown in FIG. 3: According to this possibility, an energization period BD for the injectors 15 is set to zero. For this purpose, a switching element 57 is provided, preferably in the speed control circuit 47, which can change its switching state in a binary manner depending on a logical signal SIG. The logical signal SIG can assume the values true (T) or false (F). The logical signal SIG indicates whether a quantity limit for the injection of fuel into the combustion chambers 16 via the injectors 15 is active. The logical signal SIG is set to true when the first alarm stage is set and the injection is to be stopped, and if the continued injection is to be stopped. Otherwiseand especially if the injection is to be continued with the first alarm stage setthe value of the logical signal SIG is set to false.

[0056] If the logical signal SIG has the value false, the switching element 57 is in the functional state designated in FIG. 3 with F. In this case, the energization period BD of the motor controller 53 is taken as the output variable, wherein it is predetermined by the motor controller 53, in particular calculated, particularly preferably read from a characteristic field. If, on the other hand, the logical signal SIG has the value true and, in this respect, the quantity limit for fuel injection is active, the switching element 57 takes the switching position designated in FIG. 3 with T, so that the energization period BD is set identical to the value zero. In this switching state of the switching element 57, therefore, no energization of the injectors 15 takes place, so that the injection is not carried out.

[0057] It is possible that the switching element 57 is in the form of a software switch, i.e. of a purely virtual switch. Alternatively, however, it is also possible that the switching element 57 is in the form of a physical switch, for example of a relay. The logical signal SIG can of course also adopt the numeric values 0 and 1, or other appropriate corresponding values, in a completely analogous way to the values true and false.

[0058] FIG. 4 shows a diagrammatic representation of a first embodiment of the method for operating the injection system 3. A total of seven time diagrams are shown, in which different variables are specified as a function of time t. The first, upper time diagram at a) shows the instantaneous high pressure p.sub.I as a solid curve plotted against time t. This rises initially, starting from a starting value p.sub.Start. At a first time to, the instantaneous high pressure p.sub.I reaches the first predetermined high pressure limit value p.sub.L1 and subsequently exceeds it. In the third diagram from the top at c) a current time period t.sub.A is plotted against the time t as a solid curve, which indicates the time for which the instantaneous high pressure p.sub.I continuously exceeds the first predetermined high pressure limit value p.sub.L1. At the first time t.sub.0, this current time period t.sub.A is countedstarting from the value zero. At a second time t.sub.1, the instantaneous high pressure p.sub.I reaches the first high pressure limit value p.sub.L1 again from above and subsequently falls below it. Therefore, the current time period t.sub.A is reset to zero. It has not yet reached or exceeded a predetermined limit period t.sub.L between the first time t.sub.0 and the second time t.sub.1.

[0059] At a third time t.sub.2, the instantaneous high pressure p.sub.I falls below a second predetermined high pressure limit value p.sub.L2, which is less than the first high pressure limit value p.sub.L1 by a hysteresis pressure difference value p.sub.H. The instantaneous high pressure p.sub.I drops further after the third time t.sub.2 and then rises again. At a fourth time t.sub.3, the instantaneous high pressure p.sub.I again reaches the first high pressure limit value p.sub.L1 and subsequently exceeds it. As a result, the current time period t.sub.A is counted againagain starting from zero. At a fifth time t.sub.4, the instantaneous high pressure p.sub.I again reaches the first high pressure limit value p.sub.L1 from above, so that the current time period t.sub.A, which has not yet reached the limit time t.sub.L, is reset to the value zero. The actual high-pressure p.sub.I falls even further without falling below the second high pressure limit value p.sub.L2. A subsequent increase in the instantaneous high pressure p.sub.I causes the first high-pressure limit p.sub.L1 to be exceeded again from below at a sixth time t.sub.5. This in turn leads to the current time period t.sub.A being counted up again, in particular from zero again. At a seventh time t.sub.6, the current time period t.sub.A exceeds the predetermined limit time t.sub.L, which results in the quantity limit for injection being activated and the logical signal SIG changing its value, wherein here it is set to the value true denoted by T, which is shown in the fourth diagram from the top at d). As explained in connection with FIG. 3, this means that no more fuel is injected into the combustion chambers 16. The current time period t.sub.A is set back to zero at the seventh time t.sub.6, thus being reset.

[0060] From the sixth diagram from the top at f) it becomes clear that the first alarm stage AI is set at the same time as reaching the limit time period t.sub.L and the value change of the logical signal SIG from the value F to the value T, which is shown here by a jump of a signal indicating the first alarm stage AI from the value 0 to the value 1.

[0061] At an eighth time t.sub.7, the instantaneous high pressure p.sub.I again falls below the first high pressure limit value p.sub.L1 from above, wherein at a ninth time t.sub.8 it finally falls below the second high pressure limit value p.sub.L2 from above. This causes the logical signal SIG to change its value again and to reset to false, i.e. to the value F. The injection is therefore enabled again.

[0062] Up to a tenth time t.sub.9, the instantaneous high pressure remains below the first high pressure limit value p.sub.L1. At the tenth time t.sub.9, it again exceeds the first high-pressure limit value p.sub.L1 from below, which then immediatelydue to the set first alarm stagecauses the logical signal SIG to be set to the value T again, whereby the injection of fuel into the combustion chambers 16 is stopped again.

[0063] Up to a 14th time t.sub.13, the instantaneous high pressure remains above the second high pressure limit value p.sub.L2 so that all variables and/or signals remain unchanged. At the 14.sup.th time t.sub.13, the instantaneous high pressure p.sub.I falls below the second high pressure limit value p.sub.L2 again from above, which resets the logical signal SIG to the value F. The injection is thus enabled again. At the same time, at the 14th time t.sub.13, the internal combustion engine 1 is switched off, so that as a result in the second diagram from the top the current engine speed n.sub.I drops from a speed value n.sub.Start to zero.

[0064] At a 15th time t.sub.14, it is detected that the combustion engine 1 is stopped, wherein now a logical variable MS, which indicates that the internal combustion engine is stopped, assumes the value 1. This is shown in the fifth diagram from the top at e).

[0065] At a 16th time t.sub.15, the instantaneous high pressure p.sub.I again exceeds the first high pressure limit value pL1. This causes the logical signal SIG to be set back to the value T. Thus, the injection is deactivated again, i.e. no more fuel is injected into the combustion chambers 16. At a 17th time t.sub.16, the instantaneous high pressure p.sub.I again falls below the first high pressure limit value p.sub.L1. At the 18th time t.sub.17 it finally reaches the second high pressure limit value p.sub.L2 and subsequently falls below it. The logical signal SIG is thus reset to the value F at the 18th time t.sub.17, which means that the injection is enabled again.

[0066] At the 19th time t.sub.18, an alarm reset request AR is set, which is indicated in the seventh diagram at g) by the fact that a corresponding variable takes the value 1. Since the internal combustion engine 1 is stopped at this 19th time t.sub.18, the associated first alarm stage AI is reset, i.e. the corresponding variable is set to the value zero.

[0067] The injection of fuel into the combustion chambers 16 is stopped if the instantaneous high pressure exceeds the first high pressure limit value p.sub.L1 continuously during the predetermined limit period t.sub.L.

[0068] Furthermore, FIG. 4 shows that the recording of the time period t.sub.A is always started, in particular re-initialized and started at zero, when the instantaneous high pressure p.sub.I reaches or exceeds the first high pressure limit value p.sub.L1 from below. The recorded period t.sub.A is also compared with the predetermined limit period t.sub.L. Furthermore, it becomes clear that the recorded period t.sub.A is set to zero if the instantaneous high pressure p.sub.I falls below the first high pressure limit value p.sub.L1 from above p.sub.L1. It is also clear that the first alarm stage A1 is cancelled when the internal combustion engine 1 standstill is detected and the alarm reset request AR is set at the same time.

[0069] The predetermined limit period t.sub.L is preferably selected from at least 2 s to no more than 3 s,

particularly preferably at 2.5 s.

[0070] FIG. 5 shows a schematic, diagrammatic representation of a second embodiment of the method, which, however, is preferably carried out in combination with the first embodiment explained in connection with FIG. 4.

[0071] FIG. 5 shows that the instantaneous high pressure p.sub.I, which in turn is plotted in a first, upper diagram at a) against time t, is monitored with a view to a frequency of exceeding the first high pressure limit value p.sub.L1. In the second diagram from the top at b) the current engine speed n.sub.I is plotted. In a third time diagram from the top at c) a frequency value H.sub.A is plotted, which indicates a current frequency of the instantaneous high pressure p.sub.I exceeding the first high pressure limit value p.sub.L1. In the fourth time diagram from the top at d), the logical signal SIG is again shown. In the fifth time diagram from the top at e), the logical variable M.sub.S is again shown. In a sixth time diagram from the top at f), a second alarm stage A2 is shown as a corresponding variable with the logical values 0 and 1. In the seventh time diagram from the top at g), the first alarm stage AI is shown as the corresponding logical variable with the values 0 and 1. In the eighth diagram from the top at h) the alarm reset request AR is shown again.

[0072] The first time diagram at a) shows that the instantaneous high pressure p.sub.I first increases from the starting value p.sub.Start and at a first time t.sub.0 reaches and then exceeds the first high pressure limit value p.sub.L1. The third time diagram at c) shows that the frequency value H.sub.A is incremented from 0 to 1 due to this limit violation. At a second time t.sub.1, the instantaneous high pressure again reaches the first high pressure limit value p.sub.L1 from above, wherein at a third time t.sub.2 it also falls below a third high pressure limit value, which is identical here with the second high pressure limit value p.sub.L2 according to FIG. 4. In principle, the third high pressure limit value can also be selected differently from the second high pressure limit value p.sub.L2. However, it corresponds to a preferred design for the third high-pressure limit value to be chosen equal to the second high pressure limit value p.sub.L2, wherein the third high pressure limit value is then also just smaller than the first high pressure limit value p.sub.L1 by the hysteresis differential pressure value p.sub.H. As a result, the instantaneous high pressure p.sub.I rises again and at a fourth time t.sub.3 again exceeds the first high pressure limit value p.sub.L1. This results in the frequency value H.sub.A being incremented again, from the value 1 to the value 2. At a fifth time t.sub.4, the instantaneous high pressure p.sub.I falls below the first high pressure limit value p.sub.L1 again from above. At a sixth time t.sub.5, the instantaneous high pressure p.sub.I again exceeds the first high pressure limit value p.sub.L1 from below, without first reaching or falling below the second high pressure limit value p.sub.L2 from above. Therefore at the sixth time t.sub.5 the frequency value H.sub.A is not incremented.

[0073] At a seventh time t.sub.6, the first high-pressure limit value p.sub.L1 is again exceeded by the instantaneous high pressure p.sub.I, wherein the second high pressure limit value p.sub.L2 is then also exceeded at an eighth time t.sub.7. As a result, the instantaneous high pressure p.sub.I exceeds or falls below the first high-pressure limit value p.sub.L1 even more times, as well as the second high pressure limit value p.sub.L2. This is indicated in FIG. 5 by a dotted representation of all time diagrams.

[0074] At a ninth time t.sub.8, the actual high pressure p.sub.I, i.e. the instantaneous high pressure, exceeds the first high pressure limit value p.sub.L1 again. It is assumed here for the explanation that the frequency value H.sub.A is incremented to the value 30. At a tenth time t.sub.9, the instantaneous high pressure p.sub.I again falls below the first high pressure limit value p.sub.L1 and also reaches or falls below the second high-pressure limit value p.sub.L2 at an eleventh time t.sub.10. At a twelfth time t.sub.11, the instantaneous high pressure p.sub.I again exceeds the first high pressure limit value p.sub.L1, which results in the frequency value H.sub.A being incremented to the value 31.

[0075] This now results in the second alarm stage A2 being set, wherein the corresponding logical variable is set from the value 0 to the value 1, which is shown in the sixth time diagram at f). The second alarm stage A2 is therefore set when the first high pressure limit value p.sub.L1 is exceeded for the first time by the actual high pressure, i.e. the instantaneous high pressure p.sub.I, with a predetermined second limit frequency, which is less than a first limit frequency, which is defined for setting the first alarm stage AI, which will be explained below. The second limit frequency is selected here to be 31. It can also preferably be selected to be 30. Preferably, the second limit frequency is chosen between 25 and 35. The frequency value H.sub.A is also compared with the second limit frequencyand as explained belowwith the first limit frequency. The second alarm level A2 corresponds in particular to a yellow alarm, by which an operator of the internal combustion engine 1 is warned of possible damage to the injectors 15.

[0076] At a 13th time t.sub.12, the first high-pressure limit value p.sub.L1 is exceeded and at a 14th time t.sub.13, the second high-pressure limit is reached p.sub.L2 and subsequently also undershot. The instantaneous high pressure p.sub.I subsequently exceeds and falls below the first high pressure limit value p.sub.L1 and also the second high pressure limit value p.sub.L2 further times, which in turn is indicated by a dotted representation of all time diagrams.

[0077] At a 15th time t.sub.14, the instantaneous high pressure p.sub.I exceeds the first high-pressure limit value p.sub.L1 again. It is assumed for the explanation that the frequency value H.sub.A is incremented to 50. At a 16th time t.sub.15, the instantaneous high pressure p.sub.I again falls below the first high pressure limit value p.sub.L1. At a 17th time t.sub.16, the actual high-pressure p.sub.I again exceeds the first high pressure limit value p.sub.L1 without having previously reached or exceeded the second high-speed limit p.sub.L2. Therefore no increment of the frequency value H.sub.A is carried out at this time. At an 18th time t.sub.17, the first high pressure limit value p.sub.L1 is again exceeded. At a 19th time t.sub.18, the second high pressure limit value p.sub.L2 is reached and then undershot.

[0078] At a 20th time t.sub.19, after a further increase the instantaneous high pressure p.sub.I again exceeds the first high-pressure limit p.sub.L1, wherein the frequency value H.sub.A is incremented to the value 51. This now results in the first limit frequency being reached, wherein the first alarm stage AIsee diagram g)is set. The first limit frequency is therefore preferably selected to be 51 here. It can also be selected to be 50. In general, the first limit frequency is preferably selected to be between 45 and 55.

[0079] Setting the first alarm stage AI in turn causes the energization of the injectors 15 to be stopped, whereby no more fuel is injected into the combustion chambers 16. This is done by changing the logical signal value SIG from F to Tsee diagram d).

[0080] At a 21st time t.sub.20, the instantaneous high pressure p.sub.I again falls below the first high pressure limit value p.sub.L1. At a 22.sup.nd time t.sub.21, the instantaneous high pressure p.sub.I reaches the second high pressure limit value p.sub.L2, which results in the injection being enabled again by the logical signal SIG changing its value from T to F. At a 23rd time t.sub.22, the instantaneous high pressure p.sub.I again exceeds the first high pressure limit value p.sub.L1, which means that the fuel injection into the combustion chambers 16 is stopped again by the logical signal SIG again assuming the value T. At a 24th time t.sub.23, the internal combustion engine 1 is switched off, which leads to a drop of the current engine speed n.sub.1. At the same time, the actual high-pressure p.sub.I falls below the first high-pressure limit value p.sub.L1. As a result, the instantaneous high pressure p.sub.I continues to drop and then rises again without having previously reached or exceeded the second high pressure limit value p.sub.L2. At a 25th time t.sub.24, the instantaneous high pressure p.sub.I again exceeds the first high pressure limit value p.sub.L1. At a 26th time t.sub.25, the current engine speed n.sub.I reaches the value 0, that is, the internal combustion engine 1 is now at a standstill. As a result, the logical variable M.sub.S also changes value from 0 to 1. At a 27th time t.sub.26, the instantaneous high pressure p.sub.I again falls below the second high pressure limit value p.sub.L2 from above, which means that the logical signal SIG is changed to the value F. At a 28th time t.sub.27, the alarm reset request AR is set. Since the internal combustion engine 1 is stationary, this causes all alarms, i.e. the first alarm stage AI and the second alarm stage A2, to be reset. At the same time, the frequency value H.sub.A is also reset to zero after triggering the alarm reset request AR with the internal combustion engine 1 at a standstill.

[0081] It can therefore be seen that the frequency value H.sub.A, which indicates the current frequency of the instantaneous high pressure, i.e. the actual high pressure p.sub.I, exceeding the first high pressure limit value p.sub.L1, is incremented when the instantaneous high pressure reaches or exceeds the first high pressure limit value p.sub.L1 from below the second high pressure limit value p.sub.L2. The frequency value H.sub.A is compared with the predetermined limit frequency, in particular with both the first limit frequency and the second limit frequency.

[0082] The second alarm stage A2 is also cancelled if both the standstill of the internal combustion engine 1 is detected and the alarm reset request AR is set.

[0083] The control unit 21 is specially set up to carry out the method described here.

[0084] This is now explained in more detail in connection with FIG. 6.

[0085] FIG. 6 shows a schematic representation of another embodiment of the method in the form of a flowchart. This embodiment may also be provided cumulatively with the embodiments according to FIGS. 4 and 5, wherein preferably all steps and features of the method explained in connection with FIGS. 4 to 6 are carried out in combination with each other.

[0086] Before the method starts in a start step S0, the value of a variable M, which represents a marker and is also referred to below as a marker variable, and which can take the values 0 and 1, is initialized to 1. The current time period t.sub.A is updated to the value zero, and the frequency value H.sub.A is also initialized to zero.

[0087] In a first step SI a query is carried out as to whether the first alarm stage AI is set. If this is not the case, the method is continued in a second step S2, in which a query is carried out as to whether the instantaneous high pressure p.sub.I is greater than the first high pressure limit value p.sub.L1. If this is not the case, the method is continued in a third step S3, in which a check is carried out as to whether the marker variable M has the value 1, i.e. is set, which is the case according to the aforementioned initialization at a first start of the method. If the variable M is set, the method is continued in a sixth step S6. If, on the other hand, the variable M is not set, i.e. it has a value of 0, the method continues at a fourth step S4. In this a check is carried out of whether the instantaneous high pressure p.sub.I is less than or equal to the second high pressure limit value p.sub.L2. If this is not the case, the method continues with the sixth step S6. However, if this is the case, in a fifth step S5 the marker variable M is set to the value 1, then the method proceeds with the sixth step S6. In the sixth step S6, the current time period t.sub.A is set to zero. After the sixth step S6, a seventh step S7 is executed, wherein the logical signal SIG is set to the value F. Then the method proceeds with a 33rd step S33.

[0088] If the result of the query in the second step S2 is positive, i.e. the instantaneous high pressure p.sub.I is actually greater than the first high pressure limit value p.sub.L1, the method will be continued in an eighth step S8. In this eighth step S8, a check is carried out as to whether the current time period t.sub.A is greater than the predetermined limit period t.sub.L. If this is the case, the method continues with a ninth step S9, a tenth step S10, an eleventh step SII and then the 33rd step S33. In the ninth step S9, the frequency value H.sub.A is set to the value zero. In the tenth step S10, the first alarm stage AI is set. In the eleventh step S11, the logical signal SIG is set to the value T.

[0089] If, on the other hand, the result of the query in the eighth step S8 is negative, i.e. if the current time period t.sub.A is less than or equal to the limit period t.sub.L, the method is continued in a twelfth step S12. In this step, the time variable t.sub.A is incremented by a process-inherent sampling time Ta.

[0090] In a 13th step S13, the marker variable M is queried again. If this is not set, the method continues with a 16th step S16. If it is set however, i.e., it has the value 1, the frequency value H.sub.A is incremented in a 14th step S14. The marker variable M is then set to zero in a 15th step S15.

[0091] In the 16th step S16, a query is carried out as to whether the second alarm stage A2 is set. If this variable is set, i.e. it has a value of 1, the method is continued with a 19th step S19. If it is not set, i.e. if it has a value of zero, the method is continued with a 17th step S17. In this 17.sup.th step S17, a check is carried out as to whether the frequency value H.sub.A is greater than the second limit frequency H.sub.L2 reduced by 1. If this is not the case, the method is continued with the 19th step S19, otherwise with the 18th step S18, in which the second alarm stage A2 is set. In the 19th step S19, a query is carried out as to whether the frequency value H.sub.A is greater than the first limit frequency H.sub.L1 reduced by 1. If this is the case, the method is continued with a 20th step S20, a 21st step S21, a 22nd step S22 and then the 33rd step S33. If, on the other hand, this is not the case, the method is continued with a 23rd step S23 and then the 33rd step S33. In the 20th step S20, the frequency value H.sub.A is set to zero. In the 21st step S21 the first alarm stage AI is set. In the 22nd step S22, the logical signal SIG is set to the value T. In the 23rd step S23, on the other hand, the logical signal SIG is set to the value F.

[0092] If the result of the query in the first step S1 is positive, i.e. the first alarm stage AI is set, the method is continued with a 24th step S24. In this 24th step S24, the variable M is queried. If this is set, the method is continued with a 25th step S25, otherwise with a 29th step S29. In the 25th step S25, a query is carried out as to whether the instantaneous high pressure p.sub.I is greater than the first high pressure limit value p.sub.L1. If this is the case, the method is continued with a 26th step S26, a 27th step S27 and then with the 33rd step S33. If, on the other hand, the high pressure p.sub.I is less than or equal to the first high pressure limit value p.sub.L1, the method is continued with a 28th step S28 and then the 33rd step S33.

[0093] In the 26th step S26, the marker variable M is set to zero. In the 27th step S27, the logical signal SIG is set to the value T. In the 28th step S28, the logical signal SIG is set to the value F.

[0094] In the 29th step S29, a check is carried out as to whether the instantaneous high pressure p.sub.I is less than or equal to the second high pressure limit value p.sub.L2. If this is the case, the method is continued with a 30th step S30, a 31st step S31 and then the 33rd step S33. If this is not the case, the method is continued with a 32nd step S32 and then with the 33rd step S33. In the 30th step S30, the marker variable M is set to the value 1. In the 31st step S31, the logical signal SIG is set to the value F. In the 32nd step S32, the logical signal SIG is set to the value T.

[0095] In the 33rd step S33 a check is carried out as to whether the following conditions are met at the same timei.e. cumulatively: The alarm reset request AR is set, the internal combustion engine 1 is at a standstill, i.e. the logical variable M.sub.S is set, and either the first alarm stage AI or the second alarm stage A2 is set. If these conditions are met cumulatively, the method is continued with a 34th step S34, a 35th step S35, a 36th step S36 and a 37th step S37. In the 34th step S34, the second alarm stage is reset. In the 35th step S35, the first alarm stage is reset. In the 36th step S36, the current time period t.sub.A is set to zero. In the 37th step S37, the frequency value H.sub.A is set to zero. The program then ends in an end step S38. If one of the cumulative conditions of the 33rd step S33 is not met, the program sequence ends in the end step S38 without passing through steps S34 to S37.

[0096] The method is preferably carried out continuously and iteratively, so that it starts again with the starting step S0 as soon as it has finished at the end step S38. The initialization of the marker variable M, the current period t.sub.A and the frequency value H.sub.A with the values mentioned in the figure description of FIG. 6 is carried out only at a very first start of the program sequence, but by no means for each pass, but rather the values from the previous pass are carried over for these variables for each new pass following a previous pass, otherwise the logic of the method would not work. The duration of a pass through the method is preferably the duration of the sampling step Ta in each case, wherein this ensures in particular that the current period t.sub.A is always correctly updated in the twelfth step S12.

[0097] In particular, the following advantages arise in connection with the invention: injectors 15 can be damaged if their components are overloaded due to excessive fuel pressures in the high-pressure accumulator 13. Such an excessive loading occurs when the instantaneous high pressure is either above a first limit value for too long a period of time, or if this limit is exceeded too frequently. The method proposed here makes it possible to protect the injectors 15 against further damage by disabling the injection of fuel into combustion chambers 16 in both cases. The injection of fuel is only enabled again when the high pressure falls below the first limit by a hysteresis differential pressure value. This allows the internal combustion engine 1 to continue operating in a kind of emergency mode despite possible prior damage until the operator has the possibility to carry out a maintenance measure, in particular to replace the injectors 15. The fact that replacement of the injectors 15 or maintenance is required is indicated to the operator by the triggering of the first alarm stage AI, i.e. of the red alarm, preferably with a corresponding error message. In order to warn the operator in advance, the second alarm stage A2, i.e. a yellow alarm, is triggered at an early stage, namely when a certain number of limit values that is still permissible have been detected.