Method for operating an injection system of an internal combustion engine, injection system for an internal combustion engine and internal combustion engine with such an injection system

Abstract

Method for operating an injection system (3) of an internal combustion engine (1), wherein the injection system (3) has a high-pressure accumulator (13), wherein a high pressure in the high-pressure accumulator (13) is regulated in normal operation by controlling a low-pressure side intake throttle (9), wherein the high pressure is regulated in a first operating mode of a protective operation by controlling at least one high-pressure side pressure control valve (19), wherein switching from normal operation to the first operating mode of the protective operation occurs when the high pressure reaches or exceeds a first pressure limit value, and wherein switching from the first operating mode of the protective operation to normal operation occurs when the high pressure, starting from above a pressure setpoint value, reaches or falls below the pressure setpoint value, wherein the pressure setpoint value is a high pressure value,to which the high pressure in the high-pressure accumulator is regulated both in the first operating mode of the protective operation and in the normal operation, and wherein the pressure setpoint is less than the first pressure limit value.

Description

The invention relates to a method for operating an injection system of an internal combustion engine, an injection system for an internal combustion engine and an internal combustion engine having such an injection system.Injection systems and methods for operating them are disclosed, for example, in DE 10 2014 213 648 B3 and DE 10 2015 209 377 B4.An injection system of the type discussed here has at least one injector, which is designed in particular for introducing a fuel into a combustion chamber of an internal combustion engine, and a high-pressure accumulator, which is in fluidic connection on the one hand with the at least one injector and on the other hand via a high-pressure pump with a fuel reservoir. In this way, fuel or fuel, wherein these terms are used interchangeably, can be conveyed from the fuel reservoir into the high-pressure accumulator by means of the high-pressure pump. The high-pressure pump is assigned a low-pressure-side suction throttle. In particular, the suction throttle can be controlled as a first pressure control element and is arranged in the fluidic connection between the fuel reservoir and the high-pressure accumulator, preferably upstream of the high-pressure pump. The delivery rate of the high-pressure pump and thus at the same time the pressure in the high-pressure accumulator can thus be influenced via the suction throttle. The injection system also has at least one high-pressure-side pressure regulating valve, via which the high-pressure accumulator is connected in terms of flow to the fuel reservoir-in particular parallel to the flow path running via the high-pressure pump-with the fuel reservoir. Fuel can thus be diverted from the high-pressure accumulator into the fuel reservoir via the pressure regulating valve.In the fluidic connection between the fuel reservoir and the high-pressure accumulator, a fuel filter can be provided, which serves to filter water out of the fuel. However, air is also filtered from the fuel at the same time, which can collect in the flow path to the high-pressure accumulator, so that an air column forms. The air can in turn be conveyed by the high-pressure pump together with the fuel into the high-pressure accumulator, where it can lead to undesirable pressure oscillations. In this case, it is possible in particular for the high pressure in the high-pressure accumulator to exceed a first pressure limit value on account of these undesired oscillations.In the context of a method for operating the injection system, it is provided that the high pressure in the high-pressure accumulator is regulated in a normal operation by actuating the low-pressure-side suction throttle, wherein the high pressure is regulated in a first operating mode of a protective operation by actuating the at least one high-pressure-side pressure regulating valve. It is switched from normal operation to the first operating mode of the protective operation when the high pressure reaches or exceeds the first pressure limit value. Since this constitutes a protection mechanism, it is typically provided that the protection operation is maintained until the internal combustion engine having the injection system is shut off. If there is now no actual fault, but the first pressure limit value is exceeded briefly only on account of undesirable pressure oscillations of the high pressure, continued pressure regulation via the first pressure regulating valve has proven to be disadvantageous, in particular since the fuel is excessively heated in this operating mode, as a result of which the efficiency of the internal combustion engine decreases and the emissions increase.DE 10 2013 202 266 A1 discloses a method and a device for monitoring a high-pressure injection system, in particular of an auto-igniting internal combustion engine of a motor vehicle, wherein a reaction chain is provided for detecting and treating overpressure situations, wherein pressure values detected for detecting an overpressure situation are bounced, and wherein in the case of detecting an overpressure situation, a fault reaction of at least one actuator of the high-pressure injection system is generated. In particular, it is provided that a preventive shutdown is generated as a preliminary fault response, overlapping at least partially with the debouncing, that the preliminary fault response is maintained if the presence of an overpressure situation is confirmed after debouncing has taken place, and that the preliminary fault response is canceled again if the presence of an overpressure situation is not confirmed after debouncing has taken place.The object of the invention is to create a method for operating an injection system, an injection system for an internal combustion engine and an internal combustion engine with such an injection system, wherein the disadvantages mentioned do not arise.The object is achieved by providing the present technical teaching, in particular the teaching of the independent claims and of the preferred embodiments disclosed in the dependent claims and the description.The object is achieved in particular by switching from the first operating mode of the protective mode into the normal mode within the scope of a method for operating an injection system when the high pressure reaches or falls below the pressure setpoint from above a pressure setpoint value, in particular starting from the first pressure limit value, wherein the pressure setpoint value is smaller than the first pressure limit value. In this way, a return of the injection system from the protective mode to the normal mode is made possible before the internal combustion engine is switched off, that is to say during the running operation of the internal combustion engine. The fact that the high pressure reaches or falls below the pressure setpoint again from above the latter, in particular starting from the first pressure limit value, indicates that no technical problem or defect of the injection system continues permanently, but rather that the exceeding of the first pressure limit value is based on a time-limited, non-critical event, such as, for example, an undesired high-pressure oscillation, so that the protective operation can be left and switched back to the normal operation without risk. In this case, in particular the disadvantages resulting from the operation of the injection system in the protective mode, such as impermissible heating of the fuel, can be avoided. In particular in the case of high-pressure oscillations which are caused by air in the injection system, this only briefly switches to the protective mode and can subsequently return to normal operation again, in particular when the air has leaked again from the high-pressure accumulator by shutting down by means of the pressure regulating valve, in which normal operation the high pressure is regulated by means of the suction throttle as the first pressure actuator. This prevents unnecessary heating of the fuel and unnecessary loading of the pressure regulating valve. The durability of the internal combustion engine is prolonged and the efficiency is improved. In addition, emissions are reduced.According to the invention, the pressure setpoint value is a high-pressure value to which the high pressure in the high-pressure accumulator is regulated both in the first operating mode of the protective mode and in the normal mode.The at least one pressure regulating valve is actuated in the first operating mode of the protective mode, in particular as a second pressure control element, in order to regulate the high pressure.In normal operation, a high-pressure disturbance variable is preferably generated by means of the at least one pressure regulating valve in order to stabilize the high-pressure regulation.The high-pressure accumulator is preferably designed as a common high-pressure accumulator, with which a plurality of injectors are in fluid communication. Such a high-pressure accumulator is also referred to as a rail, the injection system preferably being designed as a common-rail injection system.For comparison with the first pressure limit value, a dynamic rail pressure is preferably used, which results from filtering of the high pressure measured by means of a high-pressure sensor, in particular with a comparatively short time constant. Alternatively, however, it is also possible to compare the measured high pressure directly with the first pressure limit value. In contrast, the filtering has the advantage that short-term overshoots above the first pressure limit value do not directly lead to the switching into the first operating mode of the protective mode.It is possible for the injection system to have exactly one high-pressure-side pressure regulating valve. Alternatively, however, it is also possible for the injection system to have a plurality of high-pressure-side pressure regulating valves, in a preferred embodiment exactly two high-pressure-side pressure regulating valves. In this case, it is possible that, in the first operating mode of the protective operation, a plurality of high-pressure-side pressure regulating valves, in particular both high-pressure-side pressure regulating valves, are actuated as pressure control elements in order to regulate the high pressure in the high-pressure accumulator. According to a preferred embodiment, it is provided that the first operating mode of the protective mode is divided into a first operating mode range of the first operating mode, in which exactly one first high-pressure-side pressure regulating valve is controlled as a pressure actuator for regulating the high pressure, wherein a high-pressure disturbance variable is preferably generated via at least one other high-pressure-side pressure regulating valve for stabilizing the regulation. In a second operating mode range of the first operating mode, at least one second pressure regulating valve of the plurality of pressure regulating valves is actuated in addition to the first pressure regulating valve as a pressure actuator in order to regulate the high pressure in the high-pressure accumulator. A switch is preferably made between the first operating mode range and the second operating mode range as a function of pressure, in particular a switch is preferably made from the first operating mode range to the second operating mode range when the high pressure reaches or exceeds an operating mode range change pressure limit value which is greater than the first pressure limit value. In this way, the at least one second pressure regulating valve can be used for regulation if the regulation via the first pressure regulating valve is no longer sufficient to regulate the high pressure, in particular because not enough fuel can be diverted from the high-pressure accumulator via the first pressure regulating valve.According to a further development of the invention, it is provided that an integral component for a high-pressure regulator, which is configured to actuate the suction throttle for regulating the high pressure in normal operation, is initialized with an integral initial value when switching from the first operating mode of the protective operation to normal operation. The integral initial value is determined as a leakage characteristic value of the injection system as a function of a current operating point of the internal combustion engine. This advantageously ensures that the suction throttle is suitably controlled by the high-pressure regulator immediately after the changeover to normal operation, in particular in such a way that an operating-point-dependent leakage of the injection system can be compensated for by delivering an adapted fuel quantity into the high-pressure accumulator. Otherwise, due to the interruption of the high-pressure control by the high-pressure controller effected in the first operating mode of the protective operation, there would be the risk that the high-pressure controller actuates the suction throttle in an unsuitable manner immediately after the switchover to the normal operation, so that either too little or too much fuel is delivered into the high-pressure accumulator.An operating point of the internal combustion engine is understood here to mean, in particular, a value pair of a momentary rotational speed of the internal combustion engine and a variable which determines the momentary power of the internal combustion engine, in particular a momentary torque, a momentary power, or a momentary setpoint injection quantity of fuel. It is obvious that the momentary leakage of fuel from the high-pressure accumulator depends on the one hand on the rotational speed and on the other hand on the momentary power, since these are the essential variables which determine how much fuel flows out of the high-pressure accumulator.According to a further development of the invention, it is provided that the integral initial value is determined by reading a leakage value from a leakage characteristic map of the internal combustion engine as a function of the instantaneous operating point. This represents a particularly simple way of determining the leakage value. In this case, according to one configuration, it is possible for the leakage value to be used as a leakage characteristic value. In particular, it is possible for the leakage value to be used directly as an integral initial value for initializing the high-pressure regulator. In this case, no further computing steps are required, so that the method is particularly simple. Alternatively, it is possible that the leakage value is calculated with at least one control factor in order to obtain the leakage characteristic value. This allows an additional influence on the control behavior of the high-pressure regulator, in particular in order to influence a transient process of the high pressure to the pressure setpoint value. The control factor is preferably selected to be less than 1, in particular to be 0.8, in order to cause the high pressure to undershoot below the pressure setpoint value when switching from the first operating mode of the protective mode to the normal mode and thus to ensure a robust transition to the high pressure control by means of the suction throttle as a pressure actuator.According to a further development of the invention, it is provided that a constant characteristic map is used as the leakage characteristic map. In a particularly simple manner, the leakage characteristic map can thus be mapped once. The leakage characteristic map is preferably populated with data obtained from test bench tests. Alternatively or additionally, the leakage characteristic map is updated during operation of the injection system. In this way, it is advantageously possible to keep the leakage characteristic map always up-to-date and thus adapt it in particular to changed operating conditions of the internal combustion engine, for example aging effects or the like. The leakage characteristic map is preferably populated with instantaneous values of the integral portion of the high-pressure regulator-during normal operation-as leakage values. Values of the integral component from stationary operating points of the internal combustion engine are preferably used for this purpose. In this case, the integral portion of the high-pressure regulator during steady-state operation corresponds at least substantially to the instantaneous leakage of the injection system and is therefore suitable in a particular manner as a leakage value for requiring the leakage characteristic diagram. On the other hand, it significantly simplifies the use of the leakage characteristic field within the scope of the method proposed here if values of the integral component are stored in the latter, which values can then in turn also be used easily for initializing the integral component for the high-pressure regulator, that is to say as integral values. It is possible here for the instantaneous integral components to be calculated with at least one factor before they are stored in the leakage characteristic map, in particular in order to possibly compensate for effects which arise as a result of later application of factors to the leakage values after they have been read out from the leakage characteristic map. Particularly preferably, the leakage characteristic map is coated with filtered values of the instantaneous integral component. This advantageously allows filtering out short-term fluctuations; in this respect, low-pass filtering is particularly preferably used.According to a further development of the invention, it is provided that a check is made as to whether the suction throttle has a defect before switching from the first operating mode of the protective mode to the normal mode. It is switched to normal operation only if no defect of the suction throttle is detected, or-in other words-if it is detected that the suction throttle can operate properly. This advantageously prevents the switch to normal operation, if appropriate, although a defect is present and it is not ensured that the high pressure can actually be regulated in normal operation. It is thus advantageously only switched over into normal operation if it is actually ensured that the suction throttle can be controlled for regulating the high pressure in normal operation. Thus, not least damage to the internal combustion engine can be avoided.Preferably, the suction throttle is permanently open in the first operating mode of the protective mode.According to a further development of the invention, it is provided that a second operating mode of the protective mode is switched to when the high pressure exceeds a second pressure limit value, wherein in the second operating mode of the protective mode the at least one pressure regulating valve and the suction throttle are permanently opened. The second pressure limit value is in particular greater than the first pressure limit value and preferably greater than the operating mode range change pressure limit value. In the second operating mode of the protective operation, it is ensured that, in the event of excessively high high pressure in the high-pressure accumulator, a sufficiently large fuel quantity can be permanently diverted from the high-pressure accumulator by the at least one pressure regulating valve being permanently opened. In this case, for protecting the injection system and the internal combustion engine from excessively high pressures, regulation of the high pressure is dispensed with. At the same time, the suction throttle is permanently open in order to ensure that sufficient fuel is delivered into the high-pressure accumulator even in the medium power range and in low load points of the internal combustion engine, when the high-pressure pump is running at low rotational speed, so that the operation of the internal combustion engine is not interrupted by too little fuel delivery. Otherwise, as a result of the permanent leakage from the high-pressure accumulator via the permanently open pressure regulating valve, an undersupply of fuel to the combustion chambers could occur, so that the internal combustion engine is ultimately stalled. The second operating mode of the protective mode represents, in particular, a safety function which is intended to ensure continued operation of the internal combustion engine in an emergency operating mode without damage as possible, in particular in order to provide a so-called limp home function. In particular, the at least one pressure regulating valve can fulfil the function of a pressure relief valve, so that a mechanical pressure relief valve can be advantageously dispensed with.According to one embodiment, it is possible for the pressure regulating valve and / or the suction throttle to be actively permanently opened, that is to say to be actuated to a permanently opened state. According to an alternative embodiment, it is possible for the pressure regulating valve and / or the suction throttle to be opened passively and permanently. This is possible in particular if at least one of these elements is designed to be open when current is present. In this case, the corresponding element is preferably not activated, so that it is permanently open-in particular completely open. It is also possible for the at least one pressure regulating valve to be configured to be closed without current and without pressure, but to be configured to be open without current and under pressure. That is, the pressure regulating valve is closed in a state of being not energized and not pressurized, and opens in a de-energized state from a predetermined limit opening pressure value. In this case, the pressure regulating valve can be permanently open in the second operating mode of the protective operation without activation, since it keeps the high pressure in the high-pressure accumulator in the open position. In addition, in a starting operation of the internal combustion engine, if a sufficient high pressure is not yet built up in the high-pressure accumulator, the pressure regulating valve can be closed without current, which enables a more rapid pressure build-up without having to actively actuate the pressure regulating valve in a closed state. Actuation of the pressure regulating valve under pressure causes the pressure regulating valve to close.An embodiment of the method is preferred which is characterized in that a normal function is set for the pressure regulating valve in normal operation, in which function the pressure regulating valve is controlled as a function of a setpoint volume flow. In normal operation, the normal function provides an operating mode for the pressure regulating valve, in which the pressure regulating valve generates the high-pressure disturbance variable by diverting fuel from the high-pressure accumulator into the fuel reservoir.The normal function is preferably set for the pressure regulating valve even in the first operating mode of the protective mode, so that the pressure regulating valve is controlled as a function of a setpoint volume flow. The normal operation on the one hand and the first operating mode of the protection area on the other hand differ in this case in the manner in which the setpoint volume flow for actuating the pressure regulating valve is calculated:In normal operation, the setpoint volume flow is preferably calculated from a static and a dynamic setpoint volume flow. The static setpoint volume flow is in turn preferably calculated as a function of a setpoint injection quantity and a rotational speed of the internal combustion engine via a setpoint volume flow characteristic diagram. In a torque-oriented structure, a setpoint torque or a setpoint power can also be used instead of the setpoint injection quantity. Constant leakage is simulated via the static setpoint volume flow by the fuel being discharged only in a low-load range and in a small quantity. It is advantageous that no significant increase in the fuel temperature and also no significant reductions in the efficiency of the internal combustion engine occur. As a result of the replication of a constant leakage for the injection system via the pressure regulating valve, the stability of the high-pressure regulation in the low-load range is increased, which can be detected, for example, from the fact that the high pressure remains approximately constant in the overrun mode. The dynamic setpoint volume flow is calculated via a dynamic correction as a function of a setpoint high pressure and an actual high pressure or the control deviation derived therefrom. If the control deviation is negative, for example in the event of a load shedding of the internal combustion engine, the static setpoint volume flow is corrected by means of the dynamic setpoint volume flow. Otherwise, i.e. in particular in the case of a positive control deviation, there is no change in the static setpoint volume flow. A pressure increase of the high pressure is counteracted by means of the dynamic setpoint volume flow, with the advantage that the settling time of the system can be improved once again.This procedure is described in detail in German patent specification DE 10 2009 031 529 B3.In the first operating mode of the protective operation, on the other hand, the setpoint volume flow is calculated by a pressure regulating valve pressure regulator for regulating the high pressure. In this case, the setpoint volume flow represents a control variable for the regulation of the high pressure.Alternatively or additionally, it is preferred that a standstill function is set for the pressure regulating valve in the second operating mode of the protective operation, wherein the pressure regulating valve is not activated in the standstill function. This is the case in particular when a pressure regulating valve is used which is open when no current is present or closed when no current is present and no pressure is present. Because the pressure regulating valve is then not actuated in the standstill function, i.e. is not energized, a maximum opening of the valve results-possibly on account of the high pressure present on the input side-so that a maximum fuel volume flow from the high-pressure accumulator into the fuel reservoir is diverted via the pressure regulating valve. In this way, the pressure regulating valve can completely assume the functionality of an otherwise provided mechanical pressure relief valve, so that the mechanical pressure relief valve can be dispensed with. The configuration of the pressure regulating valve that is open without current or open without pressure and closed without current has the advantage that it reliably opens completely even if it is no longer energized due to a defect.A transition from the normal function to the standstill function is preferably carried out when the high pressure, in particular the dynamic rail pressure, exceeds the second pressure limit value, or when a defect of the high pressure sensor is detected. If the high-pressure sensor is defective, the high pressure can no longer be regulated, and it is also no longer possible to detect an unacceptably high pressure in the high-pressure accumulator. For safety reasons, the standstill function for the pressure regulating valve is therefore set in this case, so that the pressure regulating valve opens to the maximum extent and thus brings the injection system into a safe state, which corresponds to a state in which the mechanical pressure relief valve would otherwise be open. An impermissible increase in the high pressure can then no longer occur. The standstill function is preferably set starting from the normal function even if a standstill of the internal combustion engine is detected. Specifically, when the rotation speed of the internal combustion engine decreases below a predetermined value for a predetermined time, a standstill of the internal combustion engine is detected, and the standstill function for the pressure control valve is set. This is the case in particular when the internal combustion engine is shut down. A transition between the standstill function and the normal function occurs upon a start of the internal combustion engine preferably when it is determined that the internal combustion engine is running, wherein at the same time the high pressure exceeds a starting pressure value. Thus, a certain minimum pressure build-up in the high-pressure accumulator preferably takes place first, before the pressure regulating valve is actuated in the normal function for generating the high-pressure disturbance variable. The fact that the internal combustion engine is running can preferably be detected by a predetermined limit rotational speed being exceeded for a predetermined time.According to a further development of the invention, it is provided that only the first operating mode of the protective mode is switched back to normal operation. This means in particular that the switch from the second operating mode of the protective mode is not switched back to the normal mode. This takes into account the idea that the second pressure limit value is preferably selected such that it is exceeded by the high pressure only when a serious defect is actually present in the injection system, so that a return to normal operation can no longer be justified subsequently. It is preferably additionally provided that the switching from the second operating mode of the protective mode is not switched back into the first operating mode of the protective mode. The second operating mode of the protective mode thus advantageously remains until the internal combustion engine is shut down, and preferably also continues until it is signaled or confirmed in a suitable manner that the defect on the injection system has been corrected, for example by actuating a switch, an electronic input or the like.The object is also achieved by providing an injection system for an internal combustion engine, which injection system has at least one injector and a high-pressure accumulator, which is in fluidic connection on the one hand with the at least one injector and on the other hand via a high-pressure pump with a fuel reservoir, wherein a suction throttle is assigned to the high-pressure pump as a first pressure actuator. The injection system also has at least one pressure regulating valve, via which the high-pressure accumulator is fluidically connected to the fuel reservoir. In addition, the injection system has a control unit which is operatively connected to the at least one injector, the suction throttle and the at least one pressure regulating valve-in each case for the purpose of controlling the latter. The control device is configured to carry out a method according to the invention or a method according to one of the embodiments described above. In connection with the injection system, the advantages result in particular, which have already been explained in connection with the method.The control unit is preferably designed as an engine control unit (ECU) of the internal combustion engine. Alternatively, however, it is also possible for a separate control device to be provided specifically for carrying out the method.A low-pressure pump is preferably arranged upstream of the high-pressure pump and the suction throttle in order to convey fuel from the fuel reservoir to the suction throttle and the high-pressure pump.A pressure sensor is preferably arranged on the high-pressure accumulator, which is configured to detect a high pressure in the high-pressure accumulator and is operatively connected to the control unit, so that the high pressure can be recorded in the control unit. The control device is preferably configured to filter the measured high pressure, in particular to filter with a first, longer time constant, in order to calculate an actual high pressure to be used within the scope of the pressure regulation, and to filter the measured high pressure with a second, shorter time constant, in order to calculate the dynamic rail pressure.According to a preferred embodiment, the injection system has exactly one pressure regulating valve.According to another preferred embodiment, the injection system has a plurality of pressure regulating valves, particularly preferably exactly two pressure regulating valves, wherein the high-pressure accumulator is fluidically connected to the fuel reservoir via each of the pressure regulating valves, preferably fluidically parallel to one another.Preferably, the at least one pressure regulating valve is open when there is no current. This embodiment has the advantage that the pressure regulating valve opens to a maximum extent in the event that it is not actuated or energized, which enables particularly reliable and reliable operation in particular when a mechanical pressure relief valve is dispensed with. An impermissible increase in the high pressure in the high-pressure accumulator can then also be avoided if an energization of the pressure control valve is not possible due to a technical fault.The at least one pressure regulating valve is particularly preferably designed to be closed in a pressure-free and non-energized manner. In this case, it is preferably designed such that it is closed up to a predetermined limit opening pressure value in the case of a pressure applied on the input side, wherein it opens when the pressure applied on the input side reaches or exceeds the limit opening pressure value in the non-energized state. This results in particular in the advantages already explained in connection with the method.According to a further development of the invention, it is provided that the injection system is free of a mechanical overpressure valve. Rather, as explained in connection with the method, its function can be advantageously taken over by the at least one pressure regulating valve in the second operating mode of the protective mode.Finally, the object is also achieved by providing an internal combustion engine which has an injection system according to the invention or an injection system according to one of the exemplary embodiments described above. In connection with the internal combustion engine, there are in particular the advantages which have already been explained in connection with the injection system and the method.The internal combustion engine preferably has a plurality of combustion chambers, preferably identically designed. Each combustion chamber is preferably assigned at least one injector of the injection system for introducing fuel into the combustion chamber. The injection system thus preferably has at least as many injectors as the internal combustion engine has combustion chambers, according to a preferred embodiment in particular exactly the same number, wherein it is however likewise possible for two or more injectors to be assigned to each combustion chamber, for example. The internal combustion engine may have, in particular, four, six, eight, ten, twelve, fourteen, sixteen, eighteen or twenty combustion chambers. However, a different, in particular smaller or larger number of combustion chambers is also possible. The internal combustion engine is preferably designed as a reciprocating piston engine. The internal combustion engine is preferably designed as a diesel engine.The invention is explained in more detail below with reference to the drawings. The following are shown: FIG. 1 shows a schematic illustration of a first exemplary embodiment of an internal combustion engine having an exemplary embodiment of an injection system; FIG. 2 shows a schematic illustration of a second exemplary embodiment of an internal combustion engine having a second exemplary embodiment of an injection system; FIG. 3 shows a detailed illustration of a method for operating an injection system according to the prior art; FIG. 4 shows a schematic detailed illustration of a method for operating an injection system; FIG. 5 shows a detailed illustration of a method for operating an injection system according to the prior art; FIG. 6 shows a detailed illustration of an exemplary embodiment of a method for operating an injection system; FIG. 7 shows a detailed illustration of an exemplary embodiment of a method for operating an injection system; FIG. 8 shows a detailed illustration of an exemplary embodiment of a method for operating an injection system; FIG. 9 shows a detailed illustration of an exemplary embodiment of a method for operating an injection system; FIG. 10 shows a detailed illustration of an exemplary embodiment of a method for operating an injection system, and FIG. 11 shows a diagrammatic illustration of the mode of operation of an exemplary embodiment of a method for operating an injection system.FIG. 1 shows a schematic illustration of a first exemplary embodiment of an internal combustion engine 1 with a first exemplary embodiment of an injection system 3. It has a low-pressure pump 5 for delivering fuel from a fuel reservoir 7, an adjustable, low-pressure-side suction throttle 9 for influencing a fuel volume flow flowing through it, a high-pressure pump 11 for delivering the fuel with an increase in 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 designed with individual accumulators, wherein then, for example, an individual accumulator 17 is integrated in the injector 15 as an additional buffer volume. An in particular electrically controllable pressure regulating valve 19 is provided, via which the high-pressure accumulator 13 is fluidically connected to the fuel reservoir 7. A fuel volume flow is defined via the position of the pressure regulating valve 19, which is diverted from the high-pressure accumulator 13 into the fuel reservoir 7. This fuel volume flow is designated VDRV in FIG. 1 and in the text which follows and represents a high-pressure disturbance variable of the injection system 3.The injection system 3 does not have a mechanical pressure relief valve, which is conventionally provided and connects the high-pressure accumulator 13 to the fuel reservoir 7. The mechanical pressure relief valve can be dispensed with, since its function is preferably completely taken over by the pressure relief valve 19.The operating mode of the internal combustion engine 1 is determined by an electronic control unit 21, which is preferably designed as an engine control unit of the internal combustion engine 1, in particular 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). In the memory modules, the operating data relevant for the operation of the internal combustion engine 1 are applied in characteristic maps / characteristic curves. Via these, the electronic control unit 21 calculates output variables from input variables. The following input variables are shown by way of example in FIG. 1: a measured, still unfiltered high pressure p, which prevails in the high-pressure accumulator 13 and is measured by means of a high-pressure sensor 23, a momentary engine rotational speed n I, a signal FP for the power specification by an operator of the internal combustion engine 1, and an input variable E. Further sensor signals are preferably combined under the input variable E, for example a charge air pressure of an exhaust gas turbocharger. In an injection system 3 with individual accumulators 17, an individual accumulator pressure p E is preferably an additional input variable of the control unit 21.FIG. 1 shows, as output variables of the electronic control unit 21, by way of example, a signal PWMSDfor actuating the suction throttle 9 as a first pressure actuator, a signal vefor actuating the injectors 15-which in particular specifies an injection start and / or an injection end or else an injection duration-a signal PWMDRVfor actuating the pressure control valve 19 as a second pressure actuator and an output variable A. The position of the pressure regulating valve 19 and thus the high-pressure disturbance variable VDRV is defined via the preferably pulse-width-modulated signal PWMDRV. The output variable A is representative of further actuating signals for controlling and / or regulating the internal combustion engine 1, for example an actuating signal for activating a second exhaust gas turbocharger during register charging.FIG. 2 shows a schematic illustration of a second exemplary embodiment of an internal combustion engine 1 with a second exemplary embodiment of an injection system 3; a first, in particular electrically controllable, pressure regulating valve 19 is provided here, via which the high-pressure accumulator 13 is fluidically connected to the fuel reservoir 7. A fuel volume flow is defined via the position of the first pressure regulating valve 19, which is diverted from the high-pressure accumulator 13 into the fuel reservoir 7. This fuel volume flow is denoted by VDRV 1 in FIG. 2 and represents a high-pressure disturbance variable of the injection system 3.The injection system 3 additionally has a second, in particular electrically controllable, pressure regulating valve 20 here, via which the high-pressure accumulator 13 is likewise fluidically connected to the fuel reservoir 7. The two pressure regulating valves 19, 20 are accordingly arranged parallel to one another in particular in terms of flow. A fuel volume flow can also be defined via the second pressure regulating valve 20, which can be diverted from the high-pressure accumulator 13 into the fuel reservoir 7. This fuel volume flow is denoted in FIG. 2 by VDRV2.It is possible for the injection system 3 to have more than two pressure regulating valves 19, 20.In contrast to FIG. 1, a first signal PWMDRV 1 for actuating a first pressure regulating valve of the two pressure regulating valves 19, 20 and a second signal PWMDRV 2 for actuating a second pressure regulating valve of the two pressure regulating valves 19, 20 are shown here as output variables of the electronic control unit 21. The assignment of the first signal PWMDRV 1 to the first pressure regulating valve 19 and of the second signal PWMDRV 2 to the second pressure regulating valve 20 as illustrated in FIG. 2 is preferably not fixed for all times, rather the pressure regulating valves 19, 20 are preferably activated alternately with the signals PWMDRV 1, PWMDRV 2. The signals PWMDRV 1, PWMDRV 2 are preferably pulse width modulated signals, by means of which the position of a pressure regulating valve 19, 20 and thus the volume flow VDRV 1, VDRV 2 assigned to the pressure regulating valve 19, 20 in each case can be defined.If the second pressure regulating valve 20 is added, the method explained below for exactly one pressure regulating valve 19 preferably only changes as follows: the second pressure regulating valve 20 is actuated in a normal operation and in a first operating mode range of a first operating mode of a protective operation in order to generate the high-pressure disturbance variable. In a second operating mode range of the first operating mode of the protective operation, the second pressure regulating valve 20 is preferably controlled for pressure regulation in addition to the first pressure regulating valve 19, in particular by a pressure regulating valve pressure regulator. In a second operating mode of the protective mode, the second pressure regulating valve 20 is preferably also permanently opened. On the basis of the following explanations in connection with the first pressure regulating valve 19 as the single pressure regulating valve, this functionality can be implemented easily. In addition, a corresponding use of a second pressure regulating valve is disclosed in German patent specification DE 10 2015 209 377 B4.For the sake of simpler illustration, the mode of operation of the injection system 1 is explained below with reference to the exemplary embodiment illustrated in FIG. 1, which has exactly one pressure regulating valve 19.FIG. 3 shows at a) a schematic representation of an example of a method for operating the injection system 3 according to FIG. 1, a first high-pressure regulating circuit 25 is provided, via which the high pressure in the high-pressure accumulator 13 is regulated by means of the suction throttle 9 as the first pressure actuator during normal operation of the injection system 3. The first high-pressure regulating circuit 25 is explained in more detail in conjunction with FIG. 5, where it is illustrated in detail. The first high-pressure regulating circuit 25 has, as input variable, a pressure setpoint p S, also referred to below as setpoint high pressure p S for the injection system 3. This is read from a characteristic map preferably as a function of a rotational speed of the internal combustion engine 1, a load or torque request to the internal combustion engine 1, and / or as a function of further variables, in particular variables serving for correction. Further input variables of the first high-pressure control circuit 25 are, in particular, the instantaneous rotational speed n I of the internal combustion engine 1 and a setpoint injection quantity Q S. preferably calculated by a rotational speed regulator. As an output variable, the first high-pressure regulating circuit 25 has, in particular, the high pressure p measured by the high-pressure sensor 23, which is preferably subjected to a first filtering with a greater time constant in order to determine the actual high pressure p I and at the same time is preferably subjected to a second filtering with a smaller time constant in order to calculate a dynamic rail pressure p dyn. These two pressure values p I, p dyn represent further output variables of the first high-pressure regulating circuit 25.FIG. 3 a) illustrates the actuation of the pressure regulating valve 19. A first switching element 27 is provided, with which switching can be performed between normal operation and the first operating mode of the protective operation as a function of a first logic signal SIG 1. Preferably, the first switching element 27 is implemented completely on an electronic or software level. In this case, the functionality described below is preferably switched depending on the value of a variable corresponding to the first logic signal SIG 1, which variable is in particular designed as a so-called flag and can assume the values "true" or "false". Alternatively, however, it is of course also possible for the first switching element 27 to be designed as a real switch, for example as a relay. This switch can then be switched, for example, depending on a level of an electrical signal. In the embodiment specifically shown here, normal operation is set when the first logic signal SIG 1 has the value "false" (false). On the other hand, the first mode of the protection mode is set when the first logic signal SIG 1 has the value "true" (true).A second switching element 29 is provided, which is configured to switch the actuation of the pressure regulating valve 19 from a normal function to a standstill function and back. In this case, the second switching element 29 is controlled as a function of a second logic signal SIG 2 or the value of a corresponding variable. The second switching element 29 can be designed as a virtual, in particular software-based, switching element which switches between the normal function and the standstill function as a function of the value of a variable, in particular designed as a flag. Alternatively, however, it is also possible for the second switching element to be designed as a real switch, for example as a relay, which switches as a function of a signal value of an electrical signal. In the embodiment shown here in concrete terms, the second logic signal SIG 2 corresponds to a state variable which can assume the values 1 for a first state and 2 for a second state. Here, the normal function for the pressure control valve is set when the second logic signal SIG2 assumes the value 2, and the standstill function is set when the second logic signal SIG2 assumes the value 1. Of course, a different definition of the second logic signal SIG 2 is possible, in particular in such a way that a corresponding variable can assume the values 0 and 1.First, the control of the pressure regulating valve 19 in normal operation and with the normal function set will now be described. A first calculation element 31 is provided, which outputs as output variable a calculated setpoint volume flow V S,ber wherein the instantaneous rotational speed n I, the setpoint injection quantity Q S, the setpoint high pressure p S, the dynamic rail pressure p dyn, and the actual high pressure p I enter the first calculation element 31 as input variables. The mode of operation of the first calculation element 31 is described in detail in the German patent specifications DE 10 2009 031 528 B3 and DE 10 2009 031 527 B3. It is found in particular that in a low load range, for example, during idling of internal combustion engine 1, a positive value for a static setpoint volume flow is calculated, while outside the low load range, a static setpoint volume flow of 0 is calculated. The static setpoint volume flow is preferably corrected by adding up a dynamic setpoint volume flow which is in turn calculated via a dynamic correction as a function of the setpoint high pressure p S, the actual high pressure p I and the dynamic rail pressure p dyn. The calculated setpoint volume flow V S,ber is finally the sum of the static setpoint volume flow and the dynamic setpoint volume flow. The calculated setpoint volume flow V S,ber in this respect, is a resulting setpoint volume flow.In normal operation, when the first logic signal SIG 1 has the value "false", the calculated setpoint volume flow V S,ber is transferred as the setpoint volume flow V S to a pressure control valve characteristic diagram 33. The pressure control valve characteristic field 33 maps an inverse characteristic of the pressure control valve 19, as described in German patent specification DE 10 2009 031 528 B3. The output variable of this characteristic map is a pressure regulating valve setpoint flow I S, input variables are the setpoint volume flow V S to be controlled and the actual high pressure p I.Alternatively, it is also possible that the setpoint volume flow V S is not calculated by means of the first calculation element 31, but rather is predefined to be constant in normal operation.The pressure regulating valve setpoint current I S is supplied to a current regulator 35, which has the task of regulating the current for actuating the pressure regulating valve 19. Further input variables of the current regulator 35 are, for example, a proportional coefficient kp I,DRV and an ohmic resistance R I,DRV of the pressure regulating valve 19. The output variable of the current regulator 35 is a setpoint voltage U S for the pressure regulating valve 19, which is converted, by reference to an operating voltage U B in a manner customary per se, into a switch-on duration for the pulse-width-modeled signal PWMDRV for controlling the pressure regulating valve 19 and is fed to the latter in the normal function, i.e. if the second logic signal SIG2 has the value 2. For the current control, the current at the pressure control valve 19 is measured as current variable I DRV filtered in a first current filter 37 and fed back to the current controller 35 as filtered actual current I I.As already indicated, the duty cycle PWMDRVof the pulse width-modulated signal for actuating the pressure regulating valve 19 is calculated in a manner customary per se according to the following equation from the setpoint voltage U S and the operating voltage U BIn this way, in normal operation, a high-pressure disturbance variable, namely the diverted setpoint volume flow V S is generated via the pressure regulating valve 19.When the first logic signal SIG 1 becomes "true", the first switching element 27 switches from the normal operation to the first operation mode of the protection operation. Under what conditions this is the case, it will be explained in conjunction with FIG. 3 b). With regard to the actuation of the pressure regulating valve 19, in the first operating mode of the protective operation, there is no difference insofar as the pressure regulating valve 19 is actuated here as well with the setpoint volume flow V S at least as long as the normal function is set by the switching element 29. In this respect, in FIG. 3 a) to the right of the switching element 27 there is no change to the explanations given above. However, the setpoint volume flow V S is calculated differently in the first operating mode of the protective mode than in the normal mode, namely via a second high-pressure control circuit 39.The setpoint volume flow V S is set in this case identically to a limited output volume flow V R of a pressure regulating valve pressure regulator 41. This corresponds to the upper switching position of the first switching element 27. the pressure regulating valve pressure regulator 41 has as input variable a high-pressure control deviation e p which is calculated as the difference between the setpoint high pressure p S and the actual high pressure p I. Further input variables of the pressure control valve pressure regulator 41 are preferably a maximum volume flow V max for the pressure control valve 19, the setpoint volume flow V S,ber calculated in the first calculation element 31 and / or a proportional coefficient kp DRV. The pressure regulating valve pressure regulator 41 is preferably implemented as a PI(DT 1)- algorithm. An integral component (I component) at the time when the first switching element 27 is switched from its lower switching position, shown in FIG. 3 a), to its upper switching position, is initialized with the calculated setpoint volume flow V S,ber. At the top, the I-component of the pressure regulating valve pressure regulator 41 is limited to the maximum volume flow V max for the pressure regulating valve 19. In this case, the maximum volume flow V max is preferably an output variable of a two-dimensional characteristic curve 43 which has the maximum volume flow passing through the pressure control valve 19 as a function of the high pressure, wherein the characteristic curve 43 receives the actual high pressure p I as input variable. The output quantity of the pressure regulating valve pressure regulator 41 is an unlimited volume flow V U, which is limited to the maximum volume flow V max in a first limiting element 45. The first limiting element 45 finally outputs the limited setpoint volume flow V R as output variable. The pressure control valve 19 is then actuated with this as the setpoint volume flow V S by the setpoint volume flow V S being supplied to the pressure control valve characteristic diagram 33 in the manner already described.FIG. 3 shows at b) under which conditions the first logic signal SIG1 assumes the values "true" and "false". As long as the dynamic rail pressure p dyn does not reach or exceed a first pressure limit value p G1 the output of a first comparator element 47 has the value "false". At the start of the internal combustion engine 1, the value of the first logic signal SIG 1 is initialized to "false". As a result, the result of a first OR gate 49 is also "false" as long as the output of the first comparator element 47 has the value "false". The output of the first OR gate 49 is fed to an input of a first OR gate 51, the other input of which is fed the NO, shown by a cross line, of a variable MS, the variable MS having the value "true" when the internal combustion engine 1 is at a standstill and the value "false" when the internal combustion engine 1 is running. During operation of the internal combustion engine 1, the value of the negative connection of the variable MS is accordingly "true". Overall, it now turns out that the output of the first negative-phase element 51 and thus the value of the first logic signal SIG 1 is "incorrect" as long as the dynamic rail pressure p dyn does not reach or exceed the first pressure limit value p G1.If the dynamic rail pressure p dyn reaches or exceeds the first pressure limit value p G1, the output of the first comparator element 47 jumps from "false" to "true". Thus, the output of the first OR gate 49 also jumps from "false" to "true". However, the output of the first negative-profile element 51 also jumps from "false" to "true", so that the value of the first logic signal SIG 1 becomes "true". This value is fed back to the first OR gate 49, which, however, does not alter anything from its output remaining "true". Even a drop in the dynamic rail pressure p dyn below the first pressure limit value p G1 can no longer change the truth value of the first logic signal SIG 1. Instead, this remains "true" until the variable MS, and thus also its negative, changes its truth value, namely when the internal combustion engine 1 is no longer running.The following is thus shown: Normal operation is implemented as long as the dynamic rail pressure p dyn falls below the limit value p G1. In this case, the setpoint volume flow V S is identical to the calculated setpoint volume flow V S,ber since the first logic signal SIG 1 assumes the value "false" and the switching element 27 is thus arranged in its lower position in FIG. 3. If the dynamic rail pressure p dyn reaches or exceeds the limit value p G1, the first logic signal SIG1 assumes the value "true", and the first shift element 27 assumes its upper shift position. Thus, the setpoint volume flow V S in this case becomes identical to the limited volume flow V R of the second high-pressure control circuit 39. This means that in normal operation a high-pressure disturbance variable is generated via the pressure regulating valve 19, wherein the first operating mode of the protective operation is activated when the dynamic rail pressure p dyn reaches the first pressure limit value p G1 and the high pressure is subsequently regulated by the pressure regulating valve pressure regulator 41, and this until a standstill of the internal combustion engine 1 is detected, since only in this case is the variable MS the value "true", thus their negative oscillation the value "false" and thus ultimately the first logic signal SIG 1 again assumes the value "false", whereby the first switching element 27 is brought back into its lower switching position.In the first operating mode of the protective operation, the pressure control valve 19 takes over the control of the high pressure via the second high-pressure control circuit 39.It is also clear that in this method no return to normal operation from the first operating mode of the protection area is possible as long as the internal combustion engine 1 is running. Undesirable, air-induced oscillations of the high pressure can therefore disadvantageously lead to the first operating mode of the protective operation being set without it being possible for said operating mode to be exited again when the high pressure falls again.Returning to FIG. 3 a), a second operating mode of the protective mode is explained below: the second operating mode is switched to when the second logic signal SIG 2 assumes the value 1 here. In this case, the second shift element 29 is arranged in its upper shift position shown in FIG. 3, whereby a standstill function for the pressure regulating valve 19 is set as a result. In this, the pressure regulating valve 19 is not driven, that is to say the signal PWMDRV is set to 0. Since a normally open pressure regulating valve 19 is preferably used, this now permanently controls a maximum volumetric fuel flow from the high-pressure accumulator 13 into the fuel reservoir 7.If, on the other hand, the second logic signal SIG2 has the value 2, the normal function for the pressure regulating valve 19 is set-as already explained-and this is controlled by means of the setpoint volume flow V S and the signal PWMDRV calculated therefrom.FIG. 4 schematically shows a state transition diagram for the pressure regulating valve 19 from the normal function to the standstill function and back. The pressure regulating valve 19 is particularly preferably designed such that it is normally closed and normally closed, wherein it is further designed such that it is closed up to a limit opening pressure value at a pressure applied on the input side, wherein it opens when the pressure applied on the input side reaches or exceeds the limit opening pressure value in the normally open state. The limit opening pressure value may be 850 bar, for example.In FIG. 4, the standstill function is symbolized by a first circle K 1, wherein the normal function is symbolized at the top right by a second circle K 2. A first arrow P 1 represents a transition between the standstill function and the normal function, while a second arrow P 2 represents a transition between the normal function and the standstill function. An initialization of the internal combustion engine 1 after the start is indicated by a third arrow P 3, wherein the pressure regulating valve 19 is initially initialized in the standstill function. Only when a continuous operation of the internal combustion engine 1 is simultaneously detected and the actual high pressure p I exceeds a starting value p St is the normal function set for the pressure regulating valve 19-along the arrow P1-and the standstill function reset. The normal function is reset and the standstill function is set along the arrow P 2 if the dynamic rail pressure p dyn exceeds a second pressure limit value p G2 or if a fault of a high-pressure sensor-represented here by a logic variable HDSD-is detected or if it is detected that the internal combustion engine 1 is at a standstill. In the standstill function, the pressure regulating valve 19 is not activated, wherein in the normal function, as explained in connection with FIG. 3, it is activated by means of the setpoint volume flow V S.The following functionality now results: When the internal combustion engine 1 starts, there is initially no high pressure in the high-pressure accumulator 13, and the pressure regulating valve 19 is arranged in its standstill function, so that it is pressure-free and non-conductive, i.e. closed. When the internal combustion engine 1 is running up, a high pressure can therefore rapidly form in the high-pressure accumulator 13, which high pressure eventually exceeds the starting value p St. This is preferably lower than the limit opening pressure value of the pressure regulating valve 19, so that the normal function is initially set for this valve before it opens. This advantageously ensures that the pressure regulating valve 19 is activated in any case when it opens for the first time. Since it is closed without pressure, it remains closed even under actuation until the actual high pressure p I also exceeds the limit opening pressure value, wherein it then opens and is actuated in the normal function, namely either in the normal operation or in the first operating mode of the protective operation.However, if one of the cases described above occurs, the standstill function for the pressure regulating valve 19 is set again.This is the case in particular when the dynamic rail pressure p dyn exceeds the second pressure limit value p G2 wherein this is preferably selected to be greater than the first pressure limit value p G1 and in particular has a value at which a mechanical overpressure valve would open in a conventional configuration of the injection system 3. Since the pressure regulating valve 19 is open when no current is present under pressure, it opens completely in this case in the standstill function and thus securely and reliably fulfills the function of a pressure relief valve.The transition from the normal function to the standstill function also takes place if a defect is detected in the high-pressure sensor 23. If a defect is present here, the high pressure in the high-pressure accumulator 13 can no longer be regulated. In order to still be able to operate the internal combustion engine 1 reliably, the transition from the normal function to the standstill function for the pressure regulating valve 19 is brought about, so that the latter opens and thus prevents an impermissible increase in the high pressure.Furthermore, the transition from the normal function to the standstill function takes place in a case in which a standstill of the internal combustion engine 1 is detected. This corresponds to a resetting of the pressure regulating valve 19, so that upon a renewed start of the internal combustion engine 1, the cycle described here can begin again.If the standstill function is set for the pressure regulating valve 19 under pressure in the high-pressure accumulator 13, this is opened to a maximum extent and shuts off 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 this protective function can replace, in particular, the absence of a mechanical pressure relief valve.It is important here that the pressure regulating valve 19 has only two states, namely the standstill function and the normal function, wherein these two states are fully sufficient to represent the entire relevant functionality of the pressure regulating valve 19 including the protective function for replacing a mechanical pressure relief valve.FIG. 5 shows at a) a schematic representation of logic for calculating the value of a third logic signal SIG 3, which is used to ensure that in the first and second operating modes of the protective operation the suction throttle 9 is controlled to a permanently open operation. This procedure is explained in more detail in conjunction with FIG. 5 b). The value of the third logic signal SIG3 results from a second negative-profile element 61, whose first input in turn receives the negative-profile of the variable MS, the second input receiving the result of a preceding calculation, which is explained in more detail below. The third logic signal SIG 3 is initially initialized with the value "false" at the start of the internal combustion engine 1. A first input of a second OR gate 63 is entered with the result of a second comparator element 65, in which a check is made as to whether the dynamic rail pressure p dyn is greater than or equal to the first pressure limit value p G1. The second input of the second OR element 63 is entered with the result of a comparison element 67 which checks whether the value of the logical variable HDSD which indicates a sensor defect of the high-pressure sensor 23 is equal to 1, in which case a sensor defect is present, and in which no sensor defect is present if the value of the variable HDSD is equal to 0. It is thus shown that the output of the second OR gate 63 assumes the value "true" if at least one of the outputs of the second comparator element 65 or of the comparator element 67 assumes the value "true". Thus, in order for the output of the second OR gate 63 to assume the value "true", at least one of the following conditions must be fulfilled: the dynamic rail pressure p dyn must have reached or exceeded the first pressure limit value p G1 and / or a sensor defect must have been detected in the high-pressure sensor 23, so that the variable HDSD assumes the value 1. If none of these conditions is fulfilled, the output of the second OR gate 63 has the value "false".The output of the second OR gate 63 is input into a first input of a third OR gate 69, into the second input of which the value of the third logic signal SIG3 is input. Since this is originally initialized with the value "false", the output of the third OR gate 69 has the value "false" until the output of the second OR gate 63 assumes the value "true". If this is the case, the output of the third OR gate 69 also jumps to the value "true". In this case, the value of the second negative-profile element 61 also jumps to true when the internal combustion engine 1 is running, i.e. the negative-profile of the variable MS has the value 1, so that the value of the third logic signal SIG 3 also jumps to "true". It can be seen from FIG. 5 a) that the value of the third logic signal SIG 3 remains "true" until a standstill of the internal combustion engine 1 is detected, wherein in this case the variable MS assumes the value "true" and thus its negative agreement assumes the value "false".FIG. 5 shows at b) a schematic representation of the first high-pressure regulating circuit 25 including a third switching element 71 for representing the permanently open operation of the suction throttle 9 in the first and second operating mode of the protective operation, wherein the third switching element 71 receives for its activation the third logic signal SIG 3, the calculation of which was described in conjunction with FIG. 5 a). It is possible for the third switching element 71 to be designed as a software switch, that is to say as a purely virtual switch, as has already been described in connection with the switching elements 27, 29. Alternatively, it is of course also possible for the third switching element 71 to be designed as an actual switch, for example as a relay.As already explained, input variables of the high-pressure control circuit 25 are the setpoint high pressure p S, which is compared with the actual high pressure p I in order to calculate the control deviation e P. This control deviation e p is an input variable of a high-pressure regulator 73, which is preferably designed as a PI(DT 1)- algorithm and is explained in more detail in connection with FIG. 10. A further input variable of the high-pressure regulator 73 is preferably a proportional coefficient kp SD. The output variable of the high-pressure regulator 73 is a fuel volume flow V SD for the intake throttle 9, to which a desired fuel consumption V Q is added at an addition point 75. This desired fuel consumption V Q is calculated in a second calculation element 77 as a function of the instantaneous rotational speed n I and the desired injection quantity Q S and represents a disturbance variable of the first high-pressure control circuit 25. The sum of the output variable V SD of the high-pressure regulator 73 and the disturbance variable V Q is an unlimited desired fuel volume flow V U,SD. This is limited in a second limiting element 79 as a function of the instantaneous rotational speed n I to a maximum volume flow V max,SD for the intake throttle 9. As the output of the second limiting element 79, a limited desired fuel volume flow V S,SD for the intake throttle 9 results, which enters a pump characteristic curve 81 as an input variable. This converter converts the limited desired fuel volume flow V S,SD into a characteristic curve intake throttle flow I KL,SD.If the third switching element 71 has the upper switching state shown in FIG. 5 b), which is the case when the third logic signal SIG 3 has the value "false", a suction throttle setpoint current I S,SD is set equal to the characteristic curve suction throttle current I KL,SD. This suction throttle setpoint current I S,SD represents the input variable of a suction throttle current regulator 83, which has the task of regulating the suction throttle current through the suction throttle 9. A further input variable of the suction throttle current regulator 83 is, inter alia, an actual suction throttle current I I,SD. The output quantity of the suction throttle current regulator 83 is a suction throttle setpoint voltage U S,SD, which is finally converted in a third calculation element 85 in a manner known per se into a switch-on duration of a pulse-width-modulated signal PWMSDfor the suction throttle 9. The suction throttle 9 is actuated with this, the signal thus acting overall on a control section 87 which has, in particular, the suction throttle 9, the high-pressure pump 11 and the high-pressure accumulator 13. The intake throttle current is measured, resulting in a raw measured value I R,SD which is filtered in a second current filter 89. The second current filter 89 is preferably designed as a PT 1- filter. The output of this filter is the actual suction throttle current I I,SD, which is in turn fed to the suction throttle current regulator 83.The controlled variable of the first high-pressure regulating 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 first high-pressure filter element 91, which has the actual high pressure p I as output variable. In addition, the raw values of the high pressure p are filtered by a second high-pressure filter element 93, the output variable of which is the dynamic rail pressure p dyn. Both filters are preferably implemented by a PT 1- algorithm, wherein a time constant of the first high-pressure filter element 91 is greater than a time constant of the second high-pressure filter element 93. The time constant of the second high-pressure filter element 93 can also be identical to the value zero, so that the dynamic rail pressure p dyn then corresponds to the measured raw values of the high pressure p or is identical thereto. With the dynamic rail pressure p dyn there is thus a highly dynamic value for the high pressure, which is advantageous in particular whenever a rapid reaction to certain events that occur is to take place.Output variables of the first high-pressure regulating circuit 25 are therefore, in addition to the unfiltered high pressure p, the filtered high-pressure values p I, p dyn.If the third logic signal SIG 3 assumes the value "true", the third shift element 71 switches into its lower shift position shown in FIG. 5 b). In this case, the suction throttle setpoint current I S,SD is no longer identical to the characteristic curve suction throttle current I KL,SD, but rather is set equal to a suction throttle emergency current I N. The emergency suction throttle current I N preferably has a predetermined, constant value, for example 0 A, wherein the preferably normally open suction throttle 9 is then opened to a maximum extent, or it has a low current value, for example 0.5 A, compared to a maximum closed position of the suction throttle 9, so that the suction throttle 9 is not fully opened, but is largely opened. In this case, the emergency intake throttle current I N and the opening of the intake throttle 9 connected thereto reliably prevent the internal combustion engine 1 from remaining stationary when it is operated in the second operating mode of the protective operation with the pressure regulating valve 19 opened to the maximum. The opening of the suction throttle 9 has the effect that even in a medium to low rotational speed range, sufficient fuel can still be conveyed into the high-pressure accumulator 13, so that operation of the internal combustion engine 1 without stalling is possible.It is clear that a return from the second operating mode of the protective mode to the normal operation-and otherwise also to the first operating mode of the protective mode-is not provided as long as the internal combustion engine 1 is running. A return to normal operation is possible only after a shutdown and a restart of internal combustion engine 1, and preferably also only after a confirmation that any defect present is corrected.FIG. 6 shows a schematic illustration of an embodiment of a method for operating the injection system 3, wherein the high pressure in the high-pressure accumulator 13 is regulated in the normal operation by actuating the low-pressure-side suction throttle 9, wherein the high pressure is regulated in the first operating mode of the protective operation by actuating the high-pressure-side pressure regulating valve 19, wherein switching from the normal operation to the first operating mode of the protective operation is effected if the high pressure reaches or exceeds the first pressure limit value p G1. According to the invention, it is now provided that the first operating mode of the protective mode is switched back to the normal mode when the high pressure reaches or falls below the pressure setpoint p S starting from above the pressure setpoint p S, in particular from the first pressure limit value p G1 wherein the pressure setpoint p S is smaller than the first pressure limit value p G1. Thus, according to the method proposed here, a return from the first operating mode of the protective mode to the normal mode is advantageously possible while the internal combustion engine 1 is running. This makes it possible in particular to prevent the injection system 3 from being operated permanently in the first operating mode of the protective operation after inherently undesirable pressure oscillations of the high pressure due to air, even though, for example, the air conveyed into the high-pressure accumulator 13 has already leaked again via the pressure regulating valve 19.In FIG. 6, different values of a variable BM are assigned to different operating modes. Without limiting generality, the injection system 3 is operated in normal operation when the variable BM has the value 0; the injection system 3 is operated in the first operating mode of the protective operation when the variable BM has the value 1; the injection system 3 is operated in the second operating mode of the protective operation when the variable BM has the value 2. A switchover of the operating mode is preferably effected upon a change in the value of the variable BM, in particular upon such a change.In this case, the second operating mode of the protective mode is switched in particular when the high pressure exceeds the second pressure limit value p G2 wherein the pressure regulating valve 19 and the suction throttle 9 are permanently opened in the second operating mode of the protective mode.FIG. 6 now shows in particular the logic on which the method is based for switching between the different operating modes. The method starts in a starting step S 0. In a first step S1, it is queried whether variable BM has the value 2. If this is the case, the program execution ends in a twelfth step S 12.Preferably, the program sequence shown in FIG. 6 is continuously iterated; this means that the program always restarts in the starting step S 0 if it has ended in the twelfth step S 12 while the internal combustion engine 1 is running.If it is determined in the first step S 1 that the variable BM does not have the value 2, the program flow continues in a second step S 2, in which it is checked whether the dynamic rail pressure p dyn is greater than the second pressure limit value p G2. If this is the case, the value of the variable BM is set to 2 in a third step S 3. Thus, the second operating mode of the protective mode is switched to. The program execution then ends in the twelfth step S 12. The program sequence according to FIG. 6 shows that a return from the second operating mode of the protective operation is no longer possible as long as the internal combustion engine 1 is running. Rather, the value 2 for the variable BM is retained once it has been set. When the internal combustion engine 1 is started and / or after confirming that a defect or malfunction of the injection system 3 has been corrected, the variable BM is initialized to the value 0.If, on the other hand, it is determined in the second step S2 that the dynamic rail pressure p dyn is not greater than the second pressure limit value p G2, it is queried in a fourth step S4 whether the variable BM has the value 1. If this is the case, a check is made in a fifth step S 5 as to whether the suction throttle 9 is defective. If this is the case, the program sequence ends again in the twelfth step S 12. If, on the other hand, no defect is detected in the suction throttle 9 in the fifth step S 5, the program sequence is continued in a sixth step S 6, in which it is checked whether the dynamic rail pressure p dyn is less than or equal to the pressure setpoint value-or synonymous setpoint high pressure-ps. If this is not the case, the program execution ends in the twelfth step S 12. If this is the case, on the other hand, the program sequence continues in a seventh step S 7, in which the value 0 is assigned to the variable BM, whereby the operation of the injection system 3 is switched back to normal operation. It is thus checked, in particular before the switchover from the first operating mode of the protection zone to the normal operation, whether the suction throttle 9 is defective, wherein the method is switched to the normal operation only if the suction throttle 9 is not defective.In an eighth step S 8, the integral component for the high-pressure regulator 73 is initialized with an integral initial value I init as explained in more detail with reference to FIG. 10. The integral initial value I init is determined in particular as a leakage characteristic value of the injection system 3 as a function of a current operating point of the internal combustion engine 1, which is explained in more detail with reference to FIG. 7. After the eighth step S 8, the method ends in the twelfth step S 12.If it is determined in the fourth step S 4 that the value of the variable BM is not equal to 1, the program flow continues in a ninth step S 9, in which it is checked whether the dynamic rail pressure p dyn is greater than or equal to the first pressure limit value p G1. If this is the case, in an eleventh step S 11 the value of the variable BM is set to 1 and is thus switched to the first operating mode of the protective operation. On the other hand, if the result of the determination in the ninth step S 9 is negative, the value of the variable BM is set to 0 in a tenth step S 10. In this case, the tenth step S 10 can also be omitted according to another embodiment, since after the queries in the first step S 1 and in the fourth step S 4, only the value 0 for the variable BM remains set at this point anyway and therefore no renewed setting of this value may be required. Nevertheless, the tenth step S 10 can be provided in particular for safety or redundancy reasons. After the eleventh step S 11 or the tenth step S 10, the program execution ends in the twelfth step S 12 again.The program sequence according to FIG. 6 also shows, in particular, that switching back to normal operation is effected only from the first operating mode of the protective mode. In particular, as already explained, the switch is not switched from the second operating mode back to the normal operating mode as long as the internal combustion engine 1 is running.FIG. 7 shows a schematic representation of the procedure for determining the integral initial value I init for the high-pressure regulator 73 in the eighth step S 8 of the program sequence according to FIG. 6, Since the high-pressure regulator 73 is, in a preferred embodiment, a PI(DT 1)- algorithm, its output variable V SD is identical to the integral component of the high-pressure regulator 73 in stationary operation. In order to obtain an approximate value for this output variable V SD during the transition from the first operating mode of the protective mode to the normal mode, suitable values are preferably stored-as will be explained below-in a leakage characteristic diagram 95 as a function of a current operating point of the internal combustion engine 1. In the exemplary embodiment illustrated here, the instantaneous operating point is characterized on the one hand by the instantaneous rotational speed n I and on the other hand by the setpoint injection quantity Q S. Instead of the setpoint injection quantity Q S it is also possible to use a different power-determining variable, for example a setpoint torque or a setpoint power. From a physical point of view, the integral component of the high-pressure regulator 73 approximately corresponds to the instantaneous, operating-point-dependent leakage of the injection system 3. According to one configuration, this can be used directly as a leakage characteristic value and thus as an integral initial value I init. In the exemplary embodiment shown here, however, it is provided that the leakage value is calculated using at least one control factor f L in order to obtain the leakage characteristic value. In this case, the control factor f L is preferably selected to be less than 1, in particular to be 0.8, in order to achieve an undershooting of the high pressure below the pressure setpoint p S during the transition from the first operating mode of the protective mode into the normal mode and thus to enable a robust transition into the normal mode. In the exemplary embodiment shown here, a scaling factor fScaleufof the leakage characteristic value is also applied in order then ultimately to obtain the integral initial value I init. This scaling factor fSkalkanen, for example, serves to convert different physical units into one another, in particular if the high-pressure regulator 73 for the integral initial value I init requires units other than those used for the leakage characteristic map 95.The leakage characteristic map 95 can be mapped once and then used as a constant characteristic map. In particular, it is possible for the leakage characteristic map 95 to be populated with measured values for the integral component of the high-pressure regulator 73 from test bench tests on a preferably new-grade engine in stationary operation over the entire operating range. Alternatively, it is possible for the leakage characteristic map 95 to be updated during operation of the injection system 3, wherein it is preferably populated with instantaneous-preferably filtered-values of the integral portion of the high-pressure regulator 73-possibly taking into account factors, in particular a unit conversion factor-as leakage values. Thus, the leakage characteristic map 95 can always be kept at a current state and in particular also take account of aging effects of the injection system 3 and / or of the internal combustion engine 1.FIG. 8 shows a further detailed illustration of an embodiment of the method for operating the injection system 3, here specifically the actuation of the pressure regulating valve 19. the illustration according to FIG. 8 is based on the illustration of FIG. 3 a), with the following modification-reference being made otherwise to the explanations relating to FIG. 3 a): the first switching element 27 is here replaced by a first operating mode switching element 97. The pressure regulating valve 19 is therefore no longer actuated as a function of the first logic signal SIG 1, but rather as a function of the instantaneous value of the variable BM. If this has the value 1, the first operating mode of the protective mode is thus set, the first operating mode switching element 97 assumes the upper switching position shown in FIG. 8, wherein in this case the high pressure is regulated by means of the pressure regulating valve 19, as explained in conjunction with FIG. 3 a). If, on the other hand, the value of the variable BM is not 1, i.e. either equal to 0 or equal to 2, wherein accordingly either the normal mode or the second mode of operation of the protective mode is set, the first mode of operation switching element 97 assumes the lower switching position shown in FIG. 8, wherein either the high-pressure disturbance variable is generated by the pressure regulating valve 19-in normal mode-or-in the second mode of operation of the protective mode-the pressure regulating valve 19 is not actuated and is thus permanently open on account of the high pressure present. This in turn depends on the value of the second logic signal SIG 2, by which it is decided whether the normal function or the standstill function is set for the pressure regulating valve 19, as explained in connection with FIGS. 3 a) and 4, wherein in particular the state transition diagram according to FIG. 4 indicates in what way the value for the second logic signal SIG 2 is selected. This is in particular equal to 1 in the standstill function and equal to 2 in the normal function of the pressure regulating valve 19.It is thus also clear from FIG. 8 that, according to the technical teaching disclosed here, a return from the first operating mode of the protective mode into the normal mode is possible during the operation of the internal combustion engine 1, namely when the value of the variable BM is set from 1 back to 0 and the switching position of the first operating mode switching element 97 changes accordingly.FIG. 9 shows a further detailed illustration of an embodiment of the method for operating the injection system 3. the illustration according to FIG. 9 is based here on the illustration according to FIG. 5 b) and relates to the actuation of the suction throttle 9, which-apart from the modifications explained below-corresponds to the procedure explained in connection with FIG. 5 b), so that reference is made to this: As explained in more detail below in connection with FIG. 10, the high-pressure regulator 73 according to the technical teaching disclosed here receives the value of the variable BM on the one hand and the integral initial value I init on the other hand as additional input variables. In addition, the third switching element 71 is replaced here by a second operating mode switching element 99, so that the actuation of the intake throttle 9 between the characteristic curve intake throttle current I KL,SD and the emergency intake throttle current I N is now no longer switched as a function of the third logic signal SIG 3, but rather as a function of the value of the variable BM. In this case, the suction throttle 9 is actuated with the characteristic curve suction throttle current I KL,SD if the variable BM has the value 0, therefore if the normal operation is set, wherein it is actuated with the emergency suction throttle current I N if the value of the variable BM is different from 0, that is to say in particular is equal to 1 or equal to 2, and therefore if either the first operating mode of the protective operation or the second operating mode of the protective operation is set.FIG. 10 shows a schematic illustration of the high-pressure regulator 73, which is designed here as a PI(DT 1)- pressure regulator. It is thereby shown that the output variable V SD of the high-pressure regulator 73 consists of three summed regulator components, namely a proportional component A P, an integral component A I, and a differential component A DT1. These three components are added together in a summation point 101 to form the output variable V SD. The proportional component A P represents the product of the control deviation e p with the proportional coefficient kp SD in this case. The integral component A I is dependent on a switching position of a third operating mode switching element 103 and thus on the value of the variable BM. If this is equal to zero, i.e. the injection system 3 in normal operation, the integral component A I results from the sum of two summands. The first summand is the current integral component A I. delayed by a sampling step T a. The second summand is the product of a gain factor r2 p and the sum of the current control deviation e p. delayed by one sampling step. The sum of the two summands is in this case limited upward in a third limiting element 105 as a function of the instantaneous rotational speed n I and optionally further variables. The gain r2 p is calculated according to the following formula, in which tn p is a reset time:When the value of the variable BM is not 0, the integral portion A I is set equal to the integral initial value I init. This means in consequence that the third operating mode switching element 103 switches over to the integral initial value I init, when a switch is made from normal operation, in particular to the first operating mode of the protective operation. Since the suction throttle 9 is not activated in this case-compare FIG. 9-this initially has no effects. If, however, a switch is then made back to normal operation, the first value used for the integral component A I is the integral initial value I init, before new, different values for the integral component A I can be generated on the basis of the switching of the third operating mode switching element 103. Thus, as a result, the integral portion A I is initialized with the integral initial value I init when switching from the first mode of the protection operation to the normal operation.FIG. 10 also shows that the integral component A I is branched off, in particular in order to be able to store it in the leakage characteristic diagram 95 as a function of the operating point, so that this can be updated.The calculation of the differential component A DT1 is shown in the lower part of FIG. 10. This proportion results as the sum of two products. The first product results from a multiplication of the factor r4 p by the differential component A delayed by one sampling step DT1. The second product results from the multiplication of the factor r3 p by the difference of the control deviation e p and the control deviation e p. correspondingly delayed by one sampling step.In this case, the factor r3 p is calculated according to the following equation, in which tv p is a lead time and t1 p is a delay time:The factor r4 p is calculated according to the following equation:It is thus shown that the amplification factors r2 p and r3 p depend on the proportional coefficient kp SD. The amplification factor r2 p additionally depends on the adjustment time tn p, the amplification factor r3 p on the lead time tv p and the delay time t1 p. The gain r4 p also depends on the delay time t1 p.FIG. 11 shows a diagrammatic explanation of the technical teaching disclosed here on the basis of two time diagrams. In this case, the upper time diagram shows the dynamic rail pressure p dyn as a function of the time t. In particular, the profile of the dynamic rail pressure p dyn is shown here for the case in which air which has accumulated in the low-pressure region reaches the high-pressure accumulator 13 with the aid of the high-pressure pump 11. This results in oscillations in the high pressure, which slowly build up-starting from the setpoint high pressure p S-. At a first time t 1 the dynamic rail pressure p dyn finally reaches the first pressure limit value p G1, which has the result that the high pressure is now regulated by means of the pressure regulating valve 19 and no longer, as before, by means of the suction throttle 9. The lower diagram shows for this purpose the time profile of the value of the variable BM, which changes from 0 to 1 at the first time t 1 so that the switch is made from normal operation to the first operating mode of the protective operation.In this first operating mode of the protective operation, the high pressure is influenced by shutting off fuel via the pressure regulating valve 19 and is preferably regulated to the setpoint high pressure p S. As fuel is diverted from the high-pressure accumulator 13, the high pressure falls in the direction of the setpoint high pressure p S, until it finally reaches at a second point in time t 2 and is subsequently also undershot. When the setpoint high pressure p S is reached from above, i.e. from the first pressure limit value p G1, the value of the variable BM is set to 0 again, and thus the system switches over to normal operation, as can be seen from the lower diagram. As a result, the high pressure is now also regulated again with the aid of the suction throttle 9. Since air is also simultaneously diverted from the high-pressure accumulator 13 with the fuel, a stable transient process of the high pressure to its setpoint value occurs as a result, wherein in the case shown here the high pressure has once again fully settled to the setpoint high pressure p S at a third point in time t 3.It is thus advantageously achieved that the internal combustion engine 1 changes only briefly into the first operating mode of the protective operation in the event of high-pressure oscillations which are caused by air in the injection system 3 and then, when the air has leaked from the high-pressure accumulator 13 by shutting off the pressure regulating valve 19, returns to normal operation again, wherein the high pressure is regulated again by the suction throttle 9. This prevents unnecessary heating of the fuel and unnecessary load on the pressure control valve 19, thereby prolonging the durability of the internal combustion engine 1 and improving the efficiency thereof.

Claims

Method for operating an injection system (3) of an internal combustion engine (1), wherein the injection system (3) has a high-pressure accumulator (13), wherein a high pressure in the high-pressure accumulator (13) is regulated in a normal operation by actuating a low-pressure-side suction throttle (9), wherein the high pressure is regulated in a first operating mode of a protective operation by actuating at least one high-pressure-side pressure regulating valve (19), wherein switching from the normal operation to the first operating mode of the protective operation is effected when the high pressure reaches or exceeds a first pressure limit value, and wherein switching from the first operating mode of the protective operation to the normal operation is effected when the high pressure reaches or falls below the pressure setpoint value starting from above a pressure setpoint value, wherein the pressure setpoint value is a high-pressure value to which the high pressure in the high-pressure accumulator is regulated both in the first operating mode of the protective operation and in the normal operation, and wherein the pressure setpoint is less than the first pressure limit value.Method according to Claim 1, characterized in that an integral component for a high-pressure regulator (73) for actuating the suction throttle (9) is initialized with an integral initial value when the switch is made from the first operating mode of the protective operation to the normal operation, the integral initial value being determined as a leakage characteristic value of the injection system (3) as a function of a present operating point of the internal combustion engine (1).Method according to one of the preceding claims, characterized in that the integral initial value is determined by reading a leakage value from a leakage characteristic diagram (95) as a function of the instantaneous operating point, wherein a) the leakage value is used as a leakage characteristic value, or b) the leakage value is calculated with at least one control factor in order to obtain the leakage characteristic value.Method according to Claim 3, characterized in that the leakage characteristic diagram (95) uses a) as a constant characteristic diagram, or b) is updated during operation of the injection system (3), in particular with instantaneous values of the integral proportion of the high-pressure regulator (73) as leakage values.Method according to one of the preceding claims, characterized in that before the changeover from the first operating mode of the protective mode to the normal mode, it is checked whether the suction throttle (9) is defective, wherein the operation is switched to the normal mode only if the suction throttle (9) is not defective.Method according to one of the preceding claims, characterized in that a second operating mode of the protective mode is switched to when the high pressure exceeds a second pressure limit value, wherein in the second operating mode of the protective mode the at least one pressure regulating valve (19) and the suction throttle (9) are permanently opened.Method according to one of the preceding claims, characterized in that only the first operating mode of the protective mode is switched back to normal operation.Injection system (3) for an internal combustion engine (1), having - at least one injector (15), - a high-pressure accumulator (13) which is in fluid communication on the one hand with the at least one injector (15) and on the other hand via a high-pressure pump (11) with a fuel reservoir (7), wherein - a suction throttle (9) is assigned as the first pressure actuator to the high-pressure pump (11), and having - at least one pressure regulating valve (19) via which the high-pressure accumulator (13) is connected in terms of flow to the fuel reservoir (7), and having - a control unit (21) which is operatively connected to the at least one injector (15), the suction throttle (9) and the at least one pressure regulating valve (19), wherein the control unit (21) is configured to carry out a method according to one of Claims 1 to 7.Injection system (3) according to Claim 8, characterized in that the injection system (3) is free from a mechanical overpressure valve.Internal combustion engine (1) having an injection system (3) according to either of Claims 8 and 9.

Images