DE102005043684A1 - Fuel system controlling method for e.g. diesel engine, involves controlling fuel pump during overrun fuel cut off of engine with pre-control value, such that output pressure of fuel is set above null discharging pressure - Google Patents

Abstract

The method involves discharging fuel from an intake side to a pressure side by a fuel pump of a fuel system. A pre-control value is adaptatively determined and stored in a non-volatile storage of a control device, where the pre-control value is an electrical input signal of the fuel pump. The fuel pump of the fuel system is controlled during an overrun fuel cut off of an internal combustion engine with the value in such a manner that an output pressure of the fuel is set above a null discharging pressure. An independent claim is also included for a fuel system for an internal combustion engine.

Description

The The present invention relates to a method for controlling a Fuel system for an internal combustion engine, wherein the fuel system is a fuel pump includes the fuel from a suction side to a pressure side promote can.

From the EP 0 299 337 A2 . EP 0 837 986 B1 . EP 0 974 008 B1 . EP 0 299 337 A2 and DE 196 12 413 A1 are known devices for controlling a fuel pressure.

at Current fuel supply systems (KVS) become mechanical Fuel pressure holding valves (DHV) u.a. used to hold pressure. Sinks the pressure in the KVS decreases, so it will be at least on a DHV specific Pressure value, which is always below the regular system operating pressure held.

at Demand-controlled KVS with DHV is the pressure control in the operating state Shut-off (SA) switched off and the electric fuel pump (EKP) to "zero feed position" (= constant, low Speed without or with low flow rate and pressure) operated.

Problems of the State of the art

At the transition from the operating state of fuel cut to the operating state Restart (WE) suddenly resumes engine consumption, causing pressure oscillations occur that need to be corrected. The fact that the Fuel pressure during the fuel cut to the DHV pressure drops and abruptly increased to a target pressure in the WE must be strengthened These pressure fluctuations and makes the controllability of the fuel pressure and the amount

task The present invention is the controllability of the fuel pressure at the transition from operating state fuel cut to pressure-controlled operation to improve when reinserting.

Advantages of invention

This Problem is solved by a method for controlling a fuel system for an internal combustion engine, wherein the fuel system comprises a fuel pump, the fuel from a suction side to a pressure side, characterized that the control of the fuel pump during a fuel cut the internal combustion engine takes place with a precontrol value, in which an output pressure above a zero delivery pressure sets. The Method is in principle in all types of internal combustion engines, So direct injection or suction tube injection gasoline engines as well as diesel engines

One The main advantage of the process is that the fuel pressure For overrun fuel cutout targeted to a re-establishment target pressure is held and so the pressure transient behavior during the transition of overrun fuel cut to the operating state reclosing (WE) is reduced. The controllability of the fuel system in this Operating condition is significantly improved. In a suction tube injection Otto engine is also an increase reached the metering accuracy.

Preferably, the control of the fuel pump ( 3 ) takes place during a fuel cut of the internal combustion engine with a precontrol value, wherein adjusting the pressure of the re-insertion target pressure. The pilot control value is an electrical input signal of the pre-feed pump. The reset target pressure is the pressure required at the beginning of the pressure-controlled operation during the reinstatement on the output side of the pressure-maintaining valve.

Preferably the pre-control value is determined adaptively. As a result, this is constantly on e.g. age-related changes customized. In addition, an optimal value over time one. For this purpose, the precontrol value is preferably in a non-volatile Memory of a control unit saved. At initial startup or reinitialization of the control unit the precontrol value becomes a non-volatile value as a constant value Memory removed. In the simplest case, here is the value zero, so the zero promotion, be used as start value.

A Diagnosis of the pressure holding valve is possible by the control the fuel pump e.g. while a fuel cut of the internal combustion engine is increased until a pressure increase of Pressure on the high pressure side of a check valve occurs and the differential pressure the opening pressure the check valve is assigned. The diagnosis allows a measurement of the opening pressure the check valve, so that with allowable Thresholds can be compared and the check valve if necessary as a defect can be classified.

A diagnosis of the pressure holding valve is possible by the control of the fuel pump -zB is initially brought to a value during a fuel cut of the internal combustion engine or at idle, at which a pressure on the high pressure side of the check valve above egg It adjusts nes opening pressure of a pressure relief valve and then reduced until the pressure on the high pressure side remains constant and the differential pressure p_A - p_E is assigned to the opening pressure of the pressure relief valve. The diagnosis allows a measurement of the opening pressure of the pressure relief valve, so that it can be compared with permissible thresholds and the pressure relief valve can possibly be classified as defective.

In a development of the method is provided that when exceeded or falling below thresholds of the opening pressure of the pressure relief valve or the opening pressure the check valve the respective valve is output as defective. It can simultaneously the pressure relief valve as such are issued as defective. It upper and lower limits are set for the two opening pressures, within whose limits they have to move.

Of the Inlet pressure of the pressure relief valve is advantageous from the Control of the fuel pump determined. This can be model based respectively. In the simplest case, the pressure is the same when switched off Fuel pump the tank pressure (and thus usually the ambient pressure).

The The problem mentioned earlier is also solved by a fuel system for one Internal combustion engine, which includes a fuel pump, the fuel from a suction side to a pressure side, characterized that the control of the fuel pump during a fuel cut the internal combustion engine takes place with a precontrol value, in which an output pressure above a zero delivery pressure sets.

The Fuel system preferably comprises a pressure-holding valve, which the pressure side of the pre-feed pump connects to an input of a high pressure system. Includes the pressure holding valve preferably a check valve and a pressure relief valve.

drawings

following is an embodiment of Present invention explained in more detail with reference to the accompanying drawings. there demonstrate:

1 an injection system for an internal combustion engine;

2 a first embodiment of a pressure holding valve;

3 an alternative embodiment of the pressure holding valve;

4 the pressure curve during execution of the method according to the invention;

5 - 7 Flowcharts of the method according to the invention;

In 1 is a fuel system for an internal combustion engine using the example of a diesel engine or direct injection gasoline engine (BDE) shown. The fuel tank 1 is via a support line 2 with a pre-feed pump 3 connected. The pre-feed pump 3 pumps the fuel with a delivery Q and an output pressure p_E via a delivery line 4 to a control valve 5 , Here, the fuel is metered and over another line 6 to a high pressure pump 7 forwarded. The fuel is compressed to high pressures and passes through a delivery line 8th in a high-pressure accumulator 9 (Common rail). The high pressure accumulator is equipped with injectors 10 connected to the fuel in cylinders of an internal combustion engine 11 inject.

For pressure relief, the high-pressure side is hydraulically via a pipe 12 with the control valve 5 connected. At the high-pressure accumulator 9 is still a pressure sensor 13 arranged over a data line 14 with a control unit 15 connected is. The control unit 15 is also via another data line 16 with an actor 17 of the control valve 5 and via a third data line 18 with the internal combustion engine 11 connected. Furthermore, the control unit 15 input lines 19 on, with which various input values, such as the accelerator pedal position of the motor vehicle, are recorded.

The high pressure control is via the control valve 5 , The actual pressure in the high-pressure accumulator is determined by the pressure sensor 13 taken and to the control unit 15 forwarded. In the control unit, the respectively required position of the control valve 5 recalculated and the actor 17 controlled accordingly.

In 1 further shown is a pressure holding valve (DHV) 20 , which by the following 2 and 3 will be described in more detail. Finally, a return line 21 illustrated as used in one embodiment of the pressure relief valve. In the following discussion, reference is made exclusively to the low-pressure circuit provided with a dashed box with the reference numeral ND 1 Referenced. This essentially comprises the fuel tank 1 , the pre-feed pump 3 and the pressure-holding valve 20 ,

Based 2 First, a first embodiment of a pressure-holding valve 20 described. From a fuel tank 1 Fuel is by means of the feed pump 3 compressed and with the flow Q and the pressure p_E the pressure-holding valve 20 fed. The suction side of the pre-feed pump 3 (Fuel tank 1 ) is with S, the pressure side (pressure relief valve 20 ) denoted by D. The output of the pressure holding valve is referred to as the high-pressure side H for easier discrimination from the pressure side D. The pre-feed pump 3 For this purpose, it is controlled with an electrical drive A, this can be, for example, an analog or digital control signal of its own pump control or directly, for example, the armature voltage of a drive motor. The pressure holding valve 20 includes a check valve (RSV) 21 as well as a pressure relief valve with pressure maintenance function (DBV) 22 , At the outlet of the pressure holding valve 20 is a pressure sensor 25 arranged, the pressure p_A at the outlet of the pressure relief valve 20 can measure. This is preferably a pressure sensor which can deliver an electrical signal representing the pressure p_A. Optional is also on the pressure side of the pre-feed pump 3 a pressure sensor 24 arranged one of the pressure p_E at the output of the feed pump 3 , this is identical to the pressure at the inlet of the pressure holding valve 20 or in the embodiment of the 4 the check valve 21 , can give representative electrical size. The inlet pressure at the pressure retention valve 20 or check valve 21 is in the 2 and 3 denoted by p_E.

Of the Output pressure of the pressure holding valve p_A is measured in each case a measurement of the inlet pressure p_E of the pressure holding valve is optional.

The opening pressure of the check valve 21 is higher than the opening pressure of the pressure relief valve. As a result, as soon as the prefeed pump 3 over the check valve 21 Fuel feeds, while at the same time the fuel flows back through the pressure limiting valve. Is the promotion by the pre pump 3 set so long as fuel flows back through the pressure relief valve until it closes at a holding pressure p_H. On the pressure side (p_A), this pressure is maintained - as long as fuel is not withdrawn in the direction of the high pressure side - thus p_A = p_H.

3 shows an alternative embodiment of the pressure holding valve 20 with a throttle 23 to limit the amount of fuel in the return for direct fuel return to the fuel tank 1 , Across from 2 identical components are identically labeled. In the following, therefore, only the differences of the embodiments will be discussed.

In contrast to the embodiment of 2 is at the pressure holding valve 20 of the 3 the low-pressure side connection of the pressure relief valve 22 over the throttle 23 directly with the fuel forward tank 1 connected.

The operation of the pressure relief valve 20 according to the embodiments of the 2 and 3 is explained below.

In both variants, the check valve 21 open as long as the inlet pressure p_E greater than the outlet pressure p_A plus the opening pressure P_OE of the check valve 21 is (p_E> p_A + p_OE), otherwise the check valve 21 closed.

The pressure relief valve 22 is in the embodiment according to 2 opened, as long as the output pressure p_A greater than the input pressure p_E plus the opening pressure of the pressure relief valve 22 p_DB is (p_A> p_E + p_DB). In the embodiment according to 3 is the pressure relief valve 22 opened, as long as the output pressure p_A is greater than the tank pressure p_0 plus the opening pressure of the pressure relief valve p_DB (p_A> p_0 + p_DB), otherwise the pressure relief valve is closed.

In 4 is the pressure profile indicated in carrying out the method according to the invention. Shown is the pressure curve of the output pressure p_A over time. In this case, between the times t_0 to t_1, a pressure-controlled operation (denoted by D_G) of the internal combustion engine takes place. Between the time t_1 and the time t_2, the internal combustion engine is operated in the overrun cutoff mode SA; in this mode, the pre-feed pump 3 operated with a pre-controlled control value (ie a constant control value A). At time t_2, the overrun SA operating mode is abandoned, and the system returns to a pressure-controlled operation D_G. In the curve I is shown to understand the process of the invention, the pressure curve according to the prior art. The curve II denotes the pressure curve when using the method according to the invention. The pressure p_A_0 designates the pressure, which at zero delivery of the pre-feed pump 3 established. In a pressure-holding valve according to 3 this is the opening pressure of the pressure relief valve plus one of the prefeed pump 3 generated pressure at zero feed position. In a pressure-holding valve 20 according to the embodiment of the 4 the pressure p_A_0 falls to the tank pressure p_0. Is transferred back to the pressure-controlled operation D_G at the time t_2, wherein the output pressure at the pressure-holding valve P_A is to be controlled to a re-establishment target pressure WE_Z, so according to the prior art, a pressure build up, the difference of the re-insertion target pressure WE_Z and adjusting at zero flow pressure WE_Z (So the difference WE_Z - p_A_0) results. Since fuel is already taken from the high-pressure side, this leads to greater pressure fluctuations during pressure build-up, as indicated by a pronounced wave-shaped line in FIG 4 is shown.

In contrast, in the method according to the invention, the pressure at the outlet of the pressure holding valve p_A is regulated to the reintroduction target pressure WE_Z at the time t_1 at the start of the fuel cut. The pre-feed pump 3 is therefore not in the zero operation, in which no flow rate is present, transferred, but continues to promote fuel. The delivery rate of the pre-feed pump 3 is adjusted so that at the output of the pressure relief valve 20 the pressure p_A has the setpoint of the reset target pressure WE_Z. If, at time t_2, the overrun fuel cutoff switches to pressure-controlled operation D_G, then there is the output of the pressure-maintaining valve 20 already desired for further operation pressure. The pre-feed pump 3 must no longer build up the re-establishment target pressure WE_Z, but only provide the additionally requested on the high-pressure side fuel quantity, since now fuel is injected. This manifests itself in much lower pressure fluctuations, as well as in the curve II in 4 is shown.

The inventive method for controlling the prefeed pump 3 So it is that the prefeed pump 3 operated pilot-operated. This means that at the beginning of the operating state, overrun fuel cutoff is switched over from the controlled actuation variable to the pre-controlled actuation variable. The pre-controlled control variable is an unregulated, only slowly changing value, which is formed and adapted as follows:
The fuel cut pre-control value, this is the re-set target value WE_Z, is adapted, ie, for deviations during the fuel cut-off operation between actual and set pressure at the output of the pressure holding valve 20 is based on the measured fuel pressure sensor signal, this is the pressure p_A on the sensor 25 , an adaptive learning process started.

The Adaptation, z. B. by means of an integrator, takes place in the operating state Fuel cut SA instead. It preferably takes place when the vehicle after the start of the internal combustion engine is a stationary operating state has reached, for example, is warm. Physical quantities that Effects on the pressure in the fuel supply system during the Schubabschaltungen have such. As the fuel temperature, can also considered become.

After switching off the internal combustion engine, the adaptation value in a non-volatile memory of the control unit 15 are stored and so made available again at the next start of the engine.

After an initial start of the control unit (initial power-on, power-on after removal, reset or the like) or after a power failure of the control unit, the adaptation value must be readjusted. This can be done either by setting a first fixed value in the controller 15 can be stored, done, or by values that can be derived from the conventional feedforward control of the demand-controlled fuel supply system or even by merely learning the Erst-adaptation value zero.

In an expanded embodiment of the method according to the invention may also be the pressure-holding function of the pressure holding valve 20 be monitored or diagnosed. It can both the check valve 21 as well as the pressure relief valve 22 be diagnosed. "Diagnosis" is understood here in particular as meaning the functional capability, in particular the respective opening pressure, to be determined.

To the function of the check valve 21 to monitor the pressure value at which the check valve opens, this is hereinafter referred to as check valve opening pressure p_RSV. The electrical control of the pre-feed pump 3 must be done so that p_A> p_E + p_RSV. In principle, the operating states in which the engine consumption amount is constant and known or, better still, zero are suitable. This is z. B. during overrun fuel cutoff the case. For this purpose, the adapted re-establishment target pressure WE_Z is used. If the latter exceeds or exceeds a specific threshold value, then it can be assumed with great probability that the check valve opening pressure p_RSV has increased or decreased markedly and the component has to be replaced. A diagnosis is also possible before the engine start or in the engine trailing, since this is driving conditions without fuel injection of the internal combustion engine. In other operating states is a monitoring at constant injection quantity based on a defined control of the feed pump 3 possible. When the at the outlet of the pressure relief valve 20 taken fuel quantity is known, the check-opening pressure based on a defined control of the feed pump 3 be determined.

At its core, the diagnosis of the pressure limiting valve is running 21 in the operating states of the internal combustion engine without injected fuel (for example, during fuel cut-off) on it addition, the flow rate and the delivery pressure of the prefeed pump 3 so far increase until an increase in pressure at the outlet of the pressure relief valve 20 is perceived.

Furthermore, a diagnosis of the pressure relief valve 22 by a pressure rating in different operating states without engine consumption amount of fuel (ie without injection of fuel) possible. To the function of the pressure relief valve 22 To monitor the pressure value must be evaluated at which the pressure relief valve 21 closes. This value is usually approximately equal to the opening pressure of the pressure relief valve. The activation of the pre-feed pump 3 Therefore, it is done so that in the embodiment of 2 the pressure value p_A = p_E plus the opening pressure of the pressure relief valve 21 p_OE is. In the embodiment according to 3 must p_A = the tank pressure p_0 + the pressure relief valve opening pressure p_OE.

The diagnosis is preferably made at an operating point of the internal combustion engine, in which the output pressure mainly by the opening pressure of the pressure limiting valve 21 and not determined by the engine consumption amount, that is, the injection amount. For this purpose, every operating point offers in which the internal combustion engine does not inject fuel. If this operating point of the internal combustion engine, for example the fuel cut-off, is set, then the function of the check valve 22 be checked in two ways.

If the actual pressure p_A before this operating point above the holding pressure of the pressure holding valve, so the control A of the pre-supply pump 3 reduced to a certain value, for example, the value of the zero-feed position.

If the actual pressure p_A is above the holding pressure of the pressure holding valve before the operating point in which a diagnosis of the pressure limiting valve is to take place, then the feed pump must be actuated 3 done so that the pressure relief valve 22 once open. The actual pressure p_A must therefore be above the half of the holding pressure of the pressure holding valve p_DHV. By reducing the flow rate or the delivery pressure of the prefeed pump 3 Now it is ensured that the pressure relief valve 22 closes again; the actual pressure p_A is then below the holding pressure of the pressure holding valve 20 , The drive signal A of the prefeed pump 3 is z. B. reduced to the value of the zero-feed position.

Subsequently, the resulting output pressure p_A is evaluated. If the difference between the output pressure p_A and the inlet pressure p_E exceeds an applicable threshold value upwards or downwards, then it can be assumed with great probability that the opening pressure of the pressure limiting valve 22 has significantly increased or decreased and the pressure relief valve 20 overall is broken.

If the input pressure value p_E is not available in the latter method portion, the drive signal A of the prefeed pump must be present 3 be converted into an equivalent input pressure value p_E; this can be z. B. model-based.

The method according to the invention is also suitable for emergency operation in the event of a fault of the pressure-maintaining valve 20 , In certain cases of failure of the pressure relief valve 20 the method described above is used to ensure a sufficient pressure in the operating state of the overrun fuel cutoff and further a good control quality at the transition to reconnection. The error cases that can be detected with the diagnostic method presented above and for which the method according to the invention is an emergency operation, are a sharp drop in the opening pressure of the pressure relief valve 22 , so that a threshold is exceeded, a clamping of the pressure relief valve 22 in the open position and a clamping of the check valve 21 in the open position.

The 5 - 7 show flowcharts of the inventive method. First, based on the 5 the inventive method for controlling the fuel system in the transition to the fuel cut and then described during the transition to the pressure-controlled operation. With the beginning of the operating state of the fuel cut in step 101 according to 5 will be in step 102 the setpoint pressure p_A_Soll of the pressure at the outlet of the pressure holding valve 20 , this is the pressure p_A, regulated to a target pressure when resetting the pressure-controlled operation WE_Z. This regulation takes place by setting a suitable control A for the prefeed pump 3 , At the end of the fuel cut in step 103 remains in step 104 Although the pressure p_A_Soll is equal to the pressure WE_Z, however, the delivery rate of the pre-feed pump 3 now not kept constant, but controlled by the control A, ie usually increased to compensate for the fuel consumption by the successful injections and the pressure value WE_Z at the output of the pressure holding valve 20 maintain.

In 6 is the procedure for diagnosing the check valve 21 shown. decisions are in the presentation of 6 and 7 each represented as diamonds, wherein the branches "no" is each marked with the letter "n", the branches "yes" each marked with the letter "j".

First, in step 201 checked whether an engine consumption amount m_v is constant or equal to zero. The engine consumption amount m_v corresponds to the amount of the injected fuel. If the engine consumption amount is not constant or not equal to zero, the diagnosis is terminated. Otherwise, in step 202 checks whether the operating state fuel cutoff exists. If so, then the pilot pump 3 in step 203 controlled so that the pressure at the output of the pressure holding valve p_A corresponds to the expected pressure when restarting WE_Z. The pressure WE_Z is used as an adapted value to a permanent memory of the control unit 15 taken. In step 204 then the pressure at the outlet of the pre-feed pump 3 or the thus substantially identical pressure at the inlet of the pressure holding valve 20 either measured, this is possible if a pressure sensor 24 according to the 2 or 3 is present, or from the activation value A of the pre-supply pump 3 calculated. In step 205 it is then checked whether the pressure p_E is greater than a maximum pressure p_E max_1 or less than a minimum pressure p_E_min_1. If this is the case, then the pressure-holding valve 20 malfunction; if this is not the case, then the pressure-maintaining valve is not defective and the diagnosis is terminated.

Was in step 202 to the option "no" branches, the internal combustion engine is thus not in the fuel cut, so in step 213 First, it checks whether there is an operating state with a constant consumption quantity, for example a so-called operating state "drive." If this is the case, then in step 213 the prefeed pump 3 controlled by the control A so that the pressure p_E is greater than p_A plus the opening pressure of the check valve p_RSV. Then in step 214 according to step 204 the pressure p_E determined. In step 215 it is then checked whether the pressure p_E lies within the thresholds p_E max_2 or p_E min_2. The test corresponds to that of the step 205 but other pressure thresholds are set up and down here. Was in step 212 the branch chosen "no" will be in step 222 checked whether the internal combustion engine in engine pre-and-Nachlauf and thus also the pre-feed pump 3 in the lead or lag. If this is not the case, the system branches to the diagnosis end. If this is the case, then in step 223 the control A of the pre-feed pump 3 according to step 213 ge chooses. Then in step 224 in turn determines the pressure p_E, and in step 225 compared with upper and lower limits p_E_min_3 and p_E_max_3. If the pressure p_E lies within these limits, the diagnosis is terminated. If the pressure is outside these limits, this is the variant "j", then the pressure holding valve is defective.

7 shows a flow chart for diagnosis of the pressure relief valve 22 , In one step 301 First, it is checked whether the engine consumption amount is zero. If this is not the case, the diagnosis is terminated; If this is the case, it will be in step 302 first checked if the pressure at the outlet of the pressure relief valve 20 greater than the pressure at the inlet plus the opening pressure of the pressure relief valve, p_A> p_E + p_DBV. If this is the case, then the pressure relief valve 22 currently open and the control A of the feed pump 3 will be in step 303 reduced, for example, to the zero-flow position. In step 304 will according to the steps 204 . 214 such as 224 in 6 the inlet pressure p_E at the pressure holding valve 20 determined. In step 305 it is then checked whether the input pressure p_E lies within the thresholds p_E_min_4 or p_E_max_4. If this value is within the thresholds, the diagnosis is terminated. If the value is outside the threshold, then the pressure-holding valve is 20 malfunction. The branch was in step 302 "No", so the pressure relief valve is closed immediately, then the control A of the pre-supply pump 3 in step 306 initially raised until the pressure relief valve opens. If this has happened, then the control A of the feed pump in step 307 again reduced and the further procedure according to the steps 304 and 305 in the steps 308 and 309 with changed pressure thresholds p_E_min_5 and p_E_max_5 go through.

Claims (10)

Method for controlling a fuel system for an internal combustion engine, wherein the fuel system is a fuel pump ( 3 ), which can convey fuel from a suction side (S) to a pressure side (D), characterized in that the activation of the fuel pump ( 3 ) takes place during a fuel cut of the internal combustion engine with a pilot value, in which an output pressure (p_E) above a zero feed pressure (p_0) is established. Method according to claim 1, characterized in that that the control of the fuel pump during a fuel cut takes place with a precontrol value, in which the pressure of the re-insertion target pressure (WE_Z). Method according to one of the preceding claims, characterized in that the precontrol value is determined adaptively. Method according to one of the preceding Claims, characterized in that the precontrol value in a non-volatile memory of a control unit ( 15 ) is stored. Method according to one of the preceding claims, characterized in that the pre-control value at initial startup or reinitialization of the control unit ( 15 ) is taken as a constant value from a nonvolatile memory. Method according to one of the preceding claims, characterized in that the control of the fuel pump ( 3 ) is increased during a fuel cut of the internal combustion engine until a pressure increase of the pressure (p_A) on the high pressure side (H) of a check valve ( 21 ) and the differential pressure (p_A - p_E) the opening pressure (p_OE) of the check valve ( 21 ). Method according to one of the preceding claims, characterized in that the control of the fuel pump of the internal combustion engine is first brought to a value at which a pressure (p_A) on the high-pressure side (H) of the check valve ( 21 ) above an opening pressure (p_DBV) of a pressure limiting valve ( 22 ) and then reduced until the pressure (p_A) on the high pressure side (H) remains constant and the differential pressure (p_A - p_E) the opening pressure (p_DBV) of the pressure relief valve ( 22 ). Method according to one of the preceding claims, characterized in that when exceeding or falling below thresholds of the opening pressure (p_DBV) of the pressure relief valve ( 22 ) or the opening pressure (p_OE) of the check valve ( 21 ) the respective valve is issued as defective. Method according to one of the preceding claims, characterized in that the inlet pressure (p_E) of the pressure limiting valve ( 22 ) from the control of the fuel pump ( 3 ) (eg pump off = p_0) Fuel system for an internal combustion engine, which is a fuel pump ( 3 ), which can convey fuel from a suction side (S) to a pressure side (D), characterized in that the activation of the fuel pump ( 3 ) can be carried out during a fuel cut of the internal combustion engine with a pilot value, in which an output pressure (p_E) above a zero delivery pressure (p_0) is established.