Abstract:
In an internal combustion engine, in particular with direct injection, a first fuel pump delivers fuel from a fuel tank to a second fuel pump. This second fuel pump delivers the fuel to a fuel accumulation line. A metering unit is triggered by a control and/or regulating unit and meters the fuel quantity traveling into the inlet of the second fuel pump. In order to simplify the design of the engine and to reduce its cost, the invention proposes that during normal operation, the metering unit is closed when it is without power and that when the control and/or regulating unit is “dead”, the metering unit is without power and the fuel delivery to the metering unit is cut off.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method for operating an internal combustion engine, in particular with direct injection, in which a first fuel pump delivers fuel from a fuel tank to a second fuel pump that delivers fuel to a fuel accumulation line, and in which a metering unit is triggered by a control and/or regulating unit and meters the fuel quantity traveling into the inlet of the second fuel pump. 
     2. Description of the Prior Art 
     A method of operating an internal combustion is known from DE 199 26 308 A1, which discloses a pump apparatus for fuel, which has a main delivery pump embodied as a high-pressure pump, preceded by a presupply pump. The presupply pump is embodied as a mechanical fuel pump and delivers a fuel flow from a tank via a fuel line. The total fuel flow delivered is conveyed through the drive/crank chamber of the main delivery pump. Downstream of the drive/crank chamber, the total fuel flow is divided into a lubricating flow and a delivery flow. The lubricating flow travels via a return back to the tank. The delivery flow travels via a delivery circuit first to a metering unit and then on to the main delivery pump. 
     The main delivery pump is a radial piston pump driven by a camshaft. The radial piston pump feeds into a fuel accumulation line, which is also commonly referred to as a “rail”. From the fuel accumulation line, the fuel travels to injection valves, which supply the fuel to combustion chambers of the engine. 
     It in order to be able to assure a reliable operation of injection valves, the pressure in the fuel accumulation line must not exceed a particular value. To this end, a pressure control valve is provided in the fuel accumulation line. If the fuel accumulation line is supplied with more fuel than is drawn by the injection valves, then above a particular pressure in the fuel accumulation line, this pressure control valve conveys the excess fuel out of the fuel accumulation line and returns it to the fuel tank. 
     One instance in which the pressure control valve is active, for example, is when the engine is being overrun while the control unit is simultaneously “dead”. Here and in the following, a “dead” control unit is understood to mean that it is no longer possible for the control unit to adjust the fuel quantity traveling to the main delivery pump. This can be the case both in the event of an electrical failure of the control unit and in the event of a mechanically jammed metering unit. During such an overrunning, generally no fuel whatsoever is injected by the fuel injection valves into the combustion chambers of the engine. Without a pressure control valve, when the control unit is “dead”, the pressure in the fuel accumulation line could increase sharply and lead, for example, to an unwanted entry of fuel into the combustion chambers. 
     OBJECT AND SUMMARY OF THE INVENTION 
     The object of the current invention is to modify a method of the type mentioned above so that the correspondingly operated internal combustion engine and here in particular, the fuel system of the engine, can be more simply designed and produced at a lower cost. 
     This object is attained with a method of the type mentioned above in that during normal operation, the metering unit is closed when it is without power and that when the control and/or regulating unit is “dead”, the metering unit is without power and the fuel delivery to the metering unit is cut off. 
     The method according to the invention has the advantage that the internal combustion engine operated with it no longer requires a pressure control valve in the fuel accumulation line. Eliminating the pressure relief valve or pressure control valve in the fuel accumulation line simplifies the design of the engine and allows it to be produced at a lower cost. 
     An excessive increase of pressure in the fuel accumulation line and a resulting unwanted entry of fuel into the combustion chambers, for example during overrunning of the engine, is instead prevented by the method according to the invention by virtue of the fact that the fuel supply to the fuel accumulation line is reliably cut off as soon as a malfunction occurs in the control and/or regulating unit or in the metering unit. Since the powerless state of the metering unit is also its closed state, then this alone provides a highly reliable assurance that no more fuel travels to the second fuel pump. In addition, the fuel delivery to the metering unit is also cut off. This prevents the occurrence of any further delivery by means of the second fuel pump, even when the metering unit is in fact without power, but is mechanically jammed in the open state. This is based on the consideration that the pressure drop upstream of the second fuel pump and the opening pressure of the spring-loaded intake valves, which are generally used in the second fuel pump, cause this fuel pump to no be longer filled. 
     Another advantage of eliminating the pressure control valve lies in the fact that it is difficult or impossible to test its operational readiness. To be precise, a spring-loaded ball valve is usually used as the pressure relief valve or pressure control valve. But if the operational readiness cannot be regularly tested by the system, for example by means of a self test, then there can be no assurance in each case that the maximal permissible pressure in the fuel accumulation line will not be exceeded all the same. 
     With the method according to the invention, however, the operational readiness can be tested at any time. An interruption in the power supply to the metering unit, just like a discontinuation of the fuel delivery to the metering unit can be detected at any time by appropriate sensors. Consequently, the method according to the invention also increases the operational reliability of an internal combustion engine. 
     For example, the invention proposes that the first fuel pump be switched off when the control and/or regulating unit is “dead”. This is easy to accomplish. Furthermore, an interruption of the power supply to the first fuel pump can be easily tested. 
     It is also possible to cut off the fuel supply to the first fuel pump by means of a shutoff valve device when the control and/or regulating unit is “dead”. 
     This measure is particularly suitable if the first fuel pump cannot easily be switched off. This is the case, for example, with a first fuel pump that is mechanically driven, i.e. driven directly by the engine. An additional component is in fact required to execute this method, but its reliable operation can be tested at any time during the operation of the engine. 
     It is also advantageous to decouple a drive unit of the first fuel pump from the first fuel pump when the control and/or regulating unit is “dead”. In particular, with a mechanically driven fuel pump, a clutch could be provided, which could decouple the drive unit from the pump as needed. 
     The invention also relates to a computer program, which is suitable for executing the method mentioned above, when it is run on a computer. It is particularly preferable if the computer program is stored in a memory, in particular a flash memory. 
     The invention also relates to a control and/or regulating unit for controlling and/or regulating at least one function of an internal combustion engine. With a control and/or regulating unit of this kind, it is advantageous if it is provided with a computer program of the type mentioned above. 
     The invention also relates to a fuel system for an internal combustion engine, in particular with direct injection, having a fuel tank, a first fuel pump that delivers from the fuel tank, and a second fuel pump, which is connected on the inlet side to the first fuel pump and is connected on the outlet side to a fuel accumulation line, and having a metering unit that meters the fuel quantity traveling into the inlet of the second fuel pump. 
     A fuel system of this kind is also known from DE 199 26 308 A1 and has already been explained above. In order to be able to design a fuel system of this kind more simply and produce it at a lower cost, the invention proposes that the first fuel pump be electrically driven and that the metering unit be closed when it is without power. 
     An electrically driven first fuel pump can be shut off in a simple manner: in particular, a malfunction of a control and/or regulating unit that controls and/or regulates the fuel system or a general power failure automatically causes the first fuel pump to be shut off and this alone cuts off the delivery in the direction of the fuel accumulation line. In order to achieve a redundancy, the metering unit is designed so that it is closed when it is without power. 
     Alternatively, it is possible to drive the first fuel pump mechanically and to provide a shutoff valve device between the fuel tank and the first fuel pump. This has the advantage that a very simply designed and rugged fuel pump can be used and the shutoff valve device can still cut off the fuel supply in the direction of the fuel accumulation line, for example when the engine is being overrun and a malfunction simultaneously occurs in the operation of the engine. 
     It is particularly advantageous if the shutoff valve device is closed when it is without power. Primarily in the event of a failure of a control and/or regulating unit, this reliably cuts off the fuel supply to the fuel accumulation line. A fuel system of this kind operates redundantly if the metering unit is also closed when it is without power. 
     In a preferred modification of a fuel system of the type mentioned above, the outlet side of the first fuel pump is initially connected to a drive/crank chamber of the second fuel pump and the outlet side of the drive/crank chamber of the second fuel pump is connected to an overflow line that contains an overflow valve, which adjusts the pressure in the drive/crank chamber to a particular value. This modification is based on the following concept: 
     When the drive means in the drive/crank chamber rotates or moves, a large amount of thermal and/or mechanical stress is generated in this region. The high fuel flow according to the invention in the vicinity of the drive/crank chamber makes a particularly good lubrication and heat dissipation possible. In particular, a positive lubrication of the drive/crank chamber is possible since the drive/crank chamber is subjected to the total fuel flow with the full delivery pressure of the first fuel pump. The fuel system according to the invention is therefore distinguished by a long service life and a reliable operation, without requiring an additional coolant circuit and/or lubricant circuit. 
     A particularly advantageous modification of a fuel system is the one in which a zero-delivery line branches off between the metering unit and the second fuel pump, is connected to the fuel tank or the inlet of the first fuel pump, and carries away an overflow that occurs when the metering unit is closed. Such a zero-delivery line assures that when a residual quantity of fuel nevertheless is still emerging from the outlet of the metering unit, this quantity is not forced into the fuel accumulation line, but travels back into the fuel tank via the zero-delivery line. 
     To that end, the opening pressure of possibly provided intake valves of the second fuel pump should be selected (preferably greater than 2 bar) so that the pressure drop due to the zero-delivery line does not cause the intake valves to open. 
     Placing a zero-delivery throttle in the zero-delivery line assures that during normal operation of the fuel system, i.e. when the second fuel pump is delivering fuel in the direction of the fuel accumulation line, as little fuel is possible travels back to the fuel tank through the zero-delivery line. 
     The metering unit can be produced in a particularly inexpensive matter if it includes an electric sliding valve. The same is true for a shutoff valve unit, which includes a magnetic on-off valve. 
     The fuel system according to the invention permits a pressure control valve to be eliminated. This is explicitly expressed in a modification of the fuel system according to the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of preferred embodiments taken in conjunction with the drawings, in which: 
     FIG. 1 shows a schematic representation of a first exemplary embodiment of a fuel system; and 
     FIG. 2 shows a depiction similar to FIG. 1 of a second exemplary embodiment of a fuel system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, a fuel system is labeled as a whole with the reference numeral  10 . It includes a fuel tank  12 , from which fuel  14  is delivered by an electric fuel pump  16 . The fuel pump  16  is connected to the fuel tank  12  by means of a fuel line  18 . 
     A fuel line  22 , containing a filter  20 , leads from the electric fuel pump  16  to a mechanically driven high-pressure fuel pump  24 . The fuel line  22  leads to a drive/crank chamber  26  of the high-pressure fuel pump  24 . This chamber contains a crankshaft  28 , which sets a piston  32  into a reciprocating motion by means of a connecting rod  30 . The piston  32  is guided in a housing  34  of the high-pressure fuel pump  24 . 
     A return line  36  leads from the drive/crank chamber  26  back to the fuel tank  12 . A return throttle  38  is disposed in the return line  36 . From the drive/crank chamber  26 , a delivery line  40  also leads first to a metering unit  42  and from there, via a spring-loaded check valve  44  that is also referred to as an “intake valve”, into a working chamber  46  of the high-pressure fuel pump  24 . A working chamber  46  is defined, among other things, by the piston  32 . 
     From the working chamber  46 , a high-pressure fuel line  48  leads via a spring-loaded check valve  50  to a fuel accumulation line  52 , which is commonly also referred to as the “rail”. The fuel can be stored under very high pressure in the fuel accumulation line  52 . The fuel accumulation line  52  is connected to high-pressure injection valves  54 , which can inject the fuel under very high pressure into combustion chambers  56 . 
     A zero-delivery line  58  branches from the section of the delivery line  40  disposed between the metering unit  42  and the check valve  44 . This zero-delivery line  58  leads to the return line  36  downstream of the return throttle  38 . The zero-delivery line  58  contains a zero-delivery throttle  60 . In addition, a connecting line  62  branches from the section of the delivery line  40  disposed between the drive/crank chamber  26  and the metering unit  42 . This connecting line  62  feeds into the zero-delivery line  58  downstream of the zero-delivery throttle  60 . An overflow valve  64 , which opens toward the zero-delivery line  58 , is disposed in the connecting line  62 . 
     The fuel system  10  also includes a control and regulating unit  66 . On the output side, this unit is connected to magnetic actuator  68  of the metering unit  42 . The metering unit  42  can be embodied as a proportional sliding valve of the kind with two end positions  70  and  72 , as shown in FIG. 1, or can be a highly dynamic on-off valve with two switch positions. A spring  74  presses the metering unit  42  into the neutral position  70  in which the metering unit  42  is closed. In the actuated position  72 , however, the metering unit  42  is open. 
     On the output side, the control and regulating unit  66  is also connected to the electric fuel pump  16 . This connection can be used to influence the operation of the electric fuel pump  16 . In particular, the power supply to the electric fuel pump can be interrupted. 
     The fuel system shown in FIG. 1 is operated according to a method, which is stored as a computer program in the control and regulating unit  66 . 
     During normal operation, the fuel travels through the electric fuel pump  16  to the drive/crank chamber  26  of the high-pressure fuel pump  24 . It is divided there into a delivery flow that travels into the delivery line  40  and a lubricating flow that travels into the return line  36 . The pressure in the drive/crank chamber  26  is determined by the spring force of the overflow valve  64 . It is usually approximately 3 to 4 bar. 
     Through the metering unit  42 , the fuel travels into the working chamber  46 , where, with an upward motion of the piston  32 , it is compressed and displaced into the fuel accumulation line  52 . The fuel quantity, which is supplied to the working chamber  46  and is pumped from there into the fuel accumulation line  52 , is adjusted through a corresponding triggering of the metering valve  42  by the control and regulating unit  66 . 
     In an exemplary embodiment that is not shown, the metering unit is embodied as a quantity control valve, which allows the inlet and the outlet of the working chamber to cancel each other out when it is without power. 
     In the event of a malfunction, which requires a reliable discontinuation of the fuel supply into the fuel accumulation line  52 , the control and regulating unit  66  on the one hand, switches off the power to the magnetic actuator  68  and on the other, switches off the power supply to the electric fuel pump  16 . Consequently, the delivery of fuel to the drive/crank chamber  26  of the high-pressure fuel pump  24  is already reduced or cut off since a presupplying of fuel is no longer taking place by means of the electric fuel pump  16 . Furthermore, the spring  74  presses the metering unit  42  into the neutral position  70  so that the metering unit  42  is closed. 
     Any overflow that passes through even when the metering unit  42  is closed is conveyed back in the direction of the fuel tank  12  through the zero-delivery line  58 . In this manner, the supply of fuel into the fuel accumulation line  52  is cut off. Since no fuel is injected into the combustion chambers  56  by the high-pressure injection valves  54  during overrunning, this measure prevents the pressure in the fuel accumulation line  52  from increasing. 
     Likewise, even in the event of an operational failure of the control and regulating unit  66 , the delivery through the electric fuel pump  16  is automatically cut off. Furthermore, the metering unit  42  is automatically brought into the closed neutral position  70 . Consequently, the delivery of fuel to the fuel accumulation line  52  is redundantly cut off in this instance as well. 
     FIG. 2 shows a second exemplary embodiment of a fuel system  10 . In FIG. 2, those elements and regions, which serve functions equivalent to elements and regions that have already been explained in connection with FIG. 1, are provided with the same reference numerals. They will not be discussed again in detail. 
     In contrast to the fuel system  10  shown in FIG. 1, in the fuel system  10  shown in FIG. 2, a mechanically driven fuel pump  16  is provided as the presupply pump. The drive shaft (not shown) of this fuel pump  16  is connected, for example, to the crankshaft or camshaft (not shown) of the internal combustion engine. A shutoff valve  78  is provided in the fuel line  18  between the filter  20  and the fuel pump  16 . This is a 2/2-port directional-control valve with a closed neutral position  80  and an open actuated position  82 . The shutoff valve  78  is actuated by means of a magnetic actuator  84  and is pressed into the closed neutral position  80  by a spring  86 . The magnetic actuator  84  is triggered by the control and regulating unit  66 . 
     Also in contrast to the first exemplary embodiment, the zero-delivery line  58  does not feed into the return line  36 , but into the fuel line  18  between the shutoff valve  78  and the fuel pump  16 . The connecting line  62 , which contains the pressure control valve  64 , feeds into the zero-delivery line  58  downstream of the zero-delivery throttle  60 . 
     In the event of a malfunction, which requires a reliable discontinuation of the fuel delivery into the fuel accumulation line  52 , in the fuel system shown in FIG. 2, the control and regulating unit  66  switches off the power to the shutoff valve  78  so that the fuel supply to the fuel pump  16  is cut off. The metering unit  42  is also closed in the manner that has already been described in connection with the first exemplary embodiment. 
     This also provides the redundant assurance that when no more fuel should travel from the high-pressure accumulation chamber  52 , through the high-pressure injection valves  54 , and into the combustion chambers  56 , and there is a malfunction in the control and regulating unit  66  and/or in the metering unit  42 , no fuel flows from the high-pressure fuel pump  24  into the fuel accumulation line  52 . This reliably prevents the pressure in the fuel accumulation line  52  from assuming an excessive value. 
     At the same time, as can be seen from FIGS. 1 and 2, a separate pressure control valve, which is connected to the fuel accumulation line  52  so that it limits the pressure in this accumulation line, can be eliminated. 
     The foregoing relates to preferred exemplary embodiments of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.