Abstract:
In a method for operating an internal combustion engine ( 10 ) equipped with a piston pump as a high-pressure pump ( 18 ), which is driven by a drive shaft ( 28 ) of the engine ( 10 ), in which the high-pressure pump ( 18 ) delivers fuel from a low-pressure region ( 16 ) to a high-pressure side ( 38 ) and a quantity control valve ( 44 ) sets the quantity of fuel delivered by the high-pressure pump ( 18 ), the acoustic emission of the high-pressure pump is reduced by virtue of the fact that the high-pressure pump ( 18 ) functions in a two-point operation, alternating between full delivery for individual or successive piston strokes and idle delivery for individual or successive piston strokes and, when the pressure falls below a lower pressure threshold, the full delivery is activated until an upper pressure threshold is reached.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. 119(a)-(d) to German Patent Application DE 102005014093.3, filed Mar. 29, 2005. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for operating an internal combustion engine equipped with a piston pump as a high-pressure pump, which is driven by a drive shaft of the engine; the high-pressure pump delivers fuel from a low-pressure region to a high-pressure side and a quantity control valve sets the quantity of fuel delivered by the high-pressure pump. 
     2. Related Art 
     In direct-injection gasoline engines (GDI=gasoline direction injection), one-cylinder high-pressure pumps are used to raise the pressure from the preliminary pressure of the presupply pump (EFP=electric fuel pump) to the pressure required for the direct injection (50 to 200 bar). These one-cylinder pumps are operated with 2, 3, or 4 pump strokes per camshaft rotation, depending on the amount of fuel that the motor requires. Usually, the driving action is provided by a cam on the camshaft. During normal operation, each pump stroke is used and the required quantity is set, for example, by a quantity control valve. In other words, when operating in idle mode and in the partial load range, only part of the possible quantity per pump stroke is delivered. 
     EP-1327766-A2 has disclosed a method in which only a part of the delivery strokes is used at low supply quantities. The motivation for this is the better controllability at very low supply quantities. In this method, a fixed pattern of used and unused delivery strokes in relation to the camshaft rotation is set, e.g. only 2 out of 4 delivery strokes are used. 
     3. Problems of the Prior Art 
     When in delivery mode, the high-pressure pump generates structure-borne acoustic vibrations, which generate airborne sound that is perceived as acoustic noise. The method is intended to reduce the acoustic emission of the high-pressure pump and to change this acoustic emission so that it is not perceived as annoying. 
     BRIEF SUMMARY OF THE INVENTION 
     This problem is solved by a method for operating an internal combustion engine equipped with a piston pump as a high-pressure pump, which is driven by a drive shaft of the engine; the high-pressure pump delivers fuel from a low-pressure region to a high-pressure side and a quantity control valve sets the quantity of fuel delivered by the high-pressure pump; the high-pressure pump functions in a two-point operation, alternating between full delivery for individual or successive piston strokes and idle delivery for individual or successive piston strokes and, when the pressure falls below a lower pressure threshold, the full delivery is activated until an upper pressure threshold is reached. 
     The term “full delivery” is understood to mean that the high-pressure pump delivers the maximum quantity, i.e. the quantity control valve remains closed during the entire piston stroke. The term “idle delivery” is understood to mean the exact opposite: the high-pressure pump does not deliver any fuel over the entire piston stroke, i.e. the quantity control valve remains continuously open. The term “partial delivery” is understood to mean a delivery quantity between idle delivery and full delivery; in this case, the quantity control valve is opened intermittently during the piston stroke of the piston pump so that a delivery quantity of between zero and the maximum delivery quantity can be achieved. The upper pressure threshold and the lower pressure threshold depend on the pressure in the accumulator required to reliably execute an injection. The two pressure thresholds can be identical and correspond to the desired pressure of the high-pressure side or can be slightly higher and lower, respectively, than the desired pressure. 
     An essential aspect of this method is to limit the frequency of delivery by the high-pressure pump to the absolute amount required. This is achieved by switching to two-point control in idle mode and executing each activated delivery with the maximum delivery quantity. This brings to bear the effect that a full delivery of the high-pressure pump is quieter than a partial delivery. The two effects cause the acoustic emission of this control method to be significantly lower than that of the method currently in use. 
     Preferably, the two-point operation is activated when the engine speed falls below a minimum speed and/or when the injection quantity falls below a minimum quantity. The decrease to below a minimum speed can, for example, be when the idling speed is reached. In one embodiment of the method, when not in idle mode, the high-pressure pump is operated with partial delivery. 
     The term “idle mode” here is defined on the one hand by a speed range typical of internal combustion engines and on the other hand by the speed requested by the driver during operation, for example when the gas pedal of an automobile is brought into the idle position. Other requests of the operator that signal idle mode as the requested engine speed include, for example, when the selector lever is moved into the park position in an automatic transmission or in an automated manual transmission. 
     In another embodiment of the method, after the upper pressure limit is reached, the high-pressure pump is switched to idle delivery until the pressure falls back below the lower pressure limit. The high-pressure pump is operated in the full delivery mode when the quantity control valve is closed and is operated in the partial delivery mode when the quantity control valve is intermittently or continuously open. The quantity control valve remains open down to a lower pressure threshold and, once the lower pressure threshold has been reached, remains closed until the upper pressure threshold is reached. 
     In another embodiment of the method, the quantity control valve is opened when the upper the pressure threshold is reached. 
     The problem mentioned at the beginning is also solved by an internal combustion engine equipped with a piston pump as a high-pressure pump, which is driven by a drive shaft of the engine; the high-pressure pump delivers fuel from a low-pressure region to a high-pressure side and a quantity control valve sets the quantity of fuel that the high-pressure pump delivers to the accumulator, characterized in that in idle mode, the high-pressure pump can be operated in full delivery mode and in idle delivery mode. 
     The problem mentioned at the beginning is also solved by a control unit for an internal combustion engine, characterized in that it is able to execute a method as described herein. 
     The problem mentioned at the beginning is also solved by a piece of software for a stored program control unit for an internal combustion engine, characterized in that it is able to execute a method as described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the present invention will be explained in detail below in conjunction with the accompanying drawings. 
         FIG. 1  is a schematic depiction of an internal combustion engine equipped with a fuel pump and a quantity control valve; 
         FIG. 2  is a detailed depiction of the fuel pump and the quantity control valve from  FIG. 1  during an intake stroke; 
         FIG. 3  is a depiction similar to  FIG. 2  at the beginning of a delivery stroke; 
         FIG. 4  is a depiction similar to  FIG. 2  toward the end of a delivery stroke; 
         FIG. 5  is a graph of the curve of the process over time. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An internal combustion engine  10  according to FIG.  1 —this can in particular be a direct-injecting gasoline engine—includes a fuel tank  12  from which an electrically driven prefeed pump  14  delivers fuel via a low-pressure line  16  to a high-pressure pump  18 . The fuel travels onward via a high-pressure line  20  to an accumulator  22  (also referred to as the common rail) in which the fuel is stored at high pressure. The accumulator  22  has a number of injection devices  24  connected to it that inject the fuel directly into combustion chambers  26 . The combustion of the fuel in the combustion chambers  26  sets a crankshaft  28  into rotation. Via a mechanical coupling  30  that is only depicted schematically in  FIG. 1 , the high-pressure pump  13  is driven by the crankshaft  28  serving as a drive shaft. The high-pressure pump  18  is a one-cylinder piston pump in which a drive cam  32  on a shaft  33  sets a piston  34  into a reciprocating motion. The piston  34  is guided in a housing  36  and delimits a delivery chamber  38 . An inlet valve  40  can connect the delivery chamber  38  to the low-pressure fuel line  16 . The inlet valve  40  is embodied in the form of a spring-loaded check valve. An outlet valve  42  can connect the delivery chamber  38  to the high-pressure line  20 . The outlet valve  42  is also a spring-loaded check valve. A quantity control valve  44  can also connect the delivery chamber  38  to the low-pressure chamber  16 . The quantity control valve  44  is a 2/2-way switching valve. A spring  46  brings it into the open, neutral position. An electromagnetic actuating device  48  brings it into the closed, switched position. This actuating device includes a magnetic armature  52  that is connected to a valve element  50  and is encompassed by a magnetic coil  54 . The magnetic coil  54  is supplied with current by the driver stage, not shown, of a control unit  56 . The control unit  56  receives signals from a speed sensor  58 , which senses the speed of the crankshaft  23  of the internal combustion engine  10 . The input side of the control unit  56  is also connected to a pressure sensor  60  that detects the pressure prevailing in the accumulator  22  and transmits corresponding signals to the control unit  56 . The principal of adjusting the fuel quantity delivered by the high-pressure pump  18  will now be explained in conjunction with  FIGS. 2 through 4 . During the intake stroke depicted in  FIG. 2 , the piston  34  moves downward so that fuel flows into the delivery chamber  38  via the inlet valve  40 . After reaching the bottom dead center, the piston  34  moves upward again ( FIG. 3 ). During the intake stroke of the piston  34 , the magnetic coil  54  of the quantity control valve  44  is supplied with current so that, at the very latest, this valve closes when the piston  34  reaches the bottom dead center. The inlet valve  40  also closes. During the delivery stroke of the piston  34 , if the pressure in the delivery chamber  38  exceeds the opening pressure of the outlet valve  42 , then the outlet valve opens. The fuel can thus be pushed into the accumulator  22 . If the delivery of fuel into the accumulator  22  must be terminated during the delivery stroke of the piston  34 , then the supply of current to the magnetic coil  54  of the quantity control valve  44  is disconnected so that the quantity control valve switches back into the neutral position. This is shown in  FIG. 4 . The fuel can thus escape from the delivery chamber  38  into the low-pressure line  16  via the open quantity control valve  44 . Correspondingly, the outlet valve  42  also closes. The maximum fuel quantity that can be delivered during a delivery stroke of the piston  34  is essentially independent of the speed of the crankshaft  28  and the related duration of a delivery stroke. During each ci th  delivery stroke, the quantity control valve  44  can close off the delivery chamber  38  from the low-pressure line  16  for a certain duration. 
     When not in idle mode, the quantity control valve  44  is actuated so that each delivery stroke of the pump is used. The quantity is controlled by using partial strokes through intermittent opening of the quantity control valve  44 , as described above. In idle mode, however, the operation switches over to a two-point control with full delivery. This means that a delivery and therefore the actuation of the quantity control valve  44  is only triggered if the pressure falls below a pressure threshold on the high-pressure side. In this operating state, the delivery is always executed as a full delivery so that the pressure in the high-pressure system increases by a relatively large amount. The injections that follow cause the pressure to decrease again steadily. But since the injection quantities are low in idle mode, it takes a relatively long time before the pressure falls below the lower pressure threshold that triggers the next delivery. 
       FIG. 5  is a graph of the curve of the process over time. The pressure pHd in the accumulator  22 , i.e. the pressure in the common rail, is plotted over time t. The pressure curve is shown between an arbitrarily selected time t 0  and an arbitrarily selected time t 4 . At time t 0 , the pressure pHd should equal the value of a lower pressure threshold pU. At this time, the quantity control valve  44  is closed so that the high-pressure pump delivers for the entire piston stroke and is operated in an operating mode that is referred to below as a full delivery. The quantity control valve  44  remains closed until an upper pressure threshold pO is reached; this occurs at time t 1 . At time t 1 , the quantity control valve  44  is completely open so that the high-pressure pump  18  no longer delivers any fuel to the high-pressure side. This operating mode is referred to below as idle delivery. Because the injection devices  24  continue to execute injections, the pressure pHd in the accumulator  22  (common rail) decreases with each injection. For the sake of simplicity, this is depicted as a continuous line in  FIG. 5 , but in reality, this is not continuous, but is instead more or less step-like in the depiction over time. At time t 2 , the lower pressure threshold pU is reached again so that the closing of the quantity control valve  44  switches the high-pressure pump  18  back into the full delivery operating mode. When the upper pressure threshold pO is reached at time t 3 , the high-pressure pump  18  is switched back into the idle delivery mode so that the pressure pHd falls again. In the time spans t 0  to t 1  and t 2  to t 3 , one or more piston strokes are executed, depending on the maximum delivery quantity of the high-pressure pump  18 . The duration of the idle delivery mode, i.e. between times t 1  and t 2 , essentially depends on the storage capacity of the accumulator  22  and the respective quantity injected. The operating mode depicted in  FIG. 5  is only selected in the idle mode of the internal combustion engine. When not in idle mode, the high-pressure pump  18  is operated in a partial delivery operating mode. In this operating mode, fuel is delivered to the high-pressure side with each piston stroke of the fuel pump  18 . The quantity control valve  44  controls the fuel quantity by intermittently opening as needed (e.g. partial load) during the piston stroke of the fuel pump  18 .  FIG. 5  also shows a desired pressure Pso, to which the rail pressure (on the high-pressure side) should be set in the respective operating range. The lower pressure threshold pU and upper pressure threshold pO are close to the desired pressure. The activation condition for the above-explained two-point control can be selected, for example, to be when the engine speed falls below a minimum speed (e.g. when it reaches the idling speed) or when the injection quantity falls below a minimum quantity. In this connection, the Lambda regulation should be active, the engine temperature should be within a permissible range (normal temperature), and the engine should have been started long enough ago for the starting oscillations to have reached a steady state.