Abstract:
A fuel supply system comprises a piston pump having a working chamber and an adjustable throttle device, arranged upstream of the working chamber and adapted to modify the flow of fuel supplied to the working chamber. The invention is characterized in that the throttle device, in the closed position, allows a leakage volume to arrive at the working chamber. The piston pump comprises a leakage pump device which supplies at least a part of the leakage volume from the working chamber to a low-pressure zone located upstream of the throttle device.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 35 USC 371 application of PCT/EP 2006/062671 filed on May 29, 2006. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an improved fuel supply system for an internal combustion engine. 
     2. Description of the Prior Art 
     German Patent Disclosure DE 102 20 281 A1 describes a fuel system for an internal combustion engine, in which fuel is pumped from a prefeed pump to a high-pressure pump and from there into a high-pressure fuel rail. A plurality of injectors are connected to this rail and inject the fuel directly into combustion chambers of the engine. The feed quantity of the high-pressure pump that is mechanically driven by the engine is determined by an upstream throttle device. To limit the production costs, the throttle device is designed such that even in the completely closed state, it allows a certain leakage quantity of fuel to pass through. This quantity is returned to a low-pressure region via a zero-feed line, in which there is a zero-feed throttle restriction, and thus does not reach the actual piston pump. 
     SUMMARY AND ADVANTAGES OF THE INVENTION 
     The present invention has the object of refining a fuel supply system of the type defined above in such a way that its construction is as simple as possible. 
     In the fuel supply system of the invention, it is expressly permitted that the leakage quantity allowed through by the throttle device reaches the work chamber of the piston pump. Thus the zero-feed line and a zero-feed throttle restriction located in it that were previously required can be dispensed with, which simplifies the construction of the fuel supply system of the invention considerably. This also reduces costs. 
     In order nevertheless to assure that, for instance in the overrunning mode of an engine for which the fuel supply system is intended, no fuel will be pumped by the piston pump, according to the invention a special leakage pump device is provided, by which the leakage quantity is at least in part pumped away from the work chamber to a low-pressure region located upstream of the throttle device. The low-pressure region may for instance be directly upstream of an inlet valve of the piston pump, so that the fuel supply system is very compact, and additional long lines are unnecessary. 
     If the return of the leakage quantity discharges upstream of a filter, then abrasion generated in piston pump operation is reliably kept away from the piston pump, the throttle device, and other valve devices, which improves the service life and the reliability of the fuel supply system of the invention. 
     Especially simply and with little additional expense, the leakage pump device can be implemented by the pump piston of the piston pump itself and by the guide gap between the pump piston and the pump housing. A leakage quantity reaching the work chamber is in this case carried away simply by the pressure difference between the work chamber and the drive side of the pump piston. Good efficiency of the fuel supply system is maintained, if the guide gap is embodied such that, with the throttle device closed, precisely only the leakage quantity of the throttle device is pumped back to the low-pressure region. 
     Typically, the fuel supply system pumps into a high-pressure chamber, for instance a high-pressure rail. To enable reducing the pressure in this high-pressure chamber in certain operating situations in a simple way, the high-pressure chamber may communicate with the work chamber of the piston pump via a throttle restriction that is fluidically parallel to an outlet valve of the piston pump. In this way, a separate pressure relief valve on the high-pressure chamber can be dispensed with, which further simplifies the construction and reduces the corresponding costs of the fuel supply system of the invention. 
     For reliable operation of the fuel supply system of the invention, it is appropriate if the differential opening pressure of an inlet valve of the piston pump amounts to at least approximately 1 bar, since in that case the formation of fuel vapor from pressure pulsations in piston pump operation between the throttle device and the inlet valve is prevented. 
     Overall, the leakage quantity is kept low if, in the case of a throttle device with a throttle slide, a control opening is present not on the throttle slide but rather on the throttle housing. The guide gap between the throttle slide and throttle housing should be smaller than or equal to the guide gap between the pump piston and the pump housing. Typical values are 4 and 7 μm, respectively. 
     To avoid an unnecessary pressure buildup in the high-pressure chamber, the leakage quantity of the throttle device should be less than the fuel demand of an idling engine, that is, when only a minimal fuel quantity is being injected into the combustion chambers of the engine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Especially preferred exemplary embodiments of the present invention will be described in further detail below in conjunction with the accompanying drawings. In the drawings: 
         FIG. 1  is a schematic illustration of a first exemplary embodiment of a fuel supply system and an internal combustion engine; 
         FIG. 2  is an enlarged view of a region of the fuel supply system of  FIG. 1 ; 
         FIG. 3  is a graph in which a pressure difference via a pump piston of the supply system of  FIG. 1 , a piston stroke, and a leakage quantity of a throttle device are plotted over the angle of a driveshaft; 
         FIG. 4  is a view similar to  FIG. 1  of an alternative embodiment of a fuel supply system; and 
         FIG. 5  is an enlarged view of a region of the fuel supply system of  FIG. 4 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment of a fuel supply system is identified overall in  FIG. 1  by reference numeral  1 . It includes a fuel tank  2 , from which a prefeed pump  3  pumps the fuel via a line  4  to a high-pressure pump unit  10 . This high-pressure pump unit includes a throttle device  20  and a high-pressure piston pump  6 . The throttle device  20  is located fluidically between the prefeed pump  3  and the high-pressure piston pump  6 , and on the low-pressure region it regulates the pumping quantity of the high-pressure piston pump  6 . This piston pump, in the present exemplary embodiment, is driven via a cam  30 , which in turn is driven mechanically, in a manner not shown in detail in  FIG. 1  by an internal combustion engine  7 , for instance by its camshaft or crankshaft. The cam  30  may also be part of the camshaft or crankshaft. 
     The high-pressure piston pump  6  compresses the fuel delivered to it to a comparatively high pressure and pumps it via a line  5  into a high-pressure chamber  40 . In it, the fuel is stored at high pressure; it is also known as a “high-pressure reservoir” or “rail”. A plurality of injectors  41  are connected to the high-pressure chamber  40  and inject the fuel directly into combustion chambers  42  associated with each of them. 
     The pressure prevailing in the high-pressure chamber  40  is detected by a pressure sensor  43 . The rpm of a crankshaft, not shown, of the engine  7  is detected by an rpm transducer  44 , and a temperature of the engine  7  is detected via a temperature sensor  45 . A control and regulating unit  46  controls or regulates, among other things, the operation of the throttle device  20 ; the signals of the sensors  43 ,  44  and  45 , and optionally still other sensors, enter into the control or regulation as applicable. A computer program for triggering the throttle device is stored in memory on a memory medium  47  of the control and regulating unit  46 . 
       FIG. 2  will now be described, in which the high-pressure pump unit  10  is shown enlarged. Upstream of the throttle device  20  in the high-pressure pump unit  10 , there is a filter  101 , and in a conduit  8  that is part of the low-pressure region, there is a pressure damper  102 . The latter is intended to damp pulsations of the high-pressure piston pump  6 , for instance in the line  4 . It is also intended to assure a high degree of delivery of the high-pressure piston pump  6 , even at high rpms and high numbers of cams. 
     The throttle device  20  includes a cylindrical throttle slide  201  and a cylindrical throttle housing  202 . The throttle slide  201  is actuated by an electromagnetic actuator  203 , against which the throttle slide  201  is urged by a compression spring  204 . The throttle slide  201  has a portion of lesser diameter (not identified by reference numeral), by which an inlet chamber  205  is formed between the throttle slide  201  and the throttle housing  202 . On the throttle slide  201 , there is an encompassing control edge  206 , which cooperates with control openings  207  embodied on the throttle housing  202 . Via a connection  208 , these control openings lead to the high-pressure piston pump  6 . Between the throttle slide  201  and the throttle housing  202 , there is a guide gap  209 . 
     The high-pressure piston pump  6  in turn includes an inlet valve  103 , by way of which the fuel from the connection  208  of the throttle device  20  can reach a work chamber  104  that is formed between a pump piston  105  and a pump housing  106 . The pump piston  105  is sealed off from the work chamber, in which the cam  30  is located, via a piston seal  107 . From the work chamber  104 , the fuel reaches the high-pressure chamber  40  via an outlet valve  108 . Parallel to it but with an opposite opening direction, a pressure limiting valve  109  is disposed between the work chamber  104  and the high-pressure chamber  40 . In normal operation of the fuel supply system  1 , this valve is closed. 
     Fluidically parallel to the throttle device  20  and to the high-pressure piston pump  6 , a bypass valve  110  is also disposed in the high-pressure pump unit  10 ; it connects the high-pressure chamber  40  with the conduit  8  located between the filter  101  and the pressure damper  102 , and it opens toward the high-pressure chamber  40 . This bypass valve  110  is likewise closed in normal operation. In the event of a malfunction, for instance if the throttle device  20  sticks in the closed position, however, fuel can reach the high-pressure chamber  40  via this bypass valve  110 , so that in the high-pressure chamber, at least the pressure generated by the prefeed pump  3  prevails, which makes a certain emergency operation of the engine  7  possible. 
     Between the pump piston  105  and the pump housing  106 , there is a guide gap  111 . Directly upstream of the piston seal  107 , a leak fuel line  112  branches off from the guide gap  111 ; this line leads to the low-pressure region  8  directly upstream of the filter  101 . The orifice of the leak fuel line  112  is accordingly covered by the pump piston  105 , while conversely the orifices of the pump piston  105  coming from the inlet valve  103  into the work chamber  104  and from the work chamber  104  to the outlet valve  108 , are always open. 
     In normal operation, the prefeed pump  3  generates a prefeed pressure at the level of approximately 6 bar. Depending on the position of the throttle slide  201  of the throttle device  20 , and depending on the corresponding overlap of the control edge  206  and the control openings  207 , more or less fuel reaches the high-pressure piston pump  6 . During the intake phase, the fuel is aspirated into the work chamber  104  via the inlet valve  103 . Depending on the throttling, more or less vapor is then created in the work chamber  104 . In this way, the pumping quantity of the high-pressure piston pump  6  to the high-pressure chamber  40  is adjusted. It should be pointed out that in the present exemplary embodiment, the throttle device  20  is “closed when without current”, which means that the throttle slide  201  is pressed by the compression spring  204  into the closed position when the electromagnetic actuator  203  is without current. To avoid the creation of fuel vapor between the connection  208  and the inlet valve  103 , the differential opening pressure of the inlet valve is approximately 1 bar. If diesel fuel, which has a different vapor pressure, is used, then the differential opening pressure can also be markedly less. 
     Even in the closed position of the throttle device  20 , however, fuel can pass through the guide gap  209  between the throttle slide  201  and the throttle housing  202  and via the inlet valve  103  reach the work chamber  104  of the high-pressure piston pump  6 . In order nevertheless to prevent the pressure in the work chamber  104  from reaching a level at which the outlet valve  108  opens and fuel gets into the high-pressure chamber  40 , this fuel quantity, also called the “leakage quantity”, is pumped out of the work chamber  104  via a leakage pump device  113  from the work chamber  104 . 
     This leakage pump device  113 , in the present exemplary embodiment, is formed simply by the pump piston  105  and the guide gap  111  between the pump piston  105  and the pump housing  106 . This gap is specifically dimensioned such that, because of the pressure difference between the work chamber  104  and the prefeed pressure prevailing directly upstream of the piston seal  107 , in a pumping stroke of the pump piston  105  precisely the leakage quantity that reaches the work chamber  104  when the throttle device  20  is closed is pumped away via the leak fuel line  112 . Functionally, the “zero-feed throttle restriction” that was previously usual is thus formed by the guide gap  111 . 
     In this way, zero feeding of the high-pressure piston pump  6  can reliably be attained. Zero feeding is wanted for instance whenever the engine  7 , which serves to drive a motor vehicle, is in an overrunning mode, in which while the cam  30  is driven, still no fuel reaches the combustion chambers  42  via the injectors  41 . In such a case, to avoid an unwanted pressure increase in the high-pressure chamber  40 , and to enable dispensing with a separate pressure relief device, it must be assured that the pumping of fuel by the high-pressure piston pump  6  can be suppressed entirely. Thanks to the leakage pump device  113  provided, this is possible even with a throttle device  20  that does not close completely in the closed state. 
     For this purpose, it is necessary to adapt the guide gap  209  of the throttle device  20  and the guide gap  111  of the high-pressure piston pump  6  to one another in such a way that the leakage quantity allowed through by the throttle device  20  is pumped back, by means of the pumping motion of the pump piston  105 , past the guide gap  111  without the opening pressure of the outlet valve  108  in the work chamber  104  being reached, at a very specific pressure in the high-pressure chamber  40 . This very specific pressure in the high-pressure chamber  40  may for instance be a pressure at which the injectors  41  can still reliably inject the fuel into the combustion chambers  42 . In  FIG. 2 , the course of the leakage flow is illustrated by arrows  114 . 
     The functional principle of the leakage pump device  113  is also shown by the graph in  FIG. 3 . It can be seen that in the region of top dead center of the pump piston  105  (the stroke of the pump piston  105  is represented by the curve  115 ), a “pressure crest” (curve  116 ) is embodied in the work chamber  104 . Zero feeding of the high-pressure piston pump  6  exists whenever the maximum pressure of this pressure crest is at most equal to the current pressure in the high-pressure chamber  40 . That is assured only whenever the entire leakage quantity that is allowed to pass by the throttle device  20  is carried away by the leakage pump device  113 . Otherwise, the pressure in the work chamber  104  would increase in each work cycle of the high-pressure piston pump  6 , until finally the outlet valve  108  would close. 
     The leakage quantity carried away by the leakage pump device  113  via the guide gap  111  is represented by the curve  117 . It can be seen that at the high pressure prevailing in the region of top dead center, a relatively large leakage quantity passes through the guide gap  111  and is carried away by the leak fuel line  112 . Outside top dead center of the pump piston  105 , however, a lower pressure than in the conduit or low-pressure region  8  prevails in the work chamber  104 , so that a certain fuel quantity, although very slight, flows back into the work chamber  104  via the guide gap  111 . In a typical adaptation, the guide gap  111  has a value of 7 μm, while conversely the guide gap  209  has a value of 4 μm. 
     In order also to be able to attain operating states of the engine  7  in which a reduced pressure is desired in the high-pressure chamber  40 , for instance in idling, the leakage quantity of the throttle device  20  should be less than the fuel demand of the engine  7  while it is idling. This is associated with the fact that at such a low pressure in the high-pressure chamber  40 , the outlet valve  108  already opens even at a correspondingly low pressure, so that the maximum pressure in the work chamber  104  likewise corresponds at most to the reduced pressure in the high-pressure chamber  40 . At this kind of reduced pressure, however, the pressure past the guide gap  111  also drops, which reduces the “pumping output” of the leakage pump device  113 . Optionally, because of this reduction in the pressure difference past the guide gap  111 , it would even be possible for the entire leakage quantity that is allowed through by the throttle device  20  to be pumped by the high-pressure piston pump  6  to the high-pressure chamber  40 . Therefore, as noted, this leakage quantity should at most correspond to the fuel demand of the engine  7  while idling. 
     The pressure in the high-pressure chamber  40  can be reduced by providing that more fuel is injected by the injectors  41  into the combustion chambers  42  than is pumped by the fuel supply system  1  into the high-pressure chamber  40 . This can be regulated by means of the throttle device  20 . It is understood that the maximum pressure in the high-pressure chamber  40  that is established in the overrunning mode of the engine  7  should in principle be no greater than a pressure at which the injectors  41  still function reliably. If such a reduction in the pressure in the high-pressure chamber  40  is not possible, then this must be compensated for by a suitably altered triggering of the injectors  41 . 
     In the event of error, for instance if the throttle device  20  sticks in the open state, the pressure in the high-pressure chamber  40  is limited to a defined maximum value by the pressure limiting valve  109 . 
     It will furthermore be pointed out that in the above-described, especially advantageous exemplary embodiment, the guide gap  111  functions as a flow throttle restriction between the work chamber  104  and the leak fuel line  112 . However, it is conceivable, although not shown, that the leak fuel line branch off directly from the work chamber  104  and that a separate flow throttle restriction be present in it that takes on the hydraulic function of the guide gap  111 . However, if the guide gap  111  functions as a flow throttle restriction, then this has the advantage that a variable throttling action can be attained, which is at its least at bottom dead center of the pump piston  105  and at its greatest at top dead center. 
     An alternative embodiment of a fuel supply system  1  is shown in  FIGS. 4 and 5 . In these drawings, those elements and regions that have equivalent functions to elements and regions of the exemplary embodiment described in conjunction with  FIGS. 1 through 3  are identified by the same reference numerals. They will not be described again in detail. 
     In the fuel supply system  1  shown in  FIGS. 4 and 5 , the pumping output of the prefeed pump  3  is adjustable. In this way, the pressure in the line  4  and in the low-pressure region  8  can be adjusted to suit a desired pilot pressure. For this purpose, the prefeed pump  3  is triggered by the control and regulating unit  46 . This has the advantage first that the prefeed pump  3  is always operated with the least possible output. Second, an adjustable pilot pressure has the advantage that the regulation sensitivity of the throttle device  20  is improved. The pressure difference at the throttle device  20  can be adjusted optimally as a function of the load and rpm of the engine  7 , given an adjustable pilot pressure. An increased fuel temperature and an increased vapor pressure can furthermore be compensated for. 
     A variable pilot pressure can, however, also be utilized in order to control or regulate the leakage quantity of the throttle device  20  and thus a high pressure that comes to be established in the high-pressure chamber  40 . For instance, if in the overrunning mode of the engine  7  the pilot pressure is lowered, then the leakage quantity of the throttle device  20  decreases as well, since the pressure difference at the guide gap  209  decreases to the same extent. With a small leakage quantity at the guide gap  209  of the throttle device  20 , a likewise lower pressure is established in the high-pressure chamber  40  in the overrunning mode of the engine  7 . Conversely, this means that given an adjustable pilot pressure, the demands of the throttle device  20  with regard to regulation sensitivity and the allowable leakage quantity can be lessened. For instance, the guide gap  209  can be enlarged, thus simplifying production. 
     A further difference between the exemplary embodiment of a fuel supply system  1  shown in  FIGS. 4 and 5  and the exemplary embodiment preceding is that a flow throttle restriction  118  is disposed parallel to the outlet valve  108 . By means of this throttle restriction, a “passive” pressure reduction in the high-pressure chamber  40  is made possible. In the overrunning mode of the engine  7  or when the engine  7  has been shut off the pressure in the high-pressure chamber  40  can be reduced in this way down to the pressure prevailing in the low-pressure region  8 , via the flow throttle restriction  118  and the guide gap  111  of the high-pressure piston pump  6 . Particularly in the overrunning mode of the engine  7 , the pressure in the high-pressure chamber  40  can thus be lowered to a desired value and regulated, by means of the variable pilot pressure, in such a way that it is ideal for the resumption of injection by the injectors  41 . 
     If in the overrunning mode of the engine  7 , an inconsistent aspiration of the leakage quantity admitted by the throttle device  20  should occur because the vapor pressure between the connection  208  and the inlet valve  103  has been undershot, then a stepwise increase, threatened as a result, in the pressure prevailing in the high-pressure chamber  40  is avoided by means of the throttle restriction  118 . 
     In an exemplary embodiment not shown, the fuel supply system includes a high-pressure piston pump with a plurality of pump pistons and work chambers connected fluidically parallel to one another. In this case as well, the metering of the fuel can be effected via a throttle device. In designing the guide gaps, however, the guide gaps of all the pump pistons must be taken into account. 
     The foregoing relates to a preferred exemplary embodiment 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.