Patent Publication Number: US-7219659-B2

Title: Fuel injection system comprising a pressure intensifier and a delivery rate-reduced low-pressure circuit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a 35 USC 371 application of PCT/DE 03/02175 filed on Jun. 30, 2003. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to an improved fuel injection system for internal combustion engines, including a pressure booster and a reduced quantity low pressure circuit. 
   2. Description of the Prior Art 
   Both pressure-controlled and stroke-controlled injection systems are known for supplying combustion chambers of self-igniting internal combustion engines with fuel. As fuel injection systems, not only unit fuel injectors and pump-line-nozzle units but also reservoir injection systems are also used. Reservoir injection systems (common rails) advantageously make it possible to adapt the injection pressure to the load and rpm of the engine. To attain high specific outputs, a high injection pressure is required. The higher the attainable injection pressure, the less are the emissions from the engine. 
   BACKGROUND OF THE INVENTION 
   German Patent Disclosure DE 199 10 970 A1 discloses a fuel injection system having a pressure boosting unit, which is located between a pressure reservoir and a nozzle chamber and whose pressure chamber communicates with the nozzle chamber via a pressure line. A bypass line connected to the pressure reservoir is also provided. The bypass line communicates directly with the pressure line. The bypass line can be used for a pressurized injection and is located parallel to the pressure chamber, so that the bypass line is passable, regardless of the motion and position of a displaceable pressure fluid in the pressure boosting unit. This makes greater flexibility in terms of the injection possible. 
   German Patent Disclosure DE 101 23 911.4 relates to a fuel injection system with a pressure boosting device. A fuel injection system for internal combustion engines includes a fuel injector, which can be supplied from a high-pressure fuel source and has a pressure boosting device. The pressure boosting device includes a movable piston, which divides a chamber connected to the high-pressure fuel source from a high-pressure chamber communicating with the injector. The high-pressure chamber communicates with a differential pressure chamber via a fuel line, so that the high-pressure chamber can be filled with fuel via the differential pressure chamber of the pressure boosting device. The triggering of the fuel injection system with the pressure boosting device known from DE 101 23 911.4 is effected via a pressure relief of the differential pressure chamber of the pressure boosting device. The systems known from DE 199 10 970 A1 and DE 101 23 911.4, include a stroke-controlled fuel injector. Each fuel injector is assigned a pressure booster, for elevating the injection pressure as needed. The triggering of the pressure boosting device is effected via a simple 2/2-way valve and leads to reduced depressurization losses, since the differential pressure chamber of the pressure boosting device is pressure-relieved for its actuation. Moreover, these systems make it possible to perform multiple injections and to shape the injection course flexibly. 
   The use of a pressure boosting device in a fuel injection system that includes a common rail leads to a greatly increased fuel quantity demand per fuel injector within the injection system. For a high-pressure pumping unit, the result is an increased pumping quantity at a reduced pressure level. For a low-pressure pump, the pumping quantity also increases. The pressure level of the low-pressure pumping unit, however, does not decrease, since good filling of the pump chambers of the high-pressure pumping unit and exact meterability of the pumping quantity by the metering unit in the fuel system must be assured. Designing the prefeed pump for the large-quantity flows required in fuel injectors with a pressure booster is therefore a problem. In a fuel injection system with a common rail with an integrated pressure booster, high return quantities occur because of the pressure boosting, and these quantities amount to multiple times the fuel quantity to be injected via the respective fuel injector. In the systems known from the prior art, this fuel quantity is depressurized completely and is delivered to the fuel tank, which is exposed only to atmospheric pressure. The entire quantity demanded by the fuel injection system must then be compressed by the low-pressure pump to the prefeed pressure, to enable filling of the pump chambers of the high-pressure pumping unit. 
   SUMMARY OF THE INVENTION 
   According to the provisions of the invention, to reduce the prefeed quantity, the return from the pressure booster is not depressurized completely and pumped back into the fuel tank. As proposed, a compensation container can be integrated with the return from the pressure booster, and a return line can discharge into the low-pressure circuit, for instance directly downstream of the compression-side outlet from the prefeed unit into the low-pressure circuit. As a result, the fuel quantity returning from the pressure booster can depressurize only down to the relatively low pressure level of the prefeed pump, that is, the prefeed pressure. As a result, the quantity to be pumped by the prefeed pump decreases in accordance with the pressure boosting ratio of the pressure booster. 
   The return from the pressure booster can be fed into the low-pressure circuit, acted upon by the prefeed pump, at any arbitrary point. The return can be fed in upstream of a fuel filter, on the one hand, to assure cleaning of the fuel, but on the other it is also possible for the pressure booster return, flowing back from the pressure booster, to be fed into the low-pressure circuit downstream of the fuel filter, to reduce the filter size. It is furthermore possible, downstream of a metering unit that is upstream of the high-pressure pumping unit, to feed the pressure booster return into the low-pressure circuit, in order to reduce the flow cross section required in a metering unit for regulating the demand of the high-pressure pumping unit. A further possible embodiment that may be mentioned is for the return from the fuel injector also to be depressurized only down to the pressure level that can be built up by the prefeed pump and to feed it into the low-pressure circuit downstream of the prefeed pump. This variant embodiment can be employed in fuel injection systems with a common rail without a pressure booster, to reduce the low-pressure pumping quantity, since depending on the design of the fuel injector and the pressure level prevailing in the common rail, the return quantity from the fuel injector may represent a considerable proportion of the total quantity. However, it is also possible to feed in only a partial quantity of the injector return into the low-pressure circuit downstream of the prefeed pump. As a result, pressure-sensitive chambers in a fuel injector or a pressure booster module, such as a magnet valve armature chamber, can continue to be depressurized down to a lesser pressure level. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in further detail below in conjunction with the drawings, in which: 
       FIG. 1  is the hydraulic layout of the high-pressure and low-pressure circuits in a high-pressure common rail injection system with a pressure booster; 
       FIG. 2  is a schematic illustration of the hydraulic mode of operation of a fuel injection system with a common rail and a pressure booster; and 
       FIG. 3  is a schematic illustration of the hydraulic interconnection according to the invention of the low-pressure circuit of a fuel injection system with a pressure booster and a common rail. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows the hydraulic interconnection of the components of a fuel injection system with a common rail and a pressure booster, along with the components used in it. The fuel injection system with a high-pressure reservoir or common rail  4  and a pressure booster  7  upstream of a fuel injector  10  includes a high-pressure pumping unit  1 . A metering unit, not shown in further detail, precedes the high-pressure pumping unit  1 , and by way of it fuel is metered as needed to the high-pressure pumping unit. From a fuel tank  14 , which contains fuel whose fuel level is shown at 15, fuel flows via an inlet  16  to a prefeed pump upstream of the high-pressure pumping unit  1 . The fuel is compressed in that pump to the prefeed pressure. Next, the compressed fuel travels through a fuel filter  17  and is metered, controlled by demand, by a metering unit not shown in further detail to the high-pressure pumping unit  1 . Control, scavenging and lubrication quantities are returned to the fuel tank  14  via a return line  19 . 
   The fuel compressed to the prefeed pressure is further compressed in the high-pressure pumping unit  1  and stored in the common rail  4 . The high-pressure pumping unit  1  communicates with the common rail  4  via a high-pressure supply line  2  which is accommodated at a high-pressure connection  3  on the common rail  4 . 
   From the common rail  4 , fuel flows, at the pressure built up by the high-pressure pumping unit  1 , via the supply line  6  to the pressure booster  7 . Via a return line  5  to the fuel tank  14 , the common rail  4  communicates with the fuel tank  14 . 
   From a high-pressure chamber  9  contained in the pressure booster  7 , fuel at a still further elevated pressure level flows to the fuel injector  10  and can be injected into the combustion chamber, not shown in  FIG. 1 , of a self-igniting internal combustion engine at an injection nozzle  12  of the fuel injector  10 . 
   In the configuration shown in  FIG. 1 , all the return quantities that occur in the system, that is, the return quantity from the metering unit as well as the return quantity from the pressure booster  7  and the return quantity from the fuel injector  10 , are completely depressurized and returned to the fuel tank  14 . In the system configuration shown in  FIG. 1 , the pressure booster  7  is in the form of a separate component, but it may also be integrated with either the common rail  4  or the fuel injector  10 . 
   The leakage or triggering quantities that are returned to the fuel tank  14 , which is at atmospheric pressure, all flow back into the fuel tank  14  via the returns  13  from the fuel injector  10 , the return line  8  from the pressure booster  7 , the return line  5  from the common rail  4 , and the return line  19  from the metering unit. 
   From  FIG. 2 , the hydraulic mode of operation of a fuel injection system which includes a pressure booster can be seen. Via the supply line  6 , fuel, which is at the pressure level prevailing in the common rail  4  (not shown here) is delivered to the pressure booster  7 . The fuel flows into a work chamber  26  of the pressure booster  7  via the supply line  6 . Both a first conduit  23  and a second conduit  24  extend parallel to the supply line that acts on the work chamber  26  of the pressure booster  7 . A filling valve  20  is accommodated inside the first conduit  23 ; the second conduit  24  includes a throttle restriction  21 . Both the first conduit  23  and the second conduit  24  and an overflow line  25  that contains a check valve  22  all communicate with a differential pressure chamber  27  of the pressure booster  7 . A restoring spring  30  is accommodated inside the differential pressure chamber  27  and acts upon the lower face end of a booster piston  28  that divides the work chamber  26  from the high-pressure chamber  9 . On the booster piston  28 , there is a face end  29 , which upon pressure relief of the differential pressure chamber  27  of the pressure booster  7  moves into the high-pressure chamber  9 . The face end  29  that moves into the high-pressure chamber  9  upon pressure relief  27  of the pressure booster  7  brings about a still-further pressure increase of the fuel contained in the high-pressure chamber  9 , in accordance with the boosting ratio of the pressure booster  7  inside the high-pressure chamber  9 . A pressure relief of the differential pressure chamber  27  of the pressure booster  7  is effected by a triggering of an actuating valve identified by reference numeral  31 . The actuating valve  31  for pressure relief of the differential pressure chamber  27  may for instance be embodied as a 2/2-way valve and communicates with a low-pressure region, not shown in further detail here in  FIG. 2 . 
   Upon pressure relief of the differential pressure chamber  27  via the return line  8  after actuation of the valve  31 , a positive displacement of the fuel at high pressure, contained in the high-pressure chamber  9  of the pressure booster  7 , is effected into a high-pressure supply line  33 , which extends to the fuel injector  10 . By means of the check valve  22  contained in the overflow line  25  for refilling the high-pressure chamber  9 , a return flow of the fuel volume, positively displaced out of the high-pressure chamber  9 , into the differential pressure chamber  27  of the pressure booster  7  is prevented. 
   The high-pressure supply line  33  extending from the high-pressure chamber  9  of the pressure booster  7  to the fuel injector  10  discharges into a nozzle chamber  38  embodied in the injector body  11  of the fuel injector  10 . Moreover, via the high-pressure supply line  33 , a control chamber  34  of the fuel injector  10  is acted upon via an inlet throttle  35 . A pressure relief of the control chamber  34  for actuating an injection valve member  37 , preferably embodied as a nozzle needle, is effected by the triggering of an actuating valve  32 , which may be embodied as a 2/2-way valve. A pressure relief of the control chamber  34  is effected via an outlet throttle  36  into the return  13 , which adjoins the actuating valve  32  for triggering the fuel injector  10 . 
   As shown in  FIG. 2 , besides a nozzle chamber  38 , a nozzle spring chamber  39  is provided in the injector body  11  of the fuel injector  10 . The nozzle spring chamber  39  accommodates a nozzle spring  40 . A leakage line, by way of which fuel flowing out of the nozzle chamber  39  upon an opening motion of the injection valve member  37  can flow away into the low-pressure region of the fuel injection system also extends from the nozzle spring chamber  39 . 
   Via the high-pressure supply line  33 , the fuel, compressed in accordance with the boosting ratio of the pressure booster  7 , flows into the nozzle chamber  38 . Because of the pressure buildup in the nozzle chamber  38 , this boosted pressure prevails at a pressure shoulder  42 , which is embodied on the injection valve member  37  in the region of the nozzle chamber  38 . The injection valve member  37  is kept in its closing position via both the nozzle spring  40  and the pressure level prevailing in the control chamber  34 . 
   Upon pressure relief of the differential pressure chamber  27  via the actuating valve  31 , the booster piston  28  moves with its face end  29  into the high-pressure chamber  9 . An elevated fuel pressure is reached in this chamber, in accordance with the boosting ratio of the pressure booster  7 . From the high-pressure chamber  9 , the fuel flows to the nozzle chamber  38  via the high-pressure supply line  33  and acts on the pressure shoulder  42  embodied on the injection valve  37 . The control chamber  34  is pressure-relieved via the outlet throttle  36  upon switching of the actuating valve  32 . Upon switching of the actuating valve  32 , the control chamber  34  is relieved, and injection valve member  37  moves upward counter to the action of the nozzle spring  40  causing an injection of fuel into the combustion chamber  44 . For the hydraulic function of the pressure boosting, it does not matter whether the fuel in the differential pressure chamber  27  of the pressure booster is depressurized completely or has a residual pressure that is approximately equivalent to the prefeed pressure. The preservation of a slight residual pressure level inside the differential pressure chamber  27  of the pressure booster is more likely advantageous, for preventing cavitation effects in the differential pressure chamber  27 . 
   By actuation of the switching valve  31  to its closing position, that is, the interruption of the low-pressure-side communication with the return, filling of the differential pressure chamber  27  of the pressure booster  7  takes place, via the first conduit  23  and the second conduit  24 . After that, the booster piston  27 , reinforced by the restoring spring  30  accommodated in the differential pressure chamber  27 , returns to its position of repose, so that the high-pressure chamber  9  of the pressure booster  7  is pressure-relieved. As a consequence, the pressure in the nozzle chamber  38  drops. The closing motion of the injection valve member  37 , embodied as a nozzle needle, is initiated by switching the switching valve  32 , which pressure-relieves the control chamber  34 , into its closing position, so that a pressure buildup is effected in the control chamber  34 , by way of the inlet throttle  35  that branches off from the high-pressure supply line  33 . 
     FIG. 3  shows the circuitry proposed according to the invention for a low-pressure region of a fuel injection system with a pressure booster and a common rail. In this fuel injection system the high-pressure pumping unit  1 , via the high-pressure line  2 , pumps fuel into the common rail  4 . Six supply line connections are shown for the common rail  4 , and by way of them a 6-cylinder self-igniting internal combustion engine is supplied with fuel. Instead of the six high-pressure line connections shown in  FIG. 3 , either four, five, eight, ten or twelve high-pressure line connections may be provided on the common rail, in accordance with the number of cylinders of the engine to be supplied with fuel. Via the supply line  6  from the common rail  4 , the work chamber  26  of the pressure booster  7  is subjected to pressure. The pressure booster  7  includes a booster piston  28 , which divides the work chamber  26  from the differential pressure chamber  27 . A restoring spring, which returns the booster piston  28  to its position of repose, may be accommodated in the differential pressure chamber  27  of the pressure booster  7 . A subjection of the differential pressure chamber  27  of the pressure booster  7  to fuel is effected via the supply line  6 , which discharges into the second conduit  24  that includes the throttle restriction  21 . The pressure relief of the differential pressure chamber  27  is effected via the return line  8 , which by means of the switching valve  31  with a return line  50 , assigned to the pressure booster. 
   The face end  29  of the booster piston  28  acts on the high-pressure chamber  9  of the pressure booster  7 , so that in it, an elevated fuel pressure can be achieved, in accordance with the pressure boosting ratio of the pressure booster  7 . The check valve  22 , connected parallel to the pressure booster  7  in a bypass line, prevents a return flow into the supply line  6  of the fuel volume contained in the high-pressure chamber  9  of the pressure booster  7 . 
   The high-pressure chamber  9  of the pressure booster  7  communicates with the high-pressure supply line  33 . From it, a line segment containing an inlet throttle  35  branches off to the control chamber  34 , and moreover, via the high-pressure supply line  33 , the nozzle chamber  38  inside the body  11  of the fuel injector  10  is acted upon by fuel at elevated pressure, that is, boosted pressure. If the fuel injector  10  is actuated by switching of the switching valve  32 , fuel, that is, the injector control quantity, flows via the open outlet throttle  36  away to the return  13 . At the same time, as a result of the subjection of the nozzle chamber  38  to pressure, a force acting in the opening direction of the injection valve member  37  builds up at the pressure shoulder  42 , which is operative as a hydraulic surface, on the injection valve member  37 . The injection valve member  37  moves upward counter to the nozzle spring  40  let into the nozzle spring chamber  39 , so that the injection openings  43  of the injection nozzle  12  are opened, and from the nozzle chamber  38 , via the annular gap  45  surrounding the injection valve member  37 , fuel can be injected into the combustion chamber, not shown in  FIG. 3 , of a self-igniting internal combustion engine. 
   With the switching valve  32  open, the injector control quantity flows out of the control chamber  34  via the outlet throttle  36 . Via the return line  13 , the injector control quantity flows away into the pressureless fuel tank  14 . Arrows  53  indicate further return lines  13  of the further fuel injectors  10  for supplying fuel to the self-igniting engine. These lines likewise discharge through the return  13  into the pressureless fuel tank  14 . The return  50  associated with the pressure booster  7 , however, discharges into a compensation container  51  inside a low-pressure circuit  64  of the fuel injection system shown in  FIG. 3 . Arrows  52  indicate further pressure booster returns  50 , associated with further pressure boosters  7 , which also flow back into the compensation container  51 . For the hydraulic function of the pressure booster  7 , it does not matter whether the fuel in the differential pressure chamber  27  of the pressure booster  7  is depressurized completely or to a residual pressure approximately equivalent to the pressure built up by a prefeed pump  55 . A slight residual pressure in the differential pressure chamber  27  of the pressure booster  7  is more likely to be advantageous, for avoiding cavitation effects. 
   Upon depressurization of the differential pressure chamber  27  to a residual pressure, which is approximately equivalent to the pressure level attainable with a prefeed pump  55  on the low-pressure side, fuel flows from the compensation container  51  into a line segment  60 . The line segment  60  includes a plurality of infeed points  61 ,  62 ,  63 , where the fuel, at residual pressure, in the compensation container  51  can be fed back into the low-pressure circuit  64 , that is, upstream of the high-pressure pumping unit  1 . 
   A first possibility is for the fuel at residual pressure to be fed from the compensation container  51  into the line segment  60  at a first infeed point  61 , located downstream of the outlet  56  on the compression side of the prefeed pump  55 . A first infeed portion  66 . 1  can be provided for this purposed. All of the infeed points  61 ,  62  and  63  are downstream of the compression side  56  of the prefeed pump  55 , so that the fuel volume to be pumped by the prefeed pump  55  is reduced considerably. This is due to the fact that the pressure booster  7  produces relatively high return quantities, which are the product of the boosting ratio multiplied by the injection quantity. In the compensation container  51 , in which the return quantities of the pressure booster  7  are accommodated, pressure fluctuations in the return path of the pressure boosters  7  can be damped. Moreover, the compensation container  51  develops a certain cooling action, which favorably influences the temperature level of the fuel inside the low-pressure circuit  64 . 
   For safety purposes, an overpressure valve  54  is provided downstream of the compensation container  51 , in the direction of outflow of the fuel contained in the compensation container. This overpressure valve  54 , analogously to the returns  13  extending from the fuel injectors  10 , communicate with the pressureless fuel tank  14 . The return quantity originating in the six pressure boosters  7  of a six-cylinder self-igniting engine may be fed into the line segment  60  at a first infeed point  61 . If the fuel quantities diverted from the pressure boosters  7  upon pressure relief of the differential pressure chambers  27  are fed in upstream of the fuel filter  17 , then advantageously, cleaning of the diverted return quantities from the pressure boosters  7 ,  52  can be achieved. Alternatively, it is possible for the return quantities from the pressure boosters  7 , accommodated in the compensation container  51 , to be fed in at a second infeed point  62 , which is downstream of the fuel filter  17 . Feeding the return quantities from the pressure boosters  7  in at the second infeed point  62  via a second infeed portion  66 . 2  offers the advantage that the size of the fuel filter  17  can be reduced, which is favorable in terms of the structural volume. 
   The return quantities flowing back into the compensation container  51  from the pressure boosters  7  can finally also be delivered at a third infeed point  63  via a third infeed portion  66 . 3  into the introduction portion  60  in the low-pressure circuit  64 . The third infeed point  63  is downstream of a metering unit  59 , which takes on the metering of fuel to the high-pressure pumping unit  1  outside the low-pressure circuit  64  in a demand-controlled fashion. By means of a third infeed point  63  downstream of the metering unit  59 , it can be attained that the return quantities from the pressure boosters  7  are introduced into the introduction portion  60  downstream of the metering unit  59 , which is upstream of the high-pressure pumping unit  1  outside the low-pressure circuit  64 , so that the requisite flow cross section of the metering unit  59  can be kept small. By feeding the return quantity, contained in the compensation container  51 , from the pressure boosters  7  in the introduction portion  60 , the volumetric flow of fuel to be pumped by the prefeed pump  55  can be reduced considerably in all three feeding variants, that is, positions  61 ,  62  and  63 . This makes a smaller size of the prefeed pump  55  possible, since the line output to be produced by the prefeed pump  55 , in terms of the volumetric flow of fuel that is delivered to the high-pressure pumping unit  1  outside the low-pressure circuit  64 , is supplemented by the return quantities, diverted from the pressure boosters  7  and delivered from the compensation container  51  inside the introduction portion  60  to the infeed points  61 ,  62 ,  63 . The pressure level prevailing in the low-pressure circuit  64 , which level is built up by the prefeed pump  55 , is preferably in the range between  5  and  7  bar, which corresponds to the residual pressure level that remains in the differential pressure chamber  27  upon relief of the differential pressure chamber  27  of the pressure booster  7  upon triggering of its actuating valve  31 . Pressure fluctuations inside the introduction portion  60  can be compensated for by a pressure regulating valve  57 , which is accommodated in a line segment that discharges into the fuel tank  14  and that branches off inside the introduction portion  60 , between the fuel filter  17  and the metering unit  59 . 
   By means of the configuration, proposed according to the invention, of the low-pressure circuit  64  of the fuel injection system in accordance with  FIG. 3 , it is furthermore possible, when the fuel volume flowing out of the compensation container  51  is delivered to the second infeed point  62  immediately downstream of the fuel filter  17 , to design the fuel filter  17 , for smaller volumetric flows of fuel, which has a very favorable effect on the structural size of pump components and filter components inside the low-pressure circuit  64  of the fuel injection system proposed according to the invention. 
   A further reduction in the volumetric flow of fuel to be delivered to the high-pressure pumping unit  1  by the prefeed pump  55 , the filter  17  and the metering unit  59  can be implemented by depressurizing the leakage quantity, flowing as shown in  FIG. 3  into the fuel tank  14  via the return line  13  assigned to the fuel injectors  10  and via a partial-quantity return  65 , likewise only down to the prefeed pressure to be produced by the prefeed pump  55 . This volumetric flow of fuel, flowing away from the fuel injector  10  or fuel injectors  10  via the return line  13 , is preferably fed into the low-pressure circuit  64  downstream of the compression side  56  of the prefeed pump  55 . As a result, even in fuel injection systems that are embodied without pressure boosters, the fuel quantity to be pumped by the prefeed pump  55  can be reduced. Depending on the design of the fuel injectors  10  and on the fuel pressure produced in the common rail  4  by the high-pressure pumping unit  1 , the return quantity from the fuel injector or fuel injectors  10  may make up a considerable proportion of the total fuel quantity. The return quantity flowing away from the fuel injector  10  is composed essentially of the volumetric flow of fuel diverted into the nozzle spring chamber  39  upon the opening motion of the injection valve member and the control volume flowing out of the control chamber  34  via the outlet throttle  36  upon actuation of the switching valve  32 . In the fuel injection system shown in  FIG. 3  for supplying a 6-cylinder self-igniting internal combustion engine, the returns  53  from further fuel injectors  10 , which are not shown in detail here, are represented by arrows pointing to the return line  13 . 
   With the configuration proposed according to the invention of the low-pressure circuit  64  of a fuel injection system, a complete depressurization of the large return quantity flowing back from the pressure boosters  7 , which can amount to multiple times the injection quantity, to atmospheric pressure can be avoided. In previously known pressure boosters, this return quantity is depressurized completely and returned to the pressureless fuel tank  14 . After that, the entire quantity needed in this system must be compressed by the prefeed pump  55  to the prefeed pressure (5 to 7 bar) to assure filling of the pump chambers of the high-pressure pumping unit  1 . If conversely the return quantity flowing back from the pressure boosters  7  is not completely depressurized, but instead is kept at a pressure equivalent to the prefeed pressure of the prefeed pump  55  and is delivered back to the low-pressure circuit  64  at the first infeed point  61 , the second infeed point  62  and the third infeed point  63  inside the introduction portion  60 , then the fuel filter  17  or  58  can be designed and the metering unit  59  and prefeed pump  55  can be dimensioned for lesser volumetric flows. Although the lesser pumping output of the prefeed pump  55  is as a rule not designed for demand-oriented regulation, the high overflow quantities that occur in certain performance graph pumps and that can contribute to a not inconsiderable loss of efficiency of the entire fuel injection system can be avoided. 
   The foregoing relates to a 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. 
   The foregoing relates to a 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.