Abstract:
A high-pressure fuel pump for an internal combustion engine is proposed, in which the regulating behavior in idling and lower partial-load operation is improved to such an extent that a separate pressure regulating valve in the high-pressure region of the fuel injection system can be dispensed with.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to an improved high-pressure fuel pump for an internal combustion engine. 
   2. Description of the Prior Art 
   As general prior art in this field, German Patent Disclosures DE 102 36 314 A1 and DE 198 53 103 A1 are cited. 
   The invention is based especially on a so-called common rail system (CR system). The special feature of such CR systems is that the fuel quantity needed for injection must be brought to a variable pressure, dependent on the operating state of the engine at the time, by a high-pressure fuel pump. The high-pressure fuel pump is driven as a function of engine rpm, which can be done for example by means of the engine camshafts. The possible supply quantity of the high-pressure fuel pump is designed such that excess fuel, that is, more than the rail needs for the desired pressure buildup, can be pumped in every operating state. 
   From DE 198 53 103 A1, a fuel metering unit is known in which the flow quantity is controlled by a valve piston. For controlling the flow rate, the valve piston partially or entirely uncovers one or more control openings. 
   In this fuel metering unit, which is embodied as a slide valve, leakage, also called leak fuel, occurs in the overrunning mode of the engine, that is, when the high-pressure fuel pump is not supposed to pump any fuel, during the intake phase of the pump elements of the high-pressure fuel pump, and as a result in an unwanted way the high-pressure fuel pump aspirates fuel. This fuel is then put at high pressure in the ensuing working stroke and is pumped into the common rail. A pressure buildup, which is unwanted in the overrunning mode of the engine, occurs there and is reduced or limited by a pressure regulating or pressure limiting valve that is triggered by the control unit of the injection system. Since the fuel metering unit must be triggered by the control unit of the injection system, and in normal operation the regulation of the supply quantity of the high-pressure fuel pump is effected solely via the fuel metering unit, in this fuel injection system there are two structural groups, namely the pressure limiting valve and the fuel metering unit, that must be triggered by the control unit. 
   OBJECT AND SUMMARY OF THE INVENTION 
   In a high-pressure fuel pump for an internal combustion engine, having at least one pump element for pumping fuel from an intake side to a compression side of the high-pressure fuel pump, having an device on the intake side for controlling the supply quantity of the high-pressure fuel pump, and having a first check valve disposed on the intake side and a second check valve disposed on the compression side, a relief device is provided according to the invention, which—counter to the flow direction of the first check valve—enables an at least partial pressure equilibrium between the inlet and outlet of the first check valve. 
   With this provision it is possible, for instance in the overrunning mode of the engine, to prevent an unwanted pressure buildup during the supply stroke into the pumping chambers of the pump elements, so that no fuel is pumped into the common rail. As a consequence, the pressure in the common rail does not increase, either, and the unwanted pressure increase in the common rail is reliably prevented. It is therefore possible to dispense with a pressure regulating valve triggered by the control unit. A simple pressure limiting valve suffices. 
   Moreover, after the end of the overrunning, because the pressure has not risen in the common rail, it is also possible for an extremely small fuel quantity to be injected into the combustion chambers of the engine at low rail pressure, making a gentle transition from overrunning to normal operation of the engine possible. 
   Because of the provision proposed by the invention, it is additionally possible to expand the supply quantity regulation of the high-pressure fuel pump to lesser supply quantities, since the leakage that occurs for instance during engine idling in the fuel metering unit is not pumped into the common rail but instead is pumped back to the intake side of the high-pressure fuel pump by the relief device during the intake stroke of the pump elements. This is of major significance because the fuel leaking from a fuel metering unit may be on the same order of magnitude as the fuel quantity injected during idling. This means that without the provision according to the invention, in some unfavorable configurations, satisfactory idling regulation of the engine is impossible. By the provision proposed by the invention, it is reliably assured in all operating states that the supply quantity of the high-pressure fuel pump can be adapted to the fuel demand of the engine. 
   In a first variant embodiment, it is provided that the relief device includes a first bypass line, connecting an inlet and outlet of the first check valve, with a first throttle restriction. It is thus possible in a simple way for the leakage that occurs in idling or overrunning of the engine in the fuel metering unit, which leakage is aspirated during the intake stroke of the pump element, to be thrust back to the intake side of the high-pressure fuel pump in the ensuing supply stroke of the pump element. 
   Alternatively, the relief device may also be integrated with the first check valve. This can for instance be done by embodying the first check valve as a seat valve and providing a notch that has the function of a throttle restriction. The notch may have a rectangular, half-round or oval cross section and may be produced for instance by electrochemical machining or by creative shaping, in particular embossing. This variant embodiment is very simple to produce and very robust in operation and can easily be retrofitted in fuel injection systems already in mass production, whose first check valve is embodied as a seat valve. 
   In particular, it has proved advantageous if the first check valve has a counterpart plate with a valve seat and has a valve member cooperating with the valve seat, and the notch is provided between the valve seat and the valve member, in particular in the counterpart plate. 
   Alternatively, the relief device of the invention may also be furnished by providing that a fuel metering unit serves as the device on the intake side for controlling the supply quantity of the high-pressure fuel pump; that the fuel metering unit has a regulating valve, actuated by an electromagnet by means of an armature bolt, with a valve piston; and that the first check valve can be opened by the armature bolt. 
   By this provision, the functionality of the fuel metering unit can be expanded without additional effort or expense. Specifically, as is known in the prior art, the fuel metering unit is responsible, during normal engine operation, for regulating the supply quantity of the high-pressure fuel pump. If the supply quantity of the high-pressure fuel pump is to be reduced further than the leak fuel quantity of the fuel metering unit, then the supply quantity of the high-pressure fuel pump can be reduced further by triggering the first check valve during the supply stroke of the pump element, with the aid of the fuel metering unit. Particularly in engine overrunning, it can as a result be reliably assured that no fuel will be pumped into the common rail, and thus no pressure increase occurs in the common rail. 
   For the case where the metering unit sticks in the open position, however, it is recommended that a pressure limiting valve be provided on the common rail that need not be triggered by the control unit but instead is controlled directly via the pressure in the common rail. 
   It has also proved advantageous if the valve piston of the fuel metering unit is guided in a valve housing, and at least one and preferably a plurality of radial control openings are disposed in the wall of the valve housing, and the first check valve is not opened by the armature bolt until the control openings are closed. 
   In this exemplary embodiment, care must be taken in particular to assure that the valve piston is fitted into the valve housing with only very slight play, so that the leak fuel quantity is as slight as possible. In that case, it is merely necessary from time to time, during idling, to open the first check valve, in order to prevent the pressure increase in the common rail from fuel pumped unintentionally. 
   To assure that the fuel metering unit will operate independently of the position of the valve piston in the valve housing, the valve piston has a turned recess on its cylindrical circumferential surface, and a radial bore communicating with the intake side of the high-pressure fuel pump is disposed in the valve housing in such a way that the control openings can be made to communicate with the radial bore in the valve housing through the turned recess in the valve piston. 
   In a further augmentation of the invention, it is provided that a second bypass line, connecting the interior of the regulating valve and the intake side of the high-pressure fuel pump, is provided with a second pressure limiting valve. The opening pressure of the second pressure limiting valve may for instance be 0.3 bar, so that high pressures in the pumping chamber of the pump elements and in the fuel metering unit are avoided when the leak fuel is expelled to the intake side of the high-pressure fuel pump. 
   In a further advantageous feature of the invention, a second bypass line, connecting the inlet and outlet of the device for controlling the supply quantity, is provided that has a second pressure limiting valve. This device makes it possible, above all in conjunction with the inlet valve control by the armature bolt of the fuel metering unit, to maintain a specified pressure level in the interior of the fuel metering unit. 
   In conjunction with a third bypass line, which connects the inlet and outlet of the second check valve and has a second throttle restriction, a targeted pressure reduction in the rail can be effected during engine overrunning. As a result, the injection of the initially small fuel injection quantities at the transition from overrunning to normal operation can be done from a lower pressure level in the common rail, which improves the metering precision of the fuel injection. 
   In a further advantageous feature of the high-pressure fuel pump of the invention, a third pressure limiting valve and a third check valve are provided hydraulically in line with the second throttle restriction. This third pressure limiting valve has an opening pressure of 5 to 10 MPa, for instance, which is approximately equivalent to the pressure level in high-pressure starting of the engine. The advantage of this embodiment is that the pressure reduction function is not operative during the high-pressure start, and thus the volumetric efficiency of the high-pressure fuel pump is markedly improved. 
   If the high-pressure fuel pump has a plurality of pump elements, it suffices to trigger the first check valve of a pump element as described above, or to provide a pressure compensation device on a pump element. 

   
     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  is a block circuit diagram of an injection system; 
       FIGS. 2-4 ,  6 ,  8  and  9  show exemplary embodiments of high-pressure fuel pumps of the invention; 
       FIG. 5  shows a counterpart plate embodied according to the invention; and 
       FIG. 7  is a graph, showing the triggering of the fuel metering unit. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In  FIG. 1 , an injection system, as an example, is shown in the form of a block circuit diagram, but the invention is not limited to this injection system. A high-pressure fuel pump  101  according to the invention has an intake side  103  and a compression side  105 . On the compression side, both a common rail  107  and a plurality of injectors  109  are connected to the high-pressure fuel pump  101 . Both a pressure sensor  111  and a first pressure limiting valve  113  are disposed on the common rail  107 . From the first pressure limiting valve  113 , a connecting line  115  leads to the intake side  103  of the high-pressure fuel pump  101 . 
   The intake side  103  of the high-pressure fuel pump  101  communicates with a low-pressure pump  119 , disposed in a tank  117 . The low-pressure pump  119  may for instance be an electric fuel pump. Between a compression side  121  of the low-pressure pump  119  and the intake side  103  of the high-pressure fuel pump  101 , a pressure regulating valve  123  is disposed in the tank  117 . The pressure regulating valve  123  assures that the pressure on the intake side  103  of the high-pressure fuel pump  101  is kept virtually constant during engine operation. Typically, the pressure on the intake side  103  is between 3 and 6 bar. 
   The pressure limiting valve  113  prevents excessively high pressures in the common rail  107  and is triggered directly (not shown) via the pressure in the common rail  107 . 
   In  FIG. 2 , a first exemplary embodiment of a high-pressure fuel pump  101  of the invention is shown schematically. In this exemplary embodiment, the high-pressure fuel pump comprises a pump element  127 , which has a piston  129 , a cylinder bore  131 , and a pumping chamber  133  defined by the piston  129  and the cylinder bore  131 . 
   On the intake side  103 , which connects the high-pressure fuel pump  101  to the tank  117  (see  FIG. 1 ), a fuel metering unit ZME and a first check valve  135  are connected in series. 
   The piston  129  executes an oscillating motion, which is represented in  FIG. 2  by a double arrow. During the intake stroke, the piston  129  moves from top to bottom in terms of  FIG. 2 ; that is, a volume of the pumping chamber  133  increases. During this intake stroke, the piston  129  aspirates fuel from the intake side  103  into the pumping chamber  133 , through the metering unit ZME and the first check valve  135 . In the ensuing supply stroke, when the piston  129  moves from its bottom dead center to its top dead center, the volume in the pumping chamber  133  decreases; the fuel located in the pumping chamber  133  is put under pressure, until finally, through a second check valve  137 , it is expelled to the supply side  105  and then into the common rail  107  (see  FIG. 1 ). 
   The metering unit ZME is embodied as a slide valve and serves to regulate the fuel quantity aspirated during the intake stroke. As a rule, it is embodied as a proportional valve, with continuously variable adjustment of its cross section. 
   If during engine idling, for instance (not shown), only a very slight fuel quantity is to be aspirated into the pumping chamber  133 , then leakage occurs at the fuel metering unit ZME, as is true of any slide valve. This means that fuel passes uncontrollably from the intake side  103  into the pumping chamber  133 , bypassing the control openings of the fuel metering unit ZME. Since this leakage is approximately as great in volume as the quantity of fuel pumped by the high-pressure fuel pump  101  during engine idling, the engine idling cannot be regulated with sufficient precision. 
   In engine overrunning, when no fuel is supposed to be injected, the leakage in the fuel metering unit ZME of the prior art causes fuel to be pumped via the compression side  105  into the common rail  107 , without fuel being injected from the injectors  109  into the combustion chambers of the engine. As a consequence, the pressure in the common rail  107  rises, until after an allowable maximum pressure is reached the first pressure limiting valve  113  opens and limits the pressure in the common rail  107  to the maximum allowable value. 
   If a change is to be made from overrunning to normal operation, it is desirable as a rule to have a low pressure in the common rail, since it is then more easily possible, in the transition phase from overrunning to normal operation, to inject extremely small quantities of fuel and thus to achieve a gentle transition between these two operating states of the engine. 
   This high pressure in the common rail  107  makes it difficult to inject extremely small fuel quantities, so that the pressure increase in overrunning caused by the leakage from the fuel metering unit ZME is also unwanted. 
   In a first embodiment of a high-pressure fuel pump  101  according to the invention, a first bypass line  139  with a first throttle restriction  141  is provided, which bypasses the first check valve  135 . The flow resistance of the first throttle restriction  141  is selected such that at partial load and full load of the engine, the fuel quantity returned to the intake side  103  by the first throttle restriction  141  is negligible, compared to the fuel quantity pumped into the compression side  105 . 
   In engine idling, the fuel quantity pumped by the piston  129  is split between the compression side  105  and the intake side  103  in such a way that the fuel quantity to be injected is pumped into the compression side  105 , while a fuel quantity corresponding to the leak fuel quantity from the fuel metering unit ZME is pumped back into the suction line  103 . Thus it is possible to regulate the supply quantities of the high-pressure fuel pump  101  in engine idling as well, despite the leakage occurring in the fuel metering unit ZME. 
   In overrunning as well, when no fuel is to be injected into the combustion chambers of the engine, the leak fuel quantity of the fuel metering unit ZME can be returned into the suction line during the supply stroke of the piston  129 , via the first bypass line  139  and the first throttle restriction  141 , bypassing the first check valve  135 . As a result, the unwanted pressure increase in the common rail  107  is prevented effectively and in the simplest possible way. 
   In the exemplary embodiment of  FIG. 3 , the operation of the high-pressure fuel pump of the invention is further improved by the provision of a second bypass line  143  having a second pressure limiting valve  145 , which bypass the fuel metering unit ZME. 
   The second pressure limiting valve  145  has a relatively low opening pressure, for instance of 0.3 bar. As a result, the leak fuel quantity that is returned from the pumping chamber  133  into the suction line  103  is not pumped through the fuel metering unit, with its relatively high flow resistance in the closed state, but rather via the second bypass line  143 . As a result, the load on the pump element  127  in idling decreases, and the pressure loads in the fuel metering unit ZME and in the pump element  127  are reduced. 
   The pressure regulation in the common rail  107  in high-pressure starting is effected via the fuel metering unit ZME. 
   The fuel metering unit ZME of  FIG. 4  is based on an electromagnet  10  with an integrated regulating valve  11 . In detail, the electromagnet  10  essentially comprises a magnet coil  12 , an armature  13  with an armature bolt  14 , and a magnet cup  15  that partly surrounds the magnet coil  12  and the armature  13 . 
   The entire structural unit of the electromagnet  10 , with the integrated regulating valve  11 , is disposed in a high-pressure fuel pump  101 . The magnet cup  15  acts simultaneously here as a sealing element, a magnetic short circuit, and a securing element  16  of the electromagnet  10  in the high-pressure fuel pump. 
   Once the magnet coil  12  is inserted into the magnet cup  15 , it is spray-coated on all sides. The spray coating  17  assures an optimal heat transfer from the coil  12  to the magnet cup  15 . Overheating in critical operating states can be counteracted as a result. Moreover, the spray coating  17  leads to good resistance to vibration and jarring, making it possible for the fuel metering unit ZME to be secured to points, for instance on the high-pressure fuel pump, that are heavily loaded with respect to vibration, temperature, and environmental factors. 
   Moreover, the spray coating  17  of the magnet coil  12  in cooperation with two sealing points  18 ,  19  assures that the contact points of the coil  12  with the plug lugs (not shown) are “dry”. The magnet coil winding and contact points are thus optimally protected against the attack of corrosive media. 
   To check that the spray coating  17  completely surrounds the magnet coil  12 , “overflow bores”  20 ,  21  are provided on the circumference of the magnet cup  15 . 
   The regulating valve  11  has a valve housing  22 , which changes into a flangelike widened portion  23  that at the same time forms the termination on the face end of the magnet cup  15 . An axial bore  24  is embodied in the valve housing  22  and is disposed coaxially to the armature bolt  14  of the electromagnet  10 . The axial bore  24  receives a displaceable, sleevelike valve piston  25 , in the interior  26  of which a compression spring  27  is disposed. The compression spring  27  is braced on the front end on a bottom  28  of the valve piston  28  and on the back on a spring plate  29  that is located in the axial bore  24  of the valve housing  22 . A shoulder  30  on the inner wall of the valve piston  25  assures that the compression spring  27  rests in the valve piston  25  largely without contact with the inner wall. On the outside, the valve piston bottom  28  and thus the valve piston  25  are in contact with the front end of the armature bolt  14 . 
   A plurality of radially oriented control openings, of which in  FIG. 4  only one is visible and is identified with reference numeral  32 , are disposed in the valve housing  22 . The control openings  32  are in hydraulic operative communication with the intake side  103  of the high-pressure fuel pump  101 . Accordingly, the intake side  103  forms the inlet to the fuel metering unit ZME. 
   An opening  31  connects the interior  26  of the valve piston  25  to the first check valve  135 , which in this exemplary embodiment is embodied as a seat valve. 
   The upper half of FIG.  4 —above the common center axis  33  of the valve bore  24 , valve piston  25 , and armature bolt  14 —shows the regulating valve  11  in the open position, in which the control opening  32  is completely uncovered by the valve piston  25 . Conversely, in the lower half of  FIG. 4 , the regulating valve  11  is shown in the completely closed position. The magnetic force of the electromagnet  10 , when it is supplied with current, acts via the armature bolt  14  on the valve piston  25  and moves it, counter to the resistance of the compression spring  27 , into the aforementioned closed position of the regulating valve  11 . Conversely, the compression spring  27  is capable of displacing the valve piston  25  into the open position (upper half of  FIG. 4 ) when the supply of current to the electromagnet  10 , and hence its magnetic force acting on the armature  13  and the armature bolt  14 , is reduced accordingly. 
   In the open position of the regulating valve  11 , the fuel supplied to the regulating valve  11  from the intake side  103  and through the control opening  32 , flows through the opening  31  to the first check valve  135  into the pumping chamber  133  of the pump element  127 . 
   The first check valve  135 , in the exemplary embodiment of  FIG. 4 , comprises a counterpart plate  147  with a bore  149  and with a valve seat  151 . By means of a second compression spring  153 , which is braced on one end on the housing of the high-pressure fuel pump  101 , a valve member  155  is pressed against the valve seat  151  of the counterpart plate  147 . 
   In  FIG. 5 , the counterpart plate  147  is shown enlarged and in perspective. In this view, the bore  149  and the encompassing sealing seat  151  can be seen quite well. A notch  157  is machined into the valve seat  151  and takes over the function of the first throttle restriction  141  of the exemplary embodiments of  FIGS. 2 and 3 . 
   Looking at  FIGS. 4 and 5  together makes it readily clear that when the first check valve  135  is closed, when the valve member  155  rests on the valve seat  151  of the counterpart plate  147 , a hydraulic communication between the pumping chamber  133  and the intake side  103  exists through the notch  157 . The notch  157  thus has the function of the first throttle restriction  141  (see  FIGS. 2 and 3 ). Depending on how large the cross section of the notch  157  is, the throttling action of the notch  157  can be adjusted. 
   It is especially advantageous in this embodiment that the cross section of the notch  157  changes hardly at all over the service life of the first check valve  135 , since the contact area of the valve member  155  on the valve seat  151  is relatively large. Moreover, because of the pinch flows that necessarily occur each time the first check valve  135  closes, it is assured that even after many years of operation no contaminants, which can decrease the cross section of the notch  157  or even close the notch  157  completely, can become deposited in the notch  157 . As a result, it is assured that the pressure compensation function of the notch  157  is virtually constant over the entire service life of the injection system of the invention. A filter, or other protective devices that are both expensive and vulnerable to malfunction, for the notch  157  are unnecessary. Moreover, compared to the exemplary embodiments of  FIGS. 2 and 3 , a first bypass line  139  can be omitted. 
   In  FIG. 6 , a further exemplary embodiment of a high-pressure fuel pump  101  of the invention is shown. In this exemplary embodiment, the first check valve  135  is embodied similarly to the exemplary embodiment of  FIG. 4 . However, the valve seat  151  in the counterpart plate  147  of the first check valve  135  has no notch  157 . This means that the valve member  155  disrupts the hydraulic communication between the interior  26  of the fuel metering unit ZME and the pumping chamber  133  completely as soon as it rests on the valve seat of the counterpart plate  147 . 
   In this exemplary embodiment, the armature bolt  14  is lengthened through both the valve piston  25  and the compression spring  27 . The total stroke of the armature bolt  14  is marked S Z  in  FIG. 6 . In order to open and close the control openings  32  completely, an adjustment distance S R  of the armature bolt  14  is necessary. This adjustment distance S R  is shorter than the total adjustment distance S Z  of the armature bolt  14 . When the control openings  32  are completely closed, as is shown in the lower part of  FIG. 6 , the end of the armature bolt  14  is located in the immediate vicinity of the valve member  155 , but without touching it. 
   If, in engine idling or overrunning, the supply quantity of the piston  129  is now supposed to be less than the leak fuel quantity of the fuel metering unit ZME, then the armature bolt  14  is moved to the right by the armature  13  and the magnet coil  12  far enough that the armature bolt  14  lifts the valve member  155  from the valve seat  151  of the counterpart plate  147 , and thus fuel from the pumping chamber  133  can flow back into the fuel metering unit ZME and thus to the intake side  103  during the supply stroke of the piston  129 . As a result, the pressure buildup in the pumping chamber  133  is prevented, and no pumping of fuel to the compression side  105  of the high-pressure fuel pump  101  takes place. 
   The spacing between the armature bolt  14  and the valve member  155  when the control openings  32  are closed is indicated in  FIG. 6  by the symbol S V . So that the first check valve  135  can be opened by the armature bolt  14 , the total adjustment distance S Z  of the armature bolt  14  must be longer than the adjustment distance S R , required for regulating the supply quantity, plus the spacing S V  between the armature bolt  14  and the valve member  155  when the control openings  32  are closed. 
   Preferably, the control of the supply quantity of the high-pressure fuel pump  101  is effected in engine idling by varying the supply onset of the pump element. This control is shown in graph form in  FIG. 7 . In the upper part of  FIG. 7 , a first line  159  represents the position of the piston  129 . The piston  129  moves back and forth between top dead center TDC and bottom dead center BDC. When the piston  129  moves from top dead center to bottom dead center, the volume of the pumping chamber  133  increases, and the so-called intake stroke takes place. In this phase, despite the closed fuel metering unit, or if the fuel metering unit ZME is only slightly open, the piston  129  aspirates fuel from the intake side  103 , which unless other provisions in the ensuing supply stroke are taken, in which stroke the piston  129  moves from bottom dead center BDC to top dead center TDC, is pumped into the compression side  105 . However, as already noted several times, this is unwanted in some operating states of the engine and is therefore prevented by a suitable triggering of the fuel metering unit ZME. 
   In this triggering, the fuel metering unit ZME is triggered such that the first check valve  135  is open from top dead center to bottom dead center and beyond it, until the supply onset FB. This period of time is represented by the double arrow  135   open  in  FIG. 7 . During the motion of the piston  129  from bottom dead center BDC until the fuel supply onset FB, the leak fuel quantity is forced back into the suction line  103  by the opened first check valve  135 . This time interval is represented in  FIG. 7  by the double arrow  161 . Next, the fuel metering unit ZME is triggered such that the first check valve  135  closes, which is indicated in  FIG. 7  by the double arrow  135   closed . In this time interval between the supply onset FB and top dead center TDC, the fuel required for engine idling is pumped to the compression side  105 . 
   In the overrunning mode of the engine, the first check valve remains open during the entire supply stroke (FB=TDC), so that no pumping to the compression side  105  takes place. The control voltage of the fuel metering unit is represented in  FIG. 7  by a second line  163 . 
   The exemplary embodiments of  FIGS. 8 and 9  are based on the exemplary embodiment of  FIG. 6  and pertain to provisions for further improving the operating performance of the high-pressure fuel pump  101 . For the sake of simplicity, not all the reference numerals are entered in  FIGS. 8 and 9 . Reference is therefore made to the reference numerals in the preceding drawings. 
   In the exemplary embodiment of  FIG. 8 , a third bypass line  169  with a third throttle restriction  171  is provided, which bypasses the second check valve  137  on the compression side  105 . 
   On the compression side  105 , in conjunction with the control of the first check valve  135  as described in conjunction with  FIG. 6 , a targeted pressure reduction can be done in the common rail  107  during overrunning. This is advantageous especially whenever the engine was operated at full load immediately before the overrunning mode, and thus the pressure in the common rail  107  is high. At the end of the overrunning mode, it is in fact desirable for the pressure in the common rail  107  to be relatively low, so that the transition from overrunning to operation under load can be made as gentle and comfortable as possible. To that end, the low pressure in the common rail  107  is helpful, since it makes the accurate, precise metering of extremely small injection quantities easier. 
   In the exemplary embodiment of  FIG. 9 , in addition to the second throttle restriction  171 , a third pressure limiting valve  173  and a third check valve  175  are provided. The third pressure limiting valve  173  has an opening pressure of 5 to 10 MPa, for instance, which is approximately equivalent to the pressure level in high-pressure starting. The advantage of this variant embodiment is that the pressure reduction function of the second throttle restriction  171  is not operative in high-pressure starting, and thus the volumetric efficiency of the high-pressure fuel pump  101  is improved further. In the exemplary embodiment shown in  FIG. 9 , the third pressure limiting valve  173  is disposed closer to the pumping chamber  133  than the second throttle restriction  171 , which further improves the volumetric efficiency, because of the resultant reduced idle volume. However, the reverse arrangement is readily possible as well. 
   In  FIG. 9 , a further pump element  177  with a check valve  181  on the intake side and a check valve  179  on the compression side is shown schematically. This illustration is meant to indicate the fact that the high-pressure fuel pump  101  of the invention is not limited to high-pressure fuel pumps with one pump element  127 , but instead a plurality of pump elements  127  and  177  may be present in the high-pressure fuel pump  101 . The provisions according to the invention for returning the leakage to the suction line  103  need not, however, be performed for all the pump elements  127  and  177 . As a rule, it suffices for one pump element  127  or a first check valve  135  and a second check valve  137  to be embodied according to the invention. 
   The provisions made on the compression side for improving the operating performance, particularly the second throttle restriction  171 , the third pressure limiting valve  173  and the third check valve  175 , are not limited to the exemplary embodiments of  FIGS. 8 and 9  but instead can be employed in all the exemplary embodiments of the invention described above. 
   As already known and as can be seen as an example from  FIG. 2 , the regulating valve  11  is integrated with the magnet cup  15 , of the electromagnet  10 , and the complete fuel metering unit ZME is screwed directly into the high-pressure fuel pump. As a result, an optimally small installation space and production at favorable cost are guaranteed. The minimal idle volume that is attainable as a result assures exact metering of whatever fuel quantity is required at the time as well as fast reaction times to changing demands for quantity on the part of the high-pressure fuel pump or the engine. 
   From the above description it is already clear that exact regulatability is important for the valve of a fuel metering device. The triggering of the electromagnet  10  is done with pulse width modulation. This leads to reduced frictional hysteresis and good dynamics for the fuel metering unit. 
   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.