Abstract:
Disclosed is a fuel metering unit for high-pressure pumps of fuel injection systems, whose leakproofness in zero delivery operation has been further improved and whose manufacture and assembly have been simplified.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a 35 U.S.C 371 application of PCT/DE 03/02420 filed on Jul. 18, 2003. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a fuel metering unit for a fuel injection system for internal combustion engines, with a high-pressure pump driven as a function of the engine speed, wherein the fuel metering unit has a control valve that is actuated by an electromagnet and has a valve piston, wherein the valve piston is guided in a valve housing, wherein the valve piston is embodied as sleeve-shaped and its inner chamber contains a compression spring that holds it in contact with the armature pin, wherein the rear end of the compression spring rests against a spring plate disposed in the valve bore of the valve housing, wherein the wall of the valve housing has at least one, preferably several radial control openings that are formed and/or disposed so that the fuel quantity flowing through the fuel metering unit can be adjusted as a function of the valve piston stroke. 
   2. Description of the Prior Art 
   In the fuel metering unit described above, that is known from DE 198 53 103 A1, the spring plate is disposed behind the valve piston in the axial direction. In the exemplary embodiment described in conjunction with FIG. 8 of DE 198 53 103 A1, the valve disc can be provided with an axial sealing seat. In the closed position of the control valve, the valve piston comes into contact with the axial sealing seat and should thus tightly close the control valve so that no fuel travels through the fuel metering unit to the high-pressure fuel pump. 
   SUMMARY AND ADVANTAGES OF THE INVENTION 
   The fuel metering unit according to the invention for a fuel injection system for internal combustion engines includes the provision that the inner chamber of the valve piston is provided with a shutoff sleeve and that the valve piston and the shutoff sleeve constitute a shutoff valve. 
   Integrating the shutoff valve into the interior of the valve piston reduces the structural length of the metering unit and also significantly reduces the diameter of the axial sealing seat in comparison to the annular gap seal known from the prior art. As a result, in the closed position of the control valve, the fuel metering unit according to the invention is much more effective in shutting off the fuel supply to the high-pressure pump. 
   As a result, it is also possible to eliminate additional measures for avoiding an undesired pressure buildup during overrunning of the engine, for example the provision of a zero delivery throttle or of a second pressure control valve on the pressure side of the high-pressure pump. This contributes significantly to reducing costs. 
   It has turned out to be particularly advantageous for the shutoff valve to be embodied in the form of a ball valve and/or for a ball to be disposed between the shutoff sleeve and the valve piston and for the shutoff valve to have a sealing seat. 
   The use of a ball valve simplifies production since the ball can compensate for misalignments and angle errors that are virtually inevitable in manufacturing. 
   In order to assure that the ball is always disposed in its intended position, in another embodiment of the invention, the valve piston has a ball retainer that holds the ball in a definite position in relation to the valve piston. 
   In order to further simplify the manufacture and assembly of the fuel metering unit according to the invention, the spring plate can also be integrated into the shutoff sleeve. For example, this can be done by lathe-cutting an annular groove into the shutoff sleeve, which groove serves to fix the compression spring in place. 
   This reduces the number of parts and further reduces the structural length of the fuel metering unit according to the invention. It is also possible to adjust the control valve by axially shifting and subsequently fixing the shutoff sleeve in the valve bore. If the spring plate and the shutoff sleeve are two different components, then the axial position of both the spring plate and the shutoff sleeve in each control valve must be adjusted before they are initially placed in service, which increases the costs and the time required. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other advantages and advantageous embodiments and modifications of the invention can be inferred from the description contained herein below, taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  shows a vertical longitudinal section through one embodiment of a fuel metering unit according to the invention, and 
       FIG. 2  shows an enlargement of the detail “A” from  FIG. 1 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The fuel metering unit according to  FIG. 1  is based on an electromagnet  10  with an integrated control valve  11 . In particular, the electromagnet  10  is comprised of a magnetic coil  12 , an armature  13  with armature pin  14 , and a magnetic cup  15  that partially encompasses the magnetic coil  12  and the armature  13 . 
   The entire structural unit of the electromagnet  10  with the integrated control valve  11  is disposed inside a high-pressure fuel pump that is not shown. The magnetic cup  15  here serves simultaneously as a sealing element, as a magnetic yoke, and as a fastening element (see reference numeral  16 ) for attaching the electromagnet  10  into the high-pressure pump. 
   After the magnetic coil  12  has been inserted into the magnetic cup  15 , it is completely extrusion coated. The extrusion coating labeled  17  assures an optimum heat transfer from the coil  12  to the magnetic cup  15 . This makes it possible to counteract an overheating in critical operating states. The extrusion coating  17  also provides an advantageous fatigue strength and resistance to vibration, which in terms of oscillations as well as thermal stresses and environmental influences, allows the fuel metering unit  10 ,  11  to be installed in high-stress locations, e.g. of the high-pressure fuel pump. 
   In addition, the extrusion coating  17  of the magnetic coil  12 , in cooperation with two sealing points  18 ,  19 , assures that the contact points of the coil  12  with the connector lugs (not shown) are “dry”. The magnetic coil winding and the contact points are thus optimally protected from being attacked by corrosive mediums. 
   “Overflow bores”  20 ,  21  are provided on the circumference of the magnetic cup  15  in order to assure that the extrusion coating  17  completely encompasses the magnetic coil  12 . 
   The control valve  11  has a valve housing  22 , which transitions into a flange-like extension  23  that simultaneously serves as the end seal for the magnetic cup  15 . The valve housing  22  is provided with a valve bore  24  that is aligned coaxial to the armature pin  14  of the electromagnet  10 . The valve bore  24  contains a movable, sleeve-shaped valve piston  25 , whose inner chamber  26  contains a compression spring  27 , a shutoff sleeve  51 , and a ball  52 . 
     FIG. 2  gives a detailed depiction of the inner chamber  26 , which clarifies the interaction of the above-mentioned components of the pressure control valve  11 . 
   The front end of the compression spring  27  rests against a ball retainer  54  and its rear end rests in an annular groove  29  of the shutoff sleeve  51 . The shutoff sleeve  51  is attached in the valve bore  24 , for example by means of being press-fitted into it. 
   The ball retainer  54  in turn rests by means of the spring  27  against a bottom  28  of the valve piston  25 , thus holding the valve piston  25  in contact with the front end of the armature pin  14 . 
   In the position of the valve piston  25  shown in  FIG. 1 , an opening  31  of the shutoff sleeve  51  connects the inner chamber  26  of the valve piston  25  to a presupply pump (not shown) of the fuel injection system. 
   The valve housing  22  is also provided with a number of radially aligned control openings, two of which are shown in  FIG. 1 , labeled  32 . The control openings  32  are connected in a hydraulically operative way to the low-pressure region of the high-pressure pump (not shown). A number of radial openings  34  are provided in the valve piston  25 , which cooperate with the control openings  32  in the valve housing  22 . 
     FIG. 1  shows the control valve  11  in the open state, in which the control openings  32  and the openings  34  in the valve piston  25  are hydraulically connected. In this position, the electromagnet  10  is without current and the compression spring  27  brings the valve piston  25  into the position shown in  FIG. 1 . 
   In the open position of the control valve  11  shown in  FIG. 1 , the fuel supplied to the control valve  11  at  31  flows through the shutoff sleeve  51 , changes its flow direction at the end of this sleeve, and flows through the openings  34  in the valve piston  25  and the control openings  32  in the valve housing  22  in the direction of the high-pressure pump, not shown. 
   As has already been mentioned above, it has in practice turned out to be useful to provide not one, but several radial control openings  32  distributed over the circumference of the valve housing  22 . DE 198 53 103 A1 describes the design of the control openings  32  in detail, and this description is incorporated herein by reference. 
   When the vehicle is being operated in overrunning mode, the control valve  11  does not have to be tightly closed to prevent it from leaking into the high-pressure pump during the overrunning operation. Such leaks of the control valve  11  lead to an undesired supply of fuel to the high-pressure pump and consequently to a pressure increase in the common rail of the fuel injection system. As a result of the pressure increase in the common rail, an undesirable noise generation can occur during overrunning operation (“hard combustion noise”). 
   In order to assure the leakproofness of the control valve  11 , a shutoff device comprised essentially of the shutoff sleeve  51  and the ball  52  is integrated into the valve piston  25 . The function of this shutoff device will be explained below in conjunction with  FIG. 2 , which shows an enlargement of a detail from  FIG. 1 . 
   As is clear from  FIG. 2 , the shutoff sleeve  51  is provided with a sealing seat  53  against which the ball  52  rests in the closed position (not shown) of the control valve  11 . The armature pin  14  presses the ball  52  into the sealing seat  53  by means of the bottom  28  of the valve piston  27 . This breaks the hydraulic connection between the opening  31  and the control opening  32 . The adjusting movement of the armature pin  14  in opposition to the compression spring  27  is achieved by providing the electromagnet  10  with a current that corresponds to the desired adjusting force. This adjusting movement is indicated by an arrow  35  in  FIG. 2 . When the ball  52  is resting against the sealing seat  53  of the shutoff sleeve  51  (position not shown), the control valve  11  is closed. 
   A suitable triggering of the electromagnet  10  can be used to set any number of intermediate positions of the valve piston  25  between the position that is shown and the closed position that is not shown. In these intermediate positions (not shown), the control valve  11  controls the fuel quantity aspirated by the high-pressure pump (not shown) and therefore controls the delivery capacity of the high-pressure pump. 
   The advantages of a ball valve are sufficiently known from the prior art and therefore do not require extensive explanation. In the context of the invention, however, it is important for the diameter D of the sealing seat  53  to be relatively small so that even a relatively low contact force of the armature pin  14  produces a high surface pressure between the ball  52  and the sealing seat  53  of the shutoff sleeve  51 . This increases the leakproofness of the shutoff valve according to the invention. 
   Starting from the closed position of the control valve  11 , not shown, if the power supply to the armature pin  14  is reduced, then the compression spring  27  moves the valve piston  25  in the direction of the armature pin  14  so that the ball  52  lifts up from the sealing seat  53 . As a result, the ball retainer  54  lifts the ball  52  away from the sealing seat  53 . Consequently, the ball  52  always has a definite position in relation to the valve piston  25  and cannot move back and forth between the bottom  28  of the valve piston  25  and the sealing seat  53 . This back-and-forth motion would be disadvantageous to the performance quality of the control valve  11 . 
   The direction of the flow through the control valve  11  can also be reversed. In this case, the opening  31  would be hydraulically connected to the low-pressure region of the high-pressure pump, while the control opening  32  would be connected to the pressure side of the presupply pump and would thus constitute the inlet into the metering unit. 
   Before the fuel metering unit is initially placed into service, it is necessary to adjust the control valve  11 . This is done by appropriately shifting the shutoff sleeve  51  axially in the valve bore  24  and then fixing it in place. In particular, the adjusting process occurs as follows: 
   First, the electromagnet  10  is supplied with a definite current. Then, the shutoff sleeve  51  is slid into the valve bore  24  until a definite volumetric flow is passing through the control openings  32 . In this position, the shutoff sleeve  51  is fixed in place in the valve bore  24 , e.g. in that the shutoff sleeve  51  is embodied as a press-fitted part or in that the valve housing  22  is plastically deformed from outside. It is advantageous if the valve adjusting point is located in the region of minimal fuel flow quantities since this makes it possible to precisely adjust the tolerance-sensitive idle range. 
   The axial position of the sealing seat  53  in relation to the bottom  28  of the valve piston  25  and the ball  52  is established at the same time as the adjustment of the control valve  11 . This assures that in the closed position of the valve piston  25 , the ball  52  closes the shutoff sleeve  51  and as a result, no fuel travels through the control opening  32 . Integrating the annular groove  29  into the shutoff sleeve can reduce to 1 the number of required adjustment steps, which significantly reduces production costs of the metering unit. p 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.