Abstract:
The invention relates to a high-pressure fuel pump comprising a drive shaft supported by bearings, and fuel flows through the bearings in a forced manner in such a way that the mechanical and thermal load-carrying capacity of the bearings, and thus the entire high-pressure fuel pump, is significantly increased.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 35 USC 371 application of PCT/EP 2006/062119 filed on May 8, 2006. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention is based on a high-pressure fuel pump for a fuel injection system of an internal combustion engine, and more particularly to such a pump having a pump housing in which a drive shaft is supported by a first bearing and a second bearing; having at least one pump element disposed radially relative to the drive shaft; having a fuel feed line, with a prefeed pump that pumps fuel into the fuel feed line; having a fuel return line; having a metering unit for regulating the pump capacity of the pump element or elements; and having a pressure regulating valve. 
     2. Description of the Prior Art 
     In these high-pressure fuel pumps known from the prior art, the pressure regulating valve serves to regulate the pressure in the low-pressure circuit of the high-pressure fuel pump. The pump capacity of the prefeed pump is typically split into three partial flows. A first partial flow flows through the metering unit to the intake side of the pump element or elements. The second partial flow as a rule flows through the pump housing via a lubricating throttle restriction and serves there to cool and lubricate the pump. From the pump housing, the second partial flow reaches the fuel return line of the fuel injection system. A third partial flow flows through the pressure regulating valve, which may also be embodied as an overflow valve, and likewise reaches the fuel return line. 
     With increasing injection pressures, the mechanical and thermal loads on both the drive shaft and the bearing of the drive shaft in the pump housing also increase. Conventional high-pressure fuel pumps are not capable of withstanding these increasing loads. 
     The object of the invention is to furnish a high-pressure fuel pump for a fuel injection system that makes do with the same installation space as conventional high-pressure fuel pumps and nevertheless is superior with regard to thermal and mechanical bearing capacity, to the high-pressure fuel pumps known from the prior art. Moreover, the high-pressure fuel pump of the invention should be simple in construction and capable of being produced economically. 
     In a high-pressure fuel pump for a fuel injection system of an internal combustion engine, having a pump housing, having a drive shaft with the drive shaft supported in the pump housing by a first bearing and a second bearing, having at least one pump element disposed radially relative to the drive shaft, having a fuel feed line, where a prefeed pump pumps fuel into the fuel feed line, having a fuel return line, having a metering unit for regulating the pump capacity of the pump element or elements, and having a pressure regulating valve, this object is attained in that the pressure regulating valve is disposed in the fuel return line. 
     SUMMARY AND ADVANTAGES OF THE INVENTION 
     Because according to the invention the pressure regulating valve is disposed in the fuel return line, it is attained among other advantages that the majority of the fuel pumped by the prefeed pump flows through the pump housing and as a result contributes to improved cooling of the pump housing and of the drive shaft. Moreover, the pressure in the pump housing is elevated compared to conventional constructions, which reduces the tendency to cavitation in the interior of the pump housing. Finally, the formation of vapor bubbles and local overheating (so-called hot spots) is effectively prevented. 
     In the high-pressure fuel pump of the invention, a lubricating throttle restriction between the prefeed pump and the pump housing can be dispensed with, so that despite the advantages cited, the high-pressure fuel pump of the invention is constructed even more simply than conventional high-pressure fuel pumps. 
     In an advantageous embodiment of the invention, it is provided that the first bearing is lubricated by fuel under pressure; and that the first bearing is in hydraulic communication with both the fuel feed line and the fuel return line. This means that on one side of the first bearing, approximately the same pressure prevails as on the compression side of the prefeed pump, while the other side of the first bearing is in pressure equilibrium with the virtually pressureless fuel return line. As a result, the first bearing necessarily experiences a flow through it of fuel, and thus adequate lubrication and cooling of the first bearing is assured at all operating points. 
     In a further advantageous feature of the invention, a first flow limiting device is provided, which is connected in series with the first bearing. 
     The first flow limiting device serves to keep the fuel flow, which flows through the first bearing, within predetermined limits. In mass production of high-pressure fuel pumps, it can happen, because of production variations and wear at the first bearing, that the thickness of the lubrication gap and hence the fuel flow through the bearing and its bearing capacity will vary within very wide limits. This means that if the tolerance situation is unfavorable, the bearing capacity of the first bearing and its cooling and lubrication by the fuel are not adequate at all operating points. If the first flow limiting device according to the invention is now connected in series with the first bearing, it is possible because of the very narrow production tolerance with which the first flow limiting device can be produced to adjust the fuel flow by means of the first bearing. As a result, the aforementioned production variations have only a slight effect on the bearing capacity, so that even given an unfavorable tolerance situation, the bearing capacity of the first bearing is assured at all operating points of the high-pressure fuel pump. 
     The flow limiting device limits the fuel flow that flows through the bearing. As a result, in unfavorable tolerance situations of the bearing, the demands made of the prefeed pump are reduced. 
     The object stated at the outset is attained, in a high-pressure fuel pump for a fuel injection system of an internal combustion engine, having a pump housing, having a drive shaft with the drive shaft supported in the pump housing by a first bearing and a second bearing, having at least one pump element disposed radially relative to the drive shaft, having a fuel feed line, where a prefeed pump pumps fuel into the fuel feed line, having a fuel return line, having a metering unit for regulating the pump capacity of the pump element or elements, and having a pressure regulating valve, in that the first bearing is lubricated by fuel under pressure; that the first bearing is in hydraulic communication with both the fuel feed line and the fuel return line; a first flow limiting device is provided; that the first flow limiting device is connected in series with the first bearing; and that between the pump housing and the fuel return line, a bypass throttle restriction is provided. 
     In this exemplary embodiment of a high-pressure fuel pump of the invention, it is possible by the suitable adaptation of the first flow limiting device and the bypass throttle restriction to assure in a simple and effective way that an adequate quantity of fuel will flow through the first bearing, thus assuring the cooling and lubrication of this bearing at all operating points. 
     In the high-pressure fuel pumps of the invention, the metering unit can alternatively be disposed either between the prefeed pump and the high-pressure fuel pump in the fuel feed line, or between the pressure regulating valve and the high-pressure fuel pump in the fuel return line. Both arrangements have specific advantages, which should be weighed against one another in an individual case. 
     A factor in favor of disposing the metering unit between the prefeed pump and the high-pressure fuel pump in the fuel feed line is that with this arrangement, the fuel flowing into the high-pressure region of the high-pressure fuel pump will not have first flowed through the pump housing, so that any chips or other particles may be present there cannot get into the high-pressure fuel region. 
     An advantage of disposing the metering unit in the fuel return line is that the entire fuel quantity pumped by the prefeed pump is available at every operating point for cooling and lubricating the pump housing or the drive shaft of the high-pressure fuel pump as well as the associated bearings. As a result, the bearing capacity of the low-pressure region of the high-pressure fuel pump of the invention is increased still further. 
     Alternatively, it is possible to dispose the first flow limiting device either upstream or downstream of the first bearing. Which disposition will be preferred in an individual case depends on the circumstances and peripheral conditions of the individual case. 
     In a further augmentation of the high-pressure fuel pumps of the invention, it may furthermore be provided that the second bearing is lubricated by fuel under pressure, and that the second bearing is in hydraulic communication with both the fuel feed line and the fuel return line. 
     Moreover, a second flow limiting device may be provided, which is disposed upstream or downstream of the second bearing. The advantages of the forced lubrication of the second bearing and of the second flow limiting device correspond essentially to the advantages mentioned above, in conjunction with the first bearing and the first flow limiting device. 
     The first flow limiting device and/or the second flow limiting device may be embodied as a throttle restriction, diaphragm, or flow regulating valve. Which of these alternatives will be preferred in the individual case depends on the ranges of tolerance of the various components, the loads, and of course commercial reasons and must be decided in the individual case. 
     An especially advantageous feature of the invention provides that the second bearing is supplied with fuel under pressure by a leak fuel line of the prefeed pump. This is especially advantageous whenever the prefeed pump is embodied for instance as a vane cell pump, external gear-wheel pump, or internal gear-wheel pump. In vane cell pumps or gear pumps, leakage that must be carried away through a leak fuel line occurs at the interface with the drive shaft, in the gap between the vane wheel or gear wheel and the housing. If this leak fuel line is now used for lubricating and cooling the second bearing, then firstly the lubrication and cooling of the second bearing can be assured under all operating conditions, and secondly, because of the counterpressure, the leak fuel quantity from the prefeed pump is reduced. This leads to improved hydraulic efficiency of the prefeed pump. 
     It is especially advantageous if the first bearing and/or the second bearing is embodied as a slide bearing. Then, by the provision according to the invention of fuel under pressure to the bearing, a stable hydrostatic film of lubrication is formed, which assures a very high bearing capacity of the bearings in the most various rpm ranges. 
     Advantageously, the fuel connection discharges into an interior of the pump housing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages and advantageous features of the invention can be learned from the description contained herein below, taken in conjunction with the drawings, in which: 
         FIG. 1  shows a first exemplary embodiment of a high-pressure fuel pump  1  of the invention in a block circuit diagram and 
         FIGS. 2 through 7  are views similar to  FIG. 1  showing alternative embodiments of the Invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The high-pressure fuel pump  1  is part of a fuel injection system that essentially comprises a tank  3 , a prefeed pump  5 , a filter  7 , a rail  9 , and a pressure limiting valve  11 . The injectors, which are connected to the rail  9 , are not shown in the drawings. The pressure limiting valve  11  discharges into a return line  13 , into which the leak fuel quantities from the injectors are also carried away. The return line  13 , in this first exemplary embodiment, discharges into the tank  3 , where it drives a jet pump (not identifiedby reference numeral). 
     In the interior of the high-pressure fuel pump  1 , there is a temperature sensor T. The high-pressure fuel pump  1  communicates hydraulically with the tank  3  via a fuel feed line  15 , the filter  7 , and the prefeed pump  5 . 
     Inside the high-pressure fuel pump  1 , a first branch line  17 , in which a metering unit  19  is disposed, branches off from the fuel feed line  15 . The metering unit  19  serves to control the quantity of fuel aspirated by pump elements  21  of the high-pressure fuel pump, and thus also to control the pump capacity thereof. To that end, the intake sides of the pump elements  21  communicate hydraulically with the outlet of the metering unit  19  via a distribution line  23 . 
     The pump elements  21  essentially comprise suction valves  25 , check valves  27  on the high-pressure side, and a piston  29  that oscillates in a cylinder bore (not identified by reference numeral). The pistons  29  of the pump elements  21  are driven via roller tappets  31  by cams  33  of a drive shaft  35 . The pump elements  21  pump fuel, which is at high pressure, into the rail  9  via a high-pressure line  37 . 
     The cams  33  are part of a drive shaft  35  that is supported rotatably on both sides of the cams  33  in a first bearing and in a second bearing in a pump housing (not shown). The drive shaft  35  is disposed in the interior  38  of the pump housing. The bearings of the drive shaft  35  are shown as throttle restrictions in the block circuit diagram in  FIG. 1 . In  FIG. 1 , the first bearing is identified by reference numeral  39 , while the second bearing has been provided with reference numeral  41 . 
     A fuel return line  43  acts as a hydraulic communication between the interior  38  of the pump housing and the return line  13 . A pressure regulating valve  45  is disposed in the fuel return line  43 . Various throttle restrictions (not shown) may be integrated with the pressure regulating valve  45 . 
     In the exemplary embodiment, shown in  FIG. 1 , of a high-pressure fuel pump according to the invention, the pressure regulating valve  45  is disposed downstream of the interior  38  of the high-pressure fuel pump  1 . This means that in the interior  38 , virtually the same pressure as on the compression side of the prefeed pump  5  prevails. As a rule, the pressure on the compression side of the prefeed pump  5  and thus in the interior  38  amounts to approximately 3 bar to approximately 6 bar. 
     This pressure prevailing in the interior  38  leads to a lessening of the tendency to cavitation, and thus to suppression of vapor bubbles, especially at high rotary speeds. Moreover, the elevated internal pressure in the interior  38  of the pump housing causes fuel to be forced through the first bearing  39  and the second bearing  41 . As a result, depending on the pressure prevailing in the interior  38 , the viscosity of the fuel, and the flow resistance of the first bearing  39  and the second bearing  41 , a defined quantity of fuel is forced through the bearings  39  and  41 . This leads to a marked increase in the bearing capacity of both the first bearing  39  and the second bearing  41 . 
     Since the first bearing  39  and the second bearing  41  are as a rule embodied as slide bearings, the forced flow through the bearings  39  and  41  leads to the development of a hydrostatic lubrication wedge in the bearings  39  and/or  41 . As a result, the bearing capacity of the first bearing  39  and second bearing  41  increases considerably, and at the same time the heat dissipation from the first bearing  39  and the second bearing  41  is improved. 
     To reduce the variation in the fuel quantity that flows through the first bearing  39 , and thus also to reduce the variation in the bearing capacity of the first bearing, a first optional flow limiting device  47  is disposed in series with the first bearing  39 . This first flow limiting device may, as indicated in  FIG. 1 , be embodied as a throttle restriction. Alternatively, it may be embodied as a diaphragm or as a flow regulating valve. 
     In experiments, it has been found that because of the production tolerances, for instance in the diameter of the bearing journal (not shown) of the drive shaft  35  for the first bearing  39  and the associated bearing plate (not shown) in the pump housing, given an unfavorable tolerance situation, the quantity of fuel that flows through the first bearing  39  can vary considerably within one series of high-pressure fuel pumps  1 . This unwanted effect is reduced, if necessary, to a non-critical amount by the first flow limiting device  47  according to the invention. 
     Because of the serial connection of the first bearing  39  and the first flow limiting device, it can be assured that the quantity of fuel that flows through the first bearing  39  can be kept within a relatively narrow range. This can be ascribed above all to the fact that the flow resistance of the first flow limiting device  47  can be adjusted with very high precision. By a suitable adaptation of the flow resistance of the first flow limiting device  47  and the pressure prevailing in the interior  38 , it is possible in the high-pressure fuel pump  1  of the invention to keep the fuel quantity flowing through the first bearing  39  within a predetermined range under all tolerance conditions that occur in mass production. 
     If needed, a suitable second flow limiting device (not shown) may also be provided for the second bearing  41 . 
     In  FIG. 1 , a filter is provided in the fuel feed line  15 ; this filter also takes on the function of a damping device  49 . Thus any pressure fluctuations in the low-pressure region can be damped. Alternatively, the damping device  49  may be embodied with a gas cushion, or may be omitted. 
     The high-pressure fuel pump  1  of the invention has the following advantages, among others: 
     Because of the disposition of the pressure regulating valve  45  in the fuel return line  43 , the pressure level prevailing in the interior  38  of the pump housing is increased, which reduces the danger of cavitation and the danger of vapor bubble formation. 
     Moreover, both the first bearing  39  and the second bearing  41  as a result necessarily experience a flow through them of fuel, which markedly increases their bearing capacity with regard to both mechanical and thermal stresses. 
     Any fluctuations in the flow quantity that may occur between various examples of mass-produced high-pressure fuel pumps  1  according to the invention can be reduced by means of a series-connected first flow limiting device  47  and/or a second flow limiting device. 
     The quantity of fuel flowing through the pump housing and the bearings  39  and  41  for lubricating and cooling purposes is increased sharply. 
     A lubricating throttle restriction for adjusting a defined quantity of lubricant can be omitted. Because of the high lubrication quantities, any particles that may be present are rapidly floated out of the interior. 
     The pumping capacity of the prefeed pump can often be reduced, which improves the efficiency of the injection system. 
     In  FIGS. 2 through 7  further exemplary embodiments of high-pressure fuel pumps according to the invention and fuel injection systems according to the invention are shown, also in the form of block circuit diagrams. Only the essential differences will now be explained. Identical components are provided with the same reference numerals, and what has been said for the exemplary embodiment above applies accordingly. In  FIGS. 2 through 7 , for the sake of simplicity, not all the components are provided with the reference numerals of  FIG. 1 , and with respect to these components, reference is made to what is said in conjunction with the first exemplary embodiment. 
     The essential distinction between the first exemplary embodiment of  FIG. 1  and the second exemplary embodiment of  FIG. 2  is that the first branch line  17  in the second exemplary embodiment branches off from the fuel return line  43 . This means that the entire amount of fuel pumped by the prefeed pump  5  through the fuel feed line  15  reaches the interior  38  of the high-pressure fuel pump first and branches off only after that. As a result, an even better flow through the high-pressure fuel pump  1  and even better pump cooling are attained. 
     To damp any pressure fluctuations that occur in the low-pressure region, a damping device  49  is provided in the fuel return line  43 . The damping device  49  is disposed upstream of the pressure regulating valve  45  and the metering unit  19 . In  FIG. 2 , the damping device  49  is embodied as a filter with an increased flow resistance if needed (not shown). Alternatively, the damping device  49  may be embodied as a damper with a gas cushion. 
     The third exemplary embodiment in  FIG. 3  corresponds in wide areas to the second exemplary embodiment of  FIG. 2 . An essential distinction is that, unlike in the preceding exemplary embodiments, the prefeed pump  5  is driven not by an electric motor (not shown) but rather directly by the engine. The details of this drive mechanism are not shown in  FIG. 3 . 
     Upstream of the prefeed pump  5 , namely between the filter  7  and the prefeed pump  5 , a suction throttle restriction  51  is provided, which limits the pump capacity of the prefeed pump  5 , above all at high rotary speeds. 
     The prefeed pump  5  may be embodied as a vane cell pump, external gear-wheel pump or internal gear-wheel pump, and in particular as a Gerotor pump. In these pumps, there is a gap that causes leakage losses between the rotating components and the pump housing. This gap is represented in  FIG. 3  by the symbol for a throttle restriction (see reference numeral  53 ). 
     The leak fuel quantity flowing out through the gap is carried away through a leak fuel line  55 . 
     In the third exemplary embodiment, a diversion line  56  is provided, which begins at the pressure regulating valve  45  and discharges into the fuel feed line  15  upstream of the suction throttle restriction  51 . Via the diversion line  56 , the excess fuel quantity from the pressure regulation is carried away into the feed line  15 . 
     The first bearing  39  is supplied with fuel from the interior  38 . From the leak fuel line  55 , a second branch line  57  branches off, which discharges into the fuel return line  43 . Through the second branch line  57 , the quantity of lubricant in the first bearing  39  is also carried away. A bypass throttle restriction  59  may be provided in the second branch line  57 . 
     In  FIG. 4 , fourth exemplary embodiment of a high-pressure fuel pump  1  of the invention is shown that has many parallels with the third exemplary embodiment of  FIG. 3 . In this exemplary embodiment as well, a leak fuel line  55  is located at the prefeed pump  5 . 
     In this exemplary embodiment as well, the first bearing  39  is supplied with fuel from the interior  38 . From the leak fuel line  55 , a second branch line  57  branches off, which discharges into the fuel return line  43 . Through the second branch line  57 , the quantity of lubricant in the first bearing  39  is also carried away. A bypass throttle restriction  59  may be provided in the second branch line  57 . In this exemplary embodiment, the metering unit  19  is disposed in the fuel feed line  15 , as is also the case in the first exemplary embodiment of  FIG. 1 . 
     In the exemplary embodiment of  FIG. 4 , the fuel return line  43  is returned not to the tank  3 , as in the exemplary embodiments of  Figs. 1 and 2 , but rather into the fuel return line  15  as in the third exemplary embodiment, specifically upstream of the suction throttle restriction  51 . 
     The essential distinction of the fifth exemplary embodiment of  FIG. 5  compared with the fourth exemplary embodiment of  FIG. 4  is that in the fifth exemplary embodiment, the metering unit  19  and the optional damping device  49  are disposed in the fuel return line  43 . The pressure relief for the motion of the piston of the pressure regulating valve  45  can be selectively connected into the fuel feed line  15  or the fuel return line  43 . 
     In addition, the pressure regulating valve  45  has a separate diversion line  56 , which, similarly to the third exemplary embodiment, discharges into the fuel feed line  15  upstream of the suction throttle restriction  51 . 
     In this exemplary embodiment, the fuel return line  43  is returned to the tank  3  via the return line  13 . In a third branch line  63 , which connects the interior  38  of the high-pressure fuel pump  1  to the fuel return line  43 , there is a second bypass throttle restriction  61 . In series with the second bypass throttle restriction  61 , a pressure limiting valve  65  is also provided in the third branch line  63 . The pressure limiting valve  65  assures that if a predetermined pressure difference between the pressure in the interior  38  of the high-pressure fuel pump  1  and the fuel return line  43  is exceeded, the third branch line  63  is opened, and thus the excess fuel can flow out of the interior  38 . 
     In the sixth exemplary embodiment of  FIG. 6 , the leak fuel line  55  discharges into the interior  38  of the pump housing. The first bearing  39  is supplied with fuel under pressure from the interior  38  of the pump housing, and this fuel then flows through the first flow limiting device  47  and then reaches the fuel return line  43 . In this exemplary embodiment as well, the fuel metering unit  19  is disposed on the side toward the fuel return line  43 , with the advantages already mentioned several times; however, it may also be disposed on the side of the fuel feed line  15 . 
     In the exemplary embodiment of  FIG. 7 , the prefeed pump  5  is embodied as a fuel pump which is driven by an electric motor and is disposed in the vicinity of the tank  3 . The metering unit  19  is disposed on the fuel feed line side  15  of the high-pressure fuel pump  1 . The pressure regulating valve  45  is connected on the inlet side to the fuel feed line  15 . The outlet side of the pressure regulating valve  45  discharges into the fuel return line  43 . Also discharging into the fuel return line  43  is the third branch line  63 , in which there is not only a second bypass throttle restriction but also a damping device  49 , such as filter. Also in the fuel return line  43 , the fuel that flows through the first bearing  39  and the first flow limiting device  47  is carried away. The same applies to the second bearing  41 , which in the exemplary embodiment of  FIG. 7  is provided with a second flow limiting device  67 , whose mode of operation corresponds to that of the first flow limiting device  47 . 
     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.