Abstract:
The invention relates to a fuel overflow valve for a fuel injection system, particularly for limiting the pressure in a low-pressure region of the fuel injection system. The fuel overflow valve has a valve housing, in which a valve member is disposed in a stroke-moving manner, the stroke movement of the valve member controlling a connection of an inlet to the valve housing to a release region. The valve member is loaded by a valve spring in the direction of a locking position in which the connection of the inlet to the release region is interrupted, and is loaded by the pressure present in the inlet in the opening direction. The valve member may carry out a further stroke in the locking direction beyond the locking position thereof, where the valve spring does not act upon the valve member. Due to the increased stroke of the valve member, an improved balance of pressure and volume fluctuations is enabled in the low pressure region and the stroke of the valve spring, and thus the stress thereof, may be kept low.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 35 USC 371 application of PCT/EP2008/062443 filed on Sep. 18, 2008. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention is based on a fuel overflow valve for a fuel injection system and on a fuel injection system having a fuel overflow valve. 
     2. Description of the Prior Art 
     One such fuel overflow valve and one such fuel injection system are known from German Patent Disclosure DE 100 57 244 1. This fuel overflow valve serves to limit pressure in a low-pressure region of the fuel injection system. The fuel overflow valve has a valve housing, in which a valve member is reciprocatably disposed. By means of the valve member, upon its reciprocating motion, the connection of an inlet from the low-pressure region with an outlet to a relief region is controlled. The valve member is urged by a valve spring in the direction of a closing position in which the connection of the inlet with the outlet is interrupted, and is urged in the opening direction by the pressure prevailing in the inlet. If the pressure in the low-pressure region exceeds the opening pressure determined by the valve spring, the fuel overflow valve opens, and fuel can flow from the inlet out of the low-pressure region via the outlet into a relief region, such as a return to the fuel tank. The fuel injection system has a high-pressure pump, by which fuel is delivered by high pressure to at least one injector at least indirectly, for instance via a reservoir. By means of a feed pump, fuel is delivered to the high-pressure pump. The high-pressure pump has at least one pump piston that is driven in a reciprocating motion by a drive mechanism disposed in a drive region. The low-pressure region of the fuel injection system extends between the feed pump and the high-pressure pump, and in this low-pressure region, a low pressure generated by the feed pump prevails. The low-pressure region communicates with the drive region of the high-pressure pump. Because of the reciprocating motion of the at least one pump piston, the volume of the drive region varies, since in the outlet-oriented stroke of the pump piston, the volume of the drive region is increased, and in the inlet-oriented stroke of the pump piston, the volume of the drive region is decreased. As a result, pressure fluctuations are created in the drive region. Especially in the case of a high-pressure pump with only one pump piston, relatively strong pressure fluctuations are created. As a result, pressure fluctuations are generated in the entire low-pressure region as well, and they can impair the function of the fuel injection system. To compensate for these pressure fluctuations, the valve member of the fuel overflow valve must be capable of executing a long stroke, which accordingly necessitates a long stroke of the valve spring as well. This in turn means that a large amount of space is necessary for the valve spring, and the valve spring is heavily loaded and can therefore break. 
     ADVANTAGES AND SUMMARY OF THE INVENTION 
     The fuel overflow valve according to the invention has the advantage over the prior art that the valve member, independently of the valve spring, can execute a longer stroke, making improved compensation for the pressure fluctuations possible. The valve spring needs to execute only a limited stroke in order to move the valve member into its closing position, and as a result the installation space for the fuel overflow valve can be kept small and the load on the valve spring can be kept slight. Corresponding advantages result for the fuel injection system according to the invention whose function is improved by the reduced pressure fluctuations in the low-pressure region. 
     One aspect of the invention, in a simple way, enables the increased stroke of the valve member compared to the valve spring stroke. By another aspect of the invention, damping of the reciprocating motion of the support element and thus of the valve member and the valve spring is attained, thus reducing the load on the valve spring. Another aspect of the invention likewise enables damping of the reciprocating motion of the support element and thus of the valve member and the valve spring. Another aspect of the invention, without modifications to the valve housing, makes a two-stage embodiment of the fuel overflow valve possible. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Two exemplary embodiments of the invention are described in further detail in the ensuing description in conjunction with the drawings, in which: 
         FIG. 1  shows a fuel injection system in a simplified schematic illustration; 
         FIG. 2  shows a fuel overflow valve of the fuel injection system in a first exemplary embodiment in the closed state, in a longitudinal section, with a valve member in a first position; 
         FIG. 3  shows the fuel overflow valve in the closed state with the valve member in a second position; 
         FIG. 4  shows the fuel overflow valve in the open state; and 
         FIG. 5  shows the fuel overflow valve in a second exemplary embodiment in the closed state. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIG. 1 , a fuel injection system for an internal combustion engine is shown. The fuel injection system has a feed pump  10 , which aspirates fuel from a fuel tank  12  and delivers it to the intake side of a high-pressure pump  18 . By the feed pump  10 , the fuel is compressed to a delivery pressure of approximately 4 to 6 bar, for example. The feed pump  10  may be driven electrically or mechanically. Between the feed pump  10  and the intake side of the high-pressure pump  18 , there can be a fuel metering device  16 , by which the quantity of fuel aspirated by the high-pressure pump  18  and delivered at high pressure can be variably adjusted. The fuel metering device  16  may be a proportional valve that is capable of adjusting variously large flow cross sections, or it may be a clocked valve, and it is triggered mechanically or electrically by an electronic control device  17 . 
     The high-pressure pump  18  has a housing, in which in an inner chamber  19  a rotationally driven drive shaft  20  is disposed. The inner chamber  19  of the housing having the drive shaft  20  forms a drive region of the high-pressure pump  18 . The drive shaft  20  has at least one cam  22  or eccentric element, and the cam  22  may also be embodied as a multiple cam. The high-pressure pump has at least one or more pump elements, each with one pump piston  26  that is driven indirectly by the cam  22  of the drive shaft  20  in a reciprocating motion in a direction that is at least approximately radial to the axis of rotation of the drive shaft  20 . The pump piston  26  is guided tightly in a cylinder bore  28  and, with its side remote from the drive shaft  20 , it defines a pump work chamber  30 . The pump work chamber  30  has a connection with the fuel inlet from the feed pump  10 , via an inlet valve  32  that opens into the pump work chamber  30 . Furthermore, via an outlet valve  34  opening out of the pump work chamber  30 , the pump work chamber  30  has a connection with an outlet that is in communication with a high-pressure reservoir  110 , for instance. One or preferably more injectors  120 , disposed at the cylinders of the engine, communicate with the high-pressure reservoir  110  and through them the fuel is injected into the cylinders of the engine. The high-pressure reservoir  110  may also be omitted; in that case, the high-pressure pump  18  communicates with the injectors  120  via hydraulic lines. In its intake stroke, the pump piston  26  moves into the inner chamber  19  and in the process aspirates fuel, via the open inlet valve  32 , from the inlet from the feed pump  10  into the pump work chamber  30 . In its delivery stroke, the pump piston  26  moves out of the inner chamber  19  and delivers fuel at high pressure from the pump work chamber  30 , via the open outlet valve  34 , into the high-pressure reservoir  110  or to the injectors  120 . 
     The region between the feed pump  10  and the fuel metering device  16  forms a low-pressure region, in which the pressure generated by the feed pump  10  prevails. By means of the feed pump  10 , the same fuel quantity is constantly delivered, but as a function of the setting of the fuel metering device  16 , a variable fuel quantity is aspirated by the high-pressure pump  18 . For that reason, a fuel overflow valve  36  is provided, by which the pressure in the low-pressure region is limited. The fuel overflow valve  36  opens if the pressure in the low-pressure region exceeds its opening pressure, and via the open fuel overflow valve  36 , the quantity of fuel delivered by the feed pump  10 , but not aspirated by the high-pressure pump  18 , is diverted into a relief region, which is for instance a return  11  to the fuel tank  12 . 
     The fuel overflow valve  36 , in a first exemplary embodiment, will now be described in further detail in conjunction with  FIGS. 2 through 4 . The fuel overflow valve  36  has a tubular valve housing  38 , which has one tubular portion  39  of lesser diameter and one tubular portion  40  of greater diameter. In the portion  39  of the valve housing  38 , a pistonlike valve member  42  is guided displaceably tightly in a longitudinal bore  41 . In the portion  39  of the valve housing  38 , at least one opening  43  is provided, which connects the longitudinal bore  41  to the outer jacket of the portion  39 . The opening  43  is preferably embodied as a bore; for example, two diametrically opposed bores are provided. Via the openings  43 , the longitudinal bore  41  can be made to communicate with a relief region, such as a return to the fuel tank  12 . The pressure prevailing in the low-pressure region acts via the open end of that region in the longitudinal bore  41  in the valve housing  38  as well and thus acts on the face end of the valve member  42 . Hence the open end of the longitudinal bore  41  forms an inlet from the low-pressure region into the fuel overflow valve  36 . When the valve member  42  covers the openings  43 , the inlet, that is, the low-pressure region, is disconnected from the relief region, and when the valve member  42  at least partially uncovers the openings  43 , the inlet, or in other words the low-pressure region, is in communication with the relief region. Hence the valve member  42 , with the openings  43 , forms a slide valve. On the open end of the longitudinal bore  41  on the valve housing  38 , a filter screen  44  may be disposed, by which dirt particles are prevented from being able to enter the longitudinal bore  41  from the low-pressure region. The filter screen  44  may be fixed to the valve housing  38  by means of an annular securing element  45 , and the securing element can be connected to the valve housing  38  by means of a crimp, for instance. 
     A valve spring  48 , which acts on the valve member  42  via a support element  50 , is disposed in a longitudinal bore  46  of the portion  40  of the valve housing  38  that extends at least approximately coaxially to the longitudinal bore  41  but has a greater diameter than the latter. The support element  50  is embodied in cuplike fashion, and its bottom  52  points toward the valve member  42 , and its open end points away from the valve member  42 . The support element  50  is guided displaceably in the longitudinal bore  46 , and protruding into this bore, from its open end, is the valve spring  48 , which is embodied as a helical compression spring and rests on the bottom  52 . The end, remote from the valve member  42 , of the longitudinal bore  46  of the valve housing  38  is closed by means of an insert part  54 , which also acts as a brace for the valve spring  48 . The insert part  54  may be embodied in cuplike fashion like the support element  50 , and its open end points toward the valve member  42 , and the valve spring  48  protrudes into the insert part  54  and is braced on the bottom thereof. The insert part  54  is fixed in the longitudinal bore  46 , for instance being press-fitted into it. The support element  50  and/or the insert part  54  may be embodied as a shaped sheet-metal part. 
     The support element  50  is not connected to the valve member  42 ; instead, it only comes to rest with its bottom  52  on the valve member  42  as a result of the action of the valve spring  48 . Toward the valve member  42 , the support element  50  can execute a maximum stroke which is limited by contact of the support element  50  with an annular shoulder  56 , formed at the transition from the longitudinal bore  46  to the smaller-diameter longitudinal bore  41 . There is at least one opening  58  of large cross section in the bottom  52  of the support element  50 . In the peripheral region of the bottom  52 , near its transition to the jacket face of the support element  50 , at least one opening  60  of small cross section is provided. The longitudinal bore  46  can be made to communicate with a relief region, which may for instance be a return to the fuel tank  12 , via at least one opening  62  that opens out at the outer jacket of the portion  40  of the valve housing  38 . When the support element  50  is in contact with the annular shoulder  56 , it does not cover the opening  62 , and thus the longitudinal bore  46  is in communication with the relief region. When the support element  50 , beginning at its contact with the annular shoulder  56 , moves into the longitudinal bore  46 , then the opening  62  is increasingly covered by it, and thus the cross section is reduced and may be closed entirely, so that the longitudinal bore  46  now communicates with or is disconnected from the relief region via only a small, throttling flow cross section. 
     The fuel overflow valve  36  with the valve housing  38 , the valve member  42 , the valve spring  48 , the support element  50 , and the insert part  54  as well as the filter screen  44 , forms a preassembled unit that is inserted into a receiving housing  70 . The receiving housing  70  may be a separate housing or a part of the housing of the high-pressure pump  18 . 
     The function of the fuel overflow valve  36  will now be described in further detail. The length of the valve member  42  and the position of the annular shoulder  56  in the valve housing  38  for limiting the stroke of the support element  50  are adapted to one another such that the valve member  42 , when the support element  50  is in contact with the annular shoulder  56 , just covers the openings  43  and thus undoes the connection of the low-pressure region with the relief region. The valve member  42  is shown in that position in  FIG. 2 . Beginning at that position, the valve member  42  can move still farther in the direction toward the open end of the longitudinal bore  41 , whereupon the valve member  42  is no longer in contact with the support element  50 , and thus the valve spring  48  no longer acts on the valve member  42 . The valve member  42  is thus freely movable in the longitudinal bore  41 , in accordance with the difference between the pressure in the low-pressure region acting on one face end of the valve member and the pressure in the longitudinal bore  46  acting on its other face end. By means of the valve spring  48 , the valve member  42  can be moved into its closing position, and independently of the valve spring  48 , the valve member  42  can execute a still further stroke past its closing position, and thus stroke can be limited for instance by the filter screen  44  or the securing element  45 , in order to prevent the valve member from moving out of the longitudinal bore  41 . The valve member  42  is shown in  FIG. 3  in this terminal position. 
     If the pressure prevailing in the low-pressure region is not sufficient to displace the valve member  42 , counter to the force of the valve spring  48 , so far in the longitudinal bore  41  that the openings  43  are opened by the valve member  42 , then the low-pressure region is disconnected from the relief region. If the pressure prevailing in the low-pressure region attains the opening pressure of the fuel overflow valve  36 , then the valve member  42  is displaced in the longitudinal bore  41  counter to the force of the valve spring  48 , so that the openings  43  are opened by the valve member  42 , and the low-pressure region is in communication with the relief region, so that fuel can flow out of the low-pressure region into the relief region. The valve member  42  is shown in  FIG. 4  in this open position. 
     If the openings  43  are covered by the valve member  42 , or in other words the low-pressure region is disconnected from the relief region, then the valve member  42  can nevertheless execute a further stroke toward the open end of the longitudinal bore  41  and can thus at least partially compensate for fluctuations in pressure and volume in the low-pressure region. The stroke executed by the support element  50  and the valve spring  48  is shorter than the possible stroke of the valve member  42 . This leads to lesser loads on the valve spring  48 , which can accordingly be dimensioned more weakly. The maximum stroke of the support element  50  and of the valve member  42 , and thus the maximum spring travel of the valve spring  48 , are limited by the fact that the support element  50  comes to rest on the insert part  54 . At this point, the valve spring  48  is preferably not yet compressed to a block. 
     By means of the at least one opening  58  in the bottom  52  of the support element  50 , it is ensured that the valve member  42  can easily come loose from the support element  50  and come into contact with it again. Through the at least one opening  60  in the support element  50 , a pressure compensation between the two sides of the support element  50  is ensured, so that the support element can move within the fuel-filled longitudinal bore  46 . By means of the stroke-dependent control of the opening  62  by the support element  50 , damping of the opening reciprocating motion of the valve member  42  and of the support element  50  is also attained, as a result of which the load on the valve spring  48  is reduced, since the opening reciprocating motion is damped by the fuel pressure that builds up in the longitudinal bore  46 . 
     In  FIG. 5 , the fuel overflow valve  36  is shown in a second exemplary embodiment, in which it opens in two stages and controls two connections of the low-pressure region. The valve housing  38 , the support element  50 , the valve spring  48 , the insert part  54 , and the filter screen  44  and its securing element  45  are embodied identically to the first exemplary embodiment. Only the valve member  142  is embodied differently from the first exemplary embodiment, but the outer dimensions of the valve member  142 , that is, its diameter and length, are identical to those in the first exemplary embodiment. The valve member  142 , in a departure from the first exemplary embodiment, is embodied as hollow and has a blind bore  176 , originating at the end remote from the valve spring  48 , and the bottom  178  of the valve member  142  that comes to rest on the support element  50  is embodied as closed. Near the closed end of the valve member  142 , at least one opening  180  is provided on it, for instance in the form of a bore, through which the blind bore  176  communicates with the outer jacket of the valve member  142 . The opening  180  is preferably embodied as a throttle bore of defined cross section. The interior of the blind bore  176  is constantly acted upon by the pressure prevailing in the low-pressure region. 
     If by the action of the valve spring  48  the valve member  142  is located in its closing position, then it covers the openings  43 , and the orifice of the opening  180  is located inside the longitudinal bore  41  and is covered by it. The low-pressure region is thus disconnected from the relief regions. If the pressure in the low-pressure region suffices to move the valve member  142  counter to the force of the valve spring  48 , then initially at a slight opening stroke of the valve member  142 , the opening  180  emerges from the longitudinal bore  41 , so that the low-pressure region communicates with the opening  62  via the blind bore  176 , the opening  180 , and the at least one opening  60  in the support element  50 , and fuel can flow out of the low-pressure region via this opening  62 . At this slight opening stroke of the valve member  142 , the openings  43  continue to be covered by the valve member and remain closed, so that no fuel can flow out of the low-pressure region via the openings  43 . Upon a further opening stroke of the valve member  142 , the openings  43  are uncovered by it, so that fuel can flow out of the low-pressure region into the return  11  ( FIG. 1 ) via the openings  43  as well. 
     It is advantageous for the two-stage version of the fuel overflow valve  36  to be employed in a fuel injection system in which only a portion of the fuel quantity delivered by the feed pump  10  is supplied to the inner chamber  19  of the high-pressure pump  18  for the sake of lubricating and cooling its drive mechanism. If the pressure prevailing in the low-pressure region is not sufficient to open the fuel overflow valve  36 , then the entire fuel quantity delivered by the feed pump  10  is supplied via the fuel metering device  16  to the high-pressure pump  18  for delivery. If the pressure prevailing in the low-pressure region reaches a first limit value, then the fuel overflow valve  36  opens in the first stage, and the quantity of fuel flowing out, upon opening of the first stage, of the blind bore  176 , the opening  180 , the at least one opening  60  in the support element  50 , and the opening  62  is supplied in accordance with  FIG. 1  to the inner chamber  19  via a line  13 . This ensures first a rapid fuel delivery by means of the high-pressure pump  18  upon starting of the engine, and after that, it ensures adequate lubrication and cooling of the drive region of the high-pressure pump  18 . When the pressure prevailing in the low-pressure region reaches a second, higher limit value, then the second stage of the fuel overflow valve  36  opens as well, because the valve member  142  uncovers the openings  43 , and fuel can flow out of the low-pressure region into the fuel tank  12  via the return  11 . 
     The foregoing relates to the 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.