Abstract:
A high pressure fuel pump encompasses at least one delivery chamber and one high pressure outlet. in addition, a pressure limiting valve with a valve that is actuated by a pressure differential is provided that can open from the high pressure outlet to the delivery chamber. On a high pressure side of a valve seat of the pressure limiting valve, it is advantageous that a throttle device is provided, whose free cross section is at most approximately equal to a desired maximum opening cross section of the pressure limiting valve.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 35 USC 371 application of PCT/EP 2007/053682 filed on Apr. 16, 2007. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a high pressure fuel pump with a pressure relief valve. 
     2. Description of the Prior Art 
     A high pressure fuel pump of the type mentioned at the beginning is known from DE 10 2004 013 307 A1. In this one-cylinder piston pump, the delivery chamber can be connected to a high pressure outlet by means of a spring-loaded outlet valve. Fluidically parallel to the outlet valve, a pressure relief valve is provided, which has a spring-loaded valve ball as a valve element. The pressure relief valve opens toward the delivery chamber and, when open, connects the high pressure outlet to the delivery chamber. A pressure relief valve situated in such a way has the advantage that it protects the high pressure region from impermissibly high pressures, but simultaneously does not reduce the volumetric efficiency of the high pressure fuel pump since the pressure relief valve only opens when the pressure prevailing in the delivery chamber is significantly lower than the pressure in the high pressure outlet. 
     OBJECT AND SUMMARY OF THE INVENTION 
     The object of the present invention is to create a high pressure fuel pump of the type mentioned at the beginning that functions in a particularly reliable fashion. 
     According to the invention, the realization was reached that when the pressure relief valve opens, there is a danger of dynamic pressure impacts causing the valve element to lift away from the valve seat so far that it is pushed out of the valve seat and becomes jammed between the valve seat body and the spring plate. As a result, the pressure relief valve would no longer be able to close, thus rendering it impossible for pump delivery to occur. The measures taken according to the invention prevent this entire scenario: the throttle device limits the maximum volumetric flow coming out of the pressure relief valve so that the valve element of the pressure relief valve cannot exceed a maximum opening stroke. The throttle device functions more or less as a hydraulic stroke limitation. 
     This is achieved by special matching of the free cross section of the throttle device to the desired maximum opening cross section of the pressure relief valve, which corresponds to a stroke of the valve element at which the valve element is still assured of not becoming jammed. In most cases, it would be permissible for this maximum opening cross section to be an annular surface. The measure taken according to the invention prevents the valve element from coming out of the valve seat region when the maximum flow is passing through the pressure relief valve and assures that the valve element easily finds its way back to the valve seat again when the pressure relief valve closes. The throttle device also reduces the dynamic behavior of the pressure relief valve, which has a positive effect on the wear. Pressure peaks are only transmitted to the valve element in a damped fashion. 
     If the throttle device includes a part that is situated on the high pressure side in relation to the pressure relief valve, is separate from the pressure relief valve, and is equipped with a flow throttle, then it is possible for the previously used pressure relief valves to remain unchanged. This reduces the manufacturing costs. 
     The same aim is shared by the modification in which the separate part is secured in a press-fitted fashion in an overflow conduit of a pump housing. 
     The separate part can be embodied as cup-shaped and having a bottom section, with the flow throttle embodied in the farm of at least one opening in the bottom section. A part of this kind can be inexpensively manufactured as a formed and stamped sheet metal part. 
     With a throttle device that is situated on the high pressure side in relation to the pressure relief valve, it is advantageous if its free cross sectional area is at least approximately 0.6 to 1.1 times the cross sectional area of a valve seat of the pressure relief valve. 
     Alternatively or in addition to a flow throttle that is separate from the pressure relief valve, the throttle device can also include a flow throttle that is situated in a valve seat body of the pressure relief valve near or immediately adjacent to the valve seat and on the high pressure side in relation to it. This eliminates the handling of the separate part, which simplifies the assembly of the high pressure fuel pump according to the invention. 
     The flow throttle can be simply embodied in the form of a constriction in an inlet conduit in the valve seat body. 
     In a throttle device of this kind, the free cross sectional area of the flow throttle should be at least approximately 0.5 to 0.75 times the cross sectional area of the valve seat of the pressure relief valve. Such a design assures a good function of the pressure relief valve reliably prevents the valve element from jamming. 
     It is possible for the valve element of the pressure relief valve to be a spring-loaded ball that can be loosely installed, which is very inexpensive. The valve seat for such a ball is advantageously conical, with a cone angle of between approximately 30° and 50°. The more acute the angle, the better the seal when the pressure relief valve is closed. 
     It is also preferable for a free cross sectional area of an influx conduit directly upstream (i.e. to the high pressure side) of the valve seat (the term upstream here refers to the flow direction through the pressure relief valve) to be at least approximately 0.8 to 0.95 times the cross sectional area of the valve seat of the pressure relief valve. Such a narrow valve seat is advantageous for assuring that the pressure relief valve has a favorably low sensitivity to dirt. Such a narrow valve seat also permits a particularly favorable molding to the seat itself during operation. 
     In a particularly advantageous embodiment of the high pressure fuel pump according to the invention, a valve seat body of the pressure relief valve includes a securing section for the valve element that extends in the opening direction of the valve element and is embodied as an essentially annular collar. This securing section secures the valve element in a lateral direction when it is in the open position, i.e. lifted away from the valve seat, so that even with the occurrence of dynamic pressure impacts and a large opening stroke, it is impossible for the valve element to become jammed between the valve seat body and a valve spring that acts on the valve element. Finally, this measure according to the invention improves the operational reliability of the high pressure fuel pump since it prevents the pressure relief valve from jamming in the open position, thus preventing a buildup of high pressure in the high pressure fuel pump. Finally, the securing section assures that the valve element reliably finds its way back to the valve seat again, even when executing a large stroke. 
     In a modification of this, the securing section is formed onto a valve seat region of the pressure relief valve in the vicinity of its valve seat. This reduces the number of parts to be handled during assembly, thus simplifying the assembly. In addition, the manufacturing costs for the securing section are reduced since it is necessary for the valve seat region of the pressure relief valve to be machined anyway. 
     It is particularly advantageous if at least one flow conduit, in particular a flow pocket, preferably extending essentially the length of the securing section, is embodied on the radial inside of the securing section. When the pressure relief valve is open, a flow conduit of this kind—which is introduced, for example, by means of a recess permits a low-resistance flow between the valve element and the inside of the securing section with a simultaneously close guidance of the valve element through the securing section. The fluid can easily flow through the flow conduit between the inside of the securing section and the open valve element and can flow past a valve element holder possibly provided to hold the valve element. 
     The same aim is shared by the embodiment of the high pressure fuel pump according to the invention in which the securing section has at least one slot preferably extending essentially over its length. Such a slot is particularly inexpensive to manufacture. 
     Also according to the invention, the radial inside of the securing section includes a conical surface that widens out in the opening direction of the pressure relief valve. When the pressure relief valve is open, this creates the open space that permits a low-resistance flow of the fluid between the securing section on the one hand and the valve element and valve element holder on the other. In this context, the cone angle of the conical surface can at least approximately correspond to the cone angle of the valve seat, which permits a relatively simple manufacture. The cone angle of the conical surface can, however, also be greater than the cone angle of the valve seat, which, with a small opening stroke of the valve element, results in a comparatively large free space between the radial inside of the securing section on the one hand and the valve element and valve element holder on the other. 
     It is also particularly advantageous if the valve seat body has a shoulder that is adjacent to the valve seat and extends at least approximately in the radial direction, from which the radial inside of the securing section extends in the opening direction of the pressure relief valve. This measure can be used in combination both with the above-mentioned flow pockets or flow slots and with the above-mentioned conical surface. The presence of the shoulder avoids the exertion of closing flow forces on the valve element in its open position. 
     The pressure relief valve can include a piston-like valve element holder that acts on the valve element in the closing direction and protrudes into the securing section both when the pressure relief valve is closed and when it is open. This assures a particularly reliable guidance of the valve element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Particularly preferred exemplary embodiments of the present invention will be explained in greater detail with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic depiction of a fuel system equipped with a high pressure fuel pump; 
         FIG. 2  is a partial section through the high pressure fuel pump from  FIG. 1 , with a first embodiment of a pressure relief valve and a throttle device; 
         FIG. 3  is an enlarged detailed depiction of a region of the high pressure fuel pump from  FIG. 2 ; 
         FIG. 4  shows a detail IV from  FIG. 3 ; 
         FIG. 5  is a depiction similar to  FIG. 3  of a second embodiment; 
         FIG. 6  is a depiction similar to  FIG. 5  with the pressure relief valve open; 
         FIG. 7  is a depiction similar to  FIG. 5  of a third embodiment; 
         FIG. 8  is a section along the line VIII-VIII from  FIG. 7 ; 
         FIG. 9  is a depiction similar to  FIG. 7  of a fourth embodiment; 
         FIG. 10  is a section along the line X-X from  FIG. 9 ; 
         FIG. 11  is a depiction similar to  FIG. 7  of a fifth embodiment; 
         FIG. 12  is a depiction similar to  FIG. 7  of a sixth embodiment; 
         FIG. 13  is a depiction similar to  FIG. 7  of a seventh embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIG. 1 , a fuel system is labeled as a whole with the reference numeral  10 . The fuel system  10 , which is depicted only in simplified fashion in  FIG. 1  includes a fuel receptacle  12  from which a presupply pump  13  delivers fuel into a low pressure fuel line  14 . This line leads to a high pressure fuel pump  16  that compresses the fuel further and delivers it to a fuel accumulator  18  in which the fuel is stored at high pressure and which is also referred to as a “rail.” The rail  18  is connected to a plurality of injectors  20  that inject the fuel directly into associated combustion chambers (not shown) of an internal combustion engine to which the fuel system  10  belongs. 
     It is clear from  FIG. 2 , the high pressure fuel pump  16  has a housing  22  with a low pressure inlet  24  and a high pressure outlet  26 . The low pressure inlet  24  has an inlet conduit  28  leading from it to an inlet valve  30  (not visible in  FIG. 2 ) and onward to a delivery chamber  32  that is delimited by a pump piston  34 . An outlet conduit  36  leads via an outlet valve  38  to the high pressure outlet  26 . The inlet valve  30  is integrated into a quantity control valve  40  that is able to forcibly connect the delivery chamber  32  to the region of the inlet conduit  28  situated upstream of inlet valve  30 . In this way, it is possible to convey fuel back to the low pressure inlet  24  during a delivery stroke and thus to adjust the delivery quantity of the high pressure fuel pump  16 . 
     A pressure relief valve  42  is situated fluidically parallel to the outlet valve  38 . This pressure relief valve is depicted in greater detail in  FIG. 3 : it includes a valve seat body  44 , which is situated in an overflow conduit  46  leading from the high pressure outlet  26  to the delivery chamber  32  and has a press-fitted fastening region  48 . Toward the delivery chamber  32 , the outer diameter of the valve seat body  44  tapers to form a valve seat region  50 . The outer contour of the valve seat body  44  in this region can also be described as resembling a bottleneck. This prevents this valve seat region  50  from being deformed as the valve seat body  44  is being press-fitted into the overflow conduit  46 . 
     The valve seat body  44  has an inlet conduit  52  passing through it in the longitudinal direction, which is embodied in the form of a stepped bore whose inner diameter in the valve seat region  50  is smaller than in the fastening region  48 . The actual valve seat  54  for a valve element  56  embodied in the form of a valve ball is machined into the end of the inlet conduit  52  to the right in  FIGS. 3 and 4 . The valve seat  54  is conically embodied, with a cone angle of approximately 30° in the present instance. The half cone angle is indicated in  FIG. 4  by an arrow labeled with the reference numeral  58 . In principle, the cone angle should be between approximately 30° and 50°, a smaller cone angle having advantages with regard to the seal. The contact point of the valve element  56  with the valve seat  54  is linear, with a diameter d 1 . The diameter d 2  of the inlet conduit  52  is smaller than the diameter d 1 . In this way, a free cross sectional area F d2  of the inlet conduit  52 , which is situated toward the high pressure connection  26  in relation to the valve seat  54  and therefore on the high pressure side of it and is also situated immediately adjacent to the valve element  56 , is at least approximately 0.8 to 0.95 times the cross sectional area F d1  that is defined by the valve seat diameter d 1  at the valve seat  54 . 
     The valve element  56  is acted on in the direction toward the valve seat  54  by a valve element holder  60  that is in turn engaged by a valve spring  62 . An insertion depth of the valve element  56  into the inlet conduit  52  of the valve seat body  44  is labeled T in  FIG. 3 . 
     Toward the high pressure outlet  26  in relation to the pressure relief valve  42  and its valve seat  54 , i.e. on the high pressure side of the pressure relief valve  42 , a throttle device  64  is press-fitted into the overflow conduit  46 . In the embodiment shown in  FIGS. 2 through 4 , this throttle device  64  is embodied as a cup-shaped part  65  that is separate from the pressure relief valve  42 ; it has a bottom region  66  and a circumferential wall region  68  extending approximately perpendicular to this bottom region. For example, the part  65  can be manufactured as a formed and stamped sheet metal part. In the bottom section  66 , an opening is provided  70 , which has a diameter D 1  and constitutes a flow throttle. In the present exemplary embodiment, the free cross sectional area F D1  on the basis of the diameter D1 of the flow throttle  70  is 0.6 times the cross sectional area F d1  on the basis of the diameter d 1  of the valve seat  54  of the pressure relief valve  42 . In principle, however, values of between 0.6 and 1.1 times the latter are also conceivable. 
     The high pressure fuel pump  16  functions as follows: during an intake stroke of the pump piston  34 , the inlet valve  30  opens and fuel flows out of the low pressure fuel line  14  into the delivery chamber  32 . During a subsequent delivery stroke, the fuel enclosed in the delivery chamber  32  is compressed until finally, the outlet valve  38  opens and the fuel is pressed into the rail  18  at high pressure. if an excessively high pressure is built up in the rail  18  and therefore also in the region of the high pressure outlet  26 , then the valve element  56 , due to the pressure difference then prevailing, lifts away from the valve seat  54  during an intake stroke of the pump piston  34  and in opposition to the force of the valve spring  62 . In this way, filet can flow out of the rail  18  and the high pressure outlet  26 , through the overflow conduit  46  and the pressure relief valve  42 , and into the delivery chamber  32 . This relieves the pressure in the rail  18  and the high pressure outlet  26 . 
       FIGS. 5 and 6  show an alternative embodiment. In this case and in the embodiments that follow, elements and regions that have functions equivalent to those of elements and regions described above are provided with the same reference numerals and are not explained again in detail. 
     In the embodiment of a high pressure fuel pump  16  shown in  FIGS. 5 and 6 , the throttle device  64  is not embodied as a separate part, but is instead integrated into the valve seat body  44  of the pressure relief valve  42 , in the form of a constriction  70  situated on the high pressure side of and very near or immediately adjacent to the valve seat  54 . In this instance, its free cross sectional area F D1  in relation to its diameter D 1 , is approximately 0.5 times the cross sectional area F d1  of the valve seat  54  of the pressure relief valve  42  in relation to the diameter d 1 . 
     In both the embodiment according to  FIGS. 2 through 4  and the embodiment according to  FIGS. 5 and 6 , the free cross section of the flow throttle  70  is designed so that when the pressure relief valve  42  is open, i.e. when the valve element  56  has lifted away from the valve seat  54  (see  FIG. 6 ), this free cross section of the flow throttle at most corresponds approximately to the annular opening cross section F R  then produced by the gap  72  between the valve element  56  and the valve seat  54 . This assures that the stroke H of the valve element  56  thus occurring is smaller than the insertion depth T, thus preventing the possibility of the valve element  56  becoming jammed between the valve seat body  44  and the valve element holder  60 . 
       FIG. 7  shows a region of another alternative embodiment of a high pressure fuel pump  16 . With regard to the embodiment of the flow throttle  70 , this pump corresponds to the one in the embodiment shown in  FIGS. 5 and 6 . In addition, however, the valve seat body  44  of the pressure relief valve  42  has an annular collar  76 , which constitutes a securing section for the valve element  56 , extending in the opening direction (arrow  74 ) of the valve element  56 , i.e. in the axial direction of the pressure relief valve  42 . The collar  76  here has a radial outside  78  with which it rests against the inside of the overflow conduit  46 . A radial inside  80  of the collar  76  leads from a radially extending shoulder  82  to the protruding end of the collar  76 . The shoulder  82  here extends in the radial direction starting approximately from the valve seat  54 , i.e. is adjacent to the latter. 
     In the embodiment shown in  FIG. 7 , the valve element holder  60  is embodied as piston-like, with an annular flange  84  situated approximately in its axial middle, against which the valve spring  62  rests. In a fashion similar to the embodiments shown in  FIGS. 3 ,  5  and  6 , a peg-like section  86  of the valve element holder  60  leading from the annular flange  84  extends into the (unnumbered) annular chamber delimited by the valve spring  62 . A region  88  of the peg-like section  86  situated close to the annular flange  84  has an outer diameter that is only negligibly smaller than the inner diameter of the valve spring  62 . The valve element holder  60  is thus held against the valve spring  62  in a fashion that prevents tilting. 
     On the opposite side of the annular flange  84 , a holding section  90  extends from the flange to the valve element  56 . In the embodiment shown in  FIG. 7 , the holding section  90  has a cylindrical outer contour with a diameter that remains the same over its entire length. A blind hole (unnumbered) serves to radially secure the valve element  56  to the valve element holder  60 . The outer diameter of the holding section  90  is selected so that the holding section  90  is still spaced slightly apart from the radial inside  80  of the collar  76  in the closed position of the pressure relief valve  42  depicted in  FIG. 7 . This prevents the holding section  90  from striking against the collar  76  before the valve element  56  has come to rest completely against the valve seat  54 . 
     The length of the collar  76  and of the holding section  90  are, however, matched to each other so that both when the pressure relief valve  42  is closed and when it is open, the holding section  90  of the valve element holder  60  protrudes into the interior of the collar  76  delimited by the radial inside  80 . In this way, the collar  76  assures that even in the event of dynamic pressure impacts and the resulting large opening strokes of the valve element  56 , the valve element is not able to come out of the chamber delimited by the collar  76  and instead is able to reliably find its way back to the valve seat  54  again when the pressure relief valve  42  closes. 
     In order to assure as unhindered as possible an outflow of the fluid to the delivery chamber  32  when the valve element  56  has lifted away from the valve seat  54 , three flow pockets  92  distributed around the circumference of the collar  76  are provided on the radial inside  80  of the collar  76 . Starting from the shoulder  82 , these pockets extend the entire length of the collar  76  to its protruding end and have a semicircular edge contour. This is particularly visible in  FIG. 8 . 
     An alternative embodiment shown in  FIGS. 9 and 10  differs from the one in  FIGS. 7 and 8  in that in lieu of the flow pockets in the collar/securing section  76 , slots  94  are provided that extend over its entire thickness, likewise extending from the shoulder  82  over the entire length of the collar  76  to its protruding end. 
       FIG. 11  shows another variant: in this case, the radial inside  80  of the collar  76  is embodied in the form of a conical surface that widens out in the opening direction  74  of the pressure relief valve  42 . The holding section  90  of the valve element holder  60  is embodied in a similarly conical fashion, but with a smaller cone angle than the radial inside  80  of the collar  76 . An opening motion of the valve element  56  and the valve element holder  60  in the opening direction  74  produces an increasing distance between these elements on the one hand and the radial inside  80  of the collar  76  on the other, through which the fluid can flow out to the delivery chamber  32 . The conical surface here can have approximately the same cone angle as the valve seat  54  (see  FIG. 4  in particular) or a larger cone angle than the valve seat  54 . 
     In the embodiment shown in  FIG. 11 , the valve seat  54  transitions directly into the radial inside  80 . hi the embodiment shown in  FIG. 12 , however, the valve seat  54  is first adjoined by a shoulder  82  that extends in the radial direction and the conical surface of the radial inside  80  of the collar  76  starts only after this shoulder. Here, too, the shoulder  82  eliminates or at least reduces a force acting on the valve element  56  in the closing direction when the valve element  56  is open. 
     An additional variant to  FIG. 12  is shown in  FIG. 13 , in which the cone angle of the conical surface that constitutes the radial inside  80  of the collar  76  is relatively steep and the holding section  90  is embodied as cylindrical, with a uniform diameter. This variant has the advantage that when the pressure relief valve  42  is open, the outflow behavior is largely independent of the opening stroke of the valve element  56 . 
     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.