Abstract:
A fuel pump for an internal combustion engine, comprising a housing ( 14 ) in which a piston ( 16 ) is guided. A working chamber is confined in areas by the piston ( 16 ). An eccentric shaft or camshaft acts on the piston ( 16 ) on its end furthest from the working chamber. A prestressing element ( 30 ) loads the piston ( 16 ) against the eccentric shaft or camshaft. In order to reduce the fabrication costs of the fuel pump, the invention provides that a support member ( 40 ) that is separate from the piston ( 16 ) is provided, which said support member is interconnected with the end region ( 36 ) of the piston ( 16 ) closest to the eccentric shaft or camshaft, and against which the prestressing element ( 30 ) bears.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention relates to a fuel pump for an internal combustion engine, with a housing, with at least one piston, with a working chamber that is confined in areas by the piston, with an eccentric shaft or camshaft that acts on the piston, at least indirectly, on its end furthest from the working chamber, and with a prestressing element that loads the piston, at least indirectly, against the eccentric shaft or camshaft.  
           [0002]    A fuel pump of this nature is known from the marketplace, as a radial-piston pump, for example. In the case of said fuel pump, a central eccentric shaft is supported in a housing. A cam ring is mounted on an eccentric section of the eccentric shaft. Flat surfaces are formed around the circumference of the cam ring, against which slippers rest. The slippers are interconnected with a cylindrical piston; the end of said cylindrical piston that is furthest from the eccentric shaft confines a working chamber. The piston is loaded by a compression spring against the slipper or against the flat surface of the cam ring.  
           [0003]    Three cylinders are provided in the known fuel pump, each comprising one corresponding piston and one corresponding working chamber. When the eccentric shaft moves, the piston is set to moving to and fro, and the fuel present in the working chamber is compressed and ejected via appropriate valve devices, e.g., into a fuel manifold (“rail”) of an internal combustion engine.  
           [0004]    In the case of the fuel pump known from the marketplace, the compression spring bears against an end section of the piston that has a markedly greater diameter than the shaft of the piston. The end section and the piston shaft are a single piece and are worked out from a whole unit on a lathe, for example. To guide the compression spring and reduce tension in the transition regions between the piston shaft and the end section, it is necessary to provide grooves and shoulders with different diameters. In combination with the associated great differences in diameter, in particular, this makes costly chip-removal machining necessary, which has a disadvantageous effect on the costs to fabricate the fuel pump.  
           [0005]    The object of the present invention, therefore, is to further develop a fuel pump of the type stated initially in such a way that it can be fabricated less expensively.  
           [0006]    The object is attained with a fuel pump of the type stated initially by providing a support member that is separate from the piston, which said support member is interconnected with the end region of the piston closest to the eccentric shaft or camshaft, and against which the prestressing element bears.  
         ADVANTAGES OF THE INVENTION  
         [0007]    In the case of the fuel pump according to the invention, the piston of the fuel pump has a markedly simpler configuration. In the simplest case, it can be composed of a simple straight cylinder that has no changes in diameter. This eliminates the need for costly chip-removal machining of the piston, which greatly reduces the costs of the fuel pump according to the invention.  
           [0008]    The support of the prestressing element, usually a compression spring that loads the piston against the eccentric shaft or camshaft, takes place in the case of the fuel pump according to the invention via a separate support member that is securely interconnected with the piston. The connection of the support member with the piston can take place in different ways, including, for example, by means of a press fit, a shrinkage fit, welding, bonding, and other fastening variants described in detail hereinbelow. Although an additional working step is required to secure the support member on the piston, it is more cost-effective than the chip-removal machining of the piston described hereinabove.  
           [0009]    Advantageous further developments of the fuel pump according to the invention are stated in the dependent claims.  
           [0010]    The invention provides, first of all, that the support member comprises a support ring. A support ring of this type is very easy to fabricate, which is another favorable factor in terms of the cost of the fuel pump.  
           [0011]    The invention thereby provides that the support member is two-pieced and comprises a support ring interconnected with the end region of the piston and an intermediate element pushed onto the piston between prestressing element and support ring. This makes it possible to select a material for the support ring that can be secured on the piston in optimal fashion. For the intermediate element, on the other hand, a material can be selected that can be formed in a simple manner in such a way that the prestressing element is retained and guided in optimal fashion. This further development therefore increases the operational reliability of the fuel pump according to the invention as well.  
           [0012]    In another further development, the invention provides that one section of the support member abuts a retainer that is accommodated, in areas, in a groove in the end region of the piston closest to the eccentric shaft or camshaft. This method of securing is very simple, and it can be undone by destroying the retainer, for example.  
           [0013]    The invention also provides that the fuel pump comprises a guide sleeve capable of moving with the piston, which said guide sleeve cooperates with a guide section secured to the housing. By way of this, the fact that lateral forces must be dissipated into the pump housing during operation is taken into account.  
           [0014]    Although these lateral forces can also be absorbed in the housing via the guiding of the piston, it is more favorable in terms of sealing if the piston is loaded in the axial direction only. This is made possible by means of the guide sleeve according to the invention, since it dissipates the lateral forces from the eccentric shaft or camshaft directly into the guide section secured in the housing while bypassing the piston. A fuel pump having a configuration of this type therefore functions with high efficiency.  
           [0015]    In particular, a fuel pump of this type can be fabricated cost-effectively when the support member is integral with the guide sleeve.  
           [0016]    The support member can also comprise a radially inwardly directed annular collar that rests against a radially outwardly directed annular collar of the piston. The support element is therefore pressed by the prestressing element with its annular collar against the corresponding annular collar of the piston. The connection of the support member with the piston created as a result enables the piston to return reliably after a compression stroke without lateral forces being introduced into the piston by the support element. This is particularly advantageous when the support member is integral with the guide sleeve.  
           [0017]    Another advantageous embodiment of the fuel pump according to the invention provides that the fuel pump comprises a base part that abuts the end surface of the piston closest to the eccentric shaft or camshaft. A base part of this type also makes it possible to develop the piston out of a material that is optimal for its function as compression plunger. On the other hand, it is possible to design the base part out of a material that can absorb the relative motions produced by the rotation of the camshaft or eccentric shaft relative to the piston without incurring excess wear. Furthermore, appropriate material pairs can be used in order to also reduce the frictional forces between the base part and the eccentric shaft or camshaft, which results in lower lateral forces. This further development therefore has additional advantages in terms of the function of the fuel pump according to the invention.  
           [0018]    It is also preferred if the guide sleeve comprises a radially inwardly directed annular collar that extends into an annular space located between support member and base part. By way of this, the guide sleeve is held securely in the axial direction, and, simultaneously, assembly of the fuel pump according to the invention is simple and, therefore, cost-effective.  
           [0019]    It is also advantageous if the base part comprises an upwardly extending section that covers the support member axially in areas, and the support member is securely interconnected with the base part via the upwardly extending section. A base part of this type is therefore designed in the shape of a bucket and is automatically centered relative to the support member during assembly by means of the upwardly extending section. The base part can be secured on the support member via a retainer, for example, that is inserted in the lateral surface of the support member, on the one hand and, on the other, in the lateral surface of the upwardly extending section.  
           [0020]    An advantageous further development of the fuel pump according to the invention is also unique in that the support member comprises at least one bent-over section that grips around the outside of the base part and provides axial support for said base part. A bent-over section of this type is easy to fabricate and simplifies overall assembly of the fuel pump, since the base part is captively interconnected with the support member. 
       
    
    
     SUMMARY OF THE DRAWINGS  
       [0021]    Particularly preferred exemplary embodiments of the present invention are described in detail hereinbelow with reference to the attached drawings.  
         [0022]    [0022]FIG. 1 is a sectional drawing through a region of a first exemplary embodiment of a radial-piston fuel pump;  
         [0023]    [0023]FIG. 2 is a partial sectional drawing through a detail of the fuel pump in FIG. 1;  
         [0024]    [0024]FIG. 3 is a partial sectional drawing through components of a second exemplary embodiment of a fuel pump;  
         [0025]    [0025]FIG. 4 is a partial sectional drawing through components of a third exemplary embodiment of a fuel pump;  
         [0026]    [0026]FIG. 5 is a partial sectional drawing through components of a fourth exemplary embodiment of a fuel pump;  
         [0027]    [0027]FIG. 6 is a partial sectional drawing through components of a fifth exemplary embodiment of a fuel pump;  
         [0028]    [0028]FIG. 7 is an enlarged detail of FIG. 6; and  
         [0029]    [0029]FIG. 8 is a sectional drawing through components of a sixth exemplary embodiment of a fuel pump. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    In FIG. 1, a fuel pump as a whole is labelled with reference numeral  10 . Said fuel pump is a radial-piston pump having three cylinders  12   a ,  12   b  and  12   c . Only the components of cylinder  12   a  will be described in detail hereinbelow. In the drawings, only the components of cylinder  12   a  are labelled with reference numerals. The components of cylinders  12   b  and  12   c  are identical to those of cylinder  12   a.    
         [0031]    The radial-piston pump  10  comprises a housing  14 . A piston  16  is accommodated in a bore (not labelled with a reference numeral) in axially displaceable fashion. The piston  16 , with its end surface shown at the top in FIG. 1, confines a working chamber  18 . An intake valve  20  can connect the working chamber  18  with a not-shown fuel line. A not-shown outlet valve can connect the working chamber  18  with a fuel line and, further, with a high-pressure manifold (“rail”).  
         [0032]    An eccentric shaft with an eccentric section  22  is supported in the housing  14  of the radial-piston pump  10 . A cam ring  24  is mounted on the eccentric section  22 , which said cam ring comprises a machined-flat contact area  26  in the region of each of the individual cylinders  12   a - 12   c . A base part  28  configured as slipper is loaded—indirectly and in a manner to be described in greater detail—by a compression spring  30  against the contact area  26 . The compression spring  30  is accommodated in an annular space  32  in the housing  14 . Said annular space is confined radially inwardly by a guide section  34  designed in the shape of a tubular section. In turn, the piston  16  is accommodated in gliding and liquid-tight fashion in said guide section. The connection of the piston  16  with the base part  28  and the piston-side support of the compression spring  30  will now be described in detail in conjunction with FIG. 2:  
         [0033]    The piston  16  comprises an end region  36  facing the eccentric section  22  and the cam ring  24  and the cam ring  24 , which said end region has a larger diameter than the rest of the piston  16 . The end region  36  is accommodated in areas in a complementary recess  38  in the base part  28 . A support member  40  designed in the shape of a washer abuts a projection  41  formed between the piston shaft  39  and the end region  36  of the piston  16 . The support element  40  comprises a radially inwardly extending holding section  42  and an axially extending guide section  44 . The support member  40  is centered relative to the longitudinal axis of the piston  16  by these two sections  42  and  44 .  
         [0034]    In its radially outward region, the support member  40  comprises a groove-like recess  46  extending in the circumferential direction that is “open” in the axial direction, in which said recess the lower end—as shown in FIG. 2—Of the compression spring  30 —is accommodated. On its radially outward edge, the support member  40  comprises a plurality of hook-like bent-over sections  48  distributed around its circumference that grip around—with some play—a projection  50  formed on the outer lateral surface of the base part  28 . In this manner, the base part  28  is held axially against the piston  16 .  
         [0035]    The radial-piston pump  10  functions as follows: When the eccentric shaft with the eccentric section  22  rotates, the center of the cam ring  24  moves along a circular path. As a result of this, the contact areas  26  of the cam ring  24  move in the axial direction of the respective cylinder  12  on the one hand and, on the other, laterally relative to the longitudinal axis of the respective cylinder  12 . As a result of the axial movement of the contact areas  26  and the return force of the compression spring  30 , the piston  16  is set into an axial to-and-fro motion via the base part  28 .  
         [0036]    As a result of this, fuel is either drawn into the working chamber  18 , or the fuel present in the working chamber  18  is compressed and ejected in the direction of the fuel manifold.  
         [0037]    A variation of the region of the piston  16  facing the eccentric section  22  is shown in FIG. 3. The elements and areas having functions equivalent to the embodiment shown in FIGS. 1 and 2 are labelled with the same reference numerals. They are not described in detail again.  
         [0038]    In contrast to FIGS. 1 and 2, the diameter of the end region  36  of the piston  16  shown in FIG. 3 is not different from the diameter of the shaft  39  of the piston  16 . The piston  16  is therefore even easier to fabricate. Furthermore, the support member  40  is configured as two pieces. It comprises a support ring  52  that is pressed onto the piston  16 . In this fashion, the support ring  52  is secured on the piston  16  in an axially non-displaceable fashion. An intermediate element  54  is pushed onto the piston  16  from the top down as viewed in FIG. 3, which said intermediate element is centered relative to the support ring  52  with a holding section  42  and a guide section  44 .  
         [0039]    In FIGS. 2 and 3, a certain amount of play exists between the support element and the base part  28 . In other, not-shown exemplary embodiments, the base part is interconnected with the piston  16  without play.  
         [0040]    As a result of the above-described movement of the contact areas  26  laterally relative to the longitudinal axis of the respective cylinder  12 , lateral forces are also introduced into the base part  28  by the contact areas  26  due to the friction that is present. In the case of the radial-piston pump  10  described hereinabove (FIGS. 1 and 2), and in the case of the embodiment of the components according to FIG. 3, these lateral forces are introduced into the piston  16  and dissipated by said piston into the guide section  34  of the housing  14 . In the exemplary embodiments described hereinbelow according to FIGS.  4 - 8 , possibilities for holding the piston  16  in a manner that is free of transverse forces are presented. In this case as well, components and parts that have functions that are equivalent to those of previously-described components and parts have the same reference numerals and shall not be described in detail again.  
         [0041]    In FIGS. 4 and 5, exemplary embodiments having a “bucket guide” are presented. Said bucket guide comprises a guide part  56  configured in the shape of a bucket that rests via the outside of a base  58  against the contact area  26  of the cam ring  24 . The end region  36  of the piston  16  rests against the inside of the base  58  of the bucket-shaped guide part  56 . A circumferential wall  60  of the bucket-shaped guide part  56  is guided via its outer side into a guide section  62  of the housing  14 . The lateral forces introduced into the base  58  of the bucket-shaped guide part  56  by the contact area  26  when the radial-piston pump  10  operates are dissipated directly into the guide section  62  of the housing  14  via the circumferential wall  60 . This enables the piston  16  to remain free of transverse forces.  
         [0042]    The support member in FIG. 4 is configured as a single-component support ring  40  that is pressed onto the piston  16 . There are no changes in diameter on the end region  36  of the piston  16 . In the case of the exemplary embodiment according to FIG. 5, the support member  40  is configured with two components once more, namely a support ring  52  that is pressed onto the piston  16 , and an intermediate element  54 .  
         [0043]    In the case of the exemplary embodiment presented in FIGS. 6 and 7, the end region  36  of the piston  16  once more has a greater diameter than the shaft  39  of the piston  16 , thereby forming a radially outwardly directed annular collar  64 .  
         [0044]    This annular collar  64  is guided axially, on the one hand, between a radially inwardly directed annular collar  66  of a guide sleeve  68  and a base part  28  bearing against the end surface of the piston  16 .  
         [0045]    On its radially outward edge, the base part  28  comprises a circumferential, upwardly extending section  70  that covers the guide sleeve  68  in areas in the axial direction. As shown in FIG. 7, a circumferential groove  72  is provided on the inner side of the upwardly extending section  70  on the one hand and, on the other, a circumferential groove  74  is provided on the outer side of the guide sleeve  68 . A retainer  76  is accommodated in groove  72  on one side and in groove  74  on the other side, so that the guide sleeve  68  and the base part  28  are securely interconnected.  
         [0046]    The guide sleeve  68  is guided in sliding fashion in the axial direction on a guide section  62  of the housing  14 . This permits transverse forces—that are introduced into the base part  28  via the contact area  26  of the cam ring  24 —to be introduced into the guide section  62  of the housing  14  via the guide sleeve  68 . As shown in FIG. 6, the guide section  62  is part of a bushing  14   a  that is shrink-fit into a housing region  14   b . The guide sleeve  68  comprises a radially outwardly directed annular collar  44  against which the compression spring  30  bears. In this manner, the piston  16  is loaded indirectly with the return force of the compression spring  30 . The guide sleeve  68  therefore simultaneously forms the support element  40 .  
         [0047]    Yet another exemplary embodiment is presented in FIG. 8. In this exemplary embodiment, the end region  36  of the piston  16  once more has the same diameter as the shaft  39  of the piston  16 . The support element  40  is configured as support ring, the holding section  42  of which abuts a retainer  78  that lies in a circumferential groove (not labelled with a reference numeral) in the end region  36  of the piston  16 . In this case as well, transverse forces are kept away from the piston  16  by the fact that a guide sleeve  68  cooperates with a guide section  62  of the housing  14 .  
         [0048]    The base part  28  comprises a section  80 —shown at the top in FIG. 8—having a smaller diameter, and a section  82 —shown at the bottom in FIG. 8—having a larger diameter. The top side of section  80  of the base part  28  bears against the end face—facing said section  80 —Of the piston  16 . An annular space  84  is provided between the section  82  of the base part  28  and the support member  40 , in which said annular space a radially inwardly directed annular collar  66  of the guide sleeve  68  extends. Furthermore, the radially outward edge of section  82  of the base part  28  bears against the inner wall of the guide sleeve  68 . In this manner, transverse forces are once again kept away from the piston  16 . A roller  86  is also provided between the base part  28  and the cam ring  24  not shown in FIG. 8. Said roller minimizes the transverse forces as well.