Abstract:
A fuel pump for use in a fuel injection system includes a housing at least one piston defining a work chamber and drive means which put the piston into a reciprocating motion in the chamber. An inlet conduit and an outlet conduit can be made to communicate with the work chamber. A first valve device is provided between the inlet conduit and the work chamber, and a second valve device is provided between the work chamber and the outlet conduit. The valve element of one valve device has a guide portion, which is received in a guide opening embodied in the valve element of the other valve device, and the circumferential face of the guide portion and/or of the guide opening has at least one recess, by which the contact area between the guide portion and the guide opening is reduced.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates first to a fuel pump, in particular for an internal combustion engine with direct injection, having a housing, having at least one piston which is received in the housing, having drive means, which put the piston into a reciprocating motion, having a work chamber which is defined in some regions by the piston, having an inlet conduit and having an outlet conduit, which can be made to communicate with the work chamber, having a first valve device between the work chamber and the inlet conduit and a second valve device between the work chamber and the outlet conduit, wherein the valve element of one valve device has a guide portion, which is received at least in some regions in a guide opening.  
           [0003]    2. Description of the Prior Art  
           [0004]    A fuel pump known on the market serves as a high-pressure fuel pump for Diesel engines of motor vehicles. The fuel is pumped at high pressure by the high-pressure fuel pump into a fuel collection line (“rail”), in which it is stored under high pressure. From the rail, the fuel reaches injection valves, which inject directly into the combustion chambers of the engine.  
           [0005]    In the known fuel pump, the two valve devices are accommodated in a compact unit. The valve element of the valve device (“inlet valve”) between the work chamber and the inlet conduit is braced on the valve element of the valve device (“outlet valve”) between the work chamber and the outlet conduit. The valve element of the inlet valve is also guided, via a cylindrical guide peg, in a guide opening of the valve element of the outlet valve.  
           [0006]    In the known engine, it has been found that in operation, pressure surges repeatedly occur in the inlet conduit of the fuel pump and the components located downstream of it. These pressure surges reduce the efficiency of the fuel pump. The valves are also complicated to manufacture. Moreover, regulation is difficult because of the pressure fluctuations.  
           [0007]    Furthermore, in internal combustion engines there is the fundamental necessity of being able to suppress the pumping of fuel into the rail completely (“zero feeding”). Since a metering unit also used for the purpose, disposed upstream of the high-pressure fuel pump, even in the closed state always allows a certain leakage quantity of fuel to reach the high-pressure fuel pump, so-called zero-feed throttles are used between the metering unit and the high-pressure fuel pump, which are intended to return the leak fuel emerging from the outlet of the metering unit. By means of these zero-feed throttles, however, the starting performance of the engine is adversely affected. Without such zero-feed throttles, on the other hand, even with the metering unit completely closed, fuel would continue to be pumped from the high-pressure fuel pump to the rail.  
         OBJECT AND SUMMARY OF THE INVENTION  
         [0008]    The present invention has the object of refining a fuel pump of the type defined at the outset in such a way that the engine in which it used can be manufactured more economically and has better starting performance, and in which a secure shutoff of fuel pumping is assured.  
           [0009]    In a fuel pump of the type defined at the outset, this object is attained in that the guide opening is embodied in the valve element of the other valve device, and the circumferential face of the guide portion and/or of the guide opening has at least one recess, by means of which the contact area between the guide portion and the guide opening is reduced.  
           [0010]    In the fuel pump of the invention, in the inlet conduit and upstream of it, pressure pulses originating in the fuel pump no longer occur. Thus valves that can be made more economically can be used.  
           [0011]    The starting performance of an engine which is equipped with the fuel pump of the invention is also improved considerably, since the zero-feed throttles can be markedly smaller than before. In the fuel pump of the invention, specifically, even if a metering unit disposed upstream of the fuel pump allows a certain leakage quantity of fuel to reach the fuel pump, still no fuel is pumped.  
           [0012]    The reason for both provisions is that the functions of the inlet valve and the outlet valve in the fuel pump of the invention are decoupled from one another. In the known fuel pump, it has in fact been found that whenever the outlet valve opens during a pumping stroke, the effects of friction and a delayed pressure buildup in the receiving opening of the valve element of the outlet valve causes the inlet valve to open briefly. This leads to the pressure surge or pressure pulse in the inlet region of the fuel pump, which is avoided in the fuel pump of the invention.  
           [0013]    In a multi-cylinder fuel pump whose inlet conduits communicate with one another, this pressure pulse tripped during a pumping stroke of one cylinder causes opening of the valve element of the inlet valve at the other cylinder, which at that instant is in the intake phase. Thus during the intake phase of this cylinder, fuel reaches the work chamber, and during the ensuing pumping stroke it is transported onward to the rail.  
           [0014]    This is avoided in the fuel pump of the invention, since because of the reduced friction between the valve elements of the inlet and outlet valves, the aforementioned slaving effect cannot occur. The inlet valve of a cylinder that at that instant is in the pumping phase remains reliably closed.  
           [0015]    The decoupling of the valve element of the inlet valve from the valve element of the outlet valve is attained by reducing the contact area between the two elements. As a result, the friction between two elements is reduced, which finally means that the valve element of the outlet valve is not slaved upon a motion of the valve element of the inlet valve. According to the invention, a “mechanical” decoupling is accordingly created.  
           [0016]    In an advantageous feature of this fuel pump, it is proposed that it has a connecting conduit, which connects the guide opening with the work chamber. A connecting conduit of this kind prevents a pressure drop, upon a motion of the valve element of the outlet valve, in the guide opening, which could also provoke a motion of the valve element of the inlet valve, from occurring. As a result of this connecting conduit, the harmful pressure pulses are prevented even better. According to the invention, a “hydraulic” decoupling is accordingly created.  
           [0017]    As an alternative, in a fuel pump of the type defined at the outset, it is possible for the guide portion to be embodied on the valve element of the first valve device and the guide opening to be embodied in the inlet conduit. As a result, the two valve elements are completely decoupled from one another, so that an opening motion of the valve element of the inlet valve provoked by the opening motion of the valve element of the outlet valve is precluded. In contrast to the above two embodiments of the invention, however, the valve element of the outlet valve must be made somewhat shorter under some circumstances, so as to assure the requisite spacing between the two valve elements upon an opening motion of the valve element of the inlet valve.  
           [0018]    In a first refinement, the circumferential face of the guide portion and/or of the guide opening has at least one longitudinally extending recess, by which the contact area between the guide portion and the guide opening is reduced and which acts as a flow conduit when the valve device is open. In this refinement, the complete decoupling of the two valve elements and especially low-friction guidance of the valve element of the inlet valve are thus combined with one another.  
           [0019]    It is possible for there to be a plurality of recesses. The more recesses there are, the smaller is the contact area between the guide portion and the guide opening, which finally leads to a reduction in the frictional forces. Furthermore, the flow in the region of the valves is improved by a plurality of recesses.  
           [0020]    It is especially preferred if the recesses are embodied such that between the guide portion and the guide opening, only an essentially linear contact exists. In this case, the frictional forces between the guide portion and the guide opening are minimal. However, care must be taken to assure that the contact areas are still large enough that excessive wear does not occur.  
           [0021]    In an especially preferred feature of the fuel pump of the invention, it is proposed that the guide portion is embodied such that it includes at least three radially extending vanes, which are preferably distributed over the circumference. With such vanes, on the one hand secure guidance of the valve element is possible, and on the other, the recesses or openings between the vanes make a largely unhindered flow of the fuel possible. Such a fuel pump thus operates at high efficiency.  
           [0022]    The same is true for a fuel pump in which the guide opening is embodied such that it includes at least three radially extending vanes, which are preferably distributed over the circumference.  
           [0023]    It is advantageous if the vanes are ground hollow. This increases the stability of the valve element and creates a larger flow cross section.  
           [0024]    It is also proposed that the valve element of the first valve device is braced on the valve element of the second valve device via a clamping element in such a way that it is pressed against the associated seat. Such a fuel pump is very compact in structure.  
           [0025]    The engagement of the clamping element with the first valve element is facilitated by the provision that the first valve element includes a plate-shaped support portion, preferably a disk, on which the clamping element is braced. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description, taken in conjunction with the drawings, in which:  
         [0027]    [0027]FIG. 1 is a schematic illustration of a fuel system embodying the invention in a direct-injection internal combustion engine with a high-pressure fuel pump;  
         [0028]    [0028]FIG. 2 is a view, partly in section, of the high-pressure fuel pump of the fuel system of FIG. 1;  
         [0029]    [0029]FIG. 3 is a detail of the high-pressure fuel pump of FIG. 2, in which one inlet valve and one outlet valve are shown;  
         [0030]    [0030]FIG. 4 is a plan view of a valve element of the inlet valve of FIG. 3;  
         [0031]    [0031]FIG. 5 is a side view of the valve element of FIG. 4;  
         [0032]    [0032]FIG. 6 is a graph showing the pressure in an inlet conduit of the high-pressure fuel pump of FIG. 2 over time;  
         [0033]    [0033]FIG. 7 is a detail similar to FIG. 3 of an alternative exemplary embodiment of a high-pressure fuel pump; and  
         [0034]    [0034]FIG. 8 is a section through a region of a third exemplary embodiment of a high-pressure fuel pump. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]    In FIG. 1, a fuel system identified overall by the reference numeral  10  serves to supply an internal combustion engine  12  with fuel. The engine  12  is in this case a Diesel engine, but in principle the fuel system  10  shown can also be used for gasoline engines.  
         [0036]    The fuel system  10  includes a fuel tank  14 , from which a mechanical fuel pump  16 , embodied as a geared pump, pumps fuel via a filter  18 . From the fuel pump  16 , via a metering unit  20  and a fuel line  21 , the fuel reaches a high-pressure fuel pump  22 . From there, it is pumped onward via a fuel line  23  into a fuel collection line  24  (“rail”), in which the fuel is stored at high pressure.  
         [0037]    A plurality of injectors  26  are connected to the rail  24  and inject the fuel directly into combustion chambers  28 . From the fuel line  21 , a zero-feed line  30  in which a zero-feed throttle  32  is disposed branches off between the metering unit  20  and the high-pressure fuel pump  22 . The essential functions of the engine  12  are controlled and regulated by a open- and closed-loop control unit  34 . Thus the metering unit  20  is likewise connected to the open- and closed-loop control unit  34  and is triggered by it.  
         [0038]    The high-pressure fuel pump  22  is a 4-die high-pressure pump in a V arrangement (FIG. 2). This is used especially in fuel systems with a high fuel demand. In FIG. 2, the two cylinders of one cylinder plane can be seen. They are identified by the reference numerals  36   a  and  36   b . The cylinders  36   a  and  36   b  are part of a housing  38 . Pistons  40   a  and  40   b , respectively, are received in them. These pistons are set into a reciprocating motion by a camshaft  42 . The pistons  40   a  and  40   b  define respective work chambers  44   a  and  44   b.    
         [0039]    The work chambers  44   a  and  44   b  are bounded radially outward by valve blocks  46   a  and  46   b , respectively. The structure of these valve blocks is described in further detail hereinafter. The metering unit  20  is seated on the housing  38  between the cylinders  36   a  and  36   b . From the metering unit, inlet conduits  48   a  and  48   b  in the housing  38  lead to the valve blocks  46   a  and  46   b , respectively. Outlet conduits  50   a  and  50   b  are present in the respective valve blocks  46   a  and  46   b . They lead to the fuel line  23  and on to the rail  24 .  
         [0040]    The valve blocks  46   a  and  46   b  will now be described in conjunction with FIG. 3, taking one valve block  46  as an example. This valve block includes a cylindrical valve body  52 . In it, there is a valve chamber  54 , which communicates with the work chamber  44  via a connecting conduit  56 . A bore  57  that is coaxial with the axis of the valve body  52  leads, in the installed position, from the valve chamber  54  in the direction of the work chamber  44 . It forms a guide opening  58  for a guide portion  60  of a valve element  62 .  
         [0041]    A bevel (not identified by reference numeral) in the transition region between the guide opening  58  and the valve chamber  54  forms a valve seat for the valve element  62 . The valve seat and the valve element  62  together form an inlet valve  64 , through which fuel from the metering unit  20 , via the inlet conduit  48  with its portions  66  and  57  embodied in the valve body  52 , can reach the valve chamber  54  and beyond to the work chamber  44 .  
         [0042]    Opposite the guide opening  58 , a bore  68  extends from the valve chamber  54 ; a guide portion  70  of a valve element  72  is guided in it. A bevel (not identified by reference numeral) in the transition region between the bore  68  and the outside of the valve body  52  forms a valve seat for the valve element  72 . The valve seat and the valve element  72  together form an outlet valve  74 , by way of which the fuel from the work chamber  44 , via the connecting conduit  56 , the valve chamber  54 , and the outlet conduit  50 , can reach the fuel line  23  and beyond to the rail  24 .  
         [0043]    A blind bore  76  is made in the valve element  72  of the outlet valve  74 , toward the valve chamber  54 . A compression spring  80  is braced on the bottom  78  of this bore. The other end of the compression spring rests on a shoulder  82  of the valve element  62  of the inlet valve  64 . In this way, the valve element  62  of the inlet valve  64  is pressed against its valve seat. The valve element  72  of the outlet valve  74  is urged against its valve seat by a compression spring  84 .  
         [0044]    The guide portion  60  of the valve element  62  of the inlet valve  64  is embodied, as can be seen from FIGS. 4 and 5, in the form of vanes  86   a ,  86   b  and  86   c , which extend radially in a star pattern and are distributed over the circumference. On their radially outer ends, the vanes  86   a ,  86   b  and  86   c  are embodied such that a markedly reduced contact with the wall of the guide opening  58  in the valve body  52  results. Between the vanes  86   a ,  86   b  and  86   c , recesses  88   a ,  88   b  and  88   c , respectively, that are ground hollow are present.  
         [0045]    The fuel system  10  with the high-pressure fuel pump  22  functions as follows: The metering unit  20  is triggered by the open- and closed-loop control unit  34  in such a way that only the quantity of fuel that is injected by the injectors  26  into the combustion chambers  28  reaches the high-pressure fuel pump  22  and from there reaches the rail  24 . During the intake phase of a cylinder  36   a  and  36   b , the piston  40   a  and  40   b , respectively, moves radially inward, so that the pressure in the corresponding work chamber  44   a  and  44   b  drops. As a result, the pressure in the valve chamber  54  drops as well, and in turn causes the valve element  62  of the inlet valve  64  of the corresponding cylinder  36   a  and  36   b  to lift from its seat.  
         [0046]    Thus fuel can flow from the metering unit  20  into the work chambers  44   a  and  44   b . The reaction of the valve element  62  of the inlet valve  64  takes place quite spontaneously, since the friction between the guide portion  60  and the guide opening  58  is only very slight. At the same time, the valve element  62  is centered exactly in the guide opening  58  by the guide portion  60 , so that in the closed state, it reliably seals off the communication between the valve chamber  54  and the inlet conduit  48 . The recesses  88   a ,  88   b  and  88   c , when the inlet valve  64  is open, enable a largely unhindered inflow of the fuel to the work chamber  44   a  and  44   b.    
         [0047]    During the pumping phase of a cylinder  36   a  and  36   b , the corresponding piston  40   a  and  40   b  moves radially outward. As a result, the pressure in the valve chamber  54  rises, so that the valve element  62  of the inlet valve  64  comes back into contact with its valve seat. Once the pressure difference between the valve chamber  54  and the outlet conduit  50  is great enough, the valve element  72  of the outlet valve  74  lifts from the corresponding valve seat, so that the fuel from the work chamber  44  can reach the rail  24  via the valve chamber  54 . It is clear from this that a motion of the valve element  72  of the outlet valve  74  has no direct effect on the valve element  62  of the inlet valve  64 . Only the compression spring  80  is relaxed somewhat, but because of the high pressure prevailing in the valve chamber, this has no influence on the position of the valve element  62 .  
         [0048]    When no fuel from the injectors  26  reaches the combustion chambers  28  (as in the overrunning mode, for instance), the metering unit  20  is closed by the open- and closed-loop control unit  34 . However, for systematic reasons, when the metering unit  20  is closed a certain leakage quantity of fuel occurs, which via the fuel line  21  reaches the inlet conduits  48   a  and  48   b . However, since the inlet valve  64  is decoupled from the outlet valve  74 , the inlet valve  64  remains reliably closed in this case as well, and so no fuel is pumped into the rail  24 . The corresponding pressure course is identified by reference numeral  90  in FIG. 6.  
         [0049]    The decoupling assures that during the pumping stroke of the cylinder  36   a , for instance, the valve element  62  of the inlet valve  64  in this cylinder does not lift from its valve seat, and thus does not trip any pressure pulse in the inlet conduits  48   a  and  48   b . Since the cylinder  36   b  is in an intake phase when the cylinder  36   a  is in a pumping phase, such a pressure pulse could easily cause the valve element  62  of the inlet valve  64  of the cylinder  36   b  to lift from its seat.  
         [0050]    The result would be that leak fuel from the metering unit  20  would reach the work chamber  44   b  of the corresponding cylinder  36   b  and be pumped onward to the rail  24 . These pressure pulses, which in the high-pressure fuel pump  22  are avoided in the inlet conduits  48   a  and  48   b , are represented by dashed lines in FIG. 6 and identified by reference numeral  92 .  
         [0051]    In FIG. 7, a valve body  52  of a second exemplary embodiment of a high-pressure fuel pump  22  is shown. In FIG. 7, those elements and regions that have functions equivalent to the exemplary embodiment described above are identified by the same reference numerals. They will not be described again in detail.  
         [0052]    In the exemplary embodiment shown in FIG. 7, the valve element  62  of the inlet valve  64  is guided in the blind bore  76  of the valve element  72  of the outlet valve  74 . This blind bore accordingly forms the guide opening  58 . This has the advantage that a conventional valve element  72  can be used for the outlet valve  74 . For bracing the compression spring  80  on the valve element  62 , a disk  82  is provided, which rests on the axial edges of the vanes  86   a ,  86   b  and  86   c . For equalizing the pressure in the guide opening  58 , a pressure equalizing bore  94 , which communicates with the valve chamber  54  via a longitudinal groove  96 , is provided in the wall of the valve element  72  surrounding the guide opening.  
         [0053]    [0053]FIG. 8 shows one region of a modification of the inlet valve  64  shown in FIG. 7 and of the outlet valve  74  shown in FIG. 7. Here there are no vanes on the guide portion  90  of the valve element  62  of the inlet valve  64 . Instead, ribs  86  tapering to a point extend from the radially inner circumferential wall of the blind bore  76  of the valve element  72  of the outlet valve  74 .  
         [0054]    The foregoing relates to preferred exemplary embodiments of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.