Abstract:
A fuel pump comprising a plunger member reciprocable within a plunger bore, wherein the plunger member is cooperable with a drive arrangement to cause inward movement of the plunger member within the plunger bore to increase fuel pressure therein. The pump comprises an accumulator for fuel, and a valve arrangement controlling communication between the plunger bore and the accumulator to permit fuel under pressure to flow into the accumulator. The pumping plunger is moved in an outward direction under the action of the fuel pressure within the accumulator. The invention also relates to a fuel injection system comprising the fuel pump.

Description:
TECHNICAL FIELD 
     The invention relates to a fuel pump and, in particular, a fuel pump for use in supplying fuel under high pressure to a fuel injection system. 
     BACKGROUND OF THE INVENTION 
     Commonly, a common rail fuel system is used to supply fuel under high pressure to a plurality of fuel injectors for injection into the associated engine, the common rail being charged with fuel at high pressure by means an appropriate high pressure fuel pump. Typically, the pump comprises a pumping plunger which is reciprocable within a plunger bore, movement of the pumping plunger within the plunger bore being controlled by means of a cam arrangement including a cam member and a roller. The plunger bore is supplied with fuel from a low pressure pump, movement of the roller over the surface of the cam member resulting in inward movement of the pumping plunger within the bore to reduce the volume of the plunger bore, thereby increasing fuel pressure therein. The pumping plunger has an associated spring which serves to bias the pumping plunger towards its outermost position such that, on completion of inward movement of the plunger member within the bore, the pumping plunger is returned to its outermost position under the force of the spring ready for the start of the next pumping sequence. 
     A disadvantage of this type of pump is that a large spring force is required to bias the pumping plunger outwardly following inward movement, particularly when the engine is running at relatively high speeds. For this purpose, a large, heavy-duty spring is required. This may require the use of a pump body of relatively large dimensions and may have an impact upon the dimensions of other components of the pump, and may increase the cost of the pump. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an alternative fuel pump which can be manufactured with reduced cost. 
     According to the present invention, there is provided a fuel pump comprising a plunger member reciproble within a plunger bore, the plunger member being cooperable with a drive arrangement to cause inward movement of the plunger member within the plunger bore to increase fuel pressure therein, the pump further comprising an accumulator for fuel, and a valve arrangement controlling communication between the plunger bore and the accumulator to permit fuel under pressure to flow into the accumulator, and wherein the pumping plunger is moved in an outward direction under the action of the fuel pressure within the accumulator. 
     The invention provides the advantage that, as fuel pressure within the accumulator serves to bias the pumping plunger outwardly within the plunger bore, the need for a large and expensive spring component is removed. The pump can therefore be manufactured with reduced cost. 
     Conveniently, the plunger bore and the plunger member define a pumping chamber for fuel. The fuel pump may include a first valve member, for controlling communication between an inlet chamber or passage and the pumping chamber, and a second valve member for controlling communication between the pumping chamber and the accumulator. Conveniently, the first and second valve members may take the form of annular plates. 
     The accumulator may include an accumulator chamber, defined within an accumulator housing. The accumulator chamber may be substantially coaxially aligned with the pumping chamber. In this way, the pump can easily be formed as a single unit to minimise space. 
     Preferably, the accumulator housing is engageable with a seating surface defined by a seating member, the accumulator housing and the seating member being arranged such that the accumulator housing disengages the seating surface, in use, when the pressure of fuel within the accumulator chamber exceeds a predetermined amount, so as to relieve fuel pressure within the accumulator chamber. 
     The plunger member may be associated with a piston member, a surface of the piston member being exposed to fuel pressure within the accumulator, the force applied to the surface due to fuel pressure within the accumulator causing outward movement of the plunger member within the bore. The piston member may be integrally formed with the plunger member or may be a separate component. 
     Conveniently, the drive arrangement takes the form of a cam arrangement. 
     The plunger member is arranged to be driven in a forward direction to pressurise fuel pressure within the plunger bore. Preferably, the cam arrangement includes a cam member defining first and second cam surfaces, the first and second cam surfaces being shaped to provide a driving force to the plunger member in the forward direction for a prolonged period of time. By shaping the cam surfaces to have different forms, the period of time for which the driving force is applied to the plunger member can be increased and the driving torque can be minimised. 
     The accumulator may supply fuel directly to a fuel injection system, for example a plurality of fuel injection units, such that the need for a separate supply line or common rail is removed. This reduces the cost of the fuel system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described, by way of example only, with reference to the following figures in which: 
     FIG. 1 is a sectional view of a fuel pump in accordance with an embodiment of the present invention; 
     FIG. 2 is an enlarged sectional view of a part of the fuel pump in FIG. 1; and 
     FIGS. 3 and 4 are sectional views of a fuel pump in accordance with alternative embodiments. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, the fuel pump of the present invention includes a plunger member  10  which is reciprocable within a bore  12  provided in a first pump housing  14 , the bore  12  and an end surface of the plunger member defining, in part, a pumping chamber  13 . At the end of the plunger member  10  remote from the pumping chamber  13 , the plunger member  10  includes an end region  10   a  which engages a tappet member  16 , the tappet member  16  being moveable within a second pump housing  18  under the action of a roller member  20 . The roller member  20  is cooperable with a cam surface of a cam arrangement (not shown), the cam arrangement being driven by means of an engine drive shaft such that, in use, the roller member  20  is caused to ride over the cam surface and impart an inward force (in an upwards direction in the view shown in FIG. 1) to the tappet member  16 . As the tappet member  16  is engaged with the end region  10   a  of the plunger member  10 , inward movement of the tappet member  16  is transmitted to the plunger member  10 , thereby causing the plunger member  10  to move inwardly within the bore  12 . 
     The bore  12  includes an enlarged diameter region  12   b  which communicates with a restricted drilling  21  provided in the housing  18  such that, in the event that fuel leaks past the plunger member  10  from the pumping chamber  13 , the leakage fuel is returned through the drilling  21  to the inlet chamber  42 . The provision of such an arrangement is advantageous in that engine oil can be used to lubricate the tappet member  16  for movement without a significant quantity of fuel mixing with the engine oil. 
     The plunger member  10  is engaged with a piston member  22 , the piston member  22  including a lower region  22   a  having a diameter less than the diameter of the plunger member  10 , an intermediate, enlarged region  22   b  and an upper end region  22   c . The enlarged region  22   b  of the piston member  22  is engaged with one end of a light compression spring  24 . The region  22   a  of the piston member  22  is slidable within a through bore  26  provided in a seating member  28 , the diameter of the region  22   a  being substantially the same as the adjacent part of the bore  26  so as to guide sliding movement of the region  22   a  within the bore  26 . 
     The seating member  28  is in abutment with the housing  14 , the surface of the seating member  28  remote from the housing  14  defining a seating surface  30  which engages an accumulator housing  32 . The accumulator housing  32  is provided with a through bore  36  including an enlarged diameter region  36   a  and a smaller diameter region  36   b , the enlarged diameter region  36   a  defining an accumulator chamber  34  which houses the compression spring  24 . The region  22   c  of the piston member  22  is slidable within the bore region  36   b , the diameter of the bore region  36   b  being substantially the same as the diameter of the region  22   c  of the piston member  22  such that the bore region  36   b  also serves to guide sliding movement of the piston member  22 . In this way, the length of the region  22   a  of the piston member  22 , which also guides sliding movement of the piston member  22 , need only be relatively small. The region  22   c  of the piston member  22  defines a fuel flow passage whereby fuel is able to flow from the accumulator chamber  34  to an outlet defined by an end of the bore region  36   b.    
     The end of the compression spring  24  remote from the region  22   b  of the piston member  22  abuts a step defined by the bore  36 , the spring  24  serving to bias the piston member  22  and the plunger member  10  in an outwards direction (downwards in the view shown in FIG.  1 ), the spring  24  applying only a relatively low biasing force to the plunger member  10  in the outwards direction. The accumulator housing  32  is located within and is in screw threaded engagement with the pump housing  18 , the pump housing  18  being received within a part  38  of an engine housing. 
     The part  38  and the housing  18  are arranged to define an annular inlet passage  40  which permits fuel from a low pressure fuel pump or fuel reservoir (not shown) to flow into an annular inlet chamber  42 . 
     The upper surface of the housing  14  is provided with a recess  12   a  which communicates with the pumping chamber  13 , the seating member  28  being provided with a plurality of passages  46  (only one of which is shown in FIG. 1) which communicate with the recess  12   a  to permit fuel within the inlet chamber  42  to flow into the recess  12   a . An inlet valve member  48  is located within the recess  12   a , the inlet valve member  48  being engageable with a seating defined by the lower surface of the seating member  28  to control fuel flow between the inlet chamber  42  and the recess  12   a  such that, when the inlet valve member  48  is open, fuel delivered to the inlet chamber  42  is able to flow, via the passages  46 , into the recess  12   a  and into the pumping chamber  13 . When there is no, or only a limited fuel pressure difference between the pumping chamber  13  and the inlet chamber  42 , the inlet valve member  48  adopts an open position, in which it is spaced from the seating defined by the lower surface of the seating member  28  to permit fuel to flow from the inlet chamber  42  into the recess  12   a  and the pumping chamber  13 . When the pressure within the pumping chamber  13  is increased, the net force on the valve member  48  urges the valve member  48  towards a position in which it closes the passages  46  breaking communication between the pumping chamber  13  and the inlet chamber  42 . 
     An outlet valve member  50  is located within the accumulator chamber  34 , the outlet valve member  50  being engageable with a seating defined by the upper surface of the seating member  28  to control fuel flow between the pumping chamber  13  and the accumulator chamber  34 . The seating member  28  is provided with a plurality of drillings  54  which communicate with an annular groove such that, when the outlet valve member  50  is lifted away from the seating defined by the upper surface of the seating member  28 , fuel is able to flow from the pumping chamber  13  into the accumulator chamber  34 , engagement of the valve member  50  with the seating member  28  breaking such communication. The position adopted by the outlet valve member  50  is dependent upon the fuel pressures within the pumping and accumulator chambers  13 ,  34 , and the areas of the member  50  exposed to those pressures. 
     As indicated in FIG. 2, the housing  18  is provided with a drilling  58 , the tappet member  16  being provided with an elongate aperture or slot  56 . The drilling  58  provided in the housing  18  is aligned with a recess  60  provided in the part  38 , the drilling  58  and the recess  60  being arranged to receive a pin member  62  which extends through the slot  56  and serves to prevent angular movement of the tappet member  16  relative to the part  38 , thus ensuring that the axis of rotation of the roller member  20  remains substantially parallel to that of the cam. 
     In use, starting from a position in which the plunger member  10  occupies its outermost position within the plunger bore  12 , fuel is delivered to the inlet chamber  42  through the inlet passage  40  from the low pressure fuel pump. During this stage of operation, as there is only low fuel pressure within the pumping chamber  13 , the inlet valve member  48  is in its open position, spaced away from the seating defined by the lower surface of the seating member  28 , such that fuel within the inlet chamber  42  is able to flow, via the passages  46 , into the recess  12   a  and into the pumping chamber  13 , charging the pumping chamber  13  to a low pressure. 
     From this position, movement of the cam causes the roller member  20  to move over the cam surface, and the tappet member  16  is moved axially within the housing  18 , thereby imparting axial movement to the plunger member  10  within the bore  12  to reduce the volume of the pumping chamber  13 . Fuel pressure within the pumping chamber  13  is thereby increased and a point will be reached when fuel pressure within the pumping chamber  13  is sufficient to close the inlet valve. Continued movement of the plunger member  10  pressurises the fuel within the pumping chamber, and subsequently the pressure will rise to a level sufficient to urge the outlet valve member  50  away from its seating, defined by the upper surface of the seating member  28 , against the action of the fuel pressure within the accumulator chamber  34 , and fuel under high pressure is able to flow into the accumulator chamber  34 . 
     As fuel pressure within the accumulator chamber  34  increases, the force applied to the outlet valve member  50  due to fuel pressure within the chamber  34  increases and serves to urge the outlet valve member  50  towards the seating defined by the upper surface of the seating member  28 . A point will be reached when the outlet valve member  50  moves against the seating to close communication between the pumping chamber  13  and the accumulator chamber  34 , this point occurring shortly after the roller member  20  has ridden over the peak of the cam. Fuel under high pressure within the accumulator chamber  34  acts on the exposed parts of the piston member, the effective area of the exposed parts being sufficient to apply a force to the piston member urging the piston member  22  in an outwards direction (downwards in the view shown in FIG.  1 ). The piston member  22  thereby imparts movement to the plunger member  10  to return the plunger member  10  to its outmost position within the bore  12  ready for the next pumping cycle. The accumulator chamber  34  therefore provides an accumulator volume for fuel, fuel pressure within the accumulator volume acting on the piston member  22  to bias the plunger member  10  towards its outermost position. Thus, there is no need to provide a large spring within the chamber  34 , the light spring  24  being required to bias the piston member  22  and the plunger member  10  in an outwards direction upon engine start-up when the fuel pressure within the accumulator chamber  34  is relatively low. The pump can therefore be manufactured at a relatively low cost. 
     Fuel under high pressure from the accumulator volume is delivered to the remainder of a fuel injection system, for example to the common rail and injectors of a common rail fuel system. 
     It will be appreciated that, following engine start-up while the engine is still running at a relatively low speed, the fuel pressure within the accumulator chamber  34  may not increase to an amount which is sufficient to return the plunger member  10  to its outermost position within the plunger bore  12 . However, during this stage of operation, the relatively low force of the spring  24  is sufficient to urge the plunger member  10  outwardly, ready for the next pumping cycle. 
     The inlet and outlet valve members  48 ,  50  conveniently take the form of large diameter annular plates, an opening being provided through the center of each valve member  48 , 50  to permit fuel flow into the pumping chamber  13  or the accumulator chamber  34  respectively when the respective valve member  48 ,  50  is lifted away from its seating. The outer peripheries of the inlet and outlet valve members  48 ,  50  are conveniently also provided with slots, flats or grooves to permit fuel to flow between the inlet chamber  42  and the pumping chamber  13 , and between the pumping chamber and the accumulator chamber  34  at a sufficiently high rate. 
     The accumulator housing  32  and the seating member  28  may be arranged such that, when fuel pressure within the accumulator chamber  34  exceeds a predetermined amount, the wall of the bore  36  provided in the accumulator housing  32  dilates and, in addition, the seating member  28  is compressed. The accumulator housing  32  therefore disengages the surface  30  defined by the seating member  28  to permit fuel within the accumulator chamber  34  to flow into the inlet chamber  42 , thereby reducing fuel pressure within the chamber  34 . This prevents damage being caused to the pump and the engine due to an excessive increase in fuel pressure within the accumulator chamber  34 , without requiring the provision of a separate pressure relief valve. 
     The part  38  shown in FIG. 1 may form part of the engine cylinder head or part of the engine block. Referring to FIG. 3, in an alternative embodiment of the invention the housing  18  is received within a first housing  60  and a second housing  62 , the housing  62  including a projection  62   a  housing fuel inlet passages  64  which communicates with the inlet passage  40  defined, in part, by the housing  18 . The provision of the housing  62 , including the projection  62   a  for the fuel inlet passages  64 , removes the need to integrate a fuel inlet passage into the engine block, thereby reducing the cost of the engine. The embodiment of the invention shown in FIG. 3 also includes a fuel distribution manifold  64  which permits fuel under high pressure within the accumulator chamber  34  to be delivered directly to inlet passages  66 , four of which are shown in FIG. 3, for delivery to associated fuel injection units. In this way, the need for a separate common rail which supplies fuel under high pressure to the fuel injection units is removed. 
     FIG. 4 shows a further alternative embodiment of the invention, with similar parts to those shown in FIGS. 1 to  3  being denoted with the same reference numerals. FIG. 4 shows the cam arrangement which includes a cam member  70  which defines cam surfaces  72 , of relatively shallow rising form, and cam surfaces  74  of steeper falling form, the roller member  20  riding over the cam surfaces  72 ,  74  to impart axial movement to the tappet member  16  and, hence, the plunger member  10  within the bore  12 . By using a cam member  70  including cam surfaces  72 ,  74  of different form, the period of time for which the roller member  20  drives the tappet member  16  inwardly can be increased whilst minimising the driving torque. Conventionally, when a spring force is used to return the plunger member  10  outwardly following fuel compression, it is not possible to extend the pumping period in this way for relatively high engine speeds. However, it is made possible by the present invention in which the plunger member  10  is returned to its outermost position by means of fuel pressure within the accumulator chamber  34 . 
     In the embodiment shown in FIG. 4, the tappet member  16 , the housing  14 , the seating member  28  and the accumulator housing  32  are housed within an outer housing  76 , the outer housing  76  also housing the cam member  70  and an inlet passage  78  for fuel, fuel being delivered through the inlet passage  78  through a filter arrangement  80  to lubricate the tappet member  16  within the housing  76 . The filter arrangement  80  ensures dirt and other debris which may be carried by fuel delivered through the inlet passage  78  does not reach the components of the pump or the parts of the fuel injection system. 
     It will be appreciated that the pump of the present invention may be housed directly within the engine block, or may be mounted within the cylinder head of the associated engine.