Abstract:
A fuel pump comprises a pumping plunger reciprocable within a plunger bore under the action of a cam drive arrangement. The cam drive arrangement comprises first and second surfaces defining therebetween a chamber, the volume of which can be controlled to control the spacing of the first and second surfaces so as to permit control of the axial length of the cam drive arrangement. The first surface may be defined by a tappet member which acts to transmit a force from the cam drive arrangement to the pumping plunger, the second surface being defined by a piston member which is slidable within a tappet bore formed in the tappet member. The invention also relates to an arrangement for use in a fuel pump comprising a pumping plunger which is reciprocable within a plunger bore under the action of a drive arrangement and a tappet member which is slidable within a further bore provided in a housing for transmitting a force from the drive arrangement to the pumping plunger. The arrangement comprises a yoke secured to the housing, the yoke having at least one projection extending into the bore provided in the housing, the or each projection cooperating with the tappet member so as to substantially prevent angular movement of the tappet member within the further bore.

Description:
TECHNICAL FIELD 
     This invention relates to a fuel pump for use in supplying fuel under high pressure for injection into a combustion space of an associated compression ignition internal combustion engine. The invention is particularly applicable to unit pump/injectors or to unit pumps, each being intended to deliver fuel to a single associated fuel injector. It will be appreciated, however, that the invention is also applicable to other types of fuel pump, for example in-line pumps. 
     BACKGROUND OF THE INVENTION 
     The pump of a unit pump/injector typically comprises a pumping plunger reciprocable within a bore under the action of a cam drive arrangement. Motion of the plunger, in use, is dependent upon the cam profile and the speed of operation of the cam drive arrangement. It is desirable to be able to modify the operation of the pump, for example to permit the timing of the delivery of fuel to be controlled, thereby permitting a reduction in the levels of white smoke, particulate emissions and noise produced when the engine is cold, and reducing the risk of miss-fires. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a fuel pump comprising a pumping plunger reciprocable within a plunger bore under the action of a cam drive arrangement, the cam drive arrangement comprising first and second surfaces defining therebetween a chamber, the volume of which can be controlled to control the spacing of the first and second surfaces, thereby permitting control of the axial length of the cam drive arrangement. 
     Where the pump is of the type in which a filling port is provided, the filling port being obscured, in use, by the pumping plunger, adjustment of the axial length of the drive arrangement permits the timing at which the filling port is closed by the pumping plunger, and hence the timing of commencement of pressurisation of fuel by the pump, to be controlled. By controlling the timing of commencement of pressurisation of fuel, depending upon the nature of the fuel injector with which the pump is to be used, the timing of commencement of fuel injection can be controlled. As a result, an arrangement is possible in which the timing of fuel delivery can be advanced, for example when the associated engine is cold. 
     The first surface is conveniently defined by a tappet member which acts to transmit a force from the cam drive arrangement to the pumping plunger. 
     The second surface may be defined by a piston member slidable within a tappet bore formed in the tappet member. 
     Preferably, the fuel pump comprises a spring arrangement arranged to apply a biasing force to the piston member which tends to minimise the volume of the chamber. 
     Conveniently, the plunger bore defines a pumping chamber for fuel, reciprocal movement of the pumping plunger within the plunger bore causing fuel pressurisation within the pumping chamber, in use. The fuel pump may further comprise a further spring arrangement which is arranged to urge the tappet member in a direction to withdraw the pumping plunger from the plunger bore, thereby tending to maximise the volume of the pumping chamber. 
     The cam drive arrangement may include a drive member which carries the piston member, a force from the piston member being transmitted to the pumping plunger through the drive member. The drive member and the piston member may be integrally formed. 
     In an alternative embodiment, the piston member may be secured directly to the pumping plunger. 
     The tappet member may be shaped to define, at least in part, a relief passage which communicates with the chamber upon movement of the piston member beyond a predetermined position. 
     Alternatively, the tappet member may be provided with a circlip which is engageable with the piston member upon movement of the piston member beyond a predetermined amount. 
     Preferably, the tappet member is slidable within a further bore provided in a housing, the fuel pump preferably comprising means for substantially preventing angular movement of the tappet member within the further bore. 
     For example, one of the tappet member and the housing may be provided with a member which extends into a recess provided in the other of the tappet member and the housing so as to substantially prevent angular movement of the tappet member within the further bore. 
     The member may take the form of a peg carried by the tappet member, the peg extending through an opening provided in a drive member forming part of the drive arrangement so as to substantially prevent angular movement of the tappet member within the further bore. 
     Alternatively, the pump may comprise a yoke secured to the housing, the yoke having at least one projection extending into the further bore provided in the housing, the or each projection cooperating with the tappet member so as to substantially prevent angular movement of the tappet member within the further bore. 
     The piston member may include a region of part-spherical form which is cooperable with the tappet bore to permit axial misalignment between the piston member and the tappet member. 
     Preferably, the chamber is arranged to receive fluid through a supply passage, fluid pressure within the chamber applying a force to the first and second surfaces which serves to increase the volume of the chamber. 
     The fuel pump may further comprise a temperature sensitive valve arrangement, the pressure of fluid supplied to the chamber being controlled in response to an output from the temperature sensitive valve arrangement. 
     According to a second aspect of the present invention, there is provided an arrangement for use in a fuel pump comprising a pumping plunger which is reciprocable within a plunger bore under the action of a drive arrangement and a tappet member which is slidable within a further bore provided in a housing for transmitting a force from the drive arrangement to the pumping plunger, the arrangement comprising a yoke secured to the housing, the yoke having at least one projection extending into the bore provided in the housing, the or each projection cooperating with the tappet member so as to substantially prevent angular movement of the tappet member within the further bore. 
     It will be appreciated that this aspect of the invention is not limited to use in a fuel pump driven by means of a cam drive arrangement, nor is it limited to use in a fuel pump in which first and second surfaces of the drive arrangement define a chamber, the volume of which can be controlled to control the spacing of the first and second surfaces. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will further be described, by way of example, with reference to the accompanying drawings, in which: 
     FIGS. 1 and 2 are sectional views illustrating part of a fuel pump in accordance with an embodiment of the invention; 
     FIG. 3 is a diagrammatic view of another part of the fuel pump of FIGS. 1 and 2; 
     FIGS. 4 to  8  are views similar to FIG. 2 illustrating alternative embodiments; 
     FIGS. 9 and 10 illustrate a further embodiment; and 
     FIG. 11 is a view of a piston member according to another embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The fuel pump illustrated in FIGS. 1,  2  and  3  is intended to form part of a unit pump/injector and comprises a pumping plunger  10  which is reciprocable within a bore  11  formed in a pump body  12 . The plunger  10  and bore  11  together define a pumping chamber  13  which communicates with an outlet passage  14  and, depending upon the axial position occupied by the pumping plunger  10 , with a feed or filling port  15 . The feed port  15  communicates with a suitable low pressure fuel reservoir  16 . 
     At its end remote from the end located within the bore  11 , the plunger  10  is secured to a drive member  17  forming part of a drive arrangement  18 . The drive member  17  carries a piston member  19 , a shim  20  being located between the piston member  19  and the drive member  17  such that the axial position of the piston member  19  relative to the drive member  17  can be set during the assembly process of the drive arrangement  18 . 
     The piston member is slidable within a bore  21  formed in a tappet  22 , the tappet  22  being slidable within a bore  23  formed in a housing member  24 . The tappet  22  carries a roller  25 , the outer periphery of which is engageable with a cam  26  mounted upon a drive shaft  27 . 
     The bore  21  provided in the tappet  22  defines, with the piston member  19 , a chamber  28  of variable volume. The tappet  22  is provided with drillings defining a flow passage  29  whereby engine lubricating oil or another fluid can be supplied to the chamber  28 . A ball valve arrangement  30  is provided to restrict the flow of fluid from the chamber  28  along the passage  29 , but to permit fuel flow towards the chamber  28  at a substantially unrestricted rate. The ball valve arrangement  30  is conveniently spring-biased closed. 
     The flow passage  29  communicates with an annular recess  31  defined between the tappet  22  and the wall of the bore  23 , the recess  31  communicating, in turn, with a supply passage  32  which is arranged such that, in use, lubricating oil or another fluid at a desired pressure can be applied thereto. The manner in which the oil or other fluid pressure is controlled does not form part of the invention and so will be not described in great detail. 
     The tappet  22  is shaped to define a relief passage  33  which communicates with the chamber  28  upon movement of the piston member  19  beyond a predetermined position. The relief passage  33  communicates with the interior of the cam housing and is thus under relatively low oil pressure. 
     As illustrated in FIGS. 1 and 2, the open end of the bore  21  provided in the tappet  22  is closed by means of a cap  34  which is retained in position using a spring clip  35  or any other suitable technique. A spring  36  is engaged between the cap  34  and a spring abutment member  37  carried by the drive member  17  to apply a biasing force to the piston member  19  urging the piston member  19  into engagement with the blind end of the bore  21  formed in the tappet  22 . The spring  36  therefore urges the piston member  19  towards a position in which the chamber  28  is of minimum volume. The cap  34  further engages a return spring  38  which is positioned to urge the tappet  22  in a direction to withdraw the plunger  10  from the bore  11 , urging the plunger  10  towards a position in which the pumping chamber  13  is of maximum volume. 
     The housing  24  is provided with a screw-threaded pin  39 , an end of which projects into the bore  23  and rides within a slot formed in the tappet  22  to hold the tappet  22  against angular movement within the bore  23 , but to allow axial movement of the tappet  22  in a substantially unrestricted manner. It will be appreciated that by holding the tappet  22  against angular movement, the roller  25  is held such that its axis of rotation lies substantially parallel to the axis of rotation of the drive shaft  27 . 
     In use, with the supply passage  32  being supplied with engine lubricating oil at a relatively low pressure, the chamber  28  is at relatively low pressure and the spring  36  urges the piston member  19  towards the position shown in which the end surface thereof engages the surface defined by the blind end of the bore  21 . The roller  25  is in engagement with the base of the cam  26 , the roller  25  being urged into engagement with the cam  26  by the return spring  38 . It will be appreciated that the tappet  22  occupies its outermost position, and likewise the plunger  10  occupies its outermost position. As illustrated in FIG. 3, in this position the pumping chamber  13  communicates with the feed port  15 , the pumping chamber  13  being charged with fuel to a relatively low pressure. 
     Rotation of the drive shaft  27  causes the roller  25  to move into engagement with the cam lobe, causing the tappet  22  to move upwardly in the orientation illustrated in FIGS. 1 and 2, the movement of the tappet  22  being transmitted directly to the piston member  19 , and through the shim  20  to the drive member  17  from where it is transmitted to the pumping plunger  10  urging the pumping plunger  10  in an upwards direction in the orientation illustrated in FIG.  3 . The initial movement of the pumping plunger  10  displaces fuel from the pumping chamber  13  through the feed port  15  to the reservoir  16 . Displacement of fuel from the pumping chamber  13  in this manner continues until the pumping plunger  10  has moved by a sufficient distance to close the feed port  15 . Once this position has been reached, as denoted by the dashed line in FIG. 3, further movement of the pumping plunger  10  under the action of the drive arrangement  18  pressurizes the fuel within the pumping chamber  13  and supplies fuel under high pressure to the outlet passage  14  from where it is supplied to an injection nozzle forming part of the pump injector. It will appreciated that when the pump is operating in this manner, the timing of commencement of fuel pressurization is dependent upon the shape of the cam lobe, commencement of pressurization occurring once the plunger  10  has moved by a sufficient distance to cover the feed port  15 . Fuel delivery at high pressure continues until either a spill valve connected to the outlet passage  14  is opened or the plunger  10  reaches an inner position, or more preferably by a spill passage or groove, conveniently of angled or helical form, provided in the plunger aligns with the feed port provided in the housing to allow fuel to escape from the pumping chamber to a low pressure reservoir. Once the plunger reaches an inner position, movement of the plunger  10  to the position illustrated occurs under the action of the return spring  38 . 
     Where it is determined that the timing of commencement of fuel delivery by the pump should be advanced, lubricating oil under relatively high pressure is applied to the supply passage  32 . Such an application of lubricating oil under pressure permits oil to flow through the passage  29  past the non-return valve  30  to the chamber  28 . The oil under pressure acts upon the surfaces of the piston member  19  and the blind end of the bore  21  formed in the tappet  22  urging these surfaces away from one another, movement of the piston member  19  relative to the tappet  22  continuing until a position is reached in which the chamber  28  communicates with the relief passage  33 . The movement of the piston member  19  away from the blind end of the bore  21  formed in the tappet  22  occurs against the action of the spring  36 . The movement of the piston member  19  is intended to occur when the tappet  22  occupies substantially the position illustrated in FIGS. 1 and 2 and the movement of the piston member  19  is transmitted to the plunger  10 , thereby moving the plunger  10  from its outermost position by a small distance, for example to the position illustrated by a dotted line in FIG.  3 . 
     Once the piston member  19  has been moved relative to the tappet  22 , rotation of the drive shaft  27  causes movement of the tappet  22  as described hereinbefore. The motion of the tappet is transmitted through the lubricating oil located within the chamber  28  to the piston member  19  and from the piston member  19  through the shim  20  and drive member  17  to the plunger  10 . It will be appreciated that the oil within the chamber  28  will be pressurized and the piston member  19  will move to a position in which the relief passage  33  is closed and the non-return valve  30  will occupy a closed position preventing oil from escaping through the passage  29  to the supply passage  32 . 
     The inward movement of the tappet  22  causes inward movement of the plunger  10  as described hereinbefore. However, as the axial length of the drive arrangement  18  is increased by moving the piston member  19  relative to the tappet  22  as described hereinbefore, the point at which the feed port  15  is closed by the pumping plunger  10  will occur at an earlier instant than would be the case if the drive arrangement  18  were of shorter axial length. As a result of the timing at which the feed port  15  is closed by the plunger  10  being advanced, the timing of commencement of fuel pressurization is advanced, and this can be used to advance the timing at which fuel is delivered by an associated fuel injector. 
     When it is desired to return the pump to its original operating setting, the supply passage  32  is no longer supplied with lubricating oil at high pressure, but rather is connected to a low pressure source of lubricating oil. The piston member  19  will return to the position illustrated, oil from the chamber  28  escaping either past the non-return valve  30  or leaking between the piston member  19  and the tappet  22 . FIGS. 4 to  10  illustrate modifications to the arrangement described hereinbefore. Only the differences will be described, and it will be appreciated that, for the most part, the manner in which the arrangements operate will be as described hereinbefore. 
     FIG. 4 illustrates a modification to the arrangement illustrated in FIGS. 1 to  3 . In the arrangement of FIG. 4, the distance through which the piston member  19  can move relative to the tappet  22  is not controlled by controlling the point at which the chamber  28  moves into communication with a relief passage, but rather by providing the tappet  22  with a circlip  40  which is engageable with the piston member  19  upon movement of the piston member  19  by a predetermined distance. A further distinction between the arrangement of FIG.  4  and that of FIGS. 1,  2  and  3  is that the screw-threaded pin  39  is replaced by a peg  39   a  carried by the tappet  22  and slidable within a slot formed in the bore  23 . Operation of the embodiment of FIG. 4 is substantially identical to that of FIGS. 1,  2  and  3  with the exception that distance through which the piston member  19  can move is governed by the piston member  19  engaging the circlip  40  rather than by the chamber  28  moving into communication with a relief passage. The operation of the arrangement will, therefore, not be described in further detail. 
     FIG. 5 illustrates an arrangement which is similar to that of FIG. 4 but in which the piston member  19  and drive member  17  are formed integrally with one another, and denoted by reference numeral  19   a . As it is no longer possible to introduce a shim  20  between the piston member  19  and drive member  17 , a shim  20   a  is provided to set the normal outermost position for the pumping plunger  10  and a shim  20   b  is provided to set the position occupied by the pumping plunger  10  when the pump is operating under circumstances in which the timing of fuel delivery by the pump is advanced. 
     In the arrangement of FIG. 6, the peg  39   a  is of increased length and extends across the diameter of the tappet  22 , extending through an opening formed in the drive member  17 . The dimensions of the opening formed in the drive member  17  are chosen to limit the distance through which the piston member  19  can move relative to the tappet  22 , thereby avoiding the necessity to provide the circlip or stop  40 . FIG. 7 illustrates a modification to the arrangement of FIG. 6 in which the drive member  17  and piston member  19  are formed integrally with one another. Operation of these embodiments is substantially as described hereinbefore and so will not be described in further detail. 
     FIG. 8 illustrates an arrangement which operates in a manner similar to that of FIGS. 1 to  3 , but in which a peg  39   a  is used to ensure that the tappet  22  cannot move angularly relative to the housing  24 , the peg  39   a  extending into a recess formed in the piston member  19  to retain the piston member  19  in position, during assembly, but to allow free movement of the piston member  19  in use. The piston member  19  is of sufficient axial length that the drive member  17  can be omitted, the piston member  19  being secured to the pumping plunger  10 . Rather than providing the relief passage  33  within the tappet  22 , the relief passage  33  is defined by drillings formed in the piston member  19 , the relief passage  33  being closed by the tappet  22  when the piston member  19  occupies a lower position relative to the tappet  32  and opening when the piston member  19  occupies a raised position relative to the tappet  22 . 
     FIGS. 9 and 10 illustrate an alternative technique for securing the tappet  22  against rotation or angular movement relative to the housing  24 . In the arrangement illustrated in FIGS. 9 and 10, the tappet  22  is shaped to define a pair of flats  22   a , and the housing  24  has secured thereto a yolk  41  including a pair of projections  42  which extend into the recesses defined between the flats  22   a  of the tappet  22  and the bore  23 . The cooperation between the flats  22   a  and the projections  42  holds the tappet  22  against angular movement relative to the housing  24 . One advantage of using this technique for holding the tappet  22  against angular movement is that there is no necessity to provide a tappet location feature at the upper end of the tappet  22  in the orientation illustrated. Where the tappet  22  is located within a bore  23  which is of relatively short axial extent, then the provision of a feature upon the tappet  22  riding within a formation provided in the bore  23  is not always possible. Although this tappet location technique is illustrated for use with the arrangement of FIG. 4, it will be appreciated that it may be used with any of the other embodiments described hereinbefore, and may be used in fuel pumps of other types including those which do not have a cam drive arrangement of controllable axial length. 
     In any of the embodiments described hereinbefore, if there is concern that it may not be possible to exactly align the piston member  19  with the bore formed in the tappet  22 , particularly during assembly, then one possible solution may be to form the piston member  19   a  to include a region of part-spherical form, as shown in FIG.  11 . The part-spherical region engaging or cooperating with the surface of the bore formed in the tappet in such a manner that the axis of the piston member  19   a  need not be exactly coaxial with that of the tappet, but may be angled slightly thereto. 
     The invention is particularly suitable for use in controlling the timing of fuel delivery and permitting the timing of such delivery to be advanced when an associated engine is cold. As shown in FIG. 1, in such an arrangement the oil pressure supplied to the supply passage  32  may be controlled using a suitable temperature sensitive valve  50 . It will be appreciated, however, that the invention is suitable for use in other arrangements in which the timing of fuel delivery is to be controlled, and is not restricted to arrangements in which the timing of fuel delivery is modified to compensate for the engine operating temperature.