GB2384529A

GB2384529A - Fuel pump assembly

Info

Publication number: GB2384529A
Application number: GB0201626A
Authority: GB
Inventors: Peter Alban George Collingborn
Original assignee: Delphi Technologies Inc
Current assignee: Delphi Technologies Inc
Priority date: 2002-01-25
Filing date: 2002-01-25
Publication date: 2003-07-30
Anticipated expiration: 2022-01-25
Also published as: GB0201626D0; GB2384529B

Abstract

A fuel pump assembly for use in an engine includes a plurality of pump heads (13a, 13b, 13c) mounted upon a main pump housing (10), each of the pump heads (13a, 13b, 13c) including a pumping plunger (26) which is reciprocable, in use, within a plunger bore (28) under the influence of an associated shoe and roller arrangement (24, 32) so as to cause pressurisation of fuel within a pumping chamber (30) defined within a pump head housing (14a, 14b, 14c). Each of the shoe and roller arrangements includes a shoe (24) and a roller (32), wherein the roller member (32) is cooperable with a cam surface (34) common to each of the pump heads (13a, 13b, 13c) so as to impart reciprocable movement to the shoe (24) upon rotation of the drive shaft (12). Each pump head housing (14a, 14b, 14c) is shaped to define a guide path (20, 22) for guiding reciprocal movement of the associated shoe (24). The pump assembly further includes a back plate (41) which is a separate and distinct component from each of the pump head housings (14a, 14b, 14c) and the main pump housing (10). The back plate (41) is provided with a common delivery passage (52) for receiving high pressure fuel delivered from each of the pumping chambers, in use.

Description

FUEL PUMP ASSEMBLY The invention relates to a pump assembly for use in supplying high pressure fuel to the fuel injection system of a compression ignition internal combustion engine.

In a known fuel pump for use in a compression ignition internal combustion engine, a plurality of plungers are reciprocable within respective plunger bores so as to pressurise fuel within respective pumping chambers for delivery to the fuel injection system associated with the engine. It is common to provide three plungers which are equi-angularly spaced around a drive shaft, the plungers being moveable under the influence of a cam drive arrangement. The cam drive arrangement includes a common eccentric cam surface, cooperable with all three of the plungers, to cause reciprocal movement of the plungers within their respective plunger bores.

In an alternative known arrangement, each of the plungers is in connection with an associated tappet member which serves to drive movement of the respective plunger within its bore. The tappet members are slidable within tappet bores under the influence of respective roller members driven by means of an eccentric cam surface. The eccentricity of the cam surface causes the tappet members, and hence the plungers, to be driven inwardly within their respective bores, the plungers thereby performing a forward stroke in which fuel within the respective pumping chamber is pressurised. The tappet members are urged outwardly from their respective tappet bores by means of fluid pressure within a working chamber, thereby urging the respective plunger in an outward direction to perform the return stroke.

Pump arrangements of the aforementioned type can be difficult to assemble.

Furthermore, the tappet members require a relatively large accommodation space and are relatively expensive components. Parasitic pumping power losses are also an inherent feature of hydraulic tappet operation.

Our co-pending UK patent application, GB 0100667.5, describes a multiplunger fuel pump in which three plungers are equi-angularly spaced around a drive shaft and driven under the influence of a reciprocable shoe and roller arrangement. The roller is cooperable with the cam surface of the cam arrangement so as to impart reciprocal movement to the shoe upon rotation of the drive shaft. The pump includes three pump assemblies housed within a unitary, main pump housing, each of the pump assemblies including a plunger which is slidable within a respective bore in the housing to pressurise fuel within an associated pumping chamber. Inlet and outlet valves to control fuel flow to and from the pumping chambers respectively are provided in flow passages defined within the main pump housing. A tubular member, arranged coaxially with the drive shaft, is housed within the main pump housing and is provided with a plurality of apertures, each of which is shaped to guide reciprocal movement of an associated shoe as the roller rides over the cam surface.

One disadvantage of this type of pump is that, as the high pressure flow paths for pressurised fuel, the plunger bores and the high pressure inlet and outlet valves are housed or defined within the main pump housing, the housing must be formed from strong steel. Not only does this present manufacturing difficulties, but the housing is also a heavy and expensive component. It is also necessary for the tubular member to be formed from strong steel which, again, has manufacturing and cost disadvantages.

It is an object of the present invention to provide an improved pump assembly for use in supplying high pressure fuel which alleviates at least one of the aforementioned problems.

According to the present invention, there is provided a fuel pump assembly for use in an engine, the fuel pump assembly comprising; a plurality of pump heads mounted upon a main pump housing, each of the pump heads including a pumping plunger which is reciprocable, in use, within a plunger bore under the influence of an associated shoe and roller arrangement so as to cause pressurisation of fuel within a pumping chamber defined within a pump head housing, each of the shoe and roller arrangements including a shoe and a roller, wherein the roller is cooperable with a cam surface, common to each one of the pump heads, so as to impart reciprocable movement to the shoe upon rotation of the drive shaft, and wherein the pump head housing is shaped to define a guide path which serves to guide reciprocal movement of the associated shoe, and wherein the pump assembly further comprises a back plate, being a separate and distinct component from each of the pump head housings and the main pump housing, the back plate being provided with a common delivery passage for receiving high pressure fuel delivered from the pumping chambers, in use.

The invention provides the advantage that the main pump housing can be formed from a relatively light material, such as aluminium, as the common delivery passage for high pressure fuel is defined within a back plate of the assembly. Only the pump head housings and the back plate need therefore be formed from a material (for example, steel) capable of withstanding high fuel pressures.

In one preferred embodiment, each pump head housing is provided with a pair of projections or arms, facing surfaces of which define the guide path for the associated shoe.

The common delivery passage defined in the back plate is preferably arranged to receive fuel from a plurality of primary flow passages, each being arranged to receive fuel from an associated pumping chamber, and is arranged to deliver high pressure fuel to a common pump outlet.

In a preferred embodiment, the common delivery passage includes first, second and third radially extending arms such that the common delivery passage has a generally Y-shaped configuration. Each one of the first, second and third arms is conveniently arranged to receive high pressure fuel from an associated pump head delivery passage in communication with a respective pumping chamber.

By forming the common delivery passage in such a Y-shaped configuration, stress concentrations within the back plate are reduced.

Inward movement of a pumping plunger within its respective plunger bore causes the pumping plunger to perform a forward stroke, in which pressurisation of fuel within the pumping chamber occurs. Each pump head may be provided with a return spring which acts on the shoe so as to ensure the roller member remains in engagement with the cam surface during a return stroke of the pumping plunger.

The roller member may preferably take the form of a cylindrical roller.

Preferably, the pump assembly includes a first constraining means for constraining movement of the shoe and roller arrangement of each pump head in a first axial direction.

Preferably, for example, the back plate may be shaped to define a plurality of formations, for example axially projecting lugs, each of which is arranged to constrain movement of a respective shoe and roller in the first axial direction.

The pump assembly may also include a second constraining means, for example in the form of a further plate mounted upon the main pump housing, for constraining movement of the shoes and rollers in a second, opposing axial direction.

In a further preferred embodiment, each pump head housing is provided with a recess or groove which defines, in part, a return flow path for fuel displaced by the reciprocating shoe, in use.

The back plate may also be provided with at least one groove or recess which defines a part of the return flow path.

If the return flow path for fuel is of relatively large flow area, excessive back pressures can be avoided and, thus, reciprocating movement of the shoe and roller is substantially unimpeded.

The back plate is preferably adapted to have mounted thereon a transfer pump for supplying fuel at a pressure dependent upon the speed of operation of the engine to the pumping chambers of each pump head.

It will be appreciated that the pump assembly may, but need not, be manufactured to include the transfer pump. It will also be appreciated that the pump assembly may, but need not, be manufactured to include the cam drive shaft.

The invention will now be described, by way of example only, with reference to the accompanying drawings in which ; Figure 1 is an isometric view of a fuel pump assembly in accordance with an embodiment of the present invention ; Figure 2 is a sectional view of the pump assembly in Figure 1 ; Figure 3 is an end view of the pump assembly in Figures 1 and 2; Figure 4 is an isometric view of the back plate of the pump assembly in Figures lto3, Figure 5 is an isometric view of a pump head forming part of the pump assembly in Figures 1 to 3 and sectioned on its plane of symmetry; Figure 6 is a view, shown part in section, of the pump head housing in Figure 4 in combination with a back plate of the pump assembly, and Figure 7 is an alternative sectional view to that shown in Figure 6 of a part of the pump head housing and back plate.

Referring to Figures 1 to 3, there is shown a high pressure fuel pump assembly suitable for use in the fuel injection system of a compression ignition internal

combustion engine. In particular, the fuel pump assembly is suitable for use in delivering high pressure fuel to the common rail of a common rail fuel injection system. The pump assembly includes a main pump housing 10 through which a cam drive shaft (not shown) of the engine extends along a drive shaft axis extending perpendicularly to the plane of the page. First, second and third pump heads 13 a, 13b, 13c respectively are mounted upon the main pump housing 10 at approximately equi-angularly spaced radial locations around the drive shaft axis. As the pump heads 13 a, 13b, 13c are substantially identical to one another, only the first one of the pump heads 13a will therefore be described in detail hereinafter.

As can be seen most clearly in Figures 2 and 3, the first pump head 13a includes a pumping plunger 26 which is reciprocal within a blind bore 28 provided in a pump head housing 14a to cause pressurisation of fuel within a pumping chamber 30 defined at the blind end of the bore 28. The pumping plunger 26 is driven axially within the bore 28, in use, under the influence of a drive arrangement including a shoe 24 and an associated roller 32. The roller 32 is co-operable with a cam surface of a cam member 34 carried by the drive shaft 12 such that, upon rotation of the drive shaft 12 in use, the roller 32 rides over the cam surface, thereby imparting movement to the shoe 24 and, hence, axial movement to the pumping plunger 26 within the bore 28. The pump head housing 14a is shaped to define first and second flanges or arms 16,18, facing surfaces 20,22 of which define a guide path for the shoe 24 as it is driven by the roller 32 such that, as the roller 32 rides over the cam surface, movement of the shoe 24 is guided by the surfaces 16,18.

Each of first, second and third pump head housings 14a, 14b, 14c of the first, second and third pump heads 13a, 13b, 13c respectively is received within a main axial bore 40 provided in the main pump housing 10, and is mounted

upon a back plate 41 secured to a rear end surface of the main pump housing 10. The back plate 41 has three spokes 41b and is provided with a central through bore 43 through which the drive shaft 12 extends. The drive shaft 12 also includes an end face provided with a plurality of formations in the form of lugs 42 which project along the drive shaft axis. Each one of the lugs 42 cooperates with a respective shoe and roller arrangement 24,32 so as to constrain movement of the shoe 24 and roller 32 along the axis of the drive shaft 12 in a first direction (i. e. to the right in the illustration shown in Figure 2). A means for constraining movement of the shoe and roller arrangements of each of the three pump heads 13, 1312, 13f in a second, opposing direction along the axis of the drive shaft 12 is also provided in the form of a further plate 44 mounted upon the main pump housing 10.

It will be appreciated that the shoe and roller arrangement of each pump head 13a, 13b, 13c co-operates with the cam surface which is thus common to all three pump heads. As the drive shaft 12 rotates, the rollers co-operate with the common cam surface to cause reciprocating motion of the shoes in a phased, cyclic manner depending on the cam surface profile.

Referring to Figure 4, the lugs 42 of the back plate 41 are received within the main axial bore 40 provided in the main housing 10, and the plate 41 is located within the main axial bore 40 by means of a boss 37. The surface of the through bore 43 provided in the back plate 41 provides a mount for a rear bearing 39 of the cam drive shaft 12. The drive shaft 12 is also provided with an annular seal member 33 located towards the end of the shaft 12 remote from the rear bearing 39.

As shown in Figure 5, the pump head housing 14a is provided with a screw threaded drilling 55 to permit the back plate 41 to be secured thereto by means

of screws (not shown). The back plate 41 is provided with a plurality of drillings 45 into which heads of the screws may be received, if desired.

A low pressure transfer pump 46 is mounted upon the back plate 41 at an end face of the plate 41 remote from the pump head housings 14a, 14b, 14c. The transfer pump 46 is arranged to supply fuel at a pressure dependent upon the speed of rotation of the engine, the rate. of flow of fuel from the transfer pump being controlled by means of a metering valve arrangement (not shown) in a conventional manner. If a transfer pump is not fitted to the pump assembly, the receiving of the screw heads within the drillings 45 in the back plate 41 serves to minimise the axial length of the assembly. A back leak connector 15 is mounted upon the main pump housing 10 in a conventional manner for use in the return of fuel from the pump assembly to a low pressure fuel reservoir (not shown).

The metered flow of fuel from the transfer pump 46 is supplied to the pumping chamber 30 of the first pump head 13 a through one or more inlet drillings 47 provided in the pump head housing 14a. The inlet drilling 47 communicates with the pump chamber 30 through an inlet valve arrangement 48, the pumping chamber 30 also being provided with an outlet valve arrangement 50 which is arranged to open when fuel pressure within the pumping chamber 30 is pressurised to a level greater than a predetermined amount.

Fuel delivered to the outlet valve arrangement 50 is supplied to a high pressure delivery passage 52 defined within the back plate 41 (as can be seen most clearly in Figures 2 and 4) which is common to all three of the pump heads 13a, 13b, 13c. The common delivery passage 52 defined within the back plate 41 includes first, second and third radially extending arms 52a, 52b, 52c which define a flow passage of generally'Y'shaped configuration, fuel being

delivered through the first one of the arms 52a to a common outlet port 53 through which high pressure fuel is delivered to the downstream parts of the fuel injection system, for example a common rail. Axially extending drillings 49a, 49b, 49c are provided in the back plate 41, each one of which defines a part of a flow path between the pumping chamber of the associated pump head 13 a, 13b, 13c and the associated one of the arms 52a, 52b, 52c of the common delivery passage 52, as described in further detail below.

Further details of the inlet and outlet valve arrangements 48,50 are shown in Figures 6 and 7 respectively. Referring to Figure 6, the inlet valve arrangement 48 takes the form of a ball valve including a ball 54 which is urged against a valve seating by means of a first spring 56 arranged within a passage 57, of stepped form, defined in the pump head housing 14a. One end of the first spring 56 is in abutment with the ball 54 and the other end of the first spring 46 is in abutment with a surface of an abutment member 58 which includes an annular region 58a of enlarged diameter. The abutment member 58 is retained in a fixed position within the passage 57, in which the enlarged region 58a engages a step in the passage 57, by means of a circular piece 60 located within an end of the passage 57 remote from the ball valve by means of a retaining screw 61. If fuel pressure within the inlet drilling 47 exceeds a predetermined amount, the ball 54 is caused to lift away from its seating, thereby permitting a metered flow of fuel (at"metered fuel pressure") to be supplied through the inlet drilling 47 to the pumping chamber 30.

As shown in Figure 7, the outlet valve arrangement 50 also takes the form of a second ball valve including a second ball 62 which is urged against a second valve seating by means of a second spring 64 in abutment with a second abutment member 66. The second ball valve controls communication between the pumping chamber 30 and a pump head delivery passage 69,70. The pump

head delivery passage includes a first flow region 69 defined within a stepped bore 71 provided in the pump head housing 14a. A high pressure sealing member 72 is located within the bore 71 and is shaped to define a second flow region 70 of the pump head delivery passage which communicates with the common delivery passage 52 in the back plate 41 through the axially extending drilling 49a in the back plate 41. Conveniently, the sealing member 72 may be integrally formed with the second abutment member 66. If the second ball valve is urged open, fuel is able to flow from the pumping chamber 30, past the second valve seat, through the first and second flow regions 69,70, into the drilling 49a and, hence, into the first arm 52a of the common delivery passage 52. The high pressure sealing member 72 has first and second so called"knifeedge"sealing faces 74,76 to ensure substantially no leakage of high pressure fuel from the pumping chamber 30 occurs, whilst eliminating the need for a separate washer between the housing 14a (and likewise the housings 14b, 14c) and the back plate 40.

An additional seal member 81 is also shown in Figures 6 and 7, and this provides a fluid tight seal between the pump head housing 14a and the main pump housing 10.

As can be seen in Figure 4, the axially extending drillings 49b, 49c also form part of corresponding flow paths between respective pump head delivery

passages of the pump heads 13b, 13c and a respective one of the arms 52b, 52c of the common delivery passage in a similar manner. It is necessary to seal the open ends of the arms 52b, 52c (i. e. the ends remote from the first arm 52a) and this may achieved by providing a screw plug (not shown) within an enlarged diameter end region of each of the arms 52b, 52c. The enlarged diameter end regions are threaded to cooperate with the screw thread on the associated plug, and the interface between each enlarged end region and the remainder of the

arm defines a conical surface identified at 79b, 79c. An associated ball (not shown) is inserted into the open end of each arm 52b, 52c and is pressed

against the associated conical surface 79b, 79c by means of the screw plug, thus providing a seal to high pressure fuel flowing through the first and second arms 52b, 52c of the common delivery passage.

In known pump designs, in which the back plate 40 and the pump head housings form an integral unit, each pump head abuts and is secured to a face of the main pump housing. Drilling within the main pump housing extend from a face of each pump head approximately radially inwards in the main pump housing, each of these drillings being connected to one another and to the pump outlet to the common rail. With such a configuration, it is necessary to provide a seal at the mating faces of each pump head and the main pump housing (i. e. for a three-pump-head pump, three seals are required). The present invention is advantageous in that only two, separate high pressure seals are required, at the open ends of arms 52b, 52c of the common delivery passage, and not three.

In use, metered fuel is delivered from the transfer pump 46 to the inlet drilling 47. If inlet fuel pressure exceeds an amount which is sufficient to overcome the force due to the first spring 56 and any pressure in the pumping chamber 30, the first ball 54 will be urged away from its seating to permit fuel flow into the pumping chamber 30. As the drive shaft 12 rotates, the rollers 32 of each of the pump heads 13 a, 13b, 13c are caused to move in a radially outward direction, thereby imparting movement to the associated shoe, such movement being guided by co-operation between the shoe and the surfaces 20,22 of the arms 16,18 respectively. The pumping plunger 26 is caused to move axially within its bore 28 to pressurise fuel within the pumping chamber 30. When fuel pressure within the pumping chamber 30 exceeds an amount which is sufficient to overcome the combined force acting on the second ball 62 due to the second

spring 64 and any pressure within the common rail connected to the common outlet port 53, the outlet valve arrangement 50 is caused to open to permit high pressure fuel to flow from the pumping chamber 30, through the first and second passages 69,70 into the drilling 49a and, hence, into the delivery passage 52.

Fuel is also supplied by the transfer pump 46 to. the pumping chambers of the second and third pump heads 13b, 13c such that, as the drive shaft 12 rotates and the respective rollers ride over the cam surface, the shoes reciprocate cyclically within their respective bores to pressurise fuel within the pumping chambers of the second and third pump heads 13b, 13c also.

The shoe 24 is arranged such that, throughout at least part of its range of movement, a part thereof extends into or is exposed to fuel within a volume, radially inward of the shoe 24, defined within the respective pump head housing 14a and filled with fuel at relatively low pressure (commonly referred to as"housing pressure"). Fuel pressure within this volume applies a force to the shoe 24, and hence to the associated pumping plunger 26, which serves to oppose outward movement of the pumping plunger 26 from the plunger bore 28. As the shoe 24 reciprocates between the facing surfaces 20,22 of the arms 16,18, it is important that fuel within the volume radially inward of the shoe 24 can escape relatively easily as, if this volume is not vented, fuel pressure within the volume will tend to increase upon radially inward movement of the shoe 24. As can be seen most clearly in Figure 5, the pump head housing 14a is therefore shaped to include a pair of recesses 73 of part cylindrical form, one recess 73 being arranged on each side of the pumping plunger 26, which serve to define, in part, a return flow path for fuel displaced by the shoe 24 to a low pressure drain. Additional recesses 74 provided in the main pump housing 10 also define a part of the return flow path. In addition, the surface of the back

plate 41 facing the pump head housings 14a, 14b, 14c is provided with first, second and third arc recesses or grooves 77a, 77b, 77c, each one of said grooves 77a, 77b, 77c being located between neighbouring ones of the lugs 42 and further defining the return flow path. The return flow path therefore presents a relatively large flow area to fuel displaced by reciprocating motion of the shoes to ensure movement of the shoes is substantially unimpeded.

The pump head 13a also includes a return spring 78 (as shown in Figure 7), or other resilient bias means, which serves to urge the shoe 24 and the roller 32 in a radially inward direction such that the roller 32 maintains contact with the cam surface 34 throughout a complete rotation of the drive shaft 12. The force due to the return spring 78 is aided by the force due to fuel pressure within the pumping chamber 30 acting on the pumping plunger 26 in providing a return force which urges the pumping plunger 30 outwardly from the bore 28 to perform a return stroke. It will be appreciated that this only occurs if the pump is operating at its maximum displacement and, hence, at maximum filling.

One advantage of the pump assembly of the present invention is that the flow path for high pressure fuel is defined within the back plate 41, and not within the main pump housing 10. Thus, it is only necessary to form the back plate 41 and pump head housings 14a, 14b, 14c from a material capable of withstanding such high fuel pressures, for example steel, whereas the main pump housing 10 can be formed from a less expensive and lighter material, for example, aluminium. This provides both a manufacturing cost advantage and also enables more convenient machining of the pump assembly parts.

A further advantage is obtained by means of the'Y'shaped high pressure delivery passage 52 defined within the back plate 41 in that the sharp edge formed during machining by the acute angle between the second and third arms

52b, 52c can be removed by means of a tool inserted through the first arm 52a in the back plate 41. By removing this sharp edge, stress concentrations within the back plate 41 are reduced.

In order to assemble the pump assembly, initially each of the pump heads 13 a, 13b, 13c is assembled separately to include the respective pumping plunger 26, shoe 24, roller 32, and inlet and outlet valve arrangements 48,50. The back plate 41 is introduced into the main axial bore 40 and is secured thereto by means of screws threaded connections (not shown). The annular shaft seal 33 and the shim 35 are also located during this stage of assembly. Each assembled pump head 13a, 13b, 13c may then be presented to the main pump housing 10, now secured to the back plate 41, radially such that the shoe and roller arrangement 24,32 of each pump head is engaged between the lugs 42 of the back plate 41 and the further plate 44.

For each pump head, a controlled force is used to clamp the pump head housing 14a resiliently and temporarily against the main pump housing 10, to compress the return spring 78 and the seal 81 between the pump head housing 14a, 14b, 14c and the main pump housing 10. Conveniently, two screw threaded connections are used to secure the back plate 41 to the pump head housings 14a, 14b, 14c, such that the position of each housing relative to the back plate 41 takes up the correct alignment by pulling the back face of each housing firmly against the front face of the back plate 41. The temporary resilient clamping permits small movement of the pump head housing 14a, 14b, 14c during this operation. The force provided by the screw-threaded fasteners is much greater than that provided by the resilient clamping. A small clearance results between the lower face of the pump head housing and the main pump housing 10 because of movement of the housing, and this may be accommodated by the resilience of the seal under the pump head housing. The

temporary clamping is then removed. The screw threaded connections which secure the back plate 41 to the pump head housings dz 14b, 14c serve to compress the high pressure sealing members 72 to ensure a substantially fluid tight seal is provided at the first and second sealing faces 74,76 of each pump head. Finally, screws 17 (as shown in Figure 1) are inserted into through holes (not shown) provided in the pump head housings 14a, 14b, 14c and into screwthreaded drillings in the main pump housing 10 to secure the pump head housings 14a, 14b, 14c directly to the main pump housing 10.

A further advantage of the present invention is that the shoe and roller arrangements 24,32 may be assembled within each individual pump head 13a,

13b, 13c before the pump head housings 14a, 14b, 14c are mounted upon the main pump housing 10. Thus, manufacturing difficulties in assembling the shoe and roller arrangement 24, 32 from inside the main pump housing 10 are avoided. As the pump assembly is of multi-part construction, access for machining precision is also greatly improved and the assembly is readily adaptable for different user requirements.

Claims

CLAIMS 1. A fuel pump assembly for use in an engine, the fuel pump assembly comprising ; a plurality of pump heads mounted upon a main pump housing, each of the pump heads including a pumping plunger which is reciprocable, in use, within a plunger bore under the influence of an associated shoe and roller arrangement so as to cause pressurisation of fuel within a pumping chamber defined within a pump head housing, each of the shoe and roller arrangements including a shoe and a roller, wherein the roller is cooperable with a cam surface, common to each one of the pump heads, so as to impart reciprocable movement to the shoe upon rotation of the drive shaft, and wherein the pump head housing is shaped to define a guide path for guiding reciprocal movement of the associated shoe, and wherein the pump assembly further comprises a back plate which is a separate and distinct component from each of the pump head housings and the main pump housing, the back plate being provided with a common delivery passage for receiving high pressure fuel delivered from each of the pumping chambers, in use.
2. A pump assembly as claimed in Claim 1, wherein each pump head housing is provided with a pair of projections or arms, facing surfaces of which define the guide path for the associated shoe.
3. A pump assembly as claimed in Claim 1 or Claim 2, wherein the common delivery passage is arranged to receive fuel from a plurality of primary flow passages, each being arranged to receive fuel from an associated

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pumping chamber, and is arranged to deliver fuel high pressure fuel to a common pump outlet.
4. A pump assembly as claimed in any of Claims 1 to 3, wherein the common delivery passage includes first, second and third radially extending arms, each one of the first, second and third arms being arranged to receive high pressure fuel from an associated pump head delivery passage.
5. A pump assembly as claimed in any of Claims 1 to 4, wherein the back plate is provided with a first constraining means for constraining movement of the shoe and rollers in a first axial direction.
6. A pump assembly as claimed in Claim 5, wherein the back plate is shaped to define a plurality of formations, each of which is arranged to constrain movement of a respective shoe and roller in the first axial direction.
7. A pump assembly as claimed in Claim 5 or Claim 6, further comprising a second constraining means for constraining movement of the shoe and rollers in a second, opposing axial direction.
8. A pump assembly as claimed in Claim 7, further including a further plate, mounted upon the main pump housing, which is arranged to constrain movement of the shoes and rollers in the second axial direction.
9. A pump assembly as claimed in any of Claims 1 to 8, wherein the pump assembly includes a transfer pump for supplying fuel to the pumping chamber of each pump head at a pressure dependent upon engine speed.

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10. A pump assembly as claimed in any of Claims 1 to 9, wherein the main pump housing and the back plate are formed from different materials.
11. A pump assembly as claimed in Claim 10, wherein the back plate is formed from steel and the main pump housing is formed from aluminium.
12. A pump assembly as claimed in any of Claims 1 to 11, wherein each pump head housing is provided with a recess or groove which defines, in part, a return flow path for fuel displaced by the reciprocating shoe, in use.
13. A pump assembly as claimed in Claim 12, wherein the back plate is provided with at least one groove or recess which defines a part of the return flow path.
14. A pump assembly substantially as hereindescribed with reference to the accompanying drawings.