GB2385386A

GB2385386A - Pump assembly

Info

Publication number: GB2385386A
Application number: GB0203615A
Authority: GB
Inventors: George Nicholas Felton; Robert A Trickett
Original assignee: Delphi Technologies Inc
Current assignee: Delphi Technologies Inc
Priority date: 2002-02-15
Filing date: 2002-02-15
Publication date: 2003-08-20
Also published as: EP1336752A3; EP1336752B1; EP1336752A2; EP2261498B1; EP2261498A1; HUE026302T2; GB0203615D0

Abstract

A pump assembly 18, for use in an accumulator fuel system , includes a single plunger 30 which is reciprocable in a bore 32 under the influence of a drive arrangement 40, 42, 46 to cause fuel pressurisation within a pumping chamber 36. The pump assembly has an inlet metering valve arrangement 50 which controls the rate of flow of fuel into the pumping chamber 36. An outlet valve 64 controls the delivery of pressurised fuel from the pumping chamber directly to an accumulator volume such as a common rail, through a high pressure fuel line 70, 20. An inlet check valve 56 may be arranged between the inlet metering valve and the pumping chamber. The drive arrangement may include a tappet assembly.

Description

1 2385386

PUMP ASSEMBLY

The invention relates to a pump assembly for use in supplying high pressure fuel to a fuel injection system of a compression ignition internal combustion engine. In particular, the invention relates to a unit pump assembly having a pumping plunger which is driven by means of a cam shaft to cause pressurization of fuel within a pumping chamber for supply to an accumulator volume, such as a common rail.

One known common rail fuel pump assembly includes a plurality of plungers which are driven by means of a cam drive arrangement so as to pressurise fuel within respective pumping chambers for delivery to the fuel injection system associated with the engine. The cam drive arrangement includes a common eccentric cam surface, rotatable by means of a drive shaft, and it is common for such pumps to include three (or more) plungers which are equi-angularly spaced around the drive shaft. The cam surface is cooperable with all three of the plungers to cause phased, cyclical movement of the plungers and, hence, pressurization of fuel within the pumping chambers. High pressure fuel is delivered to a common rail for supply to the downstream fuel injectors.

Pump assemblies are also known in which a plurality of unit pumps are provided, each of which delivers fuel at high pressure to a separate high pressure fuel line. Each unit pump typically includes a tappet which is driven by means of a cam to drive pumping plunger movement, and is arranged to supply fuel to an injector for delivering fuel to an associated cylinder of the engine. In such pump assemblies it is therefore necessary to provide each engine cylinder with a set of separate pump components, consisting of a cam, a tappet, a unit pump, a high pressure line and an injector, with the cams for each set of pump components being carried on a common drive shaft.

A - 2 Unit injectors are also known in which a pumping element and a nozzle are incorporated within a unitary housing, with one unit injector being provided for each engine cylinder.

So called " in line" pumps include a camshaft, a set of tappets and a set of pumping elements arranged within a unitary housing. Each pumping element has an associated pumping chamber which is connected to an associated injector through a separate high pressure fuel line.

Known Mel pumps of the aforementioned type each have their own advantages and disadvantages, and which type of pump is most suitable for use in any given engine will depend on many factors. It is desirable, however, for engine systems to be adaptable and interchangeable, and this, in particular, is one of the considerations addressed by the present invention.

According to a first aspect of the present invention, there is provided a pump assembly for use in an accumulator fuel system for an internal combustion engine, the pump assembly comprising: a single pumping plunger which is reciprocable within a plunger bore under the influence of a drive arrangement to cause fuel pressurization within a pumping chamber, an inlet metering valve arrangement which is arranged to control the rate of flow of fuel into the pumping chamber, thereby to control the quantity of fuel to be pressurised within the pumping chamber during a pumping cycle, and

- 3 an outlet valve arrangement which is arranged to control the delivery of pressurised fuel from the pumping chamber directly to the accumulator volume through a high pressure fuel line.

Preferably, the accumulator volume takes the form of what is commonly referred to as a "common rail" containing fuel at high pressure for delivery to a plurality of injectors. The common rail may be of tubular configuration (i.e. axially extending), or may be of generally spherical configuration (i.e. of the type having a central hub from which radially extending delivery flow paths extend to the injectors).

The pump assembly may, but need not, be manufactured to include the drive arrangement. In one embodiment the drive arrangement may include a tappet assembly including a tappet member which is cooperable with a roller member to cause reciprocating motion of the pumping plunger upon rotation of a driven cam, in use.

The pump assembly of the present invention provides the advantage that it can be readily incorporated into existing engine installations originally intended for use with separate unit fuel injection pumps whilst preserving the existing engine layout, as there is no requirement to modify the existing pump mounting, cam drive shaft location or pump drive arrangement. Production costs associated with re-tooling an engine production line can also therefore be avoided. The pump assembly also provides the advantage that only that quantity of fuel required for an injection event is pumped during a pumping cycle. In existing pump designs, it is known to pump an excess quantity of fuel on each pumping stroke, with the excess being spilled to a drain port prior to delivery to the _

- 4 injectors. The pump assembly of the present invention provides improved mechanical efficiency over such known pump designs as the quantity of fuel pumped within a pumping cycle is controlled by means of the inlet metering valve arrangement.

In a preferred embodiment, the pumping plunger is arranged such that it can move axially relative to the tappet member.

The pump assembly preferably includes a high pressure fuel line which communicates with the pumping chamber through the outlet valve arrangement, wherein the high pressure fuel line is substantially coaxially aligned with the pumping plunger.

The plunger bore may be provided in a main pump housing, and the high pressure fuel line is preferably defined within an insert member mounted, at least in part, within the main pump housing.

Preferably, the inlet metering valve arrangement includes a valve housing which is adapted to be mounted to the main pump housing. Alternatively, the inlet metering valve arrangement may be housed in a common housing with the pumping plunger and other components of the pump assembly.

According to a second aspect of the present invention, there is provided an accumulator filet system comprising: a pump assembly having a pumping plunger which is reciprocable within the plunger bore under the influence of a drive arrangement to cause fuel pressurization within a pumping chamber,

- s an accumulator volume to which pressurised fuel is delivered directly through a high pressure fuel line, and from which pressurised fuel is delivered to an injector of the fuel system, an inlet metering valve arrangement which is arranged to control the quantity of fuel to be pressurised within the pumping chamber during a pumping cycle, and an outlet valve arrangement which is arranged to control the flow of pressurised fuel from the pumping chamber directly to the accumulator volume. Preferably, the accumulator volume takes the form of a common rail containing fuel at high pressure for delivery to a plurality of injectors.

An inlet of the common rail is in communication with an outlet of the pump assembly by means of the high pressure fuel line, the pump outlet thereby being remotely spaced from the common rail inlet. Thus, the pump assembly delivers fuel to a separate, intermediate Mel volume, in the form of a common rail, through the high pressure fuel line, the fuel system including a high pressure delivery line through which fuel is delivered from the common rail to the injector(s). The injector(s) of the fuel system is therefore spaced apart from the pump assembly.

According to a third aspect of the invention, a multi pump assembly includes a first pump assembly as herein described, and a second pump assembly having a second, single pumping plunger which is reciprocable within a second plunger bore under the influence of a second drive arrangement to cause fuel pressurization within a second pumping chamber, and a second outlet valve

- 6 arrangement which is arranged to control the delivery of pressurised fuel from the second pumping chamber to the accumulator volume through a second high pressure fuel line, and wherein the second pump assembly is provided within an inlet in communication with the inlet metering valve arrangement of the first pump assembly, such that the inlet metering valve arrangement controls the quantity of fuel which is pumped within both the first and second pumping chambers during a pumping cycle.

The multi pump assembly may include three or more pump assemblies, wherein a first one of the pump assemblies is provided with an inlet metering valve arrangement and each of the other pump assemblies is arranged to receive a metered flow of fuel from the inlet metering valve arrangement of the first pump assembly.

The multi pump assembly is advantageous in that it permits an increased quantity of fuel to be supplied to the common rail. Additionally, the assembly is efficient as only the quantity of fuel required during an injection is pumped by virtue of the inlet metering valve arrangement. A further advantage is obtained in that only a single inlet metering valve arrangement is required to control the fuel flow rate to both the first and second pumping chambers. The multi pump assembly may be extended to include third or further pumps, if required, each of which is arranged to receive a metered flow of fuel from the inlet metering valve arrangement of the first pump assembly.

Optional and/or preferable features of the pump assembly of the first aspect of the invention may also be incorporated within the second or third aspects of the invention.

- 7 The invention will now be described, by way of example only, with reference to the accompanying figures in which: Figure 1 is a schematic diagram of a fuel system in accordance with one aspect of the present invention, Figure 2 is a view, shown part in section, of a part of a common rail pump assembly in accordance with a second aspect of the present invention, Figure 3 is a sectional view of an inlet metering valve which may form part of the common rail pump assembly in Figure 2, Figures 4 and 5 illustrate two different cam surface profiles which may be used in a drive arrangement for the pump assembly in Figure 2, and Figure 6 is a schematic diagram of a fuel system in accordance with an alternative aspect of the present invention.

Referring to Figure 1, a common rail fuel system for an internal combustion engine includes a low pressure pump 10 which receives fuel from a low pressure reservoir 12 through a filter arrangement 14. The low pressure pump 10 supplies fuel through a first supply line 16 to an inlet (not shown in Figure 1) of a high pressure pump, referred to generally as 18. The high pressure pump 18 is arranged to cause pressurization of a controllable quantity of fuel to a relatively high level, and delivers high pressure fuel through a second supply line 20 to an accumulator volume in the form of a common rail 22. The common rail 22 is of generally spherical configuration and includes a plurality (four in the illustration shown) of high pressure fuel lines 24 which extend from a cenka1 hub of the rail 22. Each one of the high pressure fuel lines 24 is

- 8 arranged to supply fuel to an injector 26 of the fuel system (only one of which is shown), from where fuel is delivered to an associated engine cylinder or other combustion space. The injector may be of conventional type, the design and operation of which would be well known to a person familiar with this field of technology. For example, the injector may be of the

electromagnetically or piezoelectrically actuable type, may be of the direct actuation type or may be of the type including a hydraulic amplifier arrangement for controlling injector valve needle movement.

Figure 2 shows the high pressure fuel pump 18 in more detail, and from which it can be seen that the pump 18 includes a single pumping plunger 30 which is slidable within a plunger bore 32 provided in a pump housing 34 to cause pressurization of fuel within a pumping chamber 36. The pumping plunger 30 is driven, in use, by means of a drive arrangement, referred to generally as 38, including a generally cylindrical tappet member 40, a roller member 42 and a cam carried by a drive shaft (not shown). The roller 42 is arranged to cooperate with a surface 46 of the cam such that, as the drive shaft rotates, the cam is driven and the roller 42 is caused to ride over the cam surface 46. The roller 42 and the tappet 40 are reciprocable within a guide bore 44 provided in an engine housing 39 which is secured to the pump housing 34. An internal surface of the tappet 40 is provided with an annular groove, within which an abutment plate 47 for a return spring 48 is mounted. The return spring 48 is arranged to urge the tappet and roller arrangement 40, 42 outwardly from the guide bore 44 (downward in the orientation shown in Figure 2) into engagement with the cam surface and, hence, serves to allow the pumping plunger 30 to be urged outwardly from the plunger bore 32 to perform a return stroke of a pumping cycle, as described in further detail below. The tappet 40 and pumping plunger 30 are arranged such that they are able to move axially relative to one another.

Thus, as the tappet 40 is urged inwardly within the guide bore 44 upon rotation

- 9 - of the cam surface, a point will be reached in its range of travel at which it moves into engagement with the pumping plunger 30 to urge the pumping plunger inwardly within the plunger bore 32.

At the end of the pumping plunger 30 remote *om the tappet 40, the pump 18 is provided with an inlet metering valve arrangement, referred to generally as 50, which is located within a separate valve housing 52 secured to a face of the pump housing 34. The inlet metering valve 50 is in communication with a pump inlet 54 which communicates with the first supply line 16 in Figure 1, such that a supply of low pressure fuel is delivered to the inlet metering valve 50 from the low pressure pump 10. The inlet metering valve 50 is arranged to control the rate of flow of fuel delivered to the pumping chamber 36 through an inlet check valve, referred to generally as 56, under the control of an Engine Control Unit (ECU, not shown).

The inlet metering valve 50 may typically be of the type shown in further detail in Figure 3, in which a metering valve member 75 is movable under the influence of an electromagnetic actuator, referred to generally as 77, to control the extent of opening of an orifice or restriction 79 in a flow path between the pump inlet 54 and the inlet check valve 56, thereby to vary the rate of flow of fuel through the orifice 79 to the pumping chamber 36. The metering valve member 75 is movable between a closed position in which communication between the pump inlet 54 and the inlet check valve 56 through the orifice 79 is closed, and a fully open position in which a maximum rate of flow of fuel through the orifice 79 is permitted. Movement of the metering valve member 75 is effected by energising and de-energising a winding 81 of the actuator 77 under the control of the ECU. Further details of the operation of a metering valve of the type shown in Figure 3 would be familiar to a person skilled in the field of engine fuel system design.

- 10 The inlet check valve 56 includes a valve abutment member 60 defining a valve seat 62 with which a check valve member 58 is engageable to control the metered flow of fuel from the inlet metering valve 50 to the pumping chamber 36. The valve abutment member 60 is provided with axially and radially extending passages which communicate with one another such that, when the check valve member 58 is caused to lift from the valve seat 62, fuel delivered to the pump inlet 54 and passing through the inlet metering valve 50 is able to flow into the radially extending passage in the valve abutment member 60, into the axially extending passage and past the valve seat 62 into the pumping chamber 36. Although not shown in Figure 2, in practice it may be desirable to provide the inlet check valve 56 with a relatively low spring pre-load to urge the check valve member 58 into a position in which it engages the valve seat 62. Whilst the flow into the pumping chamber 36 is controlled by means of the inlet metering valve 50 and the inlet check valve 56, the flow of Mel out of the pumping chamber 36 is controlled by means of an outlet delivery valve arrangement, referred to generally as 64. The outlet valve arrangement 64 takes the form of a ball valve having a ball 66 which is engageable with a further valve seat 68 to control fuel flow between the pumping chamber 36 and a high pressure supply line 70. The outlet valve arrangement 64 may be provided with an outlet valve spring (not shown) having a relatively low pre-load, which serves to urge the ball 66 into engagement with the further valve seat 68.

The high pressure flow line 70 is defined by a passage provided in an insert member 72 located, in part, within a further bore 73 provided within the pump housing 34 and partially extending from the pump housing 34. The high pressure flow line 70 is substantially coaxially aligned with the pumping

- 11 plunger 30 and is arranged to communicate, at its end remote from the pump housing 34, with an end of the second supply line 20 to the common rail 22.

I'd Thus, in use, high pressure fuel delivered from the pumping chamber 36 to the high pressure flow line 70 is able to flow into the second supply line 20, and into the common rail 22, for delivery to the injectors 26.

In use, as the drive shaft is rotated and the roller 42 rides over the cam surface, the tappet 40 is caused to reciprocate within the guide bore 44, thereby causing axial movement to be imparted to the pumping plunger 30 as the tappet 40 is moved into engagement with, and moves with, the pumping plunger 30. A pumping cycle consists of two phases. During a filling phase, the inlet check valve 56 is open to permit fuel delivery from the inlet metering valve 50 to the pumping chamber 36, and the outlet valve arrangement 64 is held closed by means of high pressure fuel within the high pressure flow line 70 to the common rail. During the filling phase, the pumping plunger 30 is urged outwardly from the plunger bore 32 to perform a return stroke due to the pressure exerted on the plunger 30 by the flow of fuel from the inlet metering valve 50, through the inlet check valve 56 and into the pumping chamber 36.

During a subsequent pumping phase of the pumping cycle, the inlet check valve 56 is caused to close due to increasing fuel pressure within the pumping chamber 36 as the plunger 30 starts to move inwardly under the drive of the tappet 40, to prevent further flow of fuel into the pumping chamber 36 from the inlet metering valve 50. Additionally, as fuel pressure within the pumping chamber 36 increases further, the outlet valve arrangement 64 is caused to open to permit pressurised fuel within the pumping chamber 36 to flow into the high pressure flow line 70. During the pumping phase the pumping plunger 30 is urged inwardly within the plunger bore 32, under the influence of the tappet 40

- 12 cooperating with the roller 42 and the driven cam surface, to cause fuel pressurization within the pumping chamber 36.

The sequence of events during a pumping cycle will now be described in further detail. At the start of the pumping cycle, the pumping plunger 30 adopts its innermost position within the plunger bore 32 (i.e. uppermost position in the orientation in Figure 2) and fuel pressure within the pumping chamber 36 is high due to the pressurization caused by the previous pumping stroke. The outlet valve arrangement 64 is closed due to the equalization of fuel pressures in the pumping chamber 36 and the high pressure flow line 70. The tappet 40 is also at its innermost position in the guide bore 44, and high fuel pressure within the pumping chamber 36 serves to urge the pumping plunger 30 into contact with the tappet 40.

Upon commencement of its return stroke, the plunger member 30 is initially allowed to retract from the plunger bore 32 due to decompression within the pumping chamber 36 and retraction of the tappet 40 under the force of the return spring 48 as the roller 42 rides over the cam surface. As the pumping chamber 36 is decompressed, a point will be reached at which the pressure in the pumping chamber 36 falls below the pressure required to lift the check valve member 58 from the valve seat 62 due to the flow of fuel from the inlet metering valve 50, and the next filling phase commences.

Further movement of the pumping plunger 30 outwardly from the plunger bore 32 is effected by a force due to pressure within the pumping chamber 36 caused by the flow of fuel from the inlet metering valve 50, through the radially and axially extending passages in the valve abutment member 60 and through the inlet check valve 56 into the pumping chamber 36. Further retraction of the tappet 40 from the guide bore 44 (i.e. outward movement of the tappet 40 from

- 13 the bore 44) occurs under the force of the return spring 48, causing the roller 42 to ride over the cam surface.

During the filling phase, the ball 66 of the outlet valve arrangement 64 remains seated against the further valve seating 68 due to high pressure fuel within the high pressure flow line 70 and due to the force of the outlet valve spring.

After the tappet 40 reaches its outermost position within the guide bore 44, the roller 42 is urged in an upward direction (in the illustration shown in Figure 2) as it follows the cam surface, and a point will be reached at which the tappet 40 moves into engagement with the plunger member 36, thereby causing the pumping plunger 30 to be driven inwardly within the plunger bore 32. As the pumping plunger 30 is driven inwardly within the plunger bore 32, fuel within the pumping chamber 36 is pressurized.

As fuel pressure within the pumping chamber 36 starts to increase, a point will be reached part way through the pumping stroke at which the check valve member 58 of the inlet check valve 56 is urged against its seating, due to increasing fuel pressure within the pumping chamber 36, to prevent further flow of fuel into the pumping chamber 36 and return flow from the pumping chamber 36 towards the inlet metering valve 50.

As the plunger pumping stroke continues, fuel within the pumping chamber 36 is pressurised to a sufficiently high level to cause the ball 66 to lift from the further valve seating 68, thereby permitting pressurised fuel to flow from the pumping chamber 36 into the high pressure flow line 70 and, hence, to the supply line 20 to the common rail 22.

- 14 At the end of the pumping stroke, when the pumping plunger 30 reaches the end of its range of travel, the ball 66 will be urged against the further valve seating 68 due to high pressure fuel within the high pressure flow line 70 and the force of the outlet valve spring, thereby holding high Mel pressure within the high pressure flow line 70, the second supply line 20 and, hence, within the common rail 22.

The extent of plunger movement during the pumping stroke will be determined by the quantity of fuel delivered to the pumping chamber 36 during a filling phase, as this determines the extent to which the pumping plunger 30 is retracted from the plunger bore 32 during the return stroke. The quantity of fuel delivered to the pumping chamber 36 during the filling phase therefore determines the point in the range of travel of the tappet 40 at which it engages the pumping plunger 30 to commence the plunger pumping stroke.

The quantity of fuel delivered to the pumping chamber 36 during one pumping cycle is therefore determined by the rate of flow of fuel through the inlet metering valve 50, and the time for which the inlet check valve 56 is held open to permit fuel flow into the pumping chamber 36. The time for which the inlet check valve 56 is held open is determined by the spring rate of the inlet valve spring (if provided), the hydraulic force acting on the check valve member 58 as fuel is pressurized within the pumping chamber 36 and the speed of the associated engine which determines the rate of movement of the tappet 40. The quantity of fuel delivered to the pumping chamber 36 can therefore be varied by adjusting the inlet metering valve setting to vary the fuel flow rate through the inlet check valve 56. The inlet metering valve 50 is operable by means of the ECU between a fully open state, corresponding to maximum filling and a maximum pumping plunger stroke, and a fully closed state corresponding to zero filling and zero pumping plunger stroke, and has a range of settings

- 15 between its fully open and closed states to vary the extent of filling of the pumping chamber 36 and, hence, the quantity of fuel delivered to the common rail 22 during any given pumping cycle.

The provision of the inlet metering valve 50 provides the advantage that only that quantity of fuel required for an injection event is pumped during a pumping cycle. In existing pump designs, it is known to pump an excess quantity of Mel on each pumping stroke, with the excess being spilled to a drain port prior to delivery to the injectors. The pump assembly of the present invention provides improved mechanical efficiency over such known pump designs. The cam drive arrangement for the pump assembly is typically arranged to have a cam surface with one cam lobe per injection event. Figure 4 shows the cam drive arrangement which may be used in a two cylinder engine, and in which the cam 80, driven by means of an engine drive shaft 82, has a cam surface provided with two cam lobes. Figure 5 shows the cam drive arrangement for a three cylinder engine, in which the cam 80 has a three lobed cam surface.

If the quantity of fuel required for an injection event is greater than that which can be provided by a single pump assembly, two or more pump assemblies may be used. Referring to Figure 6, there is shown dual pump arrangement in which first and second pump assemblies 100_, 100_ respectively are arranged to deliver fuel at high pressure through respective supply lines 20_, 20_ to the common rail 22. The first pump assembly lOOa has a first inlet 54a which is supplied with fuel through a first supply passage 16 (as shown in Figure l). As described previously with reference to Figure 2, low pressure fuel delivered to the inlet check valve 56 is regulated by means of the inlet metering valve SO to

- 16 control the quantity of fuel pumped within the pumping chamber 36 during a pumping cycle.

The first and second pump assemblies lOOa, lOOb are structurally substantially identical, except that the second pump assembly 100_ need not be provided with an inlet metering valve. Instead, the second pump assembly 1 OOb is arranged such that an inlet 54_ to its pumping chamber is supplied with fuel from the inlet metering valve 50 of the first pump assembly 100_.

Conveniently, the metered flow of fuel from the first pump assembly l DO_ may be tapped off from an outlet 102 (also shown in Figure 2). Conveniently, a port 104 of the second pump assembly 1 lOb, which is equivalent to the inlet 54_ of the first pump assembly 100_, may be used to provide a backleak connection to low pressure. The use of a multi pump scheme such as that shown in Figure 5 is advantageous in that it permits an increased, yet controlled, quantity of fuel to be supplied to the common rail by means of the multiple pumping cycles in an efficient manner, whilst only requiring a single inlet metering valve SO to be provided. It will be appreciated that, although the pump assembly of the present invention is shown to include a tappet drive arrangement which cooperates with the cam, other drive arrangements are also envisaged, for example shoe and roller arrangements.

Claims

- 17 CLAIMS

1. A pump assembly for use in an accumulator fuel system for an internal combustion engine, the pump assembly comprising: a single pumping plunger which is reciprocable within a plunger bore under the ini:luence of a drive arrangement to cause fuel pressurization within a pumping chamber, an inlet metering valve arrangement which is arranged to control the rate of flow of fuel into the pumping chamber, thereby to control the quantity of fuel to be pressurized within the pumping chamber during a pumping event, and an outlet valve arrangement which is arranged to control the delivery of pressurized fuel from the pumping chamber directly to an accumulator volume through a high pressure fuel line.

2. A pump assembly as claimed in Claim 1, further comprising the drive

arrangement.

3. A pump assembly as claimed in Claim 1, wherein the drive arrangement includes a tappet assembly including a tappet member which is cooperable a roller member to cause reciprocating motion of the pumping plunger upon rotation of the cam shaft, in use.

4. A pump assembly as claimed in any of Claims l to 3, further comprising an inlet check valve for controlling the flow of fuel between the inlet metering valve arrangement and the pumping chamber, the inlet check valve being arranged such that during a pumping stroke of the pumping plunger, the inlet

- 18 check valve is closed to prevent fuel flow into the pumping chamber and, during a return stroke of the pumping plunger, the inlet check valve is opened to permit filling of the pumping chamber through the inlet metering valve arrangement.

5. A pump assembly as claimed in Claim 3 or Claim 4, wherein the pumping plunger is arranged such that it can move axially relative to the tappet member.

6. A pump assembly as claimed in any of Claims l to 5, including a high pressure fuel line which communicates with the pumping chamber through the outlet valve arrangement, wherein the high pressure fuel line is substantially coaxially aligned with the pumping plunger.

7. A pump assembly as claimed in Claim 6, wherein the plunger bore is provided in a main pump housing, and wherein the high pressure fuel line is defined within an insert member mounted, at least in part, within the main pump housing.

8. An accumulator fuel system for an internal combustion engine, the fuel system comprising: an injector, a pump assembly as claimed in any of Claims l to 7, an accumulator volume to which pressurised fuel is delivered from a high pressure fuel line, and from which pressurised fuel is delivered to the injector, wherein the inlet metering valve arrangement of the pump assembly is arranged

- 19 to control the quantity of fuel to be pressurized within the pumping chamber during a pumping cycle, and the outlet valve arrangement is arranged to control the flow of pressurised fuel from the pumping chamber directly to the accumulator.

9. A fuel system as claimed in Claim 8, wherein the accumulator takes the form of a common rail of tubular configuration, or of generally spherical configuration having a central hub from which a plurality of radially extending delivery flow paths extend to respective injectors.

10. A fuel system as claimed in Claim 9, wherein an inlet of the common rail is in communication with an outlet of the pump assembly by means of the high pressure fuel line, such that the pump assembly outlet is remotely spaced from the common rail inlet.

11. A multi pump assembly comprising a first pump assembly as claimed in any of Claims I to 7, and a second pump assembly having a second, single pumping plunger which is reciprocable within a second plunger bore under the influence of a second drive arrangement to cause fuel pressurization within a second pumping chamber, and a second outlet valve arrangement which is arranged to control the delivery of pressurised fuel from the second pumping chamber directly to an accumulator volume through a second high pressure fuel line, and wherein the second pump assembly is provided within an inlet in communication with the inlet metering valve arrangement of the first pump assembly' such that the inlet metering valve arrangement controls the quantity of fuel which is pumped within both the first and second pumping chambers during a pumping cycle.

- 20

12. A multi pump assembly as claimed in Claim 11, including three or more pump assemblies, wherein a first one of the pump assemblies is provided with an inlet metering valve arrangement and each of the other pump assemblies is arranged to receive a metered flow of fuel from the inlet metering valve arrangement of the first pump assembly.

13. A pump assembly substantially as herein described with reference to the accompanying drawings.

14. A fuel system substantially as herein described with reference to the accompanying drawings.

15. A multi pump assembly substantially as herein described with reference to the accompanying drawings.