GB2574620A

GB2574620A - Fuel pump

Info

Publication number: GB2574620A
Application number: GB1809646.1A
Authority: GB
Inventors: T D Mchattie James; J Pedley Toby
Original assignee: Delphi Technologies IP Ltd
Current assignee: Delphi Technologies IP Ltd
Priority date: 2018-06-13
Filing date: 2018-06-13
Publication date: 2019-12-18
Anticipated expiration: 2038-06-13
Also published as: GB201809646D0; GB2574620B

Abstract

A plunger 50 for a pump assembly for delivering pressurised fuel to a common rail fuel volume is disclosed. The piston 50 comprises a cap 54 and body 52 portion positioned in coaxial alignment with concentric bores (84,68, Figure 3) in their adjacent faces. A flexible connector 58 if fixedly engaged in the bores and is displaceable during movement of the plunger to allow axial movement of the cap with respect to the body. A conical interface 56 between the cap and body converts the axial movement of the plunger cap in to a radial expansion to reduce the clearance between the plunger and the pump cylinder bore. The plunger mitigates against the diameter of the piston reducing during compression and the associated fuel leakage, loss of volumetric efficiency, energy loss and increased C02 emissions. A pump for delivering fuel to a common rail is also disclosed.

Description

FIELD OF THE INVENTION

This invention relates to a fuel pump, in particular to a fuel pump for delivering pressurised fuel to a common rail fuel volume.

BACKGROUND

A fuel pump used for delivering fuel to a common rail fuel volume of a compression-ignition internal combustion engine is required to pressurise the fuel to around 2,000 bar or higher. This extreme pressure causes, amongst other things, the plunger of the fuel pump to compress. As the plunger’s diameter reduces during this compression, the clearance between its outer radial surface and the bore in which it reciprocates increases, opening up a larger surface area for the pressurised fuel to leak past. This reduces the volumetric efficiency of the fuel pump, costing energy and therefore CO₂.

Other designs to reduce this effect have been proposed but often their manufacturability or durability does not allow them to be taken forward.

It is against this background that the invention has been devised.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a plunger for a pump assembly for delivering pressurised fuel to a common rail fuel volume, the plunger being configured to reciprocate within an axial bore of the pump assembly between a bottom-dead-centre position and a top-dead-centre position. The plunger comprises a plunger cap comprising a concentric bore in its lower end, and a plunger body comprising a concentric bore in its upper end. The plunger cap is positioned on top of the plunger body in coaxial alignment. The plunger comprises a flexible connection member comprising a plunger cap engagement portion fixedly secured in the bore of the plunger cap and a plunger body engagement portion fixedly secured in the bore of the plunger body. The plunger cap engagement portion is configured to be displaceable during movement of the plunger from the bottom-dead-centre position to the top-deadcentre position to allow an axial movement of the plunger cap with respect to the plunger body during movement of the plunger from the bottom-dead-centre position to the top-dead-centre position. The plunger comprises a conical interface for converting the axial movement of the plunger cap into a radial expansion of the plunger cap to reduce the clearance between the plunger and the axial bore.

The plunger cap engagement portion may comprise a flexible coupling in the form of a disc adjoined to the plunger body engagement portion and a hollow cylinder extending from an upper peripheral surface of the disc to the end of the bore of the plunger cap. The plunger body engagement portion may comprise a post coaxially aligned with and having a diameter substantially equal to or smaller than the hollow section of the hollow cylinder, the post being configured to extend from the bore of the plunger body into the bore of the plunger cap to define a circumferential gap between the lower peripheral surface of the disc and an upper surface of the plunger body.

The plunger body engagement portion may be fixedly secured in the bore of the plunger body and the plunger cap engagement portion may be fixedly secured in the bore of the plunger cap by interference fits.

The conical interface may be formed between a frusto-conical section at the upper end of the plunger body and a corresponding circumferential tapered section at the lower end of the plunger cap.

The half-cone angle of the frusto-conical section may be in the range of 30 to 60 degrees with respect to the longitudinal axis of the plunger body.

The conical interface may be configured to increase the diameter of the plunger cap by up to 6.5 microns.

The plunger may comprise a circumferential shoulder formed in the plunger body at the base of the frusto-conical section for limiting the axial movement of the plunger cap with respect to the plunger body.

According to a second aspect of the invention, there is provided a pump assembly for delivering pressurised fuel to a common rail fuel volume, the pump assembly comprising the plunger as defined in the preceding paragraphs.

It will be appreciated that preferred and/or optional features of the first aspect of the invention may be incorporated alone or in appropriate combination in the second aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more readily understood, preferred non-limiting embodiments thereof will now be described, byway of example only, with reference to the accompanying drawings, in which:

Figure 1 is a sectional view of a fuel pump assembly for delivering fuel to a common rail fuel volume, in which a plunger in accordance with the invention may be incorporated;

Figure 2 is a cross-sectional view of a plunger in accordance with the invention;

Figure 3 is an exploded perspective view of the plunger of Figure 2;

Figure 4 is an exploded sectional view of the plunger of Figure 2;

Figure 5 is an exploded cross-sectional view of the plunger of Figure 2; and, Figure 6 is a cross-sectional view of a portion of the plunger of Figure 2.

In the drawings, like features are assigned like reference signs.

SPECIFIC DESCRIPTION

References in the following description to “upper”, “lower” or any other terms having an implied orientation are not intended to be limiting and refer only to the orientation of the parts as shown in the accompanying drawings.

Figure 1 shows a known pump assembly, generally designated by 10, for delivering pressurised fuel to a common rail fuel volume (not shown) of a compression-ignition internal combustion engine. The pump assembly 10 includes a pump housing 12 that includes a turret 13 extending from the main body of the pump housing 12. The pump housing 12 is provided with an inlet 14, an outlet 16 and an axial bore 18. The axial bore 18 is configured to receive a plunger 20 moveable from a bottom-dead-centre positon to a top-dead-centre position, defining a compression stroke, and from the top-dead-centre position to the bottom-dead-centre positon, defining a suction stroke. To enable reciprocating movement of the plunger 20, a small clearance 22 is provided between the inner surface 24 of the axial bore 18 and the outer radial surface 26 of the plunger 20.

The lower end of the plunger 20 extends from the turret 13 and is coupled to a tappet (not shown) through a spring plate 27. The spring plate 27 defines an abutment surface for one end of a plunger return spring (not shown), the other end of which engages a seat 29 in the outer surface of the pump housing 12 adjacent the turret 13. The upper end of the plunger 20 defines, in combination with the axial bore 18, a compression chamber 28, the volume of which increases and decreases during the suction and compression strokes respectively. The compression chamber 28 communicates with inlet and outlet valve assemblies 30, 32 through internal inlet and outlet passages 31, 33 respectively. The precise configuration of the inlet and outlet valve assemblies

30, 32 is not central to the invention, and so will not be described in detail. Instead, it is sufficient for present purposes that the inlet and outlet valve assemblies 30, 32 are considered to be conventional in that they both comprise a valve member 34 that, in the absence of a pressure drop across the valve member 34, is held against a valve seat 38 under the force of a spring 36. The inlet valve assembly 30 communicates with the inlet 14 through which fuel is supplied to the pump assembly 10. The fuel supplied to the pump assembly 10 originates from a low pressure fuel reservoir (not shown). From there, the fuel is supplied, at a pressure of around 3 to 4 bar, to an inlet metering valve (not shown), which regulates the supply of fuel into the pump assembly 10 depending on the requested engine load. The outlet valve assembly 32 communicates with the common rail fuel volume through the outlet 16.

In use, the operation of the inlet and outlet valve assemblies 30, 32 is a function of pressure drop across their respective valve members 34. When the plunger 20 is in the top-dead-centre position, both the inlet and outlet valve assemblies 30, 32 are closed, preventing fuel from flowing into or out of the compression chamber 28. Fuel enters the pump assembly 10 through the inlet 14 and is held within the inlet valve assembly 30 at a pressure controlled by the inlet metering valve. The plunger return spring provides a return spring force that acts on the spring plate 27, and so the plunger 20, to effect the suction stroke, moving the plunger 20 from the top-dead-centre position. This movement increases the volume of the compression chamber 28, resulting in a corresponding decrease in the pressure within the compression chamber 28, and establishes a pressure drop across the valve member 34 of the inlet valve 30. The pressure drop causes the valve member 34 to lift from the valve seat 38, against the biasing force of the spring 36, to permit fuel to enter the compression chamber 28. Fuel continues to enter the compression chamber 28 so long as the pressure in the inlet valve assembly 30, supplied by the inlet metering valve, exceeds the pressure in the compression chamber 28. Once the plunger 20 reaches the bottom-dead-centre position, it begins the compression stroke under the influence of a cam drive arrangement (not shown), which is mounted upon or forms part of the engine drive shaft.

The valve member 34 of the inlet valve assembly 30 returns to the valve seat 38 to close the inlet valve assembly 30 once the pressure across the valve member 34 equalises. This typically happens soon after the plunger 20 has reached the bottom-dead-centre position or later on during the compression stroke.

During the compression stroke, a small volume of fuel enters the clearance 22 between the plunger 20 and axial bore 18. The plunger 20 and axial bore 18 are subject to forces generated both by pressurised fuel contained in the compression chamber 28, and by pressurised fuel which has entered the clearance 22. Pressurised fuel in the compression chamber 28 generates axially downwards forces on the upper surface 44 of the plunger 20 and pressurised fuel in the clearance 22 generates radially inwards forces on the outer radial surface 26 of the plunger 20. The pressurised fuel in the clearance 22 also generates radially outward forces on the inner surface 24 of the axial bore 18.

The overall effect of these forces is to cause a decrease in the diameter of the plunger 20 and an increase in the diameter of the axial bore 18, resulting in an increase in the clearance 22 between the outer radial surface 26 of the plunger 20 and the axial bore 18. This is detrimental to the volumetric efficiency of the pump assembly 10, as it allows a greater volume of pressurised fuel to leak past the plunger 20 from the compression chamber 28 during the compression stroke.

A known way of addressing the above problem is to include a bore in the upper surface 44 of the plunger 20. In such an arrangement, fuel drawn into the compression chamber 28 fills the bore, generating radially outward forces on the inner surface of the bore during the compression stroke. This increases the diameter of the plunger, partially compensating for the increase in diameter of the axial bore 18. A drawback of this solution is that the introduction of the bore in the upper surface 44 of the plunger 20 increases the volume of fuel that must be compressed. This in turn increases the proportion of the compression stroke needed to pressurise the fuel to the required level. Furthermore, in this known design, the bore in the upper surface 44 of the plunger 20 must be concentric with the plunger 20. This is so that the plunger 20 undergoes a symmetrical radial expansion when pressurised. If the plunger 20 does not expand symmetrically about its longitudinal axis under fuel pressure, then this will affect the fit of the plunger 20 in the axial bore 18, and in turn affect the rate of leakage past the plunger 20. This requirement for a high level of concentricity of the bore in the upper surface 44 of the plunger 20 increases the complexity of this component, making it more difficult to manufacture.

The present invention has been devised with the above considerations in mind and relates to a plunger for use in a pump assembly 10 such as that shown in Figure 1. The plunger is configured to undergo a radial expansion during its compression stroke in order to reduce leakage past the plunger through the clearance 22, improving the volumetric efficiency of the pump assembly 10. This objective is achieved without increasing the volume of the compression chamber 28.

Referring now to Figures 2 to 6, a plunger, generally designated by 50, in accordance with an embodiment of the invention is shown.

The plunger 50 comprises a plunger body 52, a plunger cap 54, a conical interface 56 and a flexible connection member 58. The flexible connection member 58 comprises a plunger body engagement portion 60 and a plunger cap engagement portion 62, and functions as a means for movably coupling the plunger cap 54 and plunger body 52. The conical interface 56 is formed between the plunger cap 54 and plunger body 52, and functions as a means for converting an axial movement of the plunger cap 54 with respect to the plunger body 52 into a radial expansion of the plunger cap 54. The plunger body 52 and plunger cap 54 are generally cylindrical in shape. In this specific embodiment, the plunger body 52, plunger cap 54 and flexible connection member 58 are made from 100Cr6 steel. However, it will be appreciated by the skilled reader that the components of the plunger 50 may be made from other suitable materials.

The plunger body 52 includes an upper end 64 and a lower end 66. The upper end 64 engages with a lower end 82 of the plunger cap 54 when the plunger 50 is assembled. Similar to the plunger of the known pump assembly described above, the lower end 66 is functionally coupled to a cam drive arrangement that is configured to drive reciprocating movement of the plunger 50 in the axial bore 18 during operation.

The upper end 64 of the plunger body 52 is provided with a concentric bore 68 extending along the longitudinal axis, A, of the assembled plunger 50. The bore 68 is dimensioned so as to receive the plunger body engagement portion 60 of the flexible connection member 58 in an interference fit when the plunger 50 is assembled.

As best seen in Figure 3, the upper end 64 of the plunger body 52 comprises a frusto-conical section 70 extending between a circumferential platform 72 and a circumferential shoulder 74 formed at the base of the frustoconical section 70. The circumferential platform 72 surrounds an opening 76 of the bore 68. The frusto-conical section 70 extends radially outwards and axially downwards from the platform 72 to the shoulder 74 to form an inclined outer surface 78. The half-cone angle of the frusto-conical section 70 is 45 degrees with respect to the longitudinal axis, A, of the plunger 50. In other embodiments, the half-cone angle may be in the range of 30 to 60 degrees with respect to the longitudinal axis, A.

The plunger cap 54 includes an upper end 80 and a lower end 82. The lower end 82 is provided with a concentric bore 84 that extends into the plunger cap 54 along the longitudinal axis, A. The bore 84 is configured to receive the plunger cap engagement portion 62 of the flexible connection member 58 when the plunger 50 is assembled. An inclined inner surface 86 extends radially outwards and axially downwards from the bore 84 to form a tapered section 88 extending to a circumferential rim 90 at the lower end of the plunger cap 54. The angle of the inclined inner surface 86 with respect to the longitudinal axis, A, is equal to the half-cone angle of the frusto-conical section 70 of the plunger body 52.

When the plunger 50 is assembled, the lower end 82 of the plunger cap 54 engages with the upper end 64 of the plunger body 52. The upper end 80 of the plunger cap 54 and the axial bore 18 together define the compression chamber 28. The diameter of the plunger body 52 is less than the diameter of the plunger cap 54 so that the forces generated on the plunger 50 by pressurised fuel from the compression chamber 28 are substantially limited to the plunger cap 54. When the pump assembly 10 is assembled, the plunger body 52 extends out of the axial bore 18 and into a volume enclosed by a cambox (not shown). The cambox is at a pressure of around 4 to 5 bar. Reducing the diameter of the plunger body 52 compared to the plunger cap 54 results in the outer surface of the plunger body 52 being exposed to the cambox pressure, thus substantially limiting the region of the plunger 50 that is exposed to high pressure to the plunger cap 54. In this example, the diameter of the plunger body 52 is 50 microns smaller than the diameter of the plunger cap 54.

Now turning to the flexible connection member 58, the plunger cap engagement portion 62 includes a hollow cylinder 92 and a flexible coupling 94. The flexible coupling 94 comprises a disc 96 that couples the hollow cylinder 92 to the plunger body engagement portion 60. The hollow cylinder 92 is formed by a protrusion 98 that extends from an upper peripheral surface 100 of the disc 96 in a direction substantially parallel to the longitudinal axis, A. The hollow cylinder 92 includes an inner radial surface 102 that defines a hollow section 104, an outer radial surface 106 and an upper surface 108. The outer radial surface 106 engages with the plunger cap 54 in an interference fit when the plunger 50 is assembled. The upper surface 108 abuts the end 110 of the bore 84 when assembled. The plunger cap engagement portion 62 is configured to be displaceable during movement of the plunger 50 from the bottom-dead-centre position to the top-dead-centre position during the compression stroke.

The plunger body engagement portion 60 is formed by a solid post 112. The solid post 112 is cylindrical and is dimensioned so as to be receivable in the bore 68 of the plunger body 52 in an interference fit. In this embodiment, the diameter of the solid post 112 is less than the diameter of the hollow section 106 of the plunger cap engagement portion 62. In other embodiments, the diameter of the solid post 112 may vary provided that it does not exceed the diameter of the hollow section 104. This is to allow the flexible coupling to bend as required about the solid post 112 to enable axial movement of the plunger cap 54 with respect to the plunger body 52, as will be explained.

To assemble the plunger 50, the flexible connection member 58 is press-fit into the bore 68 to engage the plunger body 52. The plunger cap 54 is then press-fit onto the flexible connection member 58, so that the plunger cap engagement portion 62 is received in the bore 84 of the plunger cap 54 and engages with the plunger cap 54 in an interference fit. To allow air to escape from the respective bores 68, 84 during press-fitting of the flexible connection member 58, longitudinal channels (not shown) may be formed in the inner surfaces 114, 116 of each bore 68, 84.

When assembled, the flexible connection member 58 fixedly secures the plunger body 52 and plunger cap 54 together to form the three-part plunger 50. The inclined inner surface 86 of the plunger cap 54 abuts the inclined outer surface 78 of the plunger body 52 to form the movable conical interface 56.

As illustrated in Figure 2, first and second gaps 118, 120 exist between the flexible connection member 58 and the plunger body 52 when the plunger 50 is assembled. The first gap 118 provides a first clearance 122 between a lower surface 124 of the solid post 110 and the end 127 of the bore 64. The presence of the first gap 118 provides lee-way, allowing for the inclined outer surface 78 of the plunger body 52 and the inclined inner surface 86 of the plunger cap 54 to be brought into the required positions with respect to each other during assembly of the plunger 50, without the solid post 112 reaching the end of bore 114 before the conical interface 56 is formed. The second gap 120 provides a second axial clearance 126 between a lower peripheral surface 128 of the disc 96 and the platform 72 of the plunger body 52 into which the disc 96 can flex in use. This allows for the axial movement of the plunger cap 54 towards the plunger body 52 during the compression stroke. In this embodiment, the size of the second clearance 126 is 0.5mm, but in other embodiments this size may vary.

As best seen in Figure 6, a third gap 130 is provided between the rim 90 of the plunger cap 54 and the shoulder 74 of the plunger body 52. The shoulder 74 acts as a means for limiting axial movement of the plunger cap 54 with respect to the plunger body 52, by preventing further downwards movement once the rim 90 contacts the shoulder 74.

During operation of the pump assembly 10, the plunger 50 is driven to reciprocate in the axial bore 18 as described above to draw low pressure fuel from a fuel tank into the compression chamber 28 during a suction stroke, before pressurising and expelling high pressure fuel from the compression chamber 28 during a compression stroke.

As already explained, the plunger 50 and the axial bore 18 are subject to forces generated by the pressurised fuel during the compression stroke. Downward axial forces, indicated by arrows labelled F_A in Figure 2, are generated on the upper surface 44 of the plunger cap 54 by pressurised fuel in the compression chamber 28. Inward radial forces, indicated by arrows labelled F_R in Figure 2, are generated on the outer radial surface 26 of the plunger cap 54 by pressurised fuel in the clearance 22 between the plunger 50 and axial bore 18. Outward radial forces are generated on the inner surface 24 of the axial bore 18 by pressurised fuel in the clearance 22. The overall effect of these forces would be to increase the size of the clearance 22. However, the plunger 50 counteracts this increase in clearance 22 by expanding radially during the compression stroke, as will now be described.

The axial forces F_A on the upper surface 44 of the plunger cap 54 are transmitted to the disc 96 via the hollow cylinder 82 of the flexible connection member 58, causing outer regions 136 of the disc 96 to flex downwards into the second gap 120. This allows the plunger cap 54 to move towards the plunger body 52 along longitudinal axis, A. Thus, the flexible connection member 58 enables movement of the plunger cap 54 with respect to the plunger body 52.

As the plunger cap 54 moves towards the plunger body 52, the inclined inner surface 86 of the plunger cap 54 pushes against the corresponding inclined outer surface 78 of the plunger body 52. This movement generates inward radial forces on the inclined outer surface 78 of the plunger body 52, and equal and opposite outward radial forces on the inclined inner surface 86 of the plunger cap 54. The outward radial forces acting on the inclined inner surface 86 are substantial enough so as to generate a radial expansion in the lower region of the plunger cap 54. This radial expansion minimises the clearance between the plunger cap 54 and the axial bore 18, reducing fuel leakage past the plunger 50 during the compression stroke and improving the volumetric efficiency of the pump assembly 10.

The extent of the radial expansion of the plunger cap 54 varies in dependence on the angle of the conical interface, which is equal to the half-cone angle of the frusto-conical section 70. With a half cone angle of 45°, the ratio of axial compression versus radial expansion is 1:1. In this embodiment, the distance moved by the plunger cap 54 towards the plunger body 52 during the compression stroke is substantially 3 microns, and this produces a radial expansion of substantially 1.5 microns, which results in an increase in the diameter of the plunger cap 54 in the region surrounding the conical interface 56 of around 3 microns.

It will be appreciated by a person skilled in the art that the invention could be modified to take many alternative forms to that described herein, without departing from the scope of the appended claims.

REFERENCES USED:

pump assembly 10 pump housing 12 turret 13 housing inlet 14 housing outlet 16 axial bore 18 solid plunger 20 clearance between plunger and axial bore 22 inner surface of axial bore 24 outer radial surface of plunger 26 spring plate 27 compression chamber 28 seat 29 inlet valve 30 inlet passage 31 outlet valve 32 outlet passage 33 valve member 34 valve spring 36 valve seat 38 upper surface of plunger 44 three-part plunger 50 plunger body 52 plunger cap 54 conical interface 56 flexible connection member 58 plunger body engagement portion 60 plunger cap engagement portion 62 upper end of plunger body 64 lower end of plunger body 66 concentric bore of plunger body 68 frusto-conical section 70 circumferential platform 72 circumferential shoulder 74 opening 76 of bore 64 inclined outer surface 78 upper end of plunger cap 80 lower end of plunger cap 82 concentric bore of plunger cap 84 inclined inner surface 86 circumferential tapered section 88 circumferential rim 90 hollow cylinder 92 flexible coupling 94 disc 96 circumferential protrusion 98 upper peripheral surface 100 inner radial surface of hollow cylinder 102 hollow section 104 outer radial surface of hollow cylinder 106 upper surface of hollow cylinder 108 end 110 of bore 84 solid post 112 inner surface 114 of bore 64 inner surface 116 of bore 82 first gap 118 second gap 120 first clearance 122 lower surface of solid post 124 second clearance 126 lower end 127 of bore 64 lower surface of disc 128 third gap 130 outer regions of disc 136

Claims

CLAIMS:

1. A plunger (50) for a pump assembly (10) for delivering pressurised fuel to a common rail fuel volume, wherein the plunger (50) is configured to reciprocate within an axial bore (18) of the pump assembly (10) between a bottom-deadcentre position and a top-dead-centre position, wherein the plunger (50) comprises:

a plunger cap (54) comprising a concentric bore (84) in its lower end (82);

a plunger body (52) comprising a concentric bore (68) in its upper end (64), the plunger cap (54) being positioned on top of the plunger body (52) in coaxial alignment;

a flexible connection member (58) comprising a plunger cap engagement portion (62) fixedly secured in the bore (84) of the plunger cap (54) and a plunger body engagement portion (60) fixedly secured in the bore (68) of the plunger body (52), wherein the plunger cap engagement portion (62) is configured to be displaceable during movement of the plunger (50) from the bottom-dead-centre position to the top-dead-centre position to allow an axial movement of the plunger cap (54) with respect to the plunger body (52) during movement of the plunger (50) from the bottom-dead-centre position to the top-dead-centre position; and, a conical interface (56) for converting the axial movement of the plunger cap (54) into a radial expansion of the plunger cap (54) to reduce the clearance between the plunger (50) and the axial bore (18).

2. The plunger (50) as claimed in claim 1, wherein the plunger cap engagement portion (62) comprises a flexible coupling (94) in the form of a disc (96) adjoined to the plunger body engagement portion (60), and a hollow cylinder (92) extending from an upper peripheral surface (100) of the disc (96) to the end (110) of the bore (84) of the plunger cap (54), and wherein the plunger body engagement portion (60) comprises a post (112) coaxially aligned with and having a diameter substantially equal to or smaller than the hollow section (104) of the hollow cylinder (92), the post (112) being configured to extend from the bore (68) of the plunger body (52) into the bore (84) of the plunger cap (54) to define a circumferential gap (120) between the lower peripheral surface (128) of the disc (96) and an upper surface (72) of the plunger body (52).

3. The plunger (50) as claimed in any preceding claim, wherein the plunger body engagement portion (60) is fixedly secured in the bore (68) of the plunger body (52) and the plunger cap engagement portion (62) is fixedly secured in the bore (84) of the plunger cap (54) by interference fits.

4. The plunger (50) as claimed in any preceding claim, wherein the conical interface (56) is formed between a frusto-conical section (70) at the upper end (64) of the plunger body (52) and a corresponding circumferential tapered section (88) at the lower end (82) of the plunger cap (54).

5. The plunger (50) as claimed in claim 4, wherein the half-cone angle of the frusto-conical section (70) is in the range of 30 to 60 degrees with respect to the longitudinal axis of the plunger body (50).

6. The plunger (50) as claimed in any preceding claim, wherein the conical interface (56) is configured to increase the diameter of the plunger cap (54) by up to 6.5 microns.

7. The plunger (50) as claimed in any preceding claim, further comprising a circumferential shoulder (74) formed in the plunger body (52) at the base of the frusto-conical section (70) for limiting the axial movement of the plunger cap (54) with respect to the plunger body (52).

8. A pump assembly (10) for delivering pressurised fuel to a common rail fuel volume, the pump assembly (10) comprising the plunger (50) according to any preceding claim.