WO2017174405A1

WO2017174405A1 - Fuel pump

Info

Publication number: WO2017174405A1
Application number: PCT/EP2017/057330
Authority: WO
Inventors: Engin ERDEMIR; Benjamin P. G. WEST; Gunay CICEK; Erol KAHRAMAN
Original assignee: Delphi International Operations Luxembourg S.À R.L.
Priority date: 2016-04-08
Filing date: 2017-03-28
Publication date: 2017-10-12
Also published as: GB2551951A

Abstract

A fuel pump (110) comprising a pumping plunger (116) comprising a first plunger part (116a) and a second plunger part (116b) and a pump head (112) defining a first barrel (114) in which the first plunger part (116a) is reciprocable to pressurise fuel in a pumping chamber (122). The second plunger part (116b) is reciprocable within the further barrel (141c) which may be formed in the same or a different housing to the pump head (112). The first and second plunger parts (116a, 116b) are connected by a connector part (146) located part-way along the length of the pumping plunger so that the first and second plunger parts (116a, 116b) move together.

Description

FUEL PUMP TECHNICAL FIELD

The present invention relates to a fuel pump for use in a fuel system of an internal combustion engine and, in particular, to a fuel pump for use in fuel system including an accumulator volume in the form of a common rail for supplying fuel to a plurality of injectors.

BACKGROUND TO THE INVENTION

Conventional common rail fuel injection systems for diesel engines include a high pressure pump for charging an accumulator volume, or common rail, with high pressure fuel with which to supply a plurality of injectors of the fuel system. The pressure of fuel may be up to or even exceed 2000 bar. Typically, each injector is provided with an electronically controlled nozzle control valve to control movement of a fuel injector valve needle and, thus, to control the timing of delivery of fuel from the injectors to associated combustion chambers of the engine.

Figures 1 a to 1 c illustrate a known fuel pump 10 at various stages of a pumping cycle. The fuel pump 10 includes a fuel pump housing 12, or pumping head, provided with a plunger bore, or barrel 14, within which a pumping plunger 16 reciprocates, in use, under the influence of a drive arrangement 18. The plunger 16 and its barrel 14 extend co-axially through the pump housing 12. An upper region of the barrel 14 defines a cylindrical pumping chamber 22 of the fuel pump 10. Fuel is admitted into and is discharged from the pumping chamber 22 by an inlet passage 20 and an outlet passage 21 , respectively. A fuel gallery 24 is provided in the pump housing 12 for holding low pressure fuel.

During operation of the fuel pump 10, a supply line 28 delivers low pressure fuel (e.g. below 5 bar absolute pressure) from a suitable source to the fuel gallery 24. The flow of low pressure fuel from the gallery 24 to the pumping chamber 22 is controlled by an inlet valve 26 that is provided in the inlet passage 20. A spring- biased inlet valve member 30 of the inlet valve 26 is configured to be movable within the inlet passage 20 in order to control the rate of flow of fuel from the gallery 24 to the pumping chamber 22. The inlet valve member 30 is displaced to an open or closed position in response to a change in the pressure differential between the gallery 24 and the pump chamber 22.

The drive arrangement 18 includes a tappet 32, which may be driven by means of a cam (not shown) to impart drive to a lower end of the plunger 16. The cam is typically connected to a cam shaft which is driven by the engine as would be well known by the skilled person. The tappet 32 is connected to a lower part of the pump housing 12 by a return spring 34. The return spring 34 is configured to impart a downward motion on the plunger 16 by recoiling once the force of the driving cam is removed. The tappet 32 is thereby pushed away from the pump head 12, thereby drawing the plunger 16 downwardly in the plunger barrel 14.

The pump cycle of the fuel pump consists of a pumping stroke in which the plunger 16 is driven inwardly within the plunger barrel 14 to reduce the volume of the pumping chamber 22 and a return stroke in which the plunger 16 is driven outwardly from the plunger barrel 14 to increase the volume of the pumping chamber 22.

Figure 1 a illustrates the fuel pump after the pumping stroke has been performed, and in which the plunger 16 is in its most inward position with respect to the plunger barrel 14, thereby minimising the volume of the pumping chamber 22.

With reference to Figure 1 b, the return stroke starts when the plunger 16 is pulled outwardly from within the plunger barrel 14 by the return spring 34. The downward motion of the plunger 16 causes a drop in fuel pressure within the pumping chamber 22, which results in the formation of a negative pressure differential across the inlet valve 26, thereby causing it to admit low pressure fuel from the fluid-inlet gallery 24 into the high-pressure pumping chamber 22.

The pumping stroke, as shown in Figure 1 c, starts when the plunger 16 is at its most outward position with respect to the plunger barrel 14, wherein the inlet valve 26 closes. During the pumping stroke, the plunger 16 is driven inwardly within the plunger barrel 14 by the drive arrangement 18 which pressurises fuel within the pumping chamber 22 until, at a predetermined level, a positive pressure differential is formed across an outlet valve 36 causing it to open. The pressurised fuel is then delivered through the outlet valve 36 to a downstream common rail of the fuel injection system. In this way the fuel pump 10 allows pressurised fuel to be delivered to the common rail of the fuel injection system for each revolution of the engine.

In common rail fuel injection systems, the trend is towards increasing the injection pressure In order to optimise the combustion quality and efficiency. In addition to improving combustion characteristics, higher injection pressures have enabled higher engine speeds to be reached which, in turn, leads to an increase in the power output from the engine. However, as fuel pumps are typically driven by the engine, the increase in engine speeds increases the speed envelope of the fuel pump. The increasing pump frequency leads to a reduction in the time that is available to fill the pumping chamber before each subsequent pumping cycle, which can result in a reduction in the pumping efficiency of the fuel pump when operating at higher engine speeds. This effect can be made worse with the trend of synchronising pump delivery with fuel injection.

This problem can be addressed somewhat by increasing the pressure fuel supplied to the fuel pump. However, this requires the diversion of more energy from the engine, which compromises engine efficiency and results in a subsequent increase in the carbon emissions of the vehicle, which is not desirable.

It is one aim of the present invention to provide a fuel system for a common rail fuel system which provides improvements over known common rail fuel systems and which addresses, in particular, the issue of variable injection characteristics and of parasitic fuel losses so as to provide enhanced system efficiency.

SUMMARY OF THE INVENTION

It is against this background that the invention provides, in a first aspect, a fuel pump comprising a pumping plunger comprising a first plunger part and a second plunger part, and a pump head defining a first barrel in which the first plunger part is reciprocable to pressurise fuel in a pumping chamber. The second plunger part is reciprocable within a further barrel. The first and second plunger parts are connected by a connector part located part-way along the length of the plunger so that the first and second plunger parts move together. Reciprocable movement of the second plunger part within the further barrel imparts a force to the first plunger part to drive reciprocable movement of the first plunger part within the pump head.

In one embodiment, a second pump housing is located adjacent to the pump head and defines the further barrel.

It is a particular benefit of the invention that the plunger may reciprocate both within the barrel in the pump head, and within the adjacent housing part, whilst avoiding alignment problems between the barrels due to the two-part nature of the plunger. The two-part connection between the plungers allows any lack of concentricity between the two barrels in the adjacent housing parts to be readily accommodated.

Alternatively the first barrel and the further barrel may be formed in the same housing part.

In one embodiment, the first plunger part forms an upper pumping part of the plunger, and the second plunger part forms a lower driving part of the plunger. The first plunger part and the first barrel may define a first clearance therebetween and the second plunger part and the further barrel may define a second clearance therebetween, wherein the first and second clearances are different. Preferably, the second clearance is larger than the first clearance. With a conventional plunger arrangement side load forces act along the length of the plunger, which often means that certain shapes of the barrel for the plunger are required (e.g. trumpet shaped) and/or the clearance between the plunger and the barrel needs to be increased to allow for adequate lubrication. This has the disadvantage that pump efficiency is reduced.

By separating the plunger into two parts, different clearances may be assigned to the first plunger part and the second plunger part. Only one of the plunger parts (e.g. the first plunger part) is exposed to high fuel pressures and pumps fuel in the pumping chamber, whilst the second plunger part is resolving the side loads and delivering force to the first plunger part. Only the further barrel and the second plunger part therefore need a larger clearance between them, and the clearance between the first plunger part and the first barrel can be minimised to improve pump efficiency.

A further benefit of a multi-part plunger is that the first and second plunger parts may be formed from different materials, as both have different functions and performance requirements.

By way of example, the first plunger part may be formed from a first material, such as ceramic, that is different from the material of the second plunger part. In another embodiment, the first plunger (being the pumping part of the plunger) may be provided with a coating which the second plunger does not need. The first plunger part of the plunger may abut directly the second plunger part.

In one embodiment, the connector part is an annular element carried by the plunger. The annular element may be located within at least one groove in the plunger to hold the connector part in position along the plunger length.

The connector part may, in one embodiment, be formed from the same material as the first and second plunger parts.

In another embodiment, the connector part may be formed from an engineering plastics material.

The fuel pump may further comprise a fluid-inlet path through which fuel flows into the pumping chamber during a plunger return stroke, and wherein the fluid inlet path is defined, in part, by a priming-pump chamber within which fuel is pressurised to an intermediate level prior to delivery to the pumping chamber.

The connector part may reside within the priming-pump chamber and may define a priming-pump piston to cause pressurisation of fuel within the priming-pump chamber, to the intermediate level, as the plunger moves.

The invention also resides, in a second aspect, in a fuel system comprising the fuel pump as defined above. 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 within the second aspect of the invention also.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference has already been made to Figures 1 a, 1 b and 1 c, which show a cross section of a part of a known positive displacement fuel pump for a common rail fuel injection system at different stages of a pumping cycle. Preferred embodiments of the present invention will now be described, by way of example only, with reference to the following drawings in which:

Figures 2a, 2b and 2c show a cross section of a fuel pump of an embodiment of the invention, where Figures 2b and 2c illustrate the return stroke and the pumping stroke of the fuel pump pumping cycle, respectively; and,

Figure 3 is an enlarged view of a section of the plunger in the fuel pump of Figures 2a, 2b and 2c.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

References in the following description to "upper", "lower" and other terms having an implied orientation are not intended to be limiting and refer only to the orientation of the parts shown in the accompanying drawings. Also, although the embodiments relate to a fuel pump, references will also be made to 'fluid' which term is considered synonymous with fuel in the present context. However, it should be noted that the fuel pump of the embodiments described herein could also be used to pump fluids other than fuel.

Referring to Figure 2a, a fuel pump 1 10 for use in a common rail fuel injector in a diesel engine of a vehicle includes a fuel pump head 1 12 provided with a plunger bore, or barrel 1 14, within which a pumping plunger 1 16 reciprocates, in use, under the influence of a drive arrangement 1 18. The plunger 1 16 and its barrel 1 14 extend co-axially through the pump head 1 12. The plunger 1 16 is formed in two parts, further details of which will be described later. An upper region of the plunger barrel 1 14 defines a pumping chamber 122 of the fuel pump. A fluid-inlet path 120, as will be described in further detail later, communicates with the pumping chamber for the purposes of supplying fuel to it. A fluid-outlet path 121 intersects a top region of the plunger barrel 1 14 and provides a path for fluid out of the pumping chamber 122.

The pumping chamber 122 communicates with the fluid-outlet path 121 and a downstream outlet port 124 of the pump head 1 12 via an outlet valve 126 which comprises a spring-biased valve member 128, in this embodiment. The outlet port 124 is substantially co-axially aligned with the outlet valve 126, the fluid- outlet path 121 and the plunger 1 16. The outlet valve 126 controls the flow of fuel from the pumping chamber 122 through the fluid-outlet path 121 in dependence on the fuel pressure across the valve, as would be known to the skilled person. The pump head 1 12 is further provided with a sealing means, which is located at an opening where the plunger 1 16 exits the pump head 1 12. The sealing means, in the form of an annular rubber seal 130 is configured to prevent fluid and air from entering or exiting the plunger barrel 1 14. The plunger 1 16 is reciprocally slidable within the plunger barrel 1 14 under the influence of the drive arrangement 1 18 to cause fuel pressurisation within the pumping chamber 122. The drive arrangement 1 18 includes a cam driven tappet 134, which is coupled to the plunger 1 16 to impart drive thereto, in use, so that the plunger 1 16 performs a pumping cycle including a pumping stroke and a return stroke.

The tappet 134 is connected to a lower part of the pump housing 1 12 by a return spring 136. The return spring 136 is configured to draw the plunger 1 16 downwardly once the force of the cam is removed. In so doing, the tappet 134 is pushed away from the pump head 1 12, thereby driving the plunger 1 16 outwardly from the plunger barrel 1 14.

At this point it should be noted that, in contrast to the known fuel pump described above with reference to Figures 1 a to 1 c, the fuel pump of this embodiment includes an inlet to the pumping chamber 122 that is not defined by the pump head 1 12 but instead is defined by the plunger itself. Moreover, the inlet defined in the pumping plunger is fed by a fluid-inlet path that is also defined at least in part, within the pumping plunger. One benefit of this is that it simplifies the design of the pump head since only the fluid-outlet path and associated outlet valve need to be accommodated by the pump head. So, there is freedom to locate the pump outlet in the optimal position or orientation and fewer drillings in the pump head are required which is a benefit in terms of manufacturing. Additionally, the plunger is configured to cause pressurisation of the fuel in the fluid-inlet path, and this means that a smaller lift pump is required to pressurise fuel to supply the inlet side of the fuel pump. In general, the design that will now be described improves the pumping efficiency of the fuel pump by providing substantially constant pressure at the pump inlet irrespective of pump speed.

Returning now to Figures 2a to 2c, it can be seen a middle portion of the barrel 1 14 defines an enlarged diameter region extending about a lateral portion of the plunger 1 16 that provides a priming-pump chamber 140. The priming pump chamber 140 is therefore located remotely from the pumping chamber 122. Since the pumping chamber 122 is configured to raise the pressure of fuel within it to injection pressures, it can be considered to be a primary pumping chamber, whereas the priming-pump chamber 140 can be considered to be a secondary pumping chamber as it is configured to raise the pressure of fuel within it to a relatively low 'priming' or 'filling' pressure for delivery of fuel into the primary pumping chamber 122.

In the illustrated embodiment, the priming-pump chamber 140 is defined in part by an enlarged portion of the plunger barrel 1 14 which is located remotely from the pumping chamber 122. The priming-pump chamber 140 is closed at its bottom end by a priming-pump head 141 . The priming-pump head 141 is adjacent the main pump head 1 12 and is positioned at an opening of the plunger barrel 1 14 where the plunger 1 16 exits the pump head 1 12. So, it will be appreciated that the priming-pump head 141 is a separate component in this embodiment that enables the convenient manufacture of the priming-pump chamber 140, although other configurations are possible. The priming-pump head 141 is shaped to define an annular wall 141 a that provides a socket 141 b which is received onto a complementary-shaped portion of the pump head 1 12. The priming-pump head 141 therefore mates with the pump head 1 12 to become an integral part of it. This mating of parts could be by way of a press fit or by way of a screw thread. The priming pump head 141 is further provided with a barrel or bore 141 c, which extends beneath the socket 141 b, for receiving a lower end of the plunger 1 16. The priming-pump chamber 140 is configured to receive low pressure fuel from an external supply line (not shown) at relatively low pressure (for example at less than 5 bar) from a suitable source to an inlet port 142 of the pump head 1 12. The fluid-inlet path 120 includes a fluid-supply passage 120a configured to supply low pressure fuel from the inlet port 142 to the priming-pump chamber 140. In this way, the fluid-supply passage 120a supplies fluid directly to the priming-pump chamber 140.

The fluid-supply passage 120a includes a non-return valve 144 which is operable to control fuel supplied to the priming-pumping chamber 140 during the pumping stroke of the plunger 1 16. The non-return valve 144 prevents fuel from the priming-pump chamber 140 flowing back along the fluid-supply passage 120a and out of the pump head 1 12 via the inlet port 128, thereby preventing the depressurisation of the priming-pump chamber 123 through that passage. Referring also to Figure 3, the plunger is formed in two parts 1 16a, 1 16b which are joined or connected together by means of a collet 146. The collet 146 is an annular element located securely in upper and lower annular grooves (not identified) provided in the upper and lower plunger parts 1 16a, 1 16b respectively. The collet 146 serves to prevent any longitudinal or other movement between the two plunger parts 1 16a, 1 16b as they move together, in use.

If the piston 146 is a collet, it would snap into such grooves and thus be fixed in position. Alternatively, it could be welded in place, or press fitted into position, particularly if the piston is a solid ring rather than a collet. Other techniques may also be used to combine the piston 146 and the plunger 1 16 together with the first and second plunger parts 1 16a, 1 16b held securely in abutment. The collet 146 may be metallic and could be formed through fracture splitting in order to ensure there is a good match between the collet parts when they come together, thus minimising the leak path. The diameter of the collet 146 is just less than the internal diameter of the priming-pump chamber 140 so that the collet 146 forms a "priming-pump piston" which serves to pressurise fuel within the priming-pump chamber 140 as the plunger reciprocates in use (as described in further detail below).

As can be seen in Figure 3, only a slight clearance exists between the outer diameter of the priming-pump piston 146 and the internal diameter of the priming- pump chamber 140. Whilst the upper plunger part 1 16a extends into the barrel 1 14 in the pump head 1 12, the lower portion of the lower plunger part 1 16b extends into the bore 141 c provided in the priming-pump head 141 . The clearance between the upper plunger part 1 16a and the barrel 1 14, and the clearance between the lower plunger part 1 16b and the bore 141 c need not be the same, and in fact there is a benefit in forming a smaller clearance between the upper plunger part 1 16a and the barrel 1 14 (to reduce losses) compared to that between the lower plunger part 1 16b and the bore 141 c. In conventional pumps a large clearance is necessary along the entire length of the plunger 1 16 to ensure adequate lubrication for the high side loads experienced along the full plunger length. In the present invention, because only the lower plunger part 1 16b is resolving the side loads there is no need to maintain such a large clearance along the entire length of both plunger parts 1 16a, 1 16b, and so the clearance between the upper plunger part 1 16a and the barrel 1 14 can be made smaller.

The separation of the plunger into two parts 1 16a, 1 16b also enables the two different plunger parts, which have different functions, to be made from different materials. For example, the upper plunger part 1 16a may be made from a ceramic or may be provided with a coating that the lower plunger part 1 16b, which does not have a pumping function but just delivers force to the upper plunger part 1 16a, does not need to have.

It is envisaged that various materials would be suitable for the priming-pump piston 146. For example, the piston 146 could be formed of steel of the same or similar grade to the pumping plunger 1 16, or it could also be a suitable engineering plastic. Upper and lower annular seals (not shown) may be provided in grooves on the circumference of the plunger parts 1 16a, 1 16b respectively to prevent fuel leakage between the priming pump chamber 140 and the internal diameter of the collet piston 146. In practice, however, seals such as these may not survive the forces applied to them, and it may be preferable not to use them.

It is a benefit of forming the plunger into two parts 1 16a, 1 16b that alignment can be improved when assembling the plunger 1 16 within the housing parts formed by the priming-pump head 141 and the main pump head 1 12. The means of securing the plunger parts together also readily serves as a pumping element (priming-pump piston 146) for pressurising fuel to an intermediate level, prior to high pressure pumping.

In use, the priming-pump piston 146 moves together with axial movement of the plunger 1 16. In this way the priming-pump piston 146 moves with the plunger 1 16 when it reciprocates in the barrel 1 14 to cause pressurisation of the fuel in the priming-pump chamber 140 during operation of the plunger 1 16. More specifically, the priming-pump piston 146 acts to draw fuel into the priming-pump chamber 140 when the plunger 1 16 moves upwardly in the barrel 1 14 when performing a pumping stroke, and acts to pressurise fuel in the priming-pump chamber 140 when the plunger 1 16 moves downwards (in the orientation shown).

In order to manage any fuel that makes its way past the outer surface of the priming-pump piston 146, a backleak passage 148 is provided in the form of a drilling in the pump head 1 12 that extends away from an upper end of the priming-pump chamber 140 at an oblique angle. Although not shown in the Figures, the backleak passage 148 may be connected to a suitable source of relatively low pressure in order to draw away escaped fuel from the priming-pump chamber 140.

The pumping plunger 1 16 is configured to convey fuel from the priming pump chamber 140 to the main pumping chamber 122. Beneficially, therefore, this means that there is no need to provide a fuel inlet within the pump head 1 12 which simplifies the manufacture of that part. In the illustrated embodiment, the plunger 1 16 is provided with a longitudinal passage or drilling 120b that allows fuel to flow through the interior of the plunger 1 16 from the priming-pump chamber 140 to a pumping-chamber inlet 150 located at the upper end of the plunger 1 16. The longitudinal drilling 120b communicates with the priming-pump chamber 140 via one or more cross drillings 120c. Due to this structure, the longitudinal drilling 120b can be considered to be a fluid-inlet passage for the pumping chamber 122 and will be referred to as such from now on. The fluid-inlet passage 120b thus forms a part of the fluid-inlet path 120 for the pump chamber 122.

In the illustrated embodiment, the pumping-chamber inlet 150 is defined in the upper end or tip of the plunger by an enlarged diameter region 152 (see Figures 3, 4 and 5) of the fluid-inlet passage 120b. This enlarged diameter region 152 is sized to receive an inlet valve 154 to control the flow of fuel into the pumping chamber 154 through the pumping-chamber inlet 150. The inlet valve 154 may be in the form of a spring-biased ball valve or may more simply be operable based on the pressure difference between the pumping- chamber inlet 150 and the pumping chamber 122 without the additional biasing of a spring. It is envisaged that the inlet valve 154 may be configured to permit fluid to enter the pumping chamber at a pressure of approximately 8 bar which, it should be noted, is significantly higher than the working pressure of conventional lift pumps.

In summary, therefore, the pumping chamber 122 is connected through the pumping-chamber inlet 150 to the fluid-inlet path 120, under the control of the inlet valve 154, for receiving fuel at relatively low pressure from the priming-pump chamber 140. Thus, during operation the pumping chamber 122 receives partially-pressurised fuel from the priming-pump chamber 140, through the fluid inlet path 120 and, more specifically, through the fluid-inlet passage 120b defined in the plunger 1 16, and delivers highly pressurised fuel through the fluid-outlet path 121 .

Having described the general structure of the fuel pump 1 10, the following description explains the operation of the fuel pump during pumping and return strokes. Here, references to 'pumping stroke' and 'return stroke' relate to the movement of the pumping plunger within the barrel 1 14 and it should be noted that the priming-pump piston 146 performs pressurisation of the priming pump chamber 140 (i.e. a piston pumping stroke) during a return stroke of the pumping plunger 1 16, whereas the priming-pump piston 146 causes the priming-pump chamber 140 to be filled (i.e. a piston return or filling stroke) during a pumping stroke of the plunger.

Figure 2b illustrates the plunger 1 16 during a return stroke in which it is driven outwardly in the plunger barrel 1 14 to increase the volume of the pumping chamber 122. At the beginning of the return stroke, the plunger 1 16 is at its uppermost position within the barrel 1 14 and the priming-pump piston 146 is at its uppermost position within the priming-pump chamber 140. As the plunger 1 16 moves outwardly with respect to the plunger barrel 1 14, the priming-pump piston 146 moves downwardly within the priming-pump chamber 140, thereby reducing its volume and forcing the fuel within the priming-pump chamber 140 into the fluid-inlet passage 120b of the plunger 1 16.

The fuel forced through the fluid-inlet passage 120b results in fuel pressure acting on the inlet valve 154 causing it to open against the spring force and pressure in the pumping chamber 122 thereby allowing fuel to enter the pumping chamber 122 through the open inlet valve 154.

Turning to Figure 2c, following a return stroke the plunger 1 16 performs a pumping stroke during which the plunger 1 16 is driven inwardly within the plunger barrel 1 14 to reduce the volume of the pumping chamber 122, thereby causing the pressurised fuel to be delivered through the outlet valve 126. The pumping stroke starts when the plunger 1 16 is at its lowermost position with respect to the plunger barrel 1 14. During the pumping stroke, the plunger 1 16 is driven inwardly (that is to say, upwardly in the orientation shown) within the plunger barrel 1 14 by the drive arrangement 1 18. The fuel pressure in the pump chamber 122 increases as the plunger 1 16 advances until, at a predetermined pressure level, a positive pressure differential is formed across the outlet valve 126 causing it to open. The pressurised fuel is then delivered through the outlet valve 126 to the outlet port 129 of the pump. During this movement of the plunger 1 16, the inlet valve 154 remains closed due to the pressure of fuel in the pumping chamber 122. Advantageously, movement of the plunger 1 16 results in the delivery of partially- pressurised fuel from the priming-pump chamber 140 to the high pressure pumping chamber 122 which thereby ensures that a sufficient volume of fuel is delivered to the pumping chamber 122 before each pumping stroke of the plunger 1 16. Since the operation of the priming-pump chamber 140 and the main pumping chamber 122 are coupled by movement of the plunger 1 16, consistent delivery of fuel into the pumping chamber is ensured throughout the engine speed range. Even at higher pumping frequencies, the pressurisation of fuel in the fluid-inlet path 120 is maintained thereby allowing the pumping chamber 122 to be sufficiently filled during every return stroke of the plunger 1 16. This improves volumetric efficiency of the fuel pump 1 10. It also makes the design of the fuel pump 1 10 less sensitive to the inlet pipework.

A particular advantage of configuring the fluid-inlet path 120 to pass through the plunger 1 16 is that it enables the high pressure fluid-outlet path 121 to be arranged in co-axial alignment with the plunger barrel 1 14. This avoids any need for cross hole drillings within the pump head 1 12 and also greatly reduces the inherent pumping stresses within the pumping chamber 122, as well as simplifying the machining of the fuel pump 1 10.

It will be appreciated by a person skilled in the art that the invention could be modified to take many alternative forms without departing from the inventive concept, as defined by the scope of the appended claims. For example, additional plunger parts may be incorporated in the multi-part plunger in addition to the first and second parts.

Claims

CLAIMS:

1 . A fuel pump (1 10) comprising;

a pumping plunger (1 16) comprising a first plunger part (1 16a) and a second plunger part (1 16b), and

a pump head (1 12) defining a first barrel (1 14) in which the first plunger part (1 16a) is reciprocable to pressurise fuel in a pumping chamber (122),

wherein the second plunger part (1 16b) is reciprocable within a further barrel (141 c) and wherein the first and second plunger parts (1 16a, 1 16b) are connected by a connector part (146) located part-way along the length of the pumping plunger so that the first and second plunger parts (1 16a, 1 16b) move together.

2. The fuel pump (1 10) as claimed in claim 1 , wherein the further barrel (141 c) is formed in a housing (141 ) located adjacent to the pump head (1 12).

3. The fuel pump (1 10) as claimed in claim 1 or claim 2, wherein a first clearance is defined between the first plunger part (1 16a) and the first barrel (1 14) and a second clearance is defined between the second plunger part (1 16b) and the further barrel (141 c), wherein the first clearance is different to the second clearance.

4. The fuel pump (1 10) as claimed in claim 3, wherein the first clearance is less than the second clearance.

5. The fuel pump as claimed in any of claims 1 to 4, wherein the first and second plunger parts (1 16a, 1 16b) are formed from different materials.

6. The fuel pump (1 10) as claimed in any of claims 1 to 5, wherein the connector part (146) is an annular element (146) carried by the plunger (1 16).

7. The fuel pump (1 10) as claimed in claim 6, wherein the annular element (146) locates within at least one groove in the plunger (1 16).

8. The fuel pump (1 10) as claimed in any of claims 1 to 7, wherein the connector part (146) is formed from the same material as the first and second plunger parts (1 16a, 1 16b).

9. The fuel pump (1 10) as claimed in any of claims 1 to 8, wherein the connector part (146) is formed from an engineering plastics material.

10. The fuel pump (1 10) as claimed in any of claims 1 to 9, further comprising a fluid-inlet path (148, 140) through which fuel flows into the pumping chamber (122) during a plunger return stroke, and wherein the fluid inlet path is defined, in part, by a priming-pump chamber (140) within which fuel is pressurised to an intermediate level prior to delivery to the pumping chamber (122).

1 1 . The fuel pump as claimed in claim 10, wherein the connector part (146) resides within the priming-pump chamber (140) and defines a priming-pump piston (146) to cause pressurisation of fuel within the priming-pump chamber, to the intermediate level, as the plunger (1 16) moves.

12. The fuel pump (1 10) as claimed in any of claims 1 to 1 1 , wherein the first plunger part (1 16a) abuts the second plunger part (1 16b) directly.

13. A fuel system comprising the fuel pump (1 10) of any of claims 1 to 12.