WO2017174404A1

WO2017174404A1 - Fuel pump

Info

Publication number: WO2017174404A1
Application number: PCT/EP2017/057329
Authority: WO
Inventors: Engin ERDEMIR; Benjamin P. 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: GB2549266A

Abstract

A fuel pump (110) comprising a pump head (112) defining a barrel (114) in which a pumping plunger (116) is slidable to pressurise fuel in a pumping chamber (122), and a fluid inlet path (120) through which fuel flows in to the pumping chamber (122) under control of an inlet valve (154). The fluid inlet path (120) is defined at least in part within the pumping plunger (116). The fuel may include a fluid outlet path (121) through which fuel flows out of the pumping chamber (122), preferably under control of an outlet valve (126).

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 a fuel pump comprising a pump head defining a barrel in which a pumping plunger is slidable to pressurise fuel in a pumping chamber, and a fluid inlet path through which fuel flows in to the pumping chamber under control of an inlet valve. The fluid inlet path is defined at least in part within the pumping plunger. The invention also resides in a fuel system comprising the fuel pump as defined above.

In a preferred embodiment, the fuel pump comprises a fluid outlet path through which fuel flows out of the pumping chamber. For example, the fuel may flow out of the pumping chamber under the control of an outlet valve.

A benefit of the invention is that since the fuel inlet path is defined at least in part within the pumping plunger, this avoids the need to provide an inlet path in the pump head, which therefore provides more scope for positioning the outlet valve in the optimum location and orientation. This also means that the pumping head is easier to manufacture, thus reducing costs and material waste.

In one embodiment, the inlet valve is disposed in a pumping-chamber inlet of the fluid inlet path defined within the pumping plunger. This may be at the tip of the plunger adjacent the pumping chamber which minimises the dead volume of the pumping chamber. The inlet valve may comprise a valve member that is moveable in an enlarged diameter region of the plunger so as to open and close the pumping-chamber inlet. Locating the valve member within the pumping- chamber inlet in the plunger means that a smaller valve can be used, having a low mass and, thus low inertia. This means that the valve is more responsive and so opens and closes more quickly, which improves performance of the pump, particularly at higher pumping speeds.

The enlarged diameter region may define a valve seat with which the valve member is engageable to open and close the pumping-chamber inlet.

To ensure that the valve member remains in the pumping-chamber inlet during operation, the fuel pump may include retaining means to retain the valve member. In one embodiment, the retaining means includes a retaining element which engages with a surface of the enlarged diameter region of the pumping plunger and retains the valve member in the pumping-chamber. Although the retaining means may simply be a part that holds the valve member in place, it may also include a biasing spring to bias the valve member against the valve seat. The fluid inlet path may comprise a fluid inlet passage providing fluid communication between a priming-pump chamber and the pumping-chamber inlet. The fluid inlet passage may be defined within the pumping plunger.

The priming-pump chamber may be defined by an enlarged diameter region of the barrel that extends about the pumping plunger. Moreover, the plunger may carry, be connected or otherwise be associated with a priming pump piston that moves with the plunger so as to cause pressurisation of the fuel in the priming- pump chamber. In one embodiment, the priming-pump piston is an annular element, such as a collet, carried by the pumping plunger.

The fluid inlet path may further comprise a fluid supply passage configured to supply fluid to the priming-pump chamber, the fluid supply passage including valve means for preventing depressurisation of the priming-pump chamber through the fluid supply passage.

In the above embodiments, the inlet valve may be configured to open, thereby permitting fuel to enter into the pumping chamber, during the return stroke of the pumping plunger.

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, Figures 3, 4 and 5 show alternative arrangements of inlet valves used in the fuel pump of Figures 2a to 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. 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 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.

In order to pressurise fuel within the priming-pump chamber 140, the plunger 1 16 is associated with a priming-pump piston 146. In the illustrated embodiment, the priming-pump piston 146 is an annular element, such as a collet, that the plunger 1 16 at a point part-way along its length. 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. The priming-pump piston 146 is located at a fixed position along the plunger 1 16 such that, in use, the priming- pump piston 146 is positioned in the priming-pump chamber 140 and moves within it along with axial movement of the plunger 1 16. The priming-pump piston 146 may be fixed in position by being received in an annular groove defined in the plunger 1 16, for example. If the piston is a collet, it would snap into such a groove 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. It could also be integral to the plunger. The skilled person would conceive of other techniques which could be used to combine the piston 146 and 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.

The skilled person will appreciate that the inlet valve 154 provided in the pumping-chamber inlet 150 may be implemented in various forms. The following discussion, referencing Figures 3, 4 and 5, relates to various embodiments of inlet valve that would be suitable. The main consideration, however, is that the components of the inlet valve are suitably sized to fit within the enlarged diameter region 152 of the pumping-chamber inlet. Referring firstly to Figure 3, the inlet valve 154 comprises a ball valve member 160 that is engageable with a valve seat 162 in order to control the flow of fuel through the fluid-inlet passage 120b. The valve seat 162 is formed by the shoulder defined at the step in diameters between the relatively narrow fluid-inlet passage 120b and the enlarged diameter region 152, although it could be a separate part if, for example, a different material property is required for the seating surface. The valve member 160 is biased into engagement with the valve seat 162 by way of a valve spring 163, which in this embodiment is a conical coil spring. The skilled person would appreciate that the spring constant of the valve spring 163 may be selected to permit the valve member 160 to open at a suitable pressure.

The valve member 160 is kept in place within the pumping-chamber inlet 150 by retaining means 164. In some arrangements, the retaining means 164 may be embodied by a cage-like member (not shown) which permits fuel to flow through it, yet blocks escape of the valve member 160. In the embodiment illustrated in Figure 3, however, the retaining means 164 includes a ring-shaped retaining element 166 that engages with and locks into an annular groove 168 extending around a radial wall surface of the enlarged diameter region 152.

Here, a retaining disc 170 is located below the retaining element 166 and is a separate part. The retaining plate or disc 170 provides a spring abutment surface against which the valve spring 163 bears to bias the valve member 160 against the seat 162. The retaining disc 170 includes flow holes 172 so as not to present an obstruction to the flow of fuel through the retaining disc 170.

Turning to Figure 4, here the illustrated embodiment is very similar to that of Figure 3, so the same parts will be referred to with the same reference numerals and, moreover, only the differences will be explained.

In this embodiment, instead of the valve member 160 being spherical, the valve member 160 take the general form of a tapered plug. The valve member 160 has a relatively narrow end 160a which seats against the valve seat 162 and widens into a relatively wide, upper, end 160b. The clearance that the valve member 160 defines with the radial wall surface of the enlarged diameter region 152 can be designed so as to limit fuel flow through the pumping-chamber inlet 150. The upper end of the valve member 160 includes an upstanding projection 160c that serves as a lift stop, as it will impact the retaining disc 170 to define the maximum extent of movement of the valve member 160 away from the valve seat 162. One benefit of this variant of valve member compared to the Figure 3 embodiment is that it may be more stable during fuel flow through the pumping-chamber inlet 150.

Turning now to Figure 5, in this embodiment the valve member 160 is in the form of a plate valve which seats against a lateral valve seat 162 defined in the pumping-chamber inlet 150. Once again, the valve member 160 is spring biased against the valve seat 162 but in this embodiment the valve member 160 is retained in the pumping-chamber inlet 150 by a top-hat shaped retaining element 180 comprising a base 180a and a side wall or arms 180b. The retaining element is shown as inverted such that the side wall 180b of the retaining element 180 extends down into an annular cavity 182 defined in the plunger 1 16, and is secured in place by a suitable clip ring or circlip 184.

The base 180a of the retaining element 180 overlies the valve member 160 and therefore prevents it from escaping the pumping-chamber inlet 150. A central flow hole 180c in the base 180b provides a flow passage for fuel through the retaining element.

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, in the above embodiments, the fluid-inlet path 120 and the inlet valve 154 within the plunger 1 16 are fed with partially-pressurised fuel from the priming-pump chamber 140. However, it should be noted that the priming-pump chamber 140 is not essential to the inventive concept and, instead, the inlet valve 154 could be fed directly from a feed line of a fluid supply pump that is not further pressurised by a priming-pump chamber and associated priming-pump piston. In such an embodiment, the momentum of the inlet valve and fluid flow would assist in opening and closing the inlet valve due to inertia of the components and the volume of fuel in the plunger. So, it will be appreciated that the operation of the priming-pump piston in the priming-pump chamber is not essential to the operation of the inlet valve.

Claims

1 . A fuel pump (1 10) comprising;

a pump head (1 12) defining a barrel (1 14) in which a pumping plunger (1 16) is slidable to pressurise fuel in a pumping chamber (122), and

a fluid inlet path (120) through which fuel flows in to the pumping chamber (122) under control of an inlet valve (154),

wherein the fluid inlet path (120) is defined at least in part within the pumping plunger (1 16).

2. The fuel pump of claim 1 , wherein the inlet valve (154) is disposed in a pumping-chamber inlet (150) of the fluid inlet path (120) defined within the pumping plunger (1 16).

3. The fuel pump of claim 2, wherein the pumping-chamber inlet (150) comprises an enlarged diameter region (152) of the fluid inlet path (120) within the pumping plunger (1 16), and wherein the inlet valve (154) comprises a valve member (160) that is moveable in the enlarged diameter region (152) so as to open and close the pumping-chamber inlet (150).

4. The fuel pump of claim 3, wherein the enlarged-diameter region (152) defines a valve seat (162) with which the valve member (160) is engageable to open and close the pumping-chamber inlet (150).

5. The fuel pump of claim 4, further including retaining means (164) to retain the valve member (160) within the pumping-chamber inlet (150).

6. The fuel pump of claim 5, wherein the retaining means (164) includes a retaining element (166) which engages with a surface of the enlarged diameter region (152) of the pumping plunger (1 16) and retains the valve member (160) in the pumping-chamber inlet (150).

7. The fuel pump of claim 5 or claim 6, wherein the retaining means (164) includes a biasing spring (163) to bias the valve member (160) against a valve seat (162).

8. The fuel pump of any one of claims 2 to 7, wherein the fluid inlet path (120) comprises a fluid inlet passage (120b) providing fluid communication between a priming-pump chamber (140) and the pumping-chamber inlet (150).

9. The fuel pump of claim 8, wherein the fluid inlet passage (120b) is defined within the pumping plunger (1 16).

10. The fuel pump of claim 8 or claim 9, wherein the priming-pump chamber (140) is defined by a portion of the fluid inlet path (120).

1 1 . The fuel pump of any one of claims 8 to 10, wherein the priming-pump chamber (140) is defined by an enlarged diameter region of the barrel (1 14) that extends about the pumping plunger (1 16).

12. The fuel pump of any one of claims 8 to 1 1 , further comprising a priming- pump piston (146) associated with the pumping plunger (1 16) and configured to cause pressurisation of the fuel in the priming-pump chamber (140).

13. The fuel pump of claim 12, wherein the priming-pump piston (146) is an annular element carried by the pumping plunger (1 16).

14. The fuel pump of any one of claims 2 to 13, wherein the fluid inlet path (120) further comprises a fluid-supply passage (120a) configured to supply fluid to the priming-pump chamber (140), the fluid-supply passage (120a) including valve means (144) for preventing depressurisation of the priming-pump chamber (140) through the fluid-supply passage (120a).

15. A fuel system comprising the fuel pump of any preceding claim.