FIELD OF THE INVENTION

This invention relates to a fuel pump assembly for use in an internal combustion engine. In particular, the invention relates to a fuel pump assembly having a cam driven plunger.

BACKGROUND

In current fuel pump assemblies, a return spring may be used to maintain contact between a plunger and a drive mechanism that drives pumping action of the plunger. This may entail preserving engagement between a roller and cam in pump assemblies employing a roller as a cam follower, or in variants employing a slipper- tappet mechanism the return spring must maintain contact between a tappet and a rider.

The return spring must be capable of providing a return force that is sufficient to maintain contact between the plunger and its drive mechanism for all operating conditions of the pump assembly.

In this respect, in a context of increasing demands on fuel pump designs in terms of higher pump speeds and stroke volumes, it is becoming increasingly difficult to meet the varied design constraints of providing the required dynamic force at top dead centre, maintaining fatigue resistance over a large number of cycles and keeping the pump assembly compact.

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

SUMMARY OF INVENTION

According to an aspect of the invention, there is provided a fuel pump assembly for an internal combustion engine. The fuel pump assembly comprises: a housing including a plunger bore and a plunger arranged to reciprocate within the plunger bore to perform a pumping cycle comprising a pumping stroke and a return stroke. The pumping stroke comprises movement of the plunger from a bottom dead centre (BDC) position to a top dead centre (TDC) position to pressurise fuel within a pump chamber defined, at least in part, within the plunger bore. The return stroke comprises movement of the plunger from the TDC position to the BDC position during which fuel to be pressurised enters the pump chamber. The fuel pump assembly additionally comprises a drive means configured to drive the plunger through its pumping stroke and a spring assembly configured to apply a return force to the plunger to effect the return stroke and maintain contact between the plunger and the drive means during the return stroke. The spring assembly comprises a return spring configured to contribute to the return force over the entire return stroke and an additional spring configured to contribute to the return force during an initial portion of the return stroke as the plunger moves from the TDC position towards the BDC position, and to cease contributing to the return force before the plunger reaches the BDC position.

Thus, the additional spring only acts over a portion of the pumping cycle. This means the spring arrangement can be made compact. An additional advantage of reducing the range of operation of the additional spring is that the range of travel of the additional spring is substantially less than that of the return spring. This reduces the stress range experienced by the spring arrangement for the same amount of supplementary return force at the TDC position compared to if the additional spring had the same range of operation as the return spring. This in turn improves fatigue resistance of the spring assembly. In general terms, the additional spring may contribute to the return force by engaging the plunger and the housing during the initial portion of the return stroke, and disengaging at least one of the plunger and the housing for the remainder of the return stroke. Such engagement may be direct or indirect. For example, the additional spring may be longer than a gap between the plunger or a part carried thereby and the housing or a component fixed relative to the housing when the plunger is at the TDC position, so that the additional spring engages the plunger or the part carried thereby and the housing or the component fixed relative to the housing to contribute to the return force during the initial portion of the return stroke. Correspondingly, the additional spring may be shorter than a gap between the plunger or the part carried thereby and the housing or the component fixed to the housing when the plunger is at the BDC position, so that the additional spring disengages at least one of the plunger or the part carried thereby and the housing or the component fixed relative to the housing before the plunger reaches the BDC position to cease contributing to the return force.

The fuel pump assembly may comprise a seat member that is fixed relative to the plunger wherein the return spring and/or the additional spring may engage the seat member to contribute to the return force. The seat member may be a collar carried by the plunger, for example.

The additional spring may be located at least partially within the return spring, for example in a nested arrangement.

The additional spring may be configured to contribute to the return force only during a period of the pumping cycle covering between 20 and 70 degrees around TDC. In some embodiments, the additional spring is configured to contribute to the return force only during a period of the pumping cycle covering between 40 and 50 degrees around TDC. The additional spring may be supported by the housing. For example, the additional spring may be held between a retaining member and a surface of the housing. In such embodiments, the return spring may engage the retaining member. Alternatively, the additional spring may be carried by the plunger. For example, an end of the additional spring may be fixed to the plunger or to a component fixed relative to the plunger. The fuel pump assembly may comprise a sleeve that is slidably received on the plunger. The additional spring may be arranged to engage with the sleeve to contribute to the return force.

The additional spring may be a coil spring or a leaf spring.

The fuel pump assembly may comprise multiple additional springs that collectively contribute to the return force during the initial portion of the return stroke. According to another aspect of the invention, there is provided a method of manufacturing a fuel pump assembly for an internal combustion engine. The fuel pump assembly comprises a plunger that is arranged to reciprocate within a plunger bore to perform a pumping cycle comprising a pumping stroke to pressurise fuel within a pump chamber defined, at least in part, within the plunger bore, and a return stroke. The method comprises: arranging a spring assembly within the fuel pump assembly to apply a return force to the plunger to effect the return stroke, in use, and maintain contact between the plunger and the drive means during the return stroke. Arranging the spring assembly comprises: positioning a return spring so that it contributes, in use, to the return force over the entire return stroke, and positioning an additional spring so that it contributes to the return force during an initial portion of the return stroke as the plunger moves from the TDC position towards the BDC position, and ceases contributing to the return force before the plunger reaches the BDC position.

It will be appreciated that the various features of the first aspect of the invention are equally applicable to, alone or in appropriate combination, the second aspect of the invention also.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 a is a cross-sectional view of a common rail fuel pump assembly in accordance with an embodiment of the invention with its plunger at a bottom-dead- centre position;

Figure 1 b corresponds to Figure 1a but shows the plunger at a top-dead-centre position;

Figure 2 is a graph representing a total return force exerted on the plunger of Figures 1a and 1 b as a function of the angle of rotation of a cam of the fuel pump assembly; Figure 3a is a cross-sectional view of a common rail fuel pump assembly in accordance with an alternative embodiment of the invention with its plunger at the bottom-dead-centre position;

Figure 3b corresponds to Figure 3a but shows the plunger at the top-dead-centre position; and

Figure 4 is a graph representing a total return force exerted on the plunger of Figures 3a and 3b as a function of the angle of rotation of a cam of the fuel pump assembly.

DETAILED DESCRIPTION

Figures 1 a and 1 b show a common rail fuel pump assembly 2 (“the pump 2” hereinafter), in accordance with an embodiment of the invention, for use in a compression-ignition internal combustion engine. The pump 2 comprises a housing 4, which includes a substantially tubular turret 6. The turret 6 outwardly extends from the main body of the housing 4 and defines a substantially cylindrical plunger bore 8, which extends though the turret 6 into the main body of the housing 4. The plunger bore 8 is configured to receive a plunger 10, the lower end of which extends from the turret 6.

The plunger 10 is moveable between a bottom-dead-centre positon (hereinafter, “BDC position”) and a top-dead-centre position (hereinafter, “TDC position”), defining a pump stroke, and between the TDC position and the BDC positon, defining a return stroke. A pump stroke followed by a return stroke defines a pumping cycle for the plunger 10 and pump 2. Figure 1 a shows the pump 2 with the plunger 10 at the BDC position, while Figure 1 b shows the plunger 10 at the TDC position.

It should be understood that throughout this description, references to top and bottom ends of components and other such directional or relative references, including references to upper and lower portions of said components, are made in relation to the orientations of the components shown in the Figures. A spring plate 12 forms a collar around the plunger 10 in a lower region of the plunger 10 and is affixed to the plunger 10 such that their respective motions are coupled together. The spring plate 12 defines an abutment surface for one end of a plunger return spring 14 (“return spring” hereinafter). Accordingly, the spring plate 12 acts as a seat member. The other end of the return spring 14 engages the housing 4, albeit indirectly in this case as shall become clear in the description that follows. So, the return spring 14 is permanently engaged with the spring plate 12 and the housing 4.

The embodiment of the pump 2 shown in Figures 1a and 1 b additionally comprises a retainer 40 and an additional spring 42, which in this embodiment is a coil spring. As will be explained in greater detail below, the purpose of the additional spring 42 is to provide an increased return force to the plunger 10 over a limited range of angular motion of the cam, centred on the TDC position. Accordingly, the additional spring 42 supplements a return force applied to the plunger 10 in a period of maximum load, thereby reducing the peak load that the main return spring 14 is subjected to and so enabling the use of a lower capacity spring as the main return spring 14. Alternatively, the supplementary return force provided by the additional spring 42 means a higher maximum total return force can be applied to the plunger 10 without changing the return spring 14, thus enabling a higher speed of safe operation. The return spring 14 and additional spring 42 together define a spring assembly 43 that provides a return force to the plunger 10 throughout each pumping cycle.

The retainer 40 comprises a generally tubular main body 41 that tapers slightly towards its lower end as viewed in Figure 2a, to define a major diameter 44 in an upper portion of the main body and a minor diameter 46 at a lower portion of the main body.

At an upper end of the retainer 40 a generally planar annular upper lip 48 protrudes radially outwardly from the main body 41. Correspondingly, at a lower end of the retainer 40 a planar annular lower lip 50 protrudes radially inwardly, although only partially towards a central axis of the retainer 40 so that a central aperture 52 is created in the lower end of the retainer 40. The aperture 52 has a diameter that is slightly greater than that of the plunger 10, and approximately equal to the mean coil diameter of the additional spring 42. The retainer 40 is located inside the return spring 14 and over the turret 6. The top end of the retainer 40 lies flush against the housing 4, so that a downwardly-facing planar surface of the upper lip 48 defines a seat 16 for the return spring 14. Accordingly, the return spring 14 engages the housing 4 through the upper lip 48 of the retainer 40. Compression of the return spring 14 between the upper lip 48 of the retainer 40 and the spring plate 12 generates an axial force that holds the retainer 40 in place. In certain embodiments, a light interference fit between the retainer 40 and turret 6 may be created to form a sub-assembly for handling purposes.

The taper of the main body 41 of the retainer 40 is configured such that the main body 41 follows the external contours of the turret 6. The main body 41 extends beyond the lower extremity of the turret 6 so that the lower lip 50 of the retainer 40 is spaced from a lower end face of the turret 6. Accordingly, a space is defined between the lower end face of the turret 6 and the lower lip 50 of the retainer 40, within which the additional spring 42 is housed and supported by the lower lip 50 of the retainer 40. The bottom of the additional spring 42 abuts a washer 45. The washer 45 is guided laterally on its inside diameter by the plunger 10, yet remains in clearance with the plunger 10. With the plunger 10 at the BDC position, an outer annular portion of the bottom face of the washer 45 abuts the top face of the lower lip 50 of the retainer 40.

The spring plate 12 comprises an annular protrusion 54 that extends axially upwards from the top surface of the spring plate 12, the protrusion 54 having a width not exceeding the width of the aperture 52 of the lower end of the retainer 40. The protrusion 54 is configured to enter the retainer 40 to engage an inner annular portion of the bottom face of the washer 45 (and hence indirectly the additional spring 42) as the plunger 10 rises, as shall become clear in the description that follows.

The additional spring 42 is by necessity much shorter than the return spring 14, housed as it is within a small section of the interior of the return spring 14. More specifically, the additional spring 42 is of a length that is smaller than a gap between the turret 6 and the protrusion 54 of the spring plate 12 when the plunger 10 is at the BDC position, and greater than a gap between the turret 6 and the protrusion 54 of the spring plate 14 when the plunger 10 is at the TDC position. A pump chamber 18 is defined by a combination of the upper end of the plunger 10 and the plunger bore 8; its volume decreases and increases during the pump and return strokes respectively. The pump chamber 18 communicates with inlet and outlet valve assemblies via internal inlet and outlet passages 24, 26, respectively. The configurations of such valve assemblies are well known in the art and, given that they are not central to the invention, will not be described in detail here, save that they are used to control flow of the fuel from the pump inlet 28 through to the pump chamber 18 to the pump outlet 30 and through to the common rail (not shown). Each valve assembly includes a spring, which acts to close the valve assembly to prevent the passage of fuel therethrough.

The general function of the pump 2 will now be described with respect to the position of the plunger 10 through the return and pump strokes.

When the plunger 10 is in the TDC position, both valve assemblies are closed, thereby preventing fuel from flowing into or out of the pump chamber 18. With the plunger 10 still in the TDC position, fuel is supplied to the pump 2 through the pump inlet 28 but is prevented from reaching the internal inlet passage 24 by the closed inlet valve assembly. The fuel is supplied at a pressure of around 3 bar (300 kPa). The spring assembly 43 defined by the combination of the return spring 14 and the additional spring 42 provides a return force that acts on the spring plate 12, and hence on the plunger 10, to effect the return stroke and move the plunger 10 from the TDC position towards the BDC position. This causes an increase in the volume of the pump chamber 18, decreasing the pressure within it and establishing a pressure drop across the inlet valve assembly. This pressure drop allows the inlet valve assembly to open against the force of the inlet valve spring and fuel enters the pump chamber 18 until the pressure across the valve assembly equalises, causing it to close. This typically occurs just after the plunger 10 reaches the BDC position.

Once the plunger 10 reaches the BDC position, it begins the pump stroke under the influence of a rotating cam drive arrangement (not shown) to pressurise the fuel in the pump chamber 18. The rotating cam drive arrangement is either mounted on, or forms part of, the engine drive shaft and is reciprocally connected to the plunger 10 via a tappet (not shown). During the pump stroke, the fuel in the pump chamber 18 is compressed to a pressure exceeding the pressure of the fuel held in the internal outlet passage 26, which substantially equals the pressure in the common fuel rail volume. This pressure is in the region of at least 200 bar (20 MPa) and can be as high as 2500 bar (250 MPa). A pressure drop is created across the outlet valve assembly, allowing it to open against the force of the outlet valve spring and fuel to exit the pump chamber 18 and flow into the common rail fuel volume via the pump outlet 30. As the plunger 10 reaches the TDC position, the pressure across the outlet valve assembly equalises, causing it to close.

As mentioned above, the spring assembly 43 acts to provide a return force on the spring plate 12 to effect the return stroke. In pumps employing a roller as a cam follower, the force from the spring assembly 43 is essential to maintain contact between the cam and the roller, whilst also minimising slippage between the two.

Similarly, in pumps incorporating a slipper-tappet mechanism the spring assembly 43 must maintain sufficient force between the tappet and rider, or‘slipper’, to avoid rotation of the rider relative to the housing. In such arrangements, the cam is journaled within a generally tubular rider that does not rotate, but instead translates around a circular locus defined by the eccentricity of the cam as the cam rotates. The outer surface of the rider is cylindrical aside from a planar portion of reduced radius that engages an end face of a tappet, this engagement preventing rotation of the rider in normal operation. If insufficient force is maintained between the tappet and the rider, the rider may rotate relative to the housing 4, which can lead to the tappet bearing against the cylindrical portion of the rider rather than the planar portion. As the cylindrical portion will displace the tappet to a greater extent at the TDC position than the planar portion, there is insufficient space for the tappet to engage the rider in this way, potentially leading to catastrophic failure of the pump 2.

As pumps become larger and more powerful, problems associated with the return spring 14 failing to provide sufficient return force are exacerbated. In pumps employing a roller as a cam follower, when there is insufficient return force a so- called ‘ski jump’ occurs in which contact between the roller and cam is lost momentarily. Over successive pumping cycles repeated impacts between the roller and the cam following‘ski jumps’ can cause long term wear to the cam. In pumps incorporating a slipper-tappet mechanism, the failure mode noted above becomes more likely.

To avoid these problems in the context of an increased stroke rate or stroke volume, the return spring 14 must work harder to supply sufficient return force at all times. This leads to increased stresses in the return spring 14 and so demands a spring of higher load capacity.

The function of the pump 2, with specific regard to the function of the plunger 10, spring plate 12, retainer 40, return spring 14 and additional spring 42 will now be described through the pump and return strokes.

As described above, the plunger 10 undergoes pumping cycles in use, each comprising a pump stroke and a return stroke. Figure 1 a shows the pump 2 with the plunger 10 at the BDC position and about to commence a pump stroke. In this configuration, the return spring 14 is at its minimum load and the additional spring 42 is disengaged from the spring plate 12.

The force applied to the spring plate 12 by the return spring 14 varies approximately sinusoidally with rotation of the cam, with extrema at the TDC and BDC positions and maximum force provided at TDC position when the return spring 14 is maximally compressed.

As the plunger 10 moves up the plunger bore 8 during the pump stroke towards the TDC position, the spring plate 12, being affixed to the plunger 10, moves towards the lower lip 50 of the retainer 40 and in so doing progressively loads the return spring 14. Accordingly, at this stage of the pumping cycle, a return force acting on the plunger 10 is generated by the return spring 14 alone.

As the plunger 10 approaches the TDC position, the protrusion 54 of the spring plate 12 enters the aperture 52 at the lower end of the retainer 40 and engages the inner annular portion of the bottom face of the washer 45, and thus the bottom of the additional spring 42. As the plunger 10 continues towards the TDC position, both the return spring 14 and the additional spring 42 become loaded as the spring plate 12 continues to move upwards. So, in this phase of the pumping cycle the additional spring 42 contributes to the return force applied to the plunger 10.

When the plunger 10 reaches the TDC position, both the return spring 14 and the additional spring 42 are maximally compressed and together provide the peak return force for the plunger 10 to effect the return stroke. The return force provided by the return spring 14 is supplemented by that provided by the additional spring 42, thereby increasing the total return force acting on the plunger 10 through the spring plate 12 compared to the sole action of the return spring 14.

Subsequently, as the plunger 10 moves downwardly towards the BDC position, the spring plate 12 moves down to the extent that the protrusion 54 disengages the inner annular portion of the bottom face of the washer 45 and the additional spring 42 and exits the retainer 40. At this point, the total return force is once more provided only by the return spring 14.

The additional spring 42 therefore provides an additional return force only over a specific range of angular motion R of the cam, centred around the TDC position, where it is most needed. This is the part of the pumping cycle where the demands on the spring assembly 43 are greatest to initiate the return stroke and ensure the components of the rotating cam drive arrangement remain in contact during the period in which axial acceleration of the cam is greatest. As demands on fuel pump assemblies increase, problems arise first around the point of peak loading. The use of the additional spring 42 provides an effective and convenient means for addressing those problems.

It should be understood that any reference to angular positions or ranges of the cam can be equally appropriately applied to the plunger, given the plunger’s oscillatory behaviour and the relationship between the rotation of the cam and the movement of the plunger.

The graph of Figure 2 shows the effect of the additional spring 42 on the total return force as a function of the angle of rotation of the cam. In Figure 2, the BDC position corresponds to 0° cam rotation and so the TDC position is at 180° cam rotation. The total return force is shown as a solid line 56 and the return force from the return spring 14 is shown as a dashed line 58. The step change in the rate of increase of the total return force applied to the plunger 10 at the point where the additional spring is engaged is clearly visible at around 140° cam rotation, as is the moment when the additional spring is subsequently disengaged at approximately 220° cam rotation. This defines the range of angular motion R of the cam in which the additional spring 42 contributes to the total return force.

It is noted that although the rate of change of the return force changes suddenly when the additional spring 42 engages, there is no discontinuity in the magnitude of the return force at the point of engagement or disengagement. This indicates that the additional spring 42 is not preloaded as the dimensions of the retainer 40 allow it to expand fully when not engaged. In other arrangements, however, packaging constraints may dictate that the additional spring 42 and/or the retainer 40 are configured such that the additional spring 42 is held in compression when disengaged and is therefore preloaded. This will manifest as a step increase in return force when the additional spring 42 is engaged. This situation arises in the second embodiment that is now described.

Figures 3a and 3b show an alternative embodiment of a fuel pump assembly 2 (“the pump 2” hereinafter) in accordance with the invention. Many features of this pump 2 are shared with the embodiment shown in Figures 1 a and 1 b and these features will consequently be referred to with the same reference numbers.

Much of the structure of the pump 2 shown in Figures 3a and 3b is identical to the embodiment shown in Figures 1a and 1 b. As for the first embodiment, the embodiment shown in Figures 3a and 3b includes an additional spring 142 to assist the permanently engaged return spring 14 over a portion of the plunger pumping cycle. However, the embodiment shown in Figures 3a and 3b includes an additional spring 142 in the form of a disc compression spring supported by the plunger 10, instead of a coil spring supported by a retainer as in Figures 1 a and 1 b. The additional spring 142 may be a Belleville washer, for example.

To hold the additional spring 42 securely, also positioned around the plunger 10 is a substantially cylindrical sleeve 60, above which a circlip 62 is affixed to the plunger 10 to prevent upward movement of the sleeve 60 on the plunger 10. It is noted, however, that the sleeve 60 can move downwardly on the plunger 10 by compressing the additional spring 142. The height of the sleeve 60 is such that when the plunger 10 is at the TDC position and the additional spring 142 is fully compressed, the sleeve 60 completely fills the space between the lower end face of the turret 6 and the top surface of the additional spring 142.

The circlip 62 is positioned such that when the top of the sleeve 60 abuts the bottom of the circlip 62, the additional spring 142 remains at least partially loaded, i.e. the additional spring 142 does not extend to its equilibrium length. This partial loading, for the sake of clarity, shall henceforth be referred to as a residual loading to avoid confusion with the loading of the additional spring 142 that occurs as a result of the action of the pump 2 (active loading, described below).

The annular lower end face of the turret 6 is counterbored to define an annular recess 64 at the entrance to the plunger bore 8 that creates a radial clearance around the plunger. The recess 64 is sized to admit the circlip 62 but not the sleeve 60 as the plunger 10 approaches the TDC position, to allow the sleeve 60 to engage the lower end face of the turret 6 directly. The depth of the recess 64 must be such that the plunger 10 may reach the TDC position without the circlip 62 being impeded, in use.

In use, the plunger stroke is performed in a similar manner to the embodiment shown in Figures 1a and 1 b, with the general function of the pump 2 being as described for the first embodiment. At the BDC position (Figure 3a) the return spring 14 is at its minimum load. In addition, the sleeve 60 is disengaged from the turret 6 and so abuts the circlip 62 under the action of the additional spring 142, which is loaded to its least extent (residual loading).

As the pump stroke is performed, the plunger 10 moves upwards, together with the spring plate 12, additional spring 142, sleeve 60 and circlip 62. At this stage, the return spring 14 provides all of the return force acting on the plunger 10.

As the plunger 10 continues towards the TDC position, a point is reached where the circlip 62 enters the recess 64 in the turret 6. Since the sleeve 60 cannot enter the recess 64, it engages the lower end face of the turret 6 and so rises no further. Accordingly, continued upward motion of the plunger 10 towards the TDC position results in compression of the additional spring 142 between the spring plate 12 and the sleeve 60, referred to as active loading of the additional spring 142. Once active loading of the additional spring 142 commences, the additional spring 142 contributes to the total return force applied to the plunger 10.

The active loading continues until the plunger 10 reaches the TDC position (Figure 3b), which coincides with a point at which the additional spring 142 becomes maximally compressed, along with the typical maximal compression of the return spring 14. As for the first embodiment, in this situation the additional spring 142 and the return spring 14 generate a maximum combined return force to effect the return stroke of the plunger 10.

As the plunger 10 moves away from the TDC position and back down towards the BDC position for the return stroke, the circlip 62 withdraws from the recess 64. The additional spring 142 starts to expand, which keeps the sleeve 60 pressed against the lower end face of the turret 6 as the plunger 10 moves downwards until the sleeve 60 reengages the circlip 62. From this point, the plunger 10 continues moving towards the BDC position, at which point the pump stroke begins again.

As with the embodiment shown in Figures 1 a and 1 b, in the second embodiment the effect of the addition of the additional spring 142 to the pump 2 is to provide an additional return force to the plunger 10 and spring plate 12 over a portion of the pumping cycle corresponding to the range of angular motion R of the plunger 10 in which the additional spring 142 is actively loaded by the sleeve 60. Outside of this range, the return force is provided by the return spring 14 only. However, due to the fact that the additional spring 142 retains a residual loading at the BDC position, the effect on the total return force as a function of the angle of rotation of the cam is slightly different from that shown in Figure 2.

The graph of Figure 4 shows the effect of the additional spring 142 on the total return force as a function of the angle of rotation of the cam, with the BDC position at 0° and the TDC position at 180°. The total return force is shown as a solid line 56 and the return force from the return spring 14 is shown as a dashed line 58.

As can be seen in Figure 4, there are discontinuities D in the total spring force at the limits of the range of angular motion R of the plunger 10 in which the additional spring 42 is actively loaded by the sleeve 60. These discontinuities coincide with engagement and disengagement of the additional spring 142 with the housing 4 through the sleeve 60, and are a direct consequence of the residual loading induced in the additional spring 142. A larger residual loading results in a larger discontinuity D between the two sections of the curve in Figure 4. This introduces non-steady loading of the drivetrain of the pump 2, which may lead to premature failure. It is therefore desirable to minimise the residual loading of the additional spring 142.

However, if there were enough space between the spring plate 12 and the circlip 62 for the sleeve 60 to be accommodated as well as a fully unloaded additional spring 142 (i.e. the spacing between the circlip 62 and spring plate 12 was such that a state of zero residual loading was possible), due to manufacturing tolerances and forces imparted to the additional spring 142 by the sleeve 60 due to weight and inertia, there would be the possibility of the components rattling in use, which would cause noise, wear and indeterminate dynamic loading on the additional spring 142. It is therefore preferable to have a small residual loading in the additional spring 142 in practice. The desired residual loading is created using selective assembly of the sleeve 60, such as shim selection.

In alternative embodiments of the invention, a supplementary spring means may be made integral with or mounted on the top surface of the spring plate 12 for engagement with the lower end face of the turret 6 over a limited range of angular motion R of the plunger 10. Equally, the spring means may be made integral with or mounted on the lower end face of the turret 6 for engagement with the top surface of the spring plate 12 over a limited range of angular motion R of the plunger 10. Both embodiments would result in a supplementary return force being introduced to the plunger 10 over a limited range of its angular motion R, that range R being substantially centred on the TDC position and therefore providing a combined return force that is maximised at the TDC position.

For example, a small coiled spring could be mounted on the spring plate 12 inside the return spring 14. Provided the small coiled spring is sufficiently short that at equilibrium it does not engage the turret 6 when the plunger is at BDC, and has a suitable diameter to engage the turret 6 as the plunger 10 rises, it will provide a maximum additional return force to the plunger 10 at the TDC position and operate over a limited portion of each plunger stroke. Similarly, the small coiled spring may be mounted on the turret 6 to engage the spring plate 12 instead, provided the same conditions regarding the length and diameter of the spring are accounted for.

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. It will be clear to the skilled person that there are various ways to achieve engagement of a second spring over a small angular range of angular motion R of a pump plunger 10 to increase the return force applied to the plunger 10 over that angular range R. Also, it should be noted that in most cases various spring types will be suitable as the additional spring. For example, the skilled reader will appreciate that the additional springs described for the two embodiments disclosed above could be swapped without difficulty, to employ a disc compression spring supported by a retainer, or a coil spring supported by the plunger and cooperating with a sleeve and circlip.

It is also possible to use more than one additional spring. For example, the embodiments shown in Figures 1a and 1 b and 3a and 3b are not mutually exclusive, and so could be combined. A second spring plate could be added between the sleeve and the turret to control the respective points in the pumping cycle at which the spring held in the retainer and the disc compression spring become active. Another option is to use a stack of springs to provide the required behaviour, for example a stack of Belleville washers. Although in the embodiments described above a spring plate is provided as a separate component that is attached to the plunger, it would be possible to form the spring plate integrally with the plunger. References used:

2 - fuel pump assembly

4 - housing

6 - turret

8 - plunger bore

9 - clearance channel

10 - plunger

12 - spring plate

14 - plunger return spring

16 - seat

18 - pump chamber

24 - internal inlet passage

26 - internal outlet passage

28 - pump inlet

30 - pump outlet

40 - retainer

41 - main body of the retainer

42, 142 - additional spring

43 - spring assembly

44 - major diameter of the retainer

45 - washer

46 - minor diameter of the retainer

48 - upper lip

50 - lower lip

52 - aperture

54 - annular protrusion

56 - solid line showing total return force

58 - dashed line showing return force from the return spring 60 - sleeve

62 - circlip

64 - recess

D - discontinuity

R - range of angular motion