TECHNICAL FIELD The present invention relates to a fuel pump for use in an internal combustion engine, and more particularly to an improved pumping mechanism for a fuel pump having at least one pumping element which is driven by an engine- driven cam or other appropriate drive arrangement.

BACKGROUND OF THE INVENTION

A known pumping for fuel pump such as a diesel pump, as illustrated in cross-section in Figure 1, comprises a pump housing comprising an inlet valve body and a cam box, and a pumping mechanism comprising a pumping element such as a plunger, and a return spring. The plunger is moveable within a plunger location bore provided in the inlet valve body. The pump further comprises a roller and a driving mechanism comprising a cam. During a pumping stroke, rotation of the cam applies a force to the plunger, via the roller and a roller/shoe guide, thereby urging the plunger along the location bore to pressurise fuel in a pumping chamber provided in the inlet valve body.

The return spring (also shown separately in Figure 2) comprises a cylindrical helical compression spring having a constant external diameter along its length, and is provided around the plunger, in a spring chamber provided in the cam box. The spring applies a force to the roller, via the roller/shoe guide, thereby ensuring that roller is in constant contact with the cam throughout the pump cycle. A known spring is produced using EN 10270-3 1.4568 17/7 PH stainless steel wire.

The return spring must provide a sufficiently large force to maintain contact between the cam and roller; accordingly the spring must be sufficiently compressed to provide this force. As the spring is compressed, the force applied by the spring increases, however the maximum stress incurred within the spring material during the pump cycle also increases.

In prior art pump embodiments, stress in the return spring may be too high for the force required to maintain contact between the cam and the roller. Furthermore, as the force requirement is increased, due to increased speed demands, the stress within the spring material will increase, leading to a reduction in fatigue strength of the spring.

As the spring operates under dynamic conditions, it can be caused to resonate at a natural frequency, or a harmonic, of the spring. Resonating of the spring also leads to an increase in stress levels within the spring, and ultimately to failure of the spring due to fatigue. Prior art springs therefore have a limited product life due to stress levels during resonating being higher than the capability of the spring material.

A known solution to the above problems of pump springs is to increase the size of the spring chamber, thereby allowing a spring with a larger free length to be used. This reduces stress induced in the spring, whilst maintaining the required force. However, to enable a larger spring housing, the overall size of the pump is necessarily increased; the resulting larger pump envelope represents a significant disadvantage for pump applications having tight space requirements.

A known solution to the problems caused by resonating of the spring is to provide a progressively wound spring (i.e. a variable pitched spring), as illustrated in Figure 2. Compression of the progressively wound spring causes a change in the natural frequency of the spring, as end coils start to contact each other as the spring is compressed thereby changing the spring rate during compression. However, a progressively wound spring must have a larger free length in order to produce the same required force. Therefore a larger pump envelope is again required to provide more internal space for the spring, thereby also representing a disadvantage in applications having tight space requirements. SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved pumping mechanism which at least mitigates the problems as described above.

Accordingly the present invention comprises a pumping mechanism for a fuel pump for use in an internal combustion engine, the pumping mechanism comprising: an inlet valve body comprising a pumping element location bore in which a pumping element is moveable in a longitudinal axis; a cam box comprising a guide bore in which a roller/shoe guide is moveable; wherein the roller/shoe guide is co-operable with a driving mechanism via a roller; wherein the pumping element abuts the roller/shoe guide, and in a pumping stroke, the driving means causes movement of the roller/shoe guide and pumping element, and wherein movement of the pumping element within the pumping element location bore causes fuel in within a pumping chamber provided in the inlet valve body to pressurise; wherein the pumping mechanism further comprises a return spring, located around part of the pumping element and within a spring chamber, wherein a first end of the return spring abuts a first spring seat proximate to the inlet valve body, and a second end of the return spring abuts a second spring seat proximate to the roller/shoe guide, such that the return spring urges the roller into contact with the driving mechanism; wherein the first and second ends of the spring each define an external diameter, and wherein the external diameter of the first end of the spring is greater than the external diameter of the second end of the spring.

The roller/shoe guide may comprise a recess comprising a first section, and a second section which is closer to the driving mechanism than the first section and which is of a smaller diameter than the first section, and wherein the second spring seat is located within the second section of the recess.

The spring comprises at least partially a frustoconical section. The spring may entirely comprise a frustocomcal section. Alternatively, the spring may comprise at frustocomcal section and at least one cylindrical section which is of a constant external diameter.

In one embodiment, the spring comprises one cylindrical section, which extends from the second end of the spring, and wherein the frustocomcal section extends from the first end of the spring, wherein the cylindrical section and the frustocomcal section meet at a mid-point of the spring, and wherein the frustocomcal section varies in external diameter from a maximum external diameter at the first end of the spring, to a minimum external diameter at the midpoint which is equal to the external diameter of the cylindrical section. The spring May further comprise a second cylindrical section extending from the second end of the spring, wherein the frustocomcal section separates the first cylindrical section and the second cylindrical section, and wherein a maximum external diameter of the frustocomcal section is equal to the external diameter of the first cylindrical section, and wherein a minimum external diameter of the frustocomcal section is equal to the external diameter of the second cylindrical section. alternative embodiment, the spring comprises a barrel shape.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is now described by way of example with reference to the accompanying drawings in which:

Figure 3 is a cross-sectional partial view of a fuel pump including a pumping mechanism in accordance with the present invention;

Figure 4 is a side view of the spring of the pumping mechanism of

Figure 4; and

Figures 5 to 7 are side views of alternative springs for use in pumping mechanisms in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is described below with reference to the orientation of the figures. References to relative positioning of components, such as above, below, upper and lower, are not intended to be limiting.

Referring to Figures 3 and 4, a fuel pump in accordance with a first embodiment of the present invention comprises a pump housing 104, comprising a cam box 106 and an inlet valve housing 108, and a pumping mechanism 110.

The pumping mechanism comprises a pumping element provided by a plunger 112, and a return spring 114.

The inlet valve housing 108 is provided with a plunger location bore 116, in which the plunger 112 is moveable in a reciprocating motion, along a longitudinal axis A of the plunger location bore 116. A pumping chamber 118 is defined at an end of the plunger location bore 116, above the plunger 112.

The cam box 106 is provided with a guide bore 122, in which a roller/shoe guide 124, is moveable in a reciprocating motion. Above the roller/shoe guide 124, the guide bore 122 also defines a spring chamber 126, in which the spring 1 14 is located. The spring 114 surrounds a lower part 128 of the plunger 112 which protrudes into the cam box 106.

The roller/shoe guide 124 is provided with an upper recess 130, adjacent the spring chamber 126, and a lower recess 132, in which a roller 134 is located. A driving mechanism comprising a cam 136, in contact with the roller 134, is located in a cam recess 138 below guide bore 122.

The upper recess 130 of the roller/shoe guide 124 comprises two sections; a first, upper section 140 adjacent the spring chamber 126, and a second, lower section 142, remote from the spring chamber 126, of smaller diameter and therefore smaller cross-sectional area than the first, upper section 140.

The return spring 114 extends from a first spring seat 144 provided by a lower surface of the inlet valve housing 108, to a second spring seat 146 provided in the second section 142, i.e. the reduced cross-sectional area section, of the upper recess 130 of the roller/shoe guide 124. The second spring seat 146 is reduced in size compared to prior art embodiments. As illustrated most clearly in Figure 4, the spring 114 is of a varying diameter, and therefore varying cross-sectional area, along its length. Specifically, the spring 114 has a maximum external diameter Dl at a first end 148 of the spring 114 which abuts the first spring seat 114 provided by a surface of the inlet valve housing 108, and has a minimum external diameter D2 a second end 150 which abuts the second spring seat 146 provided in the roller/shoe guide 124. In the embodiment of Figures 3 and 4, the external diameter of the spring 114 decreases from the first end 148 to the second end 150 in a linear manner (as indicated by line L in Figure 4), such that the spring 114 has a frustoconical form. The diameter of the second, lower section 142 of the upper recess 130 of the roller/shoe guide 124 is less than the maximum external diameter of the spring 114. Therefore, if the spring 114 was of equal external diameter along its length, as in prior art embodiments, it would be necessary for the spring 114 to end in the first, increased diameter section 140 of the upper recess 130 of the roller/shoe guide 124. However, as the spring 114 decreases in diameter along its length towards to the roller/shoe guide 124, to a minimum external diameter D2 which is smaller than the diameter of the second, lower section 142 of the upper recess 130, it is possible for the spring 114 to extend into this second, lower section 142. Accordingly, a spring 114 having a longer free length than prior art embodiments can be used.

The reduced cross-sectional area at the second end 150 of the spring 1 14 enables the second spring seat 146 to be located in the reduced cross-sectional area section 142 of the upper recess 130 of the roller/shoe guide 124.

During operation of the pump, the rotation of the cam 136 causes drive to be imparted to the plunger 112, via the roller 134 and the roller/shoe guide 124, thereby causing the plunger 112 to move upwardly along the plunger location bore 116 and into the pumping chamber 118, thereby causing fuel in the pumping chamber 118 to pressurise.

During a return stroke of the pump, the spring 114, which is constrained at its first end 148 by the first spring seat 144, provides a force to the roller 134, via the second spring seat 146 and the roller/shoe guide 124, thereby ensuring contact is maintained between the roller 134 and the cam 136.

The present invention provides a fuel pump in which the space within the cam box 106 is more effectively used, by enabling the extension of the spring 114 into the roller/shoe guide 124. The free length of the spring 114 is thereby increased in comparison to prior art embodiments, and lower stresses are encountered for the same force requirements as prior art embodiments. In a second embodiment of the present invention, the pumping mechanism 110 comprises an alternative spring 214. Figure 5 illustrates the alternative spring 214 in accordance with the second embodiment.

In common with the first embodiment, the alternative spring 214 of Figure 5 has an external diameter Dl at the first end 148 which is larger than the external diameter D2 at the second end 150, and the external diameter D2 at the second end 150 is less than the diameter of the second section 142 of the upper recess 130 of the roller/shoe guide 124, such that the second end 150 of the spring 214 extends into the second section 142 of the upper recess 130. However the form of the spring 214 varies along its length such that the spring 214 comprises a first, frustoconical section 152, extending from the first end 148 of the spring 214, and a second, cylindrical section 154 of uniform diameter extending from the second end 150 of the spring. The external diameter of the frustoconical section 152 reduces from a maximum external diameter Dl at the first end 148 of the spring 214, to a minimum external diameter, which is equal to the external diameter D2 of the cylindrical section, where the frustoconical section 152 meets the cylindrical section 154 at a mid-point 156 of the spring 214. The mid-point 156 may be at or near a central longitudinal point of the spring 214, or could be off-centre.

A third embodiment of the present invention comprises a spring 314 as illustrated in Figure 6. This alternative spring 314 comprises three sections. A first cylindrical section 158, having a uniform diameter, extends from the first end 148 of the spring 314, and a second cylindrical section 162, having a uniform diameter which is less than that of the first cylindrical section 158 and less than the diameter of the second section 142 of the roller/shoe guide upper recess 130, extends from the second end 150 of the spring 314. The first and second cylindrical sections 158, 162 are separated by a frustoconical mid-section 160, which has a maximum external diameter equal to the external diameter Dl of the first cylindrical section 158, and a minimum external diameter equal to the external diameter D2 of the second cylindrical section 162. An alternative spring 414 in accordance with a fourth embodiment of the present invention is illustrated in Figure 7. The spring 414 of Figure 7 comprises a barrel shape, having a maximum external diameter at a mid-point 164 may be at or close to a central longitudinal point of the spring 414, or could be off centre. In common with the first to third embodiments, the external diameter Dl of the spring 414 at the first end 148 is greater than the external diameter D2 at the second end 150. Examples of forms of the spring are provided above. The spring could also comprise other combinations of cylindrical, frustoconical, and/or curved profile sections.

All embodiments of the present invention provide a spring which is of varying diameter along its length, i.e. is not of a purely cylindrical form as in prior art embodiments. All embodiments have a reduced external diameter at one end, which allows the spring to extend further into the roller/shoe guide than prior art embodiments, thereby enabling a spring with a larger free length than prior art embodiments to be used. Accordingly, the present invention allows a spring to be used which provides the necessary force, whilst reducing the maximum stress which is encountered in the spring in use of the pump, thereby improving efficiency of the pump.

Furthermore, springs in accordance with the present invention have a variable spring rate along the length of the spring, due to the varying diameter. Accordingly, springs in accordance with the present invention do not have a single resonant frequency, thereby eliminating resonance of the spring within an application. This again prevents overstressing of the spring at certain engine speeds, thereby further increasing the reliability of the spring and the pump.

REFERENCES

Prior art: fuel pump 2

pump housing 4

cam box 6

inlet valve body 8

pumping mechanism 10

pumping element 12

return spring 14

plunger location bore 16

pumping chamber 18

roller/shoe guide 24

spring chamber 26

roller 34

cam 36

spring external diameter D

Invention:

pump housing 104

cam box 106

inlet valve housing 108

pumping mechanism 110

plunger 112

return spring 1 14, 214, 314, 414 plunger location bore 116

plunger location bore longitudinal axis A pumping chamber 118

guide bore 122

roller/shoe guide 124

spring chamber 126

plunger lower part 128

roller/shoe guide upper recess 130 roller/shoe guide lower recess 132 roller 134

cam 136

cam recess 138

roller/shoe guide upper recess first, upper section 140 roller/shoe guide upper recess second, lower section 142 first spring seat (inlet valve housing lower surface) 144 second spring seat 146

spring first end 148

spring second end 150

first end spring diameter Dl

second end spring diameter D2

second embodiment (Fig. 5):

frustoconical section 152

cylindrical section 154

spring mid-point 156

third embodiment (Fig. 6):

first cylindrical section 158

frustoconical mid-section 160

second cylindrical section 162

fourth embodiment (Fig. 7):

spring mid-point 164