Pump assembly

Abstract

 A pump assembly for use in a compression ignition engine has a pumping plunger (110) that is reciprocable within a plunger bore (126) to cause pressurisation of fuel within a pump chamber (130) defined at one end of the plunger bore (126). The plunger (110) has a main plunger body (114) with a high pressure end (122) and a low pressure end (120) and a drive stem (12) for cooperation with a cam drive to cause the plunger (110) the reciprocate, in use. A fuel collection volume (128) of the assembly communicates with a return path (132) for permitting leakage fuel from the pump chamber (130) to flow to low pressure. The plunger body (114) and the plunger bore (126) are shaped to define a first clearance between the high pressure end (122) of the main plunger body (114) and the plunger bore (126) and a second clearance between the low pressure end (120) of the main plunger body (114) and the plunger bore (126).
The first and second clearances have different sizes so that any distortion of the main plunger body (114) and/or of the plunger bore (126) can be accommodated.
The relative sizing of the clearances is selected so as to accommodate distortion at whichever of the high or low pressure ends it is focused.

Description

[0001] The present invention relates to pump assembly and in particular, but not exclusively, the invention relates to a pump assembly for use in a fuel injection system of a compression ignition internal combustion engine. The pump assembly has particular application in unit injector type or unit pump type fuel injection systems.

[0002] Electronic Unit Injectors (EUIs) include an injector that is operable under the control of an electronically controlled spill valve to control fuel pressurisation and, thus, to control the timing of delivery of fuel by the injector. The EUI also includes a dedicated pumping element comprising a pump housing for a pumping plunger. The pumping plunger is driven, in use, be means of a cam drive so as to cause pressurisation of fuel within a pump chamber. Pressurised fuel is supplied from the pump chamber to the injector for injection to the associated engine cylinder or other combustion space. In EUI-type systems the pumping element and the dedicated injector are located within a common housing.

[0003] Electronic Unit Pumps (EUPs) are similar to EUIs, in that they include a pumping element having a pump housing and a plunger that is driven to pressurise fuel within a pump chamber by means of a cam drive. In EUP systems the injector associated with the EUP is not arranged within the same unit as the pumping element, but is instead supplied with fuel by the associated EUP through a separate high pressure fuel line. In both EUI and EUP systems fuel within the pump chamber is pressurised to an injectable pressure level and can be very high (typically 150 - 3000 bar).

[0004] Other types of fuel pump also employ pumping elements of the aforementioned type, having a plunger that is driven within a plunger bore so as to pressurise fuel within a pump chamber to a level that is suitable for injection. High pressure fuel pumps, such as those used in common rail fuel injection systems, typically include a plurality of pumping elements for supplying pressurised fuel to an accumulator or rail volume for subsequent delivery to the injectors.

[0005] Operation of the pumping element is similar in EUI, EUP and common rail type fuel pumps. In use, a cam is driven by a drive shaft and is co-operable with the plunger, typically through a separate drive member, so that as the shaft rotates the plunger is caused to reciprocate within its plunger bore. The plunger performs a forward stroke, during which it moves inwardly within its bore and the volume of the associated pump chamber is reduced to pressurise fuel therein, and a return stroke during which the plunger is urged outwardly from its bore, typically under a spring force, so that the pump chamber volume increases.

[0006] It is a recognised problem with EUI, EUP and common rail type pump assemblies of the aforementioned type that during the plunger forward stroke the high fuel pressures that are generated within the pump chamber can cause the plunger and the walls of the pump chamber to distort so that hydrodynamic lubrication of the plunger is compromised. The requirement to clamp the components of the pumping element also causes distortion. In the worst case the loss of hydrodynamic lubrication can cause seizure of the plunger and failure of the pump. Depending on the application, distortion of the plunger may either be focussed at the high pressure end (i.e. the pump chamber end of the plunger) or the low pressure end (i.e. the drive end of the plunger).

[0007] It is an object of the present invention to provide a pump assembly which removes or alleviates the aforementioned problem.

[0008] According to a first aspect of the present invention there is provided a pump assembly for use in a compression ignition engine, the pump assembly comprising a pumping plunger that is reciprocable within a plunger bore to cause pressurisation of fuel within a pump chamber, the plunger having a main plunger body with a high pressure end which defines, in part, the pump chamber, and a low pressure end in connection with a plunger drive stem which is co-operable with a cam drive, in use, to cause the plunger to reciprocate, a first clearance defined between the high pressure end of the main plunger body and the plunger bore and a second clearance defined between the low pressure end of the main plunger body and the plunger bore, wherein the first and second clearances have different sizes with one of the first and second clearances being enlarged relative to the other.

[0009] The first and second clearances are sized so as to accommodate any distortion of the plunger and/or of the plunger bore that may occur in use. Either the first clearance is enlarged relative to the second clearance, or vice versa, depending on whether distortion of the plunger and/or of the plunger bore is focused towards the high or low pressure end of the main plunger body.

[0010] In a preferred embodiment, the pump assembly includes a fuel collection volume in communication with a return path for permitting leakage fuel from the pump chamber to flow to low pressure.

[0011] In one embodiment, the fuel collection volume may be defined by a groove provided on the plunger body surface.

[0012] In a preferred embodiment, the fuel collection volume is defined by a recess provided in the plunger bore.

[0013] In use, the plunger is driven to perform a forward stroke during which the volume of the pump chamber is decreasing and a return stroke during which the volume of the pump chamber is increasing.

[0014] In one embodiment, the main plunger body has a variable or non-uniform diameter along its length, that is along the length of the plunger body between the low pressure end at the drive stem and a plunger end face which defines the pump chamber. For example, the main plunger body may have a step along its length separating the high and low pressure ends thereof.

[0015] In one embodiment the high pressure end of the main plunger body has a diameter that is greater than the diameter of the low pressure end of the main plunger body, thereby to accommodate distortion of the main plunger body at its low pressure end and/or distortion of the plunger bore on a low pressure side of the fuel collection volume.

[0016] Preferably, the step in the plunger body is positioned so as to align with the fuel collection volume defined in the plunger bore when the plunger is at the end of the return stroke.

[0017] In an alternative embodiment, the high pressure end of the main plunger body has a diameter that is less than the diameter of the low pressure end of the main plunger body, thereby to accommodate distortion of the main plunger body at its high pressure end and/or distortion of the plunger bore on a high pressure side of the fuel collection volume.

[0018] In this alternative embodiment the step in the main plunger body is positioned so as to align with the fuel collection volume when the plunger is at the end of its forward stroke.

[0019] Alternatively, the plunger bore may be of variable or non-uniform diameter along its length to define, to one side of the fuel collection volume (e.g. a low pressure side), an enlarged diameter region and, to the other side of the fuel collection volume (a high pressure side), a reduced diameter region.

[0020] If the enlarged diameter region of the plunger bore is on the low pressure side of the fuel collection volume, any distortion of the main plunger body at its low pressure end and/or distortion of the plunger bore on the low pressure side of the fuel collection volume can be accommodated.

[0021] Alternatively, the enlarged diameter region of the plunger bore may be on the high pressure side of the fuel collection volume, thereby to accommodate distortion of the main plunger body at its high pressure end and/or distortion of the plunger bore on the high pressure side of the fuel collection volume.

[0022] In a still further alternative embodiment the plunger bore may be tapered to define a relatively large diameter towards one end thereof compared to a relatively smaller diameter at its other end.

[0023] According to a second aspect of the invention there is provided a pumping plunger for use in a pump assembly as set out in the accompanying claims, the plunger having a main plunger body and a drive stem for co-operation with a cam drive, wherein the plunger body is of varying or non-uniform diameter along its length.

[0024] The drive stem of the plunger and the main plunger body define a first step between them (e.g. the main plunger body and the drive stem have differing diameters) and the main plunger body is itself shaped to define a second step, part way along its length.

[0025] The invention will now be described, by way of example only, with reference to the following drawings in which:

Figure 1 is a diagram of a conventional pumping plunger for use in known high pressure fuel pump assemblies, such as EUI and EUP type pumps,

Figure 2 is a diagram of a pump assembly of a first embodiment of the present invention, including a plunger having a stepped main plunger body,

Figure 3 is a diagram of the stepped plunger body in Figure 2 to illustrate in further detail the variation in diameter along the plunger length,

Figure 4 is a further enlarged and exaggerated view of the plunger body in Figures 2 and 3 when at the end of its return stroke, to illustrate the positioning of the plunger body step relative to the plunger bore, and

Figure 5 is a diagram to show a part of a pump assembly of a second embodiment of the invention, having a stepped plunger bore.

[0026] Referring to Figure 1, a conventional plunger element 10 for use in a pump assembly is of generally cylindrical form and includes a drive stem 12 and a main plunger body 14 that are spaced from one another by means of a step region 16. The main plunger body 14 is of uniform diameter along its entire length, between a plunger end face 18 and the step 16. The plunger body 14 forms a sliding fit within a barrel or bore of a pump housing (not shown) having a uniform diameter along its entire length, with the plunger end face 18 defining, together with an end of the plunger bore, a pump chamber (not shown). The plunger body 14 defines what is often referred to in the art as 'the sealing length' of the plunger, that is the length of the plunger that forms a substantial seal (except for a small amount of leakage) with the plunger bore.

[0027] In use, the plunger 10 is driven to reciprocate within its bore so as to perform a pumping cycle through co-operation between the drive stem and a cam drive. Throughout its pumping cycle the plunger 10 performs a forward stroke, in which it is driven inwardly within bore by means of the cam drive so as to reduce the volume of the pump chamber. When the plunger 10 is driven into the position in which the pump chamber volume is a minimum, it is said to be at the bottom of its stroke. During a return stroke of the pumping cycle the plunger is driven outwardly from the bore under a return spring force to increase the volume of the pump chamber. When the plunger 10 is driven into the position in which the pump chamber volume is a maximum, it is said to be at the top of its stroke. During a period of the forward stroke fuel pressure within the pump chamber is increased to a high level that is suitable for injection. An external spill valve may be used to control the timing of pressurisation within the pump chamber.

[0028] Alternatively a filling port may be provided in the plunger bore for this purpose, whereby the plunger body 14 co-operates with the filling port through its pumping cycle to open and close the filling port, and thereby to determine whether pump chamber filling and/or fuel pressurisation occurs.

[0029] Due to the high level to which fuel is pressurised within the pump chamber during the forward stroke, the plunger bore may be caused to dilate so that leakage fuel is able to flow down the plunger bore. This can also cause the main plunger body 14 to distort and, due to the high axial loading of the plunger 10, contact between the surface of the plunger body 14 and the plunger bore can occur at the uppermost end (in the orientation shown) of the plunger body 14. In extreme cases this can cause the plunger body 14 to jam within the bore, resulting in failure of the pump assembly.

[0030] Figures 2 to 4 show a first embodiment of the invention which seeks to address this problem. The plunger 110 of Figures 2 to 4 includes a main plunger body (referred to generally as 114) of stepped diameter and is arranged within a plunger bore 126 within a pump assembly housing 124. The end face 18 of the main plunger body 114 defines, together with the blind end of the plunger bore 126, a pump chamber 130 within which fuel is pressurised to a high level, in use. The other end of the plunger bore 126 opens into a chamber (not identified) for receiving lubrication fluid, such as oil, for lubricating the drive stem 12 and the cam drive components. A supply passage 131 extends from the pump chamber 130 to provide a supply path for high pressure fuel to be delivered to downstream parts of the fuel system.

[0031] The end of the plunger body 114 which defines the pump chamber 130 is referred to as 'the high pressure end' 122, due to its proximity to high pressure fuel within the pump chamber 130. The other end of the main plunger body 114, in connection with the drive stem 12, is referred to as 'the low pressure end' 120 due to its proximity to lubrication oil at low pressure.

[0032] The high pressure end 122 of the main plunger body 114 has a greater diameter, X, than the diameter, Y, of the low pressure end 120 so as to define a step 134 along the main plunger body length. It will be appreciated that the scale shown in Figures 2 and 3 is such that the step along the plunger body length is not clearly visible, but the step 134 can be seen in Figure 4. The diameters X and Y are marked on Figure 3.

[0033] A fuel collection volume 128 in the form of leakage groove is provided in the plunger bore 126. The fuel collection volume 128 communicates with a back flow or return path 132 in communication with a low pressure fuel drain, so that any fuel leakage from the pump chamber 130 which collects in the volume 128 is returned to the drain. The fuel collection volume 128 may be referred to as "the mid recess", as it defines a recessed bore region of enlarged diameter approximately mid way along the plunger bore length, and may be considered to separate or interpose high and low pressure sides of the main plunger bore 126.

[0034] As the plunger body 114 has a step 134 along its length, the length of the plunger body 114 at the high pressure end 122 defines a clearance or gap with the bore 126 which is smaller than that defined by the plunger body 114 at its low pressure end 120. Thus, the size of the clearance between the plunger body 114 and its bore 126 varies along the longitudinal plunger axis, stepping from a smaller clearance on the high pressure side to an enlarged clearance on the low pressure side. In use, therefore, any distortion of the plunger body 114 tending to focus at the low pressure end 120 thereof will be accommodated within the increased clearance in this region. Contact between the facing surfaces of the main plunger body 114 at the low pressure end 120 and the bore 126 is therefore substantially prevented.

[0035] The axial position of the step 134 in the plunger body 114 is selected so that the step 134 aligns approximately with the fuel collection volume 128 when the plunger 110 is at the end of its return stroke (i.e. the top of its stroke when pump chamber volume is a maximum), that is at the point in the pumping cycle when axial loading of the plunger 110 is a minimum, fuel pressure within the pump chamber 130 is at substantially its minimum level and there is minimum distortion. As can be seen most clearly in Figure 4, by positioning the step 134 to align with the volume 128 at this stage of the pumping cycle, the step 134 in the plunger body 114 does not travel past the edge of the volume 128 (the edge being identified at 137) on the low pressure side during the forward and return strokes of the plunger and only travels past the edge of the volume 128 on the high pressure side (the edge being identified as 136) during that part of the forward stroke when the plunger 110 is not doing substantial work to pressurise fuel within the pump chamber 130.

[0036] It is a further benefit of this embodiment of the invention that the additional clearance volume at the low pressure end 120 of the plunger body 114 enables improved lubrication of the plunger body 114 at the low pressure end 120 thereof.

[0037] For the avoidance of doubt it is noted that the step 16 along the plunger 110 between the drive stem 12 and the main plunger body 114 is of no relevance to this invention, and it is the step change in diameter along the length of the main plunger body 114 (i.e. at step 134) that provides the aforementioned advantages.

[0038] Figure 5 shows a part of an alternative embodiment to that shown in Figures 2 to 4, in which the diameter of the plunger body (not shown in Figure 5) is substantially uniform from the point at which it connects with the drive stem 12 to its end face 18. In this embodiment it is the plunger bore 126 that is of stepped diameter so that the diameter, Q, on the low pressure side of the fuel collection volume 128 is larger than the diameter, W, on the high pressure side of the fuel collection volume 128. The step change in diameter occurs in the region of the fuel collection volume 128. As a result, the clearance between the bore 126 and the plunger body 114 at its low pressure end 120 is greater than that at its high pressure end 122 so that any plunger distortion at the low pressure end 120 can be accommodated without surface to surface contact. This distortion is particularly prominent during the plunger forward stroke, when axial loading of the plunger 110 is greatest, but is accommodated by the increased clearance between the plunger bore 126 and the main plunger body 114. Again, the advantage of providing an increased clearance volume for lubrication oil at the low pressure end 120 of the plunger body 114 is also realised.

[0039] The embodiments described previously provide a solution to problems encountered as a result of distortion of the plunger body 114 at its low pressure end 120 (i.e. towards the drive stem 12). In other pump applications, however, plunger distortion may be greater at the high pressure end of the plunger body 114 (i.e. in the region of the end face 18 defining the pump chamber 130). This may arise, for example, where clamping of the pump assembly distorts the pump housing 124 and its internal components. In order to address this latter problem, either the plunger body 114 or the plunger bore 126 may be of stepped diameter so as to provide an enlarged clearance between the plunger body 114 and the plunger bore 126 at the high pressure end 122 of the plunger body 114 compared to that at the low pressure end 120.

[0040] By way of example, the main plunger body 114 may be of uniform diameter along its length (i.e. from the step 16 at the end of the drive stem 12 to the plunger end face 18), and the plunger bore 126 may be of enlarged diameter on the high pressure side of the fuel collection volume 128 and of reduced diameter on the low pressure side. Any plunger distortion at its high pressure end 122 is therefore accommodated by the increased clearance between this end 122 of the plunger body 114 and the bore 126.

[0041] Alternatively, the plunger bore 126 has a uniform diameter along its length and the main plunger body 114 is of stepped diameter, having a smaller diameter at its high pressure end 122 and a larger diameter at its low pressure end 120. The step 134 along the plunger body 114 is positioned so that it aligns approximately with the fuel collection volume 128 when the plunger 110 is at the end of its forward stroke (i.e. at the bottom of its stroke when the pump chamber volume is a minimum). This ensures that the step 134 on the plunger body 114 does not travel past the groove edge 136 on the high pressure side of the volume 128 during the forward and return strokes and only passes the groove edge 137 on the low pressure side of the volume 128 during that part of the forward stroke when the plunger 110 is not doing substantial work to pressurise fuel within the pump chamber 130.

[0042] A further alternative embodiment (not shown) replaces the stepped diameter of the main plunger body 114 and/or of the plunger bore 126 with a tapered plunger bore or a tapered main plunger body. If the main plunger body 114 is tapered, it may taper from a larger diameter at the high pressure end to a smaller diameter at the low pressure end, or vice versa, depending on the particular application and the focus of plunger and/or plunger bore distortion. If the plunger bore 126 is tapered, again this may taper from a larger diameter on the high pressure side of the fuel collection volume to a smaller diameter on the low pressure side, or vice versa.

[0043] In a further alternative embodiment both the plunger body 114 and the plunger bore 126 may be of non uniform diameter along their lengths to realise the aforementioned advantages, although this provides a more complex manufacturing process.

[0044] As described previously, the fuel collection volume 128 may be defined by a groove or recess in the plunger bore 126 which communicates with the inlet opening of the return path 132 to low pressure. However any of the embodiments may include as an alternative, or in addition, a fuel collection volume that is defined by an annular groove or recess provided in the outer surface of the plunger body 114. In this case the 'fuel collection volume' with which the plunger body step 134 is aligned (either at the tope of the stroke or at the bottom of the stroke, depending on the relative size of the high and low pressure ends 120, 122), includes the volume of the inlet opening of the return path 132.

Claims

1. A pump assembly for use in a compression ignition engine, the pump assembly comprising:

a pumping plunger (110) that is reciprocable within a plunger bore (126) to cause pressurisation of fuel within a pump chamber (130), the plunger (110) having a main plunger body (114) with a high pressure end (122) defining, in part, the pump chamber (130) and a low pressure end (120) in connection with a plunger drive stem (12) for co-operation with a cam drive, in use, to cause the plunger (110) to reciprocate,

a first clearance defined between the high pressure end (122) of the main plunger body (114) and the plunger bore (126), and

a second clearance defined between the low pressure end (120) of the main plunger body (114) and the plunger bore (126),

characterised in that one of the first and second clearances is enlarged relative to the other.

2. The pump assembly as claimed in claim 1, further comprising a fuel collection volume (128) in communication with a return path (132) for permitting leakage fuel from the pump chamber (130) to flow to low pressure,

3. The pump assembly as claimed in claim 2, wherein the plunger body (114) is of a variable diameter along its length.

4. The pump assembly as claimed in claim 3, wherein the main plunger body (114) is tapered.

5. The pump assembly as claimed in claim 3, wherein the plunger body (114) has a step (134) along its length separating the high and low pressure ends (122, 120) thereof.

6. The pump assembly as claimed in claim 5, wherein the high pressure end (122) of the main plunger body (114) has a diameter that is greater than the diameter of the low pressure end (120) of the main plunger body (114), thereby to accommodate distortion of the main plunger body (114) at the low pressure end (120) and/or distortion of the plunger bore (126) on a low pressure side of the fuel collection volume (128).

7. The pump assembly as claimed in claim 6, whereby the plunger (110) is driven to perform a forward stroke during which the volume of the pump chamber (130) is decreasing and a return stroke during which the volume of the pump chamber (130) is increasing, wherein the fuel collection volume (128) is defined by a recess provided in the plunger bore (126) and the step (134) in the main plunger body (114) is positioned so as to align with the fuel collection volume (128) when the plunger (110) is at the end of the return stroke.

8. The pump assembly as claimed in claim 5, wherein the high pressure end (122) of the main plunger body (114) has a diameter that is less than the diameter of the low pressure end (120) of the main plunger body (114), thereby to accommodate distortion of the plunger (110) at the high pressure end (122) and/or distortion of the plunger bore (126) on a high pressure side of the fuel collection volume (128).

9. The pump assembly as claimed in claim 8, whereby the plunger (110) is driven to perform a forward stroke during which the volume of the pump chamber (130) is decreasing and a return stroke during which the volume of the pump chamber (130) is increasing, wherein the fuel collection volume (128) is defined by a recess provided in the plunger bore (126) and wherein the step (134) in the main plunger body (114) is positioned so as to align with the fuel collection volume (128) when the plunger (110) is at the end of the forward stroke.

10. The pump assembly as claimed in any one of claims 2 to 5, wherein the fuel collection volume (128) is defined by a groove provided on the surface of the main plunger body (114).

11. The pump assembly as claimed in claim 2, wherein the plunger bore (126) is of variable diameter along its length.

12. The pump assembly as claimed in claim 11, wherein the fuel collection volume (128) is defined, at least in part, by a recess in the plunger bore (126) and wherein the plunger bore (126) defines, on a high pressure side of said recess, an enlarged diameter region and, on a low pressure side of said recess, a reduced diameter region, thereby to accommodate distortion of the main plunger body (114) at its high pressure end (122) and/or distortion of the plunger bore (126) on the high pressure side.

13. The pump assembly as claimed in claim 11, wherein the fuel collection volume (128) is defined, at least in part, by a recess in the plunger bore (126) and wherein the plunger bore (126) defines, on a high pressure side of said recess, a reduced diameter region and, on a low pressure side of said recess, a reduced diameter region, thereby to accommodate distortion of the main plunger body (114) at its low pressure end (120) and/or distortion of the plunger bore (126) on the low pressure side.

14. The pump assembly as claimed in any one of claims 11 to 13, wherein the plunger bore (126) is tapered.

15. A pumping plunger for use in a pump assembly as claimed in any one of claims 1 to 14, having a drive stem (12) and a main plunger body (114) of non-uniform diameter along its length.

16. The pumping plunger as claimed in claim 15, wherein the drive stem (12) and the main plunger body (114) define a first step therebetween and the main plunger body (114) is shaped to define a second step (134) part way along its length.

Drawing