GB2593934A

GB2593934A - Drive assembly for a fuel pump

Info

Publication number: GB2593934A
Application number: GB2005355.9A
Authority: GB
Inventors: John Mccrindle Christopher; Guerrato Diego
Original assignee: Delphi Technologies IP Ltd
Current assignee: Delphi Technologies IP Ltd
Priority date: 2020-04-10
Filing date: 2020-04-10
Publication date: 2021-10-13
Anticipated expiration: 2040-04-10
Also published as: GB2593934B; GB202005355D0

Abstract

A drive assembly for a fuel pump for an internal combustion engine comprising a shoe (26, Fig.5) and roller 24 rolling over the surface of a cam. The shoe defines a cavity in which the roller sits with a clearance C1 between the roller and shoe, wherein an extended clearance forms a space is defined adjacent to the roller to one or both sides of the roller, where the space C2 (at the side) has a greater dimension that the clearance (at the top). The space may be on one or both sides of the roller. The space can cover an angular range A relative from the central axis defined by a relationship to the cam pressure angle (2ɸmax<D<180º).

Description

DRIVE ASSEMBLY FOR A FUEL PUMP

FIELD OF THE INVENTION

This invention relates to a drive assembly for a fuel pump. In particular, but not exclusively, the invention relates to a drive assembly for use in a fuel pump of a compression ignition internal combustion engine.

BACKGROUND OF THE INVENTION

In an internal combustion engine, common rail fuel pumps supply fuel to a common rail fuel volume where the fuel is stored at high pressure prior to delivery to the fuel injectors of the engine. Common rail fuel pumps generally comprise a plunger which is reciprocable within a plunger bore to cause fuel pressurisation within a pump chamber. The pressurised fuel is then pumped from the pump chamber to the common rail fuel volume. The plunger is driven by means of a cam drive mechanism including a cam which is driven by a rotating drive shaft. A roller rides over the surface of the cam as the cam rotates and cooperates with a drive member in the form of a shoe, which then acts on the plunger to cause it to reciprocate through a pumping cycle comprising a pumping stroke and a return stroke. The roller rises up a leading flank of the cam surface during the pumping stroke, driving the plunger to top dead centre, and then rides down a trailing flank of the cam surface on the return stroke as the plunger moves to bottom-dead-centre. During the pumping stroke the plunger serves to pressurise fuel within a pump chamber for delivery to the common rail.

A problem exists with roller/shoe drive assemblies of this type due to elastic deformation of the roller onto the shoe, especially on the approach to plunger top dead centre. This causes wear of the shoe and/or potential seizure of the roller within the shoe, both of which are undesirable. It can also compromise the lubrication film that is required between the shoe and the roller to provide support for the shoe. The thin film effect benefits from small clearances between the roller and the shoe. However, if the elastic deformation is too large, this will adversely reduce the clearances to a point of contact, causing wear and/or seizure. There exists, therefore, an incompatibility between the requirement for narrow clearances between the roller and the shoe to provide thin film lubrication, and the requirement for large clearances accommodate the elastic deformation of the shoe and prevent contact between the shoe and the roller.

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

STATEMENTS OF INVENTION

According to the present invention, there is provided a drive assembly for a fuel pump assembly for an internal combustion engine, the drive assembly comprising a shoe and a roller, wherein the roller is configured to ride over a surface of the cam as it is driven, in use, and wherein the shoe defines an internal cavity within which the roller is rotationally received so as to define a clearance between the roller and the shoe, wherein a space is defined adjacent to the roller, on at least one of the left and right sides of the shoe, the space having a dimension which is greater than that of the clearance.

The invention provides the advantage that, because there is a space on at least one of the left and right sides of the shoe, the pinching effect which can occur when a side loading is applied to the shoe, which imparts a load to the roller to pinch the roller between the sides of the shoe, can be avoided. This reduces wear of the roller and prolongs the service life of the drive assembly.

In one embodiment, the space is defined by an enlarged region of the clearance between the shoe and the roller. The enlarged region of the clearance has a greater dimension than that of the clearance between the remainder of the shoe and the roller. In other words, the clearance has a relatively narrow region which supports thin film lubrication between the roller and the shoe, but the clearance is enlarged to define the space, on one side of the shoe or the other, or on both sides of the shoe, to avoid pinching of the roller between side shoe ears.

In some embodiments, the enlarged region of the clearance is typically defined between an ear of the shoe on at least one side of the shoe and an adjacent region of the roller.

It may be most convenient to provide the enlarged region of the clearance on both the left and right sides of the shoe. Typically, the enlarged region of the clearance on each side of the shoe extends over an equal angular range relative to a central axis of the roller. This means that the shoe can be assembled into the drive assembly in either orientation, which aids the manufacturing process.

The clearance between the roller and the shoe On the relatively narrow region) typically covers an angular range, D, which is defined by 211>max < D < 1800, where Ipmax is the maximum cam pressure angle.

In other embodiments, the enlarged region of the clearance may be provided on only one of the left and right sides of the shoe, for example by modifying the shoe only on one side.

By way of example, the enlarged region of the clearance on at least one of the left and right sides of the shoe covers an angular range relative to an axis of rotation of the roller which satisfies A + D + A < 180 with D > 20max, where A is the angular range of the clearance relative to the axis of rotation of the roller, D is the angular range of the enlarged clearance relative to the axis of rotation of the roller, and Omax is the maximum cam pressure angle.

In other embodiments, the space defined adjacent to the roller is defined on one side of the shoe, between the roller and an adjacent side portion of a guide for the shoe. This is achieved conveniently by removing one of the side ears of the shoe.

According to a further aspect of the invention, there is provided a fuel pump assembly including a drive assembly in accordance with the first aspect.

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 is a perspective view of a known fuel pump assembly for a common rail fuel system, and including a known roller/shoe drive assembly; Figure 2 is a cut-away view of the drive assembly shown in Figure 1 to illustrate the pressure angle (0) for the drive assembly; Figure 3 is a force diagram to illustrate the pressure angle (0) for the drive assembly in Figure 1, including components of the drive assembly; Figure 4 is a force diagram to show the relationship between the lift force applied to the roller, the side load from the shoe which is applied to the roller, the load applied by the plunger to the roller, and the pressure angle; Figure 5 is a cross sectional view of a part of a drive assembly of an embodiment of the invention, for use in a fuel pump assembly of the type shown in Figure 1; and Figure 6 is a cross sectional view of a part of a drive assembly of an alternative embodiment to that shown in Figure 5.

SPECIFIC DESCRIPTION

For the purpose of the following description, references to left and right, up and down, and any other references to orientation, are not intended to be limiting and refer only to the orientation of parts shown in the accompanying figures.

Figure 1 shows a part of a common rail fuel pump assembly, referred to generally as 10, for use in a compression-ignition internal combustion engine. The common rail fuel pump assembly (hereinafter "the pump") comprises a main pump housing 12 which is provided with a pump head, referred to generally as 14. A drive shaft 16 extends through the main pump housing 12 and is supported by bearings (not identified) so that the shaft 16 can rotate within the main pump housing 12.

The pump head 14 includes a turret (not identified) which extends downwardly from a top plate 18 of the head to define a plunger bore which receives a pumping plunger 20. The plunger 20 is driven to move between a bottom-dead-centre positon (hereinafter, "BDC position") and a top-dead-centre position (hereinafter, "TDC position"), defining a pump stroke for the plunger, and between the TDC position and the BDC positon, defining a return stroke for the plunger 20. Plunger motion is driven by means of a cam drive assembly (not shown in Figure 1) including a cam 22, a roller 24 and a shoe 26. The return stroke is effected by means of a return spring 28 which surrounds the plunger 20.

In order to understand the problem that has been observed by the inventors, a conventional drive assembly that is typically used in the pump assembly in Figure 1 will now be described in detail.

The cam drive assembly can be seen in more detail in Figure 2, including the components of the cam 22, the roller 24 and the shoe 26. The cam 22 is mounted on, or is integrally formed with, the drive shaft 16 in a conventional manner. The roller 24 rests of the surface of the cam 22 and rides over the cam surface as the drive shaft 16 rotates, which imparts drive to the shoe causing it to move vertically (in the illustration shown) and, in turn, impart drive to the plunger 20. A guide or barrel 30 (also shown in Figure 1) for the shoe 26 serves to guide movement of the shoe 26 as it is driven, in use. The shoe 26 has an outer cross section of generally rectangular form (as can be seen in Figure 1), but in other embodiments may be cylindrical.

The shoe 26 includes a main body portion, identified at 26a, which is provided with a cavity 32, having a C-shaped cross section, within which the roller 24 is rotationally received. A narrow radial clearance, Cl, is present between the outer surface of the roller 24 and the internal surface of the cavity 32. The radial clearance, Cl, extends around the internal cavity 32 and around that portion of the roller surface which resides within the cavity 32 (i.e. extending partially around the circumference of the roller 24). The cavity 32 further defines a left side ear 26b on the left side of the shoe 26 and a right side ear 26c on the right side of the shoe 26. The left and right side ears 26b, 26c extend downwardly from the main body portion 26a of the shoe 26.

The pump head 14 defines a pump chamber (not visible) at the top of the plunger 20 into which fuel at relatively low pressure is delivered. The fuel is supplied at a pressure of around 3 bar (300 kPa). Within the pump chamber fuel is pressurised to a relatively high level, typically in excess of 2000 bar, through the action of the driven plunger 20. The pump chamber 20 is defined by a combination of the upper end of the plunger 20 and the plunger bore; its volume decreases and increases during the pump and return strokes respectively. The pump chamber communicates with inlet and outlet valve assemblies (not identified) via internal inlet and outlet passages, respectively, in a conventional manner. 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, except to say that they are used to control flow of the fuel from the pump inlet through to the pump chamber to the pump outlet and through to the common rail (not shown).

In use, as the roller 24 rides over the cam surface, the shoe 26 acts on the plunger 20, driving the plunger 20 through its pump stroke. As the roller 24 rides up a leading flank of the lobe of the cam 22, the plunger 20 is driven to perform its pump stroke. Under the action of the return spring 28, the plunger 20 performs the return stroke where the roller 24 rides down the trailing flank of the cam 22, moving the plunger 20 from the TOO position towards the BOO position. This causes an increase in the volume of the pump chamber, 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 and fuel enters the pump chamber until the pressure across the valve assembly equalises, causing it to close. This typically occurs just after the plunger reaches the BDC position. Once the plunger reaches the BDC position, it begins the pump stroke under the influence of the cam drive assembly to pressurise the fuel in the pump chamber. During the pump stroke, the fuel in the pump chamber is compressed and fuel pressure increases. A pressure drop is created across the outlet valve assembly, allowing it to open and fuel exits the pump chamber and flows into the common rail fuel volume via the pump outlet. As the plunger reaches the TOO position, the pressure across the outlet valve assembly equalises, causing it to close.

The forces that are experienced by the drive assembly during a pumping cycle, as the roller 24 rides over the surface of the cam 22 and drives the shoe 26 to act on the plunger 20, will now be considered with reference to Figures 2 and 3. As the cam 22 rotates and the roller 24 rides over the cam surface, the roller 24 experiences a lift force applied by the driven cam 22, in addition to a force from the plunger 20 caused by pressure in the pump chamber and a side load applied by an ear of the shoe 26. Referring to Figure 3, the arrow, Fp, represents the force which is applied, along the direction of plunger motion, from the plunger 20 to the roller 24. The arrow, Fn, represents the lift force applied by the cam 22 to the roller 24: the applied lift force, Fn, varies through the pump stroke depending on the pressure angle, 0, and the pressure force, Fp. The arrow, Fs, represents the side load applied (from left to right in the illustration shown) by the side ear 26b of the shoe 26 to the roller 24 as a consequence of the shoe 26 reacting against the internal surface of the guide 30 under the applied lift force, Fn.

The pressure angle (0) is defined as the angle between the direction of plunger movement (vertical in the illustration shown) and the direction of the lift force, Fn, at any point of rotation of the cam 22. As illustrated in Figure 3, when the roller 24 is riding up the rising flank of the lobe of the cam 22, towards the highest point on the cam lobe, the pressure angle, 0, is non-zero. At this point the roller 24 will experience a side loading from the shoe ear 26b, depending on the pressure angle, 0, as the shoe 26 reacts against the guide 30. At the TDC position, with the roller 24 at the highest point on the lobe of the cam 22, the lift force, Fn, is directed vertically upwards and hence the pressure angle, 0, is zero degrees. At this point there is no side loading on the roller 24 via the shoe ear 26b.

Figure 4 illustrates a simplified force diagram to illustrate the relationship between the various forces, and the pressure angle, and in which the components of the drive assembly have been removed.

For non-zero angles of the pressure angle 0 (i.e. when the cam is at positions for which an off-axis lift force, Fn, is applied to the roller 24), the roller 24 experiences a side load, Fs, from the ear 26b of the shoe 26 as the shoe reacts against the inner surface of the guide 30 and pushes inwardly on the roller 24. Because the radial clearance, Cl, between the roller 24 and the shoe 26 is very narrow (typically in the region of several microns, for example 2 to 5 pm), this side load tends to urge the left side ear 26b of the shoe 26 onto the roller 24, causing contact between the left side ear 26b and the roller 24 on one side of the shoe 26, and consequently causing the roller 24 to be urged to the right (in the illustration shown) onto the right side ear 26c of the shoe 26. The result of the side loading on the shoe 26 in this way is therefore to provide a pinching effect on the roller 24, applied by the ears 26b, 26c of the shoe 26, which can lead to unwanted wear of the shoe and/or seizure of the roller 24. The problem is particularly aggravated because the dimension of the radial clearance, Cl, between the roller 24 and the cavity 32 of the shoe 26 (i.e. the spacing between the roller 24 and the facing internal surface of the cavity 32) is necessarily made very small to ensure a local thin film lubrication is achieved in this clearance.

What the inventors have realised is that that the structure of the shoe 26 can be modified, without compromising the desired thin film lubrication, but ensuring wear is limited and/or the risk of seizure is reduced or avoided altogether. The solution is illustrated with reference to Figure 5.

Figure 5 shows a cross section of the shoe 26 and roller 24 of the drive assembly of a first embodiment of the present invention. It will be appreciated that all other aspects of the pump assembly, with which the drive assembly of the invention is used, are identical to those features described previously, with reference to Figures 1 and 2. In Figure 5 it can be seen that the left and right shoe ears 26b, 26c have been modified, by providing a relief on the internal surface of the cavity 32. The presence of the relief means that the dimension of the radial clearance, 02, defined between the outer surface of the roller 24 and the internal surface of the cavity 32 is larger in the region of the shoe ears 26b, 26c than the dimension of the radial clearance, Cl, defined between the outer surface of the roller 24 and the internal surface of the cavity 32 in the region of the main body portion 26a of the shoe 26. This is achieved through 'thinning' the ears 26b, 26c in the side regions of the shoe 26, by relieving some of the material from the internal surface of the cavity 32 to define a space adjacent to the roller 24 on each side. Typically, the radial clearance, 02, between the roller 24 and the shoe 26 is enlarged in the region of the ears 26b, 26c by between 1-100 times the original radial clearance, Cl). The extent of the enlargement of the radial clearance is selected by considering the expected deformation of the shoe ears 26b, 26c due to the load, Fs. The enlarged clearance 02 must, however, still be small enough to ensure that the roller 24 is retained inside the shoe 26 at all times.

The angular extent of the relief provided in the shoe ear 26a, 26b is defined relative to the axis of rotation, X, of the roller 24 about which the roller rotates. Relative to the central roller axis, X, the angular range for the region, D, for which a relatively thin radial clearance Cl remains between the shoe 26 and the roller 24 is given by the following equation; 21)n,ax < D < 1800, where 1),"ax is the maximum cam pressure angle. Typically, (1) max is between 100 and 15°, and most typically is around 12°. Relative to the roller rotation axis, X, the angular coverage, A, of the enlarged region of the radial clearance, C2, between the shoe ears 26b, 26c and the roller 24 is defined by the equation: A + D + A < 180°.

The enlargement of the radial clearance, 02, in the region of the ears 26b, 26c provides the benefit that any side loading of the shoe 26 via the guide 30, as described previously, does not lead to contact between the shoe 26 and the roller 24 on the side ears 26b, 26c and avoids any pinching effect on the shoe 26 which occurs in the conventional drive assembly. The extent to which the shoe 26 is modified in the region of the ears 26b, 26c needs careful selection because the shoe ears 26b, 26c cannot be thinned to such an angular extent that the benefit of thin film lubrication between the shoe 26 and the roller 24, which provides support for the roller 24 in the main body portion 26a of the shoe 26, is lost appreciably. A selection is made to ensure that thin film lubrication between the shoe 26 and the roller 24 is still maintained between the main body portion 26a of the shoe 26 and the roller 24, whilst the pinching of the roller 24 between the shoe ears 26b, 26c is substantially avoided. The invention has therefore been devised through the inventors' realization that whilst thin film lubrication between the shoe 26 and the roller 24 is important, it is not essential around the complete internal surface of the cavity 32 and providing the thin film effect is achieved over the angular range indicated at D, a compromise can be afforded in the region of the shoe ears 26b, 26c by making the radial clearance 02 larger.

Because the loading of the roller 24 as the cam 22 is driven tends to result in a load being applied to the ear 26b of the shoe 26 from only one side (as shown in Figure 2), and hence a reaction load from the guide 30 being applied to the shoe 26 from only one side, it is possible to achieve a similar benefit to that described for Figure 5 by defining a space to the side of the roller 24 on only one side. This is illustrated in Figure 6 which shows the shoe 26 having a modified right ear 26c only, so that there is an enlarged radial clearance, 02, between the right hand side ear 26c of the shoe 26, but the radial clearance, Cl, remains the same around the remainder of the internal cavity 32 (i.e. the clearance remains narrow on the opposed side to the enlarged radial clearance, C2). Again, the extent of the shoe 26 which can be removed to define the enlarged radial clearance, 02, is carefully selected so as not to compromise the thin film lubrication effect which is achieved by the narrow radial clearance, Cl, between the main body portion 26a and the left ear 26b of the shoe 26 and the roller 24.

It is also possible for the shoe 26 to be modified only on the other side (the left hand side in the illustration shown) to avoid the roller pinching effect as any loading of the roller 24 by the right hand shoe ear 26b does not lead to any "pinching" if an enlarged radial clearance is provided on the left side of the roller 24.

In other embodiments it is also possible for the angular extent of the enlarged radial clearance on one side of the shoe 26 to be greater than that on the other side of the shoe 26 if there is a desire to retain a larger area for the thin film to support the main body portion 26a of the shoe 26. It is also possible to define a space to the side(s) of the roller 24, to avoid the roller pinching effect, by removing one or more of the shoe ears 26b, 26c altogether. Both side ears 26b, 26c may also be removed, although care must be taken in this case to ensure adequate guiding of movement of the shoe 26 is retained.

Hence, the invention provides several general embodiments, all of which provide the advantages that the effect of side loads on the shoe 26 due to the reaction against the guide 30, which tend to apply a pinching effect to the roller 24, are mitigated, whilst retaining the benefits of a thin film lubrication between the main body portion 26a of the shoe 26 and the roller 24. The various embodiments include: (1) an equal enlarged radial clearance, 02, provided between both the left and right side ears 26b, 26c and the roller 24 (Figure 5); (2) an enlarged radial clearance, 02, is defined between both the left and right side shoe ears 26b, 26c and the roller 24, but to a different angular extent on each side; (3) an enlarged radial clearance, 02, is defined between only one of the left and right side shoe ears 26b, 26c and the roller 24; (4) the left side ear 26b is removed altogether; (5) the right side ear is removed altogether or (6) both shoe ears 26b, 26c are removed.

In practice, for manufacturing and assembly reasons, it may be more convenient to machine the internal cavity 32 on both sides of the shoe 26 (as in Figure 5) so that the enlarged radial clearance 02 has the same angular extent on both sides.

This ensures all parts can be manufactured in the same way and the parts can be assembled interchangeably in either orientation.

It will be appreciated that other embodiments are envisaged in addition to the aforementioned examples, without departing from the scope of the appended claims.

Reference numerals: 10. fuel pump assembly 12. main pump housing 14. pump head 16. drive shaft 18. upper plate of pump head 20. plunger 22. cam 24. roller 26. shoe 26a. main body portion of shoe 26b. left shoe ear 26c right shoe ear 28. return spring 30. guide for shoe 32. internal cavity defined within the shoe Cl narrow radial clearance between the roller and the shoe C2 larged radial clearance between the roller and the shoe A angular extent of the enlarged clearance C2 angular extent of the narrow clearance Cl X roller axis of rotation (I) pressure angle (1)max maximum pressure angle

Claims

CLAIMS: 1. A drive assembly for a fuel pump assembly (10) for an internal combustion engine, the drive assembly comprising a shoe (26) and a roller (24), wherein the roller (24) is configured to ride over a surface of the a cam (22) as it is driven, in use, and wherein the shoe (26) defines an internal cavity (32) within which the roller (24) is rotationally received so as to define a clearance (Cl) between the roller (24) and the shoe (26), and wherein a space (C2) is defined adjacent to the roller (24), on at least one of the left and right sides of the shoe (26), the space (C2) having a dimension which is greater than that of the clearance (Cl).
2. The drive assembly as claimed in claim 1, wherein, on at least one of the left and right sides of the shoe (26), the space is defined by an enlarged region of the clearance (02) between the shoe (26) and the roller (24).
3. The drive assembly as claimed in claim 2, wherein the enlarged region of the clearance (02) is defined between an ear (26b, 26c) of the shoe (26) on at least one side of the shoe (26) and an adjacent region of the roller (24).
4. The drive assembly as claimed in claim 2 or claim 3, wherein the enlarged region of the clearance (02) is provided on both a left and right side of the shoe (26).
5. The drive assembly as claimed in claim 4, wherein the enlarged region of the clearance (C2) extends over an equal angular range relative to a central axis of the roller (24) on both the left and right sides of the roller (24).
6. The drive assembly as claimed in claim 5, wherein the clearance (Cl) between the roller (24) and the shoe (26) covers an angular range (D) which is defined by 2c13,,,..< D < 1800, where ch'ax is the cam pressure angle.
7. The drive assembly as claimed in claim 6, wherein the enlarged region of the clearance (C2) is provided on only one of the left and right sides of the shoe (26).
8. The drive assembly as claimed in claim 1, further comprising a guide (30) for the shoe, and wherein the space (02) defined adjacent to the roller (24), on at least one of the left and right sides of the shoe (26), is defined between the roller (24) and the guide (30).
9. A fuel pump assembly (10) including a drive assembly as claimed in any of claims 1 to 8.