GB2581158A - Fuel pump assembly - Google Patents

Abstract

A fuel pump assembly (2, Fig. 1) for an internal combustion engine, comprising a pump housing (4, Fig. 1) having a plunger bore (8, Fig. 1) for receiving a plunger 10 having a driven end D and a pumping end P, wherein the plunger 10 comprises an outer plunger sleeve 10a for receiving an inner plunger rod 10b which extends along the full length of the plunger sleeve 10b, at least one of the diameter 36 of the inner plunger rod 10b and the inner diameter 34 of the outer plunger sleeve 10a having a step 13 along its length so as to define a first plunger region towards the driven end D where the outer plunger sleeve 10a and the inner plunger rod 10b are in contact with one another and a second plunger region towards the pumping end P where an annular clearance 11 is defined between the rod 10b and the sleeve 10a, the annular clearance 11 receiving high pressure fuel, in use, which serves to dilate the outer plunger sleeve 10a so as to narrow a clearance channel (9, Fig. 1) between the outer plunger sleeve 10a and the plunger bore 8 as the plunger 10 performs a pumping cycle, in use.

Description

FUEL PUMP ASSEMBLY

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 with an improved volumetric efficiency.

BACKGROUND

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 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. A known problem with common rail fuel pumps is that the clearance between the plunger and the plunger bore increases with increasing pumping pressure. This causes the pressurised fuel to leak through the clearance, reducing the volumetric efficiency of the pump.

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

STATEMENTS 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 may comprise a pump housing provided with a plunger bore for receiving a plunger having a driven end and a pumping end, wherein the plunger comprises an outer plunger sleeve for receiving an inner plunger rod which extends along the full length of the plunger sleeve. At least one of the outer diameter of the inner plunger rod and the inner diameter of the outer plunger sleeve has a step along its length so as to define a first plunger region towards the driven end where the outer plunger sleeve and the inner plunger rod are in contact with one another and a second plunger region towards the pumping end where an annular clearance is defined between the inner plunger rod and the outer plunger sleeve. The annular clearance receives high pressure fuel, in use, which serves to dilate the outer plunger sleeve so as to narrow a clearance channel between the outer plunger sleeve and the plunger bore as the plunger performs a pumping cycle, in use.

The aim of the invention is to provide a small annular clearance between the inner and outer parts of a two-part plunger for use in a fuel pump assembly to aid dilation of the plunger and increase the volumetric efficiency of the pump assembly, whilst minimising the dead volume created in which fuel is pressurised. The invention also aims to ensure that the two parts of the plunger are held together securely (e.g. in an interference fit) such that there is no additional leakage of fuel as a result of the two-part nature of the plunger. The two-part nature of the plunger also provides a benefit to the ease of assembly and machining.

As acknowledged above, the issue of leakage of high pressure fuel through the clearance channel between the plunger and plunger bore is well known in the art.

By providing a clearance between two sections of the plunger, the dilative effect on the outer plunger sleeve of the high pressure fuel received therein reduces the clearance between the plunger and the plunger bore, thereby increasing the volumetric efficiency of the pump.

Optionally, the inner diameter of the outer plunger sleeve may be of uniform diameter and the outer diameter of the inner plunger rod may be of stepped diameter, In other embodiments, the outer diameter of the inner plunger rod may be of uniform diameter and the inner diameter of the outer plunger sleeve may be of stepped diameter.

In still further embodiments, both the inner diameter of the outer plunger sleeve and the outer diameter of the inner plunger sleeve may be stepped.

In the embodiments of the invention, the stepped nature of the either the outer diameter of the inner plunger rod or the inner diameter of the outer plunger sleeve allows the creation of the interference fit to keep the inner plunger rod sealed as well as allowing the clearance that is necessary for the plunger to dilate to increase volumetric efficiency. The presence of the step on the outer diameter of the inner rod has the particular benefit that contact pressure, stresses and retention force can be optimised.

Optionally, the outer diameter of the outer plunger sleeve includes a region of reduced diameter which defines an intersection with a region of the outer plunger sleeve having an enlarged outer diameter.

The annular clearance may extend beyond the intersection between the region of reduced diameter and the upper region of the outer plunger sleeve having an enlarged outer diameter.

By way of example, the annular clearance extends beyond the intersection between the region of reduced diameter and the upper region of the outer plunger sleeve having an enlarged outer diameter by at least 1 mm.

The length of the annular clearance is only dictated by the length from the pumping end of the plunger to the step in the outer diameter of the outer plunger sleeve. It is ideally longer than this length by a minimum of 1mm but can be any length beyond this, provided there is a length of interference fit section below it.

Optionally, the annular clearance defined between the inner plunger rod and the outer plunger sleeve may be less than 50 microns.

For example, the radial dimension of the annular clearance defined between the inner plunger rod and the outer plunger sleeve may be about 25 microns.

In embodiments, the inner plunger rod and the outer plunger sleeve may be formed of the same material.

Ideally, any material used for the inner plunger rod and the outer plunger sleeve must not be compliant to avoid excessive compression of the parts by the pressurised fuel.

Optionally, the outer plunger sleeve may be coated.

According to a second aspect of the invention, there is provided a method of assembling a fuel pump assembly for an internal combustion engine. The pump head assembly may comprise a plunger that is driven, in use, from a driven end to cause pressurisation of fuel in a pump chamber at a pumping end of the plunger.

The method may comprise: providing an outer plunger part having a hollow bore along its length, the outer plunger part having an outer diameter, an inner diameter and an opening at either end into which an inner plunger rod could be inserted and inserting an inner plunger rod through one of the openings of the outer plunger part so that the inner plunger rod extends along the full length of the outer plunger part.

At least one of the outer diameter of the inner plunger rod and the inner diameter of the outer plunger sleeve has a step along its length so as to define, once the inner plunger rod is fully inserted, a first plunger region towards the driven end where the outer plunger sleeve and the inner plunger rod are in contact with one another and a second plunger region towards the pumping end where a clearance is defined between the inner plunger rod and the outer plunger sleeve.

The method may further comprise grinding the outer diameter of the outer plunger sleeve once the inner plunger rod has been inserted thereinto.

The method may instead further comprise grinding the outer diameter of the outer plunger sleeve and then coating the outer surface of the outer plunger sleeve before inserting the inner plunger rod thereinto.

One feature in determining the order of operations in the method is whether or not the plunger is coated. A coated plunger is ground and then coated on the outside surface of the outer plunger sleeve before the inner plunger rod is inserted to form the annular clearance and interference fit. When the plunger is not coated, the insertion of the inner plunger rod occurs first, before subsequent grinding of the outer diameter of the outer plunger sleeve to the required size. The inner plunger rod may be inserted into the outer plunger sleeve from either end.

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 cross-sectional view of a common rail fuel pump head assembly in accordance with the invention, Figure 2 is a cross sectional view of an embodiment of a plunger of the fuel pump head assembly of Figure 1, Figure 3 is a cross sectional view of an alternative embodiment of a plunger of the fuel pump assembly of Figure 1, Figure 4 is a cross sectional view of another alternative embodiment of a plunger of the fuel pump assembly of Figure 1, and Figure 5 is a cross sectional view of another alternative embodiment of a plunger of the fuel pump assembly of Figure 1 received within a plunger bore.

SPECIFIC DESCRIPTION

Figure 1 shows an embodiment of the invention of a common rail fuel pump assembly 2 for use in a compression-ignition internal combustion engine in accordance with the invention. The common rail fuel pump assembly 2 ("the pump 2" hereinafter) comprises a housing 4, which includes a turret 6. The turret 6 outwardly extends from the main body of the housing 4 and is contained within a guide bore (not shown). The turret 6 defines a 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 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. Plunger motion is driven by means of a cam drive arrangement (not shown) including a cam and a tappet.

The lower end of the plunger 10 extends from the turret 6 and is coupled to the tappet through a spring plate 12. The spring plate 12 defines an abutment surface for one end of a plunger return spring 14, which is housed in the guide bore. The other end of the plunger return spring 14 engages a seat 16 in the outer surface of the housing 4 around the turret 6. 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 20, 22 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 20, 22 includes a spring (inlet valve spring X and outlet valve spring Y), which act to close the valve assemblies 20, 22 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 20, 22 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 20. The fuel is supplied at a pressure of around 3 bar (300 kPa).

The plunger return spring 14 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 20. This pressure drop allows the inlet valve assembly 20 to open against the force of the inlet valve spring X and fuel enters the pump chamber 18 until the pressure across the valve assembly 20 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 the cam drive arrangement 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 the tappet. 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 outlet valve passage 26, which substantially equals the pressure in the common fuel rail volume. This pressure is in the region of at least 2000 bar (200 MPa). A pressure drop is created across the outlet valve assembly 22, allowing it to open against the force of the outlet valve spring Y 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 22 equalises, causing it to close.

In order to enable the reciprocating movement of the plunger 10 between the TDC and BDC positions, there is provided an annular clearance channel (the annular clearance channel 9 is identified in Figure 5) between the plunger 10 and the plunger bore 8. During the pump stroke, pressurised fuel can leak into the clearance channel 9, exerting radial inward and outward forces on the plunger 10 and plunger bore 8. This acts to increase the size of the clearance channel 9 by dilating the plunger bore 8 and compressing the plunger 10, thereby increasing leakage of pressurised fuel past the plunger 10 during the pump stroke, reducing the volumetric efficiency of the pump 2.

As shown in Figure 2, 3, 4 and 5, the plunger 10 is comprised of two constituent parts: an outer plunger sleeve 10a (outer sleeve hereinafter) and an inner plunger rod 10b (inner rod hereinafter) in a variety of embodiments. The plunger has a driven end D, which cooperates with the cam drive arrangement, and a pumping end P, which is exposed to fuel pressure in the pump chamber 18. The following embodiments all share a number of similar features, which shall be referred to by the same reference numerals for the sake of clarity.

Turning first to the embodiment of the plunger 10 shown in Figure 2, the outer sleeve 10a is a substantially cylindrical tube with a constant inner diameter 34. The outer profile of the outer sleeve 10a has a slightly enlarged outer diameter at both its upper and lower ends, giving rise to major and minor outer diameters 32a, 32b respectively. A major outer diameter 32a extends from the lower end of the plunger to a position approximately one-quarter of the way along the length of the plunger 10, A major outer diameter 32a also extends from the upper end of the plunger to approximately the half way point along the length of the plunger 10. These relative lengths are given by way of example only and in practice are dictated by other pump/head features; relative lengths may be different to the lengths stated here. The minor diameter region 32b defines a region of reduced diameter 33 (neck region hereinafter) of the outer diameter 32 part way along the length of the plunger. The major outer diameters 32a are larger than the minor outer diameter 32b and, typically, for example, may be between 1 and 5 per cent greater than the minor outer diameter 32b. The major and minor outer diameter regions of the outer sleeve taper into one another, rather than there being an abrupt step change in diameter.

In an embodiment of the applicant's previous patent application EP 3105450 A, the concept of a groove ring, or neck region, arranged on the external face of a peripheral wall of a plunger is introduced. A fuel path extends downwardly from the groove ring to enable a pressure drop of fuel below the groove ring and thereby limit deformation of the peripheral wall of the plunger by high pressure fuel received within a clearance between the plunger and the plunger bore.

The inner rod 10b is also substantially cylindrical with a stepped outer diameter 36, having a lower region defined by a major outer diameter 36a and an upper region defined by a minor outer diameter 36b, where the major outer diameter 36a is greater than the minor outer diameter 36b. The major and minor outer diameters 36a, 36b of the inner rod 10b meet at a step 13. The difference in minor and major outer diameters 36a, 36b of the inner rod 10b should be minimised in order to minimise the dead volume inside the plunger 10.

The inner rod 10b fits inside the outer sleeve 10a, with the major outer diameter 36a of the inner rod 10b being substantially equal to the inner diameter 34 of the outer sleeve 10a such that the lower region of the inner rod 10b makes contact with the inner wall of the outer sleeve 10a and forms a close interference fit therewith. In the embodiment shown in Figure 2, the step 13 along the length of the inner rod 10b is approximately radially aligned with the region of intersection between the minor outer diameter 32b of the outer sleeve 10a and the upper region of the outer sleeve 10a major outer diameter 32a. The step 13 is at least 1 mm below the intersection between the region of reduced diameter 33 and the upper region of enlarged diameter of the outer sleeve 10a but can be further down the plunger 10 provided that beneath the step there is a length of interference fit between the outer sleeve 10a and inner rod 10b. Due to the inner rod 10b having a minor outer diameter 36b at its upper end, an annular clearance 11 is defined between the minor outer diameter region (upper region) of the inner rod 10b and the inner wall of the outer sleeve 10a. The presence of the step 13 on the diameter of the inner rod 36 has the benefit that press fit contact pressure, stresses and retention force can be optimised.

The annular clearance 11 receives high pressure fuel, which exerts an outward pressure on the outer sleeve 10a, causing it to dilate and narrow the clearance channel 9 between the plunger 10 and the plunger bore 8. This leads to a decrease in leakage of pressurised fuel past the plunger during the pump stroke and hence an increase in the volumetric efficiency of the pump 2. The close interference fit between the outer sleeve 10a and inner rod 10b is designed to seal the plunger 10 such that the high pressure fuel received within the annular clearance 11 cannot leak between the plunger parts. It should be noted that, for the sake of clarity, both the clearance channel 9 and the annular clearance 11 are shown in Figure 2 as being of much greater thickness in relation to the diameters of the inner rod 10a and outer sleeve 10b than as envisaged in practice.

In the embodiment of the invention shown in Figure 3, the outer sleeve 10a has the same outer profile as that shown in Figure 2, with a major outer diameter 32a and minor outer diameter 32b with the minor outer diameter defining a neck region 33. However, in this embodiment, the outer sleeve 10a has a stepped inner diameter 34, with a minor inner diameter 34b defined along a lower region of the outer sleeve 10a and a major inner diameter 34a defined along an upper region of the outer sleeve 10a. The major and minor inner diameters 34a, 34b of the outer sleeve 10a meet at a step 13, which is approximately radially aligned with the region of intersection between the minor outer diameter 32b of the outer sleeve 10a and its upper region of major outer diameter 32a. As with the embodiment shown in Figure 2, the step 13 must be at least 1 mm below the region of intersection between the minor outer diameter 32a of the outer sleeve 10a and its upper region of major outer diameter 32a but can be further down the plunger 10 providing there is some region of interference fit between the inner rod 10b and outer sleeve 10a below it. The major inner diameter 34a is larger than the minor inner diameter 34b. The major and minor inner diameters 34a, 34b of the outer sleeve 10a should be as close to each other in size as possible, in order to minimise dead volume inside the plunger 10.

The inner rod 10b has a substantially constant outer diameter 36 that is substantially equal to the minor inner diameter 34b of the outer sleeve 10a such that the inner rod 10b forms a close interference fit within the lower region of the outer sleeve 10a. An annular clearance 11 is defined between the inner rod 10b and the upper region of the inner wall of the outer sleeve 10a. The annular clearance 11 receives high pressure fuel in the same manner as in the embodiment described above, causing a dilation of the plunger 10 in use to narrow the clearance channel 9 between the plunger 10 and the plunger bore 8. The interference fit between the outer sleeve 10a and inner rod 10b again seals the plunger to prevent the high pressure fuel received within the annular clearance 11 from leaking through the plunger parts. Again, the sizes of both the clearance channel 9 and the annular clearance 11 are exaggerated in Figure 3.

In the embodiment of the invention shown in Figure 4, the outer sleeve 10a once again has the same outer profile as shown in Figures 2 and 3. The outer profile of the outer sleeve 10a is defined by major and minor outer diameters 32a, 32b, with the minor outer diameter 32b defining a neck region 33 while the inner diameter 34 is stepped, with a minor inner diameter 34b along a lower region of the outer sleeve 10a and a major inner diameter 34a along an upper region of the outer sleeve 10a. In contrast to Figure 3, the inner rod 10b is of stepped diameter, with a minor outer diameter 36b along a lower region of the inner rod 10b and a major outer diameter 36a along an upper region of the inner rod 10b, with the minor outer diameter 36b and the major outer diameter 36a meeting at a step 13. The minor outer diameter 36b of the inner rod 10b extends from the lower end of the inner rod 10b to a point which is approximately radially aligned with the taper between the upper major outer diameter 32a and the minor outer diameter 32b of the outer sleeve 10a. The positioning of the step in relation to the taper between the upper major outer diameter 32a and minor outer diameter 32b of the outer sleeve 10a is the same in this embodiment as for those shown in Figures 2 and 3, i.e. the step must be at least 1 mm below the taper, but may extend further down the plunger providing there is some region of interference fit between the outer sleeve 10a and inner rod 10b.

The minor outer diameter 36b of the inner rod 10b substantially equals the minor inner diameter 34b of the outer sleeve 10a such that the lower region of the inner rod 10b forms a close interference fit with the lower region of the outer sleeve 10a.

The lower region of the inner rod 10b extends further up the plunger 10 than the corresponding region of the outer sleeve 10a so as to define a small gap or chamber 15. The major outer diameter 36a of the inner rod 10b is smaller than the major inner diameter 34a of the outer sleeve 10a, with an annular clearance 11 defined between the upper region of the inner rod 10b and the upper region of the outer sleeve 10a. High pressure fuel is received within the annular clearance 11, and flows into the gap 15, which functions in the same way as described above. The close interference fit between the outer sleeve 10a and the lower minor dimeter region 36b of the inner rod 10b prevents fuel leakage between the parts 10a, 10b.

The presence of the gap can help avoid the presence of unwanted compressive stresses that may otherwise be present if the inner rod is flush against the outer sleeve at the step 13.

In embodiments of the invention, the two parts 10a, 10b of the plunger 10 are made from the same material and are assembled together by a press fit operation. In other embodiments, the two parts 10a, 10b of the plunger 10 are formed from different materials. The inner wall of the outer sleeve 10a and the lower region of the inner rod 10b form an interference fit which retains the inner rod 10b in its fitted position. After assembly, the outer diameter 32 of the outer sleeve 10a is ground to the intended size and the outer sleeve 10a is coated. In an alternative embodiment, the outer sleeve 10a is ground to the correct outer diameter 32 and coated before assembly of the two parts 10a, 10b of the plunger 10.

In the embodiments of the invention described previously, the inner rod 10b extends along the whole length of the outer sleeve 10a. In an alternative arrangement, shown in Figure 5 the inner rod may only extend a part of the way along an outer sleeve member 10a, with the outer sleeve member 10a having a solid base region 10c from which a cylindrical outer sleeve region 10d extends upwardly. The inner rod 10b is received within the cylindrical outer sleeve region 10d so that the base of the inner rod 10b engages with an upper surface of the solid base region 10c of the outer sleeve 10a.

In a further alternative arrangement (not shown), an annular groove is machined into a monolithic plunger 10 to create an outer sleeve portion 10a and an inner rod portion 10b of a one-piece plunger. This configuration allows the plunger 10 to dilate in the same way as the two-piece plungers mentioned above.

Any interference fit between the outer sleeve 10a and inner rod 10b should have a contact pressure great enough to form a hydraulic seal to prevent fuel leaking through the plunger 10.

In an embodiment of the invention, there is provided a method for the assembly of a two piece plunger. The two piece plunger comprises an outer sleeve 10a and inner rod 10b, which are configured so that an interference fit between the two pieces is formed along a lower region of the plunger 10 and an annular clearance 11 is defined between the two pieces 10a, 10b along an upper region of the plunger 10. The plunger 10 is assembled by press fit of the inner rod 10b into the outer sleeve 10a, with the outer diameter 32 of the outer sleeve then ground down to the required shape and size. The inner rod 10b may be inserted into the outer sleeve 10a from either end.

In an alternative embodiment of the invention, the outer diameter 32 of the outer sleeve 10a of the plunger 10 may be ground to the required size and shape, with the inner rod 10b subsequently inserted into either end of the outer sleeve 10b via a press fit.

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. For example, the pump head assembly in Figure 1 is simply one configuration with which the two-part plunger 10 may be used, and the inlet and outlet valve assemblies 20, 22 may be configured differently in other embodiments.

References used: 2 -fuel pump assembly 35 4 -housing 6-turret 8 -plunger bore 9 -clearance channel -plunger 10a -outer plunger sleeve 10b -inner plunger rod 10c -solid base region 10d -cylindrical outer sleeve region 11 -annular clearance 12 -spring plate 13 -step 14-plunger return spring -gap 16 -seat 18-pump chamber 20 -inlet valve assembly 22 -outlet valve assembly 24-internal inlet passage 26-internal outlet passage 28-pump inlet 30-pump outlet 32 -outer plunger sleeve outer diameter 32a -outer plunger sleeve major outer diameter 32b -outer plunger sleeve minor outer diameter 33-neck region 34-outer plunger sleeve inner diameter 34a -outer plunger sleeve major inner diameter 34b -outer plunger sleeve minor inner diameter 36-inner plunger rod outer diameter 36a -inner plunger rod major outer diameter 36b -inner plunger rod minor outer diameter D -driven end P -pumping end X -inlet valve spring Y -outlet valve spring