FIELD OF THE INVENTION

This invention relates to a roller tappet assembly for a fuel pump assembly of an internal combustion engine. The invention further relates to a fuel pump assembly comprising such a roller tappet assembly.

BACKGROUND

Common rail direct fuel injection systems use a high-pressure fuel pump to inject fuel into the engine’s combustion chamber. The high-pressure fuel pump uses one or more reciprocating plungers to feed fuel into the common rail at a pressure of typically more than 200 MPa. Each plunger is driven by a respective cam of a common rotating camshaft. In known fuel pumps, a roller tappet assembly that is coupled to the plunger causes the plunger to follow the cam movement.

Typically, the roller tappet assembly comprises a tappet body that is connected to the plunger. An annular roller assembly is provided in a cavity of the tappet body and rolls over an outer surface of the rotating cam. A spring force may be used to ensure that the tappet assembly keeps in constant contact with the camshaft. The roller assembly is mounted inside the tappet body cavity by a pin that extends through the roller assembly. As a result, the pin functions as an axle around which the roller assembly rotates.

For optimal functioning of the tappet assembly and to minimise wear, it is important that the pin is axially and radially constrained relative to the tappet body. In known roller tappet assemblies, this is achieved by deforming the pin at one or both ends after installing the roller assembly in the tappet body cavity and inserting the pin into the centre of the roller assembly. Deforming the pin after its installation is a complex manufacturing process that may, for example, involve case hardening of only the central portion of the pin or hardening the full pin, followed by tempering the ends to provide deformable end portions. In addition to requiring a complex manufacturing process, this known type of roller tappet assembly has the disadvantage that, once assembled and deformed, it is impossible to remove the pin and replace the roller assembly or a part thereof.

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

SUMMARY OF INVENTION

According to an aspect of the invention, there is provided a roller tappet assembly for a fuel pump assembly of an internal combustion engine, the roller tappet assembly comprising a tappet body with a tappet body cavity, an annular roller assembly, and a pin. The annular roller assembly is arranged inside the tappet body cavity for rolling contact with a cam surface of a camshaft of the fuel pump assembly. The pin extends through the roller assembly and is axially and radially constrained relative to the tappet body. The axial constraint of the pin is realised by means of at least two circlips, arranged at respective ends of the roller assembly.

The use of circlips to axially restrain the pin relative to the tappet body significantly simplifies the manufacturing process by obviating the need for a controlled deformation of the pin after inserting it into the centre of the roller assembly. Because the pin is not deformed, it can be removed later, for example to replace the roller assembly or a part thereof. An additional important advantage of the use of circlips is that the axially and radially constrained pin may retain some ability to rotate around its longitudinal axis. While such rotation of the pin is not needed for, and will not play a role in, the operation of the roller tappet assembly, it does vary the rotational position of the pin during operation of the fuel pump. As a consequence, all wear and bending of the lifetime of the fuel pump will be spread over the whole pin.

The use of two circlips, and not just one, results in a better axial constraint on the pin and ensures a symmetric design that avoids an uneven distribution of loads on the roller assembly or a part thereof. This, in turn, leads to reduced wear and a longer-lasting, more reliable roller tappet assembly. The two circlips may be arranged inside the tappet body cavity but are preferably arranged outside the tappet body cavity. If inside the cavity body, the circlips need to be installed before the pin is inserted, which complicates the assembly process and makes it more difficult to disassemble the roller tappet assembly afterwards. Also, contact between the roller assembly and the circlip inside the tappet body cavity may be an additional cause of wear which reduces the lifetime of the fuel pump. When provided outside the tappet body cavity, the circlips can easily be applied or removed and do not interfere with the roller assembly or any parts thereof.

The roller assembly may comprise an outer roller and an inner roller or bearing. The outer roller is arranged for rolling contact with the cam surface of the camshaft ofthe fuel pump assembly. The inner roller or bearing is arranged for rolling contact with an inner surface of the outer roller and an outer surface of the pin. In other words, the inner roller or bearing is positioned between the pin and the outer roller. With an inner roller or bearing in between the roller that contacts the cam and the pin, the roller assembly will run more smoothly and with less frictions than when the inner surface of that roller is in direct contact with the outer surface of the pin. Optionally, the pin is coated with a low friction coating, such as a DLC (Diamond Like Carbon) coating which allows an inner roller to roll smoothly over the outer surface of the pin. Lubricant is usually provided for further reducing friction in and with the roller assembly.

The pin may comprise two grooves for receiving the respective two circlips. In an embodiment, both ends of the pin protrude from the tappet body. When the circlips are arranged outside the tappet body cavity, the protruding portions of the pin can be used for engaging with the circlips.

According to a further aspect of the invention, a fuel pump assembly is provided, comprising at least one roller tappet assembly as described above.

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 side view of a fuel pump assembly which is known in the art;

Figure 2 is an enlarged section view of a part of the fuel pump assembly in Figure 1 ;

Figure 3 is an enlarged section view of the roller tappet assembly of the fuel pump assembly of Figures 1 and 2;

Figure 4 is an enlarged section view of a roller tappet assembly according to an embodiment of the invention; and

Figure 5 is an enlarged section view of a roller tappet assembly according to a further embodiment of the invention.

DETAILED DESCRIPTION

It should be understood that throughout this description, references to upper and lower ends of components, and other such directional or relative references are made in relation to the orientations of the components shown in the Figures but are not intended to be limiting. Although the invention will be described with reference to a specific type of high-pressure diesel pump, the roller tappet assemblies according to the invention are equally compatible with and useful in many other types of fuel pumps.

Figures 1 to 3 show a known common rail fuel pump assembly 10 (“the pump” hereinafter) for use in a compression-ignition internal combustion engine. The pump 10 is an in-line pump arrangement comprising a main pump housing 12 and first and second pump elements, which are driven by means of a common, engine- driven drive shaft 18 which extends through the main pump housing 12 and rotates at a speed proportionate to the speed of the engine. A low-pressure suction pump 13 is mounted to the side of the main pump housing 12 to deliver relatively low- pressure fuel to the pump 10. The drive shaft 18 carries first and second cam drive arrangements, 20, 22 respectively, which are either mounted on, or form an integral part of, the drive shaft 18. The drive shaft 18 is reciprocally connected to each of the pump elements via a respective intermediate drive assembly in the form of a first or second roller tappet assembly, referred to generally as 19, 21. Each roller tappet assembly 19, 21 includes a respective tappet body, 23, 25. As can be seen most clearly in Figures 2 and 3, and describing only the first tappet assembly 19, the tappet body 23 comprises a tappet body cavity 60 wherein an annular roller assembly is provided. The roller assembly comprises a pair of annular rollers in the form of an outer roller 27 and an inner roller 29. An outer surface of the outer roller 27 is arranged to roll over the surface of the associated cam arrangement 20. A pin 24 secures the tappet body 23 to the associated roller assembly 27, 29 and the inner roller 29 rolls on the pin 24 within the outer roller 27.

It will be appreciated that this arrangement of the roller tappet assembly 19, 21 and the roller assembly 27, 29 is just one example of how the drive assembly for a plunger is driven through rotation of the drive shaft 18. For example, in an alternative embodiment, the inner roller 29 may be replaced by a roller bearing or ball bearing that has its inner race fixedly connected to the pin 24 and its outer race to the outer roller 27 of the roller assembly.

It is helpful to consider the operation of the pump assembly in Figures 1 to 3 to understand the technical problem which the invention sets out to address. Two separate pump elements in the form of the first and second pumping plungers, 14 and 16, are shown in Figures 1 and 2, but for the purpose of the following description only one of the plungers 14 will be described in detail. The first pumping plunger 14 extends through a substantially tubular turret 28 which forms a part of a pump head housing 30 mounted to the main pump housing 12. The turret 28 downwardly extends from the pump head housing 30 and defines a substantially cylindrical plunger bore 32, the turret 28 projecting into the body of the main pump housing 12 and terminating in a lower turret surface 34. The plunger bore 32 is configured to receive the plunger 14, the lower end of which extends from the turret 28.

At the uppermost end of the plunger 14 (in the illustration shown), the plunger 14 defines, together with the bore 32 in the pump head 30, a pump chamber 36 (as shown in Figure 1) for receiving fuel to be pressurised by the plunger 14 when the pump assembly is in use. Likewise, the second plunger 16 has an associated pump chamber 38.

The pump chamber 36 is fitted with an inlet valve 40 and an outlet valve (not shown) to control, respectively, fuel flow into and out of the pump chamber 36 through the pump cycle. 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 a pump inlet 42 through to the pump chamber 36 and from the pump chamber 36 through to a pump outlet 44 to the common rail (not shown). Each valve includes a spring (not identified), which acts to close the valve to prevent the passage of fuel therethrough.

The plunger 14 is moveable between a bottom-dead-centre position (hereinafter, “BDC position”) and a top-dead-centre position (hereinafter, “TDC position”), defining a pumping stroke, and between the TDC position and the BDC position, defining a return stroke. A pumping stroke followed by a return stroke defines a pumping cycle for the plunger 14 and pump assembly 10. Figure 2 shows the plunger 14 on the right-hand side of the assembly with the plunger at the BDC position, while the plunger 16 on the left-hand side of the assembly is moving towards TDC position.

A spring abutment member in the form of an annular spring plate 50 forms a collar around the plunger 14 in a lower region of the plunger and is attached thereto such that their respective motions are coupled together. The spring plate 50 defines an abutment surface 52 for one end of a plunger return spring (“return spring” hereinafter) 54 in the form of a helical coil spring. Accordingly, the spring plate 50 acts as a seat member for the return spring 54. The other end of the return spring 54 engages a fixed abutment surface defined by the underside of the pump head housing 30. The return spring 54 is thus permanently engaged with both the spring plate 50 and the pump head housing 30.

When the plunger is in the TDC position (as for the left-hand plunger 16 in Figure 2), both the inlet valve 40 and the outlet valve to the respective pump chamber 36 are closed, thereby preventing fuel from flowing into or out of the pump chamber 36. As the drive shaft 18 rotates and the tappet assembly 19 rides over the cam 20, the return spring 54 acts on the plunger 14 to urge the plunger 14 away from the TDC position, through the return stroke to the BDC position. This causes an increase in the volume of the pump chamber 36, decreasing the pressure within it and establishing a pressure drop across the inlet valve 40. This pressure drop allows the inlet valve 40 to open against the force of the inlet valve spring and fuel enters the pump chamber 36 until the pressure across the inlet valve 40 equalises, causing it to close. This typically occurs just after the plunger 14 reaches the BDC position. During the return stroke the fuel is supplied to the pump chamber 36 at a pressure of around 3 bar (300 kPa). Throughout the return stroke the return spring 54 serves to ensure that contact is maintained between the various drivetrain components, including maintaining contact between the plunger 14 and the tappet assembly 19 and between the tappet assembly 19 and the cam 20.

Once the plunger 14 reaches the BDC position, it begins the pumping stroke as the drive shaft 18 continues to rotate. During the pumping stroke fuel in the pump chamber 36 is pressurised as the volume of the pump chamber 36 is reduced with the advancing plunger 14. During this phase of operation, the inlet valve 40 of the pump chamber 36 is caused to close due to the pressure drop across it and the pressure in the pump chamber 36 is increased, typically to at least 200 bar (20 MPa) and sometimes as high as 2500 bar (250 MPa). A pressure drop is created across the outlet valve (not shown), allowing it to open against the force of the outlet valve spring and fuel exits the pump chamber 36 and flows into the common rail fuel volume. As the plunger 14 reaches the TDC position, the pressure across the outlet valve (not shown) equalises, causing it to close.

Throughout the pumping stroke the force from the return spring 54 continues to act through the drivetrain components to ensure contact is maintained between the roller tappet assembly 19 and the cam 20, whilst importantly minimising slippage between the outer roller 27 and the cam 20. For optimal functioning of the tappet assembly 19 and to minimise wear, it is important that the pin 24 is axially and radially constrained relative to the tappet body 23. In known roller tappet assemblies 19 such as the one shown in Figure 3, this is achieved by deforming the pin 24 at one or both ends after installing the roller assembly in the tappet body cavity 60 and inserting the pin 24 into the centre of the roller assembly. Deforming the pin 24 after its installation is a complex manufacturing process that may, for example, involve case hardening of only the central portion of the pin 24 or hardening the full pin 24, followed by tempering the ends to provide deformable end portions. Once the pin 24 has been deformed, it is impossible to remove it and disassemble the roller assembly 19.

The present invention significantly simplifies the manufacturing process using two circlips 61 as shown in Figures 4 and 5. In both embodiments, the two circlips 61 are provided at either end of the pin 24. The circlips 61 clamp the pin 24 and thereby provide two ridges that protrude from the cylindrical surface of the pin 24 and constrain axial movement of the clamped pin 24 relative to the tappet body 23. In Figure 4, both circlips 61 are held between an end surface of the inner roller 29 and the tappet body cavity 60. In the Figure 5 embodiment, one circlip 61 prevents axial movement of the pin 24 in one direction and the other circlip 61 prevents axial movement of the pin 24 in the opposite direction. As shown in Figures 4 and 5, the pin 24 may comprise grooves for receiving the circlips 61. The grooves improve the clamping of the pin 24 by the circlips 61 , thereby further ensuring that the pin 24 won’t move axially, relative to the circlips 61.

The use of circlips 61 to axially restrain the pin 24 relative to the tappet body 23 obviates the need for a controlled deformation of the pin 24 after inserting it into the centre of the roller assembly. Because the pin 24 is not deformed, it can be removed later, for example to replace the roller assembly or a part thereof. An additional important advantage of the use of circlips 61 is that the axially and radially constrained pin 24 may retain some ability to rotate around its longitudinal axis. While such rotation of the pin 24 is not needed for, and will not play a role in, the operation of the roller tappet assembly 19, it does vary the rotational position of the pin 24 during operation of the fuel pump 10. As a consequence, all wear and bending of the lifetime of the fuel pump 10 will be spread over the whole pin 24.

The use of two circlips 61 , and not just one, results in a better axial constraint on the pin 24 and ensures a symmetric design that avoids an uneven distribution of loads on the roller assembly or a part thereof. This, in turn, leads to reduced wear and a longer-lasting, more reliable roller tappet assembly 19.

The embodiment of Figure 4, wherein the circlips 61 are arranged inside the tappet body cavity 60, has the advantages that both circlips 61 are constrained in both axial directions and are shielded against displacement by unintentional contact. Downsides of this embodiment may be that it will be difficult to disassemble, and that the circlips may interfere with the lubricant of the roller assembly.

In the embodiment of Figure 5, the circlips 61 are arranged outside the tappet body cavity 60. This may, e.g., be made possible by providing a pin 24 that is long enough to protrude from the tappet body 23 at both ends. It is noted that, as shown in Figure 5, the tappet body 23 is preferably shaped such that the ends of the pin

24 can protrude from the tappet body 23, without extending beyond the total width of the tappet assembly 19. Alternatively, the tappet body 23 comprises a groove that is arranged inside the tappet body 23 for receiving the circlip 61. Such a groove (not shown) will allow the circlip 61 to be retained in both axial directions, without interfering with the lubricant of the roller assembly.

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.

References used:

10 - fuel pump assembly 12 - main pump housing 14 - plunger 16 - plunger

18 - drive shaft

20 - first cam

22 - second cam

19 - first tappet assembly

21 - second tappet assembly

23 - first tappet

25 - second tappet

24 - pin

27 - outer roller

29 - inner roller

28 - turret

30 - pump head housing 32 - plunger bore 34 - lower surface of turret 36 - pump chamber 40 - inlet valve 42 - pump inlet 44 - pump outlet

50 - spring abutment plate 52 - abutment surface of spring abutment plate 54 - return spring 60 - tappet body cavity 61 - circlip