EP2639444A1

EP2639444A1 - Fuel pump assembly

Info

Publication number: EP2639444A1
Application number: EP12158980.8A
Authority: EP
Inventors: Borja Navas Sanchez; Peter Collingborn; Kevin Laity
Original assignee: Delphi Technologies Holding SARL
Current assignee: Delphi International Operations Luxembourg SARL
Priority date: 2012-03-12
Filing date: 2012-03-12
Publication date: 2013-09-18
Anticipated expiration: 2032-03-12
Also published as: ES2526599T3; EP2639444B1

Abstract

A fuel pump assembly for use in an internal combustion engine comprises a pump housing (10), at least first and second pumping plungers (16a, 16b) for pressurising fuel within a respective one of first and second pumping chambers (20a, 20b); and first and second tappets (22a, 22b), each being associated with a respective one of the pumping plungers and being driven, in use, by an engine-driven cam, so as to drive a pumping stroke of the tappet and the associated plunger during which fuel within the associated pumping chamber is pressurised. Each tappet (22a, 22b) defines an internal volume (24a, 24b) for housing, at least in part, an associated return spring (34a) which drives a return stroke of the tappet and the associated plunger during which the associated pumping chamber is filled with fuel. A return circuit (50a, 50b, 52a, 52b, 54) is provided by which the internal volume (24a) of the first tappet (22a) is in constant communication with the internal volume (24b) of the second tappet (22b) so that fuel displaced from the internal volume during the pumping stroke of the first tappet fills the internal volume of the second tappet so as to aid the return stroke of the second tappet.

Description

Technical field

The invention relates to a fuel pump assembly of the type suitable for use in common rail fuel injection systems of internal combustion engines and, in particular for use in compression ignition internal combustion engines.

Background to the invention

In a known fuel pump assembly a pumping plunger is driven for reciprocal movement within a bore provided in a pump housing by means of a cam drive arrangement including an engine-driven cam carrying a cam rider. A cam follower in the form of a tappet cooperates with the cam rider, and in turn a foot of the plunger cooperates with the tappet. The tappet is driven to perform a forward stroke, during which the plunger is driven inwardly within the bore to perform a pumping stroke and pressurise fuel within a pumping chamber by reducing the volume of the pumping chamber. The tappet performs a return stroke in which the plunger is withdrawn from the bore to expand the volume of the pumping chamber and fuel is delivered to the pumping chamber. A tappet return spring effects the return stroke of the tappet. Fuel from a low pressure fuel source is delivered to the pumping chamber via an inlet metering valve which controls the rate of flow of fuel into the pumping chamber. As the plunger reciprocates within the bore fuel within the pumping chamber is pressurised. An outlet valve controls the delivery of pressurised fuel to the downstream common rail.
As diesel fuel injection technology develops, strategies are emerging for reducing emissions which require the fuel pump of the engine to be able to run at increasingly higher speeds. However, a problem with known pump assemblies is that the force requirements for the tappet return spring make known designs unsuitable for such high speed applications. In existing designs, where the tappet takes the form of a bucket tappet having a hollow interior, the return spring is located within the internal volume defined within the tappet. The internal volume of the tappet is connected to the cam box interior in order to minimise the force required to displace fuel from the tappet interior and to cool the contacting regions of the heavily-loaded drive-train components by removing high temperature, high pressure leakage fuel. The packaging restraints limit the size of the return spring that can be accommodated within the tappet, and hence limit the force that can be achieved to return the tappet during the return stroke. Particularly at high speeds, this can result in an insufficient force being applied to hold the plunger-tappet-cam rider components together, so that the plunger and the tappet can separate from the cam rider during the return stroke. This can cause malfunction of the pump assembly. Increasing the load on the return spring provides a partial solution but, together with the high frequency of operation, causes an increase in the spring stress which in turn can lead to spring failure.
It is with a view to addressing or mitigating the aforementioned problem that the present invention provides an improved fuel pump assembly.

Statements of invention

According to a first aspect of the present invention, there is provided a fuel pump assembly for use in an internal combustion engine, the pump assembly comprising a pump housing; at least first and second pumping plungers for pressurising fuel within a respective one of first and second pumping chambers; at least first and second tappets, each being associated with a respective one of the pumping plungers and being driven, in use, by an engine-driven cam, so as to drive a pumping stroke of the tappet and the associated plunger during which fuel within the associated pumping chamber is pressurised. Each tappet defines, at least in part, an internal volume for housing, at least in part, an associated return spring which drives a return stroke of the tappet and the associated plunger during which the associated pumping chamber is filled with fuel. A return circuit is provided by which the internal volume of the first tappet is in constant communication with the internal volume of the second tappet so that fuel displaced from the internal volume during the pumping stroke of the first tappet fills the internal volume of the second tappet so as to aid the return stroke of the second tappet.
The internal volume of each tappet is preferably defined by a tappet sidewall and a tappet base plate, with the volume being open at its upper end remote from the base plate.
As the internal volumes of the tappets are in constant communication with one another, there are no hydraulic devices in the flow path between them to control or restrict the flow of fuel therethrough. Communication between the internal volumes is constant and by-passes the cam box. Moreover, fuel within the internal volume of the first tappet during its pumping stroke is supplied to the internal volume of the second tappet during its return stroke, and likewise the fuel within the internal volume of the second tappet during its pumping stroke is supplied to the internal volume of the first tappet during its return stroke, so as to apply a hydraulic force to the tappets which supplements the force of the return spring in driving the tappets on their return strokes.
The return circuit preferably includes an expansion chamber through which fuel is displaced between the internal volume of the first tappet and the internal volume of the second tappet. It is convenient for the expansion chamber to be defined within the pump housing.
The expansion chamber is of relatively large volume. By including a expansion chamber of relatively large volume within the return circuit, pressure waves that arise within the return circuit as fuel is displaced between the internal tappet volumes are damped.
The return circuit may include at least one passage defined within the pump housing. The at least one passage is defined within the pump housing to provide an unrestricted flow passage for fuel flowing between the internal volumes of the first and second tappets.
Each of the first and second tappets is movable along its axis within an associated tappet guide which may be defined by a respective bore provided within the pump housing or by a respective sleeve located within a respective bore provided within the pump housing.
Each of the first and second tappets is provided with a top-up port in a sidewall thereof which is cooperable with a top-up port provided in the associated tappet guide so as to allow fuel flow into the internal volume of the tappet when the top-up ports overlap. For this purpose the top-up port provided in the tappet guide communicates with a flow path in communication with a low pressure fuel reservoir. The benefit of this arrangement is that fuel that is lost down the tappet guide due to leakage is replenished during the period of overlap between the ports as fuel from the low pressure fuel reservoir is supplied through the flow path and the overlapping ports to the internal tappet volumes. The top-up ports overlap for a period of the pumping cycle referred to as the "replenishing period", during which the internal volumes of the tappets are replenished with fuel.
Preferably, the top-up ports associated with the first tappet are sized and positioned so as to overlap with one another on the pumping stroke of the first tappet/plunger simultaneously with the top-up ports associated with the second tappet overlapping with one another on the return stroke of the second tappet/plunger. Therefore each internal tappet volume is replenished at substantially the same time as the other.
It may be preferable to provide each of the first and second tappets with one or more additional top-up ports provided in its sidewall in the same plane as each of the other top-up ports, together with correspondingly positioned additional top-up ports in the associated tappet guides which communicate with the low pressure supply. In this way an enhanced flow of fuel is provided into the internal tappet volumes during the replenishing period.
Each tappet may have an associated further port provided in its guide for collecting leakage fuel that flows between the tappet and said guide. Each port preferably communicates, via a restriction, with the engine cam box and so allows fuel within the internal volume to "leak" to the cam box. The benefit of this is that fuel within the internal volume is cooled, whilst the leakage fuel is able to be replenished via the top-up ports during the replenishing period.
Each of the first and second tappets may include means for preventing angular movement of the tappet about its axis. This ensures overlap is maintained between the ports, if provided, during the replenishing period as any angular movement of the tappet within its guide is substantially prevented.
Preferably, the first and second plungers are opposed to one another. The fuel pump assembly may preferably comprise an even number of multiple plungers arranged, for example, in opposed plunger pairs.

Brief description of the drawings

The invention will now be described, by way of example only, with reference to the accompanying figures in which:

Figure 1 is a sectional view of a pumping unit of a pump assembly of the present invention with a plunger of the pumping unit in a first position;
Figure 2 is a sectional view of the pumping unit in Figure 1 with the plunger of the pumping unit in a second position;
Figure 3 is a sectional view of a pump assembly having two pumping units as in Figures 1 and 2 and illustrating the hydraulic flow paths between the pumping units; and
Figure 4 is a perspective external view of the pump assembly in Figure 3.

Detailed description of preferred embodiments

Referring to Figures 1 to 4, a pump assembly of the present invention includes a main pump housing 10 and first and second pumping units 12a, 12b. Only one of the pumping units 12a is illustrated in Figures 1 and 2, whereas in Figure 3 both of the pumping units 12a, 12b can be seen. For the purpose of the following description only one of the pumping units will be described in detail, although it will be appreciated that the first and second pumping units are substantially identical to one another.
The first pumping unit 12a includes a pump housing in the form of a pump head 14a which is mounted on the main pump housing 10 in a conventional manner by means of bolts (not shown). The first pumping unit 12a further includes a pumping plunger 16a which is reciprocal along its axis within a blind bore 17a provided in an elongate neck 18a of the pump head 14a. A head of the pumping plunger 16a defines, together with the blind end of the bore 17a, a pumping chamber 20a for receiving fuel at transfer pressure, provided by a transfer pump (not shown). Within the pumping chamber 20a fuel is pressurised to a high level suitable for delivery to a downstream common rail (not shown). The foot of the plunger 16a engages with an internal surface of a cam follower in the form of a bucket tappet 22a. The tappet 22a is of generally U-shaped cross section with a vertically extending sidewall 122a and a base plate 222a. The foot of the plunger 16a engages with the internal surface of the base plate 222a. The sidewall 122a and the base plate 222a of the tappet 22a together define an internal volume or chamber 24a of the tappet which is enclosed apart from its open upper end.
The lower external surface of the tappet base plate 222a engages with a cam rider 26a which is carried by a cam (not shown) mounted on an engine driven shaft (also not shown). The cam shaft resides within a cam box 29 of the engine (the cam box 29 is shown in Figure 3 only). The cam box 29 is filled with fuel at low pressure so as to provide lubrication for the cam drive components. The tappet 22a is slidable along its axis within a tappet guide defined by a main bore 32a provided in the main pump housing 10. The axis of the tappet 22a is coaxial with that of the plunger 16a. In an alternative embodiment (not shown), the tappet may be located within a sleeve that is mounted within the main bore 32a and which serves to guide movement of the tappet.
The elongate neck 18a of the pump head 14a extends into the internal volume 24a of the tappet 22a. A return spring 34a for the tappet is also located within the internal volume 24a of the tappet 22a. The lower end of the return spring 34a abuts an annular spring plate 36a carried by the foot of the plunger 16a. In use, as the cam shaft rotates, the cam rider 26a and the lower surface of the tappet 22a move relative to one another, in back and forth sliding motion, while the tappet 22a is urged inwards within the tappet guide 32a as the rider rides up the rising flank of the cam. The plunger 16a is therefore urged inwardly within the plunger bore 17a. This is referred to as the forward or pumping stroke of the plunger/tappet 16a/22a.
At the end of the pumping stroke, the cam rider 26a rides over the lobe of the cam and the plunger 16a and tappet 22a start to move outwardly from the plunger bore 17a and the tappet guide 32a, respectively, as the cam rider 26a falls down the trailing flank of the cam. The return stroke is effected by means of the return spring 34a acting in combination with a supplementary hydraulic force which will be described in further detail later. This is referred to as the return or downward stroke of the plunger/tappet 16a/22a. Figure 1 shows the plunger at the top of its pumping stroke (start of the return stroke) with the volume of the pumping chamber 20a at a minimum. Figure 2 shows the plunger part-way through its pumping/return stroke at a mid-point within the plunger bore 17a.
The pumping unit 12a is also provided with an inlet valve 38a and an outlet valve 40a. Fuel is supplied to the pumping chamber 20a from the transfer pump via the inlet valve 38a and, once pressurised, is delivered to the common rail via the outlet valve 40a. The inlet valve 38a includes a valve head 44a which extends into the pumping chamber 20a at a position directly above the head of the plunger 16a in the orientation shown in Figures 1 and 2. An inlet valve spring (not shown) serves to bias the inlet valve 38a towards a closed position in which fuel cannot flow into the pumping chamber 20a from the transfer pump. The outlet valve 40a extends laterally from the pumping chamber 20a and takes the form of a spring-biased non-return valve. The ball 46a of the valve is biased by means of an outlet valve spring 47a into a closed position in which high pressure fuel within the downstream common rail is unable to flow to the pumping chamber 20a.
At the upper end of the tappet 22a the internal volume 24a of the tappet opens into a first chamber 50a which communicates with an elongate gallery 52a defined within the pump housing 10. The gallery 52a further communicates with an expansion chamber which is also defined within the pump housing 10. The expansion chamber is not visible in Figures 1 and 2, but is identified by reference numeral 54 in Figures 3 and 4.
The main pump housing 10 is further provided with a drilling 56a, one end of which communicates with a low pressure fuel reservoir (identified as 57 in Figure 3). The other end of the drilling 56a communicates with a port 58a in the wall of the tappet guide 32a. For certain positions of the tappet 22a within the tappet guide 32a, the ports 58a, 60a overlap so that, as shown in Figure 2, when the port 60a in the tappet 22a is aligned with the port 58a in the tappet guide 32a, a communication path exists between the low pressure fuel reservoir 57, which contains fuel at higher pressure than the cam box 29, and the internal volume 24a of the tappet 22a. The communication path exists through the drilling 56a and the overlapping ports 58a, 60a. In such periods of alignment, fuel is drawn from the low pressure fuel reservoir 57 through the passage 56a and fills the internal volume 24a of the tappet 22a, replenishing any fuel that is lost to leakage through the tappet guide 32a, as will be described in further detail below.
As can be seen in Figure 3, the second pumping unit 12b is substantially identical to the first, including, for example, a second plunger 16b, a second tappet 22b, a second tappet guide 32b, a second tappet internal volume 24b, and second top-up ports 58b, 60b.
In the same manner as for the first pumping unit 12a, the internal volume of the second pumping unit 12b also includes an upper chamber 50b which communicates with an elongate gallery 52b. The gallery 52b of the second pumping unit 12b also communicates with the expansion chamber 54. The expansion chamber 54a is therefore common to both the first and second pumping units 12a, 12b and defines a part of a return circuit which also includes the upper chamber 50a, 50b and the gallery 52a, 52b associated with each of the first and second pumping units, and the internal volume 24a, 24b of each of the tappets 22a, 22b. The return circuit defines a voluminous circuit for fuel within which there are no additional valve or other flow-controlling or restricting elements. The substantial volume of the return circuit ensures that any pressure spikes which occur as fuel is displaced around the return circuit are dampened.
The external profile of the pump assembly is illustrated in Figure 4, where the position of the expansion chamber 54 between the two pumping units 12a, 12b can be seen, together with the outlet valve 40a, 40b from each pumping unit 12a, 12b.
Each tappet 22a, 22b also has an additional port 62a, 62b into the tappet guide 32a, 32b, towards the lower end of the tappet 22a, 22b, which serves to collect fuel that leaks between the sliding surfaces of the tappet sidewall and the tappet guide. Each additional port 62a, 62b communicates, via an associated restriction 64a, 64b, with the cam box 29 so that leakage fuel through the guide 32a, 32b is returned to the cam box 29 to provide a cooling effect.
The pumping cycle of the first pumping unit 12a will now be described in further detail.
On the return stroke of the pumping plunger 16a, fuel at low pressure is drawn in through the inlet valve 38a and fills the pumping chamber 20a as the pumping plunger 16a is withdrawn from the plunger bore 17a . As the cam 29 rotates further and the cam rider 26a moves upwards with the rising flank of the cam, the rider moves laterally relative to, and in sliding contact with, the base plate 222a of the tappet 22a. The tappet 22a is urged upwardly within the tappet guide 32a (in the orientation shown in Figures 1 and 2), and hence the plunger 16a moves upwards within the plunger bore 17a to reduce the volume of the pumping chamber 20a. Fuel within the pumping chamber 20a is pressurised and the inlet valve 38a is caused to close due to the force differential across the valve. A point will be reached when the pressure in the pumping chamber 20a is sufficient to overcome the force of the outlet valve spring 47a so that pressurised fuel is delivered to the common rail. In addition, during the pumping stroke of the plunger 16a, the tappet 22a displaces fuel within its internal volume 24a, through the upper chamber 50a, through the gallery 52a, through the expansion chamber 54 and into the internal volume 24b of the second pumping unit 12b via the other elongate gallery 52b and the other upper chamber 50b.
Once the plunger 16a has reached the top of the pumping stroke, and with the plunger at its innermost position within the tappet guide 32a and the pumping chamber volume at a minimum, the force due to the return spring 34a serves to urge the tappet downwards to perform the return stroke of the plunger 16a. The pumping strokes of the two plungers 16a, 16b are 180 degrees out of phase with one another so that the fuel that is displaced from the internal volume 24a of the first tappet 22a on its forward stroke is supplied, through the various connecting passages and chambers 50a, 50b, 52a, 52b, 54, to the internal volume 24b of the second tappet 22b during its return stroke. The supply of fuel from the internal volume 24a of the first tappet 22a to the internal volume 24b of the second tappet 22b serves to supplement the return force of the return spring 34b of the second pumping unit which acts on the second tappet 22b. In a similar manner, during the subsequent forward stroke of the second tappet 22b, fuel that is displaced from the internal volume 24b of the second tappet 22b is supplied through the upper chamber 50b, through the gallery 52b and through the expansion chamber 54 to the internal volume 24a of the first tappet 22a via the other gallery 52a and the other upper chamber 50a, thereby supplementing the force due to the return spring 34a of the first pumping unit 12a to drive the return stroke. The effect of fuel being delivered to the internal volume of one tappet from the internal volume of the other tappet is therefore to positively drive the return stroke of the plunger/tappet. This has the benefit that any separation of parts which may otherwise occur, for example separation of the tappet and the plunger during the return stroke, is substantially avoided due to the supplementary hydraulic force that acts on the tappet/plunger during the return stroke in the aforementioned manner. In this way, wear of the parts is minimised as the parts are prevented from separating during the return stroke.
As the upper chambers 50a, 50b, the galleries 52a, 52b and the expansion chamber 54 are of relatively large diameter, there is no restriction to flow between the internal volumes 24a, 24b of the tappets 22a, 22b throughout the pumping cycles. It is a particular benefit that pressure waves within the return circuit containing the internal tappet volumes, the upper chambers, the galleries and the expansion volume are absorbed within this relatively voluminous space. It is important that the galleries 52a, 52b and the expansion chamber 54 are of the largest cross-sectional area that can practically be achieved within the packaging and manufacturing constraints, so that the parasitic pumping losses in the return circuit are minimised. The galleries 52a, 52b and the expansion chamber 54 may be implemented as cast-in features of the pump housing, which incur no additional machining costs, as illustrated most clearly in Figure 3.
As the fuel in the gallery 52a and the expansion chamber 54 must be accelerated and decelerated along with the tappets 22a, 22b, the inertia of the fuel causes a pressure difference between the driving and driven ends of the column of fuel within the return circuit. Since acceleration of the tappet 22a, 22b is greatest at the start of the forward stroke, this pressure difference is at a maximum at this point. The pressure within the internal column of the driven tappet (on its forward stroke) must be sufficient to deliver the desired acceleration by overcoming the inertia of the combination of the tappet 22a, the plunger 16a and the spring seat 36a (referred to as "the driving pressure"), as well as cancelling out the pressure of the cam box. The driving pressure is therefore that pressure that is needed to make up any deficiency in spring force at the current speed and elapsed stroke of the tappet.
It is inevitable that some fuel leakage will occur down the tappet clearances within the guides 32a, 32b due to the pressure in the internal volumes 24a, 24b being above cam box pressure. In order to counter this fuel leakage, each pumping unit 12a, 12b is provided with a means for replenishing fuel to the internal volumes of the tappets during a portion of the pumping cycle. For this purpose the tappet of each pumping unit is provided with the top-up port 60a, 60b in its sidewall which is cooperable with the corresponding top-up port 58a, 58b provided in the tappet guide 32a.
Referring to the first pumping unit 12a shown in Figure 2, overlap between the top-up port 60a in the tappet 22a and the port 58a in the tappet guide 32a defines a flow path for low pressure fuel into the internal volume 24a through the drilling 56a from the low pressure fuel reservoir 57. The low pressure fuel reservoir is filled with fuel that is pressurised higher than the pressure of fuel within the cam box 29, so that overlap between the ports 58a, 60a defines a flow path for fuel between the low pressure fuel reservoir 57 and the internal volume 24a. This is the position of the tappet 22a shown in Figure 2. As acceleration of the tappet 22a is at or close to a minimum during the middle part of both the forward stroke and the return stroke, the tappet is approaching its highest linear speed at this point and so the inertia pressure of the fuel within the return circuit is minimal. At this point pressure within the return circuit falls to less than the supply pressure provided by the transfer pump and fuel from the low pressure reservoir 57 is drawn in through the overlapping ports 58a, 60a to replenish the internal volumes 24a, 24b of the tappets 22a, 22b.
During the period for which the top-up ports 58a, 60a are overlapping and fuel replenishes the internal volumes of the tappets (referred to as the "replenishing period"), the force that is exerted on the tappet by the return spring is more than sufficient to provide any remaining acceleration load.
The tappet motion will previously have lagged that of the cam rider 26a by a small amount due to the leakage along the tappet clearances, but the parts come firmly but gently back into full contact, before the end of the replenishing period. The sizing and positioning of the top-up ports 58a, 60a is chosen to ensure full replenishment of the return circuit during the replenishing period. It will be appreciated that the top-up ports on both pumping units (i.e. on both the first and second tappets and in their respective guides) contribute simultaneously to the replenishing process. For the first tappet the replenishment occurs through the ports of the first tappet/guide part way through its forward stroke simultaneously with replenishment via the top-up ports of the second tappet/guide part way through its return stroke, and vice versa.
Additional ports may be provided in each tappet guide, each being arranged to overlap with an additional top-up port (not shown) provided in the tappet sidewall 122a. The additional ports are arranged in the same plane as the top-up ports 60a shown in Figures 1 and 2 so that when the top-up ports overlap at the mid-point of the pumping and return strokes, the additional ports also overlap and so provide an increased flow volume for fuel from the low pressure reservoir and the return circuit. The additional top-up port in the tappet guide 32a may take the form of an annular groove with which the drilling 56a communicates.
Fuel leakage between the tappets 22a, 22b and their guides is not entirely without benefit as this provides a cooling effect for fuel within the internal tappet volumes 24a, 24b: hot fuel leaks from the internal tappet volumes 24a, 24b to the cam box 29 to be replaced by cooler fuel from the low pressure reservoir that is supplied through the top-up ports 58a, 58b, 60a, 60b. As shown in Figure 3, additional ports 62a, 62b are provided in the tappet guide to collect the fuel that leaks down the tappet guide. The leakage fuel then flows via a restriction 64a, 64b to the cam box 29.
The rate of flow of fuel from the return circuit to the cam box 29 can be further increased to provide an enhanced cooling effect by increasing the clearance between the tappet 22a and the tappet guide 32a, therefore increasing the leakage through the clearance.
In other embodiments of the invention, the drilling 56a may be arranged to communicate, at its end remote from the tappet guide 22a, with the cam box 29, instead of a separate low pressure fuel reservoir 57. Still further embodiments of the invention, a sleeve is mounted within the main bore 32a provided in the main pump housing 10 so that the sleeve defines the tappet guide within which the tappet moves.
During the second half of the return stroke of each tappet 22a, 22b, the tappet 22a, 22b is decelerated by the cam rider and the direction of the inertia-dependent pressure difference of the return circuit is reversed. This ensures corresponding deceleration of the other tappet of the pair (which is performing its forward stroke) if only a small volume of fuel is required to be pumped into the common rail during that particular stroke.
In order to ensure angular alignment of the various ports, the tappet is provided with an anti-rotation feature (not shown) which typically takes the form of a ball and recess arrangement (not shown). A ball is located within a recess provided in the outer surface of the tappet sidewall and a groove located on an inner surface of the tappet guide. The ball is trapped within the recess and the groove, the groove permitting the ball to move axially as the tappet reciprocates within the guide whilst the trapped ball prevents angular movement of the tappet within the guide. Other mechanisms are known in the art for preventing angular movement of the tappet and may also be used in this invention, in place of the ball and recess arrangement.
It will be appreciated that various modifications to the aforementioned pump assembly may be made without departing from the scope of the invention as set out in the accompanying claims. For example, the invention is also applicable to a fuel pump assembly having a multiple number of opposed plunger pairs (e.g. two, four or six pumping units).

Claims

A fuel pump assembly for use in an internal combustion engine, the fuel pump assembly comprising:
a pump housing (10);

at least first and second pumping plungers (16a, 16b) for pressurising fuel within a respective one of first and second pumping chambers (20a, 20b);

at least first and second tappets (22as, 22b), each being associated with a respective one of the pumping plungers and being driven, in use, by an engine-driven cam, so as to drive a pumping stroke of the tappet and the associated plunger during which fuel within the associated pumping chamber is pressurised,

each tappet (22a, 22b) defining, at least in part, an internal volume (24a, 24b) for housing, at least in part, an associated return spring (34a) which drives a return stroke of the tappet and the associated plunger during which the associated pumping chamber is filled with fuel, and

a return circuit (50a, 50b, 52a, 52b, 54) by which the internal volume (24a) of the first tappet (22a) is in constant communication with the internal volume (24b) of the second tappet (22b) so that fuel displaced from the internal volume during the pumping stroke of the first tappet fills the internal volume of the second tappet so as to aid the return stroke of the second tappet.
The fuel pump assembly as claimed in claim 1, wherein the internal volume (24a, 24b) of each tappet is defined by a tappet sidewall (122a) and a tappet base plate (222a).
The fuel pump assembly as claimed in claim 1 or claim 2, wherein the return circuit includes an expansion chamber (54) through which fuel is displaced between the internal volume (24a) of the first tappet (22a) and the internal volume (24b) of the second tappet (22b).
The fuel pump assembly as claimed in claim 3, wherein the expansion chamber (54) is defined within the pump housing (10).
The fuel pump assembly as claimed in any of claims 1 to 4, wherein the return circuit includes at least one passage (52a, 52b) defined within the pump housing (10).
The fuel pump assembly as claimed in claim 5, wherein the at least one passage (52a, 52b) defined within the pump housing (10) provides an unrestricted flow passage for fuel flowing between the internal volumes (24a, 24b) of the first and second tappets (22a, 22b).
The fuel pump assembly as claimed in any of claims 1 to 6, wherein each of the first and second tappets (22a, 22b) is movable along its axis within an associated tappet guide (32a).
The fuel pump assembly as claimed in claim 7, wherein each of the tappet guides is defined by a respective bore (32a) provided within the pump housing (10).
The fuel pump assembly as claimed in claim 7, wherein each of the tappet guides is defined by a respective sleeve located within a respective bore (32a) provided within the pump housing (10).
The fuel pump assembly as claimed in any of claims 7 to 9, wherein each of the first and second tappets (22a, 22b) is provided with an inlet port (60a, 60b) in a sidewall thereof which is cooperable with an inlet port (58a, 58b) provided in the associated tappet guide (32a, 32b) so as to allow fuel flow into the internal volume (24a, 24b) of the tappet when the inlet ports overlap.
The fuel pump assembly as claimed in claim 10, wherein the inlet ports (58a, 60a) associated with the first tappet (22a) are sized and positioned so as to overlap with one another on the pumping stroke of the first tappet (22a) simultaneously with the inlet ports (58b, 60b) associated with the second tappet (22b) overlapping with one another on the return stroke of the second tappet (22b).
The fuel pump assembly as claimed in claim 10 or claim 11, wherein each of the first and second tappets (22a, 22b) includes one or more additional inlet ports provided in its sidewall in the same plane as each of the other inlet ports.
The pump assembly as claimed in any of claims 7 to 12, wherein each tappet guide (32a, 32b) is provided with an additional port (62a, 62b) for collecting leakage fuel that flows between the associated tappet (22a, 22b) and the tappet guide (32a, 32b).
The pump assembly as claimed in claim 13, wherein each additional port (62a, 62b) communicates, via a restriction (64a, 64b), with the engine cam box (29).
The fuel pump assembly as claimed in any of claims 1 to 14, wherein each of the first and second tappets (22a, 22b) includes means for preventing angular movement of the tappet about its axis.