GB2332929A - A cam follower and plunger arrangement for a rotary fuel pump - Google Patents

Abstract

A rotary fuel pump comprises a rotatable distribution member 12 having a plurality of radially extending bores 18 each reciprocably receiving a plunger 19; the distributor member 12 being rotatable within a cam ring 13 such that co-operation of cam surface 14 of the cam ring 13 and cam followers 21 associated with the plungers 19 generates a pumping stroke of the plungers 19, wherein a first resilient collapsible means 25, 26 is interposed mechanically between a first on the plungers 19 and an associated one of the cam followers 21. The spring rate of the first resilient means 25, 26 is such that collapse of means 25, 26 takes place progressively during the initial phase of the pumping stroke. After the initial phase no further collapse occurs and the plunger 19 moves through the remainder of the stroke directly with the cam follower. The arrangement provides reduced pressure during the initial phase. A second resilient collapsible means, having a lower spring rate to that of the first, may also be incorporated to provide another phase to the pumping stroke at which the pressure is at a different level.

Description

FUEL PUMP This invention relates to a rotary fuel injection pump for use in the fuel injection system of an internal combustion engine, the pump being of the kind in which a rotatable distributor member having a plurality of radially extending bores each reciprocably receiving a pumping plunger, is rotatable within a cam ring such that cooperation of the cam surface of the cam ring and cam followers associated with the pumping plungers generates pumping strokes of the plungers.

In a pump of the kind specified above the bores within which the plungers reciprocate communicate with a central pumping chamber which in turn communicates through an axially extending passage provided in the distributor member with a series of radially extending inlet and delivery passages through which fuel enters and leaves the pump. In known manner fuel enters the pump from a low pressure supply system such that fuel flowing into the pumping chamber drives the plungers radially outwardly to cooperate with the cam. At a different point in the rotation of the distributor member within the cam ring the plungers are cooperating with the cams and are being driven radially inwardly to perform a pumping stroke, fuel displaced in the pumping stroke being delivered to one or more fuel injection nozzles of the internal combustion engine which open in response to fuel pressure reaching a predetermined value, whereafter the fuel enters the engine by way of the nozzle or nozzles.

The pumping plungers are equi-angularly spaced about the axis of rotation of the distribution member, and the cam ring includes an equal number of identical, equi-angularly spaced cam forms with which the plungers cooperate. The distributor member is driven at a speed related to engine speed, and thus the rate at which pressure increases, or fuel is delivered, during a pumping stroke is determined by the shape of the cam forms.

It is believed that the level of NOX emissions, and the generation of combustion noise, may be reduced in a diesel engine by providing a reduced initial pumping rate, that is to say by reducing from the norm, the amount of fuel delivered into the engine during the first part of the pumping cycle. Accordingly, it is a first object of the present invention to provide a reduced initial pumping rate in a simple and convenient manner.

In accordance with a first aspect of the present invention there is provided a rotary fuel injection pump of the kind specified in which resiliently collapsible means is interposed mechanically between a pumping plunger and an associated cam follower, the spring rate of said resiliently collapsible means being chosen such that resilient collapse of said means takes place progressively during the initial phase of the pumping stroke, as output pressure increases, the overall length of collapse being chosen in relation to the spring rate to achieve a predetermined pressure beyond which no further collapse occurs.

It is also believed that undesirable exhaust and combustion noise emissions may be reduced by the'use of injection pressures which are higher than the norm, with the fuel being injected into the engine through smaller holes in the injectors. In a pump of the kind specified the maximum pressure which can be generated directly by a plunger is limited by the stresses which the pump construction can accommodate.

It is known however that during pumping the pressure within the nozzle may exceed pumping pressure as a result of wave effects within the nozzle. Moreover, it is recognised that this effect in the nozzle can be increased by reducing the nozzle hole size, the effect generating an amplified pressure wave at the nozzle. Conventionally this pressure wave has been seen as a problem since the pressure wave can be reflected back through the system to the pump, and can then be reflected back from the pump to the injector to cause undesirable secondary injection of fuel. The inventor has recognised however that if reflection of the high pressure wave back to the nozzle can be avoided then the increased pressure within the nozzle which gives rise to the reflected wave can be beneficially employed.

In the pump defined above, at the point in time when a reflected pressure wave returns to the pumping chamber, the pumping stroke will have ceased and the pressure in the chamber will have dropped to a value such that the resiliently collapsible means will have restored, at least partially, to its rest configuration relative to its plunger and cam follower. Thus the plunger embodying the resiliently collapsible means can be moved by the pressure wave by virtue of spring collapse, to increase the volume of the pumping chamber thereby absorbing part or all of the pressure wave. However, in a preferred embodiment the resiliently collapsible means which is arranged to reduce the initial pumping rate is not ideally suited to totally absorbing a reflected high pressure wave, and thus in a preferred embodiment a second pumping plunger of the pump has a second resiliently collapsible means mechanically interposed between the plunger and its associated cam follower, the spring rate of the second resiliently collapsible means being less than the spring rate of the first mentioned resiliently collapsible means.

Desirably the stroke of collapse of the second resiliently collapsible means exceeds that of the first mentioned resiliently collapsible means.

In an alternative construction both first and second resiliently collapsible means are associated with the same piston and cam follower.

One example of the invention is illustrated in the accompanying drawings wherein: Figure 1 is a diagrammatic cross-sectional view of a rotary fuel injection pump, and Figure 2 is a diagrammatic cross-sectional view of part of the pump of Figure 1.

Referring to the drawings, the rotary fuel injection pump includes a casing 11 rotatably supporting a distributor member 12 for rotation about its longitudinal axis. Within the casing 11 and encircling part of the distributor member 12 is an annular cam ring 13 the inner surface of which defines four identical, equi-angularly spaced cam forms 14. The cam ring 13 is mounted within the casing 11 for limited rotational movement about the axis of rotation of the distributor member 12 to provide an advance/retard adjustment of the pumping cycles relative to the rotational position of the distributor member 12. An hydraulically operated timing mechanism 15 (forming no part of the present invention) is carried by the casing 11 and cooperates with the ring 1 3 for moving the ring 1 3 angularly within the casing.

The distributor member 12 has a centrally disposed pumping chamber 16 which communicates with inlet and delivery ports (not shown) by means of an axially extending passage 17. Extending radially outwardly from the chamber 16 are four equi-angularly spaced bores 18 each of which slidably receives a respective pumping plunger 19.

Slidably received in the distributor member 12 and positioned radially outwardly from each plunger 19 is a respective cam follower assembly 21 including a roller 22 engageable with the cam forms 14 within the cam ring 13 as the distributor member 12 rotates relative to the cam ring 13.

Two of the plungers 19 (the horizontally extending pair identified as 19c in Figure 1) have a direct mechanical connection with their cam follower 21 so that during rotation of the distributor member, as their respective cam followers 21 cooperate with the cam forms 14, the plungers 19c are driven inwardly to perform a pumping stroke, the rate of movement of these two plungers 19c being determined directly by the speed of rotation of the distributor member and the shape of the cam forms 14.

However, the other two plungers 19a, 19b have resiliently collapsible means interposed mechanically between the plunger and the cam follower. The resiliently collapsible means connecting each of these plungers to its respective cam follower is basically similar, but of different spring rate. It can be seen (Figure 2) that plunger 19a is engaged at its radially outermost end by a cylindrical cap 23 of "top-hat" at" form, the cap 23 being slidable radially on a cylindrical extension of the distributor member 12 within which the bore 18 is formed. The radially innermost end of the cap 23 has a peripheral, outwardly extending flange 24 against which the convex side of a first dished spring washer 25 abuts. A second, substantially identical spring washer 26 has its concave face presented to the concave face of the washer 25 such that the washers 25, 26 abut at their outer periphery. The convex face of the washer 26 abuts a cylindrical collar 27 slidably received on the outer surface of the cap 23. The shoe 28 of the cam follower 21, which carries the roller 22, is secured to the collar 27 and overlies the top of the cap 23, there being a clearance "x" between the underside of the shoe 28 and the upper surface of the cap 23 in the rest position of the spring washers 25, 26.

It will be recognised therefore that when a load is imposed on the roller 22 attempting to push the piston 19a radially inwardly of the distributor member 12, and such movement of the plunger 19a is resisted, then resilient collapse of the spring washers 25, 26 will take place absorbing the clearance x. Once clearance x has been taken up the shoe 28 abuts the cap 23 and a direct mechanical connection exists between the cam follower 21 and the plunger 19a so that the plunger 19a moves radially inwardly at a rate determined by the rotation of the distributor member 12 and the shaping of the cam forms 14. However, during a pumping stroke initially as the cam follower 21 starts to move radially inwardly, the movement of the piston 19a with the shoe 21 is opposed by fuel pressure within the chamber 16, and thus the piston 19a moves at a reduced rate by comparison with movement of the cam follower 21, and indeed by comparison with movement of the pistons 19c which have a direct mechanical connection with their cam followers 21. The reduced rate of movement is controlled by the spring rate of the washers 25, 26 until such time as the clearance x is absorbed.

The mechanical arrangement of the plunger 19b and its associated cam follower 21 is substantially identical to that described above in relation to the plunger 19a except in that the clearance x is significantly increased, and in that the spring washers 25, 26 are replaced by a helically wound compression spring 29 of lower spring rate than the washers 25, 26.

With the exception of the action of the resiliently collapsible means associated with the plungers 19a, 19b the operation of the pump is conventional. Thus, at a point in the rotation of the distributor member 12 relative to the cam ring 13 such that the rollers 22 are engaged with low points of the cam forms 14, then the passage 17 will communicate with a source of fuel at low pressure, and fuel will thus be supplied to the chamber 16 to displace the plungers 19 radially outwardly. At a later point in rotation of the distributor member 12 the rollers 22 will cooperate with the commencement of inclined portions of the cam forms 14 and the communication between the passage 17 and the source of inlet fuel will be broken and the passage 17 will communicate with one or more delivery passages leading to one or more fuel injection nozzles of the associated diesel engine. As rotation continues the cam followers 21 will be moved radially inwardly by the slopes of the cam forms 14 thus displacing their plungers 19 radially inwardly. The movement of the plungers 19a, 19b will be governed by the spring rates of their respective resiliently collapsible connections to the respective cam followers 21, but the pistons 19c will move directly in accordance with the cam forms 14.

The rate of the spring 29 is relatively low, and thus as the pressure in the chamber 16 increases at the commencement of the pumping strokes of the plungers 19 the clearance between the cap 23 of the piston 19b and the shoe 28 of its cam follower will rapidly be taken up whereafter the plunger 19b will move in accordance with the cam form 14. However, the rate of the spring washers 25, 26 of the plunger 19a is somewhat higher, and relative movement between the plunger 19a and its cam follower 21 will take place until the pressure in the chamber 16 reaches a much higher value whereafter the clearance x will have been taken up and the plunger 19a will move in accordance with the shape of the cam form 14.

It will be recognised therefore that the combined effect of the resilient collapse of the plungers 19a, 19b, but predominantly the plunger 19a, is to modify the rate of pressure rise in the chamber 16 during the initial part of the pumping stroke of the plungers 19. Since rotation of the distributor member 12 will be taking place, but without corresponding movement of all four plungers 19, then the rate at which pressure rises in the chamber 16 initially will be less than that which would be effected by the shaping of the cam forms 14 in the absence of the resilient collapse. The rates of the spring washers 25, 26 and the spring 29, together with the clearance between the pistons 19a, 19b and their cam follower shoes, is so chosen as to provide a rate of pressure rise during the first part of the pumping stroke which is half that determined by the shaping of the cam forms 14 up to a predetermined threshold pressure at which the clearance x of the plunger 1 9a has been absorbed.

In this particular example, the stiffness of the washers 25, 26 is approximately 940N/mm and the clearance x is 0.8mm. In relation to the plunger 19b the stiffness of the spring 29 is 100N/mm and the clearance of the cap 23 of the plunger 19b from the shoe 28 is 2.0mm.

The combined effect of these two resilient collapse arrangements is to ensure that the rate of pressure rise in the chamber 16 is initially half of that which would be expected given the cam shaping, up to 200 bar in the chamber 16 at which point all clearances in the resiliently collapsible mechanisms have been absorbed.

When the maximum throw of the plungers 19 has occurred, and the rollers 22 of the cam followers are traversing the high points of the cam forms 14 the communication of the passage 17 with the delivery ports will be maintained, but the pressure in the passage 17 and chamber 16 will have dropped back to a pressure below that at which the injectors are opened. The springs 25, 26 and 29 will have, at least partially, restored the clearances between the plungers and the cam followers in readiness for a subsequent pumping stroke. The plungers will not however have been moved radially outwardly to refill the chamber 16 with fuel since the rollers 22 are still on the high regions of the cam forms 14. In this situation if a pressure wave is reflected back from the or each injector to the pump then the pistons 19a and 19b can react to that pressure wave by moving against the action of their springs and in effect increasing the volume of the pumping chamber 16 to absorb the pressure wave. In such circumstances therefore the pressure wave will not be reflected back from the pump to the injector thus minimising the risk of a spurious opening of the injector. With this advantage in mind it can be recognised that the amplification of the pressure within the injector which can occur as a result of injector design can be utilised to give an increased injection pressure without an increase in the pumping pressure. The design of the injectors can be such that their outlet openings are reduced in size giving an increased amplification of pressure within the nozzle so that the benefits of injection at higher pressure through a smaller diameter aperture can be obtained without increasing the pumping pressure, and thus without increasing the complexity or structural strength of the pump, and equally without the risk of the increased pressure wave being reflected back from the pump to the injector to cause spurious opening.

A pump of the kind illustrated in Figure 1 can therefore be used with injectors designed to maximise the amplification of the injection pressure without the risk of secondary reflection of the amplified pressure wave.

Instead, in a pump of the kind illustrated in Figure 1 any pressure wave reflected back to the pump will be absorbed by movement of the plungers 19a, 19b, primarily movement of the plunger 19b which has a greater stroke against a lower spring force than is the case with the plunger 19a.

It will be recognised that in theory both the high rate and the low rate resilient collapse arrangements could be incorporated in the coupling of a single plunger 19 to its cam follower 21. However, in practice such an arrangement would probably necessitate an increase in diameter of the pump and therefore a more acceptable solution is to apply the two different rate collapse mechanisms to two different, preferably diametrically opposed, plungers.

A pump as described above can be utilised with injectors having reduced outlet apertures by comparison with conventional injectors for the same application, and the combined effects of reducing the initial pumping rate, and absorbing reflected pressure waves facilitates a reduction in the NOX levels of the engine exhaust, a reduction in combustion noise, and lower smoke emissions due to significantly higher injection pressures and smaller nozzle apertures. In addition, pumping noise is reduced by the cushioning effect of the springs in relation to the roller/cam impacts during rotation of the distributor member 12. Furthermore, it has been found that use of a pump as disclosed above in a diesel engine fuel injection system renders the system more compliant in relation to the residual pressures which exist in the delivery lines to the injectors between injection cycles. It is found that the system exhibits less variation in the residual pressure over the operating range of engine speed and load than in a conventional system, thus improving the stability and driveability of the engine in use.

Claims (5)

1. CLAIMS 1. A rotary fuel pump comprising a rotatable distributor member having a plurality of radially extending bores each reciprocably receiving a pumping plunger, the distributor member being rotatable within a cam ring such that cooperation of the cam surface of the cam ring and cam followers associated with the pumping plungers generates pumping strokes of the plungers, wherein a first resiliently collapsible means is interposed mechanically between a first one of the pumping plungers and an associated one of the cam followers, the spring rate of said first resiliently collapsible means being chosen such that resilient collapse of said means takes place progressively during the initial phase of the pumping stroke, as output pressure increases, the overall length of collapse being chosen in relation to the spring rate to achieve a predetermined pressure beyond which no further collapse occurs.
2. 2. A rotary fuel pump as claimed in Claim 1, wherein a second resilient collapsible means is interposed mechanically between a second one of the pumping plungers and an associated one of the cam followers, the spring rate of the second resiliently collapsible means being less than that of the first resiliently collapsible means.
3. 3. A rotary fuel pump as claimed in Claim 2, wherein the stroke of collapse of the second resiliently collapsible means exceeds that of the first resiliently collapsible means.
4. 4. A rotary fijel pump as claimed in Claim 1, further comprising second resiliently collapsible means interposed between the first piston and the associated one of the cam followers.
5. 5. A rotary fuel pump substantially as hereinbefore described with reference to the accompanying drawings.