GB2313630A

GB2313630A - Fuel injection pump

Info

Publication number: GB2313630A
Application number: GB9611086A
Authority: GB
Inventors: Peter Alban George Collingborn
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1996-05-28
Filing date: 1996-05-28
Publication date: 1997-12-03
Also published as: GB9611086D0

Abstract

A rotary fuel injection pump comprises a rotor 1 with at least one diametrically extending bore 4 and a pair of opposed pumping plungers 2,3 reciprocating in the or each bore and defining a pumping chamber there between. The radially inner end regions of the plungers are relieved to provide at least one land 11 for supporting the plunger for sliding movement within its associated bore 4, the relieved areas (12,13 etc. Figs. 2-8) being in fluid communication with the pumping chamber and having an axial length such that, in all possible positions which the plungers can adopt, a portion of at least one relieved area is in register with at least part of a fuel supply passage 9 to permit flow of fuel from the fuel supply passage 9 to the pumping chamber. Thus, the relieved areas prevent the fuel supply passage from becoming blocked by a pump plunger when the pump rotor 1 comes to rest with the plungers located substantially vertically. With the plungers so located, the upper plunger tends to slide down the cylinder under the action of gravity, but is prevented from blocking the fuel supply passage by the relieved areas.

Description

FUEL INJECTION PUMP This invention relates to a fuel injection pump, and more particularly to a rotary fuel injection pump.

A known type of fuel injection pump comprises a rotor which is mounted within a pump body and which is rotated by a drive shaft to effect both pumping and distribution of fuel. Pumping is effected by means of one or more pairs of pump plungers, the plungers of each pair being mounted in a common cylinder which extends diametrically across the rotor. A supply passage which extends substantially axially of the rotor supplies fuel to the central portion of the cylinder during the filling (radially outward) movement of the pump plungers. During injection (radially inward) movement of the pump plungers, the pump plungers are forced radially inwardly by an exterior ring cam and fuel is discharged from the pumping chamber formed by the central portion of the cylinder via a substantially axially extending discharge passage. The supply passage also serves as a discharge passage. Such pumps are well known and have been sold in large numbers by the Diesel division of Lucas Industries plc and its predecessors for many years under the designations DPA, DP200 and DPC.

The maximum volume of fuel which can be delivered during each injection stroke of injection pumps of the type described above is determined by the diameter and the stroke of the pump plungers. If the pump plunger diameter is increased with a view to increasing maximum delivery volume the axial force necessary to drive the pump plungers towards each other increases with the result that loading on the cams and cam followers increases producing increased wear on the cam crests. Because of this problem, the tendency in developing fuel injection pumps of the above type to provide larger maximum deliveries has been to increase the stroke of the pump plungers. It has been found, however, that a problem can arise with long pumping stroke designs in that the pump plungers can arrive at a position in which one of the pump plungers blocks the filling port which, in use, is intended to supply fuel to the pumping chamber. This undesirable situation can arise if the pump rotor comes to rest, when its associated engine stops, with a pair of pump plungers located substantially vertically. Under these circumstances, the lower plunger will tend to slide downwardly in its bore until its associated cam follower comes into engagement with the interior surface with the cam ring. The upper pump plunger will tend to follow the lower plunger and will come to rest against the lower plunger. Under these conditions, unless special precautions are taken, the upper plunger can block the port of the fuel supply passage.

One possible solution to the above problem is to provide shoulders (either integral or by way of spring clips) at the outer ends of the pump plungers so as to prevent the pump plungers moving radially inwardly beyond the limit defined by the shoulders. Such an arrangement would prevent the upper plunger as described above following the lower plunger to a position which could result in blocking of the port of the fuel supply passage. However, the provision of such shoulders would add to the cost and difficulty of manufacture and, in any event, it would be very difficult to find space for such shoulders within the confines of the rotor and driveshaft geometry.

An alternative solution would be to provide a reduced diameter portion at the inner end of both plungers to ensure that in all possible position of the plungers the fuel supply passage would be at least partially open to the pumping chamber.

However, such reduction of plunger diameter introduces the unwanted dead volume into the pumping chamber and reduces the length of the plunger which engages the rotor bore. This loss of engaged length is highly undesirable, especially in long stroke plungers.

According to one aspect of the present invention a fuel injection pump comprising: a rotor which is mounted within a pump body and is rotatable by a drive shaft to effect pumping of fuel; one or more pairs of pump plungers, the plungers of each pair being mounted in a common cylinder which extends diametrically across the rotor; and a fuel supply passage which extends substantially axially of the rotor to supply fuel to a pumping chamber constituted by a central portion of the cylinder located between the pump plungers is characterised in that the radially inner end regions of the pump plungers are relieved to provide at least one land on each plunger for supporting the plunger for sliding movement within its associated cylinder, the relieved areas being in fluid communication with the pumping chamber and having an axial length such that in all possible positions which the plungers can adopt a portion of at least one relieved area is in register with at least part of the fuel supply passage to permit flow of fuel from the fuel supply passage to the pumping chamber.

The exact number of lands and corresponding number of relieved areas is not critical to the present invention. In the preferred embodiment four axially extending lands separated by four relieved areas are preferred. The relieved areas may be of any suitable shape. The cross-section of the plungers may be constant throughout the portion having the lands and relieved areas. The shape of the surface of the plunger, when viewed in transverse cross-section, in the region of the relieved areas may be flat or concave but is preferably convex. Convex shaping reduces to a minimum the dead volume introduced by the presence of the lands whilst at the same time ensures the fuel supply passage remains open for inlet of fuel at all times.

The above and further features and advantages of the invention will become clear from the following description of a preferred embodiment thereof, given by way of example only, reference being had to the accompanying drawings wherein: Figure 1 is a schematic cross-sectional view through a preferred embodiment of the present invention; and Figures 2, 3 and 4 are cross-sections on the line X-X of Figures 2, 3 and 4 are cross-sections on the line X-X of Figure 1 showing different possible shapes for the cross-section of the plungers.

Figures 5-8 illustrate schematically alternative pump plunger designs for use in embodiments of the invention.

Referring firstly to Figure 1 there is shown schematically in transverse cross-section the pumping section of a rotary fuel injection pump of the type comprising a rotor 1 which is mounted for rotation in the body of the fuel injection pump and which carries a pair of pump plungers 2,3 mounted in a cylinder 4 which is diametric relative to the axis of rotation 5 of the rotor 1.

The rotor 1 is surrounded by a cam ring 6 which defines diametrically opposite pairs of cams 7. As the rotor rotates in normal use cam followers 8 provided at the outer ends of the plungers 2,3 engage the cam 7 during each pumping stroke of the pump to force the pump plungers 2,3 towards each other to pump fuel located in a pumping chamber which is defined by the cylinder 4 between the facing surfaces of the plungers 2,3 outwardly through a discharge port 9 which is located symmetrically about the axis 5 and is connected via a suitable distributor to the injection pipes of an internal combustion engine. During normal filling strokes of the pumping chamber fuel is supplied to the pumping chamber from the transfer pump via the same port 9 as is used to discharge fuel during injection strokes.

If the engine with which the injection pump is associated comes to rest in a position such that the cylinder 4 is orientated substantially vertically (as shown in Figure 1) the pump plungers 2,3 will tend, over time, to adopt the position illustrated in Figure 1. In this configuration, the cam follower 8 of the pump plunger 3 will rest against the interior surface 10 of the cam ring 6 and the pump plunger 2 will rest against the pump plunger 3. It will be noted that under these conditions if the pump plunger 2 is a plain cylindrical plunger the fuel inlet port 9 will be occluded by the surface of the plunger 2 with the result that fuel will not be able to flow to the interface between the plungers 2,3 at the commencement of movement of the rotor when the next attempt is made to start the engine.

With a view to removing the problem outlined above each pump plunger 2,3 is relieved over an axially inner portion thereof to provide a multiplicity of lands 11 which is separated by relieved areas 12 to ensure that the port 9 is never completely occluded.

Possible cross-sections for the relieved portion of the plungers are shown in Figures 2,3 and 4. It will be seen that the lands 11 in each case remain at the full diameter of the plunger and accordingly provide support for sliding movement of the plungers within the cylinder 4 over the full length of the plungers. Accordingly, the provision of the relieved portions does not substantially reduce the sliding support of the plungers as compared with plungers which are not provided with relieved portions. The relieved portions ensure, however, that a passage always exists between at least part of the port 9 and the interface between the pump plungers 2,3. The exact shape of the relieved portions is not critical and they may be provided by way of a flat surface 12 as shown in Figure 2, a concave surface 13 as shown in Figure 3 or a convex surface 14 as shown in Figure 4. Of these options, the use of a convex surface as shown in Figure 4 is preferred since this reduces to a minimum the dead volume introduced by relieving of the plungers.

It will be noted that the surface finish of the surfaces 12, 13,14 is not critical and accordingly the provision of these relieved areas does not materially add to the production cost of the pump plungers. The surface finish of the lands 11 is no more critical than that of the remainder of the exterior of the plungers and may be obtained by conventional centreless grinding.

It will also be noted that the modified pump plungers may be fitted in existing injection pumps of the relevant type (provided that the overall dimensions of the pump plungers are the same as those in the existing pump). Accordingly, the proposed pump replacements for existing pumps thereby obviating the need to stock both the original and modified pump plungers as inventory items.

Referring now to Figures 5-8, further designs of pump plungers suitable for use in embodiments of the present invention are illustrated.

Referring firstly to Figure 5, the illustrated plunger 15 corresponds generally to the plungers 2,3 of Figure 1 save that the relieved portions are each formed by a first relatively broad relieved area 16 and the second relatively narrow relieved area 17 interconnected by a shoulder 18. The relieved area 17 is connected to the full diameter portion of the pump plunger 15 by a further shoulder 19. The shoulders 18,19 are square to avoid the risk of wedging of the plunger by small particles of debris.

Referring now to Figure 6, the illustrated pump plunger 20 has relieved portions 21 which taper in width from a maximum adjacently the radially inner end 22 of the plunger to a minimum at the opposite end of each relieved portion. The effect of this arrangement is to provide lands 23 which increase progressively in width from the radially inner end 22 of the plunger towards the opposite end of the plunger.

Referring now to Figure 7 the relieved portions 24 of the illustrated plunger 25 each comprise a generally oval major portion 26 and a slot-like minor portion 27 which provides fluid communication between the major portion 26 and the pumping chamber defined between the plungers 25. The effect of this arrangement is to provide lands 28 which are generally hour-glass shaped. Because of the shape of the lands 28 each pump plunger 25, adjacent the radially inner end 29 thereof, is supported for sliding movement within its associated cylinder over a large proportion of the circumference thereof. In effect, the ends of lands 28 adjacent the end face 29 form a continuous annular support surface interrupted only by the slots 27. In Figure 7 the port 9 is illustrated in broken line. It will be observed that even in the worst case positioning of the plunger 25 at least some portion of the port 9 is in fluid communication with the pumping chamber defined between the plungers.

Finally, referring to Figure 8 the illustrated pump plunger 30 has a relieved portion 31 which, when the pump plunger 30 is mounted in its associated cylinder, forms a circumferentially continuous annular chamber 32. A land 33 of full plunger diameter is provided adjacent the radially inner end 34 of the pump plunger 30 to provide sliding support or the pump plunger.

One or more slots 35 are provided in the land 33 to provide communication between the chamber 32 and the pumping chamber defined between the plungers 30. Again, the port 9 is marked in Figure 8 in broken line from which it will be seen that even if the pump plunger 30 comes to rest with the land 33 in register with the port 9, the port 9 remains in fluid communication with the pumping chamber.

Claims

CLAIMS:

1. A fuel injection pump comprising: a rotor which is mounted within a pump body and is rotatable by a drive shaft to effect pumping of fuel; one or more pairs of pump plungers, the plungers of each pair being mounted in a common cylinder which extends diametrically across the rotor; and a fuel supply passage which extends substantially axially of the rotor to supply fuel to a pumping chamber constituted by a central portion of the cylinder located between the pump plungers is characterised in that the radially inner end regions of the pump plungers are relieved to provide at least one land on each plunger for supporting the plunger for sliding movement within its associated cylinder, the relieved areas being in fluid communication with the pumping chamber and having an axial length such that in all possible positions which the plungers can adopt a portion of at least one relieved area is in register with at least part of the fuel supply passage to permit flow of fuel from the fuel supply passage to the pumping chamber.