GB2275741A

GB2275741A - Fuel injection pump

Info

Publication number: GB2275741A
Application number: GB9403900A
Authority: GB
Inventors: Stuart William Nicol
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1993-03-05
Filing date: 1994-03-01
Publication date: 1994-09-07
Also published as: GB9403900D0

Abstract

A rotary distributor type of fuel injection pump includes a distributor member 10 mounting cam actuated pumping plungers 12 and a fluid pressure operable spill valve which includes a valve member 16 and an actuating piston 18 slidable in the distributor member. A trigger piston 26 is provided and is movable axially relative to the distributor member 10 by fuel pressure which is generated by auxiliary plungers 33 movable in synchronism with the pumping plungers. The movement of the trigger piston 26 due to the fuel pressure is halted by a stop surface 30 and when the movement of the trigger piston is halted the fuel pressure rises in a chamber 31 so that the pressure applied to the actuating piston lifts the valve member 16 from a seating to prevent further flow of fuel to the engine. The distributor member is axially movable to adjust the instant during the pumping stroke of the pumping plungers, when the spill valve is opened. <IMAGE>

Description

FUEL INJECTION PUMP This invention relates to a rotary distributor type fuel injection pump for supplying fuel to an internal combustion engine, the pump being of the kind comprising a rotary distributor member mounted in a pump body, a pair of pumping plungers mounted in a bore in the distributor member, a cam ring surrounding the distributor member and having inwardly extending cam lobes which impart inward movement to the pumping plungers as the distributor member rotates, passage means for delivering fuel displaced from the bore to a plurality of outlet ports in turn during successive inward movements of the pumping plungers, means for filling the bore with fuel during the periods when the plungers are allowed to move outwardly by the cam lobes and means for moving the distributor member axially to vary the quantity of fuel supplied through the outlet ports.

An example of such a pump is seen in GB-B-2037365 in which the maximum outward movement of the plungers is limited by inclined stop surfaces so that the axial position of the distributor member in the pump body determines the amount of fuel supplied to the bore and therefore the amount of fuel which is supplied through an outlet. A disadvantage of this form of pump is that delivery of fuel takes place to the associated engine all the time the plungers are moving inwardly and this leads to high stresses in the material forming the crests of the cam lobes and also in the rollers which are interposed between the cam lobes and the plungers. Moreover, towards the end of the inward movement of the plungers their rate of inward movement decreases and therefore the rate at which fuel is delivered to the engine also decreases and this can lead to problems with combustion of the fuel.A further disadvantage is that the timing of the start of fuel delivery varies as the quantity of fuel delivered to the engine varies.

The object of the present invention is to provide a pump of the kind specified in an improved form.

According to the invention a pump of the kind specified comprises a piston mounted in a cylinder extending axially in or in a part mounted in, the distributor member, pump means operable in synchronism with the pumping plungers for delivering fluid to said cylinder to urge the piston outwardly, a stop surface operable to limit the outward movement of the piston, the extent of outward movement depending upon the axial position of the distributor member within the body and fluid pressure actuated spill valve means responsive to the fluid pressure in said cylinder, said spill valve means when said piston engages the stop surface being actuated to allow fuel to spill from said bore thereby to terminate delivery of fuel to the associated engine.

An example of a fuel injection pump in accordance with the invention will now be described with reference to accompanying drawings in which: Figure 1 is a part sectional side elevation of a portion of the injection pump, Figure 2 is a view similar to Figure 1 showing a modification, and Figure 3 is a similar view showing a further modification.

Referring to Figure 1 the injection pump comprises a rotary distributor member 10 which is mounted for rotation in known manner, in a surrounding pump body (not shown). The distributor member is coupled to a drive shaft (not shown) which is journaled in the pump body and in use, is driven in timed relationship with the associated engine. The distributor member is also axially movable within the body such movement being effected in any convenient manner.

Formed in the distributor member is a transversely extending bore 11 in which is mounted a pair of pumping plungers 12. The portion of the bore which lies intermediate the plungers constitutes a pumping chamber 13 and this communicates by way of a passage 14 formed in the distributor member, with an outwardly extending passage also formed in the distributor member and positioned to register in turn with a plurality of outlet ports which are connected to the injection nozzles respectively of the associated engine. In addition, the passage 14 communicates with a plurality of outwardly extending inlet passages formed in the distributor member and these are positioned to register in turn with an inlet port or ports formed in the body and which are connected to the outlet of a low pressure fuel supply pump.

The plungers 12 are arranged to be moved inwardly by the action of cam followers not shown, with inwardly extending cam lobes formed on the internal peripheral surface of a cam ring 15 which is mounted in the pump body and the arrangement is such that when the plungers are moved inwardly by the cam lobes the delivery passage will be in register with an outlet port so that fuel will be delivered through this port to an injection nozzle of the associated engine. Following the inward movement the plungers are allowed to move outwardly by the cam lobes and during the periods when the plungers are allowed to move outwardly, an inlet passage registers with an inlet port so that a fresh supply of fuel can flow into the pumping chamber 13.

The pumping apparatus also includes a fluid pressure actuated spill valve means which comprises a spill valve member 16 movable into engagement with a seating formed at the end of a spill passage 17 which communicates with the pumping chamber 13. The spill valve member is formed integrally with a piston 18 slidable in a cylinder 19 formed in the distributor member. A spring 20 is interposed between the piston and a cup shaped end closure 21 which is secured to the distributor member, the spring 20 acting to urge the valve member 16 into engagement with the seating.

Formed in the piston 18 is a further cylinder 22 the inner end of which communicates with the pumping chamber 13 by way of a passage 23 and located in the further cylinder is a piston like pressure balancing member 24 which extends outwardly of the cylinder 22 to form a spring abutment for one end of a further spring 20A the other end of which is in engagement with the piston 18. The action of the spring 20A is to hold the balancing member 24 in a fixed position relative to the distributor member and the cylinder 22 has a diameter which is slightly larger than that of the seating so that the high pressure fuel within the pumping chamber 13 when the plungers are moving inwardly, acts to assist the action of the spring 20 in holding the valve member in engagement with the seating.

The balancing member further defines a cylinder 25 which extends axially and in which there is located a trigger piston 26 having an extension 27 which extends to the exterior of the end closure through an aperture therein. The extension 27 is provided with a flange 28 which is engagable with the end closure 21 under the action of a spring 29 interposed between the flange and a stop surface 30 which is conveniently formed on the drive shaft.

The inner end of the cylinder 25 by way of drillings in the member 24 and piston 18, communicates with an accumulator chamber 31 which is defined by the piston 18 and the inner end of the cylinder 31. Formed in the distributor member and extending into the accumulator chamber is a pair of bores 32 each of which accommodates an auxiliary pumping plunger 33 operable by the aforesaid cam lobes on the cam ring 15, in synchronism with the pumping plungers 12.

The various parts of the pump are illustrated in Figure 1 at the commencement of fuel delivery and as soon as the plungers 12 and 33 start to move inwardly, fuel will be expelled from the pumping chamber to the engine and the piston 26 will start to move out of the cylinder 25 against the action of the spring 29. The movement of the piston 26 continues until the end of the extension 27 engages the stop surface 30.

When this occurs there is a sudden increase in the pressure within the accumulator chamber 31 and this pressure acting on the piston 18 will move the piston against the action of the springs 20 and 20A. As soon as the valve member 16 is lifted from its seating, fuel can flow from the pumping chamber 13 into the accumulator chamber 31 to effect further movement of the piston. The immediate effect is to reduce the pressure in the pumping chamber thereby to terminate delivery of fuel to the associated engine.

The relative movement of the piston 26 and the distributor member which can take place before the movement of the piston 26 is halted by the stop surface 30, depends upon the axial position of the distributor member within the body and therefore by varying the axial position of the distributor member, the extent of inward movement of the pumping plungers 12 which takes place before the spill valve member 16 is opened, can be varied.

As the plungers 12 and 33 continue to move inwardly, the piston 18 is moved further against the action of the springs 20 and 20A. When the crests of the cam lobes are reached the plungers 12 and 33 can then move outwardly and fuel is returned to the pumping chamber 13 from the accumulator chamber 31 by movement of the piston 18 under the action of its springs. In order to guarantee the return motion of the piston 26, the accumulator chamber 31 is vented to a low pressure by means of a suitably positioned passage and port in the distributor member and body respectively furthermore, in order to stabilise the pressure within the accumulator chamber, the chamber after it has been vented to permit return motion of the piston 26, is connected to the outlet of the low pressure pump the pressure of which is insufficient to move the piston 26 against the action of the spring 29.

In the arrangement shown in Figure 2 the venting of the accumulator chamber 31 by the aforesaid ports and passages is assisted by a spring loaded valve. Referring to Figure 2 the valve comprises a valve member 34 which is biased into engagement with a seating defined on the balancing member 24A which in this case is provided with a through passage from the cylinder 25. When the plungers are allowed to move outwardly the piston 18 is returned by the action of its springs and whilst most of the fuel contained in the accumulator chamber will flow between the valve member 16 and its seating, as the valve member begins to close onto its seating flow of fuel will be restricted but the remaining fuel in the accumulator chamber can now flow into the pumping chamber 13 by the fact that the valve member 34 is lifted from its seating. This also permits the return movement of the piston 26.

By providing the spring loaded valve the pistons 18 and 26 can return more quickly to their initial positions and in the process there is a reduced loss of fuel to the low pressure source.

The area of the piston 18 which is subject to the pressure developed during the inward movement of the plungers 33 may be reduced in order to avoid premature movement of the piston 18 at high engine speeds.

As shown in Figure 3, the piston 18A is of stepped form and is slidable in stepped cylinder 19A. The steps defined between the wider and narrower portions of the cylinder 19A and the piston 18A are shaped to form an annular recess 35 which communicates by way of drillings in the member 24 and the piston 18A, with the inner end of the cylinder 25. In addition, the recess 35 communicates with the inner ends of a pair of blind bores 36 which accommodate the pair of auxiliary pumping plungers 33 which are operable by the aforesaid cam lobes in synchronism with the pumping plungers 12. Conveniently the passages 37 which connect the bores 36 with the recess 35 are drilled axially in the distributor member and are blocked by the pumping plungers 12.

The various parts of the pump are illustrated in the drawing at the commencement of fuel delivery and as soon as the plungers 12 and 33 start to move inwardly, fuel will be expelled from the pumping chamber 13 to the engine and the piston 26 will be urged out of the cylinder 25 against the action of the spring 29. The movement of the piston 26 continues until the end of the extension 27 engages the stop surface 30.

When this occurs there is a sudden increase in the pressure within the recess 35 and this pressure acting on the piston 18A will move the piston against the action of the springs 20 and 20A. As soon as the valve member 16 is lifted from its seating, fuel can flow from the pumping chamber 13 into the accumulator chamber 38 which is defined between the narrower end of the piston 1 8A and the narrower end of the cylinder 19A. The flow of fuel into the accumulator chamber will effect further movement of the piston. The immediate effect is to reduce the pressure in the pumping chamber 13 thereby to terminate delivery of fuel to the associated engine.

As in the previous example, the relative movement of the piston 26 and the distributor member 10 which can take place before the movement of the piston 26 is halted by the stop surface 30, depends upon the axial position of the distributor member within the body and therefore by varying the axial position of the distributor member, the extent of inward movement of the pumping plungers 12 which takes place before the spill valve is opened, can be varied.

As the plungers 12 and 33 continue to move inwardly, the piston 18A is moved further against the action of the springs 20 and 20A. When the crests of the cam lobes are reached the plungers can then move outwardly and fuel is returned to the pumping chamber 13 from the accumulator chamber 38 by movement of the piston 18 under the action of its springs and such movement will also displace fuel from the recess 35 to the bores 36. In order to guarantee the return motion of the piston 1 8A, the accumulator chamber 38 is vented to a low pressure by means of a suitably positioned passage and port in the distributor member and body respectively or by a small orifice permanently connected to a low pressure source. Moreover, in order to ensure the return motion of the piston 26, the recess 35 may be vented and then connected to the outlet of the low pressure pump to make up any fuel which may have been lost by leakage and to remove any air which may have collected in the various passages.

By the present arrangement the fuel pressure developed by the auxiliary plungers 33 acts upon a reduced area of the piston 18A as compared with the previous examples and this has the important advantage that the risk of the piston being displaced by the shock wave generated when the auxiliary plungers are initially moved inwardly is minimised.

Claims

1. A rotary distributor type fuel injection pump for supplying fuel to an internal combustion engine, comprising a rotary distributor member mounted in a pump body, a pair of pumping plungers mounted in a bore in the distributor member, a cam ring surrounding the distributor member and having inwardly extending cam lobes which impart inward movement to the pumping plungers as the distributor member rotates, passage means for delivering fuel displaced from the bore to a plurality of outlet ports in turn during successive inward movements of the pumping plungers, means for filling the bore with fuel during the periods when the plungers are allowed to move outwardly by the cam lobes and means for moving the distributor member axially to vary the quantity of fuel supplied through the outlet ports, a trigger piston mounted in a cylinder extending axially in or in a part mounted in, the distributor member, pump means operable in synchronism with the pumping plungers for delivering fluid to said cylinder to urge the trigger piston outwardly, a stop surface operable to limit the outward movement of the trigger piston, the extent of outward movement depending upon the axial position of the distributor member within the body and fluid pressure actuated spill valve means responsive to the fluid pressure in said cylinder, said spill valve means when said trigger piston engages the stop surface being actuated to allow fuel to spill from said bore thereby to terminate delivery of fuel to the associated engine.

2. A pump according to claim 1, in which said stop surface is defined on said body.

3. A pump according to Claim 2, including spring means acting between said body and said trigger piston to bias the trigger piston inwardly.

4. A pump according to Claim 3, in which said spill valve means includes a spill valve member and an actuating piston, the actuating piston defining a surface which is exposed to the fluid pressure in said cylinder.