GB2028916A

GB2028916A - Fuel Supply System for Internal Combustion Engine

Info

Publication number: GB2028916A
Application number: GB7925645A
Authority: GB
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1978-08-23
Filing date: 1979-07-23
Publication date: 1980-03-12
Also published as: GB2028916B

Abstract

A pump for use in an accumulator type fuel system for supplying fuel to an engine includes a high pressure pump to which fuel is supplied by way of a throttle 15, 16 by a low pressure pump. A valve is provided to control the output pressure of the low pressure pump and the outlet of the high pressure pump is connected to the accumulator. The setting of the throttle is determined by a piston combination comprising pistons 29, 34. The smaller of the pistons is subject to the pressure in the accumulator and the larger piston to the output pressure of the low pressure pump plus the pressure of a spring 32. The piston combination controls the throttle so that the accumulator pressure is higher than the output pressure of the low pressure pump by a predetermined ratio. <IMAGE>

Description

SPECIFICATION Fuel Supply System for Internal Combustion Engine This invention relates to a pump for use in a fuel supply system for an internal combustion engine and of the kind in which fuel is stored under pressure in an accumulator and is delivered to the engine through a nozzle the opening period of which is closely controlled to determine the amount of fuel supplied to the engine, the pump acting to supply fuel to the accumulator and being driven by the associated engine.

Fuel systems of the type set out above are known in which the pressure within the accumulator is controlled by a valve, the valve acting to spill fuel from the accumulator in order to control the pressure therein. This arrangement is wasteful of energy and furthermore, the pressure within the accumulator remains substantially constant. It has been found to be useful to be able to vary the pressure within the accumulator in accordance with the speed of the associated engine and the object of the present invention is to provide a pump for use in a system of the kind specified which can itself be used to vary the pressure within the accumulator in accordance with the speed of the associated engine, without spilling fuel at high pressure.

According to the invention a pump for use in a fuel supply system of the kind specified comprises a high pressure pump which in use is driven by the engine with which the fuel supply system is associated, the high pressure pump having an outlet connected in use to the accumulator and an inlet connected to a source of fuel, throttle means for controlling the flow of fuel through the inlet and pressure responsive means which in use is responsive to the pressure within the accumulator, for adjusting the setting of said throttle means.

According to a further feature of the invention, the pump includes a low pressure pump having an outlet connected by way of said throttle means to the inlet of the high pressure pump.

One example of a fuel system incorporating a pump in accordance with the invention will now be described with reference to the accompanying drawings in which: Figure 1 shows in diagrammatic form the fuel system, Figure 2 shows in sectional side elevation two parts of the system seen in Figure 1, Figure 3 shows in sectional side elevation another part of the system shown in Figure 1 and, Figure 4 shows a graph showing the variation of pressure with engine speed.

Referring to Figure 1 of the drawings there is indicated a compression ignition engine 21 having a plurality of injectors 22 equal in number to the number of engine cylinders. The injectors are supplied with fuel under pressure from an accumulator 23 and the duration of opening of the valves within the injectors and the timing of opening in relation to the position of the parts of the engine, is determined by a control system 24 which may be an electronic control system.

Fuel is supplied to the accumulator by a high pressure pump indicated at 25 the pump having an outlet connected directly to the accumulator and an inlet which is connected to the outlet of a low pressure pump 26. The pumps 25 and 26 are driven from the associated engine. The fuel connection between the two pumps includes an adjustable throttle 27 the setting of which is determined by a pressure responsive means 28.

The means 28 is responsive to the pressure of fuel within the accumulator 23.

Turning now to Figure 2 there is shown one example of the high pressure pump 25 combined with the low pressure pump 26 and the throttle 27. In Figure 2 the combined pump includes a body part 1 in which a rotary distributor member 1 2 is rotatably mounted. The distributor member is connected to the rotary part 2 of a vane pump which constitutes the low pressure pump 26 the vane pump having an outlet which communicates with a passage 13 in the body part 1 and an inlet 3 which is connected to a source of fuel conveniently a fuel tank. The outlet 4 of the vane pump is connected to the inlet 3 by way of a relief valve 5 and this is constructed so that the output pressure of the low pressure pump is controlled in accordance with the speed of the associated engine.

The distributor member 1 2 has an end portion 6 of enlarged diameter and in which there is formed a transversely extending bore 7 which accommodates a pair of reciprocable plungers 8.

At their outer ends, the plungers engage with shoes which carry rollers 9 respectively for engagement with the internal peripheral surface of an annular cam ring 10. The enlarged portion 6 of the distributor member is provided with connection to a drive shaft 11 which in use is driven by the associated engine.

Provided on the internal peripheral surface of the cam ring 10 are pairs of cam lobes not shown, the lobes in each pair being diametrically disposed and the bore 7 communicates with a longitudinally extending passage 1 8 formed in the distributor member. At one point, the distributor member is in communication with a plurality of inlet passages 17, these being equal in number to the number of cam lobes. In the particular example four inlet passages 1 7 are provided there being two pairs of cam lobes.

The passages 1 7 register in turn with an inlet passage 1 6 formed in the body part 1, and this communicates with the passage 1 3 by way of the adjustable throttle 27 shown in Figure 1 and which is constituted by an axially movable throttle member 1 5. The throttle member 1 5 is located within a bore and is provided with a groove intermediate its ends, the inner land defined by the groove is provided with axially extending grooves thereon so that the central groove is in communication with a circumferential groove formed on the periphery of the distributor member, and which is in constant communication with the passage 13.The land at the other end of the throttle member 1 5 can upon axial movement of the throttle member, control the effective size of the inlet 16 and thus the axial setting of the throttle member determines the amount of fuel which can flow to the bore 7 when a passage 1 7 is in register with the inlet port 1 6.

Also communicating with the longitudinal passage 1 8 is an outwardly extending delivery passage 1 9 and this is arranged to register in turn with a plurality of outlets 20 formed in the body 1.

The outlets at their outer ends, are interconnected and are connected as shown in Figure 1, to the accumulator 23.

The delivery passage 19 is brought into register with an outlet 20 immediately before the plungers 8 are moved inwardly by a pair of the cam lobes and it will be appreciated that during this time the inlet 1 6 is out of communication with the passages 1 7. There are of course as many outlets 20 as there are inlet passages 1 7.

The bore 7, the plungers 8, the rollers 9 and the cam lobes on the cam ring 10 constitute the high pressure pump which is shown at 25 in Figure 1. The amount of fuel delivered during inward movement of the plungers 8 depends upon the axial setting of the throttle member 1 5 and as seen in Figure 3, the axial setting of the throttle member can be adjusted.

The pressure responsive means 28 seen in Figure 1 comprises a piston 29 slidable within a bore 30 the axis of the bore 30 extending at right angles to the direction of movement of the throttle member 1 5. The piston 29 is provided with a cam form 31 for engagement by the rounded end of the throttle member 1 5 and the latter is urged into contact with the cam form by the force generated on the throttle member due to the fuel pressure in the passage 1 3 acting thereon. As the piston 29 is moved towards the left as seen in Figure 3 so the degree of restriction of the inlet 1 6 is reduced and therefore more fuel will be supplied to the bore 7.

The piston 29 is biassed towards the left by means of a coiled compression spring 32 and in addition by the fuel pressure in the passage 13 which is supplied to the end of the bore 30 by way of a passage 33. Movement of the piston 29 toward the left is opposed by the force acting on a further piston 34. The further piston 34 is of reduced diameter andes slideable within a bore formed in an insert 35 retained within the bore 30 by means of a cap nut 36. The cap nut 36 is provided with a drilling which communicates with the interior of the accumulator whereby the fuel pressure within the accumulator can act on the end of the piston 34 remote from the piston 29.

The space at the left hand end of the piston 29 is connected to a drain whereby any fuel which leaks along the working clearance defined between the piston 34 and the wall of the bore in which it is located, cannot cause a pressure to be applied to the piston 29. As will be apparent from Figure 3, the diameter of the piston 34 is very much smaller than the diameter of the piston 29.

In use, an equilibrium is established so that when fuel is drawn from the accumulator the pressure therein will fall slightly and the force exerted by the piston 34 against the piston 29 will decrease.

As a result the piston 29 will move toward the left thereby allowing more fuel to flow through the inlet 1 6 thereby to restore the pressure within the accumulator. Conversely, if the pressure within the accumulator should rise then the piston 29 will be moved towards the right to reduce the amount of fuel supplied to the accumulator. As has been stated the pressure in the passage 13 and therefore the pressure applied to the right hand end of the piston 29 varies in accordance with the speed at which the associated engine is operating. As a result the pressure within the accumulator also varies in accordance with the speed and the ratio of the pressure within the passage 33 to the pressure within the accumulator is determined largely by the ratio of the areas of the ends of the two pistons which are exposed to the pressures.

Figure 4 shows the variation in pressure obtained with speed. The left hand scale shows the pressure in the passage 13 whilst the right hand scale shows the pressure within the accumulator.

The spring 32 is provided to ensure that when an attempt is made to start the engine when the pressure in the accumulator may be substantially zero, the piston 29 is moved towards the right thereby ensuring that the inlet 1 6 is substantially free from obstruction. The force exerted by the spring has very little effect upon the pressure within the accumulator.

In Figure 3 it will be noted that a port 37 is provided in the insert 35. This port will be uncovered to spill fuel from the accumulator to the drain in the event that the force acting on the piston 34 due to the pressure within the accumulator, exceeds the forces acting on the piston 29. This will occur when the speed of the associated engine reduces subsequent to shut off of the fuel supply to the engine. The pressure within the accumulator will therefore be progressively reduced by spillage of fuel through the port as the engine speed decreases, the pressure being at the correct value when the fuel supply to the engine is restored.

It is possible to modify the pressure in the passage 1 3 and therefore the pressure acting on the piston 29, in accordance with the amount of fuel which is being supplied to the engine. In this manner, the fuel pressure within the accumulator will depend also upon the amount of fuel which is supplied to the engine and hence the load on the associated engine.

Claims

1. A pump for use in a fuel supply system for an internal combustion engine, the system being of the kind in which fuel is stored under pressure in an accumulator and is delivered to the engine through a nozzle the opening period of which is closely controlled to determine the amount of fuel supplied to the engine, the pump acting to supply fuel to the accumulator and comprising a high pressure pump which in use is driven by the engine with which the fuel supply system is associated, the high pressure pump having an outlet connected in use to the accumulator and an inlet connected to a source of fuel, throttle means for controlling the flow of fuel through the inlet and pressure responsive means which in use is responsive to the pressure within the accumulator, for adjusting the setting of said throttle means.

2. A pump according to Claim 1 including a low pressure pump having an outlet connected by way of said throttle means to the inlet of the high pressure pump, the low pressure pump including valve means for controlling the output pressure thereof so that it varies in accordance with the speed of the associated engine.

3. A pump according to Claim 2 in which said pressure responsive means is also responsive to the output pressure of the low pressure pump.

4. A pump according to Claim 3 in which said pressure responsive means comprises a piston combination having first and second piston portions disposed in end to end relationship within respective cylinders, one of said piston portions having a larger outer end area than the other, the outer end of said other piston portion being exposed to the pressure within the accumulator and the outer end of the one piston portion being exposed to the output pressure of the low pressure pump, the axial setting of the piston means determining the setting of said throttle means.

5. A pump according to Claim 4 in which said throttle means comprises an axially movable throttle member and said piston combination is provided with a shaped surface for engagement by said throttle member.

6. A pump according to Claim 4 or Claim 5 including a port formed in the wall of the smaller cylinder said port being positioned to be uncovered by the outer end of said other piston portion as the piston portions are moved by the pressure within the accumulator, said port communicating with a drain whereby the pressure within the accumulator is reduced until the pressure acting on the piston combination are baianced.

7. A pump according to Claim 6 in which the piston combination and the respective cylinders define a space at the inner ends of the piston portions, said space communicating with a drain.

8. A pump according to Claim 6 including a light spring acting to bias the piston combination in opposition to the force exerted by the pressure of fuel in the accumulator acting on the outer end of said other piston portion.

9. A pump according to any one of the preceding claims in which the high pressure pump and the low pressure pump comprises a rotary distributor fuel injection pump, the outlets of which are connected to the accumulator.

10. A pump for use in a fuel supply system for an internal combustion engine comprising the combination and arrangement of parts substantially as hereinbefore described with reference to the accompanying drawings.