GB2176841A

GB2176841A - Fuel supply system

Info

Publication number: GB2176841A
Application number: GB08614125A
Authority: GB
Inventors: Colin Thomas Timms
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1985-06-19
Filing date: 1986-06-10
Publication date: 1987-01-07
Also published as: GB2176841B; GB8614125D0; GB8515575D0

Abstract

A fuel supply system for an engine includes a low pressure pump 12 which draws fuel from a fuel tank 17 and supplies the fuel to a high pressure pump 11. A regulating valve 19 is provided to control the output pressure of the low pressure pump 12 and it is constructed to connect a chamber 22 to the inlet of the high pressure pump 11 so that the associated engine may be started after the fuel 17 has been allowed to run dry. The chamber 22 receives the surplus fuel which is then returned via a non-return valve 25 to the inlet of the low pressure pump 12 due to the normal regulating action of the valve 19 so that the chamber is maintained full of fuel during normal operation of the system. After tank 17 has run dry a piston part 29 is moved down by a spring 30 and thereby closes the connection between the low pressure pump 12 and the high pressure pump 11 and connects the reservoir 22 via ports 21, 27 to the high pressure pump 11. <IMAGE>

Description

SPECIFICATION Fuel supply system This invention relates to a fuel supply system for an internal combustion engine, the system being of the kind comprising a high pressure pump and a low pressure pump which supplies fuel under pressure to the high pressure pump, the latter supplying fuel at high pressure to the associated engine, the low pressure pump drawing fuel from a fuel supply tank and the two pumps being driven by the associated engine.

In one example the pumps are contained in a single housing and venting means is provided for dealing with small quantities of air drawn into the fuel system. Such small quantities of air will have little effect upon the operation of the engine this mainly being due to the fact that the low pressure pump is arranged to deliver fuel at a rate in excess of the maximum rate of fuel supply to the engine, the surplus fuel being returned to the inlet of the low pressure pump conveniently by means of a pressure regulating valve. A problem arises if large quantities of air enter the system as for example if the fuel tank is allowed to empty or if a fuel filter disposed upstream of the low pressure pump is changed.

In this situation the engine must be cranked for an extended period in order to draw fuel through the system and this imposes a severe load upon the starting motor of the engine and also the associated electrical system.

There are a number of ways in which the cranking period can be reduced for example, it is possible to increase the displacement of the low pressure pump. This has the disadvantage that during normal operation of the system the rate of fuel delivery by the low pressure pump is much higher than is required and this leads to unnecessary heating and aeration of the fuel as well as to additional power loss. Moreover, the pump itself must be physically larger.

An alternative arrangement is to provide an additional pump which can be operated manually, or an electrically operated pump. Both these forms of pump enable the system to be purged of air before an attempt is made to start the engine. The provision of either form of pump involves additional expense both in terms of the pump itself and also in terms of the cost of installation. The manual pump may be mounted on the filter. This solution enables the system to be primed before an attempt is made to start the engine. However, since it does not act as a pump during normal operation of the fuel system, the non-return valves which must be incorporated into the design of the hand operated pump, create pressure drops which may hinder the normal operation of the system. Moreover, except in the case of the electric pump manual intervention is required.

It has been proposed to provide upstream of the low pressure pump, a reservoir chamber which during normal operation of the system is maintained substantially full of fuel. The reservoir is provided with a small opening through which the fuel in the reservoir can flow at a restricted rate when the engine has stopped, the fuel flowing through the opening providing a volume of fuel at the inlet of the low pressure pump sufficient to allow the engine to be started. With this arrangement therefore once the supply tank has been replenished or the filter change completed, the engine can be started and the low pressure pump operates to clear the system of air.

Such an arrangement is described in British Published Specification 2132698A. A disadvantage with this arrangement is that the size of the reservoir has to be carefully chosen in relation to the volume of the connecting pipes in the system, the volume of the filter and also in relation to the volume of air which remains in the reservoir chamber once the transfer of fuel has been effected. Moreover, although the size of the opening is small the period during which air and fuel is delivered by the low pressure pump may be sufficiently long before the engine stops, to allow an appreciable quantity of fuel to flow through the opening. This is likely to be the case when the engine is operating at light load and low speed.

The object of the present invention is to provide a fuel system of the kind specified in a simple and convenient form.

According to the invention a fuel supply system of the kind specified comprises a pressure regulating valve including a resiliently loaded piston means which is subjected to the output pressure of the low pressure pump and is slidable in a cylinder, a port formed in the wall of said cylinder and through which fuel can escape to a low pressure source, said port being uncovered by said piston means to control the output pressure of the low pressure pump, a reservoir chamber communicating with said port and which receives fuel flowing through said port, said piston means comprising first and second parts movable relative to each other, first resilient means acting between said parts, second resilient means acting on said first part, means for limiting the extent of movement of said first part under the action of the second resilient means, the arrangement being such that in the event that the outlet pressure of the low pressure pump falls, said first part will move to the limit of its movement and the second part will continue to move under the action of said second resilient means to a position in which said reservoir is connected to the inlet of the high pressure pump and the connection between the outlet of the low pressure pump and the inlet of the high pressure pump is broken.

An example of a fuel system in accordance with the invention will now be described with reference to the accompanying diagrammatic drawing.

Referring to the drawing the fuel supply system includes a housing 10 in which is mounted a high pressure pump 11 and a low pressure pump 12. The high pressure pump 11 has outlets 13 for connection in use to the injection nozzles respectively of an associated engine and it has an inlet 14. The low pressure pump has an outlet 15 and an inlet 16.

The pumps 11 and 12 are driven from a common drive shaft (not shown) which is coupled to a rotary part of the associated engine.

The inlet 16 of the low pressure pump is connected in use to a fuel tank 17 by way of a fuel filter unit 18.

There is also provided a regulating valve generally indicated at 19 which acts to regulate the outlet pressure of the low pressure pump. The regulating valve comprises a cylinder 20 one end of which is connected to the outlet 15 of the low pressure pump. In the wall of the cylinder is formed a port 21 which communicates with the lower end of a reservoir chamber 22. The reservoir chamber has an upper outlet 23 leading into the cylinder 20 adjacent the other end thereof. This end of the cylinder also has an outlet 24 connected by way of a lightly loaded non-return valve 25, to the inlet 16 of the low pressure pump.

Also formed in the side wall of the cylinder 20 adjacent the one end thereof is a port 26 which communicates with the inlet 14 of the high pressure pump and the port 26 is connected to a further port 27 formed in the wall of the cylinder substantially opposite the port 21. Slidable in the cylinder is a piston means which comprises a first part 28 and a second part 29. The second part 29 is of cup-shaped form and is biased relative to the first part 28 by means of a first resilient means in the form of a coiled compression spring 30. The first part 28 of the piston means defines an abutment for the spring 30 and also has an outwardly directed flange 31 which forms an abutment for second resilient means in the form of a coiled compression spring 32.In addition, the flange 31 can engage with a step 33 formed in the wall of the cylinder, to limit the extent of movement of the first part of the piston means. The first part of the piston means also defines a vent 34 to the other end of the cylinder. Finally the upper end of the reservoir chamber 22 is connected by way of a restricted vent 35 to the fuel tank 17.

In operation assuming that the system is completely full of fuel, the low pressure pump 12 delivers fuel to the high pressure pump 11 through the port 26. The pressure of fuel acts upon the end face of the piston part 29 and urges the part into contact with the piston part 28 against the action of the spring 30 which is a relatively light spring as compared with the spring 32. The two parts of the piston means therefore move as one upwardly against the action of the spring 32 to uncover the port 21 to control the output pressure of the low pressure pump. The fuel which flows through the port 21 flows into the chamber 22. When the chamber 22 is full of fuel the surplus fuel is returned by way of the port 24 and the non-return valve 25, to the inlet of the low pressure pump. A small bleed of fuel takes place through the restricted vent 35.

Any bubbles of air which may be drawn from the fuel tank by the low pressure pump will tend to flow upwardly in the cylinder and flow through the port 21 into the chamber 22 and be returned to the fuel tank through the vent 35. A small quantity of fuel is taken from the cylinder 20 by way of a passage 37, for use as a coolant in the high pressure pump.

In the event that the fuel tank is allowed to run dry, the low pressure pump will start to draw air from the tank and the air will be passed into the cylinder and will rise therein and flow into the chamber 22. There will however as a result be a fall in the pressure of fuel delivered by the low pressure pump and the piston means will move downwardly under the action of the spring 32 to close the port 21. In this situation the flow of air into the chamber 22 will be halted and while some of the air can escape from the outlet of the low pressure pump by way of a capillary passage 36 which is connected to the upper end of the chamber 22, a substantial amount of the air will flow to the high pressure pump.

As a result the engine will be starved of fuel and its power output will fall. The operator of the engine should notice this loss of power and investigate the cause but if he does not do this the engine will eventually stop due to lack of fuel. With the engine halted the pressure in the lower portion of the cylinder 20 will fall to the pressure in the fuel tank 17 by reason of the capillary passage 36 and this will permit the part 29 of the piston means to move under the action of the spring 30, the part 28 having been retained against movement by the step 33. The force exerted by the spring 30 is deliberately chosen so that the movement of the part 29 of the piston means is comparatively slow. The rate of movement will also depend upon the viscosity of the fuel, the clearance between the part 29 and the wall of the cylinder and the size of the vent 34, it being arranged that the flange 31 under the action of the spring 32 forms a seal with the step 33. As the piston descends during which time the operator of the vehicle should recharge the fuel tank, a point will be reached at which the port 26 and passage 37 are closed and the ports 21 and 27 reopened to the cylinder and hence placed in communi cation with each other. Once this communication has been established the fuel in the chamber 22 is available at the inlet 14 of the high pressure pump, the volume of fuel being sufficient to allow the engine to be restarted.

With the low pressure pump now in operation fuel is drawn from the fuel tank but the air which is ahead of the fuel is delivered to the outlet 15 of the low pressure pump and flows by way of the capillary passage 36 and the vent 35, to the fuel tank. The non-return valve 25 acts to prevent the air being recirculated to the inlet 16 of the low pressure pump. As the fuel pressure at the outlet of the low pressure pump builds up, the piston part 29 will be moved upwardly thereby temporarily covering the ports 21 and 27 and uncovering the port 26 to allow the fuel to be supplied to the inlet of the high pressure pump. The vent 34 allows the piston part 29 to move upwardly against the action of the spring 30 but in the event that there is a rapid increase in the pressure, the part 28 of the piston means will be lifted to break the seal between the flange 31 and the step 33.

Although as illustrated the axis of the cylinder is vertical, it can be horizontal or at some intermediate inclination since the piston part 29 is moved by the action of the spring 30.

Claims

1. A fuel supply system for an internal combustion engine, the system being of the kind comprising a high pressure pump and a low pressure pump which supplies fuel under pressure to the high pressure pump, the latter supplying fuel at high pressure to the associated engine, the low pressure pump drawing fuel from a fuel supply tank and the two pumps being driven by the associated engine, the system further comprising a pressure regulating valve including a resiliently loaded piston means which is subjected to the output pressure of the low pressure pump and is slidable in a cylinder, a port formed in the wall of said cylinder and through which fuel can escape to a low pressure source, said port being uncovered by said piston means to control the output pressure of the low pressure pump, a reservoir chamber communicating with said port and which receives fuel flowing through said port, said piston means comprising first and second parts movable relative to each other, first resilient means acting between said parts, second resilient means acting on said first part, means for limiting the extent of movement of said first part under the action of the second resilient means, the arrangement being such that in the event that the outlet pressure of the low pressure pump falls, said first part will move to the limit of its movement and the second part will continue to move under the action of said second resilient means to a position in which said reservoir is connected to the inlet of the high pressure pump and the connection between the outlet of the low pressure pump and the inlet of the high pressure pump is broken.

2. A fuel supply system according to Claim 1 including a step formed in the wall of said cylinder, said step being engaged by the first part of said piston means to limit the movement thereof and forming therewith a seal whereby the continued movement of the second part of the piston means is at a restricted rate.

3. A fuel supply system according to Claim 2 in which said second part of the piston means is of cylindrical form and controls flow through said port, the cylinder having a second port formed therein which is aligned with said first mentioned port, said second part of the piston means uncovering said ports to allow fuel flow therebetween when at its limit of movement, the second port being connected to the inlet of the high pressure pump.

4. A fuel supply system according to Claim 3 including a third port in the wall of the cylinder, said third port being connected to the inlet of the high pressure pump and being closed at the limit of movement of the second part of the piston means, said third port during normal operation connecting the outlet of the low pressure pump with the inlet of the high pressure pump.

5. A fuel supply system according to any one of the preceding claims including a restricted vent from the upper end of said chamber to the fuel tank.

6. A fuel supply system according to Claim 5 including a return passage to the inlet of the low pressure pump and through which fuel flows from the upper end of said chamber, said return passage including a non-return valve which is opened to allow said return flow.

7. A fuel supply system according to Claim 6 including a capilliary passage connecting the outlet of the low pressure pump with the upper end of the chamber.

8. A fuel supply system according to Claim 3 including a vent in the first part of the piston means.

9. 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 drawing.