GB2177158A - I.C. engine fuel supply system - Google Patents

Abstract

A low pressure pump 14 supplies fuel to a high pressure pump 10 by way of a conduit 12. A reservoir chamber 18 has an opening from its lower end to the conduit the opening incorporating a spring loaded valve member which in normal use remains closed. A passage 25 connects the conduit with the upper portion of the chamber and from the upper portion of the chamber extends a cylindrical vent pipe 20 in which is slidable a piston 22 which defines a small clearance with the vent pipe. In the event of the low pressure pump pumping air due to fuel starvation and the engine stopping, the piston descends slowly and opens the valve to allow the fuel contained in the chamber to flow into the conduit thereby allowing the engine to be restarted. <IMAGE>

Description

SPECIFICATION Fuel supply system for an internal combustion engine 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 quanities of air will have little effect upon the operation of the engine and this is mainly 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 by the high pressure pump to the engine. The surplus fuel is returned to the inlet of the low pressure pump by way 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 but this has the disadvantage that during normal operation of the system the rate of fuel delivered 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 an additional power loss. Moreover, the low pressure 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 unit. 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 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 also 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 chamber is provided with a small opening through which the fuel in the chamber 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 element changed, 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 reservoir chamber, a spring loaded valve in the base wall of the chamber, said valve when open allowing fuel contained within the chamber to flow into a conduit connecting the outlet of the low pressure pump with the inlet of the high pressure pump, a vent pipe extending upwardly from the upper wall of the chamber, a piston slidable in the vent pipe, said piston being aligned with said valve, a restricted flow path between the ends of said vent pipe, passage means connecting said conduit with the upper portion of said chamber, said piston when the output pressure of the low pressure pump falls, slowly descending in the vent pipe into contact with the valve to open same thereby to allow fuel flow into the conduit from the chamber and opening a restricted opening from the upper portion of the chamber.

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

Referring to the drawing the fuel system comprises a high pressure pump 10 having a plurality of outlets 11 A for connection respec tively to the injection nozzles of an associated engine. The high pressure pump has an inlet 11 which is connected by way of a conduit 12 to the outlet 13 of a low pressure pump 14, the two pumps being driven from a common drive shaft which is driven in timed relationship with the associated engine by means of a rotary part thereof. The low pressure pump has a fuel inlet 15 which is connected by way of filter unit 16 to a fuel tank 17. The inlet and outlet of the low pressure pump are interconnected by a pressure relief valve not shown whereby the output pressure of the low pressure pump is controlled preferably in accordance with the speed of the associated engine.

The system includes a reservoir chamber 18 in the base wall of which is located a valve including a spring loaded valve member 19 the valve when open permitting fuel contained in the chamber 18 to flow into the conduit 12.

Extending from the upper wall of the chamber is a vent pipe 20 which is of cylindrical form and which at its end remote from the chamber is connected by means of a vent conduit 21 to the fuel tank 17 the latter being of the vented type.

Slidable within the vent pipe is a piston 22 the latter forming with the wall of the vent pipe a restricted flow path between the ends of the vent pipe. Conveniently the clearance between the piston and wall of the vent pipe is of the order of 60 microns. The piston is aligned with the valve member 19 and extending downwardly within the chamber is a guide arrangement 23 which may be of cylindrical form but which has an internal diameter substantially larger than that of the piston. The upper portion of the guide is apertured at 24 so as to allow as will be explained, communication between the upper portion of the chamber and the vent pipe. Also provided is a passage 25 which connects the conduit with the upper portion of the chamber. This passage contains a non-return valve 26 arranged to prevent flow of fuel downwardly from the top of the chamber.

In operation, when the system is functioning correctly the piston assumes the position in which it is shown in the drawing and any small bubbles of air which may be contained in the fuel delivered by the low pressure pump to the conduit, flow up the passage 25 past the non return valve 26 and escape from the upper portion of the chamber along the restricted clearance defined between the piston and the wall of the vent pipe. The air is returned to the fuel tank along the vent conduit 21. In the event that the fuel tank becomes exhausted of fuel, air will be drawn from the fuel tank by the low pressure pump and eventually will pass from the outlet 13 of the low pressure pump into the conduit 12.The initial volume of air may be discharged as described, back to the fuel tank but eventually the volume of air will increase to such an extent that it will travel along the conduit 12 into the inlet of the high pressure pump. The engine will therefore start to receive less fuel than required and its power output will fall. The drop in power should be noticed by the operator of the engine who should stop the engine to investigate the reason for the reduced power. If however he does not do this a time will come when the engine will stall due to lack of fuel.

During this time the piston 22 will remain in the position shown due to the pressure developed by the low pressure pump. With the engine at rest however the pressure at the outlet 13 of the low pressure pump falls and the piston 22 will descend gradually into the chamber which is full of fuel. In this situation the valve 26 is closed to prevent displacement of fuel down the passage 25. The rate of descent of the piston will be extremely slow but eventually it will contact the valve member 19 and urge same against the action of its spring, off its seating and thereby allow fuel to flow from the chamber into the conduit 12. The upper end of the piston forms with the openings 24 a restricted opening from the upper portion of the chamber into the vent pipe.

Once the valve member 19 has been moved to the open position fuel is available at the inlet of the high pressure pump for the purpose of starting the engine and once the engine has started the low pressure pump will start to draw fuel from the fuel tank. Initially however air will be delivered at the outlet 13 of the pump and this air will travel up the passage 25 and through the restricted opening into the vent pipe without hindrance, the air then being returned to the fuel tank. As the low pressure pump starts to draw fuel, its outlet pressure will start to increase and a pressure drop will be generated across the restricted opening defined by the piston and the openings 24. This pressure drop is applied between the ends of the piston and when the pressure drop increases to a sufficient extent the piston will move upwardly to the position in which it is shown in the drawing. The chamber 18 will stiii contain air but this will be gradually vented along the restricted clearance defined between the piston and the vent pipe and the chamber will gradually be refilled with fuel.

The aforementioned clearance between the piston and the wall of the vent pipe is important since it is desirable that the piston should not open the valve for at least five minutes after the engine has finally stopped. The reason for this is that if it were to open immediately then it would be possible for the operator of the engine to immediately restart the engine and consume the fuel contained in the chamber, without having refilled the fuel tank.

The valve 26 acts to ensure that the fuel dis placed by the piston during its downward movement, flows along the clearance between the piston and the wall of the vent pipe and not by way of the passage 25 into the conduit 12. An alternative position for the valve is in the vent conduit 21, such a valve being indicated at 27. In this case when the output pressure of the low pressure pump falls and the piston starts to descend, a depression is created above the piston and this acts to ensure that the fuel flows along the clearance so that the descent of the piston is at the required rate.

Even with the valve 26 in the passage 25 a valve 27 may be necessary to ensure that the fuel from the chamber once the valve 19 has opened, does not drain back by way of the low pressure pump to the fuel tank.

The valve member 19 is of generally truncated conical form with its narrower portion uppermost and this minimises the possibility of the valve becoming stuck in the closed position due to silt collecting in the chamber.

Moreover, the mass of the valve member in relation to the force exerted by the spring and the centre of gravity of the valve member are chosen such that the valve member will vibrate relative to its seating due to vibration of the vehicle in which the engine is mounted.

This action will tend to allow any silt collecting in the base of the chamber to fall into the conduit thereby reducing the amount of silt supplied to the high pressure pump when the engine is stopped. It will be appreciated of course that each time the engine is stopped the piston will descend into engagement with the valve member but under normal conditions of operation, will return quickly to the position in which it is shown once the engine has been started.

The piston movement in the direction to open the valve 19 can be assisted by a spring. In this event the clearance between the piston and the wall of the vent pipe 20 can be reduced. Moreover, if desired, the axis of movement of the piston need not be vertical. In this case a spring will be required to assure movement of the piston.

Claims (7)

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 reservoir chamber, a spring loaded valve in the base wall of the chamber, said valve when open allowing fuel contained within the chamber to flow into a conduit connecting the outlet of the low pressure pump with the inlet of the high pressure pump, a vent pipe extending upwardly from the upper wall of the chamber, a piston slidable in the vent pipe, said piston being aligned with said valve, a restricted flow path between the ends of said vent pipe, passage means connecting said conduit with the upper portion of said chamber, said piston when the output pressure of the low pressure pump falls, slowly descending in the vent pipe into contact with the valve to open same thereby to allow fuel flow into the conduit from the chamber and opening a restricted opening from the upper portion of the chamber.

2. A system according to Claim 1 including a one way valve in said passage means said one way valve acting to prevent fuel flow along said passage means towards said conduit.

3. A system according to Claim 1 or Claim 2 including a vent conduit connecting the upper portion of said vent pipe with the fuel tank and a non-return valve in said vent conduit, said non-return valve opening to permit flow in the direction towards the fuel tank but closing to prevent flow in the reverse direction.

4. A system according to any one of the preceding claims in which said valve includes a valve member of generally truncated conical form with is narrower portion uppermost to minimise the possibility of the valve member sticking in the closed position due to the collection of silt in the chamber.

5. A system according to Claim 4 in which the mass of the valve member, its centre of gravity and the force of its spring are so arranged that in the use of the engine, the valve member will vibrate relative to its seating to allow silt collecting in the chamber to fall into the conduit.

6. A system according to any one of the preceding claims including a spring acting on the piston to urge the piston in the direction to open the valve.

7. 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.