GB2176840A - Engine fuel supply system - Google Patents

Abstract

A low pressure pump 13 supplies fuel to a high pressure pump 10 and draws fuel from a supply tank 18. A reservoir 19 is provided which is divided into upper and lower chambers 20, 21 the lower chamber being connected to the outlet 15 of the low pressure pump and the inlet 11 of the high pressure pump. A restricted opening 22 connects the two chambers and there is a restricted outlet 24 from the upper chamber through which air can vent. When the fuel tank is allowed to run dry air is delivered to the high pressure pump and eventually the associated engine stops. The fuel in the upper chamber can then flow into the lower chamber to allow the engine to be started. <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 quantities 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 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 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 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 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 is provided with a small opening through which the fuel can flow 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 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. The volume of the reservoir chamber can therefore be considerable.Moreover, once the engine has been started all the air in the system upstream of the low pressure pump including that in the reservoir must be drawn through the low pressure pump. The rate at which fuel is drawn through the system from the supply tank depends upon the amount of fuel delivered to the engine and the amount of air expelled through the air venting arrangement of the system. It is therefore independent of the rate of displacement of the low pressure pump since excess fuel delivered by the low pressure pump and also air, is bypassed back to the inlet thereof through the relief valve.

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

According to the invention a fuel supply system of the kind specified comprises a reservoir defining upper and lower chambers, said lower chamber being connected to the outlet of the low pressure pump and the inlet of the high pressure pump, a restricted opening through which fuel can flow between the chambers and a restricted outlet from the upper chamber through which air therein can be vented.

An example of a fuel system in accordance with the invention will now be described with reference to the accompanying diagrammatic drawing. The fuel system comprises a high pressure pump 10 having a fuel inlet 11 and a plurality of outlets 12 for connection to the injection nozzles respectively of an associated engine. The high pressure pump is driven in timed relationship with the associated engine by means of a drive shaft (not shown) which also drives a low pressure pump 13 having an inlet 14 and an outlet 15. The outlet 15 is connected to the inlet 14 by way of a pres sure relief valve 16 which controls the outlet pressure of the low pressure pump conveniently so that it varies in accordance with the speed of rotation of the associated engine.

The inlet 14 of the low pressure pump is connected by way of a filter unit 17, to a fuel supply tank 18 which is of the vented variety.

Intermediate the outlet 15 of the low pressure pump and the inlet 11 of the high pressure pump there is disposed a reservoir generally indicated at 19 and this defines upper and lower chambers 20, 21. The two chambers are separated by a horizontal wall in which is formed a restricted opening 22.

Moreover, extending upwardly from the wall is a vent tube 23. The outlet 15 of the low pressure pump and the inlet 11 of the high pressure pump are connected to the lower chamber 21 and the upper chamber 20 is provided with a vent 24 connected to the interior of the fuel tank, the vent incorporating a restrictor. The reservoir is located in the same housing as the pumps.

In operation, the two chambers in the reservoir are normally filled with fuel and a small quantity of fuel is returned from the reservoir through the vent 24. The low pressure pump supplies fuel to the high pressure pump, the outlet pressure being controlled by the valve 16 whichalso serves to return the excess fuel to the inlet of the low pressure pump.

In the event that the tank becomes empty of fuel, air will start to be drawn through the inlet of the low pressure pump and will be supplied through the outlet 15 and will tend to collect in the chamber 21. The air in this chamber will pass up the vent tube 23 and will be displaced back to the fuel tank by way of the vent 24. As the proportion of fuel which is pumped decreases, the amount of air supplied at the outlet of the low pressure pump will increase to the extent that the vent 24 is no longer able to return the air to the tank and some air will flow to the high pressure pump. This air will tend to reduce the amount of fuel supplied by the high pressure pump and the operator of the engine will notice a loss of power. If he does not stop the engine to ascertain the cause, the latter will eventually stop because of lack of fuel.Once the engine has stopped the fuel in the chamber 20 will leak through the restricted opening 22 into the now empty lower chamber 21 and assuming that the fuel tank has been replenished, when the engine is cranked, the fuel now in the lower chamber 21, will be urged towards the high pressure pump which eventually will supply fuel to the engine to allow the latter to start. Once the engine has started the low pressure pump will draw fuel from the tank and supply the fuel along with air, to the lower chamber. Some air may continue to be supplied to the high pressure pump but as the amount of air in the system gradually diminishes, the vent 24 will be able to cope with all the air which flows through the outlet of the low pressure pump and gradually the two chambers 20 and 21 will become full of fuel and the system purged of air.

It will of course be appreciated that the vent 24 will allow only a small flow of fuel but a high rate of air flow and while air is flowing through the vent 24 the amount of fuel and air flowing through the outlet of the low pressure pump into the chamber 21 will be substantially equal to the theoretical displacement of the pump. As the air flow through the vent decreases, the regulating valve 16 will come into operation to control the pressure at the outlet of the pump and will bypass a proportion of the fuel flowing through the outlet of the pump.

It will be seen therefore that by positioning the reservoir between the low pressure pump and the high pressure pump, the volume of air contained in the reservoir after transfer of fuel between the upper chamber and lower chamber has taken place, does not influence the amount of fuel which is needed to start and maintain the engine in a running condition while the air purging process takes place. The size of the reservoir can therefore be substantially reduced as compared with the size of the reservoir required in the fuel system disclosed in British Published Application 2132698A.

Claims (4)

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 engine, the system further including a reservoir defining upper and lower chambers, said lower chamber being connected to the outlet of the low pressure pump and the inlet of the high pressure pump, a restricted opening through which fuel can flow between the chambers and a restricted outlet from the upper chamber through which air therein can be vented.

2. A fuel supply system according to Claim 1 including a vent tube extending between the upper portions of the chambers.

3. A fuel supply system according to Claim 1 or Claim 2 in whch said restricted outlet is connected to the fuel tank.

4. 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.