GB2110300A

GB2110300A - Fuel injection system for a compression ignition engine

Info

Publication number: GB2110300A
Application number: GB08231660A
Authority: GB
Inventors: Erick Joseph Cavanagh
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1981-11-27
Filing date: 1982-11-05
Publication date: 1983-06-15

Abstract

A high pressure fuel pump supplies fuel through a pipeline to an injection nozzle and in or adjacent the nozzle is an accumulator connected to the inlet of the nozzle. The pipeline is connected to the accumulator by a non-return valve which allows fuel flow to the accumulator but prevents flow in the reverse direction. Any secondary injection of fuel due to pressure waves in the accumulator occurs very quickly after the main delivery of fuel so that the displacement of fuel to the engine is effectively continuous.

Description

SPECIFICATION Fuel system for compression ignition engine This invention relates to a fuel system for a compression ignition engine of the kind comprising a pump for supplying liquid fuel at a high pressure, an injection nozzle through which fuel is directed into a combustion space of the engine, a pipeline connecting the outlet of the pump with the inlet of the injection nozzle and the nozzle including a resiliently loaded valve member which is moved to the open position by the fuel under pressure delivered by the pump.

Such systems are well known in the art and in general are designed so that secondary injection of fuel from the nozzle into the combustion chamber following the main delivery of fuel, is avoided. Secondary injection of fuel can occur due to a pressure wave in the column of fuel contained in the pipeline and which upon closure of the valve in the nozzle following delivery of fuel, travels back towards the pump and is reflected back towards the nozzle. Every effort is made in most systems of this type to prevent secondary injection of fuel to the reflected pressure wave since in most cases, the fuel is not properly burnt in the combustion chamber of the engine and therefore leads to smoke in the exhaust of the engine.

There are some engines where it is desirable to prolong the period of fuel delivery without reducing to any significant extent, the rate of fuel delivery and it is desirable to be able to use standard equipment. Such an engine for example is one which is designed to operate on a low quality fuel which is supplied to a combustion chamber of the engine through a separate injection system and which also requires the injection of some high quality fuel to ensure ignition and satisfactory burning of the low quality fuel. The low quality fuel which may for example be a heavy oil, will in general be injected into the combustion chamber over a period considered in terms of degrees of crankshaft rotation, which is longer than the accepted period when high quality fuel is being delivered.It is however desirable that the injection of the high quality fuel should continue for as long as possible during the delivery of the low quality fuel. Whilst a system for delivering the high quality fuel at the desired rate could be designed it would be a "special" system which might not have much application to other types of engine. As a result the system would be expensive. It is therefore an object of the present invention to provide a system of the kind specified in which the period of fuel delivery is extended using secondary injection.

According to the invention a system of the kind specified comprises an accumulator disposed at or adjacent the injection nozzle, the inlet of the injection nozzle communicating with the accumulator, an inlet to the accumulator said inlet being connected to the outlet of the pump through said pipeline and a non-return valve in said accumulator inlet, said non-return valve being arranged to allow fuel flow into said accumulator from the pump but to prevent fuel flow in the opposite direction.

In the accompanying drawings: Figure 1 is a diagrammatic view of a conventional fuel system; and Figure 2 shows the modification in accordance with the invention, to the fuel system shown in Figure 1.

The fuel system comprises a high pressure injection pump which is indicated at 10 and which has an outlet connected by way of a pipeline 11, to a fuel injection nozzle generally indicated at 12. The fuel injection nozzle is of the conventional inwardly opening type well known in the art and including a valve member which is biased to the closed position by a spring. The injection pump 10 is driven in timed relationship with the associated engine and it may be of the rotary distributor type or of the type known in the art as an "in-line" pump. It includes a pumping plunger and an outlet which is controlled by a delivery valve which may be of the unloading type.

In operation, fuel is delivered by the injection pump and the pressure wave generated at the start of fuel delivery travels along the pipeline 11 to effect opening of the spring loaded valve contained within the nozzle. The flow of fuel through an orifice of the nozzle occurs until the delivery of fuel by the injection pump terminates.

When the flow of fuel terminates the valve member in the nozzle closes quickly since it is biased by a strong spring and the effect of this is that a pressure wave is generated in the nozzle which travels back along the pipeline to the pump 1 0. The effect of the unloading delivery valve is to allow a predetermined quantity of fuel to escape from the pipeline 11 before the delivery valve closes. in this manner the residual pressure in the pipeline 11 is reduced and this helps the valve member in the nozzle to close quickly.The pressure wave in the column of fuel in the pipeline 11 and which is generated by closure of the valve member of the nozzle is reflected back along the pipeline owing to the fact that the delivery valve is closed and the pressure wave may be sufficient to cause momentary opening of the valve member of the nozzle so that so-called secondary injection of fuel occurs. Every effort is made in conventional fuel systems to minimise the risik of secondary injection since it can lead to poor combustion of the fuel which is delivered to the engine. The unloading delivery valve does help to reduce secondary injection.Moreover, because of the fact that the pressure wave has to travel twice the length of the pipeline, there is an interval between the termination of the main flow of fuel through the injection nozzle and the commencement if it takes place, of the secondary injection of fuel.

The known form of fuel system shown in Figure 1 is modified as shown in Figure 2, by the provision of an accumulator 13 connected to the inlet of the nozzle. The pipeline 11 is connected to the accumulator by way of a non-return valve 14 which is arranged to permit flow of fuel from the pipeline 11 into the accumulator but to prevent flow of fuel in the opposite direction. The valve 14 will close as soon as delivery of fuel by the pump 10 ceases. When the valve member of the injection nozzle closes the pressure wave which is generated is reflected by the closed non-return valve 14 and the reflected wave will open the valve member of the fuel injection nozzle almost immediately it has closed. The accumulator 13 stores a volume of fuel which is delivered during the secondary injection of fuel. Thus whilst the valve member iru ehe fuel injection nozzle does close following the cessation of delivery of fuel by the pump, it is opened very quickly to allow the secondary injection of fuel, and the delivery of fuel to the engine in the two stages, can be said to be continuous.

The accumulator 13 is shown in Figure 2 to be exterior of the fuel of the fuel injection nozzle. It may however be incorporated in the design of the nozzle as also may the non-return valve.

Claims

1. A fuel system for a compression ignition engine comprising a pump for supplying liquid fuel at a high pressure, an injection nozzle through which fuel is directed into a combustion space of the engine and a pipeline connecting the outlet of the pump with the inlet of the injection nozzle, the nozzle including a resiliently loaded valve member which is moved to the open position by the fuel under pressure delivered by the pump, an accumulator disposed at or adjacent the nozzle, the inlet of the injection nozzle communicating with the accumulator, an inlet to the accumulator said inlet being connected to the outlet of the pump through said pipeline and a non-return valve in said accumulator inlet, said non-return valve being arranged to allow flow of fuel into said accumulator from the pump but to prevent fuel flow in the opposite direction.

2. A fuel system for a compression ignition engine comprising the combination and arrangement of parts substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings as modified by Figure 2 thereof.