This invention relates to fuel systems for internal combustion engines of the compression ignition type and of the kind comprising an engine cam actuated pump/injector which in use, is mounted on the cylinder head of an engine, the pump/injector including a nozzle having a fuel pressure actuated valve member which is spring biased into engagement with a seating to prevent flow of fuel through an outlet orifice, the valve member defining an annular area against which the fuel under pressure can act to develop a force to lift the valve member from the seating thereby to allow fuel flow through the outlet orifice.

Fuel systems of the aforesaid type are well known in the art and in use, when the valve member is lifted from the seating the area of the valve member exposed to the fuel pressure is increased and the increased area compensates for the fall in pressure which occurs as fuel flow takes place through the outlet orifice. The practical effect is that the valve member continues to move away from the seating to its open position.

It is desirable to reduce the volume of fuel which is delivered to an engine combustion chamber during the so called ignition delay period, this period being the time interval between the start of fuel delivery to the combustion chamber and the commencement of combustion therein. The reduction of fuel delivery during the ignition delay period reduces engine noise and also reduces the hydrocarbon content of the engine exhaust.

It has been proposed to construct the injection nozzles so that the valve member has two stages of lift, the first stage providing for restricted flow of fuel to the engine. Various methods have been proposed for example it is known to provide two springs in the nozzle with one spring controlling the initial lift of the valve member and the other spring in conjunction with the one spring controlling the second stage of lift. In another construction one spring only is provided and the initial movement of the valve member is assisted by the pressure of fuel acting on a piston member which has a limited stroke. Such nozzle constructions are complex and more expensive and generally have larger dimensions than standard nozzles. The increased size of the nozzle makes it difficult to accommodate the nozzle in small to medium size engines.

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

According to the invention in a fuel system of the kind specified the spring rate of the spring in the nozzle is increased to a level such that at least up to a predetermined engine speed, the valve member following its initial movement away from the seating moves towards the seating to restrict the rate of flow of fuel to the engine.

An example of a fuel system in accordance with the invention will now be described with reference to the accompanying drawings in which:-

Figure 1 is a sectional side elevation of a fuel injection nozzle forming part of the system,

Figure 2 shows some operating characteristics of the fuel injection nozzle shown in Figure 1 , and

Figure 3 corresponds to Figure 2 with the nozzle incorporating the modification of the invention.

Referring to the drawings the fuel system comprises a pump/injector including a fuel injection nozzle generally indicated at 1 1. The injection nozzle is mounted on a body part 10 of the pump/injector and comprises a nozzle body 12 in which is formed a blind bore 13. At the blind end of the bore is formed a seating 14 and downstream of the seating is a sac volume from which extends a plurality of outlet orifices 15. Intermediate the ends of the bore there is defined an enlargement 16 which communicates with a passage 17 in the nozzle body, the passage being connected to a further passage 18 formed in the body part 10, the passage 18 communicating with the pumping chamber of the pump/injector. The pump/injector incorporates an engine cam actuated pumping plunger.

The nozzle body 12 is secured by means of a cap nut 20 to the body part 10 and a distance piece 21 is interposed between the body part and the nozzle body.

Slidable in the bore 13 is a valve member 22 which has an extension 23 projecting through an aperture in the distance piece into a chamber 24 defined in the body part. Mounted on the extension 23 is a spring abutment 25 against which bears one end of a coiled compression spring 26 the other end of which engages a further abutment 27 located against

an end wall of the chamber. The chamber is vented to a low pressure by way of a passage 28.

The valve member at its end remote from the extension is shaped for engagement with the seating 14 and the portion of the valve member which extends between the enlargement and the seating is of reduced diameter so as to permit fuel flow from the enlargement when the valve member is in the open position, to the outlet orifices 15.

In operation, when fuel under pressure is supplied from the pumping chamber, the fuel pressure acts on the annular end area of the valve member lying outside the seating 14 and a force is developed on the valve member in opposition to the force exerted by the spring. When the pressure increases to a sufficiently high value known in the art as the nozzle opening pressure, the valve member moves against the action of the spring away from the seating to permit fuel flow past the seating through the outlet orifices. As soon as the valve member is lifted from the seating the end portion of the valve member which lies within the seating area is also exposed to the high pressure of fuel to provide an additional force urging the valve member away from the seating. When the valve member is first lifted from its seating and fuel flow through the orifice takes place, the pressure of fuel falls slightly but in the conventional nozzle the increase in the area of the valve member exposed to the pressure compensates for the lowering of the fuel pressure and the valve member moves progressively away from its seating. This is shown in the lower diagram of Figure 2. The middle diagram in Figure 2 shows the rate at which fuel flows through the orifices 15 to the engine and the upper diagram shows the fuel pressure which is applied to the valve member.

In order to effect a reduction in the rate of delivery and in accordance with the invention, the rating of the spring is increased substantially a typical value lying between 5 and 6 times the standard rate. The effect is illustrated by the diagrams in Figure 3. When the valve member starts to move away from its seating there is a substantial increase in the force exerted on the valve member due to the very high rating of the spring and as a result the lowering in pressure is no longer compensated for by the increase in the area of the valve member exposed to the pressure. The result is that the valve member starts to move towards its seating and in so doing the rate of fuel delivery is reduced. The lower diagram in Figure 3 shows that the valve member starts to return towards its closed position owing to the reduction in pressure and then it again starts to move towards the fully open position as the pressure increases. The central diagram of Figure 3 shows how the rate of fuel delivery diminishes until the movement of the valve member away from its seating is re-established.

The effect described depends upon matching the spring rate and the rate at which fuel is delivered from the pumping chamber. The effect described therefore will be present up to a predetermined engine speed and above this speed the rate at which the fuel pressure increases will be sufficient to ensure that the valve member moves steadily to its fully open position as in injection nozzles having a lower rated spring. In one example the spring rate is 1250N/mm.

The pump/injector incorporates an electromagnetically operable spill valve and the higher rating of the spring enables the valve member to close onto the seating at a rate which matches the rate at which fuel spills through the spill valve. In addition as compared with a nozzle

having a standard spring there is a reduced risk of combustion gases being forced past the seating.