Fuel injector

Abstract

 A fuel injector comprising a nozzle body (10) provided with a first bore (11) defining a seating (12), a valve needle (19) being reciprocable within the bore (11) and engageable with the seating (12) to control the delivery of fuel through an outlet opening (13). The fuel injector comprises an elongate guide member (16) mounted within the bore (11), the guide member (16) being provided with a second bore (18) within which part of the valve needle (19) is received to guide the valve needle (19) for movement within the first bore (11).

Description

[0001] This invention relates to a fuel injector for use in delivering fuel under high pressure to a combustion space of an associated compression ignition internal combustion engine.

[0002] A typical fuel injector comprises a valve needle slidable within a bore formed in a nozzle body and engageable with a seating to control fuel delivery through one or more outlet openings which communicate with the bore downstream of the seating. Movement of the valve needle occurs rapidly, in use, the periods during which fuel injection occurs being of short duration. Further, in order to reduce the levels of noise and particulate contaminants produced by an engine, in some applications the injector may be required to open to deliver a small, pilot injection of fuel, to close, and to open after a short delay to deliver a main injection. It has been found that some fuel injectors are unable to operate sufficiently quickly for use in such applications. It is an object of the invention to provide a fuel injector which is suitable for use in such applications.

[0003] According to the present invention there is provided a fuel injector comprising a nozzle body provided with a first bore defining a seating, a valve needle being reciprocable within the bore and engageable with the seating to control the delivery of fuel through an outlet opening, and an elongate guide member mounted within the bore, the guide member being provided with a second bore within which at least a part of the valve needle is received to guide the valve needle for movement within the first bore.

[0004] Such an arrangement is advantageous in that the valve needle can be of relatively small dimensions and low mass. The inertia of the valve needle can thus be reduced, resulting in improved responsiveness.

[0005] The guide member conveniently further serves as a stop, limiting the distance through which the valve needle can lift from the seating. The guide member may serve as a spring abutment for a spring arranged to bias the valve needle towards its seating.

[0006] The guide member may define, with the nozzle body, a flow path for fuel flowing towards the outlet opening. The guide member may be provided with formations forming a restriction to fuel flow along the flow path.

[0007] Where the injector is to be used in, for example, a common rail type fuel system, then the valve needle conveniently defines, with the guide member, a control chamber, the guide member further defining a passage whereby fuel can be supplied to or vented from the control chamber to vary the fuel pressure within the control chamber.

[0008] The invention will further be described, by way of example, with reference to Figure 1 which is a sectional view of part of a fuel injector in accordance with an embodiment of the invention.

[0009] The fuel injector illustrated, in part, in Figure 1 comprises a largely typical nozzle body 10 provided with a blind bore 11, the blind end of which defines a seating 12. Downstream of the seating 12, a plurality of outlet openings 13 of small diameter communicate with the bore 11. The bore 11 is shaped to include a region of enlarged diameter defining an annular gallery 14 which communicates with a drilling 15 forming part of a supply passage for supplying fuel to the bore 11, in use.

[0010] Located within the bore 11 is a guide member 16 of elongate form. The guide member 16 includes, at its upper end in the orientation illustrated, a region of enlarged diameter, this region being of diameter substantially equal to the diameter of the adjacent part of the bore 11 and forming a substantially fluid tight seal between the guide member 16 and the upper end of the bore 11. The lower end of the guide member 16 is also of diameter substantially equal to that of the bore 11, and is provided with flute formations 17 permitting fuel from the supply passage to flow along a flow path defined by the bore 11 towards the seating 12.

[0011] The guide member 16 is provided with an axially extending through bore 18 of stepped form. The lower end of the bore 18 is of relatively large diameter and serves to guide a valve needle 19 for reciprocating movement within the bore 18, the valve needle 19 being engageable with the seating 12 to control the supply of fuel from the bore 11 to the outlet openings 13, thus controlling fuel delivery from the injector. The bore 18 defines a step or shoulder 20 against which the upper end of the needle 19 is engageable to limit the distance through which the needle 19 is movable, in use. The guide member 16 further defines a second step 21 which serves as a spring abutment for a spring 22 located between the spring abutment and the upper end surface of the needle 19, the spring 22 applying a biasing force to the needle 19 urging the needle 19 towards the seating 12. The upper end of the bore 18 communicates through passages (not shown) provided in other parts of the fuel injector with a source of fuel under low pressure, for example the fuel reservoir or tank associated with the engine.

[0012] In use, the supply passage of which the drilling 15 forms part may be connected to an outlet of a rotary distributor pump. When the supply passage is at relatively low pressure, then the bore 11 will be at low pressure, and the force applied by the fuel within the bore 11 to the exposed angled thrust surfaces 19a of the needle 19 will be relatively low. In these circumstances, the spring 22 will apply a sufficient force to the needle 19 to ensure that the needle 19 remains in engagement with the seating 12.

[0013] As the rotary distributor pump operates, the fuel pressure within the bore 11 will rise, thus the magnitude of the force applied to the valve needle 19 urging the valve needle 19 away from the seating 12 will rise, and a point will be reached beyond which the valve needle 19 will be able to move against the action of the spring 22, thereby lifting away from the seating 12 and allowing fuel to flow from the bore 11 to the outlet openings 13 to be delivered into the combustion space of the engine with which the injector is associated. As the needle 19 moves, fuel within the bore 18 is displaced towards the low pressure reservoir. The movement of the needle 19 is limited by the step 20. The flutes 17 are of sufficient dimensions such that the rate at which fuel is able to flow along the flow path towards the seating 12 is not significantly restricted.

[0014] It will be appreciated that the guide member 16, which is held in a substantially concentric position within the bore 11, guides the movement of the needle 19, ensuring that the needle 19 remains substantially concentric with the bore 11 whilst lifted away from the seating 12, permitting uniform fuel flow to all of the outlet openings 13.

[0015] When injection is to be terminated, the fuel pressure within the bore 11 is reduced. This may be achieved simply by terminating the supply of fuel to the bore 11, the continued delivery of fuel through the outlet openings 13 gradually causing the fuel pressure within the bore 11 to fall. Alternatively, the fuel pressure may be allowed to fall by opening a spill valve to vent fuel from the bore 11, thereby relieving the fuel pressure applied to the thrust surfaces 19a of the valve needle 19. Regardless as to how the fuel pressure within the bore 11 is reduced, the reduction in fuel pressure reduces the magnitude of the force urging the valve needle 19 away from the seating 12, and a point will be reached beyond which the valve needle 19 is returned into engagement with the seating 12 under the action of the spring 22. Once such engagement has been achieved, fuel is no longer able to flow to the outlet openings 13, thus fuel injection is terminated.

[0016] Although in the description hereinbefore, the injector is described with reference to a fuel system incorporating a rotary distributor pump, it will be appreciated that the injector could be used with other types of fuel system in which the timing of fuel injection is controlled by controlling the magnitude of the fuel pressure applied to the bore 11. The injector may alternatively be used in, for example, a common rail fuel system. In such a fuel system, the drilling 15 is connected ,through the supply passage, with a source of fuel under high pressure. The bore 11 is therefore supplied, continuously, with fuel under high pressure, thus the force applied to the valve needle 19 urging the valve needle 19 away from the seating 12 is high. In such an arrangement, the upper end surface of the needle 19 defines, with the bore 18, a control chamber, the fuel pressure within which applies a force to the needle 19 urging the needle 19 towards the seating 12. The fuel pressure within the control chamber is controlled by controlling the fuel pressure applied to the bore 18, for example using an electro-magnetically actuable valve or using a piezo-electric actuator arrangement (not shown).

[0017] Where the injector is used in a common rail fuel system, prior to commencement of injection, the fuel pressure within the control chamber is high and a relatively large magnitude downward force is applied to the needle 19 assisting the spring 22 in ensuring that the needle 19 remains in engagement with the seating 12. In this position, the effective area of the needle 19 exposed to the fuel pressure within the control chamber is greater than that exposed to the fuel pressure within the bore 11, and this difference assists in ensuring that the needle 19 remains in engagement with the seating 12. When injection is to commence, the fuel pressure within the control chamber is vented, for example, by opening an electro-magnetically actuable valve or by appropriate control of a piezo-electric actuator arrangement. The reduction in the fuel pressure within the control chamber reduces the magnitude of the downward force applied to the needle 19, and a point will be reached beyond which the needle 19 is able to lift away from the seating 12 thus permitting fuel to flow to the outlet openings 13.

[0018] In order to terminate injection, the control chamber is repressurised. Where the injector is controlled using an electro-magnetically actuable valve, this may be achieved by closing the valve, fuel flowing to the control chamber through a restricted passage which connects the bore 18 to the supply passage. Alternatively, a valve may be provided instead of the restricted passage. The repressurisation of the control chamber increases the magnitude of the downward force applied to the needle 19, and a point will be reached beyond which the valve needle 19 returns into engagement with the seating 12, thus terminating injection.

[0019] Although in the arrangement described hereinbefore, the guide member 16 is provided with formations defining flutes 17 which do not form a significant restriction to fuel flow within the bore 11, if desired the guide member 16 may be provided with formations which, in combination with the nozzle body 10, form a restriction to fuel flow along the flow path. As a result, during injection, the rate at which fuel is able to flow to the part of the bore 11 downstream of the guide member 16 is restricted, thus the fuel pressure within this region falls during injection and the magnitude of the force applied to the valve needle 19 urging the valve needle 19 away from the seating 12 reduces. This may be advantageous as it may result in more rapid movement of the needle 19 towards the seating 12 when injection is to be terminated, resulting in the fuel injector being of improved responsiveness.

[0020] Regardless as to the type of fuel system with which the fuel injector is to be used, the responsiveness of the injector is improved as the needle 19 is of relatively small dimensions and low mass compared to a typical valve needle, and as a result is of reduced inertia compared to typical arrangements. Rapid movement of the needle 19 can therefore be achieved. The injector is therefore suitable for use in fuel systems of the type in which a pilot injection is to be made, the pilot injection of fuel being followed, after a short delay, by a main injection of fuel. In such a fuel system, the valve needle is caused to reciprocate rapidly within the bore in a very short period of time.

[0021] A further advantage of the arrangement described hereinbefore is that the guide member 16 forms a substantially fluid tight seal with the nozzle body 10. As a result, the requirement of typical arrangements for the needle to form a substantially fluid tight seal with the nozzle body is removed. As the needle need not form a substantially fluid tight seal with the nozzle body, a small clearance can be formed between the needle and the bore within which the needle is to be slidable, such a clearance reducing the risk of seizure of the needle, in use.

Claims

1. A fuel injector comprising a nozzle body (10) provided with a first bore (11) defining a seating (12), a valve needle (19) being reciprocable within the bore (11) and engageable with the seating (12) to control the delivery of fuel through an outlet opening (13), characterised by an elongate guide member (16) mounted within the bore (11), the guide member (16) being provided with a second bore (18) within which at least a part of the valve needle (19) is received to guide the valve needle (19) for movement within the first bore (11).

2. The fuel injector as claimed in Claim 1, wherein the guide member (16) serves as a stop which limits the distance through which the valve needle (19) can lift away from the seating (12), in use.

3. The fuel injector as claimed in Claim 2, wherein the bore (18) provided in the guide member (16) defines a shoulder (20) with which the valve needle (19) is engageable to limit the distance through which the valve needle (19) can lift away from the seating (12), in use.

4. The fuel injector as claimed in any of Claims 1 to 3, wherein the guide member (16) serves as a spring abutment for a spring (22) arranged to bias the valve needle (19) towards the seating (12).

5. The fuel injector as claimed in any of Claims 1 to 4, wherein the guide member (16) defines, together with the nozzle body (10), a flow path for fuel flowing towards the outlet opening (13).

6. The fuel injector as claimed in Claim 5, wherein the guide member (16) is provided with at least one formation (17) which defines, together with the nozzle body (10), a restriction to fuel flow along the flow path.

7. The fuel injector as claimed in any of Claims 1 to 6 for use in a common rail type fuel system, the valve needle (19) defining, together with the guide member (16), a control chamber for fuel, the guide member (16) further defining a passage whereby fuel can be supplied to or vented from the control chamber so as to vary the fuel pressure within the control chamber, in use.

Drawing