GB2339451A - A fuel injector with control chamber and a damped needle valve - Google Patents

Description

4 6 2339451 FUEL INJECTOR This invention relates to a fuel injector for

use in supplying fuel to a combustion space of an internal combustion engine which may be of -the compression ignition or the spark igni ' tion type. Such a fuel injector may be suitable for use in, for example, a common rail type fuel system and for control by an electronic control arrangement. The present invention is particularly applicable to fuel injectors having a piezo-electric actuator and more particularly, but not exclusively, to fuel injectors of the type described in our co-pending British Patent Application Nos. 9803557.9, 9806273.0 and 9811649.4 filed on 19 February 1998, 25 March 1998 and 29 May 1998, respectively. - It is an object of the present invention to obviate or mitigate the tendency of an injector needle of the fuel injector to overshoot upon opening. This is a problem where a partial lift is required for low injection rates as the overshoot tends to produce a poorly controlled injection pulse with the resulting loss in performance and injection quality.

According to the present invention, there is provided a fuel injector comprising a valve needle moveable in a bore in opening and closing directions relative to a seating so as to control the flow of fuel from the injector in use; a fuel supply passage for supplying fuel under pressure to the bore; a control chamber which is arranged to receive fuel from the fuel passage in use, the valve needle being acted upon in use by fuel pressure in the control chamber; and an actuator disposed so as to be capable of changing the volume of the control chamber and thereby acting on the valve needle to move it in one of said directions; wherein a damper is provided for damping movement of the valve needle in said opening direction.

By dampening opening movement of the injector needle, the problem of needle overshoot can be obviated or mitigated.

Preferably, the control chamber is divided into a first part associated with the valve needle and a second part associated with the actuator, and the damper comprises a flow restrictor serving to restrict flow of fuel, in use, from the first part of the control chamber to the second part of the control chamber.

In the case where the design of the control chamber and flow restrictor is such that, in use, flow of fuel takes place through the flow restrictor from the second part of the control chamber to the first part of the control chamber, then it is preferred for there to be a relatively lower restriction to flow in this direction. This can be achieved if the flow restrictor is of the venturi or stepped orifice type. Alternatively, a valve arrangement may be provided for providing a greater restriction to flow of fuel from the first part to the second part of the control chamber than vice versa.

It is particularly preferred for the flow restrictor to be positioned opposite the valve needle so that it is masked as the latter moves in its opening direction. This increases the damping effect at large movements of the valve needle in the opening direction.

Whilst the present invention is applicable to fuel injectors of any type that suffers from valve needle overshoot, it is considered to be particularly applicable to fuel injectors of the type in which the actuator is a piezoelectric actuator.

The present invention is app licable to a fuel injector of a type in which the actuator is arranged to act on the valve needle to move it in said opening direction, and biasing means is provided to urge the valve needle in its closing direction. Examples of such fuel injectors are described in our copending British Patent Application No. 9811649.4 filed on 29 May 1998, the disclosure of which is incorporated herein by reference.

The present invention is also applicable to a fuel injector of a type in which the actuator is arranged, when activated, to apply pressure to the valve needle by applying pressure to the fuel in the control chamber so as to move the needle in said closing direction and, when de-activated, to relieve the pressure applied to the fuel in the control chamber so that fuel pressure applied to another region of the valve needle serves to move the latter in its opening direction. Examples of such fuel injectors are described in our co-pending British Patent Application Nos. 9803557.9 and 9806273.0 filed 19 February 1998 and 25 March 1998, respectively, the disclosures of which are incorporated herein by reference.

Embodiments of the present invention, will now be described, by way of example, with reference to the accompanying drawings, in which:- Fig 1 is an axial section of a fuel injector according to a first embodiment of the present invention, Fig 2 is an enlarged view of part of the injector of Fig 1, Figs 3 to 5 are views illustrating alternative embodiments to that illustrated in Fig 2, Fig 6 is an axial section of a further embodiment of fuel injector according to the present invention, and Fig 7 is an enlarged view of part of the injector of Fig 6.

Referring now to Figs 1 and 2 of the drawings, the injector comprises a valve needle 10 slidable within a bore 12 formed in a nozzle body 14. The bore 12 is a blind bore, the blind end of the bore 12 defining a seating with which an end region of the valve needle 10 is engageable to control the supply of fuel from the bore 12 past the seating to a plurality of outlet openings 16. The bore 12 is arranged to be supplied with fuel from a source of fuel under high pressure, for example a common rail or accumulator, through a supply passage 18 which communicates with an annular gallery 20 defined by part of the bore 12. The valve needle 10 is of stepped form and includes an upper end region of diameter substantially equal to the diameter of the adjacent part of the bore 12, and a lower region which is of diameter smaller than the diameter of the bore 12. In order to permit fuel to flow from the annular gallery 20 to the part of the bore 12 containing the reduced diameter region of the valve needle 10, the valve needle 10 is provided with flutes 22. The shape of the valve needle 10 is such as to include thrust surfaces 1 Oa orientated such that the application of fuel under pressure to the bore 12 applies a force to the 5- needle 10 urging the needle 10 in an opening direction (upwardly as viewed in Figs. 1 and 2) away from its seating.

The upper end of the nozzle body 14 abuts a dividing piece 23 which in turn abuts a distance piece 24. The distance piece 24 is provided with a through bore offset from the axis of the valve needle 10. A piston member 26 is slidable within the bore of the distance piece 24. The valve needle 10 has an upper end surface 1 Ob which is disposed within a shallow recess 14a formed in an upper end surface of the nozzle body 14. The dividing piece 23 overlies the recess 14a and carries an operating pin 25 which is axially sl idable and which abuts the upper end surface 1 Ob of the valve needle 10. The piston member 26 has a recess 26a in its lower surface. The recess 26a opens onto the upper surface of the dividing piece 23. The dividing piece 23 has a shallow recess 23a in its upper surface, the recess 23a opening into the recess 26a in the piston member 26. The upper end of the operating pin 25 opens into the shallow recess 23a and is engaged by an abutment member 28 under the action of a compression spring 30. The compression spring 30 acts between the abutment member 28 and a further abutment member 32 in a blind bore at the top of the recess 26a.

The dividing piece 23 is provided with a venturi-type restricted flow passage 34 therethrough which provides communication between the recesses 14a and 23a. The piston member 26, dividing piece 23 and recess 14a define, in effect, a control chamber having a first chamber part defined between the nozzle body 14 and the dividing piece 23 and a second chamber part defined between the piston member 26 and the dividing piece 23, with these first and second chamber parts being in mutual communication via the venturi flow passage 34. The orientation and design of the venturi flow passage 34 is such that it provides a greater restriction to flow from the first chamber part to the second chamber part than from the second chamber part to the first chamber part.

In use, fuel is able to flow at a restricted rate from the annular gallery 20 to the recess 14a between the valve needle 10 and the adjacent part of the wall of the bore 12. It will be appreciated that such fuel flow is at a restricted rate as the diameters of the needle 10 and the adjacent part of the bore 12 are substantially equal. From there, fuel is able to flow into the recess 26a in the piston member 26 via the venturi flow passage 34.

A gas vent passage (not shown) may be provided at the upper end of the blind bore at the top of the recess 26a so as to vent gas which may collect in the latter. The spring 30 acts in a direction such as to bias the needle 10 in a closing direction towards its seating. The spring abutment members 28 and 32 and the spring 30 itself have the effect of reducing the volume of the control chamber 30 available for occupation by fuel under pressure. The abutment member 32 also serves to limit escape of fuel through the gas vent passage when the latter is present.

The upper surface of the distance piece 24 engages a nozzle holder 36 which is of elongate form, the supply passage 18 extending through the nozzle holder 36 and including a region of enlarged diameter arranged to house an edge filter member 38 (Fig. 1). The nozzle body 14, dividing piece 23 and distance piece 24 are secured to the nozzle holder 36 by a cap nut 40 which is in screw-threaded engagement with the nozzle holder 36.

The nozzle holder 36 is provided with an elongate bore 42 which defines a chamber which, in use, communicates with a low pressure drain. An actuator comprising a stack 44 of a piezo-ceramic elements is located within the bore 420. a lower end of the stack 44 engaging an anvil member 46 which abuts the upper end of the piston member 26. The elements of the stack 44 are of the energise-to-extend Q33) type.

The stack 44 is electrically connected to an appropriate drive circuit (not shown) which is intended to be driven from the battery of the vehicle in which the engine and fuel system incorporating the injector is mounted. The drive circuit includes by-pass resistors which ensure that, when the engine is switched off, the stack 44 remains charged for a sufficiently long duration to allow the fuel pressure within the supply passage 18 and common rail or other source of fuel under pressure to decay permitting safe shut down of the fuel system without resulting in unwanted injection of fuel.

In use, upon starting the engine, the fuel pressure supplied to the supply passage 18 is relatively low, thus the force urging the valve needle 10 away from its seating is low, and the spring 30 is of sufficient strength to ensure that the valve needle 10 is maintained in engagement with its seating at this stage in the operation of the injector. As described hereinbefore, fuel is able to flow between the valve needle 10 and the wall of the bore 12 to flow to the recess 14a at a restricted rate, and from there into the recess 26a via the venturi flow passage 34. Such flow of fuel increases the fuel pressure acting upon the end surface lob of the valve needle 10, thus assisting the spring 30 in maintaining the valve needle 10 in engagement with its seating as the fuel pressure within the supply passage 18 increases.

If, at this stage in the operation of the injector, the stack 44 of piezoelectric element has not been energised, energisation of the stack 44 urges the piston member 26 to move downwards. Downward movement of the piston member 26 serves to decrease the volume of the second chamber part so as to increase the pressure within the latter. This increase in pressure is transmitted via the venturi flow passage 34 into the second chamber part defined under the dividing piece 23. This ensures that the valve needle 10 remains in engagement with its seating.

In order to commence injection, the stack 44 of piezoelectric elements is at least partially discharged, thus reducing the height of the stack 44 and permitting movement of the piston member 26 in an upward direction. The pressure in the recess 26a is relieved and the action of the fuel pressure upon the thrust surfaces 1 Oa of the valve needle 10 urges the valve needle 10 away from its seating. It will be appreciated that, as the piston member 26 is of diameter greater than the diameter of the end surface 1 Ob of the valve needle 10, a relatively smal I amount of movement of the piston member 26 results in the fuel pressure within the control chamber failing to an extent to permit a relatively large amount of opening movement of the valve needle 10. The opening movement of the valve needle 10 permits fuel to flow past the seating to the outlet openings 16. During such opening movement, fuel is discharged upwardly as viewed in Figs. 1 and 2 from the first part of the control chamber through the venturi flow passage 34 and into the second part of the control chamber. In this flow direction, the design of the passage 34 is such that there is a relatively large restriction to flow of fuel therethrough, with the result that overshoot of the valve needle 10 is obviated or mitigated. This problem can arise particularly when only partial openings of the valve needle 10 are to take place as a result of partial discharge of the stack 44 of piezoelectric elements.

During injection, fuel leaking between the valve needle 10 and the wall of the bore 12 results in the valve needle 10 moving in a downward direction towards its seating. If injection were to occur for an excessively long duration, this would result in the valve needle 10 moving into engagement with its seating to terminate injection. Clearly, the flow of fuel to the control chamber acts as a safety feature to prevent continuous injection in the event that the stack 44 of piezo- ceramic elements or the associated drive circuit should fail.

In order to terminate injection in normal operation, the stack 44 is reenergised resulting in extension of the stack 44, and hence in the piston member 26 being pushed downwards. Such movement increases the fuel pressure within the control chamber thus increasing the force applied to the valve needle 10 to an extent sufficient to urge the needle 10 into engagement with its seating. As, during injection, fuel flows to the control chamber., it will be appreciated that the drop in the position of the needle 10 during injection guarantees that the valve needle 10 is pushed back into engagement with its seating at the termination of injection.

Although the restricted flow path by which fuel flows to the recess 14a is defined by the needle 10 and adjacent part of the wall of the bore 12 in the embodiment described hereinbefore, it will be appreciated that a separate drilling may be provided, if desired, to provide such a restricted flow path.

In the embodiment of Fig. 3, similar parts to those of the injector of Figs. 1 and 2 are accorded the same reference numerals. In this embodiment, the venturi flow passage 34 is disposed opposite the valve needle 10 instead of to one side of it. Thus, masking of the passage 34 occurs to an increasing extent as the valve needle 10 moves in the opening direction towards the dividing piece 23. This provides additional damping when the valve needle 10 is close to full lift. This may be used to minimise impact stresses on the top of the valve needle 10. Slowingdownthe valve needle 10 towards the end of its lift can also help to reduce velocityinduced stress in the stack 44 at the stage when it is most compressed.

The embodiments of Figs. 4 and 5 are similar to those of Figs. 3 and 2, respectively, except that the flow passage 34 is stepped. This is more economical to manufacture than the venturi shape, but gives a less directional flow characteristic. Thus, there tends to be a significant damping in both opening and closing directions of movement of the valve needle'10. However, a conical formation at the step in the flow passage 34 as compared with the sharp edge at the opposite end of the smaller diameter region enables some directional damping to be achieved. If a nondirectional damping characteristic is required, then the flow passage 34 can have similar forms at each end and can have a plain un-tapered and unstepped side wall.

Referring now to Figs 6 and 7 of the drawings, the fuel injector illustrated is similar to that of Figs. 1 and 2 and similar parts are accorded the same reference numerals. In this embodiment, however, the bore 12 is a through bore and is shaped, adjacent its lower end, to define a seating 12a. The valve needle 10 is located within the bore 12 and is shaped, adjacent its lower end, to define a seating 12a. The valve needle also includes, at its lower end, a region 1 Oc of enlarged diameter which is engageable with the seating 12a to control communication between a part of the bore 12 upstream of the seating 12a and a chamber 12b defined between part of the bore 12 downstream of the seating 14 and a part of the enlarged diameter region 1 Od of the needle 10. A plurality of outlet openings 16 are provided in the nozzle body 14 and arranged such that, as the needle 10 moves in an opening direction downwardly away from the seating 12a, the openings 16 come into communication with the chamber 12b to permit delivery of fuel through the openings 16.

The upper end of the valve needle 10 is provided with a screw thread formation 10c which engages a corresponding formation provided upon the interior of spring abutment member 50 which is in the form of a cylindrical sleeve. The spring abutment member 50 is of outer diameter slightly smaller than the diameter of the adjacent part of the bore 12. It will be appreciated that the engagement of the spring abutment member 50 with the wal I of the bore 12, and the engagement of the region 1 Od of the needle 10 with the lower end of the bore 12 guides the needle 10 for movement along the axis of the bore 12.

The bore 12 defines a step 12c with which a second spring abutment member 52 engages. Compression spring 30 is located between the spring abutment member 50 and the second spring abutment member 52 to bias the valve needle 10 in an upward (ie closing) direction so as to bias the region 1 Oc of the needle 10 into engagement with the seating 12a. In order to allow the use of a spring 30 of relatively small diameter but constructed of relatively large diameter wire, the screw thread formation 1 Oc is conveniently of generous root radius and of a suitable pitch to allow the spring 30 to pass the screw thread formation 1 Oc by rotating the spring 30 relative to the needle 10, the spring abutment member 52 being secured to the needle 10 after the spring 30 has been located upon the needielO. Such a screw thread formation further has the advantage that stress concentrations are reduced. It has been found that the use of a small,. close fitting thread can form a reasonably good seal due to the long flow path for escaping fluid.

The spring abutment member 50 is conveniently secured to the needle 10 to avoid undesirable relative rotation therebetween, in use, by inserting a pin 54 though openings provided in the spring abutment member 50, the pin 54 extending within a groove or other formation formed in the upper end surface 10b of the needle 10. Alternatively, the pin 54 may engage within castellation-like formations provided in the spring abutment member 50. As further alternatives, a conventional lock nut, lock screw or other thread locking technique may be used.

In order to ensure that the second spring abutment 52 does not restrict the flow of fuel towards the seating 12a, in use, openings 52a are provided in the second spring abutment member 52.

The face of the nozzle body 14 remote from the end thereof including the seating 12a abuts dividing piece 23. Venturi flow passage 34 is arranged in the opposite sense to that illustrated in Fig. 2 and is axially disposed relative to the valve needle 10. In this embodiment, piston member 26 is unrecessed and the second part of the control chamber is defined between the lower end of piston member 26, bore 24a of distance piece 24, and the upper surface of dividing piece 23. Thus, fluid pressure within the control chamber applies a force to the needle 10 which acts against the action of the spring 30 to urge the needle 10 in an opening direction.

Electric actuator in the form of stack 44 of piezo-ceramic elements is located in nozzle holder 36. The lower surface of anvil member 46 has a part-spherical recess which engages a part-spherically domed upper end of the piston member 26 so as to compensate for slight misalignment between the axis of the stack 44 and that of the piston member 26. A spring 56 is engaged between the piston member 26 and the upper surface of the distance piece 24 to bias the piston member 26 towards the stack 44. The spring 56 takes the form of a wave spring, but it will be appreciated that other types of spring, for example a disc spring or a helical compression spring, could be used.

In use, fuel under pressure is supplied through the supply passage 18 to the bore 12. The diameter of the seating 12a and that of the spring abutment member 50, and the force applied to the needle 10, are chosen to ensure that the supply of fuel under pressure to the bore 12 does not cause movement of the needle 10 away from the seating 12a at this time. It will be appreciated that the force applied by the spring 30 may be reduced compared with a conventional arrangement as the diameter of the spring abutment member 50 can be relatively large.

A small amount of leakage of fuel between the bore 12 and the spring abutment member 50 occurs, thus fuel is supplied at a low rate to the recess 14a defining the first part of the control chamber. Leakage also occurs at a controlled rate between the piston member 26 and the bore 24a provided in the distance piece 24, permitting fuel to escape from the control chamber to a low pressure drain reservoir, for example the fuel tank. The fuel pressure within the control chamber is therefore relatively low.

When injection is to commence, the actuator stack 44 is at least partially energised to extend in length resulting in movement of the piston member 26 against the direct action of the spring 56. Such movement causes hydraulic pressure to be transmitted via the fuel in the control chamber to the end surface 1 Ob of the valve needle 10, thereby applying a compression force against the spring 30. Thus, a point will be reached when the valve needle 10 is able to move in the downward (opening) direction to permit fuel to flow to the chamber 12b and through one or more of the openings 16. The rate at which fuel can escape from the control chamber to the low pressure drain reservoir is chosen to be at a sufficiently low level that the pressure within the control chamber remains high throughout the desired injection period. However, because of the orientation of the venturi flow passage, the flow of fuel therethrough at this stage is relatively restricted so that overshoot of the valve needle 10 during opening is obviated or mitigated, particularly during partial energisation of the actuator stack 44.

The rate at which fuel is delivered is dependent upon the number of openings 16 which are brought into communication with the chamber 12b by the movement of the needle 16. The distance through which the needle 10 moves depends upon the magnitude of the extension of the actuator stack 44. Clearly, therefore, the rate of injection can be controlled by appropriate control of the extension of the actuator stack 44.

In order to terminate injection, the actuator stack 44 is de-energised and returns to substantially its original length. As a result, the pressure on the piston member 26 by the stack 44 is relieved so that the piston member 26 moves upwardly under the direct action of the spring 56 and assisted by hydraulic pressure applied via the fuel in the control chamber by the compressed spring 30. The flow restriction afforded by the venturi flow passage 34 to fuel moving from the first chamber part to the second chamber part is relatively unrestricted, thereby enabling prompt return of the valve needle 10 into engagement with the seating 12a under the action of the spring 30.

In the event that the actuator stack 44 fails during injection, the leakage of fuel from the control chamber to the low pressure drain will eventually cause the fuel pressure within the control chamber to fall to a sufficiently low level to terminate injection, thus the injector is fail-safe. The leakage of fuel from the bore 12 to the control chamber, in use, compensates for gradual changes in the length of the actuator stack 44, for example resulting from temperature changes.

Whilst the dividing piece 23 and the distance piece 24 are described above and shown in the accompanying drawings as being two separate items, they may be formed as a single piece having a blind bore in which the piston member 26 is located, with the flow passage 34 and the bore for the operating pin 25 being provided at the closed end of the blind bore.

Claims (5)

1. A fuel injector comprising a valve needle moveable in a bore in opening and closing directions relative to a seating so as to control the flow of fuel from the injector in use; a fuel supply passage for supplying fuel under pressure to the bore; a control chamber which is arranged to receive fuel from the fuel passage in use, the valve needle being acted upon in use by fuel pressure in the control chamber; and an actuator disposed so as to be capable of changing the volume of the control chamber and thereby acting on the valve needle to move it in one of said directions; wherein a damper is provided for damping movement of the valve needle in said opening direction.

2. A fuel injector as claimed in Claim 1, wherein the control chamber is divided into a first part and a second part, and the damper comprises a flow restrictor serving to restrict flow of fuel, in use, between the first part of the control chamber and the second part of the control chamber.

3. A fuel injector as claimed in Claim 2, wherein the flow restrictor is arranged such that, in use, flow of fuel from the second part of the control chamber to the first part of the control chamber is substantially unrestricted.

4. A fuel injector as claimed in Claim 2 or Claim 3, wherein the flow restrictor comprises a valve arrangement for providing a greater restriction to fuel flow from the first part of the control chamber to the second part of the control chamber than from the second part of the control chamber to the first part of the control chamber.

in

5. A fuel jector substantially as hereinbefore described with reference to any one of the accompany drawings.

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