Armature arrangement

Abstract

 An armature arrangement for use in an injection device of a fuel injection system comprises an armature (30), one or more of vent holes (40) in the armature, a valve body (24) associated with the armature (30) and a cavity (42) in the armature (30). At least one formation (60; 60, 64; 66; 68; 70) on an upper surface (38) of the armature (30) allows enhanced fluid flow from the one or more vent holes (40) to the cavity (42) which, in so doing, reduces the effects of cavitation on the shot-to-shot variability of the fuel delivery.

Description

Technical Field

[0001] The invention relates to an armature arrangement for use in a solenoid fuel injector. In particular, but not exclusively, the invention relates to an armature arrangement for use in a solenoid fuel injector, being arranged to deliver pressurised fuel from a high pressure accumulator to a cylinder of a diesel internal combustion engine.

Background to the Invention

[0002] EP 0798459 describes a solenoid fuel injector in which a valve needle is moveable along the centre line of a nozzle body and engageable with a valve seat to control the flow of fuel from a high pressure fuel supply line through the nozzle body. The end of the valve needle remote from the valve seat extends within a control chamber, the control chamber being arranged to receive fuel from the high pressure fuel supply line through a restrictor. The valve needle is biased into engagement with the valve seat by means of the fuel pressure within the control chamber and a spring. A control valve is moveable under the influence of a solenoid actuator which is immersed in relatively low pressure fuel. The end of the control valve in proximity to the solenoid actuator carries an armature.

[0003] In use, when the solenoid actuator is energised, the control valve is lifted thereby allowing pressurised fuel from the control chamber to escape to a suitable low pressure drain. As the control chamber communicates with the fuel supply line through the restrictor, the fuel pressure within the control chamber falls sufficiently to permit the valve needle to leave the valve seat against the biasing force of the spring.

[0004] Modern diesel internal combustion engines require fuel injectors of the sort described above to operate under increasing fuel pressures in excess of 1500 bar. Under such pressures, the control valve requires a large force to lift it. The magnetic force that the solenoid actuator can generate can be increased by reducing a gap between the solenoid actuator and the armature of the control valve. In operation, the gap contains fuel, a foam of fuel or a fuel vapour and air mixture. During fuel injections, the armature is required to move at high speeds causing fluid to be expelled from the gap when the control valve lifts and to be sucked back into the gap when the valve closes. When the gap between the solenoid actuator and the armature is reduced and the control valve is closed, fluid is no longer able to enter the gap fast enough to fill the region of the gap corresponding to an inner pole area of the solenoid actuator. Consequently, cavitation forms in the inner pole area of the solenoid actuator. The rate at which cavitation bubbles form and subsequently collapse is sensitive to fluid properties, gas content and cleanliness, and has a significant effect on the time period over which the lifting and closing stroke of the valve is carried out. These inconsistent stroke periods result in a shot-to-shot variability in the fuel delivery of the injector.

[0005] It would therefore be desirable to provide an armature arrangement which avoids or alleviates at least one of the aforementioned problems.

Summary of the Invention

[0006] Thus, in accordance with a first aspect of the present invention there is provided an armature arrangement comprising an armature having one or more vent holes, a valve body associated with the armature which is provided with a cavity. An upper surface of the armature is provided with at least one formation to allow enhanced fluid flow from the vent holes to the cavity.

[0007] Preferably, the cavity is defined by a countersink having a chamfer wherein the countersink is suitable for receiving the valve body.

[0008] It is considered desirable for the formations to have a rotational symmetry about a valve body axis in order to avoid magnetic side loads on the armature.

[0009] Furthermore, mirror symmetry of said formations about a plane along (or through) the valve body axis is also desirable to avoid rotational forces on the armature.

[0010] It is preferable for the armature to be provided with a plurality of vent holes to optimise fluid flow through the armature. Furthermore, it is preferable that the valve body is received within the cavity.

[0011] The formations to allow enhanced fluid flow from the vent holes to the cavity preferably comprises at least one slot or, more preferably, a plurality of slots.

[0012] In one embodiment, the plurality of slots are arranged to extend in a radial direction towards the outer perimeter of the armature. For example, in a first embodiment, the plurality of slots extend from the cavity to midway between alternate pairs of vent holes.

[0013] The area midway between the vent holes is known to be a low stressed area of the armature when in use. It is, therefore, considered advantageous to terminate the slots at this area in order to minimise the stresses induced by the slots.

[0014] In a second embodiment the plurality of slots extend from the centre of the valve body to midway between alternate pairs of vent holes, thereby improving the venting of the fluid during the lifting stroke of the valve body by providing an additional flow path to the centre of the valve body.

[0015] In a third embodiment, the plurality of slots are arranged to extend from the cavity to the outer perimeter of the armature. This particular slot configuration accommodates convenient manufacturing processes such as, for example, sawing or grinding.

[0016] The plurality of slots can also be arranged to provide a fluid connection between the cavity and alternate ones of the vent holes. In a fourth embodiment, the plurality of slots providing a fluid connection between the cavity and alternate ones of the vent holes are straight, whereas, in a fifth embodiment, the plurality of slots are curved. These embodiments avoid a convergence of a stress concentration at the bottom of the slots and a relatively high stress concentration at the edge of the vent holes closest to the cavity.

[0017] In a still further embodiment, the plurality of slots can be arranged to provide fluid connection between either side of alternate ones of the vent holes and the cavity. In this case the plurality of slots may be curved or straight. This embodiment avoids any possibility of the stress concentrations associated with the vent holes and the bottom of the slots accumulating.

[0018] Preferably, the armature arrangement is used in an injection device of a common rail fuel injection system.

[0019] It will be appreciated that the plurality of slots could also be arranged to extend between the cavity and adjacent vent holes thereby providing communication also between adjacent vent holes.

[0020] According to a second aspect of the invention there is provided an injection device for use in a fuel injection system. The injection device includes a valve needle which is movable under the influence of an actuator having an armature arrangement. The armature arrangement comprises an armature, one or more vent holes in the armature, a valve body coupled to the armature and a cavity defined in the armature. An upper surface of the armature is provided with at least one formation to allow enhanced fluid flow from the vent holes to the cavity.

[0021] In a third aspect, the invention extends to a method of manufacturing the armature arrangement described above wherein the at least one formation to allow enhanced fluid flow from the vent holes to the cavity is made by way of a laser machining process.

[0022] The laser machining process facilitates expeditious production of the at least one formation in a precise manner, thereby optimising any radii and minimising associated stress concentrations.

[0023] It should be appreciated that preferred and/or optional features of the first aspect of the invention may be combined with the second or third aspect of the invention, alone or in combination.

Brief Description of the Figures

[0024] The state of the art and the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is cross-sectional view of a state of the art injector;

Figure 2 is a cross-sectional view of an enlarged section of the solenoid valve of the injector of Figure 1;

Figure 3 illustrates an armature arrangement of the solenoid valve of Figure 2;

Figures 4 to 9 illustrate various embodiments of an armature arrangement of the present invention.

[0025] An example of a known fuel injector suitable for delivering pressurised fuel from a high pressure accumulator to a cylinder of a diesel internal combustion engine is the fuel injector described in EP 0798459. Referring to Figures 1 and 2, the injector takes the form of a solenoid fuel injector 2 in which a valve needle 4 is moveable along the centre line of a body 6 and engageable with a valve seat 7 to control the flow of fuel from a high pressure fuel supply line 8 (referred to as the fuel supply line) through the body 6. The fuel supply line 8 terminates at an annular gallery 10 within which the valve needle 4 is received.

[0026] The end of the needle remote from the valve seat 7 extends within a control chamber 12, the control chamber 12 being closed with an insert member 14 and arranged to receive fuel from the fuel supply line 8 through a restrictor 16. The valve needle 4 is biased into engagement with the valve seat 7 by means of a first spring 18 provided in the control chamber 12.

[0027] A drilling in the insert member 14 defines a chamber 20 which communicates with an annular chamber 22 through a passage 23. A control valve body 24 is arranged to engage a seating surface 25. Movement of the valve body 24 serves to pressurise and depressurise the control chamber 12. The valve body 24 is movable under the influence of a solenoid actuator 26 along a second axis, the second axis being offset from the centre line of the body 6. The valve body 24 is biased into engagement with the seating surface 25 by means of a second spring 28. The annular chamber 22 is arranged to communicate with a suitable low pressure drain 29 when the valve body 24 disengages the seating surface 25 such that communication between the low pressure drain 29 and the control chamber 12 is established. The end of the valve body 24 in proximity to the solenoid actuator 26 carries an armature 30 which can abut a lift stop 32 defined by an underside of a valve housing 34, the lift stop 32 being arranged to terminate the lifting stroke of the armature 30.

[0028] In use, when the solenoid actuator 26 is de-energised, as in the position shown in Figure 1, the valve body 24 engages the seating surface 25 under the influence of the second spring 28 ensuring the fuel pressure in all chambers 12, 20, 22 and the annular gallery 10 is equal. The valve needle 4 is biased into engagement with the valve seat 7 by a combined force of the pressurised fuel in the control chamber 12 and the first spring 18, the combined force being sufficient to overcome the upwards force acting on the valve needle 4 due to the fuel pressure in the annular gallery 10, acting against an angled surface 36 of the valve needle 4.

[0029] In order to permit delivery of fuel from the injector 2, the solenoid actuator 26 is energised to lift the valve body 24 against the influence of the second spring 28 so that the valve body 24 is lifted away from the seating surface 25. In so doing, fuel is allowed to flow from the annular chamber 22, and hence the chambers 12, 20, to the low pressure drain 29. Once the force acting on the valve needle 4 from the fuel pressure in the chambers 12, 20 in combination with the first spring 18 is no longer sufficient to overcome the upwards force acting on the valve needle 4, the valve needle 4 is lifted away from the valve seat 7.

[0030] Referring also to Figure 3, the armature 30 has an upper surface 38, which is provided with a six vent holes 40 that are equi-angularly spaced around a cavity 42 to extend through and emerge at the upper surface 38 of the armature 30. The cavity 42 is defined by a countersink provided in the upper surface 38 of the armature 30 and is positioned in the centre of the armature 30, the cavity 42 being arranged to receive a head 44 of the valve body 24 in a press-fit thereby securing the two components together. The head 44 of the valve body 24 bears a corrugated region and, during assembly, an annular depression 43 is formed on the underside surface of the armature 30 which causes the material of the armature 30 to conform to the corrugated profile of the head 44. The head 44 of the valve body 24 is used to impact the lift stop 32 in order to control the distance travelled by the valve body 24 during its lifting stroke.

[0031] A magnetic field is generated when the solenoid actuator 26 is energised, the magnetic field passes through an outer pole area 46, through the armature 30 and back through an inner pole area 48. An optimum magnetic force is generated when the inner and outer pole areas 48, 46 are approximately equal, thereby maximising the total magnetic flux capacity of the circuit. The armature 30 is separated from the poles 48, 46 by an axial gap 50. In operation, this gap 50 contains a fluid comprising fuel, a foam of fuel or a fuel vapour and air mixture. During injections, the armature 30 is required to move at high speeds and the fluid is expelled from the gap 50 when the valve body 24 lifts and is sucked back into the gap 50 through the vent holes 40 and/or the inner and outer pole areas 48, 46 when the valve body 24 closes.

[0032] Current fuel injection systems are required to deliver increasingly higher injection pressures and so larger forces are required to lift the valve body 24. Consequently the gap 50 which separates the armature 30 from the poles 48, 46 has had to be reduced in order to increase the magnetic force the solenoid actuator 26 can generate to lift the valve body 24. As the armature 30 is moved, the fluid in the outer pole area 46 of the gap 50 moves relatively easily as it has a short path (as indicated by arrows A in Figure 2) and is able to pass two large perimeters, namely an outer perimeter 52 of the armature 30 and an inner perimeter 54 of the outer pole area 46. The fluid in the inner pole area 48 of the gap 50 moves with more difficultly as it has a longer path (as indicated by arrows B in Figure 2) and can only pass a small outer perimeter 56 of the inner pole area 48. An inner perimeter 58 of the inner pole area 48 is of very small circumference and is blocked by the lift stop 32 when the valve body 24 is lifted.

[0033] When the fuel injector 2 is arranged so that the gap 50 is reduced, the fluid is not able to enter the gap 50 fast enough to fill the inner pole area 48 of the gap 50 when the valve body 24 is closed against the seating surface 25. Consequently, the static pressure local to the inner pole area 48 is reduced and vapour bubbles or cavitation bubbles form. The formation of these bubbles takes place over a finite time period which is sensitive to the fluid properties, gas content and cleanliness. When the cavitation bubbles are exposed to a sufficiently high static pressure, as for example during the lifting stroke of the valve body 24, said cavitation bubbles will collapse. The force exerted by the liquid rushing into the cavitation bubbles causes high localised pressures which affects the consistency of the shot-to-shot delivery of the injector and can lead to serious erosion of the boundary surfaces e.g. of the armature 30.

[0034] The present invention improves on the above-described armature 30 and valve body 24 arrangement by modifying the armature 30 to reduce the effects of cavitation on the shot-to-shot variability in the fuel delivery of the injector.

[0035] Referring to Figure 4, in the present invention the upper surface 38 of the armature 30 is not only provided with a plurality of vent holes 40 arranged in the manner described previously but is also provided with a series of radial slots 60, the radial slots 60 being machined into the upper surface 38 of the armature 30 from the cavity 42 to midway between alternate pairs of vent holes 40. Arrows C indicate the flow path that the fluid follows from the vent holes 40 to the cavity 42, at which point the fluid is distributed to the inner pole area 48. The additional fluid in the inner pole area 48 greatly reduces the fluid pressure drop experienced in this area during the closing stroke of the valve body 24, thereby eliminating or substantially reducing the formation of cavitation bubbles in all but a small area of the lift stop 32.

[0036] It is considered desirable to maintain a rotational symmetry of the radial slots 60 about an axis 61 of the valve body 24 in order to avoid magnetic side loads on the armature 30. It should be appreciated that the axis 61 of the valve body 24 is coincident with an axis of the armature 30. Furthermore, mirror symmetry of the radial slots 60 is also desirable about a plane 62 which extends vertically from the upper surface 38 of the armature 30 thereby avoiding rotational forces on the armature 30 when the fluid in the radial slots 60 flows from the vent holes 40 to the cavity 42. The plane 62 is aligned with one of the radial slots 60 and intersects the valve body axis 61. The three radial slot configuration of the armature 30 in Figure 4 provides the requisite symmetry and an adequate flow path for the fluid but without a significant reduction of the inner pole area 48, thereby maintaining an optimum magnetic force and maximising the total magnetic flux capacity of the circuit.

[0037] The radial slots 60 extend from the cavity 42 and terminate on the mid-plane between the vent holes 40. During the closing stroke of the valve body 24, when the valve body 24 impacts the seating surface 25, the mid-plane area between the vent holes 40 is known to be a low stressed area. Therefore, it is considered advantageous to terminate the radial slots 60 on the mid plane between the vent holes 40 in order to minimise the stresses induced by said radial slots 60.

[0038] The use of a three radial slot configuration provides a further benefit of minimising the stress experienced by the armature 30 during its lifting and closing strokes. A two radial slot configuration would tend to induce a bending vibration mode in the armature 30, whereas the slots used in a configuration comprising more than three radial slots 60 would have to be narrower in relation to their depth to maintain a sufficient inner pole area 48, resulting in a higher stress concentration when the armature 30 flexes as the head 44 of the valve body 24 impacts the lift stop 32.

[0039] The above-described radial slots 60 are preferably manufactured using a laser machining process. The laser machining process facilitates expeditious production of small slots in a precise manner. Furthermore, the laser machining process can form the optimum radius in the bottom of the radial slot, thereby minimising any associated stress concentrations.

[0040] Alternative manufacturing processes include: electric discharge machining; sawing with a small, narrow circular saw; or grinding with a narrow diamond or carbide coated cutting wheel.

[0041] Figure 5 shows a second embodiment of the present invention in which both the upper surface 38 of the armature 30 and the head 44 of the valve body 24 are provided with radial slots 60 which are substantially aligned along a common axis. This radial slot configuration not only minimises the formation of cavitation bubbles during the closing stroke of the valve body 24 but also improves the venting of the fluid during the lifting stroke of the valve body 24 by providing an additional flow path 64 to the middle of the head 44 of the valve body 24. A possible disadvantage of this radial slot configuration is that it reduces a beneficial squeeze film damping force that occurs on the lift stop 32 in the final stages of the lifting stroke of the valve body 24. This can result in the valve body 24 "bouncing" on the lift stop 32. The radial slot number, depth and/or width may therefore be varied to optimise the squeeze film damping force on the lift stop 32 in order to minimise the bouncing of the valve body 24, whilst maximising the venting of the fluid from the vent holes 40. For example, the radial slots 60 in the head 44 of the valve body 24 may be shallow at the lift stop 32 and either taper or step to a larger depth in the armature 30. Such variation in radial slot depth may also be applied to other designs of radial slots 60 in order to optimise the flow through said radial slots 60.

[0042] Figure 6 shows a third embodiment of the present invention in which the upper surface 38 of the armature 30 is provided with a series of radial slots 60 which extend from the cavity 42 through the outer perimeter 52 of said armature 30. An advantage of this embodiment it that it allows the radial slots 60 to be easily manufactured using manufacturing processes such as sawing or grinding.

[0043] A further advantage of this embodiment of the invention is that it provides additional flow paths in the outer pole area 40 of the armature 30. If the diameters of the vent holes 40 were reduced in order to, for example, optimise the inner and outer pole areas 48, 46, the additional flow paths provided by the radial slots 60 ensure that the fluid is expelled from the gap 50.

[0044] Figure 7 shows a fourth embodiment of the present invention in which the upper surface 38 of the armature 30 is provided with a plurality of straight slots 66, each of which is arranged to connect alternate ones of the vent holes 40 and the cavity 42) The vent holes 40 between said alternate ones connected to the cavity 42 remain isolated. This configuration of straight slots 66 ensures that the relatively high stress concentration at the edge of the vent holes 40 closest to the cavity 42 does not coincide with the stress concentration at the bottom of the straight slots 66, thereby reducing any induced stresses.

[0045] Figure 8 shows a fifth embodiment of the present invention which is similar to that shown in Figure 7. However, in this instance the upper surface 38 of the armature 30 is provided with a plurality of curved or arcuate non-radial slots 68, each of which is arranged to connect alternate ones of the vent holes 40 and the cavity 42. The vent holes 40 between said alternate ones connected to the cavity 42 remain isolated. This configuration of slots 68 further separates the stress concentration at the bottom of the slots 68 from the relatively high stress concentration at the edge of the vent holes 40 closest to the cavity 42, thereby further reducing any induced stresses.

[0046] Figure 9 shows a sixth embodiment of the present invention in which the upper surface 38 of the armature 30 is provided with a plurality of curved or arcuate slots 70, the slots 70 being arranged to connect the mid-planes either side of alternate ones of the vent holes 40 to the cavity 42. That is to say, the slots 70 do not intersect any of the vent holes 40. This configuration of slots 70 thus avoids any possibility of the stress concentrations associated with the vent holes 40 and the curved slots 70 accumulating.

[0047] A further advantage of the curved slot 70 configuration, and the slot configurations of Figures 7 and 8, is that the slots intersect the cavity 42 at a shallower angle when compared to the radial slot configurations, thereby reducing the stress concentration at said intersection 72. A reduced stress concentration at the intersection 72 of the cavity 42 and curved slot means that the curved slot can afford to be manufactured narrower and deeper, thereby minimising the reduction in the inner pole area 48 and maintaining the total magnetic flux capacity of the circuit.

[0048] The embodiments of Figures 5 to 9 maintain the advantages of having rotational and mirror symmetry of the slot configurations. However, it will be appreciated that the plane 62 described previously is applicable to the radial slot configurations 60; 60, 64 only and the specific slot configurations 66, 68, 70 illustrated in Figures 7 to 9 have a mirror symmetry about a second plane 67. With reference to Figure 8, the second plane 67 extends vertically from the upper surface 38 of the armature 30 and intersects the valve body axis 61 and the centres of opposing vent holes 40.

[0049] Other configurations and type of formation in the upper surface 38 of the armature 30 and/or the head 44 of the valve body 24 are also envisaged, whilst still providing the benefit that an enhanced fluid flow from the vent holes 40 to the cavity 42 is permitted. For example, the slots could be arranged to extend between the cavity 42 and adjacent vent holes 40 thereby providing communication also between adjacent vent holes 40. In addition, although described in relation to a common rail injector, it will be appreciated that the invention is also applicable to other types of injectors, and indeed is applicable in non-fuel injector applications where there may be a desire for vent holes in the armature to be evacuated effectively.

[0050] It should be appreciated that although the embodiments have been described above as comprising an armature arrangement having an armature 30 and a separate valve body 24 coupled to it, this is not essential to the inventive concept and, instead, the armature 30 and the valve body 24 may be a unitary component i.e. the valve body 24 is integral with the armature 30.

Claims

1. An armature arrangement comprising:

an armature (30);

one or more vent holes (40) in the armature (30);

a valve body (24) associated with the armature (30); and

a cavity (42) defined in the armature (30),

characterised in that an upper surface (38) of the armature (30) is provided with at least one formation (60; 60, 64; 66; 68; 70) to allow enhanced fluid flow from the one or more vent holes (40) to the cavity (42).

2. The armature arrangement as claimed in claim 1, wherein the cavity (42) is defined by a countersink in the upper surface (38) of the armature (30).

3. The armature arrangement as claimed in claim 1 or claim 2, wherein the at least one formation (60; 60, 64; 66; 68; 70) has a rotational symmetry about a valve body axis (61).

4. The armature arrangement as claimed in any one of claims 1 to 3, wherein the at least one formation (60; 60, 64; 66; 68; 70) has a mirror symmetry about a plane along (or through) the valve body axis (61).

5. The armature arrangement as claimed in any one of claims 1 to 4, wherein the at least one formation (60; 60, 64; 66; 68; 70) comprises a slot.

6. The armature arrangement as claimed in claim 5, comprising a plurality of slots (60) arranged to extend in a radial direction towards an outer perimeter (52) of the armature (30).

7. The armature arrangement as claimed in claim 6, wherein the plurality of slots (60) are configured to extend from the cavity (42) to midway between alternate pairs of vent holes (40).

8. The armature arrangement as claimed in claim 6, wherein the plurality of slots (60, 64) are configured to extend from the centre of the valve body (24) to midway between alternate pairs of vent holes (40).

9. The armature arrangement as claimed in claim 6, wherein the plurality of slots (60) are arranged to extend from the cavity (42) to the outer perimeter (52) of the armature (30).

10. The armature arrangement as claimed in claim 5, comprising a plurality of slots (66; 68) arranged to provide a fluid connection between the cavity (42) and alternate ones of the vent holes (40) or, alternatively, between the cavity (42) and adjacent ones of the vent holes (40).

11. The armature arrangement as claimed in claim 5, comprising a plurality of slots (70) arranged to provide fluid connection between either side of alternate ones of the vent holes (40) and the cavity (42).

12. The armature arrangement as claimed in claims 10 or 11, wherein the plurality of slots (66; 68; 70) are substantially straight or curved.

13. The armature arrangement as claimed in any one of claims 1 to 12, for use in an injection device of a common rail fuel injection system.

14. A method of manufacturing an armature arrangement as claimed in any one of claims 1 to 13, wherein the at least one formation (60; 60, 64; 66; 68; 70) of the armature that allows enhanced fluid flow from the vent holes (40) to the cavity (42) is formed by way of a laser machining process.

15. An injection device for use in a fuel injection system, the injection device including a valve needle which is movable under the control of an actuator having an armature arrangement comprising:

an armature (30);

one or more vent holes (40) in the armature (30);

a valve body (24) associated with the armature (30); and

a cavity (42) defined in the armature (30),

characterised in that an upper surface (38) of the armature (30) is provided with at least one formation to allow enhanced fluid flow from the vent holes (40) to the cavity (42).

Drawing