Fuel injector

Abstract

A fuel injector (10) for delivering gaseous fuel to an internal combustion engine, the fuel injector (10) comprising an inner valve needle (18) which is engageable with an inner valve seat (28) to control fuel injection through at least one outlet (24) of the injector; the inner valve needle (18) being received within a bore provided in an outer valve needle (20) which is engageable with an outer valve seat (26) to control fuel injection through the at least one outlet (24); and an actuator arrangement (70) for effecting movement of the inner valve needle (18) and the outer valve needle (20) away from their respective valve seats (28, 26) to commence injection into the engine, wherein the inner valve needle (18) and the outer valve needle (20) together define a volume (46) therebetween for receiving an annular seal (48, 148) which serves to prevent gaseous fuel leakage between an outer surface of the inner valve needle (18) and an inner surface of the outer valve needle (20).

Description

[0001] FUEL INJECTOR

[0002] FIELD OF THE INVENTION

[0003] This invention relates to a fuel injector for use in a gaseous fuel injection system. In particular, the invention relates to a fuel injector for gaseous fuel such as hydrogen for delivering fuel to an internal combustion engine.

[0004] BACKGROUND

[0005] In fuel injection systems for liquid fuel, it is known for a fuel pump to supply fuel to a high-pressure accumulator (or common rail), from where it is delivered into each cylinder of the engine by means of a dedicated fuel injector. Typically, a fuel injector has an injection nozzle that is received within a bore provided in a cylinder head of the cylinder, and a valve needle which is actuated to control the release of high- pressure fuel into the cylinder from spray holes provided in the injection nozzle. One way of opening and closing a valve needle is to couple a solenoid actuator directly to the valve needle, by attaching an armature of the actuator to the valve needle (or by providing a valve needle with an integral armature). The valve needle is biased towards a seating surface so that, when the solenoid is not energised, the valve needle prevents fuel flow through the spray holes. When the solenoid is actuated, the valve needle is lifted away from its valve seat and fuel injection takes place.

[0006] The unique characteristics of gases such as hydrogen as a fuel can create or exacerbate design challenges with fuel injectors such that conventional fuel injectors are in general not suitable to inject gaseous fuels in an optimum manner. This has led to a need to adapt current fuel injector technologies in order to be effective in gaseous fuel engines.

[0007] One issue in particular is that of wear of the valve seat for the injector needle which can be worse in gaseous fuel injectors because the injectable fluid does not provide the same lubricating effect as a liquid fuel.

[0008] It is against this background that the invention has been devised. SUMMARY OF THE INVENTION

[0009] According to a first aspect, there is provided fuel injector for delivering gaseous fuel to an internal combustion engine, the fuel injector comprising an inner valve needle which is engageable with an inner valve seat to control fuel injection through at least one outlet of the injector; the inner valve needle being received within a bore provided in an outer valve needle which is engageable with an inner valve seat to control fuel injection through the at least one outlet; and an actuator arrangement for effecting movement of the inner valve needle and the outer valve needle away from their respective valve seats to commence injection into the engine. The inner valve needle and the outer valve needle together define a volume therebetween for receiving a seal which serves to prevent gaseous fuel leakage between an outer surface of the inner valve needle and an inner surface of the outer valve needle.

[0010] The injector has advantages because it enables an increased nozzle flow area compared to a single seat injector and the imbalance forces on the valve needle are reduced. However, in moving into the gaseous regime with such a differential nozzle injector there is a need to accommodate the effects of a larger range of wear which may be encountered. The seal between the inner and outer valve needle addresses this problem. Firstly the seal prevents gaseous fuel leakage between the inner and outer valve needles, which prevents unwanted fuel being delivered to the combustion chamber. Secondly, the seal is configured to provide a load path for a closing force of a spring which acts on at least one of the inner and outer valve needles to cause them to seat against their respective seat(s). The flexibility of the seal accommodates different levels of seat wear across the inner and outer valve seats which may otherwise affect control of the valve needles, for example simultaneous opening or closing.

[0011] Ideally the diameter of the outer valve seat can be reduced so that it is almost identical to that of the lower valve seat. This may be achieved by using a separate seat insert to define the inner and outer valve seats, the seat insert being inserted into an open end of the nozzle body. In practice, the diameters of the outer valve seat and the inner valve seat may differ by at most 20%, preferably by at most 15% and more preferably by at most 10%. Typically, the outer valve seat will be slightly larger than the inner valve seat, but this may differ based on the construction of the seat insert and the injection valve needle. This reduces the differential area between the two valve seats thereby reducing the load required to seat the injection valve needle(s), by means of a needle spring assembly, when the fuel pressure is lower than the cylinder pressure. In turn, this minimises the required size of the spring assembly. As the spring assembly only provides a small seating load, the force required to lift the injection valve needle against the action of the spring assembly is also minimised. This enables a smaller solenoid to be used to provide the lift to the injection valve needle. The result is a more efficient and effective injector for gaseous fuels which has a low needle lift but retains a high flow rate.

[0012] In one embodiment, the inner valve needle includes a frusto-conical surface which engages with the annular seal. The frusto-conical surface is preferably forced in a recess in the outer surface of the inner valve needle.

[0013] The injector may further comprise a spring assembly which serves to urge the inner and outer valve needles against their respective inner and outer valve seats.

[0014] The spring assembly may include a spring which acts on both the inner and outer valve needles via a load path through the annular seal.

[0015] The spring load may act on the inner valve needle via a bridge piece carried on the inner valve needle.

[0016] The annular seal may include a first relatively large diameter region and a second relatively narrow diameter region, wherein the second relatively narrow diameter region is cooperable with the frusto-conical surface on the inner valve needle and a further corresponding frusto-conical surface on the outer valve needle.

[0017] The first relatively large diameter region may transition to the second relatively narrow diameter region via a region of the seal defining a curved surface.

[0018] The actuator arrangement may comprise a pull tube which cooperates with the inner valve needle and the outer valve needles to cause them to lift away from their respective valve seats to commence injection.

[0019] The pull tube may define an inner valve lift surface which cooperates with the inner valve needle to cause it to lift and an outer valve lift surface which cooperates with the outer valve needle to cause it to lift, wherein the gap defined between the inner valve lift surface and the inner valve needle and the gap defined between the outer valve lift surface and the outer valve needle are substantially the same so that the inner and outer valve needles lift at substantially the same time when the actuator arrangement is actuated.

[0020] It may be the case that the inner valve lift surface is offset perpendicularly, relative to the outer valve lift surface, about the longitudinal axis of the injector.

[0021] In an alternative embodiment, the spring assembly may include an inner valve spring which acts on the inner valve needle to urge the inner valve needle towards the inner valve seat and an outer valve spring which acts on the outer valve needle to urge the outer valve needle towards the outer valve seat.

[0022] In this case, the inner valve spring may be located within the bore defined within the outer valve needle.

[0023] The outer valve needle may define a gap with the inner valve needle so that, upon actuation of the actuator arrangement, the outer valve needle is caused to move away from the outer valve seat through a distance equal to the gap before the inner valve needle is caused to move away from the inner valve seat.

[0024] The gap may be defined between an inner valve ring carried on an outer surface of the inner valve needle and an outer valve ring carried on an inner surface of the outer valve needle.

[0025] The annular seal may include a frusto-conical surface which cooperates with the inner valve needle. For example, the annular seal may include a first relatively large diameter region and a second relatively narrow diameter region and wherein the second relatively narrow diameter region defines the frusto-conical surface.

[0026] BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order that the invention may be more readily understood, preferred non-limiting embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a cross sectional view of a fuel injector of a first embodiment of the invention, including an injection nozzle having inner and outer valve needles, when in a non-injecting state;

[0028] Figure 2 is a cross section view of a lower portion of the injection nozzle to show the detail of the seat arrangement;

[0029] Figure 3a is a cross section view of an upper portion of the injection nozzle in Figure 1 , in a first longitudinal plane of the injector;

[0030] Figure 3b is a cross section view through an upper portion of the injection nozzle in Figure 1 , similar to Figure 3a but in a second longitudinal plane of the injector perpendicular to the first longitudinal plane;

[0031] Figure 4 is an enlarged view of a part of the injector in Figure 1 to show the detail of a seal of the injector;

[0032] Figure 5 is an isometric view of an upper region of the injection nozzle in Figure 1 ;

[0033] Figure 6 is a cross section view of the injector in Figure 1 when in an injecting state;

[0034] Figure 7a is a cross section view of the upper portion of the injection nozzle in Figure 1 , similar to Figure 3a, with the injector in an injecting state;

[0035] Figure 7b is a cross sectional view the upper portion of the injection nozzle in Figure 1 , similar to Figure 3b, with the injector in an injecting state;

[0036] Figure 8 is a cross section view of the lower portion of the injection nozzle, similar to Figure 4, with the injector in an injecting state;

[0037] Figure 9 is a cross section view of an alternative embodiment of the injector to that shown in Figure 1 , with the injector in a non-injecting state;

[0038] Figure 10 is a cross section view of a lower portion of the injector in Figure 9 to show the seat arrangement in more detail; Figure 11 is a cross section view of the injector in Figure 9, with the injector in an injecting state; and

[0039] Figure 12 is a cross section view of the lower portion of the injector in Figure 11 , with the injector in an injecting state.

[0040] In the drawings, as well as in the following description, like features are assigned like reference signs.

[0041] Throughout this description, terms such as ‘upper’ and ‘lower’, and other directional references, are used with reference to the orientation of the fuel injector as shown in the accompanying drawings. However, it will be appreciated that such references are not limiting and that fuel injectors according to the invention can be used in any orientation.

[0042] DETAILED DESCRIPTION OF THE INVENTION

[0043] Figures 1 to 4 show a first embodiment of a fuel injector 10, or injector assembly, for injecting gaseous fuel in an internal combustion engine. In general terms, the fuel injector 10 is of the twin-needle type comprising a valve needle assembly having inner and outer valve needles which are operable to control injection through a plurality of outlets of the injector. The valve needle assembly has a differential seat arrangement between the inner and outer valve needles to increase nozzle flow area, which is beneficial for injecting gaseous fuel such as hydrogen.

[0044] The fuel injector includes an injection nozzle 12 having a substantially cylindrical nozzle body 14 that defines a nozzle bore 16. The nozzle bore 16 in turn defines a central bore axis which aligns with a longitudinal axis A-A of the injector. The valve needle assembly of the injector includes inner and outer valve needles 18, 20, the inner valve needle 18 being received within a bore of the outer valve needle 20 so that the inner valve needle 18 is able to slide, to a limited extent, within the outer valve needle 20 if the valve seats wear by different amounts. A valve seat insert 22 is received within the nozzle body bore 16 at the lower, open end of the bore 16. The valve seat insert 22 is provided with one more outlets 24 (only two of which are shown in the cross section of Figure 1 , and more can be seen more clearly in Figure 2) and defines first and second valve seats; an upper valve seat 26 for the outer valve needle 20 (referred to as the outer valve seat) and a lower valve seat 28 for the inner valve needle 18 (referred to as the inner valve seat). Flow directing features 25 may also be provided in the valve seat insert 22 (labelled in Figure 2).

[0045] The diameter of the outer valve seat 26 differs from the diameter of the inner valve seat 28 by only a very small amount, and ideally the outer valve seat 26 is no more than 10% bigger than the inner valve seat 28. This reduces the differential area between the two valve seats 26, 28 to reducing the load required to seat the injection valve needle(s) under a spring force, when the fuel pressure is lower than the cylinder pressure.

[0046] Figure 2 shows the valve seat assembly in enlarged detail. The inner valve needle 18 includes an enlarged head 18a, at its lower end, which engages with the inner valve seat 28. The enlarged head 18a tapers via a frusto-conical region to a narrowed region 18b of the inner valve needle and then via a further frusto-conical region towards the main stem 18c. The main stem 18c forms a sliding fit within the outer valve needle 20 to allow a small degree of movement between the two needles. The outer valve needle 20 includes an enlarged head 20a which engages with the outer valve seat 26. The enlarged head 20a tapers via a frusto-conical region into a main stem 20b of the outer valve needle. The stem 18b of the inner valve needle projects into a seat volume 30 defined within the valve seat assembly. The outlets 24 communicate with the seat volume 30.

[0047] When the valve needles 18, 20 are seated against their respective valve seats 28, 26, no fuel is able to flow into the combustion chamber through the nozzle outlet 24. The inner valve needle 18 is provided with a bore 32, and so it hollow, so that gaseous fuel can flow through the inner valve needle 18 when it is moved away from the lower valve seat 28. When the inner valve needle 18 is lifted away from the lower valve seat 28, fuel within the bore of the inner valve needle 18 is able to escape past the lower valve seat 28 and flows through the seat volume 30 and then out through the outlet 24.

[0048] The outer valve needle 20 and the nozzle body bore 16 together define a further channel for gaseous fuel as it flows towards the upper valve seat 26. An upper enlarged region 14a of the nozzle body 14 forms a guide region for the outer valve needle 20 to guide movement of the outer valve needle 20 within the bore 16. The upper end of the nozzle body 14a projects through an opening in an injector housing 34 and extends into a spring chamber 36 for housing a spring 38.

[0049] The upper end of the valve needle arrangement can be seen more clearly in Figures 3a and 3b. Each of these figures shows a cross section through a different plane of the injector, offset from each other by 90 degrees. For clarity, reference numerals are only included on one of Figures 3a and 3b for clarity. Referring also to Figure 4, the bore in the outer valve needle 20 includes an enlarged region 40 at its upper end which defines a frusto-conical or angled surface 42 (angled relative to the longitudinal axis A-A) which tapers to the main stem 20b of the outer valve needle 20. The inner valve needle 18 in this region is shaped to define a shallow recess 44 on its outer surface. The shallow recess 44 on the outer surface of the inner valve needle 18 and the enlarged region 40 of the bore in the outer valve needle 20 together define a volume or seal chamber 46 for housing a generally annular seal 48. At the lower end of the recess 44, the external surface of the inner valve needle 18 is of frusto-conical form, defining a frusto-conical or angled surface 50 which meets with the frusto-conical or angled surface of the outer valve needle 20. The frusto-conical surfaces 42, 50 together define a conical volume 46a at the tip of the seal chamber 46.

[0050] The seal 48 is located within the seal chamber 46. The seal 48 can be seen most clearly in Figure 4 and is shaped to include a first, upper region 48a of relatively larger diameter and a second, lower region 48b of relatively smaller diameter (referred to as the thin-walled region). The seal 48 also includes a curved region 48c, which defines, on its radially outer surface, a curved surface. The curved region 48c defines a transition between the upper region 48a of the seal 48 and the thin-walled region 48b of the seal 48. The radially inner surface of the seal 48 has a surface which extends along the longitudinal axis of the injector, aligned to the axis A-A of the injector. At the tip of this radially inner surface, the seal 48 engages with the frusto-conical surface 50 of the inner valve needle. The seal 48 is positioned so that the thin-walled region 48b of the seal terminates in a tip which is received within the conical volume 46a of the seal chamber 46. The seal 48 is typically made from polymer such as PEEK, PTFE, Pll or PAI. The dashed and dotted lines 49 in Figure 4 illustrate what happen to the position / orientation of the seal 48 if the inner and outer valve needles 18, 20 are not aligned, which is not desirable.

[0051] The lift mechanism for the valve needle arrangement will now be described with further reference to Figure 5. At its upper end, the inner valve needle 18 includes a pair of radially extending flanges 18d, one extending on each side of the inner valve needle 18. Each flange 18d projects radially over the upper end of a both the outer valve needle 20 and the seal volume 46. The flanges are symmetrical, one on either side of the longitudinal injector axis A-A. As seen in Figure 3a, each flange 18d defines an engagement surface 60 of the inner valve needle. The inner valve needle 18 therefore has two engagement surfaces 60 (referred to as inner engagement surfaces), one on either side of the longitudinal axis A-A of the injector (only one of the engagement surfaces 60 can be seen in the orientation shown in Figure 3a). As seen in Figure 3b, the outer valve needle 20 does not include these flanges but instead is shaped with a notch or groove 58 on its outer surface, towards its upper end, to define engagement surfaces 62 of the outer valve needle 20. The outer valve needle 20 therefore has two engagement surfaces 62 (referred to as outer engagement surfaces), one on either side of the longitudinal axis A-A of the injector (only one of the engagement surfaces 62 can be seen in the orientation shown in Figure 3b). The engagement surfaces 60 of the inner valve needle 18 are located in a plane perpendicular to the engagement surfaces 62 of the outer valve needle 20, as can be seen by comparing Figures 3a and 3b which show cross sections through planes perpendicular to one another and aligned with the direction of the longitudinal injector axis A-A.

[0052] The lift mechanism further includes a pull tube 64 which extends through the spring chamber 36 to cooperate with the upper ends of the inner and outer valve needles 18, 20. The upper end of the inner valve needle 18 and the upper end of the outer valve needle 20 are both received within a lower portion of the pull tube 64 (as seen in Figures 3a and 3b). Each of the inner and outer engagement surfaces 60, 62 is cooperable with a corresponding lift surface defined by the lower portion of the pull tube. The lift surfaces are constituted by inner valve lift surfaces 66 which are engageable with the engagement surfaces 60 on the inner valve needle 18 and outer valve lift surfaces 68 which are engageable with the engagement surfaces 62 on the outer valve needle 20. As for the respective engagement surfaces 60, 62, the inner valve lift surfaces 66 are located in a plane perpendicular to the outer valve lift surfaces 68 and aligned with the direction of the longitudinal injector axis A-A. The inner and outer valve lift surfaces 66, 68 are located at the same axial height along the pull tube 64.

[0053] The pull tube 64 extends upwardly from the valve needle arrangement 18, 20 and is operable by means of an actuator arrangement, referred to generally as 70. The pull tube 64 is provided with elongate openings 72 which extend along the longitudinal axis of the injector. Four elongate openings are provided in the pull tube 64 (as can be seen in Figure 1 , although only three can be readily identified in this section). A bridge piece 74 is located at the upper end of the inner valve needle 18, above the flanges 18d, and defines an abutment surface for a spring seat 76. The openings 72 in the pull tube 64 are positioned to allow the bridge piece 74 to extend through them and also to define an exit path for fuel which flows through the pull tube 64. The spring seat 76 defines a seat for the lower end of the spring 38 located within the spring chamber 36. The spring 38 surrounds the upper part of the pull tube 64 and provides a biasing force to the valve needles 18, 20 which serves to urge them against their respective valve seats 26, 28.

[0054] A yoke 80 or load transmitting member is located between the upper part of the outer valve needle 20 and the upper part of the inner valve needle 18. The yoke 80 extends into the seal volume 46 and defines a part of the load path for the force of the spring 38 to act on the valve needles 18, 20. The yoke 80 takes the form of an elongate annular member which locates around the inner valve needle 18 within the bore of the outer valve needle 20, closing the seal chamber 46 at its upper end. The yoke 80 includes upward extensions (only one of which is visible in Figure 3b). The upper surface of each yoke extension abuts the underside of the bridge piece 74. The force of the spring 38 acts on the spring plate 76, through the bridge piece 74 and the yoke 80 and onto the seal 48. The shape and configuration of the bridge piece 74 and the yoke 80 ensures that the spring force is applied in a uniform manner around the full circumference of the seal 48.

[0055] The load of the spring 38 which is transmitted through the bridge piece 74, the yoke 80 and the seal 48 to the inner and outer valve needles 18, 20 is transmitted through the parts so that the valve needles 18, 20 seat against their respective valve seats 26, 28 under the force of the spring 38. To minimize the frictional forces and the required spring load, the seal 48 requires a small contact area with the frusto-conical surfaces 42, 50. This is achieved by providing the thin-walled section 48b to define the end of the seal 48. As the thin-walled section 48b makes the seal 48 difficult to handle, the seal 48 is shaped to have the increased cross section at its upper end region 48a, as seen most clearly in Figure 4 This also minimizes the amount of the seal 48 that is subject to compressive stresses and therefore minimizes the potential for creep over time.

[0056] Referring again to Figure 1 , the actuator arrangement 70 for the valve needles 18, 20 includes a solenoid 82 housed within a solenoid housing 84 and an armature 86 which is actuated in response to a current being supplied to the solenoid 82. The armature 86 is fixedly attached to the pull tube 64 so that, as the armature 86 is caused to move, the pull tube 64 moves with it. The lower surface of the armature 86 rests against an annular insert 88 which is located within the upper end of the spring chamber 36. The actuator arrangement 70 includes an actuator spring 90 which acts on the upper end of the pull tube 64 to maintain a gap between the pull tube 64 and the engagement surfaces 60, 62 of the inner and outer valve needles 18, 20. This is the position in which the injector is not injecting fuel through the nozzle outlets 24 (non-injecting state). When the actuator arrangement 70 is activated by supplying a current to the solenoid 82, the armature 86 is caused to moved upwardly, away from the collar 88, pulling the pull tube 64 upwards.

[0057] The solenoid housing 84 is arranged within an inlet housing 92 of the injector which defines an inlet passage 94 to which gaseous fuel, such as hydrogen, is supplied in use. The actuator spring 90 is located within the inlet housing 92 so that gaseous fuel flows through the inlet passage 94 and through the actuator spring 90. Fuel delivered to the inlet passage 94 flows onward into the pull tube 64 from where it can flow through the elongate openings 72 in the wall of the pull tube 64 into the spring chamber 36. Fuel is also able to flow directly from the interior of the pull tube 64 into the bore 32 of the inner valve needle 18. Openings 96 provided in the upper end of the nozzle body 14 allow fuel to flow from the spring chamber 36 into the volume between the outer valve needle 20 and the nozzle body bore 16. Fuel delivered to this volume, and to the bore 32 of the inner valve needle 18, is able to flow into the combustion chamber when the inner and outer valve needles 18, 20 are lifted. It is important that the seal 48 is loaded sufficiently so as to ensure that in circumstances where the hydrogen tanks are significantly emptied, so that the available hydrogen pressure (e.g. around 50 bar) can be lower than the peak cylinder pressure (e.g. 200 bar), the seal 48 still remains seated against the frusto- conical surfaces 42, 50 of the seal volume. In this position the seal 48 prevents gaseous fuel from passing between the inner valve needle 18 and the outer valve needle 20 when the inner and outer valve needles 18, 20 are seated. The flexible nature of the seal 48 distributes the load equally across the inner and outer valve needles 18, 20, providing the cone angles and heights of the frusto-conical surfaces 42, 50 with which it is engaged, and the seat contact areas, are substantially equal. The flexibility of the seal 48 means that even if the valve seats 26, 28 wear by slightly different amounts, the seal 48 can accommodate this difference to ensure no gaseous fuel is able to flow between the inner and outer valve needles 18, 20. The advantage of the seal 48 is therefore to prevent unwanted leakage of fuel between the valve needle 18, 20.

[0058] Operation of the injector will now be described with further reference to Figures 6, 7a, 7b and 8, with particular attention to the function of the seal 48 in the seal chamber 46.

[0059] In the non-injecting state the inner valve needle 18 is seated against the lower valve seat 28 and the outer valve needle 20 is seated against the upper valve seat 26. This is the position shown in Figures 1 , 2, 3a, 3b and 4. With the inner and outer valve needles 18, 20 seated, no fuel is able to flow from the nozzle bore 16 into the seat volume 30 past the outer valve seat 26 and no fuel is able to flow through the pull tube 64 past the inner valve seat 28. Referring to Figure 4, the seal 48 is located so that the lower end engages with the facing frusto-conical surfaces 42, 50 of the outer and inner valve needles 20, 28. The presence of the seal 48 ensures that no fuel is able to escape between the inner valve needle 18 and the outer valve needle 20.

[0060] When it is required to inject fuel, the actuator arrangement 70 is energized by supplying a current to the solenoid coil 82. The armature 86 is therefore moved upwardly, against the force of the actuator spring 90, and the pull tube 64 moves with it. The pull tube 64 is moved into the position shown in Figure 6, in which the inner and outer pull tube lift surfaces 66, 68 are brought into contact with the engagement surfaces 60, 62 on the inner and outer valve needles 18, 20, respectively. As the surfaces (60 and 66, and 62 and 68) engage and the pull tube 64 moves further upwards, both the inner and the outer valve needles 18, 20 are caused to lift simultaneously against the force of the spring 38, breaking the seal between the inner and outer valve needles 18, 20 and their respective valve seats 28, 26. Once the seals are broken, gaseous fuel is able to flow through the open end of the inner valve needle 18, past the inner valve seat 28, into the seat volume 30 and out through the outlets 24, and also between the nozzle body bore 16 and the outer valve needle 20 and past the outer valve seat 26, into the seat volume 30 and out through the outlets 24. The differential seat arrangement is advantageous because it enables a relatively large flow of gaseous fuel into the combustion chamber when the injector is injecting in this way, as is desirable for optimum hydrogen combustion, without requiring a high lift force.

[0061] In order to terminate injection, the current is removed from the solenoid coil 82 and the inner and outer valve needles 18, 20 are returned into engagement with their respective valve seats 28, 26 under the force of the springs 38, 90, so that no gaseous fuel can flow past the valve seats into the seat volume 30 and through the outlets 24. The load from the spring 38 is applied through the bridge piece 74, the yoke 80 and the seal 48 and then onto the inner and outer valve needles 18, 20. With the inner and outer valve needles 18, 20 seated, the injector is in a noninjecting state.

[0062] As the valve needles 18, 20 lift, the seal 48 lifts together with the needles 18, 20 as it nestles in the conical tip of the seal volume 46. If there is any variation in seat wear between the inner and the outer valve seats 28, 26 this would lead to a slight deviation from simultaneous lifting of the valve needles 18, 20. The shape of the frusto-conical surfaces 50, 42 of the inner and outer valve needles 18, 20 accounts for this deviation as the seal can “ride up” the frusto-conical surfaces 50, 42. Furthermore, for seating of the valve needles 18, 20, the provision of the seal 48 accounts for any deviation in seat wear due to the flexibility of the seal 48 which forms part of the load path for the spring closing force, ensuring the closing force is applied uniformly.

[0063] An alternative configuration for the injector is shown in Figures 9 and 10, which show the injector in a non-injecting state. Again the injector includes inner and outer valve needles 18, 20, with the inner valve needle 18 being received within the bore of the outer valve needle 20. The seating arrangement at the lower end of the nozzle body is the same as in Figure 1 and will not be described in further detail. The inner valve needle 18 has a different configuration from the previous embodiment and includes a main stem 18e (of shortened form compared to the previous embodiment) which cooperates with the bore in the outer valve needle 20 to guide the inner valve needle 18 as it moves relative to the outer valve needle 20. A narrower diameter region 18f of the inner valve needle 18 extends above the main stem 18e. The main stem 18e of the inner valve needle 18 and the narrower diameter region 18f together define a step 100. An outer valve needle ring 102 is carried by and attached to the inner surface of the outer valve needle 20, typically by an interference fit or welding. In a similar manner, the inner valve needle 18 carries an inner valve ring 104 which is carried by and attached to the outer surface of the inner valve needle, in the narrower diameter region 18f, typically by an interference fit or welding. The step 100 and the lower surface of the outer valve needle ring 102 define a first gap, G1 , and the upper surface of the outer valve needle ring 102 and the lower surface of the inner valve needle ring 104 define a second gap, G2, when the valve needles 18, 20 are seated against their respective valve seats 28, 26.

[0064] A seal 148 is located within the bore of the outer valve needle 20, directly above the upper surface of the inner valve needle ring 104. As before, the seal 148 is typically made from polymer such as PEEK, PTFE, Pll or PAI. One difference in position, compared to the seal 48 in the previous embodiment, is that here the seal 148 is positioned in a part of the nozzle body 14 which projects outside the injector housing 34. The seal 148 is configured to have an annular base 148a of relatively large diameter compared to the relatively narrow diameter of an elongate upward projection 148b which locates within a groove (not labelled) in the outer surface of the narrower diameter region 18f of the inner valve needle 18. The groove may be considered to divide the inner valve needle 18 into a main body 18e, 18f and a head 18g. The upward projection on the seal 148 is provided with a frusto-conical region to define a tapered or angled surface 110 which engages with a corresponding angled or tapered surface defined by a frusto-conical region 18h formed on the inner valve needle 18. An elongate, annular spring abutment member 112 is located above the seal 148 and defines, at its upper end, a spring seat for an inner valve spring 114 acting on the inner valve needle 18. The spring force acts through the spring abutment member 112, the seal 148, and the inner valve ring 104. The spring 114 is received within the bore of the outer valve needle 20 and extends to a bridge piece 116 at the upper end of the outer valve needle 20. The bridge piece 116 takes a similar form to that in the previous embodiment and defines an abutment surface for the lower end of the spring 38 (which here acts as the outer valve needle spring) within the spring chamber 36.

[0065] The remaining parts of the injector, with regard to the pull tube 64 and the actuator arrangement 70, are also the same as for the previous embodiment and are labelled with like reference numerals.

[0066] With the inner and outer valve needles 18, 20 in their seated position against their respective valve seats 28, 26, as shown in Figures 9 and 10, the gaps, G1 and G2, are open. The inner valve spring 114 provides the force to urge the inner valve needle 18 against the inner valve seat 28, via the spring abutment member 112, the seal 148 and the inner valve ring 104. The outer valve spring 38 provides the force to urge the outer valve needle 20 against the outer valve seat 26, via the spring seat 76 and the bridge piece 116. By applying axial pressure to the seal 148 near the sealing surface, the seal 148 will try to expand radially to maintain contact pressure with the bore in the outer valve needle 20. As the seal 148 wears, axial loading means creep of the polymer will also expand the seal 148 to compensate for the wear. The presence of the frusto-conical surface or angled surface 110 on the seal 148, and its cooperation with the corresponding angled surface defined by the frusto-conical region 18h on the inner valve needle 18, means that gas pressure from below generates a radial load against the spring seat 112 which increases the friction to resist upward movement when the valve needles 18, 20 are seated. The presence of the gaps, G1 and G2, ensures that the inner valve needle 18 is free to move within the outer valve needle 20 so that both can seat and apply spring loads to their respective valve seats 28, 26 as long as any differential wear of the seats is within the range of the gaps, G1 and G2.

[0067] Figures 11 and 12 show the injector of Figures 9 and 10 when in an injecting state. In order to move the inner and outer valve needles 18, 20 away from their respective valve seats 28, 26, the actuator arrangement 70 is energized so that the armature 86 is pulled in an upwards direction (in the orientation shown). Movement of the armature 86 causes the pull tube 64 to move upwardly, drawing the outer valve needle 20 upwardly with it. As the outer valve needle 20 lifts away from the outer valve seat 26, gaseous fuel is able to flow past the outer valve seat 26, into the seat volume 30 and out through the outlets 24. In addition, as the outer valve needle 20 moves so too does the outer valve ring 102, moving to close the second gap, G2, and engaging with the inner valve ring 104. Hence, after movement through the gap distance G2, the inner valve needle 18 is caused to lift too, acting against the inner valve spring 114. As the inner valve needle 18 lifts from the inner valve seat 28, gaseous fuel is also able to flow past the inner valve seat 28 into the seat volume 30 and into the nozzle outlets 24. During this movement of the valve needles 18, 20, the seal 148 has to slide within the bore of the outer valve needle 20, together with the inner valve needle 18. To minimize wear of the seal 148 (by minimising this movement) it is therefore beneficial for the gap, G2, to be only as large as that which is needed to accommodate any differential wear at the valve seats 28, 26.

[0068] The sum of the first and second gaps, G1 and G2, can be set accurately by inserting a shim (not shown) in one of the gaps G1 , G2 when the inner valve ring 104 is pressed onto the inner valve needle 18. The shim is then removed before the inner valve needle 18 is inserted into the outer valve needle 20. The individual gaps, G1 and G2, can be adjusted when pressing this inner valve needle assembly into the outer valve needle 20. For example, this could be done by applying extra load to the outer valve needle 20 which causes it to compress slightly and, when it springs back, a gap, equivalent to the first gap G1 , will be generated between the inner valve needle 18 and the outer valve ring 102. This gap, G1 , can also be created with a pressing fixture which mimics the nozzle, but with a small offset in the relative seat positions.

[0069] List of parts

[0070] 10 - fuel injector

[0071] 12 - injection nozzle

[0072] 14 - nozzle body

[0073] 14a - upper enlarged region of nozzle body

[0074] 16 - nozzle bore

[0075] 18 - inner valve needle

[0076] 18a - enlarged head of inner valve needle 18b - narrowed region of inner valve needle

[0077] 18c - main stem of inner valve needle (first embodiment)

[0078] 18d - flanges on inner valve needle

[0079] 18e - main stem of inner valve needle (second embodiment)

[0080] 18f - narrow diameter region of the inner valve needle (second embodiment)

[0081] 18g - head of inner valve needle (second embodiment)

[0082] 20 - outer valve needle

[0083] 20a - enlarged head of outer valve needle

[0084] 20b - main stem of outer valve needle.

[0085] 24 - outlets

[0086] 26 - outer valve seat

[0087] 28 - inner valve seat

[0088] 30 - seat volume

[0089] 32 - bore of inner valve needle

[0090] 34 - injector housing

[0091] 36 - spring chamber

[0092] 38 - spring

[0093] 40 - enlarged region of bore in outer valve needle

[0094] 42 - frusto-conical surface of outer valve needle

[0095] 44 - shallow recess in inner valve needle

[0096] 46 - seal chamber

[0097] 46a - conical volume for fuel

[0098] 48 - annular seal (first embodiment)

[0099] 48a - relatively large diameter region of the seal

[0100] 48b - thin-walled region of the seal

[0101] 48c - curved region of the seal

[0102] 49 - dashed / dotted lines to represent movement of the seal

[0103] 50 - frusto-conical surface of inner valve needle

[0104] 58 - notch on outer valve needle

[0105] 60 - engagement surfaces of the inner valve needle

[0106] 62 - engagement surface on the outer valve needle

[0107] 64 - pull tube

[0108] 66 - inner valve lift surfaces

[0109] 68 - outer valve lift surfaces

[0110] 70 - actuator arrangement

[0111] 72 - elongate openings in pull tube 74 - bridge piece

[0112] 76 - spring seat

[0113] 80 - yoke

[0114] 82 - solenoid

[0115] 84 - solenoid housing

[0116] 86 - armature

[0117] 88 - annular insert

[0118] 90 - actuator spring

[0119] 92 - inlet housing

[0120] 94 - inlet passage

[0121] 96 - openings in upper end of nozzle body

[0122] 100 - step on inner valve needle

[0123] 102 - outer valve ring

[0124] 104 - inner valve ring

[0125] 110 - angled surface on seal

[0126] 112 - annular spring abutment member

[0127] 114 - inner valve spring

[0128] 116 - bridge piece

[0129] 148 - seal (second embodiment)

[0130] 148a - annular base of seal

[0131] 148b - elongate upward projection of seal

[0132] G1 - first gap

[0133] G2 - second gap

Claims

CLAIMS1. A fuel injector (10) for delivering gaseous fuel to an internal combustion engine, the fuel injector (10) comprising: an inner valve needle (18) which is engageable with an inner valve seat (28) to control fuel injection through at least one outlet (24) of the injector (10); the inner valve needle (18) being received within a bore provided in an outer valve needle (20) which is engageable with an outer valve seat (26) to control fuel injection through the at least one outlet (24); and an actuator arrangement (70) for effecting movement of the inner valve needle (18) and the outer valve needle (20) away from their respective valve seats (28, 26) to commence injection into the engine, wherein the inner valve needle (18) and the outer valve needle (20) together define a volume (46) therebetween for receiving an annular seal (48, 148) which serves to prevent gaseous fuel leakage between an outer surface of the inner valve needle (18) and an inner surface of the outer valve needle (20).

2. The fuel injector (10) as claimed in claim 1 , wherein the inner valve needle (18) includes a frusto-conical surface (50; 18h) which engages with the annular seal (48, 148).

3. The fuel injector (10) as claimed in claim 1 or claim 2, further comprising a spring assembly which serves to urge the inner and outer valve needles (18, 20) against their respective inner and outer valve seats (28, 26).

4. The fuel injector (10) as claimed in claim 3, wherein the spring assembly includes a spring (38) which acts on both the inner and outer valve needles (18, 20) via a load path through the annular seal (48).

5. The fuel injector (10) as claimed in claim 4, wherein the spring (38) acts on the inner valve needle (18) via a bridge piece (80) carried on the inner valve needle (18).

6. The fuel injector (10) as claimed in claim 4 or claim 5, when dependent on claim 2, wherein the annular seal (48) includes a first relatively large diameter region (48a) and a second relatively narrow diameter region (48b), wherein the second relatively narrow diameter region (48b) is cooperable with the frusto-conical surface (50) on the inner valve needle (18) and a further corresponding frusto-conical surface (42) on the outer valve needle (20).

7. The fuel injector (10) as claimed in claim 6, wherein the first relatively large diameter region (48a) transitions to the second relatively narrow diameter region (48b) via a region of the seal (48) defining a curved surface (48c).

8. The fuel injector (10) as claimed in any of claims 1 to 7, wherein the actuator arrangement (70) comprises a pull tube (64) which cooperates with the inner valve needle (18) and the outer valve needle (20) to cause them to lift away from their respective valve seats (28, 26) to commence injection.

9. The fuel injector (10) as claimed in claim 8, wherein the pull tube (64) defines an inner valve lift surface (66) which cooperates with the inner valve needle (18) to cause it to lift and an outer valve lift surface (68) which cooperates with the outer valve needle (20) to cause it to lift, wherein the gap defined between the inner valve lift surface (66) and the inner valve needle (18) and the gap defined between the outer valve lift surface (68) and the outer valve needle (20) are substantially the same so that the inner and outer valve needles (18, 20) lift at substantially the same time when the actuator arrangement (70) is actuated.

10. The fuel injector (10) as claimed in claim 9, wherein the inner valve lift surface (66) is offset perpendicularly, relative to the outer valve lift surface (68), about the longitudinal axis of the injector.

11. The fuel injector (10) as claimed in claim 1 or claim 2, wherein the spring assembly includes an inner valve spring (114) which acts on the inner valveneedle (18) to urge the inner valve needle (18) towards the inner valve seat (28) and an outer valve spring (38) which acts on the outer valve needle (20) to urge the outer valve needle (20) towards the outer valve seat (26).

12. The fuel injector (10) as claimed in claim 11 , wherein the inner valve spring (114) is located within the bore defined within the outer valve needle (20).

13. The fuel injector (10) as claimed in claim 11 or claim 12, wherein the outer valve needle (20) defines a gap (G2) with the inner valve needle (18) so that, upon actuation of the actuator arrangement (70), the outer valve needle (20) is caused to move away from the outer valve seat (26) through a distance equal to the gap before the inner valve needle (18) is caused to move away from the inner valve seat (28).

14. The fuel injector (10) as claimed in claim 13, wherein the gap (G2) is defined between an inner valve ring (104) carried on an outer surface of the inner valve needle (18) and an outer valve ring (102) carried on an inner surface of the outer valve needle (20).

15. The fuel injector (10) as claimed in any of claims 11 to 14, when dependent on claim 2, wherein the annular seal (148) includes a frusto-conical surface (110) which cooperates with the frusto-conical surface (18h) on the inner valve needle (18).

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