Fuel injector

Abstract

A gaseous fuel injector comprising a gas delivery chamber 48 and housing body 22 defining a working fluid chamber 34 and needle guide bore 24. A valve needle 12 is movable within the guide bore, where

Description

HELD OF THE INVENTION This invention relates to a fuel Injector for gaseous fuel. In particular, but not exclusively, the invention relates to a fuel injector for use in a fuel system of an internal combustion engine for a gaseous fuel such as hydrogen. BACKGROUND Gaseous fuels such as hydrogen are promising alternative fuels to gasoline and diesel due to their potential for low or zero emissions and there has been considerable interest in developing traditional internal combustion engines to run on ecologically produced hydrogen. High pressure injection of gaseous fuels into the combustion chamber of an internal combustion engine offers benefits, including reduced compression work, reduced susceptibility to uncontrolled auto-ignition and greater flexibility in combustion strategies resulting in improved efficiency. A servo method of operating gaseous fuel injectors is generally favoured and, in principle, the working fluid for servo operation of the injector valve needle could be the same gas as the gaseous fuel that is injected. However, the use of hydraulic oil as the working fluid offers the advantage of more accurate control due to the bulk modulus of the liquid. In a known hydrogen gas injector, the hydraulic control fluid is maintained at a pressure above the pressure of the injectable gaseous fuel to ensure there is no risk of the gaseous fuel leaking into the control fluid circuit. The control fluid is separated from the gaseous fuel via a sliding seal along a guide for the valve needle, but an inevitable consequence of the rigid sliding seal is fluid leakage across the pressure differential. This can result in injection of hydraulic fluid in the gaseous fuel, which can lead to fouling of the combustion chamber surfaces, exhaust sensors and after treatment apparatus. It is an object of the invention to provide to address this leakage problem. SUMMARY OF THE INVENTION According to a first aspect of the invention, there is provided a fuel injector of a fuel injection system for delivering gaseous fuel to an internal combustion engine, the fuel injector comprising a working fluid chamber, a gas delivery chamber and a needle guide bore extending from the working fluid chamber to the gas delivery chamber. The gas delivery chamber communicates with a fuel injector outlet for delivering the gaseous fuel to the internal combustion engine. A valve needle is movable within the needle guide bore along a needle axis to open and close the outlet, wherein a gap is defined between an outer surface of the valve needle and an inner surface of the needle bore. A biasing spring acts on the valve needle to urge the valve needle to close the fuel injector outlet. A spring abutment member is provided for one end of the biasing spring. An annular seal is cooperable with the spring abutment member so that a force of the biasing spring is transmitted to the annular seal through the spring abutment member, to urge an inner surface of the annular seal into engagement with the valve needle. The injector is an inwardly opening injector in which the valve needle is moveable inwardly, within an injector housing and along the needle axis, to open fuel injector outlet. The annular seal may be arranged between the valve needle and the needle bore to seal the gap. For example, in one embodiment the annular seal may comprise an inner seal surface which may contact the outer surface of the valve needle and an outer seal surface which may contact the inner surface of the needle bore. The needle bore may be formed in an injection nozzle housing, wherein the spring abutment member may extend at least in part into the injection nozzle housing. The fuel injector may comprise a carrier assembly which is carried by the valve needle, wherein the carrier assembly houses the seal arrangement. For example, the needle bore may be formed in an injection nozzle housing, and wherein the spring abutment member extends at least in part into the injection nozzle housing. in other embodiments, the annular seal may define an inner seal surface which engages with an outer surface of the valve needle. The carrier assembly may comprise first and second parts, wherein the first part defines the spring abutment member for the biasing spring. The annular seal may define an outer seal surface which engages with the first part. Where a carrier assembly is provided, the carrier assembly may further comprise an O-ring which is housed within the carrier assembly, whereby the O-ring engages with an engagement surface of the annular seal. For example, the annular seal may comprise a tangential surface which is tangential to the longitudinal axis of the valve needle. The tangential surface may define the engagement surface of the annular seal which is engaged with the O-ring. The second part may be urged into engagement with the injection nozzle housing, under the force of the biasing spring, to provide a further seal to prevent working fluid in the gap flowing into the gas delivery chamber. The seal arrangement may be made from polytetrafluoroethylene. The fuel injector may comprise a servo-valve mechanism for controlling the movement of the valve needle by controlling the pressure of a working fluid in the working fluid chamber. The valve needle may be constructed of more than one part, connected together, so that the end of the valve needle which engages with the valve seat to control injection out of the fuel injector is not the same part as that which slides within the nozzle guide bore, even though the two (or more) parts move together simultaneously. BRIEF DESCRIPTION OF THE DRAWINGS 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 schematic diagram of a known servo-actuated fuel injector for delivering a gaseous fuel; Figure 2 is an enlarged view of a part of the fuel injector in Figure 1; Figure 3 is a schematic diagram of a part of a fuel injector of a first embodiment; Figure 4 is an enlarged view of a part of the fuel injector in Figure 3; Figure 5 is a schematic diagram of a part of a servo-actuated fuel injector of a second embodiment; and Figure 6 is an enlarged view of a part of the fuel injector in Figure 5. Throughout this description, terms such as ‘upper and ‘lower’, and other directional references, are used with reference to the orientation of the injector as shown in the accompanying drawings. However, it will be appreciated that such references are not limiting and that injectors according to the invention can be used in any orientation. DETAILED DESCRIPTION OF THE INVENTION Referring to Figures 1 and 2, a known servo-actuated gaseous fuel injector includes an injection nozzle 10 comprising an injector valve needle 12 which is controlled by means of a servo-valve mechanism, referred to generally as 14. The fuel injector is arranged to inject gaseous fuel into a combustion chamber 16 of an internal combustion engine by moving the valve needle 12 towards and away from a valve needle seating 18 to control the gaseous fuel flow through one or more injector outlets 20. The valve needle has a longitudinal axis A-A and is movable along the axis A-A within a needle bore provided in an injection nozzle housing part 22. The fuel injector is inward opening so that the valve needle 12 moves inwardly within the needle bore to move away from the valve needle seating 18 to commence injection. An upper portion of the valve needle 12 defines a head of the valve needle and is received within a bore 24 in the injection nozzle housing part 22. The bore 24 acts as a needle guide for the valve needle 12 as it moves towards and away from the valve needle seating 18 (and may therefore be referred to as a needle guide portion 24). The upper portion of the valve needle 12 has an outer surface 13 which defines an axially extending gap 26 with an inner surface of the bore 24. As best seen in Figure 2, the servo-valve mechanism 14 comprises a valve member 30 which is operable by means of an electromagnetic actuator 32 to control fluid pressure within a working fluid chamber 34 defined at an upper end of the valve needle 12. The valve member 30 is movable within a valve bore provided in a valve housing 38, the valve housing 38 being in abutment with the injection nozzle housing part 22. The valve member 30 is movable between first and second valve seats 40, 42. In the example shown the first valve seat 40 is defined by the bore within the valve housing 38 and the second valve seat 42 is defined by an upper surface of the injection nozzle housing part 22. A low pressure drain passage 44 is provided in the injection nozzle housing part 22 so that, when the valve member 30 is moved away from the second valve seat 42, into engagement with the first valve seat 40, the working fluid chamber 34 communicates with the drain 44 to allow control fluid within the working fluid chamber 34 to flow to low pressure. High pressure control fluid is supplied to the valve bore in the valve housing 38 so that when the valve member is moved away from the first valve seat 40, into engagement with the second valve seat 42, hence closing communication between the working fluid chamber 34 and the tow pressure drain 44, high pressure control fluid is abie to flow into the working fluid chamber 34. A supply passage 46 is defined within the valve housing 38 and the injection nozzle housing part 22 for supplying injectable gaseous fuel to the injection nozzle 10. The supply passage communicates with a gas delivery chamber 48 defined in the injection nozzle housing part 22, with a valve needle spring 50 being housed within the gas delivery chamber 48. The valve needle spring 50 acts on the valve needle 12 to urge the valve needle into engagement with the valve needle seating 18, to prevent fuel injection into the combustion chamber 16. A tower end of the spring is engaged with a collar 52 carried by valve needle and an upper end of the spring 50 is engaged with the lower surface of the injection nozzle housing part 22, which results in a closing biasing fore being applied to the valve needle 12 by the spring 50. The actuator 32 is actuable to allow the valve needle 12 to lift away from the valve needle seating 18, against the biasing force of the spring 50, to commence injection through the injector outlet 20. The collar 52 resides within the gas delivery chamber 48 which houses the valve needle spring 50 and is therefore exposed to gaseous fuel within the gas delivery chamber 48. The working fluid within the working fluid chamber 34 is typically hydraulic oil and the fluid for injection, within the gas delivery chamber 48, is gaseous fuel such as hydrogen. It is therefore important to isolate, as far as possible, the working chamber 34 from the gas delivery chamber 48. To this end, a sliding seal is formed in the gap 26 between the upper portion of the valve needle 12 and the adjacent region of the needle guide bore 24 within the injection nozzle housing part 22. Due to the pressure difference across the sliding seal (between working fluid in the working chamber 34 and high pressure gaseous fuel in the gas delivery chamber 48), it is inevitable that some leakage occurs through the gap 26 across the sliding seal. Whilst tight tolerancing can minimise the fluid leakage rate, it is not always possible to eliminate this to a satisfactory level or controlled level. Referring to Figures 3 and 4, embodiments of the invention overcome this problem by introducing a separate seal arrangement into the injection nozzle. Similar parts to those shown in Figures 1 and 2 are denoted with the same reference numbers in Figures 3 and 4, and detaiis will not necessarily be repeated. In more detail, the valve needle 12 comprises a head portion 12a comprising a frusto-conical portion which defines a flat end surface 12b of the valve needle. The head portion is at the upper end of a main valve needle stem 12c. Working fluid within the working fluid chamber 34 is exposed to the frusto-conical head portion 12a. The outer diameter of the main valve needle stem 12c is larger than the diameter of the end surface 12b and is uniform along the full axial length of the valve needle 12. The valve needle 12 carries a spring abutment member 54 which defines a first abutment surface for a first, upper end of the valve needle spring 50. The second, lower end of the valve needle spring 50 is in abutment with a second abutment surface defined by the collar 52. The injection nozzle housing part 22 is modified compared to Figures 1 and 2 so that the needle guide bore 24 comprises an enlarged recess portion 24a at its lower end remote from the valve housing 38. The enlarged recess portion 24a is defined within the injection nozzle housing 22 and defines a stepped diameter to the bore 24 so that there is a step 56 (as identified in Figure 4) extending perpendicularly to the axis of the valve needle 12 which separates a needle guide portion 24b of the bore from the enlarged recess portion 24a of the bore. The spring abutment member 54 has a main body portion 54a which extends into the enlarged recess portion 24a and carries an inwardly extending annular flange 54b at a lower end of the main body portion 54a to define the second abutment surface 54c for the valve needle spring 50. The fuel injector comprises a seal arrangement comprising an annular seal 62 located within the enlarged recess portion 24a of the bore 24 so that an upper surface of the annular seal 62 engages with the step 56 and a lower surface of the annular seal 62 engages with the main body portion 54a of the spring abutment member 54. The annular seal 62 extends around the valve needle 12 to define an inner seal surface which contacts the outer surface of the valve needle 12, and an outer seal surface which contacts the inner surface of the needle guide bore 24. The provision of the annular seal 62 within the enlarged recess portion 24a serves to seal the gap between the outer surface of the valve needle 12 and the inner surface of the bore 24. This ensures working fluid in the working chamber 34, which communicates with the gap, is seaied from gaseous fuel within the gas delivery chamber 48. The annular seal 62 may be a rod seal. It does not seal against a guide portion of the bore 24 for the valve needle 12 but resides along the main stem of the valve needle 12 within the gaseous delivery chamber 48. It is beneficial to provide the annular seal 62 because it avoids unwanted working fluid escaping through the needle guide bore 24 into the gas delivery chamber 48 and therefore avoids accidental injection of any working fluid into the engine, which can foul surfaces of the combustion chamber 16. It will be appreciated that the spring abutment member 54 may be formed of more than one part, so that the part which engages with the annular seal 62 need not be the same part which engages with the spring 50, but may be a part carried by the spring abutment member 54. A further benefit of the seal arrangement is that the annular seal 62 is biased against the step 56 by means of the spring 50, acting through the spring abutment member 54. As valve needle movement is controlled to control injection into the combustion chamber 16 through an injection cycle, the force acting on the annular seal 62 due to the spring 50 will vary but nonetheless is applied throughout the injection cycle. Inevitably the annular seal 62 will suffer some wear, but because it is biased securely against the step 56 by the spring 50 the impact of any wear is mitigated. This also means that the annular seal 62 can be made of a relatively soft and elastic material such as a fluoroelastomer. The biasing force of the spring 50 mitigates wear of the annular seal 62 whilst the conformability of the malleable seal means that there is no adverse impact on the sealing properties. The arrangement also benefits from a simplicity of assembly. An alternative embodiment is shown in Figures 5 and 6 in which the enlarged recess of the previous embodiment is removed so that the injection nozzle housing 22 defines a needle guide portion 24a for the bore 24, as in Figures 1 and 2. A carrier assembly 70 is carried by the main stem or rod of the valve needle 12, the carrier member defining the spring abutment member for the spring 50. The carrier assembly 70 locates entirely within the gas delivery chamber 48 and comprises a two-part split gland arrangement. The split gland arrangement is of annular form and comprises a first, lower gland part 72 and a second, upper gland part 74. The lower gland part 72 forms the spring abutment member for the upper end of the valve needle spring 50. The lower gland part 72 is of elongate form and has an inwardly extending flange 76 which extends from the lower gland part 72 to define a lower gap 78 with the outer surface of the valve needle 12. The upper gland part 74 is of relatively short, conical form and defines an upwardly facing surface 80 (identified on Figure 6) which is tangential to the longitudinal axis of the valve needle 12. A radially inner surface of the upper gland part 74 defines an upper gap 82 with the outer surface of the valve needle 12. The upper edge of the upper gland part defines an annular knife-edge seal 84 which engages with the lower surface of the injection nozzle housing part 22. It is necessary to provide a split gland, having two parts, to enable installation of the seal member 90. An annular seal member 90 is housed within the split gland arrangement 72, 74 of the carrier assembly. The annular seal member 90 is of trapezoidal form and defines an upwardly facing abutment surface 92 which is tangential to the longitudinal axis of the valve needle 12. An O-ring 94 is located within the split gland arrangement 72, 74 so as to engage with the upwardly facing surface 92 of the seal member 90 and with the lower surface of the upper gland part 74. The valve needle spring 50 is engaged between the collar 52 and the lower surface of the first gland part 72 and therefore provides an upwardly directed force which acts through the lower gland part 72 and compresses the seal member 90 onto the O-ring 94 and, consequently, the O-ring 94 onto the upper gland part 74. The spring force therefore urges the knife-edge annular seal 84 against the lower surface of the injection nozzle housing part 22 to prevent any leakage of working fluid through the nozzle guide bore 24a from entering the gas delivery chamber 48. In addition, the O-ring 94 reacts against the tangential surface 92 of the seal member 90 causing the seal member 90 to conform and press against the valve needle 12 and prevent leakage fluid between the upper gap 82 and the lower gap 78. The spring force therefore has a dual purpose: firstly it provides the closing force for the valve needle 12 to terminate injection and urge the valve needle 12 against the valve needle seat 18 and, secondly, it provides a compression force for the seal member 90, acting through the O-ring 94, to prevent fuel leakage between the seal member 90 and the valve needle 12. It will be appreciated that the tangential nature of the upper surface 92 of the seal member 90 ensures there is a radially directed component of force via the O-ring 94 to compress the seal member 90 against the valve needle 12. The O-ring 94 is typically formed from a material which has good creep and chemical resistance, for example a fluoroelastomer. It is possible to use an elastomer because the O-ring 94 is energised by the spring 50 which then presses the annular seal 90 to conform to the valve needle 12. This also means that the seal member 90 can be made of a material which is more resistant to wear, such as a thermoplastic, whilst maintaining an adequate sealing force on the valve needle 12 due to the compressing by the O-ring 94. For example, the seal member 90 may be formed from a material such as PTFE which has good abrasion resistance and a low coefficient of friction. The sealing force to the valve needle 12 does not simply rely on the sealing force created during installation, as in many conventional arrangements, and so is more resistant to creep and wear occurring in service.

Claims

1. A fuel injector of a fuel injection system for delivering gaseous fuel to an internal combustion engine, the fuel injector comprising:a working fluid chamber (34), a gas delivery chamber (48) and a needle guide bore (24) extending from the working fluid chamber (34) to the gas delivery chamber (48), wherein the gas delivery chamber (48) communicates with a fuel injector outlet (20) for delivering the gaseous fuel to the internal combustion engine;a valve needle (12) movable within the needle guide bore (24) along a needle axis to open and close the outlet (20), wherein a gap is defined between an outer surface of the valve needle (12) and an inner surface of the needle bore (24);a biasing spring (50) which acts on the valve needle (12) to urge the valve needle (12) to close the fuel injector outlet (20);a spring abutment member (54; 72) for one end of the biasing spring (50); andan annular seal (62, 90) which is cooperable with the spring abutment member (54; 72) so that a force of the biasing spring (50) is transmitted to the annular seal (62; 90) through the spring abutment member (54; 72), to urge the annular seal into engagement with an outer surface of the valve needle (12).

2. A fuel injector as claimed in claim 1, wherein the annular seal (62, 90) is arranged between the valve needle (12) and the needle bore (24) to seal the gap.

3. A fuel injector according to claim 2, wherein the annular seal (62) comprises an inner seal surface which contacts the outer surface of the valve needle (12) and an outer seal surface which contacts the inner surface of the needle bore (24).

4. A fuel injector as claimed in claim 3, wherein the needle bore (24) is formed in an injection nozzle housing (22), and wherein the spring abutment member (54) extends at least in part into the injection nozzle housing (22).

5. A fuel injector as claimed in claim 1, comprising a carrier assembly (70) which is carried by the valve needle (12), wherein the carrier assembly (17) houses the annular seal (90).

6. A fuel injector as claimed in claim 5, wherein the annular seal (90) defines an inner seal surface which engages with an outer surface of the valve needle (12).

7. A fuel injector as claimed in claim 6, wherein the carrier assembly (70) comprises first and second parts (72, 74), and wherein the first part (72) defines the spring abutment member for the biasing spring (50).

8. A fuel injector as claimed in claim 7, wherein the annular seal (90) defines an outer seal surface which engages with the first part (72).

9. A fuel injector as claimed in claim 8, wherein the carrier assembly (70) further comprises an O-ring (94) which is housed within the carrier assembly (70), whereby the O-ring (74) engages with an engagement surface of the annular seal (90).

10. A fuel injector as claimed in claim 8 or claim 9, wherein the annular seal (90) comprises a tangential surface which is tangential to the longitudinal axis of the valve needle (12).

11. A fuel injector as claimed in claim 10 when dependent on claim 9, wherein the tangential surface (92) defines the engagement surface of the annular seal (90) which is engaged with the O-ring (74).

12. A fuel injector as claimed in any of claims 7 to 11, wherein the second part (74) is urged into engagement with the injection nozzle housing (22), under the force of the biasing spring (50), to provide a further seal to prevent working fluid in the gap flowing into the gas delivery chamber (48).

13. A fuel injector according to any preceding claim, wherein the seal arrangement (62; 90) is made from polytetrafluoroethylene (PTFE).

14. A fuel injector according to any preceding claim, comprising a servo-valve mechanism (14) for controlling the movement of the valve needle (12) by controlling the pressure of a working fluid in the working fluid chamber (34).5

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