Fuel injector

Abstract

A fuel injector 8 of a fuel injection system for delivering gaseous fuel to an internal combustion engine 16. The fuel injector comprises an injector housing and a valve needle 12 that is engageable w

Description

FIELD 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 as such there has been considerable interest in developing traditional internal combustion engines to run on 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 direct-acting actuation of the injector valve needle is limited by the force and working stroke that can be generated by the actuator, typically a solenoid that must be packaged within the confines of an injector. A servo method of operating gaseous fuel injectors is therefore generally favoured and offers the ability to decouple the solenoid actuator force and the wording stroke from the requirements of the valve needle. Servoactuation is generally associated with inward-opening injectors. However, outwardopening injectors can present advantages as an outward-opening valve resists injector backflow when cylinder pressure is higher than backflow. It is an object of the invention to address the requirement for an outward-opening injector suitable for gaseous fuel delivery. SUMMARY OF THE INVENTION According to a first aspect of the invention, there is provided a fuel injector for delivering gaseous fuel to an internal combustion engine, the fuel injector comprising an injector housing; a valve needle engageable with a valve needle seat and movable outwardly from the injector housing, away from the valve needle seat, to open a fuel injector outlet, a gas delivery chamber configured to receive a gaseous fuel for injection; a control valve arrangement operable to control a fluid pressure in a working fluid chamber; and a control piston cooperable with the valve needle and exposed to working fluid in the working fluid chamber so that, as fluid pressure in the working fluid chamber is varied by operating the control valve arrangement, the valve needle is caused to move relative to the valve needle seat to control fuel injection through the fuel injector outlet. Decoupling movement of the valve needle from the opening force of the hydraulically operable control valve arrangement enables a larger valve needle lift and a heavier valve needle spring to be used than would be possible with a direct-actuation arrangement. This enables the injector to operate over a wider gaseous fuel pressure range and allows higher gaseous fuel flow rates than can be achieved with a direct actuation. Furthermore, the injector can be used at very low gas pressures, which means the injector can be operated in a purge mode to empty accumulated gaseous fuel in the system during engine shutdown. The working fluid may be hydraulic oil, advantageously permitting more accurate control due to the bulk modulus of the liquid and damping and lubrication of moving components and sliding interfaces. The working fluid chamber is isolated from the working fluid when the injector is closed by virtue of the control valve arrangement. Because the control piston is statically leakless, there is no working fluid leakage while the fuel injector is closed / at rest. When the injector is actuated, the control piston forms a sliding seal with the piston guide. If the pressure of the working fluid is higher than gas pressure, some working fluid may leak into the injector cavity; however, the overall leakage rate is minimal as injection time is relatively brief in the full injection cycle. The ability of the injector to automatically increase the amount of micro lubrication with a longer injection duration is desirable for optimising working fluid consumption and adequate valve seat lubrication. The servo-actuation of the valve needle is advantageous in this respect. The fuel injector may comprise a nozzle housing located within the gas delivery chamber, the valve needle being received within the nozzle housing. The fuel injector may further comprise a valve needle spring, housed within the nozzle housing, which serves to urge the valve needle inwardly to the injector housing to seat against the valve needle seat. A control piston spring may be provided to act on the control piston in a direction to close communication between the working fluid chamber and the gas delivery chamber. The control piston spring may be a lower spring force than the valve needle spring, but in other embodiments the control piston spring may provide a higher spring force than the valve needle spring. The force need only be set to provide a positive closing force on the control piston, with the primary closing force provided by hydraulic control. In embodiments, the control piston may be cooperable with the valve needle via an intermediate member that serves to transmit a force from the control piston to the valve needle during outward movement of the valve needle. The intermediate member may project fully through an opening in the nozzle housing to engage with the control piston. The intermediate member may define a lift stop for valve needle movement through engagement between the lift stop and the nozzle housing. The control piston may be movable within a guide bore in response to variation of fluid pressure within the working fluid chamber, and wherein a gap is defined between an outer surface of the control piston and an inner surface of the guide bore. An annular seal member may be provided within the gap so that an inner seal surface contacts the outer surface of the control piston and an outer seal surface contacts the inner surface of the guide bore, thereby to provide a seal against fluid and / or fuel flow therethrough. The control piston may be provided with an annular groove for housing the annular seal member. Many other advantages of the invention will be envisaged with reference to the following description. 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 view of an outwardly opening fuel injector in a closed phase; Figures 2 and 3 are schematic views of the fuel injector of Figure 1 in operating positions during an opening phase; Figure 4 is a schematic view of the fuel injector of Figures 1 to 3 in a closing phase; Figure 5 is an enlarged view of a part of the fuel injector of Figure 4; Figure 6 is a schematic view of the fuel injector of Figures 1 to 5 when injection is being terminated through a closing phase; Figure 7 is a schematic view of the fuel injector of Figure 6, at the end of the closing phase; Figure 8 is graph to illustrate the variable injection rate that can be achieved using the disclosed injectors; Figure 9 is a schematic view of an alternative embodiment of the injector in Figures 1 to 7; and Figure 10 is an enlarged view of the control piston of the injector in Figure 9. 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 Figure 1, a servo-actuated gaseous fuel injectors includes an injection nozzle 10 comprising an outwardly opening injector valve needle 12. The valve needle 12 is controlled by means of a hydraulically controlled valve mechanism, referred to generally as 14, to move along a longitudinal valve needle axis. The fuel injector 8 is arranged to inject gaseous fuel into an internal combustion engine 16 of an internal combustion engine by moving the valve needle 12 outwardly, away from and towards a valve needle seat 18 to control the gaseous fuel flow through one or more injector outlets 20 defined in an injector housing. The valve needle 12 is movable within a gas delivery chamber 24 defined within the injector housing. The valve needle seat 18 is defined by an external surface of an end plate 22 forming a part of the injector housing. At its lower end, the valve needle 12 includes a needle seat portion 12a that is engageable with the valve needle seat 18. At its upper end, the valve needle 12 includes an enlarged head portion 12b that defines an upper and lower engagement surfaces 12c, 12d, respectively. A nozzle housing 26 is located within the gas delivery chamber 24, with the longitudinal axis of the nozzle housing 26 being aligned with the longitudinal valve needle axis. An upper portion 26a of the nozzle housing 26 is provided with an opening 28 in which an intermediate member in the form of a lift stop 30 is slidably received. The lift stop 30 comprises a main body portion and an enlarged head, the enlarged head defining upper and lower engagement surfaces 30a, 30b, respectively. The lower engagement surface 30b extends radially to a larger diameter than the opening 28 in the nozzle housing 26 and cooperation between the lower engagement surface 30b of the lift stop 30 and the upper portion 26a of nozzle housing 26 provides a means for limiting the extent of movement of the valve needle 12 outwardly from the end plate 22, as described further below. An opening 32 is provided at the lower end of the nozzle housing 26 to provide a communication path for gaseous fuel delivered to the gas delivery chamber 24 so that it can flow into the nozzle housing 26 and around the valve needle 12. The nozzle housing 26 also houses a valve needle spring 34, one end of which acts on the head portion 12b of the valve needle 12 and the other end of which abuts the end plate 22 of the injector housing. The valve needle spring 34 is configured to urge the valve needle 12 into a closed position in which the needle seat portion 12a is engaged with the valve needle seat 18. A control piston 40 is also arranged within the gas delivery chamber 24 and is configured to apply an opening force to the valve needle 12 when actuated. The control piston 40 comprises an upper portion 40a, a lower portion 40b of reduced diameter compared to the upper portion 40a, and an annular collar 40c part-way along its length. The upper portion 40a of the control piston 40 is received within a guide bore 42 provided in a control piston housing 44. The upper portion 40a of the control piston 40 has an outer surface that defines a narrow axially extending gap 45 with an inner surface of the guide bore 42. The upper portion 40a comprises a frusto-conical portion that defines an end face of the control piston 40d. The upper end 40a of the control piston 40 is engageable with a piston seat 46 defined within the control piston housing 44. A control piston spring 48 is arranged within the gas delivery chamber 24 to engage between the annular collar 40c of the control piston 40 and an upper surface of the upper portion 26a of the nozzle valve housing 26. The control piston spring 48 serves to urge the control piston 40 towards the piston seat 46. When the control piston 40 is moved away from the piston seat 46, against the force of the control piston spring 48, it moves downwardly within the guide bore 42 until it contacts the lift stop 30. Continued movement of the control piston 40 causes the lift stop 30 to move downwardly with it, until the lift stop engages the head portion 12b of the valve needle 12 and urges the valve needle 12 away from the valve needle seat 18, against the force of the valve needle spring 34. Movement of the control piston 40 is controlled by means of a servo valve mechanism, referred to generally as 50, comprising a control valve member 52. The control valve member 52, operable by means of an electromagnetic actuator 54, controls fluid pressure within a working fluid chamber 56 defined at the upper portion of the control piston 40a. The actuator 54 controls movement of the control valve member 52 between first and second valve positions, allowing the valve needle 12 to move away from the valve needle seat 18 to commence injection through the injector outlet 20, as discussed further below. The control valve member 52 is movable within a control valve bore 53 provided in a valve housing 58 located adjacent to the control piston housing 44. The control valve member 52 is movable between first and second valve seats, 60 and 62 respectively, under the control of the actuator 54. In the example shown in Figure 1, the first valve seat 60 is defined by the control valve bore 53 and the second valve seat 62 is defined by an upper surface of the valve housing 58. A low pressure drain passage 64 is provided in the control piston housing 44 so that when the control valve member 52 is moved away from the first valve seat 60 into engagement with the second valve seat 62 the working fluid chamber 56 communicates with the drain passage 64 to allow working fluid within the working fluid chamber 56 to flow to low pressure. A supply passage 66 for working fluid is provided through the control valve housing 58 and the control piston housing 44 so that when the control valve member is moved away from the second valve seat 62 into engagement with the first valve seat 60 working fluid delivered through the supply passage 66 is supplied to the working chamber 56 and cannot flow to the drain passage 64. It is envisaged that the relative strength of the control piston spring 48 is much less than the force of the valve needle spring 34, and just enough to ensure positive closing of the control piston 40, with the primary closing force of the control piston 40 being hydraulic imbalance. The control piston spring 48 in fact increases the pressure of the working fluid that is required to open the valve needle 12, but on the other hand does not contribute to the sealing force of the valve needle 12. As an alternative, it would be possible to have a control piston spring 48 that was heavier than the valve needle spring 34 to ensure that the fluid pressure in the working fluid chamber 56 remains at or above gas pressure during the period of needle closing when the control piston 40 is not seated. This would inhibit any gas leakage into the working fluid chamber 56 during this period. A gaseous fuel supply passage 70 is also defined within the control valve housing 58 and the control piston housing 44 for supplying injectable gaseous fuel to the gas delivery chamber 24. When the valve needle 12 is moved away from the valve needle seat 18, gaseous fuel may be delivered from the gas delivery chamber 24 through the fuel injector outlet 20. Figure 1 shows the fuel injector in a closed position where the actuator 54 is deenergised so that the control valve member 52 is seated against the second valve seat 62. The working fluid chamber 56 communicates with the drain passage 64 and the pressure in the working fluid chamber 56 is therefore relatively low. In this case, the control piston 40 is urged upwardly against the piston seat 46 by means of the control piston spring 48, so that the valve needle 12 is urged into engagement with the valve needle seat 18 under the force of the valve needle spring 34. In this state of operation, there is no injection of gaseous fuel from the gas delivery chamber 24. Referring to Figure 2, when the actuator 54 is energised initially, the control valve member 52 is drawn upwards (in the orientation shown) away from the second valve seat 62 and into engagement with the first valve seat 60. In this scenario, high pressure working fluid is delivered to the working fluid chamber 56 through the supply passage 66 so that a force is exerted on the control piston 40, which serves to urge the control piston 40 downwards (as indicated by the arrow), against the force of the control piston spring 48. This causes the control piston 40 to move away from the piston seat 46 into engagement with the lift stop 30. This is an intermediate state of the injector during the opening phase where the actuator 54 is energised but the valve needle 12 has not yet moved away from the valve seat 18 to permit injection. Referring to Figure 3, as the pressure of working fluid in the working fluid chamber 56 continues to build, the control piston 40 moves further downwards, moving the lift stop 30 and the valve needle 12 together downwardly (as indicated by the arrows), against the force of the valve needle spring 34. This causes the valve needle 12 to move outwardly from the injector housing, away from the end plate 22 and the valve needle seat 18, . This is a fully open state of the injector where the actuator 54 is energised and injection has commenced through injector outlet 20 in the form of an injection spray 21. Referring to Figures 4 and 5, when it is required to terminate injection, the actuator 54 is deenergised so that the control valve member 52 moves downwards, away from the first valve seat 60 and into engagement with the second valve seat 62. In this position, the working fluid chamber 56 is brought into communication with the drain passage 64 so that pressure in the working fluid chamber 56 starts to decrease. As the force acting downwardly on the control piston 40 decreases, the control piston spring 48 and the valve needle spring 34 start to urge the control piston 40 upwardly (as indicated by the arrows in Figure 4), actively pumping working fluid from the working fluid chamber 56 to the drain passage 64 past the open first valve seat 60. This is an intermediate closing state of the injector where the actuator 54 is deenergised but injection has not yet terminated. Eventually the operating state is reached where the valve needle 12 has seated against the valve needle seat 18 and injection is terminated. With the valve needle 12 seated, the lift stop 30 and the control piston 40 continue to move upwardly (as indicated by the arrow) under the influence of the springs 48, 34 until the working fluid chamber 56 is fully discharged to the drain passage 64. This non-injecting operating state is shown in Figure 7 where the control piston 40 has again seated against the piston seat 46. The rate of closing of the valve needle 12 is controlled carefully by the flow area provided by the drain passage 64, which dictates the rate of discharge of working fluid from the working fluid chamber 56. This creates an intermediate pressure in the working fluid chamber 56 during valve needle closure (as in Figure 6). The hydraulically operated control valve 50 therefore limits the valve seat impact velocity, which can reduce seat wear and valve needle bounce. Since the gaseous fuel is isolated from the working fluid in the working fluid chamber 56 when the valve needle 12 is seated and the injector is closed, it is possible to decouple the working fluid pressure from the pressure in the gas delivery chamber 24. For any given gas pressure there is a minimum working fluid pressure that is required to open the injector, i.e., to generate enough force on the control piston 40 (and hence on the lift stop 30 and the valve needle 12) to cause the movement required against the pressure of gas in the gas delivery chamber 24. Increasing working fluid pressure to a level above this minimum results in a faster opening of the valve needle 12. Closure of the injector and seating of the valve needle 12 is governed by the total spring force (a combination of the valve needle spring 34 and the control piston spring 48) and the gaseous fuel pressure but is not sensitive to the working fluid pressure. Figure 8 is a graph to show the effect of the working fluid pressure on the injection rate over injection demand duration, i.e., the period for which the injector is open and injecting. It can be seen from Figure 8 that the injection rate increases as the working fluid pressure increases, while the closing rate does not vary. For example, the injection rate has a relatively shallow slope as a function of the injection demand duration for relatively low working fluid pressure (plot X) but has a relatively steep slope as a function of the injection demand duration for relatively high working fluid pressure (plot Y). The ability to select the opening rate of the injector by varying the working fluid pressure provides an advantage for optimising gas jet mixing and load transients. For example, during a high-low load transient a small injection quantity is important and a slower injector opening rate affords more accurate injection quantity metering. Due to the high bulk modulus of liquids, working fluid pressure can also be varied more quickly than gas pressure so the responsivity with which the injection rate can be varied, for optimum performance, is advantageous. The working fluid within the working fluid chamber 56 is typically hydraulic oil, and the fluid for injection within the gas delivery chamber 24 is gaseous fuel such as hydrogen. It may therefore be beneficial to isolate, as much as possible, the working fluid chamber 56 from the gas delivery chamber 24. With reference to Figure 1, a sliding seal is formed in the gap 45 between the upper portion of the control piston 40 and the adjacent region of the guide bore 42 within the control piston housing 44. Due to the pressure difference (between working fluid in the working chamber 56 and high pressure gaseous fuel in the gas delivery chamber 24), some leakage may occur through the gap 45 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. It may be assumed that, because the injector is non-injecting for most of the time, when no or reduced working fluid is in the working fluid chamber 56, any leakage may be very low. However, it may be desirable to maintain the working fluid at a significantly higher pressure than the gas pressure (e.g., for injection rate purposes), which tends to increase the leakage rate during injection. However, it is a characteristic of the outwardly-opening injector that the required actuation force to open the valve needle 12 decreases with increasing gas pressure. To prevent excessive valve needle opening velocity, it may therefore be desirable, in some circumstances, to maintain working fluid pressure below gas pressure. In this scenario it is feasible that gas could leak into the working fluid chamber 56. Any sealing between the working fluid chamber 56 and the gas delivery chamber 24 must therefore be symmetrical in operation so that there is a sealing function with the pressure differential in either direction. A further embodiment addresses this issue by introducing a separate annular seal member 80 into the injector, as shown in Figure 9. Similar parts to those shown in previous figures are denoted with the same reference numbers in Figure 9, and details will not necessarily be repeated. In Figure 9 the injector is shown in the noninjecting state. As shown in Figure 10, the control piston 140 is provided with an annular groove 82 so that the control piston 140 comprises three portions: an upper head portion 140a that includes a frusto-conical portion defining an upper end face 140b, a thinned portion 140c that is formed in the upper portion 140a and created by the annular groove 82, a lower portion 140d of reduced diameter relative to the upper portion 140a, and an annular collar 140e. With reference to Figure 9, the annular seal member 80 resides within the gap 45 between the control piston 40 and the internal surface of the guide bore 42. The annular seal member defines an inner seal surface that seals against the outer surface of the control piston 140, in the region of the annular groove 82, and an outer seal surface that seals against the inner surface of the guide bore 42. The annular seal member 80 provides a tight seal under varying conditions including component dimension tolerance, thermal expansion, mechanical loading, and varying working fluid and gas pressure. The injector in Figure 9 operates in the same way as described for previous embodiments by controlling the hydraulically operated control valve 50 to control movement of the valve needle 12. The leakage rate is considerably reduced through the gap 45 with the seal member within the annular grove 82, compared to the situation where the sealing relies only on the metal interfaces between the outer surface of the control piston 140 and the inner surface of the guide bore 42. The seal member 80 may be formed from PTFE, which has low sliding friction properties, excellent chemical resistance, and good tolerance in operating temperatures ranging between -40°C to 120°C especially. The PTFE may be virgin PTFE (PTFE without a filler) or filled PTFE. Advantages of the aforementioned embodiments are multiple and include the following: (1) Decoupling movement of the valve needle 12 from the opening force of the hydraulically operable control valve 50 enables a larger valve needle lift and a heavier valve needle spring 34 to be used than would be possible with a direct-actuation arrangement. This enables the injector to operate over a wider gaseous fuel pressure range and allows higher gaseous fuel flow rates than can be achieved with a direct actuation. (2) The injector can be operated at very low gas pressures, and as low as zero gauge gas pressure, which means the injector can be operated in a purge mode to empty accumulated gaseous fuel in the system during engine shutdown. (3) A hydraulically operated control valve 50 for the injector enables a much smaller solenoid to be used compared to a direct-actuation arrangement. This means there is a lower electrical power demand from the solenoid drive arrangement. (4) A hydraulically operated control valve 50 for the injector results in good valve needle dynamics with reduced maximum opening and closing velocities. This results in reduced valve bounce, injector noise, and seat wear (e.g., increased service life). (5) A hydraulically operated control valve 50 for the injector provides a means for some micro lubrication of the valve needle seat 18 without the need for a separate micro-doser for lubricant, providing lower system cost and complexity. (6) Injection rate shaping capabilities through varying the working fluid pressure enables optimised injection rates and injection metering quantity control even during large high load / low load engine transients, which is otherwise challenging in a gaseous fuel injection system. (7) A statically leakless injector as described enables the hydraulic system to maintain gas pressure within the gas delivery chamber 24 during periods of non-injection when hydraulic pump pressure may not be available, for example during an engine start-stop strategy. In addition, the statically leakless operation prevents the accumulation of hydraulic fluid in the gas delivery chamber 24 during periods of non-injection where hydraulic pressure is present, as may occur during engine overrun. It will be appreciated that various modifications may be made to the aforementioned embodiments without departing from the scope of the appended claims.

Claims

1. A fuel injector (8) of a fuel injection system for delivering gaseous fuel to an internal combustion engine (16), the fuel injector comprising:an injector housing;a valve needle (12) that is engageable with a valve needle seat (18) and movable outwardly from the injector housing, away from the valve needle seat (18), to open a fuel injector outlet (20),a gas delivery chamber (24) configured to receive a gaseous fuel for injection;a control valve arrangement (50) that is operable to control a fluid pressure in a working fluid chamber (56); anda control piston (40) cooperable with the valve needle (12) and exposed to working fluid in the working fluid chamber (56) so that, as fluid pressure in the working fluid chamber (56) is varied by operating the control valve arrangement (50), the valve needle (12) is caused to move relative to the valve needle seat (18) to control fuel injection through the fuel injector outlet (20).

2. The fuel injector as claimed in claim 1, comprising a nozzle housing (26, 26a) located within the gas delivery chamber (24), the valve needle (12) being received within the nozzle housing (26).

3. The fuel injector as claimed in claim 2, comprising a valve needle spring (34), housed within the nozzle housing (26), which serves to urge the valve needle (12) inwardly to seat against the valve needle seat (18).

4. The fuel injector as claimed in claim 2 or claim 3, comprising a control piston spring (48) that acts on the control piston (40) in a direction to closecommunication between the working fluid chamber (56) and the gas delivery chamber (24).

5. The fuel injector as claimed in claim 4 when dependent on claim 3, wherein the control piston spring (48) has a lower spring force than the valve needle spring (34).

6. The fuel injector as claimed in any of claims 2 to 5, wherein the control piston (40) is cooperable with the valve needle (12) via an intermediate member (30) that serves to transmit a force from the control piston (40) to the valve needle (12) during outward movement of the valve needle (12).

7. The fuel injector as claimed in claim 6, wherein the intermediate member (30) projects fully through an opening (28) in the nozzle housing (26) to engage with the control piston (40).

8. The fuel injector as claimed in claim 6 or claim 7, wherein the intermediate member defines a lift stop (30) for valve needle movement through engagement between the lift stop (30) and the nozzle housing (26).

9. The fuel injector as claimed in any of claims 1 to 8, wherein the control piston (40) is movable within a guide bore (42) in response to variation of fluid pressure within the working fluid chamber (56), and wherein a gap (45) is defined between an outer surface of the control piston (40) and an inner surface of the guide bore (42).

10. The fuel injector as claimed in claim 9, wherein an annular seal member (80) is provided within the gap (45) so that an inner seal surface contacts the outer surface of the control piston (40) and an outer seal surface contacts the inner surface of the guide bore (42), thereby to provide a seal against fluid and / or fuel flow therethrough.

11. The fuel injector as claimed in claim 10, wherein the control piston (40) is provided with an annular groove (82) for housing the annular seal member (80).

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