Fuel injector assembly

Abstract

A fuel injector assembly 20 for delivering gaseous fuel comprising a fuel injector 21 with main injector housing 32. The injector has an electromagnetic actuator 24 mounted on an upper region 34 of th

Description

FIELD OF THE INVENTION This invention relates to a fuel injector assembly for gaseous fuel. In particular, but not exclusively, the invention relates to a fuel injector assembly for use in a fuel system of, for example, an internal combustion engine or fuel cell for a gaseous fuel such as hydrogen. BACKGROUND Gaseous fuels, such as hydrogen, are promising alternative fuels to fossil fuels, such as 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. Injection of gaseous fuels into the combustion chamber of an internal combustion engine, especially for higher pressures, offers benefits, including reduced compression work, reduced susceptibility to uncontrolled auto-ignition, and greater flexibility in combustion strategies resulting in improved efficiency. A relatively low flow fuel injector for hydrogen fuel is already known, particularly for use with internal combustion engines, but there remains a need fora relatively high flow injector to satisfy the demands of engines with fuel cells. Injectors capable of delivering the requisite high flows for gaseous fuel delivery typically include a large flow area that requires a significant force from an actuator of the injector. For example, an electromagnetic actuator requires a significant magnetic force to open an inwardly opening injector valve needle. In addition, the injector must open against a large gaseous fuel pressure difference between the injector inlet and the injector outlet. In order to generate the necessary force from the electromagnetic actuator, it is necessary to apply a large electrical current to the solenoid coil of the actuator, which can consequently lead to a large increase in temperature within the solenoid coil and the surrounding components. In some injector applications, it is sufficient to rely on the short time between injections (i.e. the injector ‘off’ time) to allow the dissipation of thermal energy. It is also known to manage the thermal energy within the injector using active cooling means, including the use of a fuel flow through the injector during injection. One of the problems with fuel cell applications, however, is that the duration of injection is much longer than in internal combustion engine applications, which means the associated thermal loading is higher. Such injectors tend to be sidefeed injectors so that fuel enters the injector much lower along the axial length of the injector and cannot easily be routed past the actuator for cooling purposes. The mounting of an injector in a fuel cell engine also requires the injector to be positioned or mounted on one or more O-rings, and so conductive thermal dissipation does not provide adequate cooling. It is against this background that the invention has been devised. SUMMARY OF THE INVENTION According to a first aspect of the invention, there is provided a fuel injector assembly of a fuel injection system for gaseous fuel. The fuel injector assembly comprises a fuel injector, an electromagnetic actuator, an overmould, and a cooling device. The fuel injector comprises a main injector housing comprising an upper region and a lower region, the main injector housing having a longitudinal injector axis. The electromagnetic actuator is mounted on the upper region and comprises a solenoid assembly having a solenoid tube housing a solenoid coil. The overmould is mounted on the upper region of the main injector housing for encapsulating at least a part of the electromagnetic actuator. The cooling device comprises a cooling chamber for receiving a cooling fluid. The cooling device is integrated within the overmould so that the overmould defines at least internal and external sidewalls of the cooling chamber. The internal sidewall contacts or abuts at least a portion of the solenoid tube to form a thermal path extending between the actuator and the cooling chamber. The cooling device on the fuel injector provides a direct and effective means for dissipating heat from the fuel injector. Importantly, the configuration of the cooling device being integrated within the overmould means it can be readily and easily mounted on the injector without having to modify the configuration of the fuel injector or the associated engine head for accommodating the cooling device. The internal sidewall of the cooling chamber forms a part of the direct thermal path between the actuator and the cooling fluid to which heat can be dissipated through the cooling device. In embodiments, an outer surface of the fuel injector assembly may be at least partially formed by the external sidewall of the cooling chamber. In embodiments, the overmould may include an internal overmould wall that extends between the solenoid coil and the solenoid tube to define a part of the thermal path between the actuator and the cooling chamber. The internal overmould wall advantageously ensures a direct conductive path between the solenoid coil and the solenoid tube for effective heat transfer. In embodiments, the fuel injector assembly may comprise an inlet for cooling fluid to flow to the cooling chamber and an outlet for cooling fluid to flow from the cooling chamber, and the inlet and outlet may be located on substantially opposed sides of the cooling chamber. The cooling fluid advantageously circulates around a substantial proportion of the cooling chamber, allowing it to absorb a large amount of heat from the solenoid coil. In embodiments, the inlet and the outlet may be formed on a lid of the cooling chamber. The lid may form an intergral part of the overmould. In embodiments, the overmould may be formed from a plastic. The overmould, and the associated integrated components, may thus be formed together, improving the ease and cost of manufacturing of the overmould. In embodiments, the gaseous fuel may be hydrogen. It will be appreciated that the fuel injector assembly is appropriate for use with an internal combustion engine or a fuel cell application. 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 perspective view of a mounting portion of a known fuel injector assembly; Figure 2 is a cross section of a mounting portion of a fuel injector assembly of an embodiment; and Figure 3 is a perspective view of the fuel injector assembly of Figure 2. DETAILED DESCRIPTION OF THE INVENTION 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. By way of background to the invention, an upper portion of a known fuel injector assembly 1 comprising a fuel injector 2 is shown in Figure 1. Internally the fuel injector 2 comprises an injection nozzle (not shown) having an injector valve needle that is inwardly-opening within the injection nozzle and is controlled by means of a servo-valve mechanism. Externally, the injector assembly 1 can be seen to include several housing parts, including an actuator housing or overmould 4 and a main injector housing or fuel tube 6 that houses the valve needle. The injector assembly 1 may be arranged in a fuel cell application or arranged to inject gaseous fuel into a combustion chamber of an associated internal combustion engine by moving the valve needle towards and away from a valve needle seating to control the gaseous fuel flow through one or more injector outlets. An electromagnetic actuator (not shown) is positioned towards the upper end of the fuel injector 2, within the actuator housing 4. The electromagnetic actuator comprises a solenoid coil to which a current is supplied to generate an electromagnetic force that acts on the injector valve needle, directly or indirectly. Operation of the actuator therefore controls movement of the valve needle to control the injection of gaseous fuel. Gaseous fuel is introduced to the fuel injector 2 via inlet fuel ports 5 (one of which is identified), the inlet fuel ports 5 receiving gaseous fuel from an inlet that projects from the upper end of the fuel injector 2. A side projection 8 of the actuator housing 4 houses an electrical connector (not shown) electromagnetic actuator. The actuator housing 4 is formed from an overmould that is typically formed from plastic. The injector assembly 1 is clamped to an associated engine head through the overmould. Also, being positioned on multiple O-rings 10 which contact the associated engine head when the fuel injector 2 is clamped to it, there is no direct metal-to-metal contact between the fuel injector 2 and the engine head. It is important to control and manage the temperature of electromagnetic or solenoid actuators for fuel injectors as temperature changes can affect the performance of the fuel injectors in operation. Heating of the solenoid coil through operation presents a number of problems for the fuel injector. For example, an increase in the temperature of the solenoid coil can cause the resistance of the coil to increase. An increase in resistance causes further heating for a given current supplied to the solenoid coil, that again causes a rise in the resistance. An increasing temperature therefore drives a cyclical heating problem if heat is not effectively dissipated from the solenoid coil. Excessive heating to the solenoid coil can result, for example, in thermal damage to the actuator and / or the resistance of the solenoid coil reaching a threshold whereby given voltages are unable to meet corresponding current targets, thus preventing the fuel injector from operating as desired. Further, an increase in the temperature of the solenoid coil can cause thermal dissipation to other components within the fuel injector, causing them to expand despite the components being at least partially radially and / or axially constrained. For example, elastomer dampers (not shown) retained within the fuel injector may increase in length axially when heated above a standard operating temperature range. This increase in length can result in a shorter needle lift for the valve in the fuel injector, thus causing a reduction in flow. When the fuel injector cools, and thus when the elastomer dampers cool, this effect is reversed and normal operating conditions can resume. This variation in flow and performance is not desirable and may not be acceptable for certain customer applications. It is therefore necessary identify a more reliable means of managing the temperature within the fuel injector for delivering gaseous fuel to an associated engine or engine head. The invention addresses this problem by equipping the fuel injector assembly with a cooling device that provides a direct heat transfer path between the actuator and the cooling device to allow heat to dissipate from the solenoid coil. Referring to Figures 2 and 3, an upper portion of a fuel injector assembly 20. The injector assembly 20 comprises a fuel injector 21 with a main injector housing or fuel tube 32 with several different sections of varying diameter including an upper region 34 of relatively narrower diameter, and a lower region 36 of relatively large diameter. A step 38 defines a supporting ledge between the upper and lower regions 34, 36. A moveable armature (not shown) within the fuel tube 32 is coupled to an injector valve needle 22 and is configured control the injection of gaseous fuel into an associated engine or engine head. The fuel injector 21 comprises an actuator 24 comprising a solenoid coil 26 that is wound on a bobbin 28, the bobbin 28 being supported by the step 38. A solenoid tube 30 annularly surrounds the components of the actuator 24. The fuel tube 32 extends through the bobbin 28 and serves to drive movement of the armature within the fuel tube 32. When current is applied to the solenoid coil 26, an electromagnetic force is generated that acts on the armature to control the movement of the injector valve needle 22 along a main axis A-A of the fuel injector 21. Gaseous fuel is introduced to the fuel injector 21 via inlet fuel ports 65 (one of which is identified) and a filter 66 in the fuel tube 32 and as the injector valve needle 22 is controlled to move, injection of the gaseous fuel occurs through injector outlets (not shown). The fuel injector assembly 20 may operate in any pressure range - relatively low, medium, or high pressures - depending on the system requirements. For example, low pressure gaseous fuel may be supplied to the fuel injector assembly 20 in the range of approximately 10-20 bar. It will be appreciated that this range is not exhaustive and other pressures and pressure ranges may be equally applicable. An overmould 40 is applied to the upper region 34 of the fuel tube 32 of the fuel injector 21 and fills any open gaps around the solenoid tube 30, for example including the space between the solenoid coil 26 and the solenoid tube 30, forming an internal overmould wall 41 of the overmould 40. The overmould 40 is configured to cover the components of an actuator 24 of the fuel injector 21, including the majority of the solenoid tube 30 while leaving a small lower section of the solenoid tube 30 exposed. The exposed lower section abuts partially with the step 38 and partially with an outer surface of the fuel tube 32. The overmould 40 includes a side projection 45 that extends, at an angle, axially above an upper surface 44 of the overmould 40. An O-ring 42 may be retained within the overmould 40 to provide a seal against the fuel tube 32 and to prevent contamination from the external environment outside of the injector assembly 20. An end cap 46 covers the upper region 34 of the fuel tube 32 and defines an internal abutment surface for a valve needle spring (not shown) that serves to bias the valve needle 22 closed. The end cap 46 and the overmould 40 together provide an upper enclosure for the components of the fuel injector 21 that extend beyond a pocket in an engine housing component. The side projection 45 of the overmould 40 houses an electrical connector (not shown) for the solenoid coil 26. The overmould 40 comprises an integrated cooling device 50 comprising an annular water-jacket. The cooling device 50 is configured to provide heat dissipation via thermal transfer from the solenoid coil 26 to a circulating cooling medium, such as water or any suitable fluid, to prevent the components of the actuator 24, and thus the fuel injector 21, from reaching undesirable high temperatures during operation. The cooling device 50 comprises a cooling chamber, tank, or cavity 52 defined by an internal annular sidewall 53, an external annular sidewall 54, an upper lid 56 and a lower base 55. The cooling chamber 52 is configured to receive a cooling fluid. As can be seen most clearly in Figure 3, an inlet 60 and an outlet 62 to the cooling chamber 52 are disposed on an upper external surface 58 of the lid 56 of the cooling device 50 to provide flow paths for cooling fluid to enter and exit, respectively, the cooling chamber 52. The cooling chamber 52 is integrated or embedded within the overmould 40 such that the cooling device 50 and the overmould 40 therefore form one unitary structure or component. In other words, the internal annular sidewall 53, the external annular sidewall 54 and the lid 56 are formed by, and hence an integral part of, the overmould 40. The base 55 abuts the internal and external annular sidewalls 53, 54 at their respective lower end points and is connected to them, for example by ultrasonic welding, to form the cooling chamber 52. The inlet 60 and the outlet 62 are attached to the upper external surface 58 of the external annular sidewall 54, for example by ultrasonic welding, to complete the assembly. The external annular sidewall 54 forms an external surface 51 of the injector assembly 20 around the upper region 34 of the fuel tube 32. It will be appreciated that other configurations and methods of fabrication may be equally applicable in accordance with the embodiments of the invention. The internal annular sidewall 53 of the cooling chamber 52 extends along a portion of the upper region 34 of the fuel tube 32 and contacts with the outer surface of the solenoid tube 30, for example as illustrated in Figure 2. The inlet 60 and the outlet 62 may be positioned, disposed, or located on substantially opposite sides of the upper external surface 58 of the external annular sidewall 54. For example, the inlet 60 and the outlet 62 may be located on diametrically opposed sides of the longitudinal axis A-A. A pump (not shown) is configured to deliver the cooling fluid into the cooling chamber 52 via the inlet 60 and fuel then exits the tanks via the outlet 62. Other configurations may be equally applicable. The abutment of the internal annular sidewall 53 and the solenoid tube 30 forms a conductive thermal bridge or path for thermally connecting the solenoid coil 26 to the cooling chamber 52. The thermal bridge is effectively comprised of the solenoid coil 26, the internal overmould wall 41, the solenoid tube 30, and the internal annular sidewall 53 such that heat is able to transfer from the solenoid coil 26 to circulating cooling fluid within the cooling chamber 52. The solenoid tube 30 may be constructed from a conductive material, such as a metal or alloy. The internal overmould wall 41 and the internal annular sidewall 53 may be constructed from the same material as the overmould 40, such as a plastic or polymer material. In such cases, the thicknesses of the internal overmould wall 41 and of the internal annular sidewall 53 are respectively selected to enable effective heat transfer through them. In operation, current is supplied to the solenoid coil 26 which causes the solenoid coil 26 to generate heat that increases the temperature of the actuator 24 and other injector components. Cooling fluid is delivered through the inlet 60 to the cooling chamber 52 at a relatively low temperature compared to the temperature of the solenoid coil 26 during operation. Heat generated by the solenoid coil 26 during operation can therefore be advantageously absorbed due to the relatively large temperature differential between the heated solenoid coil 26 and the low temperature cooling fluid during its circulation around the cooling chamber 52. Cooling fluid that has been circulated around the cooling chamber 52, and thus has absorbed heat from the solenoid coil 26, exits the cooling chamber 52 and is replaced by additional relatively low temperature cooling fluid. Advantageously, the temperature differential between the solenoid coil 26 and the relatively low temperature cooling fluid can therefore be maintained to provide effective heat transfer and dissipation. It is a feature of the arrangement that the abutment of the internal annular sidewall 53 of the cooling device 50 and the solenoid tube 30 provides a direct thermal load path between the actuator 24 and the cooling device 50. In order to assemble the fuel injector assembly 20 in Figures 2 and 3, the following assembly steps are followed. Initially, the internal components of the fuel injector 21, including the valve needle 22 and the armature, are assembled within the fuel tube 32. Separately the components of the actuator 24, including the solenoid coil 26, the bobbin 28, and the solenoid tube 30, and the components of the cooling device 50, including the base 55, the inlet 60 and the outlet 62, are assembled onto the overmould 40. The end cap 46 is applied to the upper end of the fuel injector 21 and the assembled overmould 40 is press-fit onto the upper region 34 of the fuel tube 32. In this manner, the end cap 46 and the overmould 40 are applied over the entire assembly to encapsulate the upper region 34 of the fuel tube 32, whereby a small portion of the solenoid tube 30 is left exposed, for example as illustrated in Figure 2. Advantageously, the integrated nature of the cooling device 50 with the overmould 5 40 enables quick and easy installation onto different fuel injectors and injector assemblies without having to modify the configuration of the associated fuel injector or associated engine head for accommodating the cooling device 50. Additionally, the internal annular sidewall 53 of the cooling chamber 52 extends over the major portion of the external annular surface of the solenoid tube 30 to 10 provide optimal and effective heat dissipation. Other modifications and variations will be apparent to the skilled person without deviating from the scope of the appended claims. 15

Claims

1. A fuel injector assembly (20) of a fuel injection system for delivering gaseous fuel, the fuel injector assembly (20) comprising:a fuel injector (21) comprising a main injector housing (32) comprising an upper region (34) and a lower region (36), the main injector housing (32) having a longitudinal injector axis (A-A);an electromagnetic actuator (24) mounted on the upper region (34) and comprising a solenoid assembly having a solenoid tube (30) housing a solenoid coil (26);an overmould (40) mounted on the upper region (34) of the main injector housing (32) for encapsulating at least a part of the electromagnetic actuator (24); anda cooling device (50) comprising a cooling chamber (52) for receiving a cooling fluid, wherein the cooling device (50) is integrated within the overmould (40) so that the overmould (40) defines at least internal and external sidewalls (53, 54) of the cooling chamber (52) and wherein the internal sidewall (53) contacts at least a portion of the solenoid tube (30) to form a thermal path extending between the actuator (24) and the cooling chamber (52).

2. The fuel injector assembly (20) as claimed in Claim 1, wherein an outer surface (51) of the fuel injector assembly (20) is at least partially formed by the external sidewall (54) of the cooling chamber (52).

3. The fuel injector assembly (20) as claimed in claim 1 or claim 2, wherein the overmould (40) includes an internal overmould wall (41) that extends between the solenoid coil (26) and the solenoid tube (30) to define a part of the thermal path between the actuator (24) and the cooling chamber (52).

4. The fuel injector assembly (20) as claimed in any of claims 1 to 3, comprising an inlet (60) for cooling fluid to flow to the cooling chamber (52) and an outlet (62) for cooling fluid to flow from the cooling chamber (52), and wherein the inlet (60) and outlet (62) are located on opposed sides of the cooling chamber (52).

5. The fuel injector assembly (20) as claimed in claim 4, wherein the inlet (60) and the outlet (62) are formed on a lid (56) of the cooling chamber (52).

6. The fuel injector assembly (20) as claimed in claim 5, wherein the lid (56) is integral with the overmould (40).

7. The fuel injector assembly (20) as claimed in any of claims 1 to 6,5 wherein the overmould (40) is formed from a plastic.

8. The fuel injector assembly (20) of any preceding claim, wherein the gaseous fuel is hydrogen.