Dual fuel injector

Abstract

A dual-fuel injector for injecting two separate fuels into a combustion chamber of an internal combustion engine comprises a nozzle body 14 housing first and second needle valves 118, 120, the first n

Description

Technical field The present invention relates generally to fuel injectors and more specifically to a dual-fuel injector for an internal combustion engine. Background Internal combustion (IC) engines are typically powered by combusting a fuel, such as petrol (gasoline) or diesel, in a combustion chamber. New technologies have been developed to enable the use of renewable fuels to achieve carbon neutral energy production and, additionally, increase fuel efficiency and reduce emissions of IC engines. One such development involves the combustion of two separate fuels within the combustion chamber of a dual-fuel engine. For example, a first gaseous fuel, such as methanol or ammonia, may be supplied to the combustion chamber due to its advantageous combustion or emissions characteristics. A second liquid fuel, such as petrol or diesel, may be supplied to the combustion chamber to improve the reliability and timing of the combustion of the first fuel. In a dual-fuel engine, the first and second fuels may be supplied to the combustion chamber in a number of different ways. Some examples include: mixing the two fuels together with air in an inlet manifold before supplying the fuel / air mixture to the combustion chamber through an inlet valve; supplying air and the first fuel through the inlet valve and subsequently injecting the second fuel to the combustion chamber; or injecting each fuel to the combustion chamber using separate fuel injectors. However, each of these methods has drawbacks in terms of efficiency and / or packaging of the engine components. Dual-fuel injectors, i.e. singular injectors that inject both the first and second fuels to the combustion chamber, have been developed in an attempt to overcome these drawbacks. However, significant challenges remain. For example, dual-fuel injectors typically require separate control valves to control separate needle valves of the injector for separately injecting the first and second fuels. Challenging packaging constraints typically result in one or more of the control valves being located a substantial distance from the respective control chamber that ultimately regulates the movement of the respective needle valve. This can result in wave activity and delays between the control valve and the needle valve, which increase needle valve response times, reduce linearity of demand vs delivery, as well as decreasing the consistency and reliability of fuel delivery. Consequentially, engine combustion performance may be negatively affected. It is against this background that the present invention has been devised. Summary According to an aspect of the invention, a dual-fuel injector for injecting two separate fuels into a combustion chamber of an internal combustion engine comprises a nozzle body which houses, at least in part, a first needle valve and a second needle valve, the first needle valve defining an internal bore and being coaxial with the second needle valve. An annular insert is arranged within the internal bore of the first needle valve and within which an upper portion of the second needle valve is received. A first fuel control chamber is defined at least in part by an upper portion of the first needle valve and a surface of a an injector housing part, whereby varying a first pressure in the first fuel control chamber varies the force on the upper end of the first needle valve. A second fuel control chamber is defined at least in part by an upper portion of the second needle valve and the surface of the injector housing part, whereby varying a second pressure in the second fuel control chamber varies the force on the upper end of the second needle valve. A first fuel control valve is configured to selectively fluidly couple the first fuel control chamber to a source of high-pressure fluid or to a low-pressure fluid drain; and a second fuel control valve is configured to selectively fluidly couple the second fuel control chamber to a source of high-pressure fluid or to a low-pressure fluid drain. The annular insert is engaged with the surface of the injector housing part wherein the annular insert to ensure the first fuel control chamber and the second fuel control chamber remain isolated from one another at all times i.e. throughout injection of the first fuel and / or the second fuel (i.e. throughout a full range of movement of both the first needle valve and the second needle valve) and when the first and second needle valves are seated. The annular insert forms an intermediate part between the upper portion of the outer needle valve and an upper portion of the inner needle valve, and as such means that the inner needle valve can be guided for movement within the annular insert, but without the need for the outer needle valve to be narrowed in this region to form a guide surface. This means that an elongate and unitary outer needle valve can be used, which provides benefits for the concentricity and assembly of the dual-needle arrangement, and a two-part outer needle valve is not required. In some examples, the first fuel control valve may be a three-way valve comprising a first fuel switching chamber. Accordingly, the first fuel switching chamber may provide a fluid flow path between the first fuel control chamber and the source of high-pressure fluid and a fluid flow path between the first fuel control chamber and the low-pressure fluid drain. The first plunger may be configured to selectively block one fluid flow path whilst facilitating fluid communication through the other fluid flow path. The first fuel switching chamber may be in fluid communication with the first fuel control chamber via a first fuel control conduit. The first fuel control chamber and first fuel control conduit collectively define a first fuel control volume. In some examples, the second fuel control valve may be a three-way valve comprising a second fuel switching chamber. Accordingly, the second fuel switching chamber may provide a fluid flow path between the second fuel control chamber and the source of high-pressure fluid and a fluid flow path between the second fuel control chamber and the low-pressure fluid drain. The second plunger may be configured to selectively block one fluid flow path whilst facilitating fluid communication through the other fluid flow path. The second fuel switching chamber may be in fluid communication with the second fuel control chamber via a second fuel control conduit. The second fuel control chamber and second fuel control conduit collectively define a second fuel control volume. The annular insert may include a first end surface of frusto-conical form which engages with the surface of the housing. The first end surface may include a flat-tipped surface which engages with the surface of the injector housing part. The injector housing part may form a control valve housing to house the first and second fuel control valves. In embodiments, the second needle valve extends through the first fuel control chamber such that the first fuel control chamber forms an annulus extending around a portion of the second needle valve. The first fuel control chamber may be defined at least in part by an outer surface of the second needle valve. A first biasing means may be configured to bias the first needle valve into engagement with a first fuel valve seat portion of the nozzle body, wherein the first biasing means is disposed around the first needle valve. The dual-fuel injector may include a collar coupled to the first needle valve, wherein the first biasing means engages the collar and a portion of the nozzle body to bias the first needle valve. The annular insert may include a second end surface, remote from the first end surface, which engages the first biasing means. The first needle valve and the second needle valve may be arranged within the nozzle body housing, and wherein the second fuel control chamber is defined at least in part by the nozzle body housing. The dual-fuel injector may further comprise a second biasing means configured to bias the second needle valve into engagement with a second fuel valve seat portion of the first needle valve, wherein the second biasing means is disposed around the second needle valve within the internal bore of the first fuel needle valve. In some examples, the first fuel may be a gaseous fuel. For example, the first fuel may be a gas such as methane gas or hydrogen gas. Additionally, or alternatively, the second fuel may be a liquid fuel. For example, the second fuel may be a liquid such as liquid diesel or liquid petrol (gasoline), in some examples. In some other examples, the first and second fuels may be the same type of fuel. In other examples, the first and second fuels may both be liquid fuels. One advantage of using a liquid fuel as the second fuel is that it can serve as the hydraulic fluid for the control chambers. Brief description of the drawings Examples of the present invention will now be described by way of non-limiting example only, with reference to the accompanying figures, in which: Figure 1 is a schematic cross-sectional view of an example dual-fuel injector; Figure 2 is an enlarged view of a part of the dual-fuel injector in Figure 1; Figure 3 is a schematic view of the dual-fuel injector of a first embodiment of the invention; and Figure 4 is an enlarged view of a part of the dual-fuel injector in Figure 3 to illustrate an annular insert between inner and outer needle valves of the injector. Detailed description Figure 1 shows a schematic cross-sectional view of a dual-fuel injector 10 for injecting two separate fuels into a combustion chamber 12 of an internal combustion engine. Figure 1 is shown for the purpose of describing how a first and a second fuel needle valve of the injector can be controlled to inject a first and a second fuel quantity, respectively, into the combustion chamber. In all respects, the injector parts in Figure 1 are applicable to embodiments of the invention (as shown in Figure 2) unless explicitly stated. The injector 10 may be configured to inject a first fuel in gaseous form, such as methane or hydrogen for example, and a second fuel in liquid form, such as petrol (gasoline) or diesel for example. The first fuel may also be in liquid form e.g. alcohol fuels or ammonia for example. As explained previously by way of background, the first fuel may be selected for its advantageous combustion or emissions characteristics and the second fuel supplied to the combustion chamber 12 may help to improve the reliability and timing of the combustion of the first fuel. Other arrangements are envisaged, for example where the first and second fuels are of the same type, for use with partially premixed combustion strategies for example The dual-fuel injector 10 comprises a nozzle body 14 housing a dual needle valve arrangement 16 for regulating the separate supply of the first and second fuels to the combustion chamber 12. An upper end of the nozzle body 14 is received within a lower end of an intermediate housing part 15 which defines an accumulator volume 76 for receiving fuel. A piston guide 17 is received within the upper end of the intermediate housing part 15. An injector housing part in the form of a control valve housing 21 abuts the upper end of the piston guide 17. The control valve housing 21, the piston guide 17, the intermediate housing part 15 and the nozzle body 14 are all received within a cap nut housing 23. The dual needle valve arrangement 16 comprises the first needle valve 18, in the form of an outer needle valve, and the second needle valve 20, in the form of an inner needle valve. The outer needle valve 18 is guided for movement within the piston guide 17. The outer needle valve 18 defines an internal bore 22 and the inner needle valve 20 is slidably received within the internal bore 22 (the sliding fit between the inner needle valve 20 and the outer needle valve 18 is identified by reference number 25 in Figure 2). The outer and inner needle valves 18, 20 may be co-axial, i.e. the outer needle valve 18 defines a first needle valve axis 24, and a second needle valve axis 26 defined by the inner needle valve 20 may be co-axial with the first needle valve axis 24. Such an arrangement may be advantageous for balancing loading and reducing wear in use. In an upper region of the internal bore 22, identified as 22a, the inner needle valve 20 is guided for sliding movement within the outer needle valve 18. The internal bore 22 in the outer needle valve 18 is of a variable diameter along its axial length. Referring also to Figure 2, the dual-fuel injector 10 also includes a first fuel control chamber 28. The first fuel control chamber 28 is configured to hydraulically control the movement of the outer needle valve 18. For example, the first fuel control chamber 28 is at least partly defined by an upper end 30 of the outer needle valve 18. Accordingly, varying a first pressure in the first fuel control chamber 28 varies the force on the upper end 30 of the outer needle valve 18, and the movement of the outer needle valve 18 can therefore be controlled by controlling the pressure in the first fuel control chamber 28. When the outer needle valve 18 is engaged with a first valve seat of the injector (as described further below), there is no injection of fuel past the first valve seat. To control movement of the inner needle valve 20, the dual-fuel injector 10 also includes a second fuel control chamber 32. Similarly, the second fuel control chamber 32 is at least partly defined by an upper end 34 of the inner needle valve 20. Varying a second pressure in the second fuel control chamber 32 therefore varies the force on the upper end 34 of the inner needle valve 20, facilitating control of the movement of the inner needle valve 20. The pressure in each control chamber 28, 32 may be varied, i.e. controlled, by a respective one of first and second fuel control valves 36, 38 arranged within the control valve housing 21. The first fuel control valve 36 includes a first plunger 40 configured to move along a first control valve axis 42. Movement of the first plunger 40 selectively fluidly couples the first fuel control chamber 28 to a source of high-pressure fluid (not shown) or to a low-pressure fluid drain (not shown). Accordingly, in some examples the first fuel control valve 36 may be a three-way valve. Similarly, the second fuel control valve 38 comprises a second plunger 46 configured to move along a second control valve axis 48. The second fuel control chamber 32 is selectively fluidly coupled to a source of high-pressure fluid (not shown) or to a low-pressure fluid drain (not shown) by moving the second plunger 46 along the second control valve axis 48. The first and second fuel control valves 36, 38 may each comprise a switching chamber 52, 54, respectively, each of which is in fluid communication with the respective control chamber 28, 32, a source of high-pressure fluid (not shown) and a low-pressure fluid drain 44, 50. Accordingly, each control valve 36, 38 is configured such that movement of the respective plunger facilitates fluid communication between the respective control chamber 28, 32 and the source of high-pressure fluid, or between the respective control chamber 28, 32 and the low-pressure fluid drain 44, 50. Each fuel control valve 36, 38 may be fluidly coupled to the respective control chamber 28, 32 via a respective control conduit (not labelled). The respective control conduit and associated control chamber 28, 32 together define a control volume. As explained previously by way of background, minimising each control volume is advantageous for improving control and responsiveness of the outer and inner needle valves 18, 20, which may be further advantageous for combustion performance. The dual-fuel injector 10 of the present invention is configured to improve control and responsiveness of the outer and inner needle valves 18, 20. For example, the first and second fuel control valves 36, 38 of the dual-fuel injector 10 are arranged side-by-side. This helps to reduce the distance between the respective switching chamber 52, 54 for the control valves 36, 38 and the associated control chamber 28, 32. In particular, this means that neither control conduit needs to be routed around a control valve 36, 38, or other complex routing, and instead each control conduit can be routed directly from the respective switching chamber to the respective control chamber 28, 32, thereby minimising the respective control volume. With the first and second fuel control valves 36, 38 arranged side-by-side, the first control valve axis 42 of the first fuel control valve 36 preferably extends parallel to the second control valve axis 48. Further, both control valve axes 42, 48 preferably extend parallel to the first needle valve axis 24. However, the control valves 36, 38 are preferably arranged such that the respective axis 42, 48 of each control valve 36, 38 is offset from the first needle valve axis 24. Accordingly, neither control valve axis 42, 48 is co-axial with the first needle valve axis 24. As previously noted, the dual-fuel injector 10 is therefore configured such that neither control volume is optimised at the detriment of the other control volume. Further still, in some preferred examples, the first control valve axis 42 and the second control valve axis 48 may each be offset from the first needle valve axis 24 by the same distance. It is envisaged that in other embodiments the first and second fuel control valves 36, 38 are arranged one above the other, so that the first control valve axis 42 is co-axially aligned with the second control valve axis 48. The inner needle valve 20 projects through the first fuel control chamber 28. Accordingly, the first fuel control chamber 28 may form an annulus extending around an upper portion of the inner needle valve 20. Further, at least part of the first fuel control chamber 28 may therefore be defined by an outer surface 68 of the inner needle valve 20. With the inner needle valve 20 extending through the first fuel control chamber 28 and the upper end 34 of the inner needle valve 20 defining part of the second fuel control chamber 32, at least part of the second fuel control chamber 32 may be defined by the nozzle body 14. Configuring the dual-fuel injector 10 with first and second fuel control chambers 28, 32 in this way further helps to minimize the distance between the control valves 36, 38 and the control chambers 28, 32 to reduce the respective control volumes and thereby improve combustion performance. In some examples the dual-fuel injector 10 may include a first biasing means 70 in the form of a first spring 70 to bias the outer needle valve 18 into a closed position. The first spring 70 may be disposed around the outer needle valve 18 and resides within a first accumulator volume 60. The nozzle body 14 may define, at its lower end, a first fuel valve seat portion 72 and the first spring 70 may bias the outer needle valve 18 into engagement with the first fuel valve seat portion 72. The nozzle body 14 may define one or more first fuel injection outlets 74 downstream of the first fuel valve seat portion 72, to facilitate fluid communication between the first fuel accumulator volume 60 and the combustion chamber 12 when the outer needle valve 18 is spaced away from the valve seat portion 72. Accordingly, biasing the outer needle valve 18 into engagement with the first fuel valve seat portion 72 may cover or block the first fuel injection outlets 74, thereby interrupting the fluid communication between the first accumulator volume 60 and the combustion chamber 12. As the first spring 70 is located in the first fuel accumulator volume 7660 it will be appreciated that the first spring 70 is located in an entirely separate area of the dual-fuel injector 10 compared to the first fuel control chamber 28. Accordingly, the first fuel control chamber 28 does not need to be dimensioned to accommodate the first spring 70, which further helps to facilitate a reduction in the first fuel control chamber volume. In some examples the first spring 70 may engage both an engagement collar 62 coupled to the outer needle valve 18 to bias the outer needle valve 18. The provision of an engagement collar 62 may facilitate simpler assembly of the dual-fuel injector 10. In order to further facilitate assembly of the dual-fuel injector 10, the outer needle valve 18 is formed in two parts (not shown). The intersection of the two parts may be arranged to be in the vicinity of the collar 62. The dual-fuel injector 10 also includes a second biasing means in the form of a second spring 80 configured to bias the inner needle valve 20 as shown in Figure 1. For example, a second fuel accumulator volume 64 may be in fluid communication with the combustion chamber 12 via one or more second fuel injection outlets 84. The second fuel accumulator volume 64 may be defined within the outer needle valve 18, for example within the internal bore 22 of the outer needle valve 18. The second fuel injection outlets 84 may be defined within the outer needle valve 18 and downstream of an inner valve seat portion 86 of the outer needle valve 18. The second spring 80 is located within the second fuel accumulator volume 64 and may be configured to bias the inner needle valve 20 into engagement with the inner valve seat portion 86 of the outer needle valve 18 to interrupt the fluid communication between the second fuel accumulator volume 64 and the combustion chamber 12. First and second drillings 92, 94 communicate with the second fuel accumulator volume 64 and provide a flow path for fuel into the second fuel accumulator volume 64 from a source of high pressure fuel. The two drillings 92, 94 connect with an annular groove (not shown) which surrounds the outer needle valve 18, and this annular groove connects via further drillings (not shown) to the source of high pressure fuel. In some preferred examples, such as that shown in Figure 1, the second spring 80 may be disposed around the inner needle valve 20 within the internal bore 22 of the outer needle valve 18. Notably, as described previously the second fuel control chamber 32 may be defined at least in part by the upper end 34 of the inner needle valve 20, and the second spring 80 may therefore be located in an entirely separate area of the dual-fuel injector 10 compared to the second fuel control chamber 32. It follows that the second fuel control chamber 32 does not need to be sized to accommodate the second spring 80, further facilitating a minimized second fuel control volume. Finally, still with reference to Figure 1, the outer needle valve 18 may be slidably received in a needle guide portion 88 of the nozzle body 14. The outer needle valve 18 and needle guide portion 88 preferably interface with a matched clearance fit. Accordingly, a small gap may exist between the needle guide portion 88 and the outer needle valve 18 to enable the slidable relation. Embodiments of the invention include all of the features described previously for the dualfuel injector 10 in Figures 1 and 2, with the exception of the arrangement at the upper end of the dual-needle valve arrangement 16 and the configuration of the inner fuel needle valve 20 within the outer fuel needle valve 18 in the region 22a of the internal bore 22. A further difference exists in the arrangement of the housing parts 15, 16, 17 for the needle valves 18, 20, and how the first spring 70 is arranged in those housing parts, as described further below. Referring to Figures 3 and 4, in embodiments of the invention the outer needle valve 118 is a unitary part, rather than being formed of two parts as described previously. The outer needle valve 118 includes a lower needle valve portion or region 118a and an upper needle valve portion or region 118b. The internal bore in the outer fuel needle valve 118 includes lower and upper internal bore regions 122a, 122b, respectively, in the lower and upper needle valve regions 118a, 118b respectively. The upper internal bore region 122b is of a first diameter and is of a greater diameter than the outer diameter of an upper region 120b of the inner needle valve 120. The lower internal bore region 122a of the outer needle valve 118 is of a second diameter which is less than the first diameter of the upper bore region 122b. The transition region between the upper and lower bore regions 122a, 122b is of frusto-conical form. The first accumulator volume 60, in Figure 3, is defined partly within the nozzle body 14 and partly within the intermediate housing part 15. An upper region of the accumulator volume 60, defined within the intermediate housing part 15, communicates with a first supply passage 100 from the source of high pressure fuel. The supply passage 100 is formed within the intermediate housing part 15. The drillings 92, 94 which communicate with the second accumulator volume 64 receive fuel through a second supply passage 102 defined partly within the housing 21 and partly within the intermediate housing part 15. As described previously, the spring 70 is received within the first accumulate volume 60 so that the upper end of the first spring 70 is engaged with a surface of the intermediate housing 15. This presents a difference compared to Figure 1, where in that case the upper end of the first spring 70 engages with a surface of the piston guide 17. The nozzle body 14, the intermediate housing 15 and the valve housing 21 are received within the capnut 23. An annular insert 130 is located in the annular space between the upper region 120b of the inner needle valve 120 and the upper region 118b of the outer needle valve 118. The annular insert 130 takes the form of a cylinder which, on assembly, can be inserted into the upper end of the upper region 122b of the internal bore 122 in the outer needle valve 118. A lower surface 130a of the annular insert 130 defines a first abutment surface for one end of the second spring 80. The other end of the second spring 80 engages with a second abutment surface 136 defined by a collar 138 on the inner needle valve 120. At the end of the annular insert 130 remote from the second spring 80, the annular insert defines an end surface 130b. The radially outer diameter (D1) of the annular insert 130 is identified as D1 and the radially inner diameter is identified as D2.. The surface 130b of the annular insert is represented as a flat surface in Figure 3, but the alternative form of the frusto-conical surface is shown in the exaggerated view of Figure 4. Referring to Figure 4, where the end surface is a frusto-conical surface 1130b, the cone slopes radially inwardly towards the inner needle valve 120). In this example, the radially outer diameter D1 of the end surface 130b may define an annular seating surface 134 (or seating line) which seats against the surface 21a of the housing 21 for the first and second control valves (not shown in Figures3 and 4). The second spring 80 acts on the annular insert 130 so as to urge the annular insert 130 against the housing surface 21a. In practice the end face of the annular insert 130 may have a seat form which creates a seating line 134 which contacts with the surface 21a of the housing 21, and the diameter of this seating line may be anywhere between the inner and outer diameter D1, D2 of the annular insert 130. In Figure 4 the seating line contact is shown at the very outer diameter of the annular insert 130 (i.e. at D1), but ideally this may not be the case so as to reduce the impact of stress. In reality, therefore, the seating line 134 may be defined by the intersection of two conical surfaces and there may be a small flattened region at this intersection. The seating line 134 may be biased in position towards the outer edge of the annular insert 130 to allow more room for the second fuel control conduit 58 into the control chamber 32. The flat-tipped nature of the annular seating surface 134 reduces the effects of wear which could otherwise be problematic if the annular insert 130 was formed as a sealing tip or sealing line. The annular insert 130 remains in the seated position, against the housing 21, through the cycle of injector operation, whether the outer needle valve 118 is operated to inject fuel, whether the inner needle valve 120 is operated to inject fuel, or whether both fuel needle valves 118, 120 are operated together. The first fuel conduit 56 communicates with the control chamber 28 (i.e. the control chamber 28 to which the outer needle valve 118 is exposed) radially outside of the annular seating line 134 and the second fuel conduit 58 communicates with the control chamber 32 (i.e. the control camber 32 to which the inner needle valve 120 is exposed) radially inside the annular seating line 134. In other embodiments the frusto-conical surface 130a may be replaced with a flat surface which is aligned with the surface 21a of the housing 21, as showns in Figure 3 At the injecting end of the dual needle valve arrangement 16, the inner needle valve 120 defines a seating surface for engagement with the inner valve seat 86. The sizing of the inner needle valve 120 at the seating surface is selected to optimise the dynamics of the injector. The dual-fuel injector may be operated to deliver a pilot injection of fuel by moving the inner needle valve 120 away from the inner valve seat portion 86 and may be operated to deliver a main injection of fuel by moving the outer needle valve 118 away from the outer valve seat portion 72 . The inner needle valve 120 is held in engagement with the inner valve seat portion 86 by means of the first spring 70 acting in combination with fuel pressure within the second fuel control chamber 32 when the second fuel control valve 38 is coupled to the source of high pressure fluid. In order to move the inner needle valve 120 away from the inner valve seat portion 86, the second fuel control valve 38 is actuated to connect the second fuel control chamber 32 to the low pressure fluid drain, thereby reducing pressure in the second fuel control chamber 32 so that the inner needle valve 120 is able to lift from the inner valve seat portion 86. When the inner needle valve 12 is moved away from the inner valve seat portion 86, this allows fuel to be injected through the injection outlets 84. Similarly, the outer needle valve 118 is held in engagement with the outer valve seat portion 72 by means of the first spring 80 acting in combination with fuel pressure within the first fuel control chamber 28 when the second fuel control valve 36 is coupled to the source of high pressure fluid. In order to move the outer needle valve 118 away from the outer valve seat portion 72, the first fuel control valve 36 is actuated to connect the first fuel control chamber 28 to the low pressure fluid drain, thereby reducing pressure in the first fuel control chamber 28 so that the outer needle valve 118 is able to lift from the outer valve seat portion 72. Operation of the first and second fuel control valves 36, 38 therefore controls movement of the inner and outer needle valves 120, 118, away from their respective valve seat portions 72, 86, to commence injection through the respective injection outlet (s). When the inner needle valve 120 is seated against the inner valve seat portion 86 the annular insert 130 is urged in an upwards direction (upward in the illustration shown) by the force of the second spring 80, urging the annular insert 130 against the surface 21a of the housing 21. When the inner needle valve 120 is caused to move away from the inner valve seat portion 86, the collar 138 on the inner needle valve 120 continues to act on the lower end of the second spring 80 and the annular insert 130 remains in contact with the surface 21a of the housing 21. When the inner needle valve 120 is closed against the inner valve seat portion 86 once again, the annular insert 130 does not move due to the force of the second spring 80. As the annular insert 130 remains seated against the housing 21, the first and second fuel control chambers 36, 38 remain sealed from one another throughout the full range of movement of the inner needle valve 120, and when it is seated. Movement of the outer needle valve 118 towards and away from the outer valve seat portion 72 does not impact on the annular insert 130 which at all times remains in contact with the surface 21a of the housing 21 under the force of the second spring 80. The outer needle valve 118 will also contact the surface 21a of the housing 21 at maximum lift (i.e. the surface 21a of the housing 21 defines a lift stop for the outer needle valve 118). Likewise, the inner needle valve 120 will also contact on the surface 21a at maximum lift. Because of this a specific geometry on the upper end face of each of the needle valves 118, 120 may be required. For example, the outer needle valve 118 may have an end surface which is the intersection of two cones, intersecting at a flat tip of the end surface at a position somewhere between the inner and outer diameters of the hollow outer needle valve 118. For the inner needle valve 120 the upper end surface may be of conical form and may again terminate in a flat tip. It will be appreciated that the description provided above serves to demonstrate possible examples of the present invention. Features described in relation to any of the examples above may be readily combined with any other features described with reference to different examples without departing from the scope of the invention as defined in the appended claims.

Claims

1. A dual-fuel injector (10) for injecting two separate fuels into a combustion chamber (12) of an internal combustion engine, the dual-fuel injector comprising:a nozzle body (14) which houses, at least in part, a first needle valve (118) and a second needle valve (120), the first needle valve defining an internal bore (122) and being coaxial with the second needle valve (120);an annular insert (130) arranged within the internal bore (122) of the first needle valve (118) and within which an upper portion (120b) of the second needle valve (118) is received,a first fuel control chamber (28) defined at least in part by an upper portion (118b) of the first needle valve (118) and a surface (21a) of an injector housing part (21) whereby varying a first pressure in the first fuel control chamber (28) varies the force on the upper end (118b) of the first needle valve (118);a second fuel control chamber (32) defined at least in part by an upper portion (120b) of the second needle valve (120) and the surface (21a) of the injector housing part (21) whereby varying a second pressure in the second fuel control chamber (32) varies the force on the upper portion (120b) of the second needle valve (120);a first fuel control valve (36) configured to selectively fluidly couple the first fuel control chamber (28) to a source of high-pressure fluid or to a low-pressure fluid drain (44); anda second fuel control valve (38) configured to selectively fluidly couple the second fuel control chamber (32) to a source of high-pressure fluid or to a low-pressure fluid drain (50);wherein the annular insert (130) is engaged with the surface (21a) of the injector housing part (21) to ensure the first fuel control chamber (28) and the second fuel control chamber (32) remain isolated from one another at all times.

2. The dual-fuel injector of Claim 1, wherein the annular insert (130) includes a first end surface (130b) of frusto-conical form which engages with the surface (21a) of the injector housing part (21).

3. The dual-fuel injector of Claim 2, wherein the first end surface includes a flat-tipped surface which engages with the surface (21a) of the injector housing part (21).

4. The dual-fuel injector (10) of any preceding claim, wherein the injector housing part (21) houses, at least in part, the first and second fuel control valves (36, 38).

5. The dual-fuel injector (10) of any preceding claim, further comprising a first biasing 5 means (70) configured to bias the first needle valve (118) into engagement with a first fuel valve seat portion (72) of the nozzle body (14), wherein the first biasing means (70) is disposed around the first needle valve (118).

6. The dual-fuel injector (10) of Claim 5, further comprising a collar (78) coupled to 10 the first needle valve (118), wherein the first biasing means (70) engages the collar (78) and a portion of the nozzle body (14) to bias the first needle valve.

7. The dual-fuel injector (10) of Claim 6 when dependent on Claim 2, wherein the annular insert (130) includes a second end surface, remote from the first end surface 15 (130b), which engages the second biasing means (80).

8. The dual-fuel injector (10) of any preceding claim, further comprising a second biasing means (80) configured to bias the second needle valve (120) into engagement with a second fuel valve seat portion (86) of the first needle valve (118), wherein the second 20 biasing means (80) is disposed around the second needle valve within the internal bore (122) of the first fuel needle valve.

9. The dual-fuel injector (10) of any preceding claim, wherein the first fuel is a gaseousfuel and / or wherein the second fuel is a liquid fuel.25

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