Dual-fuel injector

Abstract

In a first aspect of the present invention there is provided a dual-fuel injector for injecting a first fuel and a second fuel into a combustion chamber of an internal combustion engine. The dual-fuel injector comprises a nozzle body housing a dual needle valve arrangement comprising a first fuel needle valve and a second fuel needle valve. The first fuel needle valve defines a first needle valve axis and an internal bore, and the second fuel needle valve is slidably received within the internal bore. The dual-fuel injector further comprises a first fuel control chamber defined at least in part by an upper end of the first fuel needle valve such that varying a first pressure in the first fuel control chamber varies the force on the upper end of the first fuel needle valve. The first fuel needle valve is a subassembly comprising a first needle section and a second needle section, wherein the first needle section is attached to the second needle section by means of a press fit connection. The first needle section defines at least part of the upper end of the first fuel needle valve. The first fuel needle valve comprises a connection region in which one of the first or second needle sections comprises a socket portion and the other of the first or second needle sections comprises a plug portion that is press fit into the socket portion, and wherein in the connection region, at least one of the socket portion and the plug portion comprises a friction element defining at least part of an interface between the socket portion and the plug portion.

Description

[0001] DUAL-FUEL INJECTOR

[0002] FIELD OF THE INVENTION

[0003] This invention relates to internal combustion engines, and in particular, to a dual-fuel injector for injecting two separate fuels into a combustion chamber of an internal combustion engine.

[0004] BACKGROUND

[0005] 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 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 fuel, such as methane or hydrogen gas, may be supplied to the combustion chamber because of its advantageous combustion or emissions characteristics. A second 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.

[0006] 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.

[0007] 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. In particular, each fuel typically requires individual valves and hydraulic systems to accurately control injection of the respective fuel into the combustion chamber. Packaging the necessary valves for injecting two fuels in a single fuel injector can be challenging.

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

[0009] In a first aspect of the present invention there is provided a dual-fuel injector for injecting a first fuel and a second fuel into a combustion chamber of an internal combustion engine. The dual-fuel injector comprises a nozzle body housing a dual needle valve arrangement comprising a first fuel needle valve and a second fuel needle valve. The first fuel needle valve defines a first needle valve axis and an internal bore, and the second fuel needle valve is slidably received within the internal bore. The dual-fuel injector further comprises a first fuel control chamber defined at least in part by an upper end of the first fuel needle valve such that varying a first pressure in the first fuel control chamber varies the force on the upper end of the first fuel needle valve. The first fuel needle valve is a subassembly comprising a first needle section and a second needle section, wherein the first needle section is attached to the second needle section. The first needle section defines at least part of the upper end of the first fuel needle valve.

[0010] In some examples, the first needle section may be attached to the second needle section by means of a threaded connection. Accordingly, the first and second needle sections may comprise cooperating threaded portions that mate to attach the first needle section to the second needle section. Alternatively, in some other examples the first needle section may be attached to the second needle section by means of a press fit connection. Press-fitting the first and second needle sections together may result in a particularly dimensionally accurate first fuel needle valve. Notably, press fitting the first and second needle sections together facilitates close control of the overall length of the first fuel needle valve. This is advantageous because the length of the first fuel needle valve affects valve timings, and press-fitting the needle sections together may therefore improve performance of the dual-fuel injector.

[0011] In some examples, the first fuel needle valve may comprise a connection region. In the connection region, one of the first or second needle sections may comprise a socket portion and the other of the first or second needle sections comprises a plug portion that is press fit into the socket portion. Accordingly, the first and second needle sections may be connected together in the connection region via a plug and socket arrangement. For example, the plug and socket portions of the first and second needle sections may be press-fitted together to thereby attach the first and second needle sections together.

[0012] The socket portion may comprise an annular structure extending around the plug portion. In some examples, the plug portion may be delimited by a shoulder of the respective first or second needle section to limit the distance to which the plug portion can be inserted into the socket portion. Accordingly, such a shoulder may be configured to engage and end of the socket portion when the first and second needle sections are fitted together, i.e. when the plug portion is inserted into the socket portion. Such a configuration may help to ensure accuracy in the overall length of the first fuel needle valve.

[0013] A plug and socket arrangement in the connection region may aid with alignment of the first and second needle sections during assembly of the first fuel needle valve. In particular such an arrangement may be beneficial in combination with a press fit connection for aligning the first and second needle sections. A plug and socket arrangement may be particularly beneficial for ensuring concentricity of the first and second needle sections relative to one another. In some examples, the first needle section may be formed of a material that is cheaper and / or easier to machine. Therefore it may be advantageous for the socket portion to be defined by the first needle section, in some examples.

[0014] In some examples, in the connection region, at least one of the socket portion and the plug portion may comprise a friction element. The friction element may define at least part of an interface between the socket portion and the plug portion. Accordingly, the friction element may be configured to increase the coefficient of friction between interfacing surfaces of the first and second needle sections.

[0015] In some examples, the friction element may be formed of a different material compared to the socket portion and / or plug portion. In some preferred examples the friction element may be integrally formed as part of the plug and / or socket portion of the respective first and / or second needle section. For example, the friction element may comprise one or more grooves and / or ribs, preferably a series of grooves and / or ribs, that define at least part of the interface between the socket portion and the plug portion.

[0016] In some examples, the first and second needle sections may be formed of different materials. For example, the first needle section may be formed of a material selected for wear resistance properties to withstand frictional forces and associated wear mechanisms at an interface between the sliding first needle section and a needle guide portion of the nozzle body. In some examples, the second needle section may be formed of a material selected for heat resistant properties to withstand the heat generated by combustion in the combustion chamber.

[0017] It follows that the or each friction element may be formed of a different material to the socket portion and / or plug portion. In some examples, the friction element may be a separate component arranged in the connection region between interfacing surfaces of the socket portion and plug portion. In such an example, the friction element may be formed of a different material to both the socket portion and the socket portion of the first and second needle sections.

[0018] In some examples, the press fit connection between the plug and socket portions may be formed at least in part by a compression collar arranged around an outer surface of the socket portion in the connection region. Accordingly, the dual-fuel injector may comprise a compression collar. Including such a compression collar in the configuration of the first fuel needle valve may facilitate a reduction in the wall thickness of the socket portion, and in some examples a reduction in the wall thickness of the respective needle section comprising the socket portion, because the compressive force for the press fit connection is provided at least in part by the compression collar, and not solely by the socket portion.

[0019] In some examples, the dual-fuel injector may further comprise a first biasing means arranged to engage the compression collar and a portion of the nozzle body. Accordingly, the first biasing means may be arranged to exert a biasing force on the first fuel needle valve, via the compression collar. In some examples, the first biasing means may be disposed around the first fuel needle valve. For example, the first biasing means may comprise a coil spring.

[0020] In some examples, the nozzle body may define a first fuel valve seat portion. The first biasing means may be configured to bias the first fuel needle valve into engaging the first fuel valve seat portion. In particular, the first biasing means may be configured to bias the first fuel needle valve such that the second needle section of the first fuel needle valve engages the first fuel valve seat portion. The first biasing means may therefore be arranged to bias the first fuel needle valve into a normally-closed position.

[0021] In some examples, the first fuel needle valve may define an accumulator volume for receiving the second fuel. For example, the second fuel may be received into the accumulator volume from a supply of second fuel via one or more supply conduits. The second fuel accumulator volume may be configured to retain an amount of the second fuel immediately prior to injection of a portion of the second fuel from the accumulator volume into the combustion chamber.

[0022] In some examples, the second needle section of the first fuel needle valve may define one or more second fuel injection outlets. Such second fuel injection outlets may be in fluid communication with the accumulator volume and may be configured for supplying the second fuel from the accumulator volume to the combustion chamber.

[0023] In some examples, the second needle section of the first fuel needle valve may define a second fuel valve seat portion. Additionally, the dual-fuel injector may further comprise a second biasing means located in the accumulator volume. Such a second biasing means may be configured to bias the second fuel needle valve into engagement with the second fuel valve seat portion.

[0024] For example, the second biasing means may be arranged to engage the second fuel needle valve and the first needle section of the first fuel needle valve to bias the second fuel needle valve into engagement with the second fuel valve seat portion. Engaging the second fuel needle valve may comprise engaging a shoulder portion or a rib of the second fuel needle valve. Engaging the first needle section of the first fuel needle valve may comprise engaging an internal surface of the first needle section. The internal surface of the first fuel needle valve may define part of the accumulator volume, in some examples. As described previously, in some examples, the second biasing means may therefore be located within the accumulator volume. In some examples, the second biasing means may be disposed around the second fuel needle valve within the accumulator volume. For example, the second biasing means may comprise a coil spring.

[0025] As previously noted, the first fuel needle valve defines an internal bore and the second fuel needle valve is slidably received within the internal bore. In some examples, the internal bore may be defined by the first needle section of the first fuel needle valve. Further, in some examples the first fuel needle valve may comprise an aperture defined in an upper surface of the first needle section. In such an example the internal bore may extend through the first needle section from the aperture. Further still, in some examples the second fuel needle valve may protrude from the aperture such that an upper end of the second fuel needle valve extends above the upper end of the first fuel needle valve. This configuration facilitates separate control and movement of the second fuel needle valve, independent of the first fuel needle valve.

[0026] Additionally, movement of the second fuel needle valve may be controlled by varying a second pressure in a second fuel control chamber defined at least in part by an upper surface of the upper end of the second fuel needle valve. Configuring the dual needle valve arrangement such that the upper end of the second fuel needle valve extends above the upper end of the first fuel needle valve therefore affords flexibility in the location of the second fuel control chamber in that said control chamber can be located external, i.e. separate, to the first fuel needle valve.

[0027] In some examples, the nozzle body may define a needle guide portion. For example, the first fuel needle valve may be slidably received in the needle guide portion. In particular, the first needle section of the first fuel needle valve may be slidably received in the needle guide portion. Configuring the first fuel needle valve as a subassembly of the first and second needle sections facilitates an optimised material selection for the first needle section to withstand wear associated with sliding in the needle guide portion.

[0028] In some examples, the first fuel control chamber may be defined in part by the needle guide portion, for example by a ceiling surface of the needle guide portion. It follows that the first fuel control chamber may therefore be defined between the needle guide portion of the nozzle body and the first needle section of the first fuel needle valve, in some examples. Configuring the first fuel needle valve as a subassembly may enable the use of an improved manufacturing method to accurately manufacture the first needle section such that the geometry of the first fuel control chamber is accurately defined.

[0029] In some examples, the first needle section of the first fuel needle valve may define both an outer surface for arrangement in sliding relation with the needle guide portion, and the internal bore in which the second fuel needle valve is slidably received. Such a configuration may facilitate high manufacturing accuracy in the location of the internal bore relative to the outer surface, i.e. the sliding surface. For example, the second fuel needle valve may be co-axial with the first fuel needle valve. The first needle section defining the outer surface and the internal bore of the first fuel needle valve may facilitate manufacture of the first fuel needle valve with a high degree of concentricity between the outer sliding surface and the internal bore. Accordingly, such a configuration may provide a dual needle valve arrangement having a high degree of concentricity between the first fuel needle valve and the second fuel needle valve. This may be advantageous both for injection performance and for longevity by reducing wear between the moving components.

[0030] In preferred examples the first needle valve axis may be a longitudinal axis of the first fuel needle valve. The second fuel needle valve may define a second needle valve axis. The second fuel needle valve axis may be a longitudinal axis of the second fuel needle valve. Arranging the first fuel needle valve and second fuel needle valve co-axially may mean that the first and second needle valve axes are co-linear.

[0031] In some examples the first fuel may be a gaseous fuel, such as hydrogen for example. Further, in some examples the second fuel may be a liquid fuel, such as petrol (gasoline) or diesel, for example.

[0032] BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The above and other aspects of the invention will now be described, by way of example only, with reference to the accompanying figures in which: Figure 1 is a schematic cross-sectional view of a dual fuel injector comprising an example of a dual needle valve arrangement;

[0034] Figure 2 is a schematic cross-sectional view of the dual fuel injector comprising another example of a dual needle valve arrangement;

[0035] Figure 3a is a schematic cross-sectional view of the dual fuel injector comprising another example of a dual needle valve arrangement; and

[0036] Figure 3b is a schematic detail view of a connection region of a first fuel needle valve of the dual needle valve arrangement of Figure 3a.

[0037] SPECIFIC DESCRIPTION

[0038] As described previously by way of background, dual-fuel injectors are configured to inject two separate fuels into a combustion chamber of an internal combustion (IC) engine. Figure 1 shows part of an example of a dual-fuel injector 10 in a schematic cross-sectional view. The dual-fuel injector 10, referred to hereafter as the “injector 10”, includes a nozzle body 12. The nozzle body 12 houses a dual needle valve arrangement 14 comprising a first fuel needle valve 16 and a second fuel needle valve 18. The second fuel needle valve 18 is slidably received within an internal bore 20 defined by the first fuel needle valve 16. The first fuel needle valve 16 and second fuel needle valve 18 are respectively configured to control the injection of a first fuel and a second fuel into a combustion chamber 22 of an internal combustion (IC) engine.

[0039] For example, the nozzle body 12 may define a first fuel valve seat portion 24 and the first fuel needle valve 16 may be configured to engage the first fuel valve seat portion 24 to thereby block one or more first fuel injection outlets 26 through which the first fuel may be selectively supplied to the combustion chamber 22. The first fuel needle valve 16 may be slidably received in a needle guide portion 28 defined by the nozzle body 12 such that the first fuel needle valve 16 is movable relative to the nozzle body 12 to selectively engage the first fuel valve seat portion 24.

[0040] Movement of the first fuel needle valve 16 into and out of engagement with the first fuel valve seat portion 24 may be controlled by varying a first pressure in a first fuel control chamber 30. For example, as shown in Figure 1 , the injector 10 includes a first fuel control chamber 30 that is defined at least in part by an upper end 32 of the first fuel needle valve 16. As such, it will be appreciated that varying a first pressure in the first fuel control chamber 30 varies the force on the upper end 32 of the first fuel needle valve 16 which may result in actuation of the first fuel needle valve 16. In preferred examples, actuation of the first fuel needle valve 16 may result in linear movement of the needle valve 16 along a first needle valve axis 34 that is defined by the first fuel needle valve 16.

[0041] With reference still to Figure 1 , in accordance with examples of the present invention, the first fuel needle valve 16 is a subassembly comprising a first needle section 36 and a second needle section 38. The first needle section 36 is attached to the second needle section 38 to form the first fuel needle valve 16. During manufacture of the injector 10, the first and second needle sections 36, 38 may be attached together to form the first fuel needle valve 16 after the second fuel needle valve 18 has been arranged within the internal bore 20. Accordingly, forming the first fuel needle valve 16 from two separate needle sections 36, 38 facilitates a simple assembly process to manufacture the dual needle valve arrangement 14.

[0042] As shown in Figure 1 , the first needle section 36 defines at least part of the upper end 32 of the first fuel needle valve 16. It follows that the first needle section 36 may be an upper needle section, relative to the second needle section 38. Accordingly in preferred examples the first needle section 36 may therefore define part of the first fuel control chamber 30. Additionally, in some examples, the first needle section 36 may define at least part of the internal bore 20 in which the second fuel needle valve 18 is slidably received. For example, the first fuel needle valve 16 may comprise an aperture 40 defined in an upper surface 42 of the first needle section 36. As shown in Figure 1 , the internal bore 20 may extend through the first needle section 36 from the aperture 40.

[0043] As shown in the example of Figure 1 , the first needle section 36 may be attached to the second needle section 38 by means of a threaded connection in some examples. Accordingly, the first and second needle sections 36, 38 may include cooperating threaded portions 44 that are configured to hold the first and second needle sections 36, 38 together when the threaded portions 44 are engaged.

[0044] Additional reference is now made to Figures 2 and 3a which show further examples of the dual-fuel injector 10 previously described with reference to Figure 1. As such it will be appreciated that features in the examples of Figures 2 and 3a that are equivalent to features previously described with reference to Figure 1 are identified by the same reference numerals and will not be described again in the interest of conciseness. As will now be described in more detail, the dual-fuel injectors 10 shown by way of example in Figures 2 and 3a include respective dual needle valve arrangements 14 each comprising differently configured first fuel needle valves 16. Whilst the first and second needle section 36, 38 may be attached together via a threaded connection in some examples, as shown in Figure 1 , in some other examples the first needle section 36 may be attached to the second needle section 38 by means of a press fit connection, as shown in Figures 2 and 3a.

[0045] The first fuel needle valve 16 may include a connection region 46 in which the first and second needle sections 36, 38 are attached together. For example, with reference to Figures 2 and 3a, in the connection region 46 the first needle section 36 may include a socket portion 48 and the second needle section 38 may comprise a plug portion 50 that is press fit into the socket portion 48. The socket and plug portions 48, 50 cooperate to align the first and second needle sections 36, 38 relative to one another to facilitate accurate assembly of the first fuel needle valve 16.

[0046] With reference in particular to Figure 3a, and the schematic detail view of the connection region 46 shown in Figure 3b, in some examples at least one of the socket portion 48 and the plug portion 50 may include a friction element 52 that defines at least part of an interface between the socket portion 48 and the plug portion 50. In the example shown in Figure 3b, the plug portion 50 of the second needle section 38 comprises a friction element 52. The friction element 52 is configured to increase the friction in the mechanical press fit connection attaching the first and second needle sections 36, 38 together. As shown in Figure 3a, the friction element 52 may therefore comprise a series of grooves and / or ribs defining at least part of the interface between the plug and socket portions 50, 48.

[0047] Referring still to Figures 3a and 3b, in some examples the press fit connection between the first and second needle sections 36, 38 may be formed, at least partly, by a compression collar 54. Such a compression collar 54 may be arranged around an outer surface 56 of the socket portion 48 in the connection region 46 to provide a compressive force clamping the socket portion 48 onto the plug portion 50. It follows that the compression collar 54 may be press- fitted to the socket portion 48. Including a compression collar 54 may facilitate a reduction in wall thickness of the socket portion 48 because at least some of the compressive force required for the press fit connection attaching the first needle section 36 to the second needle section 38 is provided by the compression collar 54. As shown in a comparison of Figures 2 and 3a, the compression collar 54 may also facilitate a reduction in the wall thickness of the needle section which comprises the socket portion 48, in this example the second needle section 38.

[0048] With reference now to each of the accompanying figures, the dual-fuel injector 10 may include a first biasing means 58 configured to exert a biasing force on the first fuel needle valve 16. In examples such as those shown in Figures 1 and 2, the first biasing means 58 may be arranged to engage a shoulder feature or a locating collar 60 associated with the first fuel needle valve 16. As shown in the example of Figures 3a and 3b, in an example comprising a compression collar 54 for forming at least part of the press fit connection, the first biasing means 58 may be arranged to engage the compression collar 54. In either example it will be appreciated that the biasing means 58 is preferably arranged to engage the first fuel needle valve 16, or a component associated with the first fuel needle valve 16, and a portion of the nozzle body 12 such that the first fuel needle valve 16 is biased relative to the nozzle body 12. In particular, the first biasing means 58 may be configured to bias the first fuel needle valve 16 such that the second needle section 38 engages the first fuel valve seat portion 24.

[0049] As described previously, the first fuel needle valve 16 is slidable relative to the nozzle body 12 to facilitate actuation of the first fuel needle valve 16 to selectively engage the first fuel valve seat portion 24. It follows that the first needle section 36 of the first fuel needle valve 16 may be slidably received in the needle guide portion 28. Accordingly, as shown in particular with reference to Figures 2 and 3a, the first needle section 36 may define an outer surface 62 for arrangement in sliding relation with the needle guide portion 28. In some advantageous examples the first needle section 36 may therefore define both the outer, i.e. sliding, surface 62 and the internal bore 20 in which the second fuel needle valve 18 is slidably received. This configuration may facilitate particularly accurate manufacture of the internal bore 20 relative to the sliding surface 62 in a single manufacturing process.

[0050] Referring still to each of the accompanying figures, the first fuel needle valve 16 may define an accumulator volume 64, i.e. a second fuel accumulator volume, for receiving and temporarily storing the second fuel immediately prior to injection into the combustion chamber 22. One or more second fuel injection outlets 66, which are in fluid communication with the accumulator volume 64, may be defined by the second needle section 38. The or each second fuel injection outlet 66 may be defined by a second fuel valve seat portion 68 of the second needle section 38. It follows that the second fuel needle valve 18 may be configured to engage the second fuel valve seat portion 68 to thereby selectively block the second fuel injection outlets 66.

[0051] As shown in the accompanying figures, in some examples, the dual-fuel injector 10 may include a second biasing means 70 configured to bias the second fuel needle valve 18 into engagement with the second fuel valve seat portion 68. As such, the second biasing means 70 may be arranged to engage the second fuel needle valve 18 and the first needle section 36 of the first fuel needle valve 16. For example, the second biasing means 70 may be located in the second fuel accumulator volume 64. The examples shown in the figures are provided by way of example only, and various other examples of the invention are also envisaged without departing from the scope of the appended claims.

[0052] For example, as described previously, in the examples shown in Figures 2 and 3a the first needle section 36 of the first fuel needle valve 16 may include a socket portion 48 and the second needle section 38 may include a corresponding plug portion 50. However, it will be appreciated that in some other examples, the first needle section 36 may comprise a plug portion 50 and the second needle section 38 may include a corresponding socket portion 48. Such a configuration is shown by way of example in Figure 1 wherein the first needle section 36 comprises a plug portion 50 that is attached to a socket portion 48 defined by the second needle section 38 by means of a threaded connection. As such it will be appreciated that either of the first or second needle sections 36, 38 may define a plug portion 50, and the other of the first or second needle sections 36, 38 may define the corresponding socket portion 48.

[0053] Further, whilst the configuration of the first needle section 36 comprising an outer, sliding surface 62 and defining at least part of the internal bore 20 has been described previously with reference to the press-fit examples of Figures 2 and 3a, it will be appreciated that in some examples a first fuel needle valve 16 comprising first and second needle sections 36, 38 attached together via a threaded connection may also comprise a first needle section 36 comprising the sliding surface 62 and defining at least part of the internal bore 20.

[0054] Finally, whilst the connection region 46 is shown in a middle region of the first fuel needle valve 16 in Figures 2 to 3b, it should be appreciated that in some other examples, the connection region 46 where the first and second needle sections 36, 38 are attached together may be located in an upper region of the first fuel needle valve 16, as shown in Figure 1 for example. It follows that in some examples the first fuel needle valve 16 may be configured such that the first and second needle sections 36, 38 are press-fit together in a connection region in an upper region of the first fuel needle valve 16.

[0055] It will be appreciated that all of the above-described examples are provided by way of example only, and that other examples of the invention may include any combination of the features described with reference to each of the examples above.

Claims

CLAIMS1. A dual-fuel injector (10) for injecting a first fuel and a second fuel into a combustion chamber (22) of an internal combustion engine, the dual-fuel injector (10) comprising: a nozzle body (12) housing a dual needle valve arrangement (14) comprising a first fuel needle valve (16) and a second fuel needle valve (18), the first fuel needle valve (16) defining a first needle valve axis (34) and an internal bore (20), and the second fuel needle valve (18) being slidably received within the internal bore (20); a first fuel control chamber (30) defined at least in part by an upper end (32) of the first fuel needle valve (16) such that varying a first pressure in the first fuel control chamber (30) varies the force on the upper end (32) of the first fuel needle valve (16); wherein the first fuel needle valve (16) is a subassembly comprising a first needle section (36) and a second needle section (38), wherein the first needle section (36) is attached to the second needle section (38) by means of a press fit connection, and wherein the first needle section (36) defines at least part of the upper end (32) of the first fuel needle valve (16); wherein the first fuel needle valve (16) comprises a connection region (46) in which one of the first or second needle sections (36, 38) comprises a socket portion (48) and the other of the first or second needle sections comprises a plug portion (50) that is press fit into the socket portion (48), and wherein in the connection region (46), at least one of the socket portion (48) and the plug portion (50) comprises a friction element (52) defining at least part of an interface between the socket portion (48) and the plug portion (50).

2. The dual-fuel injector (10) of Claim 1 , wherein the press fit connection is formed at least in part by a compression collar (54) arranged around an outer surface (56) of the socket portion (48) in the connection region (46).

3. The dual-fuel injector (10) of Claim 2, further comprising a first biasing means (58) arranged to engage the compression collar (54) and a portion of the nozzle body (12) to exert a biasing force on the first fuel needle valve (16).

4. The dual-fuel injector (10) of Claim 3, wherein the nozzle body (12) defines a first fuel valve seat portion (24), and wherein the first biasing means (58) is configured to bias the first fuel needle valve (16) such that the second needle section (38) of the first fuel needle valve (16) engages the first fuel valve seat portion (24).

5. The dual-fuel injector (10) of any preceding claim, wherein the first fuel needle valve (16) defines an accumulator volume (64) for receiving the second fuel.

6. The dual-fuel injector (10) of Claim 5, wherein the second needle section (38) of the first fuel needle valve (16) defines one or more second fuel injection outlets (66) in fluid communication with the accumulator volume (64) for injecting the second fuel from the accumulator volume (64) into the combustion chamber (22).

7. The dual-fuel injector (10) of Claim 5 or Claim 6, wherein the second needle section (38) of the first fuel needle valve (16) defines a second fuel valve seat portion (68), and wherein the dual-fuel injector (10) further comprises a second biasing means (70) located in the accumulator volume (64) and configured to bias the second fuel needle valve (18) into engagement with the second fuel valve seat portion (68).

8. The dual-fuel injector (10) of Claim 7, wherein the second biasing means (70) is arranged to engage the second fuel needle valve (18) and the first needle section (36) of the first fuel needle valve (16).

9. The dual-fuel injector (10) of any preceding claim, wherein the first needle section (36) defines the internal bore (20) in which the second fuel needle valve is slidable received.

10. The dual-fuel injector (10) of Claim 9, wherein the first fuel needle valve (16) comprises an aperture (40) defined in an upper surface (42) of the first needle section (36), and wherein the internal bore (20) extends through the first needle section (36) from the aperture (40).

11. The dual-fuel injector (10) of any preceding claim, wherein the nozzle body (12) defines a needle guide portion (28), and wherein the first needle section (36) of the first fuel needle valve (16) is slidably received in the needle guide portion (28).

12. The dual-fuel injector (10) of Claim 11 , wherein the first needle section (36) of the first fuel needle valve (16) defines both an outer surface (62) for arrangement in sliding relation with the needle guide portion (28), and the internal bore (20) in which the second fuel needle valve (18) is slidably received.

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