Fuel injector

Abstract

A fuel gaseous injector 10 for an internal combustion engine, the fuel injector comprising a fuel inlet 66; a valve needle 32 for axial movement within a nozzle bore 16, the needle engageable with a s

Description

FIELD OF THE INVENTION 5 This invention relates to a fuel injector for use in a gaseous fuel injection system. In particular, the invention relates to a fuel injector for gaseous fuel such as hydrogen for delivering fuel to an internal combustion engine. 10 BACKGROUND In fuel injection systems for liquid fuel, it is known for a fuel pump to supply fuel to a high-pressure accumulator (or common rail), from where it is delivered into each cylinder of the engine by means of a dedicated fuel injector. Typically, a fuel injector 15 has an injection nozzle that is received within a bore provided in a cylinder head of the cylinder, and a valve needle which is actuated to control the release of high-pressure fuel into the cylinder from spray holes provided in the injection nozzle. One simple way of opening and closing a valve needle is to couple a solenoid actuator directly to the valve needle, by attaching an armature of the actuator to 20 the valve needle (or by providing a valve needle with an integral armature). The valve needle is biased towards a seating surface so that, when the solenoid is not energised, the valve needle prevents fuel flow through the spray holes. When the solenoid is actuated, the valve needle is lifted away from its valve seat and fuel injection takes place. 25 Fuel injectors and injection systems may be configured in a similar manner for use with gaseous fuel, such as hydrogen. In this case, the hydrogen is typically held at high pressure in a storage tank of the vehicle, for example up to 700 bar. One of the problems with a hydrogen fuel injector is that a large nozzle flow area is 30 required in order to inject enough gaseous fuel in the time available for the combustion process. This in turn leads to the requirement for a relatively large seat area where the valve needle seals the outlet, so that a relatively high force is required to lift the valve needle away from the valve seat and to lift quickly enough to achieve full injection rate. However, once the valve needle is beyond an initial 35 lift phase, the seat forces reduce and the force requirement is reduced, whereas for a solenoid-actuated valve needle the lifting force it provides increases as the needle lifts. These two scenarios are conflicting, with the effect that the valve needle is caused to accelerate and therefore can impact the lift stop too quickly. A 07 02 25 sharp impact between the valve needle and the lift stop can lead to poor control of fuelling and excess impact stresses, and so is not desirable. It is against this background that the invention has been devised. 5 SUMMARY OF THE INVENTION According to a first aspect, there is provided an inwardly-opening fuel injector for delivering gaseous fuel to an internal combustion engine, the fuel injector 10 comprising an inlet for receiving gaseous fuel; a nozzle body having a nozzle bore; a valve needle having a longitudinal axis and disposed for axial movement within the nozzle bore, the valve needle including a valve head engageable with a valve seat to control gaseous fuel delivery through at least one outlet of the fuel injector; a fuel delivery path for receiving gaseous fuel from the inlet and for delivering 15 gaseous fuel to the at least one outlet; a return spring which serves to urge the valve needle in a direction to seat the valve head against the valve seat; and a restriction arrangement provided in the fuel delivery path including a restriction member which moves with the valve needle to define, at least in part, a variable restriction to fuel flow as the valve needle moves away from the valve seat, the 20 restriction member being configured so that as the valve needle moves further away from the valve seat the variable restriction becomes more restrictive to fuel flow. Advantageously, the restriction arrangement results in a pressure drop in the fuel 25 delivery path, which acts to oppose the lifting force acting on the valve needle. However, this pressure drop is initially insignificant when the valve needle first lifts, meaning that the lifting force is effectively unopposed when the force requirement to lift the valve needle is highest. However, as the valve needle lifts further away from the valve seat the restriction arrangement presents an increasing restriction 30 (i.e. the flow restriction becomes more restricted), and hence a greater pressure drop exists in the flow path, so that the force acting against the lifting force increases with lift. For a solenoid actuated injector the valve needle lifting force increases as the valve needle lifts and so the characteristic of the restriction arrangement in the invention counteracts this well. 07 02 25 In one embodiment, the restriction member may be configured so that the variable restriction reaches a minimum restrictive flow area for fuel flow once the valve needle has moved a predetermined distance away from the valve seat. 5 In one embodiment, the restriction member may be carried by or may form an integral part of the valve needle. By way of example, the restriction member may define a seat for the return spring. The variable restriction is defined between the restriction member and a further 10 restriction member. An internal collar carried by and fixed to the nozzle body, or integral with the nozzle body, defines the further restriction member. In other words, one part defining the variable restriction is movable with the valve needle and one part is not movable. 15 The variable restriction may be defined by an axial restrictive flow path in an axial direction along the longitudinal axis of the fuel injector and a radial restrictive flow path in a radial direction relative to the longitudinal axis of the fuel injector, for at least a part of the range of movement of the valve needle away from the valve seat. 20 The fuel injector may further comprise a by-pass passage for gaseous fuel to allow a portion of gaseous fuel flow through the fuel delivery path to by-pass the variable restriction. This ensures a good flow of gaseous fuel is maintained to the valve seat, and hence the outlets, even though the benefits of the variable restriction are achieved. 25 In one embodiment, the fuel injector may further comprise a diverging flow member which defines a diverging flow area for fuel downstream of the variable restriction. For example, the diverging flow area may be defined by a diverging flow collar which is carried by the nozzle body. This has the benefit that despite the pressure 30 drop caused upstream in the fuel delivery path, due to the variable restriction, there is no ongoing pressure drop in the onward flow through the injector. In one embodiment, the restriction arrangement may include a curved flow diffuser surface, preferably defined by the internal collar carried by the nozzle body. The variable restriction may be defined by first and second variable restrictions to define a relatively larger flow path for fuel flow when the valve needle is at relatively 07 02 25 low lift, and a relatively restrictive flow path for fuel flow when the valve needle is at relatively higher lift. In other words, there are two restrictions contributing to the pressure drop which aids the damping force acting on the valve needle. 5 For example, the first and second variable restrictions may be defined by a tubular member carried by the valve needle, the tubular member preferably defining a spring seat for the return spring. In this case, the spring seat may include an extension portion to define the first and second variable restriction to fuel flow through the fuel delivery path. 10 In one embodiment, the first variable restriction may be defined by the tubular member and an injector housing part or a part carried thereby and the second variable restriction may be defined by the extension portion and the nozzle body or a part carried thereby, with both the first and second restrictions being variable 15 as the valve needle moves away from the valve seat. BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention may be more readily understood, preferred non-limiting 20 embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view of a fuel injector of a first embodiment of the invention; 25 Figure 2 is an enlarged split cross-sectional view of the fuel injector in Figure 1, to illustrate, on left and right sides of the injector respectively, first and second injector operating positions; 30 Figure 3 is an enlarged split cross-sectional view of an alternative embodiment of the fuel injector to that shown in Figure 2, to illustrate, on left and right sides of the injector respectively, first and second injector operating positions; Figure 4 is an enlarged split cross-sectional view of an alternative embodiment of 35 the fuel injector to that shown in Figures 2 and 3, to illustrate, on left and right sides of the injector respectively, first and second injector operating positions; 07 02 25 Figure 5 is an enlarged split cross-sectional view of an alternative embodiment of the fuel injector to that shown in Figures 2 to 4, to illustrate, on left and right sides of the injector respectively, first and second injector operating positions; and 5 Figure 6 is a graph to illustrate the behaviour (“restrictiveness” characteristic versus valve needle lift) of various embodiments of fuel injectors of the invention. In the drawings, as well as in the following description, like features are assigned like reference signs. 10 Throughout this description, terms such as ‘top’, ‘bottom’, ‘upper’ and ‘lower’, and other directional references, are used with reference to the orientation of the fuel injector as shown in the accompanying drawings. However, it will be appreciated that such references are not limiting and that fuel injectors according to the 15 invention can be used in any orientation. DETAILED DESCRIPTION OF THE INVENTION Figure 1 shows a first embodiment of a fuel injector 10 or injector assembly for 20 injecting gaseous fuel in an internal combustion engine. The fuel injector 10 is of the inwardly-opening type comprising an injection nozzle 12 having a substantially cylindrical nozzle body 14 that defines a nozzle bore 16. The nozzle bore 16 in turn defines a central bore axis which aligns with the longitudinal axis A-A of the injector. 25 A valve seat 20 is disposed at a lower end (relative to the orientation of Figure 1) of the nozzle bore 16, and comprises a generally annular contact area. The contact area of the valve seat 20 is a generally annular portion of a spherical surface. A sac volume 24 is defined within a nozzle tip 28 located at the lower end of the fuel injector 10, the volume 24 extending downwards from the opening defined by the 30 valve seat 20. Several nozzle outlets 26 extend through the nozzle tip 28. In the cross section of Figure 1 only three outlets 26 of the injector are visible, but in practice the nozzle tip 28 may be provided with any number of outlets so as to ensure a high cross sectional flow area is available for fuel exiting the injector 10. This is especially important for a gaseous fuel injector where relatively high 35 injection volumes are required for combustion. 07 02 25 The nozzle tip 28 includes a projection or extension which extends into the nozzle bore 16 and defines a first guide member 30 for guiding movement of a valve needle of the injector 10, referred to generally as 32. The first guide member 30 defines a generally hollow frusto-conical shape in longitudinal section, with the 5 shape of the cone extending in a radially inward and axially upward direction from a lower end of the nozzle bore 16. A frusto-conical guide surface (the radially inner contact surface 34 of the first guide member 30) defined by the first guide member 30 therefore narrows in a direction away from the nozzle tip 28. The first guide member 30 includes several circumferentially spaced apertures 36 for passage of 10 gaseous fuel when the fuel injector 10 is in use. The apertured first guide member 30 therefore provides a guiding function while allowing fuel to flow past the first guide member 30 into the sac volume 24. An upper portion of the nozzle body 14 is received within an internal bore defined 15 within a tubular housing part 42. The internal bore comprises two portions, upper portion 40a of relatively large diameter and lower portion 40b of relatively smaller diameter. The upper portion of the nozzle body 14 includes a second guide member 44 and several radially spaced cross drillings or apertures 47 to provide a fluid path between the internal bore portion 40a and the nozzle bore 16 in the 20 nozzle body 14. The second guide member 44 is provided with an axially extending bore, which extends between the upper and lower faces of the second guide member 44, to define a guide surface 46 (a radially inner contact surface 46 of the second guide member 44) for guiding the valve needle 32. 25 The valve needle 32 extends along the bore axis through the nozzle bore 16, the internal bore 40a, 40b, and the second guide member 44. The valve needle 32 includes a valve needle seating surface 48 defined by a first widened region, or valve head 50, at an end of the valve needle 32 proximal to the valve seat 20. The valve needle seating surface 48 is convex at its lower end, shaped to be 30 complementary to the contact area of the valve seat 20. The complementary relationship between the valve needle seating surface 48 and the valve seat 20 provides a seal when the valve needle 32 is in the closed position. Towards its upper end, the valve needle 32 projects from the second guide 35 member 44 and extends through a return spring 52 which acts on the valve needle 32 and serves to urge the valve needle seating surface 48 against the valve seat 20. The return spring 52 is located within a spring chamber 54 defined within the 07 02 25 relatively larger portion 40a of the bore in the tubular housing part 42. An annular disc 56 is carried by the valve needle 32 in the vicinity of the second guide member 44. The annular disc 56 defines a spring seat member for the return spring 52 and has a first surface defining a seat for the lower end of the spring 52. The spring 52 5 is attached to the valve needle 32 by means of a circlip 58 located in an annular groove on the valve needle 32. The opposed, second surface of the spring seat 56 faces the second guide member 44. The upper end of the spring 52 abuts a spacer member 60 received within the tubular housing part 42. The spring 52 is therefore compressed between the spacer member 60 and the spring seat 56 10 carried by the valve needle 32. The actuation mechanism for the valve needle 32 includes a pull tube 62 provided at the uppermost end of the valve needle 32 which defines a part of the fuel delivery path through the injector between the injector inlet 66 and the outlets 26. The pull 15 tube extends into the upper end of the return spring 52. A flange 64 at the upper end of the valve needle 32 cooperates with a step 70 in an internal bore 68 of the pull tube 62. The pull tube 62 is hollow and includes circumferentially spaced radial apertures 72 immediately above the internal step 70. The pull tube 62 extends along the axis of the injector A-A and through an actuation means in the form of a 20 solenoid actuator, referred to generally as 74. The pull tube 62 is coupled to an armature 76 of the actuator mechanism such that forces acting on the armature 76 are coupled to the pull tube 62, and vice versa. A coil 78 is disposed within a solenoid tube 79 within a solenoid housing 80 adjacent to the armature 76. A lift stop 77 for the armature 76 is located above the armature 76 and is fixed within 25 the solenoid tube 79. In use, when a current is supplied to the coil 78 of the actuator, the armature 76, and hence the pull tube 62, is caused to move upwardly (in the orientation shown), lifting the valve needle 32 with it through the internal engagement between the pull tube 62 and the flange 64 at the step 70. The maximum lift of the valve needle 32 occurs when the pull tube 62 reaches the lift 30 stop 77, as will be clear from the following description. The apertures or cross drillings 72 provided in the pull tube 62 permit gaseous fuel within the pull tube 62 to flow onwards into the spring chamber 54 before passing through the apertures 47 in the nozzle body 14 towards the downstream parts of 35 the injector 10. The internal bore of the pull tube 62 itself, the apertures 72 in the pull tube 62, the spring chamber 54, the apertures 47 in the second guide member 44, the internal bore 16 of the nozzle body 14 and the apertures 36 in the first guide 07 02 25 member 30 therefore all form a part of the fuel delivery path through the injector to the sac volume 24 and the nozzle outlets 26. The solenoid housing 80 for the actuator is coupled to an upper end of the tubular 5 housing part 42 by a connecting sleeve 82. The solenoid housing 80 includes a threaded portion 84 on an outer surface of its lower end. An upper end of the tubular housing part 42 includes an outer flange 86. The connecting sleeve 82 includes a threaded portion 88 on an inner surface of its upper end, and a circumferential ledge 90 near its lower end. The connecting sleeve 82 is screwed 10 onto the solenoid housing 80 by way of these threaded portions. The tubular housing part 42 is axially clamped to the solenoid housing 80 due to the interaction between the outer flange 86 and the circumferential ledge 90 as the connecting sleeve 82 is screwed into place. 15 The section of the injector 10 indicated within the circle C in Figure 1 is shown in enlarged view in Figure 2. As shown in more detail in Figure 2, the injector 10 is provided with a restriction control means defined within the spring chamber 54. It will be appreciated that Figure 2 shows an enlarged view of the injector 10 in Figure 1, in the region of the restriction control means, with the left hand side of the figure 20 showing parts when the valve needle 32 is in a seated position and the right hand side of the figure showing the parts when the valve needle 32 is in a lifted position. The restriction control means serves to restrict the flow of fuel through the spring chamber 54, and hence the fuel delivery path, as the valve needle 32 moves between the seated and lifted positions, as will be described in more detail below. 25 The restriction control means includes a restriction member in the form of a collar 100 which is carried by an internal surface of the tubular housing part 42. The collar 100 may alternatively be an integral part of the tubular housing part 42. The spring seat 56 also forms a part of the restriction control means and itself defines a 30 restriction member. Together, the spring seat 56 and the collar 100 define a restriction (identified generally at 102 in the right hand side of the figure) which varies as the valve needle 32 moves towards and away from the valve seat 20. As shown most clearly on the left hand side of the figure, the restriction may be considered as comprising two restrictive elements; an axial restricted flow path (A) 35 which is defined between a lower axial surface 100a of the collar 100 and an upper axial surface 56a of the spring seat 56 and a radial restricted flow path (R) which is defined between an outer radial surface 56b of the spring seat 56 and an internal 07 02 25 radial surface 100b of the collar 100. The combined effect of the axial and radial restricted flow paths A, R defines the restrictive gap, G (also referred to as 102). Reference to a radial restricted flow path R is intended to mean that the fuel flow through the radial restricted path flows in a radial direction relative to the 5 longitudinal axis A-A of the injector 10 and reference to an axial restricted flow path A is intended to mean that the fuel flow through the axial restricted path flows in an axial direction parallel to the longitudinal axis A-A of the injector 10. Operation of the injector 10 will now be described to show the effect of providing 10 the restriction A, R, in the fuel delivery path to the nozzle outlets 30. The nozzle spring 52 is under compression in Figure 1, holding the valve needle 32 in the closed position such that the valve head 50 is engaged with the valve seat 20. When the valve needle 32 is in this seated position, no gaseous fuel is able to escape from the sac volume 24 through the nozzle outlets 26. 15 When fuel is to be delivered by the fuel injector 10, the coil 78 is energised causing the armature 76 to move upwardly. This causes the pull tube 62 and the valve needle 32 to move upwardly as well, against the force of the nozzle spring 52. The upward movement of the valve needle 32 disengages the valve head 50 from the 20 valve seat 20. This allows high-pressure hydrogen gas (or whichever gaseous fuel is in use) to pass through the interior of the pull tube 62, then through the apertures 72 in the pull tube 62 into the spring chamber 54 through the restriction control mechanism, through the apertures 47 in the nozzle bore 16, through the apertures 36 in the first guide member 30, into the sac volume 24 and then out of the fuel 25 injector 10 through the nozzle outlets 26. The presence of the restriction A, R, in the fuel delivery path, as defined by the collar 100 and the spring seat 56, plays an important part in profiling the rate of lift of the valve needle 32 away from the valve seat 20. Initially, with the valve needle 30 32 seated (the illustration shown on the left hand side of Figure 2), the restriction to fuel flowing through the fuel delivery path, as defined by the restrictive gap G, is relatively unrestricted. In other words, the flow area presented to fuel flow through the fuel delivery path is relatively large. As a result there is little pressure drop across the restrictive gap G and so the resultant closing force acting on the valve 35 needle 32 due to fuel pressure across the restrictive gap G is relatively low. In other words, the force due to fuel pressure within the spring chamber 54 which acts 07 02 25 against the force applied by the pull tube 62 is relatively insignificant and does not impede the initial lift of the valve needle 32 away from the valve seat. As the valve needle 32 starts to lift away from the valve seat 20, with the spring 5 seat 56 moving with it (moving into the position shown in the illustration on the right hand side of Figure 2), the restrictive gap G starts to close as the axial separation A between the upper axial surface 56a of the spring seat 56 and the axial lower surface 100a of the collar 100 are brought together. Eventually the axial surfaces 56a, 100a ’overlap’ so that the axial gap effectively disappears and the restrictive 10 gap G is defined only by the radial separation R between the radial outer surface 56b of the spring seat 56 and the inner radial surface 100b of the collar 100. In other words, the axial gap A gets smaller as the valve needle 32 lifts further so that eventually the radial gap R dominates the effect of the restriction. The increased effect of the restriction (i.e. the reduced flow area across the spring seat 56) leads 15 to an increased pressure drop across the restriction, so that a higher downward force is applied to the valve needle 32 as a result of fuel pressure in the spring chamber 54, opposing the force from the actuated pull tube 62 which lifts the valve needle 32. The collar 100 and the spring seat 56 are positioned relative to one another so that the effect of the restriction getting smaller (i.e. the reducing flow 20 area), and the increasing pressure drop across the restriction, is more effective at relatively high needle lifts following the initial unseating of the valve head. This downward force due to the pressure drop across the restrictive gap G is therefore used to compensate for the solenoid force characteristics and can slow or even stop the valve needle 32 before it reaches full lift. Various combinations of the radial 25 and axial gaps, R, A, which contribute to the overlap restrictive gap G can be used to provide a variety of force versus lift characteristics, as will be discussed further below. The fuel injector 10 is closed by de-energising the coil 78, which allows the return spring 52 to push the valve needle 32 downwards until the valve head 50 engages 30 the valve seat 20. The presence of the restrictive gap G acts in a complementary direction to supplement the closing force of the nozzle spring 52. The velocity of the hydrogen gas flow is so much higher than that of the moving parts that the force due to the pressure drop across the restriction gap G is always downwards irrespective of which direction the parts are moving (moving to open or moving to 35 close). 07 02 25 In an alternative embodiment to that shown in Figures 1 and 2, Figure 3 shows an embodiment where the restriction control means including one or more additional by-pass drillings 110 (two of which are shown in the cross section) provided through the spring seat 56 between the upper and lower faces. As before the left 5 hand side of Figure 3 shows the parts when the valve needle 32 is in the seated position, and the right hand side of Figure 3 shows the parts when the valve needle 32 is in the lifted position. Similar parts to those shown in Figures 1 and 2 are referred to with like reference numerals. The effect of the by-pass drillings 110 is that not all of the injected gaseous fuel has to flow through the restrictive gap, G. 10 This means that a relatively larger flow of gaseous fuel is maintained through the fuel delivery path, despite still achieving a damping effect from the pressure drop across the restrictive gap G. In this example the restrictive gap G is contributed to by both an axial restriction component A and a radial restriction component R, as for Figure 2. When the valve needle 32 is lifted beyond a certain amount (right 15 hand side), the spring seat 56 has moved to a position in which it overlaps the lower axial surface 100a of the collar 100 so that the restrictive gap 102 at this point is defined only by the radial gap between the inner radial surface 100b of the collar 100 and the outer radial surface 56b of the spring seat 56. 20 In this embodiment the second guide member 44 forming part of the nozzle body 14 is chamfered at its upper end. This complements the by-pass drillings 110 as it ensures that the flow through the by-pass drillings 110 has an exit path. In other embodiments, the chamfer may be replaced by a step or a region of reduced diameter to define this exit path. 25 The by-pass passage may be provided in other components, and not necessarily through a component which moves with the valve needle 32, as is the case in Figure 3. 30 Figure 4 is a further embodiment of the invention where the injector of Figure 2 is modified to include a diverging flow area, referred to generally as 120, immediately downstream of the restrictive gap 102, which helps to recover fuel pressure in the fuel flow once it has passed through the restrictive gap 102. As before the left hand side of Figure 4 shows the parts when the valve needle 32 is in the seated position, 35 and the right hand side of Figure 4 shows the parts when the valve needle 32 is in the lifted position. Similar parts to those shown in Figures 1 to 3 are referred to with like reference numerals. The diverging flow area 120 is defined by a diverging flow 07 02 25 member in the form of a collar 122 which is carried on the second guide member 44 of the nozzle body 14. The outer profile of the diverging flow collar 122 is profiled so that the uppermost portion of the collar 122 defines a narrower portion of the fuel delivery path whereas a lower portion of the collar 122 defines a wider portion 5 of the fuel delivery path. The use of the diverging flow collar 122 has the effect that, whilst a pressure drop occurs across the restrictive gap, G, so that the pressure immediately below the spring seat 56 is relatively low to dampen the lifting force, as described previously, there is no excessive pressure drop in the onward flow through the nozzle. 10 A further feature of the embodiment of Figure 4 is to provide a curved flow diffuser surface 100c on the lower axial surface of the collar. A curved flow diffuser surface 100c in this position serves to further reduce the pressure loss effect in the flow delivery path by aligning the flow to the diverging flow area 120 further 15 downstream. The by-pass drilling(s) 110 could be provided in the embodiment of Figure 4, although they are not shown here. 20 In another embodiment the curved flow diffuser surface 100c on the lower axial surface of the collar may be provided without the diverging flow area collar 122. Figure 5 shows a further embodiment of the invention where the restriction arrangement defines first and second restrictive gaps, G1 and G2. As before the 25 left hand side of Figure 5 shows the parts when the valve needle 32 is in the seated position, and the right hand side of Figure 5 shows the parts when the valve needle 32 is in the lifted position. Similar parts to those shown in Figures 1 to 4 are referred to with like reference numerals. The spring seat 156 takes a different form in Figure 5 compared to previous embodiments, being in the form of a tubular spring seat 30 member 156 having an extension portion 156a (the spring seat extension) extending downwards (in the illustration shown) which receives the second guide member 44 of the nozzle body 14. The spring seat 156 includes a lower base portion 156b at the base of the extension portion 156a. The lower base portion 156b has a lipped edge 156c. The spring seat 156, the extension portion 156a and 35 the lower base portion 156b therefore define, together with the outer surface 44a of the second guide member 44 of the nozzle body 14, an internal cavity 126. The wall of the extension portion 156a provides space for apertures or cross drillings 07 02 25 128 and facilitates the presence of the second restriction gap, G2. The outer surface 44a of the second guide member 44 is provided with an annular groove 130 which contributes to the second restrictive gap G2. 5 Referring to the left hand side of the illustration, the first restriction gap G1 is initially defined between the lower axial surface 100a of the collar 100 and an upper curved surface of the lipped edge 156c on the spring seat 56. This restrictive gap G1 has both radial and axial components at the very initial point of lift of the valve needle 32 and at positions of relatively low needle lift. The restrictive gap G1 is variable 10 and becomes more restrictive as the valve needle 32 lifts (into the right hand side of Figure 5), and the upper surface of the lipped edge 156c moves closer to the lower axial surface 100a of the collar 100. The second restriction gap G2 defines a variable axial restriction through the lower base portion 156b of the spring seat 156. The second restrictive gap G2 becomes more restrictive as the valve needle 15 32 lifts (into the right hand side of Figure 5), moving the surface of the lower base portion 156b closer to the outer surface 44a of the second guide member 44. The effect of having the first and second restrictive gaps G1, G2 is to enable a larger flow area for the injected flow through the delivery flow path at relatively low lift (as in the left hand illustration) but a relatively lower flow area for the injected flow 20 through the delivery flow path once the valve needle reaches higher lifts (as in the right hand side of the illustration). As can be seen by comparing the left and right hand sides of the illustration in Figure 5, both the first and second restrictive gaps G1, G2 become smaller (more restrictive) as the valve needle 32 lifts further away from the valve seat 20. 25 The flow through the restriction arrangement in Figure 5 passes into a space 130 between the collar 100 and the extension portion 156a on the spring seat 156, through the cross drillings 128 in the extension portion 156b into the internal cavity 126 and onwards through the second restriction gap G2. The flow also passes from 30 the space 130 through the first restrictive gap G1. As the valve needle 32 continues to lift, both the first and second restrictive gaps G1, G2 start to close so that eventually both have only a radial component. In practice the gaps may be dimensioned to provide either axial or restrictive 35 components, or both, providing that as the valve needle 32 continues to lift the overall restriction to the flow provided by the combination is reduced to increase the damping force acting on the valve needle as it lifts further. 07 02 25 Figure 6 illustrates a graph to show what may be referred to as the “restrictiveness” characteristic versus valve needle lift for several different embodiments of the injector. The “restrictiveness” characteristic is represented by 1 / total flow area2, 5 where the total flow area is the flow area through the overall restrictive gap G for a given needle lift, which gives an indication of the pressure drop across the restriction arrangement and, hence, the downwards force provided against the lift force. The overall restrictive gap G varies depending on the particular arrangement of the restriction, the axial gap A, the radial gap R (if present) and the by-pass 10 drilling(s) 110 (if present). The lines / curves on the graph are identified as follows: (I) Relatively small radial gap R and relatively small axial gap A; (II) Relatively small radial gap R and relatively large axial gap A; (III) Relatively large radial gap R and axial gap; 15 (IV) Average radial gap R (part way between the size of the radial gap R in (I) and (III) for example) and bypass drilling 110; (V) Relatively small radial gap and bypass drilling 110; (VI) Two restrictive gaps G1, G2 (as in Figure 5). 20 Figure 6 illustrates that by adjusting the axial gap, A (comparing lines (I) and (II)) a large range of curve Y can be achieved, but with the small radial gap R (as in line (I) the increase in force at low lift may be too high where there is a relatively small axial gap. A limiting plateau or threshold, as indicated by X, may be achieved through a combination of a radial gap R and / or a by-pass passage 110 (see lines 25 (III), (IV), (V), (VI)). The restriction arrangement is configured so that the threshold suits the operating requirements of the injector, providing enough restriction to effect a damping to the valve needle as it lifts but not overly damping the movement beyond a certain amount of lift. The steepness of the curve for lift values just before the plateau can be adjusted by adjusting the relative flow areas of each flow path 30 through the restriction (i.e. adjusting the size of the radial gap - see lines (III), (IV), (V)). It will be appreciated that other embodiments of the invention are envisaged without departing from the scope of the appended claims and the function of the 35 injector claimed therein. List of parts 07 02 25 10 - fuel injector 12 - injection nozzle 14 - nozzle body 16 - nozzle bore 5 20 - valve seat 24 - sac volume 26 - nozzle outlets 28 - nozzle tip 30 - first guide member 10 32 - valve needle 34 - radially inner contact surface of the first guide member 30 36 - apertures in radially inner contact surface 34 40a, 40b - upper and lower portions of the internal bore in the nozzle body 42 - tubular housing part 15 44 - second guide member 44a - outer surface of the second guide member 44 46 - guide surface of the second guide member 44 47 - apertures in the second guide member 44 48 - valve needle seating surface 20 50 - valve head 52 - return spring 54 - spring chamber 56 - spring seat 56a - upper axial surface of spring seat 56 25 56b - outer radial surface of spring seat 56 58 - circlip 60 - spacer member 62 - pull tube 64 - flange on the valve needle 32 30 66 - injector inlet 68 - internal bore of the pull tube 70 - step in the internal bore 68 72 - apertures in the pull tube 62 74 - solenoid actuator 35 76-armature 77- lift stop 78 - coil 07 02 25 79 - solenoid tube 80 - solenoid housing 82 - connecting sleeve 84 - threaded portion on solenoid housing 80 5 86 - outer flange on tubular housing part 42 88 - threaded portion on connecting sleeve 82 90 - circumferential ledge on connecting sleeve 82 100 - collar of restriction control means 100a - lower axial surface of collar 100 10 100b - internal radial surface of collar 100 100c - flow diffuser surface 102 - restrictive gap 110 - by-pass drillings through spring seat 56 120 - diverging flow area 15 122 - diverging flow collar 130 - annular groove in second guide member 44 156 - spring seat 156a - extension portion of spring seat 156 156b - lower base of spring seat 156 20 156c - lower axial surface of spring seat 156 A - axial restrictive flow path R - radial restrictive flow path G - restrictive gap G1, G2 - first and second restrictive gaps 25 A-A - injector axis X - limiting plateau or threshold Y - curve range 30 35 04 04 25

Claims

1. A fuel injector (10) for delivering gaseous fuel to an internal combustion engine, the fuel injector (10) being an inwardly-opening fuel injector (10), 5 and the fuel injector (10) comprising:an inlet (66) for receiving gaseous fuel;a nozzle body (14) having a nozzle bore (16);a valve needle (32) having a longitudinal axis and disposed for axial movement within the nozzle bore (16), the valve needle including a valve head (50) 10 engageable with a valve seat (20) to control gaseous fuel delivery through at least one outlet (26) of the fuel injector (10);a fuel delivery path for receiving gaseous fuel from the inlet (66) and for delivering gaseous fuel to the at least one outlet (26);a return spring (52) which serves to urge the valve needle (32) in a direction 15 to seat the valve head (50) against the valve seat (20); anda restriction arrangement (56, 100, 102) provided in the fuel delivery path including a restriction member (56; 156) which moves with the valve needle and defines, at least in part, a variable restriction (102) to fuel flow as the valve needle (32) moves away from the valve seat (20), the restriction member (56; 156) being 20 configured so that as the valve needle (32) moves further away from the valve seat (20) the variable restriction (102) becomes more restrictive to fuel flow;wherein the variable restriction is defined between the restriction member (56; 156) and a further restriction member; andwherein an internal collar (100) carried by and fixed to the nozzle body (14), 25 or integral with the nozzle body (14), defines the further restriction member.

2. The fuel injector (10) as claimed in claim 1, the restriction member (56; 156) being configured so that the variable restriction (102) reaches a minimum restrictive flow area for fuel flow once the valve needle (32) has moved a 30 predetermined distance away from the valve seat (20).

3. The fuel injector (10) as claimed in claim 1 or claim 2, wherein the restriction member (56) is carried by or forms an integral part of the valve needle.35 4. The fuel injector (10) as claimed in claim 3, wherein the restriction memberdefines a seat (56; 156) for the return spring (52).04 04 255. The fuel injector (10) as claimed in any of claims 1 to 4, wherein the variable restriction (102) is defined by both an axial restrictive flow path (A) in an axial direction along the longitudinal axis (A-A) of the fuel injector (10) and 5 a radial restrictive flow path (R) in a radial direction relative to thelongitudinal axis (A-A) of the fuel injector (10), for at least a part of the range of movement of the valve needle (32) away from the valve seat (20).

6. The fuel injector (10) as claimed in any of claims 1 to 5, further comprising 10 a by-pass passage (110) for gaseous fuel to allow a portion of gaseous fuelflow through the fuel delivery path to by-pass the variable restriction (102).

7. The fuel injector (10) as claimed in any of claims 1 to 6, further comprising a diverging flow member (122) which defines a diverging flow area (120) 15 for fuel downstream of the variable restriction (102).

8. The fuel injector (10) as claimed in claim 7, wherein the diverging flow area (120) is defined by a diverging flow collar (122) which is carried by the nozzle body (14).

209. The fuel injector (10) as claimed in any of claims 1 to 8, wherein the restriction arrangement includes a curved flow diffuser surface (100c).

10. The fuel injector (10) as claimed in any of claims 1 to 9, wherein the variable 25 restriction (102) is defined by firstand second variable restrictions (G1, G2)to define a relatively larger flow path for fuel flow when the valve needle (32) is at relatively low lift, and a relatively restrictive flow path for fuel flow when the valve needle (32) is at relatively higher lift.30 11. The fuel injector (10) as claimed in claim 10, wherein the first and secondvariable restrictions (G1, G2) are defined by a tubular member (156) carried by the valve needle (32).

12. The fuel injector (10) as claimed in claim 11, wherein the tubular member 35 (156) defines a spring seat (156) for the return spring (52).

13. The fuel injector as claimed in claim 12, wherein the spring seat (156) includes an extension portion (156a) to define the first and second variable restriction (G1, G2) to fuel flow through the fuel delivery path.

14. The fuel injector as claimed in claim 13, wherein the first variable restriction (G1) is defined by the tubular member (156) and an injector housing part (42) or a part (100) carried thereby, and the second variable restriction (G2)5 is defined by the extension portion (156a) and the nozzle body (14) or apart carried thereby.LDCM

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