Patent Publication Number: US-6338445-B1

Title: Fuel injector

Description:
This invention relates to a fuel injector for use in supplying fuel, under pressure, to a combustion space of a compression ignition internal combustion engine. 
     In order to reduce emissions levels, it is known to provide fuel injectors in which the total area of the openings through which fuel is delivered can be varied, in use. One technique for achieving this is to use two valve needles, one of which is slidable within a bore provided in the other of the needles to control the supply of fuel to some of the outlet openings independently of the supply of fuel to others of the outlet openings. 
     A known fuel injector of this type includes an outer valve needle which is provided with a through bore within which an inner valve needle is slidable, the outer valve needle being slidable within a bore provided in a fuel injector nozzle body. The nozzle body is provided with first and second outlet openings, occupying different axial positions in the nozzle body. A valve insert member is received within the through bore provided in the outer valve needle, the lower end surface of the valve insert member, the bore provided in the outer valve needle and an upper surface of the inner valve needle together defining a spring chamber which houses a compression spring, the spring serving to urge the inner valve needle against the second seating. 
     When the outer valve needle is moved away from the first seating by an amount less than a predetermined amount, fuel is delivered through the 
     first outlet opening and the inner valve needle remains seated to prevent fuel delivery through the second outlet opening. When the outer valve needle is moved away from the first seating by an amount greater than the predetermined amount, a surface of the outer valve needle engages an enlarged region of the inner valve needle, movement of the outer valve needle thereby being transmitted to the inner valve needle causing the inner valve needle to move away from the second seating to permit fuel delivery through the second outlet opening. In this way, the fuel delivery rate, or other injection characteristic, can be varied, in use, by controlling the extent of movement of the outer valve needle away from its seating. 
     Fuel injectors of this type do, however, suffer from the disadvantage that, during the non-injecting stages of the injection cycle, fuel may be able to escape from the spring chamber, thereby causing poor emissions. Additionally, exhaust gases from the engine cylinder may be able to enter the spring chamber which can degrade the performance of the fuel injector. The inner valve needle is also subjected to undesirably high stresses during operation, particularly just prior to the inner valve needle being moved away from the second seating to expose the second outlet opening. 
     It is an object of the present invention to provide a fuel injector which alleviates one or more of the aforementioned problems. 
     According to a first aspect of the present invention there is provided a fuel injector comprising a nozzle body having a first bore defining first and second seatings, an outer valve needle being slidable within the first bore and engageable with the first seating to control fuel flow from a first outlet opening, the outer valve needle being provided with a second bore within which an inner valve needle is slidable, the inner valve needle being engageable with the second seating to control fuel delivery through a second outlet opening, the outer valve needle including a deformable region which is shaped such that, in use, when the outer valve needle is urged against the first seating, the outer valve needle deforms. 
     In one embodiment of the invention, the deformable region is shaped such that, in use, when the outer valve needle is urged against the first seating, the outer valve needle cooperates with the inner valve needle to form a substantially fluid tight seal. 
     By providing the outer valve needle with the deformable region, when the outer valve needle is seated against the first seating the volume defined by the inner valve needle, the outer valve needle and the fuel injector nozzle body within which fuel can reside is significantly reduced. Thus, a reduced volume of fuel is exposed to exhaust gases from the engine cylinder or other combustion space, thereby improving the performance of the fuel injector. 
     Alternatively, or in addition, the deformable region may be shaped such that, in use, when the outer valve needle is urged against the first seating, the outer valve needle deforms to close the first outlet opening. 
     Conveniently, a chamber is defined within the second bore, cooperation between the deformable region of the outer valve needle and the inner valve needle when the outer valve needle is urged against the first seating causing the chamber to be substantially sealed. 
     As the chamber is sealed when the outer valve needle is seated against the first seating, exhaust gases from the engine cylinder or other combustion space are prevented from entering the chamber. This improves the performance of the fuel injector. Additionally, as fuel is unable to escape from the fuel injector when the outer valve needle is seated against the first seating, leakage of fuel from the fuel injector during undesirable stages of the fuel injecting cycle is substantially avoided. 
     Conveniently, the inner valve needle and the outer valve needle may be arranged such that movement of the outer valve needle away from the first seating beyond a predetermined amount is transmitted to the inner valve needle, thereby causing the inner valve needle to move away from the second seating. 
     The outer valve needle may be provided with a surface which is engageable with a first region of the inner valve needle to transmit movement of the outer valve needle to the inner valve needle. The first region and the surface are preferably of substantially frusto-conical form. 
     Preferably, the surface of the outer valve needle which is engageable with the first region is located on the outer valve needle at a position remote from the deformable region. As the surface is located remotely from the deformable region, towards the uppermost open end of the second bore, the fuel injector is easier to manufacture. 
     The inner valve needle may further comprise a second region located downstream of the first region, the second region being of substantially frusto-conical form such that stresses within the second region of the inner valve needle are minimized upon engagement between the surface of the outer valve needle and the first region of the inner valve needle. 
     The inner valve needle may be slidable within a lower region of the second bore and a valve insert member may be received within an upper region of the second bore, the valve insert member being engageable with a seating defined by the open end of the second bore remote from the inner valve needle to permit fuel upstream of the inner valve needle to vent from the second bore. 
     According to a second aspect of the present invention there is provided a fuel injector comprising a nozzle body having a first bore defining first and second seatings, an outer valve needle slidable within the first bore and engageable with the first seating to control fuel flow from a first outlet opening, the outer valve needle being provided with a second bore within which an inner valve needle is slidable, the inner valve needle being engageable with the second seating to control fuel delivery through a second outlet opening, the inner valve needle comprising a first region which is engageable with a surface defined by the second bore such that movement of the outer valve needle away from the first seating beyond a predetermined amount is transmitted to the inner valve needle when the surface engages the first region, and comprising a second region located downstream of the first region, the second region being of substantially frusto-conical form such that stresses within the second region of the inner valve needle are minimized upon engagement between the surface of the outer valve needle and the first region of the inner valve needle. By providing the inner valve needle with the second region of substantially frusto-conical form, stresses which are transmitted to the inner valve needle just prior to movement of the outer valve needle can be reduced. 
     According to a third aspect of the present invention there is provided a fuel injector comprising a nozzle body having a first bore defining first and second seatings, an outer valve needle slidable within the first bore and engageable with the first seating to control fuel flow from a first outlet opening, the outer valve needle being provided with a second bore, an inner valve needle being slidable within a lower region of the second bore and a valve insert member being received within an upper region of the second bore, the valve insert member being engageable with a seating defined by the open end of the second bore remote from the inner valve needle to permit fuel upstream of the inner valve needle to vent from the second bore. 
     As the seating with which the valve insert member is engageable is located at the open end of the second bore, the seating is easier to manufacture. 
     The fuel injector may further comprise a spacer member, received within the second bore provided in the outer valve needle, the spacer member being interposed between the inner valve needle and the valve insert member. The spacer member and the valve insert member may be integrally or separately formed. 
    
    
     The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
     FIG. 1 is a fuel injector in accordance with an embodiment of the present invention; 
     FIG. 2 is an enlarged view of a part of the fuel injector in FIG. 1; 
     FIGS. 3 and 4 are enlarged views of the fuel injector in FIGS. 1 and 2 in first and second fuel injecting positions respectively; 
     FIG. 5 is a sectional view of a fuel injector in accordance with an embodiment of the present invention; and 
     FIGS. 6 and 7 are enlarged sectional views of the fuel injector in FIG. 5 when in first and second positions respectively. 
    
    
     Referring to FIGS. 1 and 2, a fuel injector includes a nozzle body  10  having a blind bore  11  formed therein. The blind end of the bore  11  is shaped to be of frusto-conical form and defines first and second seating surfaces  13   a ,  13   b . An outer valve needle  12  is slidable within the bore  11  and is engageable with the first seating  13   a  to control fuel delivery through a first set of outlet openings  14  (only one of which is shown). The valve needle  12  and bore  11  together define a delivery chamber  15  which communicates with a source of fuel at high pressure by means of a supply passage  16  defined, in part, within an upper part of the nozzle body  10 . The outer valve needle  12  cooperates with the first seating  13   a  to control communication between the delivery chamber  15  and the first outlet opening  14 . 
     The outer valve needle  12  is reciprocable within the bore  11  under the control of an appropriate control arrangement which controls the distance through which the outer valve needle  12  can move away from the first seating  13   a . The control arrangement may comprise, for example, a piezoelectric actuator arrangement which includes a piezoelectric actuator element or stack. The outer valve needle  12  is provided with one or more thrust surfaces  12   a , fuel pressure within the delivery chamber  15  acting on the thrust surfaces  12   a  to urge the valve needle away from the first seating  13   a , in use. The outer valve needle  12  also includes an enlarged region  12   c  extending radially from one section of the outer valve needle  12 , the enlarged region  12   c  having substantially the same diameter as the adjacent part of the bore  11 . Cooperation between the enlarged region  12   c  of the outer valve needle  12  and the bore  11  serves to guide the outer valve needle  12  during axial movement and ensures that the outer valve needle  12  remains concentric with the nozzle body  10 , in use. The outer valve needle  12  may be provided with flats or slots (not shown) on the outer surface to permit fuel in the delivery chamber  15  to flow past the enlarged region  12   c . The outer valve needle  12  further includes an end region  12   b , the end region  12   b  being shaped so as to be capable of deformation when the axial load applied to the outer valve needle  12  is increased beyond a predetermined amount. 
     The outer valve needle  12  is provided with a through bore  17  including a region  17   a  of reduced diameter within which an inner valve needle  18  is slidable, the inner valve needle  18  having a tip region  18   a  which extends into a sac region  19  defined by the blind end of the bore  11 . The bore  17  is shaped to define a further seating surface  20  of substantially frustoconical form with which a region  22  of the inner valve needle  18  is engageable. The seating  20  defined by the bore  17  and the region  22  together define a clearance gap such that, in use, when the outer valve needle  12  is moved inwardly within the bore  11  away from the first seating  13   a  by an amount greater than the clearance gap, the seating  20  engages the region  22  causing movement of the outer valve needle  12  to be transmitted to the inner valve needle  18 . 
     The bore  17  defines a spring chamber  23  within which a compression spring  24  is housed, the compression spring  24  serving to urge the inner valve needle  18  downwardly against the second seating  13   b  such that the tip region  18   a  of the inner valve needle  18  covers a second set of outlet openings  26  (only one of which is shown) provided in the nozzle body  10 . In use, when the inner valve needle  18  is moved away from the seating  13   b , the tip region  18   a  of the valve needle  18  uncovers the second set of outlet openings  26  to permit fuel delivery therethrough. The inner valve needle  18  and the outer valve needle  12  together define a clearance  27  which permits fuel to enter and escape from the spring chamber  23 , in use. 
     One end of the compression spring  24  abuts the upper end surface of the inner valve needle  18 , the other end of the compression spring  24  being in abutment with the lower end surface of a spacer member  28  which is received within bore  17 . At the end of the spacer member  28  remote from 
     the chamber  23 , the spacer member  28  abuts a valve insert member  30  provided with a surface  30   b , the valve insert member  30  being received within the bore  17  and the surface  30   b  being engageable with a corresponding additional seating  32  defined by the bore  17 . It will be appreciated that the spacer member  28  and the valve insert member  30  may be integrally or separately formed. 
     The valve insert member  30  includes, at its uppermost end, a region  30   a  of enlarged diameter, the upper end surface of the valve insert member  30  therefore being of increased diameter. Typically, the enlarged upper end surface of the valve insert member  30  may be acted on by means of a spring (not shown) which serves to urge the valve insert member  30 , and hence the outer valve needle  12 , inwardly within the bore  11 . The enlarged upper end surface may also define, in part, a control chamber  31  for fuel, fuel pressure within the control chamber  31  being varied so as to control movement of the outer valve needle  12  within the bore  11 . 
     The spacer member  28  and the valve insert member  30  are slidable within the bore  17  such that, in use, if fuel pressure within the chamber  23  defined, in part, by the bore  17 , exceeds that in the control chamber  31 , the spacer member  28  is moved upwardly within the bore  17  causing the surface  30   b  to lift away from the seating  32 . Fuel is therefore able to escape from the chamber  23  to the control chamber  31  to reduce fuel pressure within the chamber  23 . Conventionally, the seating  32  defined by the bore  17  with which the valve insert member  30  is engageable to control fuel flow between the spring chamber  23  and the control chamber is provided part way along the length of the bore  17 . By providing the seating  32  at or very close to the open end of the bore  17 , manufacturability of the injector is improved. 
     As can be seen most clearly in FIG. 2, the inner valve needle  18  includes a further region  34  of substantially frusto-conical form, the further region  34  being located downstream of the region  22 . Thus, when the outer valve needle  12  is moved inwardly within the bore  11  and the surface  20  engages the region  22 , the further region  34  adopts a position downstream of the seating  20 . The region  22  of the inner valve needle  18  is also provided with one or more flats or grooves  36  such that, when the region  22  of the inner valve needle  18  is seated against the seating  20 , the fuel is able to flow to and from the chamber  23  past the region  22 . 
     In use, the fuel injector is arranged such that the delivery chamber  15  is supplied with fuel through the supply passage  16  from a source of fuel under high pressure, for example, the common rail of a common rail fuel system, the common rail being charged to a high pressure by an appropriate high pressure fuel pump. Prior to commencement of injection, the actuator arrangement is operated in such a manner that the outer valve needle  12  engages the first seating  13   a . As a result, fuel within the delivery chamber  15  is unable to flow past the seating  13   a  out through the first set of openings  14 . During this stage of the operation, the compression spring  24  biases the inner valve needle  18  against the second seating  13   b , the tip region  18   a  of the inner valve needle  18  covering the second set of outlet openings  26 . As fuel is unable to flow past the first and second seatings  13   a ,  13   b , fuel injection does not therefore take place. 
     When fuel injection is to be commenced, the actuator arrangement is operated in such a manner that the valve insert member  30 , the spacer member  28  and the outer valve needle  12  are moved in an upwards direction, causing the outer valve needle  12  to be lifted away from the first seating  13   a  to the position shown in FIG.  3 . Lifting may be aided by the action of the fuel under pressure within the delivery chamber  15  acting upon the thrust surface  12   a  of the outer valve needle  12 . Upward movement of the outer valve needle  12  away from the first seating  13   a  permits fuel to flow from the delivery chamber  15  past the first seating  13   a  and out through the first set of outlet openings  14 . Provided the outer valve needle  12  is only moved upwardly through a distance which is less than the clearance gap defined between the region  22  of the valve needle  18  and the seating  20  defined by the bore  17 , the seating  20  does not move into engagement with the region  22  of the inner valve needle  18 . The inner valve needle  18  therefore remains in engagement with the second seating  13   b  under the action of the spring  24  and fuel pressure within the chamber  23 . As a result, fuel is unable to flow past the second seating  13   b  out through the second set of outlet openings  26 . It will therefore be appreciated that, as fuel is only injected through the first set of outlet openings  14 , injection of fuel occurs at a relatively low rate for a given applied fuel pressure. 
     When the fuel is to be injected at a higher rate for a given fuel pressure, the actuator arrangement is actuated such that the valve insert member  30 , the spacer member  28  and the outer valve needle  12  are moved through a further distance into the position shown in FIG. 4, further movement of the outer valve needle  12  away from the first seating  13   a  resulting in the seating  20  moving into engagement with the region  22  of the inner valve needle  18 . Movement of the outer valve needle  12  is therefore transmitted to the inner valve needle  18  and the inner valve needle  18  lifts away from the second seating  13   b . As a result, fuel is able to flow from the delivery chamber  15  past the second seating surface  13   b  and out through the second set of outlet openings  26 . As fuel is delivered through both the first and second set of outlet openings  14 ,  26  during this stage of operating, it will be appreciated that fuel is delivered at a relatively high rate for a given fuel pressure. 
     In order to terminate injection, the actuator is operated such that the outer valve needle  12  is returned to the position illustrated in FIGS. 1 and 2 in which the outer valve needle  12  engages the first seating  13   a  and the tip region  18 a of the inner valve needle  18  engages the second seating  13   b . It will be appreciated that, prior to engagement of the outer valve needle  12  with the first seating  13   a , the tip region  18   a  of the inner valve needle  18  moves into engagement with the second seating  13   b . It will therefore be appreciated that termination of fuel injection through the second set of outlet openings  26  occurs prior to termination of injection through the first set of outlet openings  14 . 
     As the end region  12   b  of the outer valve needle  12  is deformable, when an increased axial load is applied to the valve insert member  30  to urge the outer valve needle  12  against the first seating  13   b , the end region  12   b  of the outer valve needle  12  deforms inwardly and co-operates with the inner valve needle  18  so as to form a substantially fluid tight seal. The seal formed between the inner valve needle  18  and the region  12   b  of the outer valve needle closes the clearance  27  and, thus, any fuel remaining in the chamber  23  following an injection of fuel cannot escape from the chamber  23  through the clearance passage  27 . Undesirable leakage of fuel through the first and second outlet openings  14 ,  26  during this non-injecting stage is therefore substantially avoided. Additionally, as the chamber  23  is sealed when the end region  12   b  of the outer valve needle  12  deforms, exhaust gases from the engine cylinder or other combustion space cannot flow into the chamber  23  and contaminate fuel therein. 
     During the fuel injecting stage of operation, with a reduced axial load applied to the outer valve needle  12 , the outer valve needle  12  lifts away from the first seating  13   a  and the end region  12   b  deforms outwardly so as to move away from the inner valve needle  18 , breaking the fluid tight seal and opening the clearance  27 . Thus, during this stage of operation, fuel is able to escape from the chamber  23  through the clearance  27  defined between the outer valve needle  12  and the inner valve needle  18 . Fuel is also able to enter the chamber  23  to re-pressurise the chamber  23  if the pressure in the delivery chamber  15  exceeds that in the chamber  23 . As fuel is able to enter and escape from the chamber  23  through the clearance passage  27 , fuel is prevented from becoming trapped within the chamber  23 . The effects of fuel degradation are therefore minimised. 
     The valve insert member  30  also provides a means of venting the chamber  23  during the fuel injecting cycle. In use, the amount of fuel which flows from the spring chamber  23  to the control chamber at the uppermost end of the outer valve needle  12  is determined by the fuel pressure difference between these two chambers, the length of time that the pressure difference is maintained and the fuel flow area through which the fuel flows. The fuel flow area may be increased by including further flats or slots on the surface of the valve insert member  30 . The fuel pressure difference and the length of time that the fuel pressure difference is maintained are determined by the operating conditions and the type of actuator arrangement use to control movement outer valve needle  12 . 
     The fuel injection of the present invention is also advantageous in that, just prior to the point when the outer valve needle  12  moves into engagement with the region  22  of the inner valve needle  18 , the stresses in the inner valve needle  18  are reduced due to the frusto-conical shaping of the further region  34 . Additionally, the seating  20  defined by the bore  17  and the region  22 , both being of substantially frusto-conical form, are relatively easy to manufacture. 
     By providing first and second sets of outlet openings  14 ,  26  having a different number of openings, or having openings of different size, or having openings providing a different spray pattern, the fuel injection characteristic, for example the fuel injection rate, may be varied in use by injecting fuel through one or both sets of outlet openings. 
     Referring to FIGS. 5 to  7 , there is shown an alternative embodiment of the present invention in which similar parts to those shown in FIGS. 1 to  4  are denoted with like reference numerals. The embodiment shown in FIGS. 5 to  7  differs from that shown in FIGS. 1 to  4  in that the inner valve needle  18  is of elongate form and the seating  20  is positioned relatively close to the uppermost open end of the bore  17 , and remote from the deformable region  12   b  of the outer valve needle. Manufacturability of the injector is therefore improved as it is more difficult to form the seating  20  close to the lowermost, open end of the bore  17 , as shown in FIG.  1 . It will be appreciated that, as the inner valve needle  18  is of elongate form, the need for the spacer member  28  is removed. 
     The through bore  17  provided in the outer valve needle  12  includes a region  17   a  of reduced diameter, an intermediate region  17   b  of intermediate diameter and an upper region  17   c  of enlarged diameter. The inner valve needle  18  includes a lower, tip region  18   a  of reduced diameter, an upper region  18   c  of enlarged diameter and an intermediate region  18   d  of intermediate diameter. As can be seen most clearly in FIG. 6, the region  18   a  of the inner valve needle  18  terminates in a tip portion  18   b  which extends into the sac region  19 . The diameters of the lower region  18   a  of the inner valve needle  18  and of the region  17   a  of the bore  17 , and the diameters of the enlarged region  17   c  of the bore and the enlarged region  18   c  of the inner valve needle  18 , are such that movement of the valve needle  12  within the bore  17  is guided. The interconnection between the regions  17   b ,  17   c  of the bore  17  forms a step which defines the seating  20  with which a surface of the region  18   c  of the inner valve needle  18  is engageable. 
     The spring chamber  23  communicates, by means of a clearance  27   a  defined between the region  17   b  of the bore  17  and the region  18   d  of the inner valve needle  18 , with a further chamber  29  defined, in part, within the bore  17 . The lower region  18   a  of the inner valve needle  18  and the region  12   b  of the outer valve needle  12  together define a clearance  27  which permits fuel to enter and escape from the chamber  29 , in use. Thus, fuel is able to enter and escape from the chamber  23 , in use, through the clearances  27 ,  27   a.    
     One end of the compression spring  24  abuts a part of the upper end surface of the region  18   c  of the inner valve needle  18 , the other end of the compression spring  24  being in abutment with the lower end surface of the valve insert member  30  which is slidable within a region  17   d  of the bore  17 . As described previously, the valve insert member  30  is slidable within the region  17   d  of the bore  17  such that, in use, if fuel pressure within the chamber  23  exceeds fuel pressure within the control chamber  31 , the valve insert member  30  is moved upwardly within the bore region  17   d  causing the surface  30   b  thereof to lift away from the seating  32 . Fuel is therefore able to vent from the chamber  23  to the control chamber  31  to reduce fuel pressure within the chamber  23 . 
     As indicated in FIG. 6, the outer surface of the region  12   b  of the outer valve needle  12  is shaped such that, when the outer valve needle  12  adopts a first position in which the surface of the region  12   b  engages the first seating  13   a , a clearance  35  is defined by a portion of the region  12   b  downstream of the seating  13   a  and the adjacent part of the bore  11 . The clearance  35  communicates with a limited volume  37  defined by the bore  11 , the region  18   a  and the region  12   b . Typically, the region  12   b  of the outer valve needle  12  may be shaped such that the angle  2  (as shown in FIG. 6) subtended by the region  12   b  in the region of engagement with the seating  13   a  is approximately 60° and the angle N subtended by the clearance  35  is approximately 0.125° . By shaping the surface of the region  12   b  in this way, following initial engagement between the region  12   b  and the seating  13   a  to prevent fuel flow past the seating  13   a , the outer valve needle  12  will be caused to move to a second position (as shown in FIG.  7 ), a portion of the region  12   b  downstream of the first seating  13   a  deforming to close the clearance  35 , and hence closing the first set of outlet openings  14 , as will be described in further detail hereinafter. 
     In use, with fuel under high pressure delivered through the supply passage  16  and prior to commencement of injection, the actuator arrangement is operated in such a manner that the region  12   b  of the outer valve needle  12  engages the first seating  13   a . As a result, fuel within the delivery chamber  15  is unable to flow past the seating  13   a  out through the first set of outlet openings  14 . During this stage of the operation, the compression spring  24  biases the inner valve needle  18  against the second seating  13   b , such that the lower region  18   a  of the inner valve needle  18  closes the second set of outlet openings  26 . As fuel is unable to flow past the first and second seatings  13   a ,  13   b , fuel injection does not therefore take place. 
     When fuel injection is to be commenced, the actuator arrangement is operated in such a manner that the valve insert member  30  and the outer valve needle  12  are moved in an upwards direction, causing the outer valve needle  12  to be lifted away from the first seating  13   a . Such lifting movement may be aided by the action of fuel under pressure within the delivery chamber  15  acting on the thrust surfaces  12   a  of the outer valve needle  12 . Upward movement of the outer valve needle  12  away from the first seating  13   a  permits fuel to flow from the delivery chamber  15  past the first seating  13   a  and out through the first set of outlet openings  14 . 
     Provided the outer valve needle  12  is only moved upwardly through a distance which is less than the clearance gap defined between the region  18   c  of the inner valve needle  18  and the seating  20  defined by the bore  17 , the seating  20  does not move into engagement with the region  18   c . The inner valve needle  18  therefore remains in engagement with the second seating  13   b  under the action of the spring  24  and fuel pressure within the chamber  23 . As a result, fuel within the delivery chamber  15  is unable to flow past the second seating  13   b  out through the second set of outlet openings  26 . Thus, as fuel is only injected through the first set of outlet openings  14 , injection of fuel occurs only at a relatively low rate for a given applied fuel pressure. 
     When the fuel is to be injected at a higher rate for a given fuel pressure, the actuator arrangement is actuated such that the valve insert member  30  and the outer valve needle  12  are moved through a further distance, further movement of the outer valve needle  12  away from the first seating  13   a  resulting in the seating  20  moving into engagement with the region  18   c  of the inner valve needle  18 . Movement of the outer valve needle  12  is therefore transmitted to the inner valve needle  18  such that the inner valve needle  18  lifts away from the second seating  13   b . As a result, fuel is able to flow from the delivery chamber  15  past the second seating surface  13   b  and out through the second set of outlet openings  26 . Thus, as fuel is delivered through both the first and second sets of outlet openings  14 ,  26 , fuel is delivered at a relatively high rate for a given fuel pressure. 
     In order to terminate fuel injection, the actuator is operated such that the outer valve needle  12  is returned, initially, to the position illustrated in FIG. 6 in which the region  12   b  of the outer valve needle  12  engages the first seating  13   a  and the lower region  18   a  of the inner valve needle  18  engages the second seating  13   b .With the region  12   b  of the outer valve needle  12  seated against the seating  13   a , the pressure of fuel downstream of the seating  13   a  will reduce to a value significantly less than fuel pressure within the control chamber  31 . The portion of the region  12   b  of the outer valve needle  12  downstream of the seating  13   a  will therefore deform to close the clearance  35 , as shown in FIG. 7, causing the first set of outlet openings  14  to be closed. Thus, with the first set of outlet openings  14  closed and with the region  18   a  of the inner valve needle closing the second set of outlet openings  26 , fuel injection is ceased. It will be appreciated that, upon termination of fuel injection, prior to engagement of the region  12   b  of the outer valve needle with the first seating  13   a , the lower region  18   a  of the inner valve needle  18  moves into engagement with the second seating  13   b . Thus, termination of fuel injection through the second set of outlet openings  26  occurs prior to termination of injection through the first set of outlet openings  14 . 
     Deformation of the region  12   b  to close the first set of outlet openings  14  prevents any residual fuel within the volume  37  from escaping into the engine cylinder or other combustion space. Additionally, as the outer valve needle  12  deforms to close the first set of outlet openings  14 , the volume  37  within which fuel can reside is considerably reduced compared with known fuel injectors of this type. This provides the advantage that the volume of fuel exposed to exhaust gases within the engine cylinder is reduced, thereby reducing undesirable emissions. Furthermore, as can be seen in FIG. 7, as the region  18   a  of the inner valve needle  18  covers the second set of outlet openings  26  during this stage of operation, any residual fuel within the volume  37  and the sac region  19  will be unable to escape to the engine cylinder through the second set of outlet openings  26 . 
     It will be appreciated that, if the fuel injector is operated only as a single-stage lift injector, such that the inner valve needle  18  remains seated against the seating  13   b , the sac region  19  will not refill with fuel between injections. This provides the advantage that, as no fuel will reside in the sac region  19 , there is no fuel to escape to the engine cylinder between injections. 
     The shaping of the region  12   b  of the outer valve needle  12  and of the adjacent part of the bore  11  provided in the nozzle body  10  is preferably arranged to ensure that closure of the first set of outlet openings  14  by deformation of the region  12   b  occurs at minimum rail pressure. This will vary for different fuel injector applications. However, by way of example, the region  18   a  of the inner valve needle  18  may have a diameter of 1 mm, the angle 2 subtended by the region  12   b  may be 60° , the angle N subtended by the clearance  35  may be approximately 0.125° and the first seating  13   a  may have a diameter of 2.25 mm. A fuel injector having these dimensions will cause the region  12   b  of the outer valve needle  12  to deform to close the first set of outlet openings  14  at a rail pressure of approximately 500 Bar. 
     As described hereinbefore with reference to FIGS. 1 to  4 , the region  12   b  of the outer valve needle  12  in FIGS. 5 to  7  may also be arranged such that it deforms inwardly and cooperates with the region  18   a  of the inner valve needle  18  to form a substantially fluid tight seal. The seal formed between the region  18   a  of the inner valve needle  18  and the region  12   b  of the outer valve needle  12  closes the clearance  27  and any fuel remaining within the chambers  23 ,  29  following an injection of fuel cannot therefore escape through the clearance  27 . Undesirable leakage of fuel into the volume  37  and out through the first and second outlet openings  14 ,  26  is therefore further reduced. Additionally, the seal formed between the region  12   b  of the outer valve needle and the region  18   a  of the inner valve needle and the seal formed at the seating  32  ensures the chambers  23 ,  29  are sealed when the region  12   b  of the outer valve needle  12  deforms. Thus, exhaust gases from the engine cylinder of other combustion space cannot flow into the chambers  29 ,  23  and contaminate any fuel therein. 
     It will be appreciated that a different number of outlet openings to those shown in the accompanying figures may be provided in each of the first and second sets  14 ,  26 . Although the second set of outlet openings  26  does not communicate with the sac region  19  in the embodiments of the invention described herein, it will be appreciated that the fuel injector may be of the type in which the second set of outlet openings  26  does communicate directly with the sac region  19 . 
     Although in the description hereinbefore the spring  24  has been referred to as a compression spring, it will be appreciated that any other resilient bias arrangements could be used. It will also be appreciated that, if desired, the inner valve needle  18  may itself be provided with a bore within which a further valve needle is slidable to control delivery of fuel through one or more further outlet openings or groups of outlet openings.