Abstract:
The invention relates to a fuel injection valve for internal combustion engines, comprising a valve body ( 1 ), with a drilling ( 3 ) embodied therein and defined at the combustion chamber end thereof by a conical valve seat ( 18 ) from which at least one injection opening ( 20 ) leads. A hollow valve needle ( 8 ) is arranged in the drilling ( 3 ) such as to be displaced longitudinally, comprising a valve sealing surface ( 35 ) at the end thereof facing the valve seat ( 18 ). A first sealing region ( 31; 34 ) and a second sealing region ( 32; 46; 48 ) are embodied on the valve sealing surface ( 35 ), whereby, on contact of the hollow valve needle ( 8 ) on the valve seat ( 18 ), the first sealing region ( 31; 34 ) provides a seal upstream of the at least one injection opening ( 20 ) and the second sealing region ( 32; 46; 48 ) downstream thereof.

Description:
TECHNICAL FIELD 
     The present invention relates to an injection nozzle for use in a fuel injector for an internal combustion engine. More particularly, although not exclusively, one aspect of the present invention relates to an injection nozzle for use in a compression ignition internal combustion engine in which at least one valve is operable to control the injection of fuel into a combustion space through one or more nozzle outlets. 
     BACKGROUND TO THE INVENTION 
     Due to increasingly stringent environmental regulations, a great deal of pressure is levied upon automotive manufacturers to reduce the level of vehicle exhaust emissions, for example, hydrocarbons, nitrogen oxides (NOx) and carbon monoxide. As is well known, an effective method of reducing exhaust emissions is to supply fuel to the combustion space at high injection pressures (around 2000 bar for example) and to adopt nozzle outlets of a small diameter in order to optimise the atomisation of fuel and so improve efficiency and reduce the levels of hydrocarbons in the exhaust gases. Although the above approach is effective at improving fuel efficiency and reducing harmful engine exhaust emissions, an associated drawback is that reducing nozzle outlet diameter conflicts against the requirement for high fuel injection flow rates at high engine loads and so can compromise vehicle performance. 
     So-called “variable orifice nozzles” (VONs) enable variation in the number of orifices (and therefore the total orifice area) used to inject fuel into the combustion space at different engine loads. Typically, such an injection nozzle has at least two sets of nozzle outlets with first and second valves being operable to control whether fuel injection occurs through only one of the sets of outlets or through both sets simultaneously. In a known injection nozzle of this type, as described in the Applicant&#39;s co-pending European patent application no. EP04250928, the fuel flow to a first (upper) set of nozzle outlets is controlled by an outer valve and the fuel flow to a second (lower) set of nozzle outlets is controlled by an inner valve. The inner valve is lifted by the outer valve only after the flow of fuel through the first set of nozzle outlets has reached a sufficient rate. An injection nozzle of this type enables selection of a small total nozzle outlet area in order to optimise engine emissions at relatively low engine loads. On the other hand, a large total nozzle outlet area may be selected so as to increase the total fuel flow at relatively high engine loads. 
     Although beneficial in many ways, such nozzles do have associated problems. For instance, if the valves do not lift with perfect concentricity, high side loads can be generated due to the hydraulic pressure being significantly lower on the side of the outer valve closest to the nozzle body. Under some conditions these side loads can be high enough to prevent the outer valve closing. 
     One aspect of the present invention relates to a variable orifice nozzle which aims to have the advantages of the above designs, but which serves to alleviate or overcome the aforementioned side load problem. 
     SUMMARY OF THE INVENTION 
     To this end, the invention resides in an injection nozzle for an internal combustion engine. The injection nozzle comprises: a nozzle body defining a seating surface and having a first nozzle outlet and an outer valve member received within the nozzle body and being engageable with an external seating defined by the seating surface so as to control fuel injection through the first nozzle outlet. The outer valve member is provided with a bore having an internal bore surface. An insert is received within the bore, defining an annular gap with the internal bore surface. The outer valve member is engageable with an internal seating defined by a surface of the insert to control fuel flow through the annular gap to the first nozzle outlet. The arrangement is such that the outer valve member is arranged to disengage with the external seating at the same time as it disengages with the internal seating such that the fuel which is to be ejected from the nozzle is always caused to flow simultaneously along: (a) a first path between the outer valve member and the external seating; and (b) a second path through the annular gap. 
     An injection nozzle having a combination of features as set out above has been found to provide particular benefits. For example, the outer valve member is provided with both an internal seating and an external seating, one defined being by the nozzle body and one being defined by the insert in the outer valve bore. By providing the insert to define the internal seating, there is no restriction on the seats being at different axial heights (as in the case where two external seats are provided), so that the internal and external seats can be provided at approximately the same, or similar, axial positions. This means that the vertical area of the valve member exposed to unequal side forces near the outlet is reduced. Furthermore, the external seating and the internal seating can be positioned along the axis of the nozzle body in approximate alignment, at least in circumstances in which the outer valve member is seated. 
     The insert may include a part-spherical head which spans the internal diameter of the bore to define the annular gap. Preferably, the internal seating is defined by a surface of the part-spherical head. The provision of the part-spherical head on the insert means that any misalignment at the internal seating for the valve member is accommodated by the head being able to move angularly about the centre of its sphere. As the internal seating can be located close to the centre of the sphere, any torque at the internal seating resisting the realignment is minimised. 
     In one embodiment, the injection nozzle includes a second nozzle outlet provided in the nozzle body, wherein the insert is an inner valve member which is slidable within the bore and engageable with the insert seating defined by the seating surface so as to control fuel injection through the second outlets. 
     Further, it is preferred for an annular member to be received within the bore so as to be engageable with the internal seating. It is envisaged that the annular member will be a separate component from the main body of the outer valve member. Alternatively, the outer valve member may be machined such that the annular member is formed integrally therewith. 
     The injection nozzle may further comprise a sleeve member that is coupled to the inner valve member, wherein the annular member is brought into engagement with the sleeve member when the outer valve member is moved axially through a distance that is greater than a predetermined distance so as to impart axial movement to the inner valve member also. 
     Preferably, the annular member and the sleeve member have opposed end faces which are spaced apart by the predetermined distance when the outer valve member and the inner valve member are seated against their respective seatings. 
     In one embodiment, an end face of the annular member that engages the internal seating is substantially flat. However, in some respects, it is beneficial for the end face of the annular member that engages the internal seating to be frusto-conical. A frusto-conical end face generates a distinct annular seating line against the flat upper face of the part-spherical head, which provides an improved seal that is more tolerant of flatness errors and less likely to trap dirt. 
     Further, it is preferred that inner valve member includes a valve stem, wherein the internal seating is defined by a shoulder between the part-spherical head and the valve stem. 
     In another embodiment of the invention, the insert does not take the form of a moveable valve member. Instead, the insert may remain engaged with the insert seating during all stages of nozzle operation. 
     Also in this embodiment, the outer valve member may include an annular member which is received within the bore of the outer valve member so as to be engageable with the internal seating. 
     Preferably, the nozzle body is provided with a vent passage through which fuel can escape in the event of fuel leakage past the insert seating. 
     In any embodiment of the invention, the injection nozzle may further comprise an arrangement for urging the insert against the insert seating. For instance, the arrangement for urging the insert against the insert seating may include at least one opening formed in the outer valve member which enables fuel to enter the bore, thereby to apply a hydraulic closing force to the insert. In addition, a spring may be provided to urge the insert against the insert seating. 
     The above described embodiments provide a fuel flow path past the external seating to the first outlet, and a supplementary flow path to the first outlet past the internal seating when the outer valve member is unseated. The supplementary flow path may include at least one channel provided on the insert. 
     In a second aspect, the invention resides in an injector for use in an internal combustion engine, wherein the injector includes an injection nozzle as described above and an actuator for operating the injection nozzle. 
     In order to optimise control over the volume of fuel that is delivered to the combustion chamber, it is preferred that the actuator is a piezoelectric actuator. However, another form of actuator could also be used, such as an electromagnetic actuator. 
     It will be appreciated that the preferred and/or optional features of the first aspect of the invention may be provided alone, or in appropriate combination, in the second aspect of the invention also. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       By way of example, the invention will now be described with reference to the accompanying drawings, in which: 
         FIG. 1  is a part-sectional view of a fuel injector in which an injection nozzle in accordance with the embodiments of the present invention may be incorporated; 
         FIG. 2  is a part-sectional view of the injection nozzle according to a first embodiment of the invention when in a non-injecting position; 
         FIG. 3  is an enlarged part-sectional view of the injection nozzle in  FIG. 2 ; 
         FIG. 4  is a part-sectional view of the injection nozzle in  FIGS. 2 and 3  when in a first injecting position; 
         FIG. 5  is a part-sectional view of the injection nozzle in  FIG. 2  when in a second injecting position; 
         FIG. 6  is a sectional view of the injection nozzle in  FIG. 5  along the line A-A during circumstances in which the outer valve needle lifts eccentrically; 
         FIG. 7  is an enlarged part-sectional view of an injection nozzle according to a second embodiment of the present invention when in a non-injecting position; 
         FIG. 8  is an enlarged part-sectional view of an injection nozzle according to a third embodiment of the present invention when in a non-injecting position; and 
         FIG. 9  is a part-sectional view of the injection nozzle in  FIG. 8  when in a first injecting position. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description, the terms “upper” and “lower” are used having regard to the orientation of the injection nozzles as shown in the drawings. Likewise, the terms “upstream” and “downstream” are used with respect to the direction of fuel flow through the nozzle from a fuel inlet line to fuel outlets. 
       FIG. 1  shows a piezoelectric fuel injector, referred to generally as  2 , within which an injection nozzle  4  in accordance with the invention is incorporated.  FIG. 2  shows the injection nozzle in greater detail. The fuel injector  2  is of the type described in Applicant&#39;s U.S. Pat. No. 6,776,354. 
     The injection nozzle  4  includes a nozzle body  6  provided with an axial bore  8  within which an outer valve member  10  in the form of a needle is slidably received. The nozzle body  6  also includes respective first and second sets of nozzle outlets  12 ,  14  (not shown in  FIG. 1 ) through which fuel can be injected into a combustion chamber, in use. 
     Fuel is supplied to the injector  2  via an injector inlet  16  from, for example, a common rail or other appropriate source of pressurised fuel, which is also arranged to supply fuel to one or more other injectors. Pressurised fuel is communicated from the inlet  16 , through an inlet passage  18  and an accumulator volume  20 , to an annular chamber  22  defined within the bore  8  between the nozzle body  6  and an upper end region  10   a  of the outer valve needle  10 . The upper end region  10   a  has a diameter substantially equal to that of the nozzle body bore  8  such that, in use, co-operation between these parts serves to assist in guiding movement of the outer valve needle  10  as it reciprocates within the bore  8 . Spiral flutes  24  machined into the upper region  10   a  provide a flow path for fuel to be communicated from the annular chamber  22 , through the bore  8  and into a nozzle delivery chamber  26  located towards the tip of the outer valve needle  10 . The delivery chamber  26  is defined between the outer surface of the outer valve needle  10  and the nozzle body bore  8  in a region upstream of the outlets  12 ,  14 . 
     Towards its blind end, the nozzle body bore  8  defines a conical seating surface  28  that terminates in a sac volume  30 . The seating surface  28  defines an external seat  32  with which a tip region  10   b  of the outer valve needle  10  is engageable to control fuel injection through the first set of nozzle outlets  12 . 
     As shown in  FIG. 1 , movement of the outer valve needle  10  is controlled by means of a piezoelectric actuator  40 . The piezoelectric actuator  40  comprises a stack  42  of piezoelectric elements, arranged within the accumulator volume  20 , and an electrical connector  44  which enables a voltage to be applied across the stack  42 . In use, the accumulator volume  20  forms a part of a supply passage to the injection nozzle  4  and, as it is filled with high pressure fuel, applies a hydrostatic loading to the stack  42  which increases the operational efficiency of the stack  42 . The piezoelectric actuator  40  is coupled to the outer valve needle  10  via a hydraulic amplifier arrangement  46  and movement of the outer valve needle  10  is controlled by varying the voltage applied to the stack  42  in order to cause the stack  42  to extend and contract. When the voltage across the stack  42  is reduced, the stack  42  contracts so as to reduce its length and therefore a retracting force is applied to the outer valve needle  10 . Conversely, when the voltage is increased, the length of the stack  42  increases which applies a force urging the outer valve needle  10  into engagement with the seating surface  28 . 
     The outer valve needle  10  is biased towards the external seat  32  by means of a resilient member in the form of a closing spring  45  (shown in  FIG. 1  only), and is operable to move away from the external seat  32 , against the force provided by the closing spring  45 , by means of the actuator. 
     It should be mentioned at this point that although in  FIG. 2  a single outlet is shown in each set of outlets  12 ,  14 , typically each set  12 ,  14  will include a plurality of outlets. Therefore, for the purposes of this specification, reference to an ‘outlet’ should be taken to mean one or more outlets. 
     The injection nozzle  4  also includes an insert member  50  in the form of an inner valve needle which is slidably mounted within a blind axial bore  52  provided in the tip region  10   b  of the outer valve needle  10 . The lower end of the nozzle is shown more clearly in  FIG. 3 . 
     In  FIG. 3 , it can be seen that the inner valve needle  50  is shaped to include a part-spherical head  50   a  that tapers to a generally conical pointed tip. An upper stem region  50   b  extends upwardly from the part-spherical head  50   a  and is of generally uniform cross-section along its length having a diameter less than that of the head  50   a.    
     At its widest point, where the part-spherical head  50   a  meets the stem  50   b , the head  50   a  defines an upper surface that is received within the opening of the inner bore  52  and spans virtually the entire internal diameter thereof. However, the diameter of the part-spherical head  50   a  is slightly less than that of the outer valve bore  52  such that an annular gap  55  is defined between the periphery of the head  50   a  and the inward facing surface of the bore  52 . 
     The upper surface of the part-spherical head  50   a  is substantially flat and defines a shoulder which provides an internal seating  56  for the outer valve needle  10 . The outer valve needle  10  therefore has two seats i.e. the external seating  32  and the internal seating  56 . 
     In the non-injecting position illustrated in  FIGS. 2 and 3 , the inner valve needle  50  is seated on an insert seating  60 , referred to as the inner valve seating, which is defined by a region of the seating surface  28  at a position below the first outlets  12 . Engagement between the part-spherical head  50   a  and the inner valve seating  60  thus controls fuel flow to the second outlets  14 , whilst engagement between the outer valve needle  10  and the internal and external seats  56 ,  32  controls fuel flow to the first outlets  12 . 
     The upper end of the stem region  50   b  is accommodated in a chamber  62  defined by the blind end of the outer valve bore  52 . The chamber  62  is in communication with the nozzle body bore  8  via radial passages  64 , in the form of cross drillings, provided in the outer valve needle  10  so that pressurised fuel within the nozzle body  8  is able to flow into the outer valve bore  52  and the chamber  62 . Fuel pressure within the chamber  62  therefore acts on the inner valve needle  50  and so provides an arrangement for biasing the inner valve needle  50  against the inner valve seating  60 . 
     As has been mentioned, movement of the inner valve needle  50  towards and away from the inner valve seating  60  controls fuel injection through the second set of outlets  14 . However, unlike the outer valve needle  10 , the inner valve needle  50  is not actuated directly by the piezoelectric stack  42 . Instead, and as will be described in greater detail later, once the outer valve needle  10  has moved upwards (i.e. away from the external seating  32 ) beyond a pre-determined distance, it conveys movement to the inner valve needle  50  causing it to move upwards also away from the inner valve seating  60 . 
     To this end, the outer valve needle  10  further comprises an annular member or ring  70  which is received within the outer valve bore  52 . The ring  70  is a separate and distinct part and is coupled to the outer valve needle  10  through frictional contact between the outer surface of the ring  70  and the internal surface of the outer valve bore  52 . That is to say, the ring  70  is an interference fit with the outer valve bore  52 . Together, the outer valve needle  10  and the ring  70  form a moveable valve arrangement. The ring  70  includes a first, upper end face  70   a  and a second, lower end face  70   b.    
     In the closed position, the lower end face  70   b  of the ring  70  engages the internal seating  56  defined by the upper face of the part-spherical head  50   a  such that the inner valve needle  50  is held against the inner valve seating  60  by virtue of the ring  70  acting in combination with high pressure fuel within the chamber  62 . This is the position shown in  FIG. 3 . 
     The internal diameter of the ring  70  is greater than the outer diameter of the inner valve stem  50   b , such that the stem  50   b  passes through the ring  70  and defines a clearance fit therewith such that fuel may flow past the clearance between the inner facing surface of the ring  70  and the outer facing surface of the stem region  50   b . The upper face  70   a  of the ring  70  defines fuel channels  71  in the form of slots or grooves to allow fuel to pass into the centre of the ring  70 , as will be described later. 
     In order for movement to be conveyed from the outer valve needle  10  to the inner valve needle  50 , the stem region  50   b  carries a substantially tubular member  72  in the form of a sleeve, which is a separate and distinct part from the inner valve needle  50 . The sleeve  72  has an external diameter that is less than the internal diameter of the outer valve bore  52 , such that the inner valve needle  50  is free to slide within the bore  52 . Further, the sleeve  72  has an internal diameter that is substantially equal to the outer diameter of the stem region  50   b  and, therefore, the sleeve  72  is an interference fit with the stem  50   b  and so is coupled to the stem  50   b  through frictional contact. A lower end face  72   a  of the sleeve  72  opposes the upper end face  70   a  of the ring  70 , the purpose of which will now be described in further detail. 
     When both the outer valve needle  10  and the inner valve needle  50  are seated, the lower end face  72   a  of the sleeve  72  and the upper end face  70   a  of the ring  70  are separated by a distance ‘L’ that is predetermined at manufacture. The distance ‘L’ determines the amount by which it is necessary for the outer valve needle  10  to lift away from its internal and external seatings  56 ,  32  before engaging the sleeve  72  to convey movement to the inner valve needle  50 . It should be appreciated that the lower end face  72   a  of the sleeve  72  and the upper end face  70   a  of the ring  70  are at maximum separation (i.e. predetermined distance ‘L’) when both the inner valve needle  50  and the outer valve needle  10  are seated, as shown in  FIG. 3 . 
     In use, fuel under high pressure is delivered from the common rail to the nozzle body bore  8  (and thus to the delivery chamber  26 ) via the inlet  16 , the inlet passage  18  and the stack volume  20 , as shown in  FIG. 1 . Initially, the piezoelectric actuator  40  is energised to a relatively high energisation level so that the stack  42  is in an extended state. In such circumstances, the outer valve needle  10  is held against its internal and external seatings  56 ,  32  due to the biasing force of the closing spring  45 . The inner valve needle  50  is held against the inner valve seating  60  due to the pressure of the fuel within the chamber  62  and also by the ring  70  abutting the internal seating  56 . 
     Referring to  FIG. 4 , in order to inject fuel through the first (upper) outlets  12  only, the stack is de-energised to a first, intermediate energisation level causing it to contract, resulting in a lifting force being transmitted to the outer valve needle  10 . The outer valve needle  10  is thus urged to move away from its internal and external seatings  56 ,  32  to open a fuel flow path ‘A’ past the external seating  32  and, thus, through the first outlets  12 . It will be appreciated that the flow path ‘A’ to the outlets  12  which is opened as the outer valve needle  10  lifts from the external seating  32  is an annular flow path around the outer valve needle  10 , although in the section shown it is denoted by a single arrow. 
     In addition to the first fuel flow path ‘A’, a second fuel flow path ‘B’ is created as the lower surface  70   b  of the ring  70  disengages the internal seating  56 . Fuel flows along flow path ‘B’ from the delivery chamber  26 , through the radial drillings  64  and through the channels  71  provided in the upper face  70   a  of the ring  70  into the annular gap defined between the ring  70  and the stem region  50   b . Since the ring  70  is disengaged from the internal seating  56 , fuel flows through the annular gap past the seating  56 , through the annular gap  55  between the opening of the outer valve bore  52  and to the first outlets  12 . 
     During this initial de-energisation of the stack  42 , the outer valve needle  10  is caused to move through a distance less than or equal to the distance ‘L’ (identified on  FIG. 3 ). The ring  70  is carried with the outer valve needle  10  so that the upper end face  70   a  of the ring  70  approaches the opposing lower end face  72   a  of the sleeve  72 . In  FIG. 4 , the ring  70  is moved exactly through the distance ‘L’ so that it just makes contact with the sleeve  72 . Provided the distance through which the outer valve needle  10  moves is no greater than the pre-determined distance ‘L’, movement of the inner valve needle  50  remains decoupled from the outer valve needle  10 , thus the inner valve needle  50  will remain firmly seated against the inner valve seating  60  under the influence of pressurised fuel within the chamber  62 . Fuel is therefore unable to flow past the seated part-spherical head  50   a  of the inner valve needle  50  to the second outlets  14 . 
     The above described condition represents fuel injection optimised for relatively low power applications since a relatively small volume of fuel is injected through the first set of relatively small outlets  12  only. 
     If, at this point, it is necessary to terminate injection through the first outlets  12 , the stack  42  is re-energised to its initial energisation level causing the stack  42  to extend. As a result, the outer valve needle  10  is caused to re-engage both with the external seating  32 , defined by the conical seating surface  28 , and the internal seating  56 , defined by the part-spherical head  50   a , under the influence of the biasing force of the closing spring  45  (shown in  FIG. 1 ). 
       FIG. 5  shows the injection nozzle during a subsequent, or alternative, stage of injector operation in which the stack  42  may be de-energised further to a second energisation level causing the stack length to be reduced further. As a result, the outer valve needle  10  is urged away from the internal and external seatings  56 ,  32  by a further amount, which is greater than the predetermined distance ‘L’. In such circumstances, the upper end face  70   a  of the ring  70  is caused to engage the lower end face  72   a  of the sleeve  72 , thereby causing movement of the outer valve needle  10  to be conveyed or coupled to the inner valve needle  50 . As a result, the inner valve needle  50  is caused to lift from the inner valve seating  60 . 
     As the inner valve needle  50  lifts away from the inner valve seating  60 , fuel within the delivery chamber  26  is able to flow past the internal and external seatings  56 ,  32  to the first outlets  12 , but also past the inner valve seating  60  to the second (i.e. lower) outlets  14  and into the combustion chamber via the sac volume  30 . The flow through the second outlets  14  supplements the fuel flow through the first outlets  12  to provide a higher fuel injection rate suitable for higher engine power modes. 
     Termination of injection occurs if the stack  42  is energised once again to the higher energisation level, as described previously. Alternatively, the energised level may be increased slightly to the first level so that only the outer valve needle  10  is lifted and the inner valve needle  50  returns to the inner valve seating  60  so as to close the flow path to the second outlets  14 . 
     A particular benefit of the nozzle described previously is that the second flow path ‘B’ improves the flow efficiency of the injection nozzle  4  since there is a greater flow area for fuel for a given level of lift of the outer valve needle  10  compared to conventional VONs. In addition, the second flow path ‘B’ serves to reduce the pressure drop between positions upstream and downstream of the seats,  32 ,  56 ,  60  such that lateral side loads acting on the outer valve needle  10  are also reduced. 
     Furthermore, the above described arrangement has the effect of substantially balancing the side loads on the outer valve needle  10 . By way of explanation,  FIG. 6  depicts a scenario in which the outer valve needle  10  has lifted away from the external seating  32  in an eccentric manner such that the clearance between the nozzle body bore  8  and the outer valve needle  10  at a first region ‘C’ is greater than a diametrically opposite region ‘D’. It will be appreciated that the scale of the components and the clearances in  FIG. 6  are exaggerated for the sake of clarity. Fuel flowing through the regions C and D therefore generate a side load in the direction of F 1 . However, since the part-spherical head  50   a  remains seated during relatively low needle lifts, the fuel flowing through the annular gap  80  (second fuel flow path ‘B’) between the stem region  50   b  and the outer valve bore  52  generates a side load in the direction of F 2  which opposes F 1 , and thus provides a balancing force. Therefore, the net side force acting on the outer valve needle  10  is substantially educed which reduces the tendency of the outer valve needle  10  to lift eccentrically. 
     A further benefit is achieved as the outer valve needle  10  seats against a component (the inner valve needle  50 ) which has a part-spherical surface in engagement with the inner valve seating  60 . The part-spherical nature of the inner valve needle  50  allows it to rotate, or tilt, about the centre of its sphere to correct any misalignment of the internal seating  56  on its upper face. As the centre of the part-spherical head  50   a  is paced only a short distance from the internal seating  56  (i.e. a ‘flat top’ of the part-spherical head  50   a ), any torque on the inner valve needle  50  arising from friction at the seating  56 , which would otherwise resist the realignment, is minimal. As the internal seating  56  is defined by the upper surface of the part-spherical head  50   a , this also means that the external seating  32  and the internal seating  56  can be approximately aligned along the longitudinal axis of the injection nozzle  4  when the outer valve needle  10  is seated, and only axially spaced by a relatively small amount (at most, by the predetermined lift distance L), when the outer valve needle  10  is lifted. 
       FIG. 7  shows a second embodiment of the invention, whereby instead of the lower face  70   a  of the ring  70  being flat, it is inclined at an angle to the horizontal (i.e. the lower face  70   a  is frusto-conical) in order to generate a distinct annular seating line  56  against the flat upper face of the part-spherical head  50   a . Concentrating the seating  56  to a distinct annular line, rather than a face to face contact, is likely to give an improved seal which is more tolerant of flatness errors and less likely to trap dirt. It will be appreciated that it is also possible for the part-spherical head  50   a  to be manufactured with an inclined surface and the lower surface  70   a  of the ring  70  to be flat. However, this variant may be more challenging to manufacture since a frusto-conical surface would be more susceptible to concentricity errors. 
     At higher lifts, as the outer valve needle  10  is lifted further away from its internal and external seatings  56 ,  32 , the effective location of the internal seat restriction will move towards the periphery of the outer valve bore  52  as the clearance between the part-spherical head  50   a  and the outer valve bore  52  becomes more restrictive than that at the internal seating  56 . That is to say, as the outer valve needle  10  is lifted higher the fuel flow is most restricted through the channel formed between the peripheral surface of the part-spherical head  50   a  and the inner surface of the outer valve bore  52 , as this channel becomes smaller relative to the spacing between the lower end face  70   a  of the ring  70  and the internal seating  56 . 
     Operation of the injection nozzle  4  in  FIG. 7  would be implemented in a similar manner as for  FIGS. 2 to 5 . 
       FIGS. 8 and 9  illustrate a third embodiment of the present invention. This embodiment is broadly similar to the above-described embodiments and like parts will be numbered accordingly and not described again here. 
     The third embodiment differs in that the nozzle body  4  is provided with only a single set of outlets  100  to the combustion chamber, but is however provided with an additional axially extending outlet or vent  102 , the function of which will be described later. A further modification is that the inner valve needle  50  is replaced with a substantially immovable part-spherical insert  104  having a part-spherical external surface  105  and a flat, upper surface  106 . The part-spherical surface  105  seats on the insert seating  60  and is received within the lowermost end opening of the outer valve bore  52 . 
     In this embodiment, the bore  52  in the outer valve needle  10  includes a ring  110  having a frusto-conical lower face  110   a  similar to that shown in  FIG. 7 , although a ring  110  having a flat lower face could equally be used. The frusto-conical lower surface  110   a  thus defines an internal annular seating line  112  for the outer valve needle  10 . When the nozzle  4  is in the non-injecting position, the ring  110  seats against the internal seating  56  defined by the insert  104 . 
     The diameter of the outer periphery of the insert  104  is less than the diameter of the outer valve bore  52  such that a restricted annular flow path is defined between the periphery of the insert  104  and the inner surface of the outer valve bore  52 . The dimension of the gap is selected as a compromise between providing sufficient centring force to the outer valve needle  10  and providing sufficient fuel flow through the gap. 
     In the event that the ring  110  is slightly misaligned in the outer valve bore  52 , the insert  104  can adjust its seating angle on the insert seating  60  by rotating, or tilting, about the centre of its sphere, so that its flat upper face  106  can adopt the angle of the ring  110  and, hence, account for the misalignment. The set of nozzle outlets  100  is therefore sealed effectively from high pressure fuel at both the external and internal seatings  32 ,  56  of the outer valve needle  10 . 
     High pressure fuel enters the outer valve bore  52  via the radial drillings  64  and, together with the force of the spring  45  (not shown in  FIG. 8 ), which is transmitted to the part-spherical insert  104  via the ring  110 , serves to hold the insert  104  in place against the insert seating  60 . The axial outlet  102  in the nozzle body  6  provides a vent underneath the insert  104  to ensure that any fuel leaking past the insert seating  60  into the tip of the nozzle body  6  simply vents into the combustion chamber. In this way, the insert  104  is prevented from lifting from the insert seating  60  because of fuel trapped beneath it. 
     Referring to  FIG. 9 , when it is desired to inject fuel through the outlets  100 , the outer valve needle  10  is retracted by means of the piezoelectric stack  42  (not identified in  FIG. 9 ) causing the ring  104  to disengage from the internal seating  56 . In such circumstances, a first annular flow path ‘E’ opens up past the external seating  32  and a second annular flow path ‘F’ opens up past the internal seating  56  so that high pressure fuel can flow out through the outlets  100  into the combustion chamber. 
     As the part-spherical insert  104  is effectively rooted to the inner seating  60  by virtue of the high pressure fuel in the outer valve bore  52 , fuel is unable to flow past the insert seating  60  to the outlet  102 . 
     A method by which the inner and outer valves members  50 ,  10  according to the first embodiment may be assembled within the nozzle body  6  will now be described, with general reference to the aforementioned  FIGS. 1 to 7  and the reference numerals indicated therein. 
     Initially, the ring  70  is caused to receive the stem region  50   b  of the inner valve needle  50  so that the lower face  70   b  of the ring  70  abuts the internal seating  56  defined by the part-spherical head  50   a . With the ring  70  in position, the stem region  50   b  is received in the sleeve  72  such that the ring  70  is retained on the inner valve needle  50 . 
     In order to set the predetermined distance ‘L’, a spacer tool, such as a shim of thickness ‘L’ (not shown), is positioned against the upper end face  70   a  of the ring  70 , whereby the sleeve  72  is pushed so as to engage the shim. When the shim is removed, the necessary separation of distance ‘L’ is established between the upper end face  70   a  of the ring  70  and the lower end face  72   a  of the sleeve  72 . 
     Following assembly of the inner valve needle  50 , the ring  70  and the sleeve  72 , the combined inner valve and ring/sleeve assembly is pushed into the bore  52  of the outer valve needle  10 . The inner and outer valves needles  50 ,  10  are then together inserted into the nozzle body bore  8  such that the outer valve needle  10  engages with its internal and external seatings  56 ,  32  and the inner valve needle  50  engages the inner valve seating  60 . Following assembly of the nozzle  4 , a seat bedding operation is performed in order to establish effective seals at the seatings of the inner and outer valve needles  50 ,  10 , respectively. The seat bedding operation comprises applying a constant predetermined axial force to the outer valve needle  10 , which causes it to “bed in” over the external seating  32 . As an alternative to applying a predetermined constant axial force to the outer valve needle  10 , the bedding in operation could also be dynamic. 
     Regarding the manufacture of the embodiment in  FIGS. 8 and 9 , to ensure that the outer valve needle  10  contacts with both internal and external seatings  56 ,  32  simultaneously to provide an effective seal for the outlets  100 , the ring  110  is pushed into its final position by assembling all the components within the nozzle body  6  and applying a load to the outer valve needle  10  until a seal is formed such that fluid ceases to issue from the outlets  100 . Alternatively, the outer valve needle  10  could be pushed into the bore until it makes contact with its seating with a predetermined force. It will be appreciated that the above method could also be employed during the manufacture of the first embodiment. 
     It will be understood by those who practice the invention and those skilled in the art, that various modifications and improvements may be made to the invention without departing from the scope of the invention, as defined by the claims. For example, in the first, second and third embodiments the inner valve needle  50  is forced into engagement with its seating  60  by the high pressure fuel in the outer valve bore  52  and the ring  70  in abutment with the part-spherical head  50   a . However, it is possible that, in use, the lower end face  70   a  of the ring  70  may wear such that a clearance develops at the seating  60  even when the inner and outer valve needle  50 ,  10  are seated, so compromising the seal established by the inner valve needle  50  on the nozzle body  6 . To address this, it may be desirable to provide a resilient member such as a helical spring (not shown) within the chamber  62  to provide a further biasing force to the inner valve needle  50 . Such a spring may abut against an upper end face of the sleeve  72  such that the biasing force is transmitted to the inner valve needle  50  via the frictional coupling between these parts. Alternatively the spring may abut a separate abutment member located within the chamber  62 . 
     Furthermore, although the ring  70  and the sleeve  72  are coupled to the outer valve needle  10  and inner valve needle  50 , respectively, through frictional contact, it will be appreciated that coupling may be achieved through alternative arrangement, for example by gluing or soldering. 
     In addition, although the vent  102  in the embodiment described with reference to  FIGS. 8 and 9  is axially disposed, it should be appreciated that this need not be the case. For example, the vent  102  may be parallel with the outlets  100  or at an angle to the central axis of the nozzle body  6 . 
     It should be understood that although the injection nozzle of the present invention has been described as suitable for use within an injector having a piezoelectric actuator, it is entirely possible that the injector may include an alternative form of actuator for moving the valve(s). For example, instead of a piezoelectric actuator, the outer valve may be moved by means of an electromagnetic actuator. 
     Although the nozzle body  6  has been described as defining the external seating  32  and the insert seating  60  for the outer valve needle  10  and the inner valve needle  50 , respectively, the nozzle body  6  may be provided with a lining plate, sleeve or similar so as to define these surfaces. Similarly, the ring  70  could be provided with a covering plate over its lower end face  70   a  to define that surface of the outer valve needle  10  that engages with the internal seating  56 . Also, either the inner valve needle  50  or the insert  104  could be provided with covering plates or similar so as to define the internal seating  56 . In another modification, the outer valve bore  52  may be provided with a lining sleeve, or similar component, so as to define the internal bore surface. 
     In an alternative embodiment, the inner valve needle  50  may be constructed differently so that the ring  70  forms an integral part of the outer valve needle  10 .