Patent Publication Number: US-9903327-B2

Title: Fuel injector

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to EP Application No. 14169986 filed May 27, 2014, the contents of which are hereby incorporated by reference in their entirety. 
     TECHNICAL FIELD 
     The present invention concerns a fuel injector. More specifically, it concerns a fuel injector for use with a combustion engine in a motor vehicle. 
     BACKGROUND 
     A fuel injector for injecting fuel into a combustion engine comprises a valve that can be opened by means of an electrically driven actuator against the force of a spring. Different designs are known in the art, comprising electromagnetic or piezo actuators, digital or servo models and actuators for different fuel types such as gasoline or diesel. 
     US 2006/0255185 A1 shows a fuel injector with electromagnetic actuator in which the valve comprises a needle and the valve opens when the needle is moved in a direction of a nozzle of the injector. 
     An amount of fuel running through the injector is generally dependent on the time the actuator is driven. A flow curve that shows a relationship between the drive time and the throughput has generally three successive areas. Very short drive times relate to a ballistic area where the needle is never fully open and the injection is never fully stabilized. Nevertheless, flow rates are generally repeatable. With longer drive times, the injector will be in a non-linear area. In this area, the needle reaches full opening but the flow dynamics are not stabilized as not all parts of the injector had enough time to settle. With even longer drive times, a linear area is entered, where the needle reaches its fully open position, the flow is stabilized and all the moving parts of the injector have settled. 
     The smaller the non-linear area is, the smaller are part-to-part and shot-to-shot deviations. An ideal flow curve would be monotonic with only a ballistic area and a linear area. 
     In order to help the needle to mechanically settle during an opening phase, a hydraulic dampening area may be foreseen that provides hydraulic dampening. However, extensive dampening leads to slower opening transients and much slower closing transients, which is undesirable. 
     SUMMARY 
     One embodiment provides a fuel injector for injecting fuel into a combustion engine, the injector comprising: a valve with a needle that is movable along a longitudinal axis between an open position and a closed position, for opening or closing the valve; an actuator which comprises an armature and a pole piece, the armature is axially movable and operable to interact mechanically with the needle, so that the needle is moved towards the open position by a movement of the armature in axial direction towards the pole piece; and a first spring for biasing the armature in axial direction away from the pole piece, wherein the first spring is configured and operable to stop said movement of the armature by means of its spring force when the needle is in the open position. 
     In a further embodiment, the armature is spaced apart from the pole piece when the needle is in the open position. 
     In a further embodiment, the first spring is axially movable relative to the pole piece and has an axial play towards the pole piece when the needle is in the closed position. 
     In a further embodiment, a deflection of the first spring when the needle is in the open position is small compared to the play. 
     In a further embodiment, the armature is axially displaceable relative to the needle, the needle has an upper retainer and the armature is operable to establish a form-fit engagement with the upper retainer for moving the needle towards the open position, the injector further comprises a second spring for biasing the armature away from the upper retainer, the second spring being softer than the first spring. 
     In a further embodiment, the first spring is axially displaceable relative to the needle and axially arranged between the second spring and the armature so that a spring force of the second spring is transferred to the armature via the first spring. 
     In a further embodiment, the armature is spaced apart from the pole piece by a fuel filled axial gap, such that the gap is reduced when the needle is moved towards the open position, the gap being shaped and dimensioned such as to provide hydraulic dampening to the movement of the armature. 
     In a further embodiment, the first spring comprises a hollow cylindrical body with a radial opening. 
     In a further embodiment, the radial opening is a helical cut. 
     In a further embodiment, the injector further comprises a third spring for moving the needle towards the closed position. 
     In a further embodiment, a spring seat for an end of the third spring that is remote from the needle is axially movable relative to a spring seat for an end of the first spring that is remote from the armature for calibrating a preload of the third spring. 
     In a further embodiment, the spring seat for said end of the first spring is press-fitted into an opening of the pole piece and the spring seat for said end of the third spring is press fitted into an opening of the spring seat for said end of the first spring. 
     In a further embodiment, the third spring is harder than the second spring but softer than the first spring. 
     In a further embodiment, the needle is configured to open the valve when the needle is moved away from a nozzle end of the injector. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Example embodiments are discussed below in detail with reference to the drawings, in which: 
         FIG. 1  shows a longitudinal section of an injector according to a first exemplary embodiment; 
         FIGS. 2 and 3  show enlarged details of the injector of  FIG. 1 ; and 
         FIGS. 4-9  show longitudinal sections of injectors according further exemplary embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of present invention provide an injector with improved opening and closing behaviour. 
     Some embodiments provide a fuel injector for injecting fuel into a combustion engine is disclosed. The fuel injector comprises a valve with a needle that is moveable between an open and a closed position, for opening or closing the valve. In particular, the needle is moveable between the open and the closed position along a longitudinal axis. The longitudinal axis is in particular also a longitudinal axis of a valve body of the fuel injector, the valve body in particular having a cavity in which the needle is received in reciprocatingly displaceable fashion. 
     Expediently, the needle is operable to interact with a valve seat to close the valve when it is in the closed position and axially displaceable away from the closed to the open position to open the valve, in particular to enable fluid flow from the cavity through an injection opening of the injector. The needle may be configured to open the valve when the needle is moved away from a nozzle end of the injector, i.e. in particular a direction from the injection opening towards a fluid inlet end of the valve body. 
     Further, the injector comprises an actuator. The actuator, which is in particular an electromagnetic actuator, comprises an armature and a pole piece. The armature is axially movable, in particular relative to the valve body. It is operable to interact mechanically with the needle so that the needle is moved towards the open position by a movement of the armature in axial direction towards the pole piece. The movement of the armature towards the pole piece is in particular effected by a magnetic force on the armature which is generated by the actuator, e.g., by means of a solenoid which is comprised by the actuator. 
     In addition, the injector comprises a first spring for biasing the armature away from the pole piece, in particular in axial direction. The first spring is configured and operable to stop the movement of the armature towards the pole piece when the needle is in the open position. In particular, the armature is operable to compress the first spring when it moves towards the pole piece to generate a spring force which compensates the magnetic force. In other words, the spring rate of the first spring, i.e. the stiffness of the first spring, is in particular configured such that the movement of the armature is stopped by the spring force of the first spring when the needle is in the open position. 
     In one embodiment, the armature is spaced apart from the pole piece when the needle is in the open position. In particular, the needle is not in form-fit engagement with an element other than the first spring in this situation. In other words, absent the first spring, the armature would be further displaceable axially towards the pole piece when the needle is in the open position. 
     Thus, when the needle reaches the open position, the armature will not be stopped by a stationary barrier, sometimes also called a “hard stop” by the person skilled in the art, but rather be cushioned by the elastic force of the first spring. Movement of the armature and the needle on the way from the closed to the open position and towards the pole piece may be slowed down rather gently by the first spring so that a fast and repeatable settling of the needle&#39;s quick opening movement may be achieved. This can help to reduce the above mentioned non-linear area so that better control over the amount of fuel injected into the combustion engine can be achieved over a broader range of injection times. 
     In one embodiment, an end of the first spring which is remote from the armature is positionally fix relative to the pole piece. Alternatively, the end of the first spring which is remote from the armature can be axially displaceable relative to the pole piece so that in one embodiment, the first spring is axially moveable relative to the pole piece. In an expedient development, the first spring has an axial play towards the pole piece when the needle is in the closed position. The needle may therefore be quickly accelerated by the armature before the compression of the first spring sets in and decelerates the armature. Faster opening of the valve may thus be accomplished. 
     The first spring may have very steep spring characteristics. With the high stiffness of the first spring, the force required to compress it over a predetermined length is preferred to be very high and may lie one or several magnitudes over the stiffnesses of other springs in the injector. Thus, according to another embodiment, a deflection of the first spring when the needle is in the open position is small compared to said play. Through this, acceleration and deceleration of the needle may be further improved. Control over the needle and therefore the valve may therefore be augmented. 
     In one embodiment the armature is axially displaceable relative to the valve needle. In order to enable the mechanical interaction between the armature and the needle, the needle has an upper retainer. The armature is in particular operable to establish a form-fit engagement with the upper retainer for moving the needle towards the open position. 
     In one development, the injector further comprises a second spring for biasing the armature away from the upper retainer. The second spring may also be denoted as armature return spring. By means of the bias of the second spring, surfaces of the armature and the upper retainer which abut one another for establishing the form-fit engagement may be axially spaced apart when the actuator is deenergized. In this way, a so-called free lift or blind lift of the armature is enabled. This allows opening of the needle against particularly high fluid pressures due to a large initial impulse transfer on the needle when the—already accelerated—armature hits the upper retainer. Expediently, the second spring is softer than the first spring. For example, its spring rate is smaller 50% or smaller, in particular 20% or smaller, for example 10% or smaller than the spring rate of the first spring. 
     In one embodiment, the first spring is axially displaceable relative to the needle. In one development, the first spring is axially arranged between the second spring and the armature so that a spring force of the second spring is transferred to the armature via the first spring. With advantage, the second spring also is operable to bias the first spring away from the pole piece. In this way, the position of the first spring is stabilized so that the axial play of the first spring may be particularly well defined. With the combination of the first, hard spring and the second, soft spring, high acceleration and quick deceleration of the needle may be achieved. The non-linear area of the injector&#39;s flow curve may thus be further reduced. 
     According to another embodiment, the armature is spaced apart from the pole piece by a gap, such that the gap is reduced when the needle is moved towards the open position. The gap is filled with fuel. In particular it is positioned within the cavity of the valve body. The gap is shaped and dimensioned such as to provide hydraulic dampening to the movement of the armature. 
     Through this, hydraulic dampening may help to save time in the deceleration process. The dampening may also help to further reduce a settling time of the needle in the open position. Surfaces that define said gap may be chosen to be relatively large so that the dampening effect may be controlled to be rather substantial. Preferably, opposing surfaces of the armature and the pole piece which define the gap remain spaced apart from one another—in places, preferably over the bigger part of their overlapping area or, particularly preferably, completely—when the armature is stopped by the first spring. In this way, hydraulic sticking between these two surfaces is avoided or at least largely reduced when the actuator is de-energized for initiating the closing movement of the armature-needle assembly. In this way, a particularly fast closing transient of the needle is achievable. 
     In one embodiment, the first spring comprises a hollow cylindrical body, i.e. a cylinder shell, with a radial opening. In one embodiment, it has a plurality of radial openings, such as bores through the sidewall of the cylinder shell. In another embodiment, the opening may run in a helical or transverse direction. For example, the radial opening is a helical cut through the sidewall of the cylinder shell. A spring of this type may have extremely hard spring characteristics and thus be well suited for the first spring. Springs of such type are in principle known to the skilled person under the trade name HELI-CAL or from German patent 63263, German utility model 1783503 and German patent application DE 40 33 945 A1. 
     In another embodiment, there is provided a third spring for moving the needle towards the closed position. The third spring may also be denoted as calibration spring. Preferably, the third spring has no play towards the needle. In one embodiment, the third spring is harder than the second spring and softer than the first spring. For example, the spring rate of the third spring is at most 50% of the spring rate of the first spring and the spring rate of the second spring is at most 50% of the spring rate of the third spring. 
     In one embodiment, ends of the first and third springs that are remote from to the armature and the needle, respectively, abut parts of the injector that are axially movable with respect to each other. To put it in a different way, a spring seat for an end of the third spring that is remote from the needle is axially movable relative to a spring seat for an end of the first spring that is remote from the armature for calibrating a preload of the third spring. For example, the spring seat for said end of the first spring is press-fitted into an opening of the pole piece and the spring seat for said end of the third spring is press-fitted into an opening of the spring seat for said end of the first spring. 
     It is therefore possible to adjust the tension of the third spring when the needle is in the open position independently from a position of the first spring. By adjusting said tension, a dynamic flow rate of fuel through the injector may be calibrated. Part-to-part variations between identically constructed injectors may thus be compensated during or after the manufacturing process. 
       FIG. 1  shows an injector  100  for injecting fuel into a combustion engine according to a first exemplary embodiment in a longitudinal section view. 
     The injector  100  has a longitudinal axis  105 , a nozzle end  110  and an opposed supply end  115 , sometimes also referred to as fuel inlet end and fuel outlet end, respectively. The injector  100  comprises a valve  120  and an actuator  125  for operating the valve  120 . The actuator  125  is an electromagnetic actuator which is supplied with electrical power through a connector  130 . When electrical energy is supplied to the connector  130 , fuel flows from the supply end  115  through the valve  120  and is ejected from the injector  100  at the nozzle end  110 . 
     In the shown embodiment, the valve  120  comprises a needle  135  that is movable along the axis  105  between an open position  140  in which the valve  120  is open and a closed position  145  in which no fuel can pass through the valve  120 . The needle  135  is received in a cavity of a valve body  122  and axially displaceable relative to the valve body  122  in reciprocating fashion. The needle  230  is biased towards the closing position  145  by means of a calibration spring, also denoted as third spring  230  in the following. 
     The actuator  125  in the shown embodiment comprises a solenoid  150 , a pole piece  250 , and an armature  155 . The pole piece  250  is positionally fix or in one piece with the valve body  122 . The armature  155  is axially displaceable in reciprocating fashion relative to the pole piece  250 . When the solenoid  150  is energized, it generates a magnetic field which is led along a magnetic path through the pole piece  250  and the armature  155  so that a magnetic force is exerted on armature  155  which attracts the armature  155  towards the pole piece  250  so that the armature  155  can be moved along the axis  105  towards the pole piece  250 . When energizing stops, the force of the calibration spring may bias the armature  155  into the opposite direction, in particular by means of mechanical interaction via the needle  135 . The armature  155  is mechanically coupled with the needle  135  so that the position of the needle  135  can be controlled electrically by the actuator  125  via the armature  155 . It is preferred that the needle  135  is moved towards the open position  140  when the solenoid  150  is energized and towards the closed position  145  when no current flows through solenoid  150 . 
       FIGS. 2 and 3  show enlarged details of the injector  100  of  FIG. 1 . The needle  135  is in the closed position  145  which relates to a lower position of armature  155  in the depicted embodiment. 
     The needle  135  comprises a bushing  205  and an upper retainer  210 . The bushing  205  is affixed to a shaft  202  of the needle  135  and extends circumferentially around a portion of the shaft  202 . The upper retainer  210  is affixed to the bushing  205  and extends laterally around a portion of the bushing  205 . A section of the armature  155  lies between axial surfaces of the bushing  205  and the upper retainer  210 , respectively. 
     In this, there is a predetermined play  215  of the armature  155  towards the needle  135 . In other words, the armature  155  is axially displaceable relative to the needle  135 . The axial displaceablility of the armature  155  relative to the needle  135  is limited by the upper retainer  210  in axial direction towards the pole piece  250  and is limited by the bushing  205  in axial direction away from the pole piece  250 . The armature is thus operable to establish a form-fit engagement with the upper retainer  210  for taking the needle  135  with it away from the closing position  145  when it is moved towards the pole piece  250  by means of the magnetic force generated by the solenoid  150 . 
     A first spring  220 , which may be very stiff, rests axially on a surface of the armature  155  that faces towards the pole piece  250 . The first spring  220  is axially displaceable in reciprocating fashion relative to the needle  135 , relative to the pole piece  250 , and in particular also relative to the armature  155 . It is conceivable that the first spring is a coil spring. Preferably, however, the first spring  210  may be represented by a metal tube with one or more radial openings  505 . For example, the metal tube has a cylinder-shell section comprising a helical cut through the circumferential sidewall of the cylinder-shell. Alternatively or additionally, the sidewall may have a plurality radial of bores which may be elongated in circumferential direction and which are preferably distributed in circumferential and axial direction. 
     A second spring  225  is disposed between axial surfaces of the first spring  220  and the upper retainer  210  so that it presses the first spring  220  towards the armature  155  and at the same time biases the armature  155  away from the upper retainer  210  and into contact with the bushing  205 . The third spring  230 , i.e. the calibration spring, presses down onto the assembly of needle shaft  202 , bushing  205  and retainer  210  such as to provide a closing force on the needle  135 . As will be shown later, an end of third spring  230  that is remote from said assembly is supported by a fixed part  240  that is attached to the valve body  122  or the pole piece  250 . The third spring  230  is preferred to be harder than the second spring  225  but softer than the first spring  220 . 
     The injector  100  is configured such that the first spring  220  is compressible between and by the fixed part  240  and the armature  155 . When the valve needle  135  is in the closed position  145 , an axial gap  235  is established between the fixed part  240  and the first spring  220 . 
     When the actuator  125  is energized, the solenoid  150  generates a magnetic field that attracts the armature  155  so that it starts to move axially towards the pole piece  250 . Due to the movement of the armature  155 , the play  215  is reduced to zero and the armature  155  engages with the upper retainer  210 . Further movement of the armature  155  in the same direction will move the needle  135  towards the open position  140 . 
     Due to the further movement of the armature  155 , the axial gap  235  between an axial end of the first spring  220  that is remote from the armature  155  and the fixed part  240  is reduced until the first spring  210  engages with the fixed part  240 . In particular, the fixed part  240  represents a spring seat for the axial end of the first spring  220  that is remote from the armature  155  in this way. 
     Subsequently, the first spring  220  is compressed by further movement of the armature  155  towards the pole piece, the movement still being driven by the magnetic force caused by the solenoid  150 . Through compression of the first spring  220 , the net force on the armature  155  is reduced and the armature is decelerated until the movement of the armature  155  towards the pole piece  250  is stopped when the open position  140  of the needle  135  is reached. The needle  135  may overshoot this position by no more than the amount of play  215 . In this case, the needle  135  will be pushed back by the third spring  230  into the open position  140 . 
     In one embodiment, an axial surface of the armature  155  encloses an further axial gap  245  with the pole piece  250 . When the armature  155  is moved from the closed position  145  to the open position  140 , the size of the gap  245  is reduced. By this movement fuel  255  inside the cavity of the valve body  122  is squeezed out of the further axial gap  245  so that hydraulic dampening occurs to the movement of the armature  155 . However, the further axial gap  245  is preferably non-zero when the armature  155  and needle  135  are at rest in the open position  140  of the needle  135 . 
     The first spring  220  may help to reduce a slope of a flow curve in a linear area as discussed above. The hydraulic dampening around further axial gap  245  can be used to reduce the width of a non-linear area of the flow curve. 
     In one embodiment, the fixed part  240  comprises a fuel filter. The fixed part  240  is comprises a metal tube which is press-fitted into a central opening of the pole piece  250 . the fixed part  240  may have an outer tube  405  comprising the fuel filter—for example embodied as bores in the outer tube—and an inner sleeve  305  comprising a spring seat for the end of the third spring  230  which is remote from the needle  135 . The outer tube  405  protrudes axially beyond a downstream end of the inner sleeve  305  and radially encloses a portion of the third spring  230 . A spring seat for the end of the first spring  220  which is remote from the armature  155  is preferably comprised by the outer tube  405 . The upper end of the third spring  230  that is remote from the needle  135 , rests against the sleeve  305  that is axially held by friction or otherwise fixed to the outer tube of the fixed part  240 . In turn, the outer tube of fixed part  240  may be held by friction against the pole piece  250 . The outer tube may have a constriction where it is radially spaced apart from the pole piece  250  and where the inner sleeve  305  is connected to the outer tube. In this way, easy assembly of the inner sleeve and the outer tube is achievable and a desired press-fitting force for the press-fit connection to the pole piece  250  is well adjustable. 
     During or after manufacturing, the injector  100  may be calibrated to a predetermined flow rate of fuel  255  between the supply end  115  and the nozzle end  110  when the needle  135  is in the open position  140 . To this end, tension and/or position of the third spring  230  may be adjusted. In the shown embodiment, both tension of the third spring  230  when the needle  135  is in the open position and size of the axial gap  235  when the needle  135  is in the closed position  145  may be calibrated at the same time by axially moving the fixed part  240  with respect to pole piece  250 . 
     Such a fixed part  240  may also be useful for other embodiments of injectors  100  according to this disclosure or for any other solenoid injector having a calibration spring. 
       FIG. 4  shows a further exemplary embodiment of an injector  100 . It is of the same basic construction as the injector according to the first embodiment, but differs from the embodiment of  FIGS. 1-3  in the construction details of the fixed part  240 . 
     In the present embodiment, the inner sleeve  305  comprises the fuel filter. The inner sleeve  305  is embodied, for example, according to one of the embodiments of a fluid filter which are disclosed in applicant&#39;s co-pending PCT-application PCT/EP2014/058700. The disclosure contents of this application relating to the construction of the fluid filter and in particular the embodiments of fluid filters disclosed in this application are herewith incorporated into the present description by reference for all purposes. In particular, the inner sleeve  305  may have a filter element and a fastening element which comprises a fitting portion for fastening the filter in the outer sleeve  405 . By means of such an inner sleeve  305 , a particularly reproducible press-fit connection to the outer tube  405  is achievable. 
     Further, in the present embodiment, the outer tube  405  is open at both axial ends and the inner sleeve  305  protrudes axially from an upstream end of the outer tube  405 . The upstream end of the outer tube  405  is radially spaced apart from the inner sleeve  305 . 
     The outer tube  405  may therefore be moved axially during a calibration process such as to determine the width of gap  235  when the needle  135  is in the closed position  145 . Independently from this, an axial force onto the inner sleeve  305  may be applied to adjust the axial position of the sleeve  305  with respect to the outer tube  405 . In this way, the spring seat for the end of the third spring  230  that is remote from the needle  135  is axially movable relative to the spring seat for the end of the first spring  220  which is remote from the armature  155  for calibrating a preload of the third spring  230 . Through this, tension of the third spring  230  when the needle  135  is in the open position  140  may be calibrated independently from the size of the axial gap  235  between the outer tube  405  and the first spring  220 . 
     Such a fixed part  240  may also be useful for other embodiments of injectors  100  according to this disclosure or for any other solenoid injector having a calibration spring. 
       FIG. 5  shows another exemplary embodiment of an injector  100  which in general corresponds to the embodiment disclosed above in connection with  FIG. 4 . In contrast thereto, the first spring  220  is made from a section of the fixed part  240 , specifically from a section of the outer tube  405 . For this, the hollow cylinder body of the outer tube  405  may carry one or several radial openings  505 . The openings are preferred to extend in a direction other than that of longitudinal axis  105 . In the present embodiment, the radial opening  505  is in the shape of a helical cut through the hollow cylinder body, the helical cut and the hollow cylindrical body sharing the longitudinal axis  105  as central axis. As in the embodiment shown in  FIG. 4 , independent calibration of tension of the third spring  230  and the size of the gap  235  may be carried out. 
     The spring seat for the end of the first spring  220  which is remote from the armature  155  is represented by an upstream portion of the outer sleeve  405  in the present case. The first spring  220  is not axially moveable relative to the pole piece  250  during operation of the injector  100  but for calibration purposes via press-fitting the outer sleeve  405  into the opening of the pole piece  250 . While in the previous embodiments, the downstream end portion of the outer sleeve  405  may or may not be in press-fit engagement with the pole piece  250 , the downstream end portion of the outer sleeve  405  not in press-fit engagement with the pole piece  250  but is axially displaceable relative to the latter so that the section of the outer sleeve  405  which represents the first spring  220  can be compressed by the armature  155 . The axial gap  235  is established between the downstream end portion of the outer sleeve  405  and the armature  155  in the present embodiment. 
       FIG. 6  shows yet another example embodiment of an injector  100  according to the invention. The first spring  220  is again realized as a section of the fixed part  240  and the third spring  230  presses directly onto the needle  135  as in the previous embodiments. The present embodiment differs from the embodiment discussed above with respect to  FIG. 5  in that the upper retainer  210  is integrated in the shaft  202  of the valve needle  135 , in that the armature  155  comprises a main body  600  and an insert  605 , and in that the second spring  225  is connected in series to the first spring instead of being seated against the upper retainer  210 . 
     More specifically, the upper retainer  210  is not a separate part in the present embodiment but it is represented by a radially protruding collar at the upstream end of the shaft  202  of the needle  135 . In addition, the bushing  205  is embodied as a disc element downstream of the armature  155  in the present embodiment. 
     The insert  605  is fixed to the main body  600  of the armature  155 , for example by press-fitting and/or welding. The upper retainer  210  and a portion of the shaft  202  are received in a central opening of the insert  605 . The shaft  202  of the needle  135  protrudes axially from the insert  605 , and also from the main body  600 , of the armature  155  in direction away from the pole piece  250 . 
     The insert  605  axially projects beyond the main body  600  in direction towards the pole piece  250 . The insert  605  may provide radial support for the third spring  230 , in particular by receiving an end of the third spring  230  in the central opening. Since the insert  605  embraces the upper retainer  210  and due to its dimensions, it functions as axial guide for the needle  135  via interaction with the retainer  210  and/or the shaft  202   
     An upper axial end of the insert  605  abuts the second spring  225 , the other end of which rests against an end of the first spring  220  which faces towards the armature  155 . In this way, the first spring  220  and the second spring  225  are connected in series in the present embodiment. 
     The calibration of gap and tension in the shown embodiment may be carried out like described above for instance with respect to the embodiment of  FIG. 5 . 
       FIG. 7  shows yet another exemplary embodiment of an injector  100 . The present embodiment is a variant of the embodiment described in connection with  FIG. 4  above. 
     In the present embodiment, the upper retainer is embodied as a collar of the shaft  202  of the needle  135  and the bushing  205  is embodied as a disc element as described in connection with  FIG. 6  above. The outer tube  405  of the fixed part  240  protrudes axially from the pole piece  250  and into a central opening of the armature  155  so that it overlaps axially with the armature  155 , in particular to guide the axial movement of the latter. 
     Unlike the embodiment of  FIG. 4 , the first spring  220  is not arranged axially subsequent to the fixed part  240  so that a downstream axial end of the fixed part  240  represents the spring seat for the end of the first spring  220  which is remote from the armature  155 . Rather, the first spring  220  is shifted partially into the outer tube  405  of the fixed part  240  so that only a portion of the first spring  220  projects from the outer tube  405 , i.e. the fixed part  240 . The outer tube  405  has a step representing the spring seat for the end of the first spring  220  which is remote from the armature  155 . The axial gap  235  between the fixed part  240  and the first spring  220  which is reduced by the movement of the armature  155  towards the pole piece  250  before the first spring  220  is compressed by further movement of the armature  155  is in the present embodiment established between said step of the tube  405  and the first spring  220 . 
     Further unlike the embodiment of  FIG. 4 , the second spring  220  is not seated against the needle  135  but against the fixed part  240 , specifically against a further step of the outer tube  405  upstream of the above mentioned step. The second spring  225  is clamped between the further step and the end of the first spring  220  which is remote from the armature  155 . In this way, the second spring  225  is operable to bias the first spring  220  away from the step and to bias the armature  155  away from the upper retainer  210  for maximizing the play  215  by pressing on the armature  155  via the first spring  220 . Due to the small absolute dimensions of the gap  235  and the play  215 , these are barely visible in  FIG. 7  and other figures. 
       FIG. 8  shows one more exemplary embodiment of an injector  100 . This embodiment is a variant of the embodiment described above in connection with  FIG. 6 . 
     In contrast to that embodiment, the second spring  225  is omitted in the present embodiment. Instead, the third spring  230  is seated on the insert  605  of the armature  155  instead of being seated against the needle  135 . In this way, the third spring  230  has a triple function: It is operable to bias the armature  155  away from the pole piece  250 , it is operable to bias the armature  155  away from the upper retainer  210  and at the same time it is operable to bias the needle  135  towards the closed position  145  by means of mechanical engagement via the armature  155  and the bushing  205 . 
       FIG. 9  shows yet one more exemplary embodiment of an injector  100 . The present embodiment is based on the embodiment of  FIG. 7 . However, analogously to the previously described embodiment of  FIG. 8 , no second spring  225  is provided. Consequently, also the further step of the outer tube  405  is omitted. 
     The third spring  230  is seated on the end of the first spring  220  which is remote from the armature  155 . In this way, the third spring  230  is operable to bias the first spring  220  away from the step of the outer tube  405 , it is operable to bias the armature  155  away from the pole piece  250  and from the upper retainer  210  by means of mechanical interaction via the first spring  220 , and at the same time it is operable to bias the needle  135  towards the closed position  145  by means of mechanical engagement via the first spring  220 , the armature  155  and the bushing  205 . Calibration of tension of the third spring  230  and gap size  235  can be done independently from each other.