Abstract:
A fuel injector comprises a body having a longitudinal axis, a length-changin solid state actuator that has first and second ends, a closure member coupled to the first end of the solid state actuator, and a compensator assembly coupled the second end of the solid state actuator. The solid state actuator includes a plurality of solid state elements along the axis between the first and second ends. The closure member is movable between a first configuration permitting fuel injection and a second configuration preventing fuel injection. And the compensator assembly axially positions the solid state actuator with respect to the body in response to temperature variation. The compensator assembly utilizes a configuration of at least one spring disposed between two pistons so as to reduce the use of elastomer seals to thereby reduce a slip stick effect. Also, a method of compensating for thermal expansion or contraction of the fuel injector comprises providing fuel from a fuel supply to the fuel injector; and adjusting the solid state actuator with respect to the body in response to temperature variation.

Description:
PRIORITY  
       [0001]    This application claims the benefits of provisional application Ser. No. 60/239,290 filed on Oct. 11, 2000, which is hereby incorporated by reference in its entirety in this application. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention generally relates to length-changing actuators such as a magnetorestrictive or length-changing solid state actuator. In particular, the present invention relates to a compensator assembly for a length-changing actuator, and more particularly to an apparatus and method for hydraulically compensating a solid state actuated high-pressure fuel injector for internal combustion engines.  
         BACKGROUND OF THE INVENTION  
         [0003]    Solid-state actuator such as a length-changing actuator may include a ceramic structure whose axial length can change through the application of an operating voltage. It is believed that in typical applications, the axial length can change by, for example, approximately 0.12%. In a stacked configuration, it is believed that the change in the axial length is magnified as a function of the number of actuators in the length-changing actuator stack. Because of the nature of the length-changing actuator, it is believed that a voltage application results in an instantaneous expansion of the actuator and an instantaneous movement of any structure connected to the actuator. In the field of automotive technology, especially, in internal combustion engines, it is believed that there is a need for the precise opening and closing of an injector valve element for optimizing the spray and combustion of fuel. Therefore, in internal combustion engines, length-changing actuators are now employed for the precise opening and closing of the injector valve element.  
           [0004]    During operation, components of an internal combustion engine experience significant thermal fluctuations that result in the thermal expansion or contraction of the engine components. For example, it is believed that a fuel injector assembly includes a valve body that may expand during operation due to the heat generated by the engine. Moreover, it is believed that a valve element operating within the valve body may contract due to contact with relatively cold fuel. If a length-changing actuator stack is used for the opening and closing of an injector valve element, it is believed that the thermal fluctuations can result in valve element movements that can be characterized as an insufficient opening stroke, or an insufficient sealing stroke. It is believed that this is because of the low thermal expansion characteristics of the length-changing actuator as compared to the thermal expansion characteristics of other fuel injector or engine components. For example, it is believed that a difference in thermal expansion of the housing and actuator stack can be more than the stroke of the actuator stack. Therefore, it is believed that any contractions or expansions of a valve element can have a significant effect on fuel injector operation  
           [0005]    It is believed that there is a need to provide thermal compensation that overcomes the drawbacks of conventional methods.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention provides a fuel injector that utilizes a length-changing actuator, such as, for example, an electrorestrictive, magnetorestrictive or a solid-state actuator with a compensator assembly that compensates for thermal distortions, brinelling, wear and mounting distortions. The compensator assembly utilizes a minimal number of elastomer seals so as to reduce a slip stick effect of such seals while achieving a more compact configuration for a compensator assembly. In one preferred embodiment of the invention, the fuel injector comprises a housing having a first housing end and a second housing end extending along a longitudinal axis, the housing having an end member disposed between the first and second housing ends; a length-changing solid state actuator disposed along the longitudinal axis. A closure member coupled to the length-changing actuator, the closure member being movable between a first configuration permitting fuel injection and a second configuration preventing fuel injection, and a compensator assembly that moves the length-changing actuator with respect to the housing in response to temperature changes. The compensator assembly includes a body. The body includes an interior surface defining a first fluid reservoir and a second fluid reservoir that are disposed between a first body end and a second body end, a valve spacer disposed between the first fluid reservoir and the second fluid reservoir. The valve spacer has a first spacer face and a second spacer face, and a plate contiguous to one of the first and second faces. The plate is responsive to one of a first fluid pressure in the first fluid reservoir and a second fluid pressure in the second reservoir so as to permit fluid flow from one of the first and second fluid reservoirs to the other of the first and second fluid reservoirs.  
           [0007]    The present invention provides a compensator that can be used in a length-changing actuator, such as, for example, an electrorestrictive, magnetorestrictive or a solid-state actuator so as to compensate for thermal distortion, wear, brinelling and mounting distortion of an actuator that the compensator is coupled to. In a preferred embodiment, the length-changing actuator has first and second ends. The compensator assembly includes a body having a first body end and a second body end extending along a longitudinal axis. The body has a body inner surface facing the longitudinal axis, a first piston disposed in the body proximate one of the first body end and second body end, the first piston including a first face having a first surface area, a first sealing member coupled to the first piston and contiguous to the body inner surface, a second piston disposed in the body distal to the first piston, the second piston including a second face having a second surface area, a second sealing member coupled to the second piston and contiguous to the body inner surface, a spacer disposed between the first piston and the second piston in the body. The spacer has a first spacer end and a second spacer end in fluid communication with one another, the first spacer end being disposed in a confronting arrangement to one of the first face and second face so as to define a first fluid reservoir within the body, the second spacer end being disposed in a confronting arrangement to the other of the first face and the second face so as to define a second fluid reservoir within the body. A valve is disposed in one of the first and second reservoir, the valve being responsive to one of a first fluid pressure in the first fluid reservoir and a second fluid pressure in the second reservoir so as to permit fluid flow from one of the first and second fluid reservoirs to the other of the first and second fluid reservoirs.  
           [0008]    The present invention further provides a method of compensating for distortion of a fuel injector due to thermal distortion, brinelling, wear and mounting distortion. The fuel injector includes a housing having a first housing end and a second housing end extending along a longitudinal axis, the housing having an end member disposed between the first and second housing ends, an length-changing actuator disposed along the longitudinal axis, a closure member coupled to the length-changing actuator, the closure member being movable between a first configuration permitting fuel injection and a second configuration preventing fuel injection, and a compensator assembly that moves the length-changing actuator with respect to the body in response to temperature changes. The compensator assembly includes a body having a first body end and a second body end extending along a longitudinal axis. The body has a body inner surface facing the longitudinal axis, a first piston disposed in the body proximate one of the first body end and second body end, the first piston including a first face having a first surface area, a first sealing member coupled to the first piston and contiguous to the body inner surface, a second piston disposed in the body distal to the first piston, the second piston including a second face having a second surface area, a second sealing member coupled to the second piston and contiguous to the body inner surface, a spacer disposed between the first piston and the second piston in the body. The spacer has a first spacer end and a second spacer end in fluid communication with one another, the first spacer end being disposed in a confronting arrangement to one of the first face and second face so as to define a first fluid reservoir within the body, the second spacer end being disposed in a confronting arrangement to the other of the first face and the second face so as to define a second fluid reservoir within the body. A valve is disposed in one of the first and second reservoir, the valve being responsive to one of a first fluid pressure in the first fluid reservoir and a second fluid pressure in the second reservoir so as to permit fluid flow from one of the first and second fluid reservoirs to the other of the first and second fluid reservoirs. In a preferred embodiment, the method is achieved by containing a predetermined amount of hydraulic fluid in the first and second fluid reservoirs; pressurizing the hydraulic fluid in at least one of the first and second fluid reservoirs so as to displace the first piston; and preventing communication of hydraulic fluid between the first and second fluid reservoirs during activation of the length changing actuator. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate presently preferred embodiments of the invention, and, together with the general description given above and the detailed description given below, serve to explain features of the invention.  
         [0010]    [0010]FIG. 1 is a cross-sectional view of a fuel injector assembly having a length-changing actuator stack and a compensator unit of a preferred embodiment.  
         [0011]    [0011]FIG. 2 is an enlarged view of the compensator assembly in FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0012]    Referring to FIGS.  1 - 2 , a preferred embodiment is shown. FIG. 1 illustrates a preferred embodiment of a fuel injector assembly  10  having a length-changing actuator stack  100  and a compensator assembly  200 . The fuel injector assembly  10  includes inlet-fitting  12 , spring preload adjuster  13 , injector housing  14 , and valve body  16 . The inlet fitting  12  includes a fuel filter  11 , fuel passageways  18 ,  20  and  22 , and a fuel inlet  24  connected to a fuel source (not shown). The inlet fitting  12  also includes an inlet end member  28  coupled to threaded adjuster  13 . The compensator  200  has two fluid reservoirs that are filled with fluid  36 . The fluid  36  can be a substantially incompressible fluid that is responsive to temperature change by changing its volume. Preferably, the fluid  36  is either silicon or other types of fluid that has a higher coefficient of thermal expansion than that of the injector inlet fitting  12 , the housing  14  or other components of the fuel injector.  
         [0013]    In the preferred embodiment, injector housing  14  encloses the length-changing actuator stack  100  and the compensator assembly  200 . Valve body  16  is fixedly connected to injector housing  14  and encloses a valve closure member  40 . The length-changing actuator stack  100  includes a plurality of length-changing elements that can be operated through contact pins (not shown) that are electrically connected to a voltage source. When a voltage is applied between the contact pins (not shown), the length-changing actuator stack  100  expands in a lengthwise direction. A typical expansion of the length-changing actuator stack  100 , under load, may be on the order of approximately 30-50 microns, for example. The lengthwise expansion can be utilized for operating the injection valve closure member  40  for the fuel injector assembly  100 .  
         [0014]    Length-changing actuator stack  100  is guided along housing  14  by means of guides  110 . The length-changing actuator stack  100  has a first end in operative contact with a closure end  42  of the valve closure member  40  by means of bottom  44 , and a second end of the actuator stack  100  that is operatively connected to compensator assembly  200  by means of a top  46 .  
         [0015]    Fuel injector assembly  100  further includes a spring  48 , a spring washer  50 , a keeper  52 , a bushing  54 , a valve closure member seat  56 , a bellows  58 , and an O-ring  60 . O-ring  60  is preferably a fuel compatible O-ring that remains operational at low ambient temperatures (−40 C or less) and at operating temperatures (140 C or more).  
         [0016]    Referring to FIG. 2, compensator assembly  200  includes a body  210  encasing a first piston  220 , a valve spacer portion  230 , a second piston  240 , and an elastic member or spring  260 . The body  210  can be of any suitable cross-sectional shape that provides a mating fit with the first and second pistons, such as, for example, oval, square, rectangular or any suitable polygons. Preferably, the cross section of the body is circular, thereby forming a cylindrical body.  
         [0017]    First piston  220  has a first face  222 , which is disposed in a confronting arrangement with the valve spacer portion  230  so as to define a first fluid reservoir  32 . The first face  222  can be conical, frustoconical or, preferably, a planar surface that has a first surface area.  
         [0018]    An outer peripheral surface  228  of the first piston  220  is dimensioned so as to form a close tolerance fit with a body inner surface  212  . The first piston includes a sealing member, preferably an elastomer  214  disposed in a groove  229  on the outer circumference of the second piston  240  so as to generally prevent leakage of fluid  36 . Preferably, the elastomer  214  is an O-ring. Alternatively, the elastomer  214  can be an O-ring of the type having non-circular cross-sections. Other types of elastomer seals can also be used, such as, for example, a labyrinth seal. Additionally, a groove could be formed on the body inner surface  212  instead of on the outer peripheral surface  228 .  
         [0019]    The valve spacer portion  230  includes a first spacer face  232 , a second spacer face  234 , a flow passage  236  connected to a restrictor passage  237  that allows fluid communication between the first fluid reservoir and the second fluid reservoir  34 . Although the restrictor  237  is employed in a preferred embodiment to reduce fluid pressure of fluid flowing to the first fluid reservoir, the restrictor  237  can be eliminated by extending the passage  236  along the whole length of the spacer  230 .  
         [0020]    The first spacer face  232  has a plurality of pockets or channels  238   a ,  238   b  formed on a surface that is preferably transverse with respect to the longitudinal axis A-A. The pockets or channels can be of a suitable shape, such as, for example, a cylinder, a square or a rectangle. Preferably, the pockets or channels  238   a  and  238   a  are cylindrical in shape.  
         [0021]    The spacer  230  can be coupled to the body by a suitable coupling such as, for example, a spline coupling. In one preferred embodiment, the spacer  230  and the inner surface  249  of the body  210  is provided with complementary threads formed thereon so as to permit the spacer to be threaded to the body. Also preferably, there are twelve pockets or channels formed on the first spacer face  232 .  
         [0022]    A second piston  240  includes a second face  242  that is disposed in a confronting arrangement with the second spacer face  234  so as to define a second fluid reservoir  34 . The second face  242  can be a conical, frustoconical or preferably, a planar surface with a second surface area that is approximately the same as the first surface area of the first piston. The second piston  240  also includes a sealing member, preferably an elastomer  246  disposed in a groove  248  on the outer circumference of the second piston  240  so as to generally prevent leakage of fluid  36  from the second fluid reservoir  34 . Preferably, the elastomer  246  is an O-ring. Alternatively, the elastomer  246  can be an O-ring of the type having non-circular cross-sections. Other types of elastomer seals can also be used, such as, for example, a labyrinth seal. Alternatively, a groove can also be formed in the body inner surface  212  with a sealing member disposed therein.  
         [0023]    A spring member  260  biases the second piston  240  towards the outlet end of the injector. The piston  240  is coupled to a filler plug  38  that allows fluid  36  to be introduced into the body  210 . Preferably, the filler plug  38  is coupled to the piston  220  by complementary helical threads  239  formed on the second piston  240  and the filler plug  38 .  
         [0024]    A pressure sensitive valve is disposed in the first fluid reservoir  32  that allows fluid flow in one direction, depending on the pressure drop across the pressure sensitive valve. The pressure sensitive valve can be, for example, a check valve or a one-way valve. Preferably, the pressure sensitive valve is a flexible thin-disc plate  270  having a smooth surface disposed confronting the first face  222 .  
         [0025]    The plate  270  is disposed between the spacer  230  and a boss portion  311 . The Plate  270  can be affixed to the face  232  of the spacer  230  by a suitable coupling, such as, for example, bonding, crimping, spot-welding or laser welding. Preferably, the face  232  of spacer  230  is used to retain the plate  270  between the face  232  and a boss portion  311  of the body  210  by threading the spacer  230  into the body  210  so as to retain the plate  270 .  
         [0026]    Referring to the plate  270 , by having a smooth surface on the side contiguous to the first piston  220  that forms a sealing surface with the first spacer face  232 , the plate  270  functions as a pressure sensitive valve that allows fluid to flow between a first fluid reservoir  32  and a second fluid reservoir  34  whenever pressure in the first fluid reservoir  32  is less than pressure in the second reservoir  34 . That is, whenever there is a pressure differential between the reservoirs, the smooth surface of the plate  270  is lifted up to allow fluid to flow to the channels or pockets  238   a ,  238   b . It should be noted here that the plate forms a seal to prevent flow as a function of the pressure differential instead of a combination of fluid pressure and spring force (as in a ball type check valve) in order to maintain a check valve closed against flow. The pressure sensitive valve or plate  270  includes orifices  278   a  and  278   b  formed through its surface. The orifice can be, for example, square, circular or any suitable through orifice. Preferably, there are twelve orifices formed in the plate with each orifice having a diameter of approximately 1.0 millimeter. Also preferably, each of the channels or pockets  238   a ,  238   b  has an opening that is approximately the same shape and cross-section as each of the orifices  278   a  and  278   b.    
         [0027]    Because the plate  270  has very low mass and is flexible, it responds very quickly with the incoming fluid by lifting up towards the first piston  220  so that fluid that has not passed through the plate adds to the volume of the hydraulic shim. The plate  270 , in the open position (not shown), approximates a portion of a spherical shape as it pulls in a volume of fluid that is still under the plate  270  and in the passage  236 . This additional volume is then added to the shim volume but whose additional volume is still on the first reservoir side of the sealing surface. One of the many benefits of the plate  270  is that pressure pulsations are quickly damped by the additional volume of hydraulic fluid that is added to the hydraulic shim in the first reservoir. This is because activation of the injector is a very dynamic event and the transition between inactive, active and inactive creates inertia forces that produce pressure fluctuations in the hydraulic shim. The hydraulic shim, because it has free flow in and restricted flow out of the hydraulic fluid, quickly dampens the oscillations.  
         [0028]    The through hole or orifice diameter of the orifice  278   a  or  278   b  can be thought of as the effective orifice diameter of the plate instead of the lift height of the plate  270  because the plate  270  approximates a portion of a spherical shape as it lifts away from the first spacer face  232 . Moreover, the number of orifices and the diameter of each orifice determine the stiffness of the plate  270 , which is critical to a determination of the pressure drop across the plate  270 . Preferably, the pressure drop should be small as compared to the pressure pulsations in the first reservoir  32  of the compensator. When the plate  270  has lifted approximately 0.1 mm, the plate  270  can be assumed to be wide open, thereby giving unrestricted flow into the first reservoir  32 . The ability to allow unrestricted flow into the hydraulic shim prevents a significant pressure drop in the fluid. This is believed to be important because when there is a significant pressure drop, the gas dissolved in the fluid comes out, forming bubbles. This is due to the vapor pressure of the gas exceeding the reduced fluid pressure (i.e., certain types of fluid take on air like a sponge takes on water, thus, making the fluid behave like a compressible fluid.) The bubbles formed act like little springs making the compensator “soft” or “spongy”. Once formed, it is difficult for these bubbles to re-dissolve into the fluid. The compensator, preferably by design, operates between approximately 2 and 7 bars of pressure, and it is believed that the hydraulic shim pressure does not drop significantly below atmospheric pressure. Thus, degassing of the fluid and compensator passages is not as critical as it would be without the plate  270 . Preferably, the thickness of the plate  270  is approximately 0.1 millimeter and its surface area is approximately 88 millimeter squared (mm 2 ). Furthermore, to maintain a desired flexibility of the plate  270 , it is preferable to have an array of approximately twelve orifices, each orifice having an opening of approximately 0.8 millimeter squared (mm 2 ), and the thickness of the plate is preferably the result of the square root of the surface area divided by approximately 94.  
         [0029]    The spring  260  can react against the threaded adjuster  13  (and also end member  28 ) to push the second piston  240  towards the outlet of the injector. The spring force causes a pressure increase in the fluid  36  that acts against the second face  242  of the second piston  240 . In an initial condition, hydraulic fluid  36  is pressurized as a function of the spring force of the spring  260  and the second surface area of the second face  242 . The pressurized fluid tends to flow into and out of the first reservoir  32  and the second reservoir  34  when the pressure in the first fluid reservoir is less than the pressure in the second reservoir. Where the pressure in the first reservoir  32  is lower than the second reservoir  34 , such as in an initial condition, the flapper or plate  270  operates to permit fluid  36  to flow into the first reservoir  32 . The fluid  36  that forms a hydraulic shim in the first reservoir  32  tends to expand due to an increase in temperature in and around the compensator. Prior to any expansion of the fluid in the first reservoir  32 , the first reservoir is preloaded by the second face  242  and the spring force of the spring  260  so as to form a hydraulic shim. Preferably, the spring force of spring  260  is approximately 30 Newton to 70 Newton.  
         [0030]    The force vector (i.e. having a direction and magnitude) “F out ” of the first piston  220  moving towards the stack is defined as follows:  
           F   out =( F   spring   ±F   seal246 )*( A   shim32   /A   2ndReservoir34 )± F   seal214    
         [0031]    where:  
         [0032]    F out =Applied Force (To the Piezo Stack)  
         [0033]    F spring =Spring Force (30 to 70 N)  
         [0034]    A shim32 =Area above piston (Hydraulic Shim or first fluid reservoir 32 )  
         [0035]    A 2ndReservoir34 =Area below the second piston (Second Fluid Reservoir 34 )  
         [0036]    F seal246 =Seal Friction Force of seal  246   
         [0037]    F seal214 =Seal Friction Force of seal  214 .  
         [0038]    Preferably, the spring  260  is a coil spring. Here, the pressure in the fluid is related to at least one spring characteristic of the coil spring. As used throughout this disclosure, the at least one spring characteristic can include, for example, the spring constant, spring free length, the amount of preload due to the threaded adjuster  13  and modulus of elasticity of the spring. Each of the spring characteristics can be selected in various combinations with other spring characteristic(s) noted above so as to achieve a desired response of the compensator assembly.  
         [0039]    Referring again to FIG. 1, during operation of the fuel injector  100 , fuel is introduced at fuel inlet  24  from a fuel supply (not shown). Fuel at fuel inlet  24  passes through a fuel filter  11 , through a passageway  18 , through a passageway  20 , through a fuel tube  22 , and out through a fuel outlet  62  when valve closure member  40  is moved to an open configuration.  
         [0040]    In order for fuel to exit through fuel outlet  62 , voltage is supplied to length-changing actuator stack  100 , causing it to expand. The expansion of length-changing actuator stack  100  causes bottom  44  to push against valve closure member  40 , allowing fuel to exit the fuel outlet  62 . After fuel is injected through fuel outlet  62 , the voltage supply to length-changing actuator stack  100  is terminated and valve closure member  40  is returned under the bias of spring  48  to close fuel outlet  62 . Specifically, the length-changing actuator stack  100  contracts when the voltage supply is terminated, and the bias of the spring  48  which holds the valve closure end  42  in constant contact with bottom  44 , also biases the valve closure member  40  to the closed configuration.  
         [0041]    During engine operation, as the temperature in the engine rises, inlet fitting  12 , injector housing  14  and valve body  16  experience thermal expansion due to the rise in temperature while the length-changing stack experience generally insignificant thermal expansion. At the same time, while the actuator  100  is not energized, fuel traveling through fuel tube  22  and out through fuel outlet  62  cools the internal components of fuel injector assembly  100  and causes thermal contraction of valve closure member  40 .  
         [0042]    Referring to FIG. 1, as valve closure member  40  contracts, bottom  44  tends to separate from its contact point with valve closure end  42 . Length-changing actuator stack  100 , which is operatively connected to the bottom surface of first piston  220 , is initially pushed downward due to a pressurization of the fluid by the spring  260  acting on the second piston with a force F out . The increase in temperature causes inlet fitting  12 , injector housing  14  and valve body  16  to expand relative to the actuator stack  100  due to the generally higher volumetric thermal expansion coefficient β of the fuel injector components relative to that of the actuator stack. This movement of the first piston is transmitted to the actuator stack  100  by a top  46 , which movement maintains the position of the bottom  44  of the stack constant relative to the closure end  42 . It should be noted that in the preferred embodiments, the thermal coefficient β of the hydraulic fluid  36  is greater than the thermal coefficient β of the actuator stack. Here, the compensator assembly can be configured by at least selecting a hydraulic fluid with a desired coefficient β and selecting a predetermined volume of fluid in the first reservoir such that a difference in the expansion rate of the housing of the fuel injector and the actuator stack  100  can be compensated by the expansion of the hydraulic fluid  36  in the first reservoir.  
         [0043]    When the actuator  100  is energized, pressure in the first reservoir  32  increases rapidly, causing the plate  270  to seal tight against the first spacer face  232 . This blocks the hydraulic fluid  36  from flowing out of the first fluid reservoir to restrictor passage  237  and the passage  236 . Because of the virtual incompressibility of fluid, the fluid  36  in the first reservoir  32  approximates a stiff reaction base, i.e. a shim, on which the actuator  100  can react against. The stiffness of the shim is believed to be due in part to the virtual incompressibility of the fluid and the blockage of flow out of the first reservoir  32  by the plate  270 . Here, when the actuator stack  100  is actuated in an unloaded condition, it extends by approximately 60 microns. As installed in a preferred embodiment, one-half of the quantity of extension (approximately 30 microns) is absorbed by various components in the fuel injector. The remaining one-half of the total extension of the stack  100  (approximately 30 microns) is used to deflect the closure member  40 . Thus, a deflection of the actuator stack  100  is believed to remain constant as it is energized time after time, thereby allowing an opening of the fuel injector to remain the same.  
         [0044]    When the actuator  100  is not energized, fluid  36  flows between the first fluid reservoir and the second fluid reservoir while maintaining the same preload force F out . The force F out  is a function of the spring  260 , the friction force due to the seals  214 ,  246  and the surface area of each piston. Thus, it is believed that the bottom  44  of the actuator stack  100  is maintained in constant contact with the contact surface of valve closure end  42  regardless of expansion or contraction of the fuel injector components.  
         [0045]    Although the compensator assembly  200  has been shown in combination with a length-changing actuator for a fuel injector, it should be understood that any length changing actuator, such as, for example, an electrorestrictive, magnetorestrictive or a solid-state actuator, could be used with the compensator assembly  200 . Here, the length changing actuator can also involve a normally deenergized actuator whose length is expanded when the actuator energized. Conversely, the length-changing actuator is also applicable to where the actuator is normally energized and is de-energized so as to cause a contraction (instead of an expansion) in length. Moreover, it should be emphasized that the compensator assembly  200  and the length-changing solid state actuator are not limited to applications involving fuel injectors, but can be for other applications requiring a suitably precise actuator, such as, to name a few, switches, optical read/write actuator or medical fluid delivery devices.  
         [0046]    While the present invention has been disclosed with reference to certain preferred embodiments, numerous modifications, alterations, and changes to the described embodiments are possible without departing from the sphere and scope of the present invention, as defined in the appended claims. Accordingly, it is intended that the present invention not be limited to the described embodiments, but that it have the full scope defined by the language of the following claims, and equivalents thereof.