Abstract:
A fuel injector comprises a body having a longitudinal axis, a length-changing 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 electromechanical solid state actuators such as an electrorestrictive, magnetorestrictive or 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 piezoelectrically actuated high-pressure fuel injector for internal combustion engines.  
         BACKGROUND OF THE INVENTION  
         [0003]    A known solid-state actuator includes 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 solid-state actuator stack. Because of the nature of the solid-state 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, it is believed that solid-state actuators are now employed for the precise opening and closing of the injector valve element.  
           [0004]    During operation, it is believed that the 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 solid-state 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 solid-state 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 conventional methods and apparatuses that compensate for thermal changes affecting solid-state actuator stack operation have drawbacks in that they either only approximate the change in length, they only provide one length change compensation for the solid-state actuator stack, or that they only accurately approximate the change in length of the solid-state actuator stack for a narrow range of temperature changes.  
           [0006]    It is believed that there is a need to provide thermal compensation that overcomes the drawbacks of conventional methods.  
         SUMMARY OF THE INVENTION  
         [0007]    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 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 solid-state 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 includes a first working surface distal to a first outer surface, the outer surface cooperating with the body inner surface to define a first fluid reservoir, a second piston disposed in the body proximate the first piston, the second piston having a second outer surface distal to a second working surface that confronts the first working surface, a first sealing member coupled to the second piston and contiguous to the body inner surface, and a flexible fluid barrier coupled to the first piston and the second piston, the flexible fluid barrier cooperating with the first and second working surfaces to define a second fluid reservoir.  
           [0008]    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, brineling and mounting distortion of an actuator that the compensator is coupled to. In a preferred embodiment, the self elongating actuator has a first and second ends. The compensator comprises 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 includes a first working surface distal to a first outer surface, the outer surface cooperating with the body inner surface to define a first fluid reservoir, a second piston disposed in the body proximate the first piston, the second piston having a second outer surface distal to a second working surface that confronts the first working surface, a first sealing member coupled to the second piston and contiguous to the body inner surface, and a flexible fluid barrier coupled to the first piston and the second piston, the flexible fluid barrier cooperating with the first and second working surfaces to define a second fluid reservoir.  
           [0009]    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, a length-changing actuator disposed along the longitudinal axis, a closure member coupled to the length-changing actuator, 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 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 cooperating with the body inner surface to define a first fluid reservoir, a second piston disposed in the body proximate the first piston, the second piston having a second outer surface distal to a second working surface that confronts the first working surface, an elastomer coupled to the second piston and contiguous to the body inner surface, and a flexible fluid barrier coupled to the first piston and the second piston, the flexible fluid barrier cooperating with the first and second working surface to define a second fluid reservoir. In a preferred embodiment, the method is achieved by confronting a surface of the first piston to an inner surface of the body so as to form a controlled clearance between the first piston and the body inner surface of the first fluid reservoir; engaging the elastomer between a surface of the second piston and the inner surface of the body so as to form a seal therebetween; pressurizing the hydraulic fluid in the first and second fluid reservoirs; and biasing the length-changing actuator with a predetermined force vector resulting from changes in the volume of hydraulic fluid disposed within the first fluid reservoir as a function of temperature. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    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.  
         [0011]    [0011]FIG. 1 is a cross-sectional view of a fuel injector assembly having a solid-state actuator stack and a compensator unit of a preferred embodiment.  
         [0012]    [0012]FIG. 2 is an enlarged view of the compensator assembly in FIG. 1.  
         [0013]    [0013]FIG. 3 is a view of the first and second pistons prior to assembly in the body of the compensator of FIG. 2.  
         [0014]    [0014]FIG. 4 is a view illustrating the operation of the pressure responsive valve of the compensator assembly. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]    Referring to FIGS.  1 - 4 , a preferred embodiment is shown. FIG. 1 illustrates a preferred embodiment of a fuel injector assembly  10  that has a solid-state actuator stack  1100  and a compensator assembly  200 . The fuel injector assembly  10  includes inlet fitting  12 , injector housing  14 , and valve body  17 . The inlet fitting  12  includes a fuel filter  16 , 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  (FIG. 2) with an O-ring  29 . The inlet end member has a port  30  that can be used to fill a reservoir  32  with fluid  36  after a filler plug  38  is removed. The filler plug can be coupled to the injector housing by a suitable technique such as threading, sealing or permanently bonding the filler plug  38  to the housing. 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 type of hydraulic type fluid that has a higher coefficient of thermal expansion than that of the injector inlet  12 , the housing  14  or other components of the fuel injector. Also preferably, the filler plug  38  is connected to the housing by a threaded connection.  
         [0016]    In the preferred embodiment, injector housing  14  encloses the solid-state actuator stack  100  and the compensator assembly  200 . Valve body  17  is fixedly connected to injector housing  14  and encloses a valve closure member  40 . The solid-state actuator stack  100  includes a plurality of solid-state actuators 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 solid-state actuator stack  100  expands in a lengthwise direction. A typical expansion of the solid-state actuator stack  100  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 .  
         [0017]    Solid-state actuator stack  100  is guided along housing  14  by means of guides  110 . The solid-state 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 stack  100  that is operatively connected to compensator assembly  200  by means of a top  46 .  
         [0018]    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).  
         [0019]    Referring to FIG. 2, compensator assembly  200  includes a body  210  encasing a first piston  220 , a piston stem or an extension portion  230 , a second piston  240 , bellows  250  and 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.  
         [0020]    The extension portion  230  extends from the first piston  220  so as to be linked by an extension end  232  to the top  46  of the piezoelectric stack  100 . Preferably, the extension portion  230  is integrally formed as part of the first piston  220 . Alternatively, the extension portion can be formed separate from the first piston  220  and coupled to the first piston  220  by, for example, a spline coupling, ball joint or other suitable couplings.  
         [0021]    First piston  220  is disposed in a confronting arrangement with the inlet end member  28 . 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 , i.e. a controlled clearance that allows lubrication of the piston and the body while also forming a hydraulic seal that controls the amount of fluid leakage through the clearance. The clearance between the first piston  220  and body  210  provides a leakage flow path from the first fluid reservoir  32  to the second fluid reservoir  33 , and reduces friction between the first piston  220  and the body  210 , thereby minimizing hysteresis in the motion of the first piston  220 . It is believed that side loads introduced by the stack  100  would increase the friction and hysteresis. As such, the first piston  220  is coupled to the stack  100 , preferably only in the direction along the longitudinal axis A-A so as to reduce or even eliminate any side loads. The body  210  is free floating relative to the injector housing, thus preventing distortion. Furthermore, by having a spring contained within the piston subassembly, little or no external side forces or moments are introduced in the compensator assembly  200 .  
         [0022]    To permit fluid  36  to selectively circulate between a first face  222  of the first piston  220  and a second face  224  of the first piston, a passage  226  extends between the first and second faces. 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 atop the first face  222 , shown here in FIG. 4.  
         [0023]    Specifically, by having a smooth surface on the side contiguous to the first piston  220  that forms a sealing surface with the first face  222 , 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  33  whenever pressure in the first fluid reservoir  32  is less than pressure in the second reservoir  33 . 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  228   a . 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. The pressure sensitive valve or plate  270  includes orifices  272   a  and  272   b  formed through its surface. The orifice can be, for example, square, circular or any suitable through orifice. Preferably, there are twelve orifices formed through 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 . The plate  270  is preferably welded to the first face  222  at approximately four or more different locations  276  around the perimeter of the plate  270 .  
         [0024]    Because the plate  270  has very low mass and is flexible, it responds very quickly with the incoming fluid by lifting up towards the end member  28  so that fluid that has not passed through the plate adds to the volume of the hydraulic shim. The plate  270  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  226 . 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 of the hydraulic fluid out of the first fluid reservoir  32 , quickly dampens the oscillations.  
         [0025]    The through hole or orifice diameter of the orifice  272   a  or  272   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 face  222 . 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 110 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.  
         [0026]    Pockets or channels  228   a  and  228   b  can be formed on the first face  222 . The pockets  228   a  and  228   b  ensure that some fluid  36  can remain on the first face  222  to act as a hydraulic “shim” even when there is little or no fluid between the first face  222  and the end member  28 . In a preferred embodiment, the first reservoir always has at least some fluid disposed therein. The first face  222  and the second face  224  can be of any suitable shapes such as, for example, a conic surface of revolution. Preferably, the first face  222  and second face  224  include a planar surface transverse to the longitudinal axis A-A.  
         [0027]    Disposed between the first piston  220  and the top  46  of the stack  100  is a ring like piston or second piston  240  mounted on the extension portion  230  so as to be axially slidable along the longitudinal axis A-A. The second piston  240  includes a sealing member, preferably an elastomer  242  disposed in a groove  245  formed on the outer circumference of the second piston  240  so as to generally prevent leakage of fluid  36  towards the stack  100 . Preferably, the elastomer  242  is an O-ring. Alternatively, the elastomer  242  can be an O-ring of the type having non-circular cross-sections. Other types of elastomer seal can also be used, such as, for example, a labyrinth seal.  
         [0028]    The second piston includes a surface  246  that forms, in conjunction with a surface  256  of the first bellows collar  252 , a second working surface  248 . Here, the second working surface  248  is disposed in a confronting arrangement with the first working surface, (i.e. the first working surface is the second face  224  of the first piston  220 ). Preferably, the pistons are circular in shape, although other suitable shapes, such as rectangular or oval, can also be used for the piston  220 .  
         [0029]    The second piston  240  is coupled to the extension portion  230  via bellows  250  and at least one elastic member or spring  260 . The spring  260  is confined between a boss portion  280  and the second piston  240 . Preferably, the boss portion  280  can be a spring washer that is affixed to the extension portion by a suitable technique, such as, for example, threading, welding, bonding, brazing, gluing and preferably laser welding. The bellows  250  includes a first bellows collar  252  and a second bellows collar  254 . The first bellows collar  252  is affixed to the inner surface  244  of the second piston  240 . The second bellows collar  254  is affixed to the boss portion  280 . Both of the bellows collars can be affixed by a suitable technique, such as, for example, threading, welding, bonding, brazing, gluing and preferably laser welding. It should be noted here that the first bellows collar  252  is disposed for a sliding fit on the extension portion  230 . Preferably, the first bellows collar  252  in its axial neutral (unloaded) condition has approximately 300 micrometer of clearance between the extension portion  230  and the bellows collar  252  at room temperature (approximately 20 degrees Celsius). From this position it can move approximately +/−100 microns to approximately +/−300 microns depending on the number of operating cycles that are desired for the solid state actuator. Maximum operating temperature (approximately 140 degrees Celsius or greater) could increase this clearance to approximately 400 microns. Minimum operating temperature (approximately −40 degrees Celsius or lower) would decrease the clearance to approximately 250 microns.  
         [0030]    The spring  260  can react against boss portion  280  to push the second working surface  248  towards the inlet  16 . This causes a pressure increase in the fluid  36  that acts against the first face  222  and second face  224  of the first piston  220 . In an initial condition, hydraulic fluid  36  is pressurized as a function of the spring force of the spring  260  and the second working surface  248 . The pressurized fluid tends to flow into and out of the first reservoir  32  and the second reservoir  33  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, such as in an initial condition, the pressure responsive valve  270  operates to permit fluid  36  to flow into the first reservoir  32 . Prior to any expansion of the fluid in the first reservoir  32 , the first reservoir is preloaded by the second working surface  248  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.  
         [0031]    The fluid  36  that forms a hydraulic shim tends to expand due to an increase in temperature in and around the compensator. Since the first face  222  has a greater surface area than the second working surface  248 , the first piston tends to move towards the stack or valve closure member  40 . The force vector (i.e. having a direction and magnitude) “F out ” of the first piston  220  moving towards the stack  100  is defined as follows:  
           F   out   =F   spring+ ( F   spring   +/−F   seal )*(( A   shim   /A   2ndReservoir )−1)  
         [0032]    where:  
         [0033]    F out =Applied Force (To the Piezo Stack)  
         [0034]    F spring =Spring Force (30 to 70 N)  
         [0035]    A shim =Area above piston (Hydraulic Shim)  
         [0036]    A 2ndReservoir =Area below the first piston (Second Fluid Reservoir)  
         [0037]    F seal =Seal Friction Force (sealing member  242 )  
         [0038]    Assuming frictionless seals the following mathematical relation would also apply.  
           F   out   =F   spring   *A   shim   *P   shim /( A   2ndReservoir   *P   2ndReservoir )  
         [0039]    where:  
         [0040]    F out =Applied Force (To the Piezo Stack)  
         [0041]    F spring =Spring Force  
         [0042]    A shim=(π/ 4)*Pd 2  or Area above piston where Pd is first piston diameter  
         [0043]    P shim =Pressure (Hydraulic Shim)  
         [0044]    A 2ndReservoir =(π/4)*(Pd 2 −Bh 2 ) or Area below the first piston where Bh is the hydraulic diameter of bellows  250   
         [0045]    P 2ndReservoir =Pressure (in the Second Reservoir)  
         [0046]    At rest, the respective pressures of the hydraulic shim and the second fluid reservoir tend to be generally equal. Since the friction force of sealing member  242  affects the pressure in the hydraulic shim and the second fluid reservoir equally, the sealing member  242  does not affect the force F out  of the piston. However, when the solid-state actuator is energized, the pressure in the hydraulic shim is increased because (a) the plate  270  seals tight against the face  222  and (b) the fluid  36  is incompressible as the stack expands. This allows the stack  100  to have a stiff reaction base in which the valve closure member  40  can be actuated so as to inject fuel through the fuel outlet  62 .  
         [0047]    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 and modulus of elasticity of the spring. Each of the spring characteristics can be selected in various combinations with other spring characteristic(s) described above so as to achieve a desired response of the compensator assembly.  
         [0048]    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.  
         [0049]    In order for fuel to exit through fuel outlet  62 , voltage is supplied to solid-state actuator stack  100 , causing it to expand. The expansion of solid-state 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 solid-state 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 solid-state actuator stack  100  contracts when the voltage supply is terminated, and the bias of the spring  48  which holds the valve closure member  40  in constant contact with bottom  44 , also biases the valve closure member  40  to the closed configuration.  
         [0050]    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  17  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.  
         [0051]    When the actuator  100  is energized, pressure in the first reservoir  32  increases rapidly, causing the plate  270  to seal tight against the first face  222 . This blocks the hydraulic fluid  36  from flowing out of the first fluid reservoir to the passage  236 . It should be noted that the volume of the shim during activation of the stack  100  is related to the volume of the hydraulic fluid in the first reservoir at the approximate instant the actuator  100  is activated. 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 be constant, as it is energized time after time, thereby allowing an opening of the fuel injector to remain the same.  
         [0052]    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 seal  242  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.  
         [0053]    Although the compensator assembly  200  has been shown in combination with a piezoelectric 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.  
         [0054]    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.