Abstract:
A fuel injector comprises a body having a longitudinal axis, a length-changing actuator that has first and second ends, a closure member coupled to the first end of the length-changing actuator, and a compensator assembly coupled the second end of the actuator. The length-changing actuator includes 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 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 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 or magnetic field. It is believed that in typical applications, the axial length can change by, for example, approximately 0.12%. In a stacked configuration of piezoelectric elements of a solid-state actuator, it is believed that the change in the axial length is magnified as a function of the number of elements in the actuator. 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 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 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, or that they only accurately approximate the change in length of the solid state actuator 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 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 of the 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 having an inner surface facing the longitudinal axis, a first piston coupled to the length-changing actuator and disposed in the body proximate one of the first body end and second body end, a second piston disposed in the body proximate the first piston. The first piston has a first outer surface and a first working surface distal to the first outer surface, the first outer surface cooperating with the end member of the housing of the fuel injector to define a first fluid reservoir in the body. The second piston has a second outer surface distal to a second working surface that confronts the first working surface of the first piston. A second fluid reservoir is disposed between the first working surface and the second working surface, a communication passage being disposed between the first fluid reservoir and the second fluid reservoir, and an extension portion having a first extension end coupled to one of the first piston and second piston and a second extension end coupled to the length-changing actuator. The extension portion includes a fill passage disposed within the extension portion so as to supply hydraulic fluid to the communication passage and the first and second fluid reservoirs.  
           [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, 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 thermal compensator comprises an end member, a body having a first body end and a second body end extending along a longitudinal axis, the body having an inner surface facing the longitudinal axis, a first piston coupled to the length-changing actuator and disposed in the body proximate one of the first body end and second body end, a second piston disposed in the body proximate the first piston. The first piston has a first outer surface and a first working surface distal to the first outer surface, the first outer surface cooperating with the end member of the housing of the fuel injector to define a first fluid reservoir in the body. The second piston has a second outer surface distal to a second working surface that confronts the first working surface of the first piston. A second fluid reservoir is disposed between the first working surface and the second working surface, a communication passage being disposed between the first fluid reservoir and the second fluid reservoir, and an extension portion having a first extension end coupled to one of the first piston and second piston and a second extension end coupled to the length-changing actuator. The extension portion includes a fill passage disposed within the extension portion so as to supply hydraulic fluid to the communication passage and the first and second fluid reservoirs.  
           [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. In particular, the actuator includes a fuel injection valve or a fuel injector that incorporates a length-changing actuator such as, for example, an electrorestrictive, magnetorestrictive, piezoelectric or solid state actuator. A preferred embodiment of the length-changing actuator includes a solid-state actuator that actuates a closure member of the fuel injector. The fuel injector includes a housing having an end member, a body, the body having an inner surface facing the longitudinal axis, a first piston coupled to the length-changing actuator and disposed in the body, the first piston having a first outer surface and a first working surface distal to the first outer surface, the first outer surface cooperating with the end member of the housing of the fuel injector to define a first fluid reservoir in the body, a second piston disposed in the body proximate the first piston. A second fluid reservoir is disposed between the first working surface and the second working surface. A communication passage is disposed between the first fluid reservoir and the second fluid reservoir, and an extension portion coupled to one of the first piston and second piston. The extension portion includes a fill passage disposed within the extension portion so as to supply hydraulic fluid to the communication passage and the first and second fluid reservoirs. 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; coupling an flexible fluid barrier between the first piston and the second piston such that the second piston, the elastomer and the flexible fluid barrier form the second fluid reservoir; biasing the second piston being disposed at least partly within the outer shell of the piston skirt so as to generate a hydraulic pressure in the first and second hydraulic reservoirs; and biasing the length-changing actuator with a predetermined 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 and a compensator assembly of a preferred embodiment.  
         [0012]    [0012]FIG. 2A is an enlarged view of the thermal compensator assembly in FIG. 1.  
         [0013]    [0013]FIG. 2B is an enlarged view of another preferred embodiment of the thermal compensator assembly.  
         [0014]    [0014]FIG. 3 is an illustration of the operation of the pressure sensitive valve of FIGS. 2A or  2 B.  
         [0015]    [0015]FIG. 4 is an illustration of another embodiment utilizing the nested configuration of FIG. 2B. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]    Referring to FIGS.  1 - 4 , a plurality of preferred embodiments is shown of a thermal compensator assembly. In particular, FIG. 1 illustrates a preferred embodiment of a fuel injector assembly  10  having a solid-state actuator that, preferably, includes a solid-state actuator stack  100  and a compensator assembly  200  for the stack  100 . The fuel injector assembly  10  includes inlet fitting  12 , injector housing  14 , and valve body  17 . 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 . 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 hydraulic fluid that has a higher coefficient of thermal expansion than that of the injector inlet  16 , the housing  14  or other components of the fuel injector.  
         [0017]    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  10 . That is, the lengthwise expansion of the stack  100  and the closure member  40  can be used to define an orifice size of the fuel injector as opposed to an orifice of a valve seat or an orifice plate as is used in a conventional fuel injector.  
         [0018]    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 .  
         [0019]    Fuel injector assembly  10  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 Celsius or less) and at operating temperatures (140 Celsius or more).  
         [0020]    As used herein, elements having similar features are denoted by the same reference number and can be differentiated between FIG. 2A and FIG. 2B by a prime notation. Referring to FIG. 2A, compensator assembly  200  includes a body  210  having a first body end  210   a  and a second body end  210   b . The second body end  210   b  includes an end cap  214  with an opening  216 . The end cap  214  can be a portion that can extend, transversely or obliquely with respect to the longitudinal axis A-A, from the inner surface  213  of the body  210  towards the longitudinal axis. Alternatively, the end cap  214  can be of a separate portion affixed to the body  210 . Preferably, the end cap  214  is formed as part of the second end  210   b  of the body  210 , which end cap  214  extends transversely with respect to the longitudinal axis A-A.  
         [0021]    The body  210  encases a first piston  220 , part of a piston stem or an extension portion  230 , a second piston  240 , a flexible diaphragm  250  and an elastic member or spring  260  located between the second piston  240  and the end cap  214 . The first body end  210   a  and second body end  210   b  can be of any suitable cross-sectional shape as long as it 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  210  is circular, thereby forming a cylindrical body that extends along the longitudinal axis A-A. The body  210  can also be formed by coupling two separate portions together (FIG. 2A), or by forming the body from a continuous piece of material (FIG. 2B) as shown here in the preferred embodiments.  
         [0022]    The extension portion  230  extends from the first piston  220  so as to be linked to the top  46  of the piezoelectric stack  100 . Preferably, the extension portion is formed as a separate piece from the first piston  220 , and coupled to the first piston  220  by a spline coupling  272 . Other suitable couplings can also be used, such as, for example, a ball joint, a heim joint or any other couplings that allow two moving parts to be coupled together. Alternatively, the extension portion  230  can be integrally formed as a single piece with the first piston  220 .  
         [0023]    In a preferred embodiment (FIG. 2B), a separate extension portion  230  is configured with an internal fill passage  232  that is disposed within the extension portion  230 . The fill passage  232  extends from a first fill end  232   a  through generally the whole length of the extension portion  230  to a second fill end  232   b . The first fill end  232   a  is generally a port that has its axis along the same axis as the fill passage  232  or the longitudinal axis A-A. The second fill end  232   b  is generally a port having an axis transverse to the fill passage or the longitudinal axis A-A. The cross-sections of the fill passage and ports can be of a suitable cross-section, such as, for example, circular, oval, square, or rectangular. Preferably the respective cross-sections are circular in shape.  
         [0024]    One of the many benefits of the internal fill passage  232  (or  332 ) is the ability to fill the compensator with minimal amount of fluid without overfilling the compensator. In particular, the thermal compensator  200  or  200 ′,  300  can be fully assembled and placed in the injector housing  14  but without the actuator or stack  100 . As the fluid  36 , preferably a silicone oil (Baysilone™ M350), has an affinity for gas or air, the partially assembled fuel injector is then placed in a chamber that can be placed under a vacuum (approximately −28 milliBar) so as to minimize any air or gas that can dissolve in the fluid  36  prior to filling of the compensator  200  or  200 ′,  300  with the fluid  36 . As the fluid  36  flows through the internal fill passage  230 , the first reservoir and the second reservoir become filled with fluid  36 . Since the fluid  36  is substantially incompressible, it displaces the first piston  220  towards the outlet end. As the first piston  220  moves toward the outlet end, a chamfer  234   a  on the piston side mates with a chamfer  234   b  on the extension portion side, thereby forming a seal  234  that prevents egress or ingress of fluid  36  into or out of the compensator. The stack  100  may now be installed in the injector housing  14  while still under a vacuum. Once the vacuum is removed, the first piston  220  expands tight against the extension portion so as to form a generally fluid tight seal with the chamfer seal  234 . Alternatively, an elastomeric seal  234  can be mounted in a groove formed between the first piston  220  and the extension portion  230  so as to provide another seal against leakage of the fluid  36 .  
         [0025]    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. 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 controlled clearance between the first piston  220  and body  210  provides a controlled 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 movement 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  only in the direction along the longitudinal axis A-A so as to reduce or even eliminate any side loads. The body  210  is preferably affixed to the injector housing at a first end  210   a  so as to be semi-free floating relative to the injector housing. Alternatively, the body  210  can be permitted to float in an axial direction within the injector housing. Furthermore, by having a spring contained within the piston subassembly, little or no external side forces or moments are introduced by the compensator assembly  200  ( 200 ′ or  300 ) to the injector housing. Thus, it is believed that these features operate to reduce or even prevent distortion of the injector housing.  
         [0026]    Pockets or channels  228   a  can be formed on the first face  222  that are in fluid communication with the second fluid reservoir  33  via the passage  226 . The pockets  228   a  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  32  always has at least some fluid disposed therein. The first face  222  and the second face  224  can be of any shapes such as, for example, a conic surface of revolution, a frustoconical surface or a planar surface. Preferably, the first face  222  and second face  224  include a planar surface transverse to the longitudinal axis A-A.  
         [0027]    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  220 , a passage  226  extends between the first and second faces. Facilitating the flow of fluid  36  between the passage  226  and the reservoirs is a gap  229  formed by a reduced portion  227  of the first piston  220  located on an outer peripheral surface of the piston  220 . The gap  229  allows fluid  36  to flow out of passage  226  and into the second reservoir  33 .  
         [0028]    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 (FIG. 3). 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 .  
         [0029]    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  (or  32 ′) and a second fluid reservoir  33  (or  33 ′) whenever pressure in the first fluid reservoir  32  (or  32 ′) is less than pressure in the second reservoir  33  (or  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  (or  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  228   a  has an opening that is approximately the same shape and cross-section as each of the orifices  272   a  and  272   b . The plate  270  is preferably welded to the first face  222  at four or more different locations around the perimeter of the plate  270 .  
         [0030]    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 out of the hydraulic fluid, quickly dampens the oscillations.  
         [0031]    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 thermal 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 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 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.  
         [0032]    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 third face  242  confronting the second face  224 . The second piston  240  also includes a fourth face  244  distal to the third face  242  along the longitudinal axis A-A. The fourth face  244  includes a retaining boss portion  246  which also constitute a part of a retaining shoulder  248 . The retaining boss portion  246  cooperates with a boss portion  211  (formed on an surface of the body  210  that faces the longitudinal axis A-A) so as to facilitate assembly of a flexible diaphragm  250  after the second piston  240  has been installed in the second end  210   b  of the body  210 . Preferably, the pistons are circular in shape, although other shapes, such as rectangular or oval, can also be used for the first piston  220  and second piston  240 .  
         [0033]    The second reservoir  33  is formed by a volume, which is enclosed by the flexible diaphragm  250 . The diaphragm  250  is located between the second face  224  of the first piston  220  and the second piston  240 . The flexible diaphragm  250  can be of a one-piece construction or of two or more portions affixed to each other by a suitable technique such as, for example, welding, bonding, brazing, gluing and preferably laser welding. Preferably, the flexible diagram  250  includes a first strip  252  and second strip  254  affixed to each other.  
         [0034]    The flexible diaphragm  250  can be affixed to the first piston  220  and to an inner surface of the body  210  by a suitable technique as noted above. One end of the first strip  252  is affixed to the reduced portion  227  of the first piston  220  whereas another end of the second strip  254  is affixed to an inner surface of the body  210 . Where the body  210  is of a one-piece construction, the another end can be affixed directly to the inner surface of the body  210 . Preferably, where the body  210  includes two or more portions coupled to each other, the another end of the second strip  254  is affixed to one or the other portions prior to the portions constituting the body  210  being affixed together by a suitable technique.  
         [0035]    The spring  260  is confined between the end cap  214  and the second piston  240 . Since the second piston  240  is movable relative to the end cap  214 , the spring  260  operates to push the second piston  240  against the flexible diaphragm  250 . The second piston  240  impinges on the flexible diaphragm  250 , which then forms a second working surface  248  with a surface area that is less than the surface area of the first working surface. Because the third face  242  impinges against the flexible diaphragm  250 , the working surface  248  can be thought of as having essentially the same surface area as the third face  242 .  
         [0036]    This causes a pressure increase in the fluid  36  in the second fluid reservoir  33 . In an initial condition, hydraulic fluid  36  is pressurized as a function of the product of the spring force and the surface area of the second working surface  248 . Prior to any expansion of the fluid in the first reservoir  32 , the first reservoir is preloaded so as to form a hydraulic shim. Preferably, the spring force of the spring  260  is approximately 30 Newton to 70 Newton.  
         [0037]    The fluid  36  that forms a volume of hydraulic shim tends to expand due to an increase in temperature in and around the thermal compensator. The increase in volume of the shim acts directly on the first outer surface or first face  222  of the first piston. 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 is defined as follows:  
           F   out =( A   shim   *P   shim )− F   spring    
         [0038]    where:  
         [0039]    F out =Applied Force (To the Piezo Stack)  
         [0040]    F spring =Total Spring Force  
         [0041]    A shim =(π/4)*Pd 2  or Area above piston where Pd is first piston diameter (Hydraulic Shim)  
         [0042]    At rest, the respective pressure of the pressures in the hydraulic shim and the second fluid reservoir tends to be generally equal. However, when the solid-state actuator is energized, the pressure in the hydraulic shim is increased because 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 .  
         [0043]    Preferably, the spring  260  is a coil spring. Here, the pressure in the fluid reservoirs is related to at least one spring characteristic of each of the coil springs. 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) so as to achieve a desired response of the compensator assembly  200 .  
         [0044]    Referring to FIG. 2B, the second piston  240 ′ is mounted in a “nested” arrangement of a compensator assembly  200 ,  300  that differs from the pistons arrangement of the compensator assembly  200  of FIG. 2A. In FIG. 2B, the nested arrangement requires that the first piston  220 ′ includes a piston skirt  221  of sufficient dimensions so as to permit a spring  260 ′ and the second piston  240  to be installed within a volume defined by the piston skirt  221 . The axial extent of the skirt  221  along the longitudinal axis A-A should be of a sufficient length so as to permit a spring  262  to be compressed and mounted within the piston skirt  221  without binding or interference between the springs or other parts of the pistons. The first piston  220 ′ also includes an elongated portion  223  that allows the first piston  220 ′ to be coupled to by a suitable coupling to the extension portion  230 ′. The elongated portion  223  also cooperates with the skirt  221  to define a volume for receipt of the spring  262 . The spring  262  is operable to push the second piston  240 ′ against a flexible diaphragm  250 ′. The flexible diaphragm  250 ′ is attached by any suitable technique (such as those described with reference to flexible diaphragm  250 ) to the first piston  220  and to the end cap  214 ′. Preferably, the flexible diaphragm  250 ′ is of a one-piece construction. It should be noted that although the compensator  200 ,  300  operates similarly to the compensator  200 , one of the many aspects in which the embodiment of FIG. 2B differs from that of the embodiment of FIG. 2A is in the direction at which the second piston ( 240  in FIG. 2A and  240 ′ in FIG. 2B) moves due to the spring force. In FIG. 2A, the spring force causes the piston to move towards the inlet end of the injector whereas in FIG. 2B, the spring force causes the second piston  240 ′ to move towards the outlet end. Like the second piston  220  of FIG. 2A, the second piston  220 ′ of FIG. 2B is preferably not in physical contact with the fluid  36 . The second piston  220 ′, by impinging its face  229 ′ against the flexible diaphragm  250 ′ (which is in physical contact with the fluid  36 ) causes the flexible diaphragm  250 ′ to transfer the spring force to the fluid  36  through a second working surface  248 ′ of the diaphragm  250 ′. Another aspect of the compensator  200 ,  300  includes an overall axial length that is more compact than that of the compensator assembly  200 .  
         [0045]    The compensator  200 ′ of FIG. 2B can be simplified by eliminating the pressure responsive valve and the fluid passage that extends through the first piston. This simplification results in another preferred embodiment, shown here in FIG. 4, as a thermal compensator  300 . The thermal compensator  300  includes a body  310  surrounding a first piston  320  that has a piston skirt  324 . The piston skirt  324  is disposed a facing arrangement with an inner surface  312  of the body  310  that presents a gap  326  therebetween. A second piston  340  is disposed at least partly within the piston skirt  324 . The second piston  340  includes a working face  342  and an extension  344  that extends through an opening  316  of the end cap  314 . To generally prevent fluid  36  from entering the volume between the nested pistons, a sealing member  352  is disposed in a groove formed on either the skirt of the first piston or on an exterior portion of the second piston, which for clarity, only one side of the sealing member  352  is shown. The sealing member can be a diaphragm coupled to the skirt  324  and the second piston  340  or the extension portion  344  thereof. Preferably, the sealing member  352  is an O-ring. To generally prevent fluid from escaping a second reservoir  33 , a seal  318  can be formed between the end cap  314  and the extension  344  of the second piston  340 . Specifically, a groove can be formed into either the end cap  314  or the extension  344 . The O-ring  318  is then mounted in the groove. Preferably, the groove  319  is formed on a peripheral surface of end cap  314  that faces the longitudinal axis A-A.  
         [0046]    A first fluid reservoir  32  is formed between a face  322  and an end member  28 . A second fluid reservoir  33  is formed between the working face  342  and the body. The first fluid reservoir  32  is in fluid communication with the second fluid reservoir  33  via a controlled clearance or gap  326 . Preferably, the gap  326  should be of a suitable clearance so as to 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 or gap  326 .  
         [0047]    An internal filling passage  332  (similar in operating principle to the internal passage  232  of FIG. 2B) extends between a first port  332   a  and a second port  332   b . A seal  350  is formed to preclude ingress or egress of fluid to the first reservoir  32  when a surface  350   a  of the first piston  320  contacts a surface  350   b  of the extension portion  330 . At least one spring  360  is disposed within an internal volume of the first piston  320 . The at least one spring  360  biases the second piston  340  away from the first piston  320 . This applies a force to the fluid  36  through a surface area of the working surface  342 , resulting in a first pressure that is transmitted to the first face  322  of the first piston  320 . The first pressure can be designated as a pressure that permits the first reservoir to act as a hydraulic shim. Subsequent volumetric changes to the fluid  36  (due to thermal changes) in the first or second reservoir would cause the first piston to move along the longitudinal axis. This is believed to maintain the solid state actuator in a fixed spatial relation with various components of the fuel injector.  
         [0048]    The force F out  applied to the actuator stack  100  of the embodiment shown in FIG. 4 is defined as follows:  
           F   out =[( F   spring360   ±F   seal352   ±F   seal318 )*( A   shim   /A   reservoir33 )]× F   spring   ±F   seal352    
         [0049]    Where:  
         [0050]    F out =Force applied to stack  100   
         [0051]    F spring360 =Force of spring  360   
         [0052]    F seal352 =Friction force of seal  352   
         [0053]    F seal318 =Friction force of seal  318   
         [0054]    A shim =(π/4)*Pd 2  or Area above piston where Pd is first piston diameter (Hydraulic Shim)  
         [0055]    A reservoir33 =Area of the second reservoir  33   
         [0056]    Referring again to FIG. 1, during operation of the fuel injector  10 , 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.  
         [0057]    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.  
         [0058]    During engine operation, as the temperature in the engine rises, inlet fitting  12 , injector housing  14  and valve body  17  experience thermal expansion due to the rise in temperature while the solid-state actuator stack experience generally insignificant thermal expansion. At the same time, fuel traveling through fuel tube  22  and out through fuel outlet  62  cools the internal components of fuel injector assembly  10  and causes thermal contraction of valve closure member  40 . Referring to FIG. 1, as valve closure member  40  contracts, bottom  44  tends to separate from its contact point with valve closure member  40 . Solid-state actuator stack  100 , which is operatively connected to the bottom surface of first piston  220  (or  220 ′), is pushed downward. The increase in temperature causes inlet fitting  12 , injector housing  14  and valve body  17  to expand relative to the piezoelectric stack  100  due to the generally higher volumetric thermal expansion coefficient β of the fuel injector components relative to that of the piezoelectric stack. Since the fluid is, in this case, expanding, pressure in the first fluid reservoir therefore must increase. Because of the virtual incompressibility of fluid and the smaller surface area of the second working surface  248  (or  248 ′), the first piston  220  (or  220 ′) is moved relative to the second piston  240  (or  240 ′) towards the outlet end of the injector  10 . This movement of the first piston  220  (or  220 ′) is transmitted to the piezoelectric stack  100  by the extension portion  230  (or  230 ′), which movement is believed to maintain the position of the piezoelectric stack constant relative to other components of the fuel injector such as the inlet cap  14 , injector housing  14  and valve body  18 . 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 piezoelectric stack. Here, the compensator assembly  200  (or  200 ,  300 ) 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 piezoelectric stack  100  can be compensated by the expansion of the hydraulic fluid  36  in the first reservoir.  
         [0059]    During subsequent fluctuations in temperature around the fuel injector assembly  100 , any further expansion of inlet fitting  14 , injector housing  14  or valve body  17  causes the fluid  36  to expand or contract in the first reservoir. Where the fluid is expanding, the first piston  220  (or  220 ′) is forced to move towards the outlet end of the fuel injector since the first face  222  (or  222 ′) has a greater surface area than the second working surface  248  (or  248 ′). On the other hand, any contraction of the fuel injector components would cause the hydraulic fluid  36  in the first reservoir  32  (or  32 ′) to contract in volume, thereby retracting the first piston  220  (or  220 ′) towards the inlet of the fuel injector  10 .  
         [0060]    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.  
         [0061]    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  (or  262 ), 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.  
         [0062]    Although the compensator assembly  200 ,  200 ′ or  300  has been shown in combination with a solid-state 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 thermal compensator assembly  200 ,  200 ′ or  300 . 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 thermal compensator assembly  200 ,  200 ′ or  300  and the length-changing 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.  
         [0063]    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.