Abstract:
A fuel injector for an internal combustion engine is provided. The fuel injector has a simple structure that is easy to install and remove a piezoelectric valve actuator in and from the fuel injector and to adjust fuel injection characteristics finely and that allows an overall size of the fuel injector to be decreased. The fuel injector includes a housing to be installed in the engine and a structural element serving to install the piezoelectric valve actuator in the housing to be detachable easily. The piezoelectric valve actuator is retained in an actuator casing fitted in the housing so that it can expand or contract to move a needle valve. The actuator casing has an extensible portion in itself or is coupled to a bellows in alignment for enabling the piezoelectric valve actuator to expand, thereby allowing the size of the actuator casing to be minimized.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field of the Invention  
           [0002]    The present invention relates generally to a fuel injector for internal combustion engines, and more particularly to an improved structure of a fuel injector for installation of a piezoelectric device used as a valve actuator of the fuel injector.  
           [0003]    2. Background Art  
           [0004]    Typical fuel injectors used in, for example, internal combustion diesel engines of automotive vehicles are designed to drive a three-way valve or a two-way valve connected to a common rail in which a high pressure fuel is stored for opening and closing a fuel supply passage selectively. When it is required to inject the fuel into the engine, the fuel injector changes the fuel pressure acting on a needle to lift up the needle for opening a spray hole to initiate the fuel injection.  
           [0005]    As a valve actuator to open and close the three-way valve or the two-way valve, a solenoid valve has been usually used. In recent years, however, an attempt is made to utilize an piezoelectric device which expands or contracts in response to input of an electric signal to actuate a valve for controlling the fuel injection precisely. For example, a valve actuator is proposed which consists of a piezoelectric device made up of a stack of piezoelectric layers and a piston. In operation, the voltage is applied to the piezoelectric device. The piezoelectric device then contracts or expands to move the piston to open or close, for example, a three-way valve to control the back pressure of a nozzle needle of a fuel injector. The three-way valve works to switch communications between a back pressure chamber formed adjacent the nozzle needle and a high-pressure fuel path and between the back pressure chamber and a drain passage. When the back pressure chamber communicates with the fuel passage so that the pressure in the back pressure chamber drops, it will cause the nozzle needle to be lifted up to initiate a jet of fuel from a spray hole. Alternatively, when the back pressure chamber communicates with the high-pressure fuel passage, the fuel flows from the high-pressure fuel passage to the back pressure chamber, thereby moving the nozzle needle downward to close the spray hole.  
           [0006]    The piezoelectric device is made by laminating the piezoelectric layers each having upper and lower surfaces on which electrodes are formed and applying a conductive paste to a side surface of the lamination to form side terminals which connect negative and positive sides of the electrodes, respectively. Installation of the piezoelectric device in a housing is accomplished by coupling the side terminals to a connector through leads, fitting an insulator tube on the periphery of the piezoelectric device, and inserting it into a vertical chamber of the housing. After the installation of the piezoelectric device, a hermetic seal is formed by placing the whole of the housing in a mold and forcing resin into the mold to seal an upper end of the housing.  
           [0007]    The piezoelectric device is usually made from PZT (lead zirconate titanate). The PZT contains the lead that is a harmful substance and thus needs to be withdrawn after the piezoelectric device is used up. The withdrawal of the lead requires cutting the housing because the upper end of the housing is, as described above, sealed by resin. It is, thus, quite inconvenience. Further, there is a problem that parts cannot be removed from the housing after assembly thereof, therefore, it is impossible to replace the parts and adjust characteristics of the fuel injection finely.  
           [0008]    The piezoelectric device, the insulator tube, and the connector are not secured completely during assembly thereof and thus are not easy to handle, which may lead to the breakage of the insulator tube. The connector is covered with a resin material using a mold after the fu(:l injector is assembled to insulate the connector from the injector body and thus is fixed in orientation thereof in a circumferential direction of the injector. Accordingly, it is necessary to prepare a connector mold for every type of engine, resulting in an increase ill manufacturing cost of the injector.  
           [0009]    Japanese Patent No. 3010835 discloses a piezoelectric device which is disposed hermetically within a casing which has a bellows for avoiding the ingress of moisture or foreign objects into the piezoelectric device. This structure, however, has the drawbacks in that the bellows has a larger diameter and is difficult to install in small-sized fuel injectors. If the size of the piezoelectric device is decreased to match with that of the fuel injectors, it may cause the performance thereof to be reduced. For theses reasons, fuel injectors equipped with the piezoelectric device as an actuator are not yet put into practical use.  
         SUMMARY OF THE INVENTION  
         [0010]    It is therefore a principal object of the invention to avoid the disadvantages of the prior art.  
           [0011]    It is another object of the invention to provide an improved structure of a fuel injector which is easy to install and remove a piezoelectric actuator in and from the fuel injector and to adjust fuel injection characteristics finely and which allows an overall size of the fuel injector to be decreased.  
           [0012]    According to one aspect of the invention, there is provided an improved structure of a fuel injector for an internal combustion engine. The fuel injector comprises: (a) a housing to be installed in the engine with a portion of the housing exposed outside the engine; (b) an actuator including an electrically deformable element which works to be deformed in response to input of an electric signal for opening and closing a spray hole selectively; and (c) a structural element installing the actuator in the housing detachably.  
           [0013]    In the preferred mode of the invention, the housing has a length and an end portion thereof exposed outside the engine. A nozzle needle is disposed within the housing in alignment with the actuator so as to be moved in a lengthwise direction of the housing by the deformation of the actuator to open and close the spray hole selectively. The structural element secures the actuator so that the actuator is detachable from the end portion of the housing opposite the nozzle needle across the actuator.  
           [0014]    The actuator has a length with a first end oriented toward the portion of the housing exposed outside the engine. A connector is coupled integrally with the first end of the actuator for establishing an electric connection between the actuator and a power source.  
           [0015]    The housing has firmed therein a vertical chamber which has an opening oriented to a first end of the housing exposed outside the engine. The structural element includes a fastening member which retains the actuator detachably within the vertical chamber. The nozzle needle is disposed in alignment with the actuator within a chamber formed in the housing opposite the first end across the vertical chamber so as to be moved in a lengthwise direction of the housing by the deformation of the actuator to open and close the spray hole selectively.  
           [0016]    The connector may alternatively be installed detachably in the opening of the vertical chamber.  
           [0017]    The connector may include a connector body which is coupled integrally with the actuator and has retains therein leads connecting with the actuator in an electrically insulating fashion.  
           [0018]    A spacer may be disposed between a flange coupled with the first end of the actuator and a shoulder formed in the housing for adjusting a lengthwise location of the actuator within the vertical chamber.  
           [0019]    The fastening member is fastened to the opening of the vertical chamber in the housing to hold the actuator detachably within the vertical chamber. A positioning means is provided for positioning the actuator within the vertical chamber without being subjected to torque or unbalanced load arising from fastening of the fastening member.  
           [0020]    The connector includes an electric terminal portion and a connector body extending from a surface of the electric terminal portion. The fastening member may be implemented by a retaining nut through which the connector body extends. The retaining nut is installed in the opening of the vertical chamber with an outer end facing the surface of the electric terminal portion of the connector through a gap of 5 to 10 mm so that a portion of the connector body is exposed outside the retaining nut.  
           [0021]    The structural element may alternatively be implemented by one of a screw and a structural member joined to the housing by one of staking, welding, and bonding.  
           [0022]    A joint of the structural member and the housing may be set more fragile than any other portions.  
           [0023]    At least one fragile portion may be formed on the housing for facilitating ease of cutting or breaking up the housing for withdrawing the actuator.  
           [0024]    The electrically deformable element may be implemented by a piezoelectric device designed to expand and contract in response to the input of the electric signal. The piezoelectric device is made up of a stack of piezoelectric layers and electrode layers each interposed between adjacent two of the piezoelectric layers.  
           [0025]    According to the second aspect of the invention, there is provided a fuel injector for an internal combustion engine which comprises: (a) a hollow cylindrical housing having a first and a second opening formed in opposed ends thereof, respectively; (b) an actuator disposed within the housing, the actuator including an electrically deformable element which works to be deformed in response to input of an electric signal; (c) a first plate installed on one of the ends of the housing to seal the first opening hermetically; and (d) a second plate installed on the other end of the housing to seal the second opening hermetically, the second plate being so coupled to the housing as to transform the deformation of the electrically deformable element of the actuator into a stroke of a needle for opening and closing a spray hole selectively.  
           [0026]    In the preferred mode of the invention, the second plate is coupled to the housing so as to be displaced in response to the deformation of the electrically deformable element to produce the stroke of the needle.  
           [0027]    The second plate may alternatively be coupled to the housing so as to be deformed elastically in response to the deformation of the electrically deformable element to produce the stroke of the needle.  
           [0028]    The housing includes a bellows which expands and contracts following the deformation of the electrically deformable element.  
           [0029]    A piston is coupled at an end thereof to the electrically deformable element so as to move following deformation of the electrically deformable element within the cylindrical housing. The second plate may be a diaphragm coupled to the housing in contact with the other end of the piston.  
           [0030]    The diaphragm may be coupled to the housing in contact with an end of the rod of the piston. An annular seat member is installed within the second opening of the cylindrical housing through which the rod of the piston extends. A spring member is disposed on the seat member to exert a given pressure on the electrically deformation member in a lengthwise direction thereof.  
           [0031]    The cylindrical housing may have a bellows formed on the end in which the second opening is defined. In this case, the second plate made of a diaphragm is coupled to an end of the bellows to close the second opening.  
           [0032]    The cylindrical housing may be so designed as to extend following the deformation of the electrically deformable element. At least two of the cylindrical housing, the first plate, and the second plate may be formed integrally with each other. The electrically deformable element may be isolated from fluid within the fuel injector.  
           [0033]    The electrically deformable element may be implemented by a piezoelectric device designed to expand and contract in response to the input of the electric signal. The piezoelectric device is made up of a stack of piezoelectric layers and electrode layers each interposed between adjacent two of the piezoelectric layers.  
           [0034]    According to the third aspect of the invention, there is provided a fuel injector for an internal combustion engine which comprises: (a) a hollow cylindrical housing; (b) an actuator disposed within the housing, the actuator including an electrically deformable element which works to expand and contract selectively in a lengthwise direction thereof in response to input of an electric signal; (c) a piston coupled at an end thereof to the electrically deformable element in alignment therewith so as to move following the expansion and contraction of the electrically deformable element; and (d) an extensible member in which the piston is disposed, the extensible member extending in a lengthwise direction thereof so as to allow the piston to move to displace a needle for opening and closing a spray hole selectively, the extensible member being coupled to the housing in alignment therewith in a direction of expansion and contraction of the electrically deformable element.  
           [0035]    In the preferred .mode of the invention, the extensible member is implemented by a bellows.  
           [0036]    A plate is joined to the other end of the piston. If a minimum diameter of the cylindrical housing is defined as A, a minimum diameter of the plate is defined as B, and a maximum diameter of the extensible member is defined as C, at least one of relations of A&gt;C and B&gt;C is satisfied.  
           [0037]    The end of the piston coupled the electrically deformable element is disposed within the cylindrical housing. If a maximum clearance between the end of the piston and an inner wall of the cylindrical housing is defined as d, and a minimum clearance between the piston and an inner wall of the extensible member is defined as e, a relation of d&lt;e is satisfied.  
           [0038]    A first plate is joined to a first end of the cylindrical housing. A second plate is Jointed to a second end of the cylindrical housing opposite the first end. At least two of the cylindrical housing, the extensible member, the first plate, and the second plate are formed integrally with each other. The electrically deformable element is isolated from fluid within the fuel injector.  
           [0039]    The electrically deformable element may be implemented by a piezoelectric device designed to expand and contract in response to the input of the electric signal. The piezoelectric device is made up of a stack of piezoelectric layers and electrode layers each interposed between adjacent two of the piezoelectric layers. 
       
    
    
     BRIEF DESPCRIPTION OF THE DRAWINGS  
       [0040]    The present invention will be understood more fully from the detailed description giver, hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only.  
         [0041]    In the drawings:  
         [0042]    [0042]FIG. 1 is a vertical sectional view which shows a fuel injector according to the first embodiment of the invention;  
         [0043]    [0043]FIG. 2 is a perspective view which shows a common rail system for a diesel engine using fuel injectors of the types shown in FIG. 1;  
         [0044]    [0044]FIG. 3 is a vertical sectional view which shows an actuator installed in the fuel injector of FIG. 1;  
         [0045]    [0045]FIG. 4 is a vertical sectional view which shows an actuator according to the second embodiment of the invention;  
         [0046]    [0046]FIG. 5 is a partially sectional view which shows an actuator according to the third embodiment of the invention;  
         [0047]    [0047]FIG. 6 is a partially sectional view which shows an actuator according to the fourth embodiment of the invention;  
         [0048]    [0048]FIG. 7( a ) is a side view which shows a connector for establishing an electric connection between a power supply and an actuator of FIG. 6;  
         [0049]    [0049]FIG. 7( b ) is a bottom view of the connector of FIG. 7( a );  
         [0050]    [0050]FIG. 8 is a vertical sectional view which shows a fuel injector according to the fifth embodiment of the invention;  
         [0051]    [0051]FIG. 9 is a vertical sectional view which shows a fuel injector according to the sixth embodiment of the invention;  
         [0052]    [0052]FIG. 10 is a vertical sectional view which shows an actuator installed in the fuel injector of FIG. 9;  
         [0053]    [0053]FIG. 11 is a perspective view which shows a piezoelectric device built in the actuator of FIG. 10;  
         [0054]    FIGS.  12 ( a ) and  12 ( b ) are views which show adjacent piezoelectric layers raking up the piezoelectric device of FIG. 11;  
         [0055]    [0055]FIG. 12( c ) is an exploded view which shows a stack of piezoelectric layers making up a drive portion of the piezoelectric device of FIG. 11;  
         [0056]    FIGS.  13 ( a ) and  13 ( b ) are perspective views which show modifications of the piezoelectric device of FIG. 11;  
         [0057]    [0057]FIG. 14 is a vertical sectional view which shows an actuator according to the seventh embodiment of the invention;  
         [0058]    [0058]FIG. 15 is a vertical sectional view which shows an actuator according to the eighth embodiment of the invention;  
         [0059]    [0059]FIG. 16 is a vertical sectional view which shows an actuator according to the ninth embodiment of the invention;  
         [0060]    [0060]FIG. 17 is a vertical sectional view which shows an actuator according to the tenth embodiment of the invention, FIGS.  18 ( a ),  18 ( b ),  18 ( c ), and  18 ( d ) are sectional views which show modifications of the actuator of FIG. 17;  
         [0061]    FIGS.  19 ( a ),  19 ( b ),  19 ( c ),  19 ( d ),  19 ( e ), and  19 ( f ) are sectional views which show modifications of the actuator of FIG. 10;  
         [0062]    FIGS.  20 ( a ),  20 ( b ),  20 ( c ),  20 ( d ),  20 ( e ), and  20 ( f ) are sectional views which show modifications of the actuator of FIG. 15;  
         [0063]    [0063]FIG. 21 is a partially sectional view which shows a fuel injector according to the eleventh embodiment of the invention;  
         [0064]    [0064]FIG. 22 is a partially sectional view which shows a fuel injector according to the twelfth embodiment of the invention; and  
         [0065]    [0065]FIG. 23 is a partially sectional view which shows a fuel injector according to the thirteenth embodiment of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0066]    Referring to the drawings, wherein like reference numbers refer to like parts in several views, particularly to FIG. 1, there is shown a fuel injector  100  according to the invention. The following discussion will refer to, as an example, a common rail fuel injection system, as shown in FIG. 2, in which the fuel injector  100  is provided for each cylinder of a diesel engine  300 .  
         [0067]    The common rail fuel injection system includes a common rail  200  which accumulates therein fuel supplied from a fuel tank  400  elevated in pressure by a fuel pump installed in the engine  300 . When it is required to inject the fuel into the engine  300 , the fuel stored under high pressure in the common rail  200  is supplied to the fuel injectors  100 .  
         [0068]    The fuel injector,  100  includes, as shown in FIG. 1, an upper housing  2  in which an actuator  1  is disposed and a lower housing  3  which is jointed to the upper housing  2  in alignment therewith and has a injection nozzle  4 .  
         [0069]    The upper housing  2  is made of a hollow cylindrical member and has a vertical chamber  21  formed eccentrically with a longitudinal center line thereof. In the vertical chamber  21 , the actuator  1  is disposed. The upper housing  2  has formed therein a high-pressure fuel passage  22  which extends in parallel to the vertical chamber  21  and connects at an upper end thereof to a fuel inlet connector  23 . The fuel inlet connector  23  projects outside the upper housing  2  (i.e., the cylinder of the engine  300 ) and communicates with the common rail  200 , as shown in FIG. 2. An fuel outlet connector  25  is installed in an upper portion of the upper housing  2  opposite the fuel inlet connector  23 . The fuel flowing into a drain passage  24  is discharged from the fuel outlet connector  25  to the fuel tank  400 . The drain passage  24  leads to a gap  50  between an inner wall of the vertical chamber  21  and the actuator  1  and to a three-way valve  51  through a passage (not shown) extending vertically through the upper and lower housings  2  and  3 .  
         [0070]    The injection nozzle  4  has a needle  41  and a spray hole  43 . The needle  41  is slidable vertically within a nozzle block  31  to spray fuel in a fuel sump  42  from the spray hole  43 . The fuel sump  42  is defined around a middle portion of the needle  41  and leads to a lower end of the high-pressure fuel passage  22 . The needle  41  is applied with the pressure of the fuel in the fuel sump  42  which works to move the needle  41  in an upward direction (also referred to as a valve-opening direction below) and the pressure of the fuel in a back pressure chamber  44  which works to move the needle  41  in a downward direction (also referred to as a valve-closing direction below). When the pressure in the back pressure chamber  44  drops, it will cause the needle  41  to be lifted upward to open the spray hole  43 , initiating a fuel jet.  
         [0071]    The pressure in the back pressure chamber  44  is controlled by the three-way valve  51 . This pressure control is achieved by selectively establishing communications between the back pressure chamber  44  and the high-pressure fuel passage  22  and between the back pressure chamber  44  and the drain passage  24 . The switching of these communications is achieved by moving a ball of the three-way valve  51 , as indicated by a broken line in FIG. 1. The movement of the ball is accomplished by displacing a large-diameter piston  52  and a small-diameter piston  54  through the actuator  1 . The small-diameter piston  54  is hydraulically coupled with the large-diameter piston  52  through a pressure chamber  53 . Three-way valves are known per se, and explanation thereof in detail will be omitted here.  
         [0072]    The actuator  1 , as clearly shown in FIG. 3, consists essentially of a thin-walled metallic hollow cylindrical housing  11 , a laminated piezoelectric device (also called a piezo stack)  61 , and a piston  62 . The piezoelectric device  61  is disposed within an upper portion of the housing  11 . The piston  62  is disposed slidably within the housing  11  in alignment with the piezoelectric device  61 .  
         [0073]    The piezoelectric device  61  may be of a known type which is, as will be described in detail later, made up of a stack of piezoelectric discs each having electrodes formed on both surfaces thereof. A conductive paste is applied to a side wall of the stack of the piezoelectric discs to form side terminals (not shown) connecting positive and negative sides of the electrodes, respectively. The side terminals are coupled to leads  72   a  and  72   b  of a connector  7 . The application of voltage to the piezoelectric device  61  through the connector  7  will cause the piezoelectric device  61  to contract or expand in a longitudinal direction thereof. An insulator  63  is disposed within the housing  11  so as to surround the periphery of the piezoelectric device  61  to isolate the piezoelectric device  61  electrically from the housing  11 .  
         [0074]    The connector  7  has, as clearly shown in FIG. 3, a cylindrical connector body  71  welded to an upper open end of the housing  11 . The leads  72   a  and  72   b  extend through vertical holes (not shown) formed in the connector body  71  and connect with a connector terminal or plug  73  disposed on the connector body  71 . The leads  72   a  and  72   b  are hermetically sealed in the connector body  71  for providing for airthghtness and electric insulation. The connector body  71  has a flange  71  on which a retaining nut  74  is disposed around the periphery of the connector body  71 . The retaining nut  74  is, as shown in FIG. 1, screwed into an upper end of the upper housing  2  to install the connector  7  in the upper housing  2 . The plug  73  of the connector  7  is held at an interval a of 5 to 10 mm away from an upper end of the retaining nut  74  so as to expose an upper portion of the connector body  7  outside the retaining nut  74  for facilitating, as will be described later in detail, ease of positioning the actuator  1  within the vertical chamber  21 .  
         [0075]    The piston  62  has a small-diameter rod  64  extending downward, as viewed in FIG. 3, from a lower surface thereof. An annular seat  12  is welded to an inner wall of the housing  11 . A coil spring  65  is disposed between an upper surface of the annular seat  12  and the lower surface of the piston  62  around the rod  64  to urge the piston  62  upward into constant engagement with a lower end of the piezoelectric device  61 . The rod  64  extends slidably through a central hole of the annular seat  12  and reaches a diaphragm  66  mounted on a lower end of the housing  11 . The diaphragm  66  is made of a thin metallic disc in the form of a conical spring and welded at a peripheral edge thereof to a ring formed on a lower end of the annular seat  12 , thereby sealing a lower opening of the housing  11  hermetically.  
         [0076]    The diaphragm  66  is elastically deformed by vertical movement of the rod  64 . Specifically, when energized, the piezoelectric device  61  expands vertically and pushes the piston  62  downward, as viewed in FIG. 3, to project the diaphragm  66  downward through the rod  64 . This causes the large-diameter piston  52  disposed, as shown in FIG. 1, in the upper housing  2  in contact with the diaphragm  66  to move downward. Specifically, a stoke of the piston  62  produced by the expansion of the piezoelectric device  61  is transmitted through the diaphragm  66  to the large-diameter piston  52 . The large-diameter piston  52  is installed coaxially with the vertical chamber  21  of the upper housing  2  so as to be slidable within the upper housing  2 . The downward movement of the large-diameter piston  52  is transformed into a rise in pressure in the pressure chamber  53 , as shown in FIG. 2, defined between the upper and lower housings  2  and  3 , which is, in turn, causes the small-diameter piston  54  to be shifted downward. The small-diameter piston  54  is disposed slidably within a cylindrical chamber  32  formed in the lower housing  3  coaxially with the fuel injector  100 . The vertical movement of the piezoelectric device  61  (i.e., the stoke of the large-diameter piston  52 ) is amplified as a function of a difference in diameter between the large-diameter piston  52  and the small- diameter piston  54 .  
         [0077]    The fabrication of the actuator  1  is accomplished by inserting the annular seat  12  having the diaphragm  66  welded to the bottom thereof into the housing  11  from the lower opening, welding the annular seat  12  to the inner wall of the housing  11 , putting the spring  65 , the piston  62 , and the piezoelectric device  61  covered with the cylindrical insulator  63  into the housing  11  from the upper opening, welding the connector body  71  to the upper end of the housing  11 , and placing this assembly in a mold to form a resinous block of the plug  73  of the connector  7 .  
         [0078]    The installation of the thus fabricated actuator  1  in the upper housing  2  is accomplished by inserting the actuator  1  into the vertical chamber  21  from the upper opening thereof, holding the upper portion of the connector body  71 , as indicated at a in FIG. 3, using a given jig or a tool, and fastening the retaining nut  74 . A shoulder  21   a  is formed on the inner wall of the vertical chamber  21  to define an upper large bore whose inner wall is threaded. The flange  75  of the connector body  71  is seated on the shoulder  21   a  through a ring shim  13 . The shim  13  works to seal a gap between the flange  75  and the shoulder  21   a  and also serves as a spacer for adjusting the vertical position of the actuator  1  within the vertical chamber  21  to regulate :he injection characteristics of the fuel injector  100  (e.g., the amount of fuel to be sprayed) finely.  
         [0079]    The use of the retaining nut  74  to secure the actuator  1  in the upper housing  2  facilitates ease of removal of the actuator  1  after used up and allows the plug  73  of the connector  7  to be adjusted in orientation easily. When the retaining nut  74  is fastened, the gap a of 5-10 mm is kept between the bottom of the plug  73  and the upper end of the retaining nut  74 . The upper portion of the connector body  71  is held by a tool such as a damper or nipper. This avoids application of undesirable torque or unbalanced load to the actuator  1  during installation in the upper housing  2 .  
         [0080]    The piezoelectric device  61  is protected by the housing  11 . The leads  72   a  and  72   b  connected to the piezoelectric device  61  are held by the connector body  71  welded to the housing  11 , thus facilitating ease of handing of the actuator  1  and ensuring high degrees of airtightness and electric insulation of the whole of the actuator  1 . This also enables use of the gap  50  between the inner wall of the vertical chamber  21  and the outer wall of the actuator  1  as a drain passage, thus resulting in a decrease in holes to be drilled in the upper housing  2 . The small-diameter piston  54  is formed coaxially with the upper housing  2 , thus resulting in a decrease in overall length of an eccentric hole (i.e., the vertical chamber  21  and a chamber in which the large-diameter piston  52  is disposed), thereby facilitating ease of machining of the eccentric hole.  
         [0081]    In operation of the fuel injector  1 , when it is required to inject the fuel into the engine  300 , an engine controller (not shown) applies the voltage to the piezoelectric device  61 , so that the piezoelectric device  61  extends and pushes the piston  62 , the diaphragm  66 , and the large-diameter piston  52  downward, as viewed in FIG. 1. The downward movement of the large-diameter piston  52  causes the volume of the pressure chamber  53  to be decreased, thus resulting in a rise in pressure in the pressure chamber.  53  This causes the small-diameter piston  54  to move to push the ball of the three-way valve  51  downward, so that the fuel in the back pressure chamber  44  flows to the drain passage  24 , thereby decreasing the fuel pressure in the back pressure chamber  44 . This causes the needle  41  to be lifted up to open the spray hole  43 , so that the fuel in the fuel sump  42  is sprayed into the engine  300 . When it is required to stop the spray of the fuel, the engine controller drops the voltage applied to the piezoelectric device  61  to contract it, thereby causing the piston  62  to be lifted upward by the spring pressure of the coil spring  65 . The diaphragm  66  and the large-diameter piston  52  are thus moved upward, so that the pressure in the pressure chamber  53  drops, thus causing the small-diameter piston  54  to be lifted upward. The lifting of the small-diameter piston  54  causes the ball of the three-way valve  51  to be moved upward to establish the communication between the high-pressure fuel passage  22  and the back pressure chamber  44 , so that the fuel pressure in the back pressure chamber  44  is elevated to push the needle  41  downward, thereby closing the spray hole  43 .  
         [0082]    [0082]FIG. 4 shows the actuator  1  according to the second embodiment of the invention.  
         [0083]    A bellows  11   b  is coupled with the lower end of the housing  121 . The bellows  11   b  is closed at a lower opening thereof by a diaphragm  11   a . The diaphragm  11   a  is in contact with the bottom of the rod  64  of the piston  62 . The bellows  11   b  has substantially the same length as that of the rod  64  and urges the piston  62  into constant engagement with the bottom of the piezoelectric device  61 . The downward movement of the rod  64  will cause the bellows  11   b  to expand, so that the diaphragm  11   b  moves downward.  
         [0084]    Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here.  
         [0085]    [0085]FIG. 5 shows the fuel injector  100  according to the third embodiment of the invention.  
         [0086]    The connector body  71  is fitted directly in the upper opening of the upper housing  2  with a flange  78  placed on the upper end of the upper housing  2 . A mount plate  76  is secured on the upper end of the upper housing  2  using bolts  16  to nip the flange  78  between itself and the upper end of the upper housing  2  to retain the actuator  1  in the upper housing  2  firmly. The gap a of 5-10 mm is kept, like the first embodiment, between the bottom of the plug  73  and the upper end of the mount plate  76  for avoiding application of undesirable torque or unbalanced load to the actuator  1  during installation in the upper housing  2 .  
         [0087]    Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here.  
         [0088]    [0088]FIG. 6 shows the fuel injector  100  according to the fourth embodiment of the invention.  
         [0089]    The connector body  71  of the connector  7  is machined to an illustrated shape. Specifically, a threaded portion identical with the retaining nut  74  in the first embodiment is formed on the connector body  71  to screw the connector body  71  directly into the upper opening of the upper housing  2 .  
         [0090]    The connector  7  has a plug  73 ′, as shown in FIGS.  7 ( a ) and  7 ( b ), which is fitted on an upper portion of the connector body  71 . The plug  73 ′ has formed on the bottom thereof an annular rail  77 . The annular rail  77  has a plurality of protrusions formed around an outer periphery thereof which establish firm engagement with the connector body  71  when the plug  73 ′ is fitted in the connector body  71  for holding the plug  73 ′ from rotating about the connector body  71 . A positive terminal  74   a  is, as clearly shown in FIG. 7( b ), provided in the center of the annular rail  77 . A negative annular terminal  74   b  is disposed coaxially with the positive terminal  74   a . The annular rail  77  is fitted in an annular groove formed in the upper end of the connector body  71  to establish electric connections of the positive and negative terminals  74   a  and  74   b  with the leads  72   a  and  72   b.    
         [0091]    Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here.  
         [0092]    [0092]FIG. 8 shows the fuel injector  100  according to the fifth embodiment of the invention.  
         [0093]    The upper housing  2  consists of two parts: a head  2   a  and a cylinder  2   b . The head  2   a  has, like the first embodiment, the fuel inlet connector  23  and the fuel outlet connector  25  and also has formed therein a cylindrical chamber  21 ′ and a fuel inlet passage  22 ′. When the head  2   a  is fitted on the cylinder  2   b , the cylindrical chamber  21 ′ and the fuel inlet passage  22 ′ communicate with the vertical chamber  21  and the high-pressure fuel passage  22 , respectively. The connector  7  has the plug  73 ′ identical in structure with the one in the fourth embodiment of FIG. 6. The connector body  71 ′ consists of an upper small-diameter portion and a lower large-diameter portion. The lower large-diameter portion is fitted in the cylindrical chamber  21 ′ in engagement with an upper inner wall of the cylindrical chamber  21 ′ through the shim  13 .  
         [0094]    The installation of the actuator  1  in the upper housing  2  is initiated without fitting the plug  73 ′ on the connector body  71 ′. Specifically, the actuator  1  is first inserted into the cylinder  2   b  of the upper housing  2 , after which the head  2   a  is coupled to the connector body  71 ′ in a screw fashion. The head  2   a  has formed in a bottom thereof an annular chamber  26  which has a threaded inner wall. The cylinder  2   b  has an tipper flange whose peripheral wall is threaded and engages the inner wall of the annular chamber  26 . The connector body  71 ′ is, as described above, retained in the cylindrical chamber  21 ′ through the shim  13 . Finally, the plug  72 ′ is fitted on the connector body  71 ′ in a desired orientation.  
         [0095]    The cylindrical chamber  21 ′ is formed in the head  2   a  coaxially with a vertical center line of the head  2   a , thereby resulting in a decreased in length of an eccentric hole (i.e., the vertical chamber  21  and the chamber in which the large-diameter piston  52  is disposed), thereby facilitating ease of machining of the eccentric hole.  
         [0096]    Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here.  
         [0097]    FIGS.  9  to  12 ( c ) shows the fuel injector  100  according to the sixth embodiment of the invention which is a modification of the fifth embodiment.  
         [0098]    The actuator  1 , as clearly shown in FIG. 10, includes a piston  62  coupled to the lower end of the piezoelectric device  61 , a metallic hollow cylindrical housing  11 , and an extensible member  14  coupled to the lower end of the housing  11 . The piston  62  is disposed in the extensible member  14  in alignment with the piezoelectric device  61  installed within the housing  11 . A head plate  81  is joined to a head  144  of the extensible member  14 . A plate  82  is joined to the upper end of the housing  11  to seal it hermetically.  
         [0099]    The piezoelectric device  61  may be used in any of the above described first to fifth embodiments. The piezoelectric device  61 , as clearly shown in FIGS.  11  to  12 ( c ), consists of a stack of piezoelectric layers  61 A and positive and negative inner electrodes  621  and  622  which are disposed alternately between the piezoelectric layers  61 A, respectively. Each of the positive inner electrodes  621  extends at one side thereof to a side: surface  601  of one of the piezoelectric layers  61 A, while each of the negative inner electrodes  622  extends at one side thereof to a side surface  602  opposite to the side surface  601 . Specifically, the positive and negative electrodes  621  and  622  are exposed to opposite side walls of the piezoelectric device  61 , respectively. The positive and negative electrodes  621  and  622  are coupled at the exposed sides thereof to each other through vertically extending side electrodes  631  and  632 . The side electrodes  631  and  632  are each made by baking a silver paste containing 97% of Ag and 3% of glass frit.  
         [0100]    Outer electrodes  643  are, as shown in FIG. 10, coupled to the side electrodes  631  and  632  using a conductive adhesive. The outer electrodes  643  are each made of a 18-8 stainless steel. The conductive adhesive is made from a resinous silver containing 80% of Ag and 20% of epoxide.  
         [0101]    The piezoelectric device  61 , as clearly shown in FIG. 11, consists essentially of three parts: a drive portion  611  ranging over a central portion of the piezoelectric device  61  in a lengthwise direction thereof, buffer portions  612  located on both sides of the drive portion  611 , and dummy portions  613  located at the ends of the piezoelectric device  61 .  
         [0102]    The piezoelectric device  61  is produced using known green sheets. The green sheet is made in the following manner. First, powders of lead oxide, zirconium oxide, titanium oxide, niobium oxide, and strontium carbonate that are main components of a piezoelectric material are prepared at a given rate. For compensating for a loss of the lead caused by evaporation in a subsequent process, it is preferably enriched by about 1 to 2% in a stoichometric ratio. Next, the powders are mixed and dried in a mixing chamber and then baked temporarily at 800 to 950° C. To this mixture, demineralized water and dispersant is added to produce slurry. The slurry is subjected to the wet grinding using a mill, dried, and then decreased to remove binder, after which it is mixed with solvent, binder, plasticizer, and dispersant in a ball mill. This is then agitated using an agitator within a vacuum device to be degassed and adjusted in viscosity.  
         [0103]    Next, the slurry is, shaped using a doctor blade device into a layer of a constant thickness to produce a green sheet. The green sheet withdraws from the doctor blade device is cut in a cutting machine or a press machine to a rectangular shape. Note that the drive portion  611 , the buffer portions  612 , and the dummy portions  613  are made of the same green sheets.  
         [0104]    An Ag/Pd paste containing of silver and palladium of 7:3 ratio is applied to one surface of the rectangular green sheet to print an electrode pattern (i.e., the inner electrode  621  or  622  in FIG. 12( a ) or  12 ( b )), using screen printing techniques to form each of the piezoelectric layers  61 A.  
         [0105]    Each of the inner electrodes  621  and  622 , as can be seen from FIGS.  12 ( a ) and  12 ( b ), occupies one surface of the piezoelectric layer  61 A other than a side portion  619 . Specifically, each of the inner electrodes  621  and  622  of a stack of the piezoelectric layers  61 A reaches either of the side surfaces  601  and  602 . The inner electrodes  612  and  622  may alternatively be made of copper, nickel, platinum, or silver or a mixture thereof.  
         [0106]    The piezoelectric layers  61 A of a number required to provide a desired amount of expansion of the whole of the drive portions  611  and the buffer portions  612  are prepared in the above manner. Additionally, the rectangular green sheets on which no electrodes are formed are also prepared which are employed as piezoelectric layers  61 B, as will be described below in detail, in forming the buffer portions  612  and the dummy portions  613 .  
         [0107]    The piezoelectric layers  61 A and  61 B are stacked up in the following manner to produce the piezoelectric device  61 . FIG. 12( c ) illustrates only the drive portion  611  for convenience. The drive portion  611  is made by stacking the piezoelectric layers  61 A so that the electrode-nonformed side portions  619  are alternately oriented in opposite directions. Half of the inner electrodes  621  of the piezoelectric layers  61 A exposed to the side surface  601 , as shown in FIG. 11, are used as positive electrodes, while the remainders exposed to the side surface  602  are used as negative electrodes.  
         [0108]    The buffer portions  612  are each made by stacking the piezoelectric layers  61 A and the electrode-nonformed piezoelectric layers  61 B alternately. The dummy portions  613  are each made by stacking only the piezoelectric layers  61 B. In this manner, a stack of the piezoelectric layers,  61 A and  61 B, as shown in FIG. 11, is produced.  
         [0109]    The thus produced piezoelectric stack is thermo-compressed using, for example, a hot-water rubber press, after which it is degreased at 400 to 700° C. in an electric furnace and baked at 900 to 1200° C.  
         [0110]    An Ag paste is applied to the side surfaces  601  and  602  of the piezoelectric stack and baked to form the side electrodes  631  and  632  which lead electrically to the inner electrodes  621  and  622 , respectively. The side electrodes  631  and  632  may alternatively be made of an Ag/Pd paste or using copper, nickel, platinum, or silver/palladium.  
         [0111]    External electrodes  634  are, as shown in FIG. 10, joined to the side electrodes  631  and  632  using a conductive adhesive. Next, a dc voltage is applied to the inner electrodes  621  and  622  through the external electrodes  634  to polarize a stack of the piezoelectric layers  61 A to produce the piezoelectric device  61 . The external electrodes  634  may alternatively be soldered or brazed to the side electrodes  631  and  632  or bonded directly to the inner electrodes  621  and  622 , respectively, without using the side electrodes  631  and  632 . The external electrodes  634  are preferably formed by a waved strip made of a metallic foil or a waved metallic wire which may be sheathed.  
         [0112]    The dummy portions  613  are, as described above, made up of the piezoelectric layers  61 B which are identical in material with the piezoelectric layers  61 A, thus resulting in a decrease in manufacturing cost of the piezoelectric device  61 .  
         [0113]    Finally, the thus produced piezoelectric device  61  is disposed in the housing  11  and compressed through the piston  62  and the head plate  81  by the reactive force from the extensible member  14 .  
         [0114]    The piston  62 , as shown in FIG. 10, consists of a base  62   b  substantially identical in sectional area with the piezoelectric device  61  and a rod  62   a . The rod  62  has an outer diameter of 6 mm. The piston  62  is made of a quenched stainless steel. To the end of the rod  62 , the head plate  81  is joined which is made of a disc member having an outer diameter B of 10.2 mm.  
         [0115]    The housing  11  made of a stainless steel pipe which is 0.3 mm in thickness and 10.2 mm in outer diameter A. The extensible member  14  is implemented by a bellows which is made of a stainless steel having a thickness of 0.17 mm and consists of large-diameter portions  141  and small-diameter portions  142  arrayed alternately. The large-diameter portions  141  have a diameter C of 9.5 mm. The small-diameter portions  142  have a diameter of 6.5 mm. The bellows also includes the rear end  143  joined to the end of the housing  11  and the head  144  joined to the head plate  81  The rear end  143  has substantially the same diameter as the diameter A of the housing  11 . The head  144  has substantially the same diameter as the diameter B of the head plate  81 .  
         [0116]    The extensible member  14  is joined at the rear end  143  thereof to the housing  11  and at the head  144  to the head plate  81  hermetically. The upper plate  82  is installed on the upper end of the piezoelectric device  61  to seal the upper opening of the housing  11  hermetically. The upper plate  82  has formed therein holes  821  through which the external electrodes  634  extend outside the housing  11 . The upper plate  82  has an outer diameter equal to the outer diameter A of the housing  11 . Sealing members  822  are fitted in the through holes  821  to seal gaps between the external electrodes  634  and inner walls of the holes  821 , respectively.  
         [0117]    The minimum outer diameter A of the housing  11 , the maximum outer diameter B of the head plate  81  joined to the rod  62   a  of the piston  62 , and the maximum outer diameter C of the extensible member  14  meet the relations of A&gt;C and B&gt;C. Specifically, the extensible member  14  is smaller in diameter than the housing  11  and the head plate  81  installed on both sides of the extensible member  14 , thereby avoiding physical contact with the inner wall of the vertical chamber  21  of the fuel injector  100  during operation of the actuator  1 , thus resulting in an increase in lifespan of the extensible member  14 . Note that at least one of the relations of A&gt;C and B&gt;C may be satisfied.  
         [0118]    If a maximum clearance between an inner wall of the housing  11  and the base  62   b  of the piston  62  and a minimum clearance between an inner wall of the extensible member  42  and the rod  62   a  of the piston  62  are defined as d and e, then d&lt;e. This causes the piezoelectric device  61  and the base  62   b  of the piston  62  to hit on the inner wall of the housing  11  when the piston  62  deflects horizontally during the operation of the piezoelectric device  61 , thereby avoiding physical contact of the piston rod  62   a  with the extensible member  14  thus resulting in an increase in lifespan of the extensible member  14 .  
         [0119]    The installation of the actuator  1  assembled in the above manner in the fuel injector  100  of FIG. 9 is accomplished by inserting the actuator  1  into the vertical chamber  21  while keeping the gap  50  through which the fuel flows and securing the housing  11  in the same manner as that in the fifth embodiment of FIG. 8 so that the head plate  81  may move in a lengthwise direction of the actuator  1 .  
         [0120]    As apparent from the above discussion, the actuator  1  of this embodiment has the extensible member  14  joined to the end of the housing  11  in alignment therewith so as to absorb a lengthwise movement of the piezoelectric device  61  and the piston  62 , thus eliminating the need for the housing  11  to have an extensible portion in itself. This allows the housing  11  to be minimized in thickness, so that the outer diameter of the housing  11  can be decreased, thereby allowing the size of the actuator  1  to be reduced without sacrificing the performance of the piezoelectric device  61 .  
         [0121]    The piezoelectric Crevice  61  is not limited in cross section to a square shape and may alternatively be made up of barrel-shaped piezoelectric layers  61 A and  61 B, as shown in FIG. 13( a ) or octagonal piezoelectric layers  61 A and  61 B, as shown in FIG. 13( b ).  
         [0122]    [0122]FIG. 14 shows the actuator  1  according to the seventh embodiment of the invention which is a modification of the sixth embodiment. Specifically, the piston  62  is reverse in location on the piezoelectric device  61  to that in the sixth embodiment.  
         [0123]    The piston  62  is installed on the upper end of the piezoelectric device  61 . The extensible member  14  is joined at the rear end  143  to the upper end of the housing  11  and at the head  144  to the plate  83 . The plate  83  has formed therein holes  831  through which the external electrodes  634  extend. Sealing members  832  are fitted in the holes  831  to seal gaps between inner wall of the holes  831  and the external electrodes  634  hermetically. The external electrodes  634  are welded to wires  63 A which extend outside the plate  83  and to conductive members  63 B which extend downward, as viewed in the drawing, and connect with the side electrodes of the piezoelectric device  61 . The external electrodes  634  may alternatively be soldered or brazed to the wires  63 A and the conductive members  63 B or staked to establish electric communications therewith. Each of the conductive members  631  is joined to an overall length of one of the side electrodes of the piezoelectric device  61 .  
         [0124]    The housing  11  is made of a stainless steel and stores therein the piezoelectric device  61 . The housing  11  is identical in diameter with the one in the first embodiment. The extensible member  14  is made of a stainless steel and identical in structure with the one in the sixth embodiment.  
         [0125]    A lower plate  84  having the same diameter as that of the housing  11  is joined to a lower end of the housing  11 .  
         [0126]    The actuator  1  of this embodiment is installed in the fuel injector  100  identical in structure with the one in the sixth embodiment. Specifically, the piezoelectric device  61  is so secured in the upper housing  2  that the housing  11  can be moved vertically by the activation of the piezoelectric device  61 . Other arrangements are identical with those in the sixth embodiment, and explanation thereof in detail will be omitted here.  
         [0127]    [0127]FIG. 15 shows the actuator  1  according to the eighth embodiment of the invention which is different from the sixth embodiment only in that a diaphragm  85  is used. Other arrangements are identical, and explanation thereof in detail will be omitted here.  
         [0128]    The diaphragm  85  is joined to the head  144  of the extensible member  14  in physical contact with the end of the rod  62   a  of the piston  62  so that a central portion of the diaphragm  85  may be deformed vertically by a vertical movement of the piston  62 . The diaphragm  85  has the sane diameter as that of the head  144  and is made of a metallic disc spring.  
         [0129]    [0129]FIG. 16 shows the actuator  1  according to the ninth embodiment of the invention.  
         [0130]    The piston  62  has the rod  62   a  which is shorter than that in the eighth embodiment of FIG. 15 and connects at an end thereof to a central portion of the diaphragm  11   c . The diaphragm  11   c  is formed integrally with a lower end of the housing  11 . Other arrangements are identical with those in the eighth embodiment, and explanation thereof in detail will be emitted here.  
         [0131]    [0131]FIG. 17 shows the actuator  1  according to the tenth embodiment of the invention which is different from the above embodiments in that the movement of the piezoelectric device  61  is transmitted directly to the large-diameter piston  2  without use of a piston.  
         [0132]    The housing  11 ′ is made of a metallic cylindrical bellows consisting of large-diameter portions  111  and small-diameter portions  112  arrayed alternately. The housing  11 ′ is closed by an upper plate  86  and a lower plate  87  hermetically. The upper plate  86  has formed therein holes  861  through which the external electrodes  634  extend outside the housing  11 ′. The sealing members  832  are fitted ill the holes  861  to seal gaps between inner wall of the holes  861  and the external electrodes  634  hermetically.  
         [0133]    The piezoelectric device  61  is disposed within the housing  11 ′ and compressed elastically by the upper and lower plates  86  and  87  through the housing  11 ′. When energized, the piezoelectric device  61  expands vertically along with expansion of the housing  11 ′ to push the lower plate  87  downward, as viewed in the drawing. The structure of this embodiment results in decreases in parts to be assembled and parts-joining process, thereby simplifying the parts management and production processes.  
         [0134]    The housing  11 ′ may alternatively be formed integrally, as shown by circles in FIGS.  18 ( a ) and  18 ( b ), at a lower end or an upper end thereof with the lower plate  87  or the upper plate  86 . Additionally, the lower plate  87  may be replaced, as shown in FIG. 18( c ) or  18 ( d ), with a diaphragm  85 . In FIG. 18( c ), the housing  11 ′ is formed integrally at the lower end thereof with the diaphragm  85 . In FIG. 18( d ), the housing  11 ′ is formed integrally at the upper end thereof with the upper plate  86 . The diaphragm  85  is joined to the lower end of the housing  11 ′.  
         [0135]    It is important for the structure in which the piezoelectric device  61  is disposed in the housing  11  to ensure the airtightness. However, as the number of parts making up the actuator  1  increases, the possibility that failures in joining the parts increase, resulting in leakage of air will increases, and the manufacturing costs will also increase. In order to avoid these problems, it is advisable that at least two of the housing  11 ′, the upper plate  86 , and the lower plate  87  be formed integrally with each other. This results in a decrease in joint of the actuator  1  thereby assuring the actuator remains highly airtight and also decreasing the manufacturing costs.  
         [0136]    Similarly, the actuator  1  of the sixth embodiment in FIG. 10 may also have parts formed integrally with each other, as shown in FIGS.  19 ( a ) to  19 ( f ). In FIG. 19( a ), the upper plate  82  is formed integrally with the housing  11 . In FIG. 19( b ), the rear end  143  of the extensible member  14  is formed integrally with the lower end of the housing  11 . In FIG. 19( c ), the head plate  81  is formed integrally with the extensible member  14 . At least two of these parts, as clearly shown by circles in FIGS.  19 ( d ) to  19 ( f ), may also be formed integrally with each other.  
         [0137]    Additionally, the actuator  1  of the eighth embodiment in FIG. 15 may also have parts formed integrally with each other, as shown in FIGS.  20 ( a ) to  20 ( f ). In FIG. 20( a ), the upper plate  82  is formed integrally with the housing  11 . In FIG. 20( b ), the rear end  143  of the extensible member  14  is formed integrally with the lower end of the housing  11 . In FIG. 20( c ), the diaphragm  85  is formed integrally with the extensible member  14 . At least two of these parts, as clearly shown by circles in FIGS.  20 ( d ) to  20 ( F ), may also be formed integrally with each other.  
         [0138]    [0138]FIG. 21 shows the fuel injector  100  according to the eleventh embodiment of the invention.  
         [0139]    The piezoelectric crevice  61  is disposed directly within the vertical chamber  21  of the upper housing  2  without use of the housing  11 . The connector body  71  is secured on the upper end of the piezoelectric device  61 . The installation of the piezoelectric device  61  in the vertical chamber  21  of the upper housing  2  is accomplished, similar to the first embodiment, by inserting the piezoelectric device  61  to which the connector  7  is joined into the vertical chamber  21  from the upper opening of the upper housing  2 , holding an upper end portion of the connector body  71 , as indicated by a, using a given tool, and fastening the retaining nut  74  to nip the flange  75  of the connector body  71  between the retaining nut  74  and the shoulder  21   a  of the upper housing  2  through the shim  13 . The shim  13  serves as a spacer for adjusting a vertical location of the piezoelectric device  61  within the upper housing  2 . This avoids application of undesirable torque or unbalanced load to the actuator  1  during installation in the upper housing  2 .  
         [0140]    It is impossible for this structure to define a drain passage between the outer wall of the piezoelectric device  61  and the inner wall of the vertical chamber  21 . A fuel passage (not shown) is, therefore, formed directly in the upper housing  2  which leads to the three-way valve  51 . The vertical displacement of the piezoelectric device  61  is transmitted to the large-diameter piston  52  through the rod  64 . Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here.  
         [0141]    In a case where there is no need for replacing or adjusting the piezoelectric device  61 , and it is required only to withdraw the piezoelectric device  61  from the fuel injector  100  for discarding it, the connector body  71 , as shown in FIG. 22 as the twelfth embodiment, may be secured in the upper housing  2  by staking, welding, or bonding a flange  711  formed on a middle portion of the connector body  71  in the upper end of the upper housing  2 . It is advisable that the strength of a joint of the connector body  71  and the upper housing  2  be lower than that of another portion for facilitating ease of removal of the piezoelectric device  61 .  
         [0142]    [0142]FIG. 23 shows the fuel injector  100  according to the thirteenth embodiment of the invention.  
         [0143]    The upper housing  2  has at least one fragile portion  27  formed near the lower end of the piezoelectric device  61  for facilitating ease of cutting or breaking up the upper housing  2  in order to withdraw the piezoelectric device  61 . The fragile portion  27  is defined by an annular groove formed in a peripheral outer wall of the upper housing  2 .  
         [0144]    While the present invention has been disclosed in terms of the preferred embodiments in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments witch can be embodied without departing from the principle of the invention as set forth in the appended claims. For example, the three-way valve  51  is used to open and close the injection nozzle  4 , however, the invention is not limited to the same. Another known mechanism may be used to open and close the injection nozzle  4 . Further, the actuator  1  is implemented by a piezoelectric device, however, another element may be used as long as it is so constructed as to be expand and contract in response to input of an electric signal.