Abstract:
A spark plug fabricating method and a spark plug fabricated by the same are provided. The spark plug is equipped with a piezoelectric sensor working to measure the pressure of combustion of fuel in an engine and required to apply a given preload to the piezoelectric sensor at all the time. The piezoelectric sensor is disposed within a holder and retained between the holder and a porcelain insulator. The fabrication method includes the steps of pressing the holder to apply a required preload to the piezoelectric sensor in a longitudinal direction of the porcelain insulator and joining the holder to a metal shell while maintaining the preload applied to the piezoelectric sensor as it is.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates generally to a method of fabricating a spark plug equipped with a piezoelectric sensor working to measure the pressure of combustion of fuel in internal combustion engines and a spark plug fabricated by the same. 
     2. Background Art 
     Japanese Utility Model First Publication No. 58-23194 discloses a spark plug with a piezoelectric sensor installed on the periphery of a porcelain insulator within a housing. An annular holder or nut is screwed into the housing to apply a preload to the piezoelectric sensor which is required for the piezoelectric sensor to transform mechanical deformation thereof into an electric voltage signal. 
     The above type of spark plug, however, encounters the drawback in that the nut screwed into the housing abuts directly on the piezoelectric sensor, thus resulting in distortion of the piezoelectric sensor. In the worst case, the breakage of the piezoelectric sensor results. 
     It is usually necessary to place the piezoelectric sensor in a waterproof environmental condition. It is, however, difficult to avoid entrance of water from between the nut and the housing completely. 
     SUMMARY OF THE INVENTION 
     It is therefore a principal object of the invention to avoid the disadvantages of the prior art. 
     It is another object of the invention to provide a fabrication method of spark plugs for avoiding breakage of a piezoelectric sensor subjected to a preload. 
     It is a further object of the invention to provide a fabrication method of spark plugs for avoiding entrance of water into a piezoelectric sensor. 
     It is a still further object of the invention to provide a spark plug made by either of the above fabrication methods. 
     According to one aspect of the invention, there is provided a spark plug fabricating method which comprises the steps of: (a) preparing a press jig; (b) preparing a spark plug assembly made up of a hollow cylindrical porcelain insulator having a given length in which a center electrode is disposed, a housing in which the center electrode is retained through the porcelain insulator and which has a ground electrode installed thereon, a piezoelectric sensor designed to be responsive to pressure of combustion of fuel within an engine transmitted through the porcelain insulator to provide a signal indicative thereof, and a holder put on the housing for holding the piezoelectric sensor mechanically; (c) placing the spark plug assembly within the press jig; (d) pressing the holder to apply a given preload to the piezoelectric sensor in a longitudinal direction of the porcelain insulator; and (e) joining the holder to the housing while maintaining the preload applied to the piezoelectric sensor to fabricate a spark plug completely. 
     In the preferred mode of the invention, the press jig includes a press block having opposed ends one of which abuts on the holder and a screw which abuts on the other end of the press block and is rotated to press the holder through the press block. 
     The holder may be jointed to the housing by at least one of staking and welding. 
     According to the second aspect of the invention, there is provided a spark plug which may be employed in a gas engine of a generator in cogeneration systems or automotive internal combustion engines. The spark plug comprises: (a) a center electrode; (b) a porcelain insulator having a first and a second length, the first length having the center electrode retained therein; (c) a hollow cylindrical housing in which the center electrode is retained through the porcelain insulator and which has a ground electrode installed thereon; (d) a piezoelectric sensor disposed on an outer periphery of the second length of the porcelain insulator, the piezoelectric sensor being responsive to pressure of combustion of fuel within an engine transmitted through the porcelain insulator to provide a signal indicative thereof; and (e) a holder welded to the housing so as to apply a given preload to the piezoelectric sensor at all times. 
     In the preferred mode of the invention, the holder is made of a hollow cylinder one end of which is welded to an overall circumference of the housing. A sealing member is disposed between the other end of the hollow cylinder and the porcelain insulator. The piezoelectric sensor is disposed within a chamber formed between the holder and the porcelain insulator. 
     The holder may also include an annular disc. The hollow cylinder is welded at the one end thereof to the overall circumference of the housing and at the other end to the whole of an outer circumference of the annular disc. The sealing member may be disposed between an inner periphery of the annular disc and the porcelain insulator. The piezoelectric sensor is disposed within a chamber defined by the hollow cylinder, the annular disc, and the porcelain insulator. 
     A chamber is defined within which the piezoelectric sensor is disposed and filled with a resinous material. 
     A shoulder may be formed on an outer periphery of the porcelain insulator. A seat member may be disposed between the shoulder and the piezoelectric sensor in direct contact with the shoulder for transmission of the pressure of combustion of fuel to the piezoelectric sensor through the seat member. 
     The housing may have an end portion which is far from the center electrode and staked on the porcelain insulator to retain the porcelain insulator within the housing. The seat member may alternatively be disposed between the staked end portion of the housing and the piezoelectric sensor in direct contact with the staked end portion for transmission of the pressure of combustion of fuel to the piezoelectric sensor through the staked end portion and the seat member. 
     According to the third aspect of the invention, there is provided a spark plug which comprises: (a) a center electrode; (b) a porcelain insulator having a first and a second length, the first length having the center electrode retained therein; (c) a hollow cylindrical housing in which the center electrode is retained through the porcelain insulator and which has a ground electrode installed thereon; (d) a piezoelectric sensor disposed on an outer periphery of the second length of the porcelain insulator, the piezoelectric sensor being responsive to pressure of combustion of fuel within an engine transmitted through the porcelain insulator to provide a signal indicative thereof; and (e) a holder staked on the housing so as to apply a given preload to the piezoelectric sensor at all times. 
     In the preferred mode of the invention, a chamber within which the piezoelectric sensor is disposed is filled with a resinous material. 
     The spark plug further comprises a shoulder formed on an outer periphery of the porcelain insulator and a seat member disposed between the shoulder and the piezoelectric sensor in direct contact with the shoulder for transmission of the pressure of combustion of fuel to the piezoelectric sensor through the seat member. 
     The housing may have an end portion which is far from the center electrode and staked on the porcelain insulator to retain the porcelain insulator within the housing. The seat member may alternatively be disposed between the staked end portion of the housing and the piezoelectric sensor in direct contact with the staked end portion for transmission of the pressure of combustion of fuel to the piezoelectric sensor through the staked end portion and the seat member. 
    
    
     BRIEF DESPCRIPTION OF THE DRAWINGS 
     The present invention will be understood more fully from the detailed description given 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. 
     In the drawings: 
     FIG. 1 is a longitudinal partial sectional view which shows a spark plug according to the first embodiment of the invention; 
     FIG. 2 is a top view which shows a seat for use in retaining a piezoelectric sensor; 
     FIG. 3 is a longitudinal partial sectional view of FIG. 2; 
     FIG. 4 is a plan view which shows a press jig use to provide a preload to a piezoelectric sensor; 
     FIG. 5 is a transverse sectional view taken along the line A—A in FIG. 4; 
     FIG. 6 is a longitudinal sectional view taken along the line B—B of FIG. 5; 
     FIG. 7 is a longitudinal partial sectional view which shows a spark plug according to the second embodiment of the invention; 
     FIG. 8 is a longitudinal partial sectional view which shows a spark plug according to the third embodiment of the invention; 
     FIG. 9 is a longitudinal partial sectional view which shows a spark plug according to the fourth embodiment of the invention; 
     FIG. 10 is a partially enlarged view which shows a shoulder formed on a porcelain insulator as illustrated in FIG. 9; 
     FIG. 11 is a longitudinal partial sectional view which shows a spark plug according to the fifth embodiment of the invention; 
     FIG. 12 is a longitudinal partial sectional view which shows a spark plug according to the sixth embodiment of the invention; and 
     FIGS. 13,  14 ,  15 ,  16 , and  17  are partially sectional views which show modified structures of a joint between a seat for use in retaining a piezoelectric sensor and a porcelain insulator according to the seventh to eleventh embodiments of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, particularly to FIG. 1, there is shown a spark plug  200  according to the invention which may be employed in a gas engine of a generator in cogeneration systems or automotive internal combustion engines. 
     The spark plug  200  has a hollow cylindrical housing or shell  10  made of a conductive steel material such as a low-carbon steel. The metal shell  10  has a flange  12  and a thread  11  for mounting the spark plug  200  in a cylinder head  1  of the engine. The mounting of the spark plug  200  is achieved by inserting the thread  11  of the metal shell  10  into a screw hole formed in the cylinder head  1  through a packing  13  disposed between the flange  12  and the thread  11 . The cylinder head  1  defines a combustion chamber  1   a  within the engine. 
     The spark plug  200  also includes a cylindrical porcelain insulator  20 , a center electrode  30 , a ground electrodes  50 , and a stem  40  on which a terminal  41  is installed. The porcelain insulator  20  is made of an alumina ceramic (Al 2 O 3 ) and retained within the metal shell  10 . Specifically, the porcelain insulator  20  is inserted into the metal shell  10  on the side of the center electrode  30  and exposed outside the metal shell  10  on the side of the stem  40 . the metal shell  10  has an elastically deformable annular portion  14  which is pressed or staked inwardly to hold the porcelain insulator  20  in the metal shell  10  firmly. The metal shell  10  also has a flange  15  continuing from the annular portion  14 . 
     The porcelain insulator  20  has formed therein a longitudinal central hole within which the center electrode  40  and the stem  40  are installed in an electrically insulating fashion. The center electrode  30  consists of a core portion made of a metallic material such as Cu having a higher thermal conductivity and an external portion made of a metallic material such as an Ni-based alloy having higher thermal and corrosion resistances. The center electrode  30  has a tip extending outside the top of the porcelain insulator  20 . The stem  40  is made of a metallic material and has the terminal  40  extending outside the porcelain insulator  20 . 
     The ground electrode  50  is made of an Ni-based alloy bar whose main component is nickel and welded directly to the end of the metal shell  10 . The ground electrode  50  has a tip portion bent at 90° to define a gap (usually called spark gap) between itself and the tip of the center electrode  30 . 
     A seat  60  is put around the porcelain insulator  20  above the annular portion  14  of the metal shell  10 . The seat  60  is made of metal for eliminating high-frequency noises arising from the stem  40 . The seat  60  is, as clearly shown in FIG. 3, made up of a hollow cylinder  61  and a flange  62  extending from a lower end of the cylinder  61 , as viewed in FIG. 1, outward. The flange  62  abuts on the annular portion  14  of the metal shell  10 . An insulator  63  is attached to an outer periphery of the cylinder  61  for establishing insulation of a piezoelectric sensor and an electrode  71 , as will be described later in detail, from the seat  60 . The insulator  63  may be implemented by a heat-shrinkable tubing made of tetrafluoroethylene (PTFE). 
     Referring back to FIG. 1, the electrode  71  made of a ring member is interposed between an upper piezoelectric device  70  and a lower piezoelectric device  70  around the cylinder  61  of the seat  60 . Washers  45  having rough surfaces are laid on the upper and lower piezoelectric devices  70  for avoiding breakage of the piezoelectric devices  70 . The upper and lower piezoelectric devices  70  and the electrode  71  constitutes the piezoelectric sensor. Each of the piezoelectric devices  70  is formed by a ring-shaped member made of lead titanate or lead zirconate titanate and works to produce an electric charge which changes as a function of an applied mechanical stress. The piezoelectric sensor is used to measure the pressure of combustion of fuel within the combustion chamber  1   a  of the engine. 
     The spark plug  200  also has a metallic holder  80  put on a portion of the metal shell  10  extending outside the cylinder head  1  of the engine. The holder  80  defines between itself and the porcelain insulator  20  an annular chamber within which the seat  60 , the piezoelectric devices  70 , and the electrode  71  are disposed. The holder  80  is made up of a hollow cylinder  81  and an annular plate or head  82  extending from an end of the cylinder  81  inwardly in contact with the piezoelectric devices  70 . The holder  80  is welded at an open end thereof to the overall circumference of the flange  12  of the metal shell  10 . The head  82  has formed in an inner wall thereof a groove in which a rubber O-ring  90  is disposed to establish a liquid-tight seal between the holder  80  and the porcelain insulator  20 . The holder  80  also has formed on an outer periphery thereof a hexagon head  83 , as clearly shown in FIG. 2, for facilitating ease of installation of the spark plug  200  in the cylinder head  1 . 
     The joining of the holder  80  and the metal shell  10  that is one of fabrication processes of the spark plug  200  will be described in detail with reference to FIGS. 4 to  6 . 
     The holder  80  is pressed using a press jig  100  in a lengthwise direction of the metal shell  10  to provide a preload to the piezoelectric devices  70  and welded to the flange  12  of the metal shell  10 . The press jig  100 , as clearly shown in FIG. 6, consists of a base  101 , an inner press block  102 , a cover  103 , and a bolt  104 . 
     The base  101  is made of a hollow cylinder which has formed in an upper end, as viewed in FIG. 6, a first hole  101   a  having an inner diameter greater than a maximum diameter of the spark plug  200 . The base  101  also has formed in the other end thereof a second hole  101   b  which is aligned with the first hole  101   a  and has an inner diameter slightly greater than an outer diameter of the thread  11  of the spark plug  200 . The spark plug  200  is put in the press jig  100  from the first hole  101   a . The thread  11  is inserted into the second hole  101   b , thereby holding the spark plug  200  in the base  101 . The base  101  also has four through holes  101   c  formed in a peripheral wall thereof at regular intervals. Each hole  101   c  faces the flange  12  of the metal shell  10  to which the holder  80  is welded. The base  101  has a thread  101   d  cut in a boss formed on the end of the cylinder  101 . 
     The press block  102  has a bottom and is disposed slidably within the first hole  10   a  in direct contact of an open end thereof with the head  82  of the holder  80 . The cover  103  has formed thereon an outer thread  103   a  for establishing screw engagement with the thread  101   d  of the base  101  and formed in a bottom thereof an inner thread  103   b  for establishing screw engagement with the bolt  104 . 
     The installation of the holder  80  on the metal shell  10  is accomplished by the following process. 
     First, after completion of assembling of all parts of the spark plug  200  except the packing  13 , the spark plug  200  is inserted into the press jig  100  from the first hole  101   a . The thread  11  is screwed into the second hole  101   b  to hold the spark plug  200  within the base  101 . 
     Next, the press block  102  is inserted from the first hole  101   a . After abutment of the press block  102  against the head  82  of the holder  80 , the cover  103  with the bolt  104  is attached to the base  101  in engagement of the thread  103   a  with the thread  101   d  of the base  101 . 
     The bolt  104  is tightened with a given torque to press the holder  80  through the press block  102  in the lengthwise direction of the porcelain insulator  20 , thereby providing a preload (i.e., a compressive pressure) to the piezoelectric devices  70 . 
     While keeping the preload, a laser beam is radiated through the through holes  101   c  to a plurality of portions of the holder  80 , thereby joining the open end of the holder  80  to the flange  12  of the metal shell  10  temporarily. 
     After completion of welding of the holder  80  to the metal shell  10 , the spark plug  200  is removed from the press jig  100 . A laser beam is radiated to the overall circumference of the cylinder  81  of the holder  80  to weld the holder  80  to the flange  12  of the metal shell  10  completely. Other fabrication processes of the spark plug  200  are not major part of the present invention and well known in the art. Explanation thereof in detail will, therefore, be omitted here. 
     In use, the spark plug  200  thus fabricated is installed in the cylinder head  1  through the packing  13 . When an air-fuel mixture is burned in the combustion chamber  1   a  of the engine, it will cause the combustion pressure to act on the porcelain insulator  20  to press it upward, as viewed in FIG.  1 . This upward pressure is transmitted or applied to the piezoelectric devices  70  through the annular portion  14  of the metal shell  10  and the flange  62  of the seat  60 . The piezoelectric devices  70  produces an electric signal as a function of the pressure applied thereto (i.e., the combustion pressure). 
     As apparent from the above discussion, the preload applied to the piezoelectric devices  70  may be adjusted finely by controlling the pressure tightening the bolt  104  of the press jig  100 . The use of the press block  102  disposed between the bolt  104  and the holder  80  serves to avoid direct transmission of the torque of the bolt  104  to the holder  80 , thereby minimizing twisting of the holder  80  arising from the rotation of the bolt  104  to avoid undesirable breakage of the piezoelectric devices  70 . 
     Further, the holder  80  is welded directly to the metal shell  10  without twisting the holder  80 , so that no torque is transmitted to the piezoelectric devices  70  during installation of the holder  80 . 
     The overall circumference of the cylinder  81  of the holder  80  is welded to the flange  12  of the metal shell  10 , thereby establishing a liquid-tight seal therebetween. Additionally, a gap between the inner wall of the head  82  of the holder  80  and the porcelain insulator  20  is sealed by the O-ring  90 , thus avoiding the entrance of water to the piezoelectric devices  70  completely. 
     FIG. 7 shows a spark plug  200  according to the second embodiment of the invention which is different from the one shown in FIG. 1 only in a location where the holder  80  is welded to the metal shell  10 . Other arrangements are identical, and explanation thereof in detail will be omitted here. 
     The cylinder  81  of the holder  80  is shorter than in the first embodiment for increasing the rigidity thereof. The open end of the cylinder  81  is welded to the whole of a circumference of the second flange  15  formed, as viewed in the drawing, above the flange  12  of the metal shell  10 . This results in a decrease in expansion of the cylinder  81  of the holder  80  arising when the porcelain insulator  20  is subjected to the combustion pressure, thereby improving the sensitivity of the piezoelectric devices  70 . 
     The first flange  12  may be increased in outer diameter more than a maximum diameter of the holder  80  to form a hexagon head for use in screwing the spark plug  200  into the cylinder head  1  of the engine. This avoids transmission of torque used to install or remove the spark plug  200  into or from the cylinder head  1  to a weld of the holder  80  to the metal shell  10 . This structure is, therefore, useful in a case where it is difficult to ensure a strong strength of the weld between the holder  80  and the metal shell  10 . 
     FIG. 8 shows a spark plug  200  according to the third embodiment of the invention which is different from the first embodiment in that the holder  80  is made up of a hollow cylinder  85  and an annular disc  86 . Other arrangements are identical, and explanation thereof in detail will be omitted here. 
     The installation of the holder  80  to the metal shell  10  is accomplished by the following manner. 
     First, the cylinder  85  of the holder  80  is welded to the whole of the circumference of the first flange  12  of the metal shell  10 . Next, the spark plug  200  is disposed within the base  101  of the press jig  100 . The press block  102  is placed in the base  101  in direct contact with the annular disc  86  of the holder  80 . The bolt  104  is tightened with a given torque to press the annular disc  86  through the press block  102  in the lengthwise direction of the porcelain insulator  20 , thereby providing a preload (i.e., a compressive pressure) to the piezoelectric devices  70 . 
     While keeping the preload as it is, a laser beam is radiated through the through holes  101   c  to form a plurality of discrete welds between the cylinder  85  and the annular disc  86  of the holder  80 . After completion of the welding, the spark plug  200  is removed from the press jig  100 . Finally, a laser beam is radiated to the overall circumference of the cylinder  85  to weld it to the annular disc  86  completely, thereby ensuring a liquid-tight seal between the cylinder  85  and the annular disc  86 . 
     FIGS. 9 and 10 show a spark plug  200  according to the fourth embodiment of the invention which is different from the first to third embodiment in that the seat  60  is placed directly on a shoulder  21  formed on the periphery of the porcelain insulator  20  at an interval away from the staked portion  14 . Other arrangements are identical, and explanation thereof in detail will be omitted here. 
     The porcelain insulator  20  has a cylindrical guide portion  22  and the shoulder  21  projecting outward from the cylindrical guide portion  22 . The shoulder  21  has an annular surface  21   a  extending perpendicular to the longitudinal center line of the porcelain insulator  20 . Similarly, a bottom surface  62   a  of the flange  62  of the seat  60  which abuts on the annular surface  21   a  of the shoulder  21  extends perpendicular to the longitudinal center line of the porcelain insulator  20 . 
     The outer diameter φD of the shoulder  21  is, as clearly shown in FIG. 10, greater than the inner diameter φd of the piezoelectric devices  70  so that the piezoelectric devices  70  may be laid to overlap with the annular surface  21   a , thereby increasing the efficiency of transmission of the combustion pressure produced in the engine to the piezoelectric devices  70 . An overlap between the annular surface  21   a  and the piezoelectric devices  70  is indicated by a in FIG.  10 . 
     Between the first and second flanges  12  and  15  of the metal shell  10 , a small-diameter portion  16  is formed. The staking the deformable portion  14  is accomplished by heating the small-diameter portion  16  and pressing the deformable portion  14  inward. 
     When the air-fuel mixture is burned in the combustion chamber  1   a  of the engine, it will cause the combustion pressure to act on the porcelain insulator  20  to press it upward, as viewed in FIG.  9 . This upward pressure is transmitted or applied to the piezoelectric devices  70  through the shoulder  21   a  and the flange  62  of the seat  60 . Specifically, the combustion pressure is transmitted to the piezoelectric devices  70  without passing through the metal shell  10 , thus improving the sensitivity of the piezoelectric devices  70 . 
     Usually, vibrations of the engine are transmitted to the piezoelectric devices  70  and added as electric noises to an output of the piezoelectric devices  70 . The vibrations enter at the metal shell  10 . Therefore, if the metal shell  10  lies on a combustion pressure transmission line, it will cause the vibrations of the engine to be transmitted directly to the piezoelectric devices  70 . In contrast, the structure of this embodiment avoids direct transmission of the engine vibrations to the piezoelectric devices  70 . Specifically, the engine vibrations inputted to the metal shell  10  are transmitted to the piezoelectric devices  70  through the porcelain insulator  20 , thus decreasing the noises added to the output of the piezoelectric devices  70 . 
     The annular portion  14  of the metal shell  10  is pressed inwardly so that it is staked on the periphery of the porcelain insulator  20 , as described above, while the small-diameter portion  16  is heated. The pressure applied to stake the annular portion  14  results in longitudinal compression of the metal shell  10  which causes the heated small-diameter portion  16  to contract in the lengthwise direction of the metal shell  10 . This results in firm adhesion between the metal shell  10  and the porcelain insulator  20  at the staked portion  14  and a joint of the metal shell  10  and the porcelain insulator  20 , thus ensuring hermetic sealing between the metal shell  10  and the porcelain insulator  20 . 
     FIG. 11 shows a spark plug  200  according to the fifth embodiment of the invention which is different from the fourth embodiment in that the deformable portion  14  of the metal shell  10  is pressed inwardly and staked on the porcelain insulator  20  without heating the small-diameter portion  16 . 
     The metal shell  10  has defines between an inner wall thereof and an outer wall of the porcelain insulator  20  an annular chamber in which talc powder is packed to form a hermetic seal between the metal shell  10  and the porcelain insulator  20 . Other arrangements are identical with those in the fourth embodiment, and explanation thereof in detail will be omitted here. 
     FIG. 12 shows a spark plug  200  according to the sixth embodiment of the invention which is different from the fourth embodiment in that the porcelain insulator  20  is fitted in the metal shell  10  without staking the end of the metal shell  10 . 
     The porcelain insulator  20  is retained within the metal shell  10 . The holder  80  is fitted on the metal shell  10  to install the piezoelectric devices  70  on the porcelain insulator  20  through the seat  60 . The elimination of the staked portion of the metal shell  10  enables the outer diameter of the shoulder  21  of the porcelain insulator  20  to be increased, thus resulting in an increase in the overlap a. Other arrangements are identical with those in the fourth embodiment, and explanation thereof in detail will be omitted here. 
     The staked portion  14  of the metal shell  10  in the above embodiments restrains the displacement of the porcelain insulator  20  causing the contraction of the piezoelectric devices  70 , which leads to a decrease in sensitivity of the piezoelectric devices  70 . The structure of this embodiment serves to alleviate such a problem. 
     The seventh to eleventh embodiments will be described with reference to FIGS. 13 to  17  which are different from the fourth to sixth embodiments only in structure of the shoulder  21  of the porcelain insulator  20  and the seat  60 . Other arrangements are identical, and explanation thereof in detail will be omitted here. 
     In the seventh embodiment of FIG. 13, the shoulder  21  of the porcelain insulator  20  has an annular surface  21   a  which is tapered at a given angle to the longitudinal center line of the porcelain insulator  20 . Similarly, the flange  62  of the seat has a surface  62   a  tapered to establish a close contact with the tapered surface  21   a  of the porcelain insulator  20 . 
     In the eighth embodiment of FIG. 14, the shoulder  21  of the porcelain insulator  20  has a chamfered or rounded outer corner, as indicated by d, and a rounded inner corner, as indicated by c. Similarly, the flange  62  of the seat  60  has a rounded inner corner, as indicated by c. 
     In the ninth embodiment of FIG. 15, the flange  62  of the seat  60  has an annular recess  62   a  which is fitted on the shoulder  21  of the porcelain insulator  20 , thereby establishing a positional relation between the porcelain insulator  20  and the seat  60  in a radius direction thereof accurately. The seat  60  has formed between the inner wall of the cylinder  61  and the outer wall of the cylindrical guide portion  22  of the porcelain insulator  20  a gap within which a sealing member  120  is packed. 
     The tenth embodiment of FIG. 16 is a modification of the ninth embodiment in FIG.  15 . 
     The porcelain insulator  20  has the tapered surface  21   a . The flange  62  of the seat  60  has the annular recess  62   a  which is tapered to establish a close fit with the tapered surface  21   a  of the porcelain insulator  20 . The sealing member  120  is disposed between the seat  60  and the porcelain insulator  20 . Other arrangements are identical. 
     The eleventh embodiment of FIG. 17 is a combination of the eighth and ninth embodiments of FIGS. 14 and 15. 
     The shoulder  21  of the porcelain insulator  20 , like the eighth embodiment, has rounded outer and inner corners. The flange  62  of the seat  60 , like the ninth embodiment, has the annular recess  62   a  which is fitted on the shoulder  21  of the porcelain insulator  20 . The annular recess  62   a  has rounded inner and outer corners. The outer corner of the annular recess  62   a  is fitted on the outer corner of the shoulder  21  of the porcelain insulator  20 . The sealing member  120  is disposed between the seat  60  and the porcelain insulator  20 . 
     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 which can be embodied without departing from the principle of the invention as set forth in the appended claims. For instance, the holder  80  and the metal shell  10  may be joined together by resistance welding, brazing, or soldering. Alternatively, the holder  80  is joined to the metal shell  10  by pressing or staking an end of the holder  80  inwardly. After staked on the metal shell  10 , the holder  80  may also be welded. Further, a gap between the holder  80  and the porcelain insulator  2  may be filled with a resinous material such as silicone to form a liquid-tight seal therebetween for avoiding the entrance of water into the piezoelectric devices  70 . The resinous material may be packed into the holder  80  through a hole formed in the holder  80  after the holder  80  is secured to the metal shell  10 .