Patent Publication Number: US-7714490-B2

Title: Spark plug for internal combustion engine and related manufacturing method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is based on Japanese Patent Application No. 2006-69245, filed on Mar. 14, 2006, the content of which is hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to spark plugs for use in internal combustion engines of motor vehicles, cogeneration systems and gas-pressure feed pumps or the like and, more particularly, to a spark plug having long operating life and a related manufacturing method. 
   2. Description of the Related Art 
   In the related art, attempts have heretofore been made to provide spark plugs as igniting means for internal combustion engines of motor vehicles or the like. 
   Each of these spark plugs generally includes a center electrode and a ground electrode between which a spark discharge gap is provided. Applying a high voltage across the center electrode and the ground electrode allows a spark discharge to take place in the spark discharge gap, thereby igniting an air-fuel mixture. 
   With the development of internal combustion engines with increased performances in maintenance-free statuses, the spark plugs have been required to have long operating life. To satisfy such requirements, an attempt has heretofore been made to form a spark plug with a center electrode having a spark discharge portion provided with an Ir alloy tip in face of the spark discharge gap. 
   Here, the Ir alloy tip and a center electrode base material body, made of Ni-based alloy, have a big difference in thermal expansion coefficients. Therefore, the Ir alloy tip is liable to drop off from the center electrode base material body when exposed to thermal stresses in frequent times. To address such an issue, it has been a common practice to employ laser welding to bond the Ir alloy tip to the center electrode base material body via a fused layer having a thermal expansion coefficient in a substantially intermediate level between those of the Ir alloy tip and the center electrode base material body. This allows a reduction in thermal stress acting on the center electrode, thereby permitting the Ir alloy tip and the center electrode base material body to ensure increased bonding capability. 
   In such a laser welding method, the Ir alloy tip and the center electrode base material body are preliminarily unitized to each other by resistance welding or the like, after which a laser beam is irradiated onto an entire circumferential periphery of the Ir alloy tip while turning the same about an axis thereof. 
   Here, the center electrode has laser-welding capability that remarkably depends on contours of the Ir alloy tip and the center electrode base material body exposed to a position at which the laser beam is irradiated. If the contours of the Ir alloy tip and the center electrode base material body exposed to the laser beam irradiating position are irregular, a joint portion between the Ir alloy tip and the center electrode base material body is fused in an uneven fusing pattern with the resultant difficulty of having adequate bonding capability. To address such an issue, an Ir alloy tip processed in a columnar configuration has been used to provide a fixed profile at all times during the rotation of the Ir alloy tip when performing welding step. 
   However, a large number of fabricating steps need to be performed for processing the Ir alloy tip in a precisely columnar shape. With a view to addressing such a problem, U.S. Pat. No. 6,885,137 discloses a spark plug that is manufactured in a process wherein even if an Ir alloy tip has a non-roundness shape in cross section on a plane perpendicular to an axis of the Ir alloy tip, a roundness tolerance is specified such that the Ir alloy tip is bonded to a center electrode base material body with bonding capability nearly equal to that obtained with a columnar shaped Ir alloy tip. 
   In addition, the above U.S. patent also discloses a rod-like Ir alloy tip, formed in a polygonal shape more than hexagonal shape in cross section, which is more preferable to be used as an Ir alloy tip for the purpose of satisfying the requirements mentioned above. 
   Meanwhile, from a standpoint of an increase in operating life of the spark plug depending on a wearing speed of the Ir alloy tip, it is advisable for the Ir alloy tip to have a square shape in cross section. That is, the Ir alloy tip formed in such a square shape is particularly effective for a spark plug of the sidewise-facing ground electrode type with a ground electrode placed in face of an outer circumferential periphery of a center electrode. 
   As shown in  FIGS. 29 to 31 , in welding a square shaped rod-like Ir alloy tip  92  to a center electrode base material body  91 , the square shaped rod-like Ir alloy tip  92  is welded to the center electrode base material body  91  with an outline circle of the center electrode base material body  91  placed in an area slightly outside a circumscribed circle of a square shape of the Ir alloy tip  92 . During such welding process, uneven differences occur in distance between a circumferential sidewall of the center electrode base material body  91  and sidewalls of the square shaped rod-like Ir alloy tip  92 . Thus, a fused layer  95  (see  FIG. 32 ) tends to have unevenness in thermal expansion coefficient after laser welding has been completed. 
   With a structure of a center electrode  90  as shown, for instance, in  FIG. 31 , a sidewall  921  of the square shaped rod-like Ir alloy tip  92  is spaced from a circumferential sidewall  911  of the center electrode base material body  91  by a distance L 1  in a direction vertical to the sidewall  921  of the square shaped rod-like Ir alloy tip  92 . 
   Further, a corner  922  of the square shaped rod-like Ir alloy tip  92  is spaced from the circumferential sidewall  911  of the center electrode base material body  91  by a distance L 2  in a direction along a diagonal line  923  of the square shaped rod-like Ir alloy tip  92 . 
   Thus, the distance L 1  between the sidewall  921  of the square shaped rod-like Ir alloy tip  92  and the circumferential sidewall  911  of the center electrode base material body  91  becomes greater than the distance L 2  between the corner  922  of the square shaped rod-like Ir alloy tip  92  and the circumferential sidewall  911  of the center electrode base material body  91 . 
   Accordingly, the fused layer  95  (see  FIG. 32 ) resulting from laser welding has a Ni-rich area, influenced with material components (Ni or the like) of the center electrode base material body  91 , in the vicinity of a vertical region  912  between the sidewall  921  of the square shaped rod-like Ir alloy tip  92  and the circumferential sidewall  911  of the center electrode base material body  91 . Thus, the fused layer  95  has a thermal expansion coefficient deviated to that of the center electrode base material body  91  in the vicinity of the vertical region  912 . 
   On the contrary, the fused layer  95  has an Ir rich area, influenced with material components (Ir or the like) of the square shaped rod-like Ir alloy tip  92 , in the vicinity of a diagonal region  913  of the square shaped rod-like Ir alloy tip  92 . Thus, the fused layer  95  has a thermal expansion coefficient deviated to that of the square shaped rod-like Ir alloy tip  92  in the vicinity of the diagonal region  913  thereof. 
   Thus, the fused layer  95  becomes hard to have an overall circumference having a thermal expansion coefficient in the vicinity of a value intermediate between those of the square shaped rod-like Ir alloy tip  92 , containing Ir, and the center electrode base material body  91  containing Ni or the like. 
   Further, even if laser welding is performed under a condition to minimize a difference in thermal expansion coefficients in various areas of the fused portion on an entire circumferential periphery thereof, the fused layer  95  becomes rich in Ni or the like as a whole, causing a difficulty to occur in forming the fused layer  95  so as to have the thermal expansion coefficient in the vicinity of the intermediate value between those of the square shaped rod-like Ir alloy tip  92  and the center electrode base material body  91 . 
   Furthermore, the corner  922  and a center of the square shaped rod-like Ir alloy tip  92  have a greater distance along the diagonal line than a distance between the sidewall  921  of the square shaped rod-like Ir alloy tip  92  and the center thereof. Therefore, the innermost fused region  95   a  of the fused layer  95  is hard to reach the center of the square shaped rod-like Ir alloy tip  92  with a concurrence of the unfused region  96  being left as shown in  FIG. 32 . As a result, the spark plug of the related art suffers from increased thermal stresses when exposed to thermal shocks in repeated cycles during operations in the internal combustion engine. 
   As set forth above, the spark plug formed in such a structure mentioned above has incapability of achieving a reduction in thermal stress with the resultant difficulty of ensuring adequate bonding capability. 
   Accordingly, under circumstances where the spark plug is exposed to thermal shocks in rapid heating and rapid cooling in repeated cycles, there is a fear of flaking or cracking  97  occurring in the fused layer  95  at the joint portion of the center electrode  90 . 
   Moreover, U.S. Pat. No. 6,724,132 discloses a spark plug of a sidewise-facing ground electrode type having a center electrode provided with a noble metal tip formed in a square shape in cross section. With the spark plug of the center electrode formed in such a structure, the square shaped noble metal tip has an outer diameter larger than that of a center electrode base material body. That is, an inscribed circle of the square shaped noble metal tip is larger in diameter than the outer diameter of the center electrode base material body. 
   Therefore, the noble metal tip can be welded to the center electrode base material body only upon completely assembling the center electrode base material body to a porcelain insulator. Thus, an issue arises with the occurrence of deterioration in productivity of the spark plug. In addition, the fused layer of the center electrode needs to be placed in a position protruding from an end face of the porcelain insulator. This causes a limitation to occur in the positional relationship between a welding position and the end face of the porcelain insulator. 
   SUMMARY OF THE INVENTION 
   The present invention has been completed with the above view in mind and has an object to provide a spark plug for an internal combustion engine, having increased bonding capability between a center electrode base material body and an Ir alloy tip with increased productivity while having elongated operating life, and a method of manufacturing a spark plug for use in an internal combustion engine. 
   To achieve the above object, a first aspect of the present invention provides a spark plug for an internal combustion engine, which spark plug comprises a metal shell having an outer periphery formed with a mounting thread, a porcelain insulator fixedly secured to the metal shell on a central axis thereof, a center electrode retained within the porcelain insulator along a central axis thereof, and a ground electrode extending from the metal shell and having a leading end placed in face-to-face relationship with the center electrode to provide a spark discharge gap. The center electrode includes a center electrode base material body of a substantially columnar shape having a base material leading end portion, and a substantially square shaped rod-like Ir alloy tip bonded to the base material leading end portion of the center electrode base material body. The base material leading end portion of the center electrode base material body has a columnar shape with a diameter D 2  smaller than a diameter D 1  of the center electrode base material body. The square shaped rod-like Ir alloy tip has a square shape in cross section on a plane perpendicular to an axis of the Ir alloy tip with diagonal lines among which a long diagonal line is supposed to be a diameter A of a circumscribed circle C A  of the Ir alloy tip and an inscribed circle C B  is supposed to internally touch at least two sides of the square shape coaxially with the circumscribed circle C A  of the Ir alloy tip and have a diameter B, with four diameters A, B, D 1  and D 2  lay in the relationship expressed as D 1 &gt;A&gt;D 2 &gt;B. 
   With such a structure of the spark plug, the diameter D 1  of the center electrode base material body, the diameter D 2  of the base material leading end portion, the diameter A of the circumscribed circle C A  of the Ir alloy tip and the diameter B of the inscribed circle C B  lay in the relationship expressed as D 1 &gt;A&gt;D 2 &gt;B. This allows the center electrode base material body and the Ir alloy tip to have increased bonding capability, while providing an increase in productivity of the spark plug. 
   That is, with the relevant component parts specified in dimension achieving the relationship A&gt;D 2 &gt;B, unevenness in distance between a circumferential periphery of the base material leading end portion of the center electrode base material body and a sidewall of the Ir alloy tip can be minimized over an entire circumference of a joint portion between the base material leading end portion of the center electrode base material body and the Ir alloy tip. This allows the Ir alloy tip and the center electrode base material body to be welded at the joint portion in such a way to allow materials, contained in the fused layer, to contain a homogeneous mixture through an entire circumferential area of the fused layer so as to minimize a difference thermal expansion coefficients. That is, the entire circumferential area of the fused layer can be formed of substantially and homogeneously mixed materials resulting from materials of the center electrode base material body and the Ir alloy tip. As a result, the fused layer can have a thermal expansion coefficient in the vicinity of an intermediate value between those of the center electrode base material body and the Ir alloy tip. This makes it possible to achieve a reduction in thermal stress acting on the fused layer between the center electrode base material body and the Ir alloy tip. 
   Accordingly, even under circumstances where the spark plug is exposed in use to thermal shocks in rapid heating and rapid cooling cycles in repeated number of times, no flaking or cracking takes place in the joint portion between the center electrode base material body and the Ir alloy tip, ensuring the spark plug to have increased bonding capability between the center electrode base material body and the Ir alloy tip. 
   Further, with the relationship A&gt;D 2 &gt;B, the fused layer can be formed such that a distance between an outer circumferential periphery of the joint portion between the center electrode base material body and the Ir alloy tip and a center of the Ir alloy tip can be shortened in a diagonal region along a diagonal line of the Ir alloy tip. This allows the fused layer to easily reach the center of the Ir alloy tip during a welding stage, thereby minimizing the formation of an unfused region in the fused layer between the center electrode base material body and the Ir alloy tip. This results in a reduction in thermal shock acting on between the center electrode base material body and the Ir alloy tip. 
   Furthermore, with the relationship D 1 &gt;A, the spark plug can be assembled such that the Ir alloy tip is bonded to the center electrode base material body before the center electrode base material body is assembled to an inside of the porcelain insulator. That is, the diameter D 1  of the center electrode base material body is made less than a diameter of a central through-bore of the porcelain insulator that retains the center electrode in a fixed place. In addition, the diameter A of the circumscribed circle CA of the Ir alloy tip is less than D 1 . Thus, both the Ir alloy tip and the center electrode base material body can pass through the central through-bore of the porcelain insulator. Therefore, the Ir alloy tip can be bonded to the center electrode base material body before the center electrode base material body is assembled to the inside of the porcelain insulator, providing an increase in productivity. Another advantage resides in a fact that no particular limitation occurs in a bonding position between the center electrode base material body and the Ir alloy tip. 
   Further, the Ir alloy tip formed in the substantially square shaped rod-like configuration can minimize a wearing speed of the Ir alloy tip, caused by spark discharge, making it possible to provide an ease of extending operating life of the spark plug. 
   As set forth above, the present invention can provide a spark plug for an internal combustion engine with a center electrode base material body and an Ir alloy tip bonded to each other with increased bonding capability to provide elongated operating life while having increased productivity. 
   Further, with the spark plug for the internal combustion engine of the present embodiment, the substantially square shape of the Ir alloy tip in cross section may preferably have corners formed in curved portions, respectively, and straight portions interconnecting the curved portions to each other, and wherein each straight portion may have a length L and the Ir alloy tip has a width W under the relationship expressed as 0.8×W≦L&lt;W. 
   With the spark plug of such a structure, a thermal stress can be prevented from concentrating on the corner of the Ir alloy tip, while providing increased bonding capability between the Ir alloy tip and the center electrode base material body. In addition, the Ir alloy tip can have an increased surface area facing the ground electrode, enabling the spark plug to have increased operating life. 
   With the straight portion having the length L and the Ir alloy tip having the width W falling in the relationship 0.8×W&gt;L, the Ir alloy tip has a decreased surface area facing the ground electrode, causing a fear to occur with a difficulty of having the spark plug with increased operating life. 
   Furthermore, with the spark plug for the internal combustion engine of the present embodiment, the ground electrode may preferably include a noble metal ship having an opposing surface, facing a sidewall of the Ir alloy tip to provide the spark discharge gap, which has a width M in relation to the length L of the straight portion under the relationship expressed as M&lt;L. 
   With such a structure, the Ir alloy tip and the straight portion can easily provide a spark discharge, enabling the Ir alloy tip to have increased operating life. 
   The noble metal may be made of, for instance, Ir alloy, Pt alloy or the like. 
   Further, with the spark plug for the internal combustion engine of the present embodiment, the opposing surface of the noble metal tip of the ground electrode may be preferably placed in face-to-face relationship with the sidewall of the Ir alloy tip such that the opposing surface of the noble metal tip does not protrude from the sidewall of the Ir alloy tip. 
   With such a structure, the Ir alloy tip of the center electrode and the noble metal chip of the ground electrode can face each other in an adequate opposing surface area, enabling the Ir alloy tip to have elongated operating life. 
   Further, with the spark plug for the internal combustion engine of the present embodiment, the opposing surface of the noble metal tip of the ground electrode may be preferably placed in face-to-face relationship with a sidewall of the Ir alloy tip so as not to protrude from the sidewall of the Ir alloy tip in a widthwise direction thereof, and each of the curved portions of the Ir alloy tip may have a depth less than 0.3 mm along a direction in which each curved portion faces the noble metal tip of the ground electrode. 
   Such a structure prevents a reduction in an opposing surface area between the noble metal tip of the ground electrode and the Ir alloy tip of the center electrode, enabling the Ir alloy tip to have elongated operating life. 
   In a case where the depth of the curved portion of the corner of the Ir alloy tip exceeds 0.3 mm, the curved portion on the corner of the Ir alloy tip is placed in face-to-face relationship with the opposing surface of the noble metal tip of the ground electrode. When this takes place, there is a fear of the opposing surface of the Ir alloy tip wearing on an early stage with the resultant decrease in operating life. That is, if the spark discharge gap is enlarged and exceeds 0.3 mm, misfiring is liable to occur. In addition, if the depth of the curved portion exceeds 0.3 mm, the spark discharge gap has a value exceeding 0.3 mm in an increased area on an early stage between the curved portion of the Ir alloy tip and the noble metal tip of the ground electrode, with the resultant fear of an appropriate opposing surface area reducing on an early stage. 
   Furthermore, with the spark plug for the internal combustion engine of the present embodiment, the substantially square shape of the Ir alloy tip in cross section thereof may preferably have one of a substantially quadrate shape and a rectangular shape. 
   In such a case, the Ir alloy tip and the center electrode base material body can be easily positioned with respect to each other. This provides an ease of inserting the center electrode, composed of the Ir alloy tip bonded to the center electrode base material body, through the porcelain insulator. This results in capability of obtaining a spark plug with increased productivity. 
   Moreover, with the spark plug for the internal combustion engine of the present embodiment, the substantially square shape of the Ir alloy tip in cross section may preferably have a profile including corners formed in curved portions, respectively, and straight portions interconnecting the curved portions to each other, and wherein a square shape may be supposed to be defined with four extended lines of the straight portions with a length E of a long diagonal line among diagonal lines of the substantially square shape of the Ir alloy tip under which a virtual circumscribed circle C C  is supposed to have a diameter C, corresponding to the long diagonal line among diagonal lines of the substantially square shape of the Ir alloy tip, which has the relationship expressed as D 1 &gt;C&gt;D 2 ≧E. 
   In such a case, even if the curved portions of the Ir alloy tip formed on the corners thereof are placed in an inward area of the base material leading end of the center electrode base material body, the virtual circumscribed circle C C  comes to be placed in a position between an outer profile of the base material body and an outer profile of the base material leading end. This enables the spark plug to have the same advantages effects as those described with reference to the first aspect of the present invention, ensuring increased bonding capability between the center electrode base material body and the Ir alloy tip. 
   In addition, with the spark plug for the internal combustion engine of the present embodiment, the diameter D 1  of the center electrode base material body and the diameter D 2  of the base material leading end portion of the center electrode base material body may preferably lay in the relationship expressed as 0.5×D 1 ≦D 2 ≦0.95×D 1 . 
   With such a relationship, it becomes possible to ensure bonding capability between the center electrode base material body and the Ir alloy tip, while ensuring increased operating life of the Ir alloy tip. 
   With the structure falling in the relationship 0.5×D 1 &gt;D 2 , it becomes hard for heat, developed in the Ir alloy tip, to escape to the base end of the center electrode via the base material leading end portion, causing a fear of the occurrence of deterioration in operating life of the Ir alloy tip. 
   On the contrary, in case of the relationship D 2 &gt;0.95×D 1 , the spark plug encounters a difficulty of satisfying the relationship D 1 &gt;A&gt;D 2 &gt;B, previously noted above with reference to the first aspect of the present invention, and compelling a difference between the diameter D 2  of the base material leading end portion and the diameter A of the circumscribed circle C A  of the Ir alloy tip to be nearly equal to a difference between the diameter D 2  of the base material leading end portion and the diameter B of the inscribed circle C B  of the Ir alloy tip. Thus, there is a fear of a difficulty encountered in increasing bonding capability between the center electrode base material body and the Ir alloy tip. 
   Moreover, with the spark plug for the internal combustion engine of the present embodiment, the diameter D 1  of the center electrode base material body and the diameter D 2  of the base material leading end portion of the center electrode base material body may preferably lay in the relationship expressed as 0.7×D 1 ≦D 2 ≦0.9×D 1 . 
   In such a case, increased bonding capability can be ensured between the center electrode base material body and the Ir alloy tip, providing a further ease of ensuring increased operating life of the Ir alloy tip. 
   Further, with the spark plug for the internal combustion engine of the present embodiment, the diameter D 1  of the base material body of the center electrode base material body may be preferably greater than 2.5 mm. 
   With such a dimension of the center electrode base material body, the spark plug can adequately exhibit the advantageous effects of the present invention. 
   That is, in general, with the center electrode base material body having the diameter greater than 2.5 mm, a drop is liable to occur in bonding capability between the center electrode base material body and the Ir alloy tip. Therefore, applying the present invention to such a spark plug allows the spark plug to have advantageous effects of the present invention. 
   Furthermore, with the spark plug for the internal combustion engine of the present embodiment, the Ir alloy tip may be preferably bonded to the center electrode base material body at a joint portion where a fused layer is formed with an unfused portion being formed inside the fused layer in a width F in a correlation with the diameter A of the circumscribed circle C A  of the Ir alloy tip under the relationship expressed as F≦0.2×A. 
   In such a case, an increased thermal stress is prevented from concentrating on the unfused area, ensuring bonding capability between the center electrode base material body and the Ir alloy tip. 
   Moreover, with the spark plug for the internal combustion engine of the present embodiment, the Ir alloy tip may be preferably bonded to the center electrode base material body at a joint portion formed with a fused layer having an entire circumference spaced from a bore end of a through-bore of the porcelain insulator by a distance greater than 0.1 mm. 
   In such a case, an adequate clearance can be provided between the bore end of the through-bore of the porcelain insulator and the fused layer of the center electrode. Thus, even if remnants of combustion gases accumulate on the bore end of the porcelain insulator, adverse affects of the remnants on the porcelain insulator can be suppressed. 
   That is, in a case where the fused layer is spaced from the bore end of the porcelain insulator by a distance less than 0.1 mm, there is a fear of remnants resulting from combustion of fuel gas and accumulating in an area between the bore end and the fused layer of the center electrode with the resultant occurrence of clogging. When this takes place, thermal stress occurs in between both component parts due to a difference in thermal expansion coefficients of the center electrode and the porcelain insulator. This results in a fear of thermal stress acting on the porcelain insulator with the resultant occurrence of cracking. In another case, engine vibrations cause the center electrode leading end portion to vibrate and a stress acts on the porcelain insulator due to the remnants accumulated on the bore end of the porcelain insulator, causing a fear to take place with the occurrence of cracking. 
   Thus, permitting the fused layer to be spaced from the bore end of the porcelain insulator by a distance greater than 0.1 mm prevents the porcelain insulator from damage. 
   Besides, with the spark plug for the internal combustion engine of the present embodiment, the center electrode may have an annular space defined between an insulator leading end portion and an outer periphery of the joint portion between the Ir alloy tip and the center electrode base material body, and the joint portion may preferably have an inward boundary edge exposed to the annular space of the center electrode. 
   With such a structure, the temperature rise of the fused portion can be avoided, thereby enabling the fused layer to be prevented from thermal stress in excess. 
   Further, with such a structure of the fused layer, the bore end of the porcelain insulator and the fused layer is separate from each other over an entire circumference of the fused layer by a distance greater than 0.1 mm, making it possible to effectively prevent the porcelain insulator from damage. 
   Further, with the spark plug for the internal combustion engine of the present embodiment, the ground electrode may preferably comprise a ground electrode base body having a trailing end connected to the metal shell and a leading end to which a noble metal tip is bonded in face-to-face relationship with a sidewall of the center electrode to define the spark discharge gap. 
   With such a structure, the noble metal tip of the ground electrode can face the Ir alloy tip in an increased surface area, thereby enabling the Ir alloy tip to have increased operating life. 
   Further, with the spark plug for the internal combustion engine of the present embodiment, the porcelain insulator may preferably have an insulator leading end portion axially extending through the metal shell and ended at a position axially inward of a leading end of the metal shell, and the base material leading end portion of the base material body may be spaced from the insulator leading end portion of the porcelain insulator through an annular space, wherein the joint portion between the Ir alloy tip and the base material leading end portion is exposed to the annular space between the base material leading end portion of the base material body and the insulator leading end portion of the porcelain insulator. 
   With such a structure, the base material leading end portion of the base material body is spaced from the insulator leading end portion of the porcelain insulator through the annular space and the joint portion between the Ir alloy tip and the base material leading end portion is exposed to the annular space between the base material leading end portion of the base material body and the insulator leading end portion of the porcelain insulator. In such a case, the bore end of the through-bore of the porcelain insulator and the fused layer of the center electrode can be separate from each other through the annular space. Thus, even if remnants of combustion gases accumulate on the bore end of the porcelain insulator, adverse affects of the remnants on the porcelain insulator can be suppressed. This prevents the porcelain insulator from damage due to a stress arising from such remnants. 
   Furthermore, with the spark plug for the internal combustion engine of the present embodiment, the Ir alloy tip and the base material leading end portion may be preferably bonded to each other at the joint portion through a fused layer having an inward boundary edge exposed to the annular space between the base material leading end portion of the base material body and the insulator leading end portion of the porcelain insulator. 
   With such a structure, the temperature rise of the fused portion can be avoided, thereby enabling the fused layer to be prevented from thermal stress in excess. 
   Moreover, with the spark plug for the internal combustion engine of the present embodiment, the fused layer of the joint portion may preferably have an outer circumferential periphery spaced from the leading end portion of the porcelain insulator through the annular space by a given distance. 
   With such a structure of the fused layer, the fused layer is separate from the bore end of the porcelain insulator over an entire circumference of the fused layer by a given distance, making it possible to effectively prevent the porcelain insulator from damage. 
   Besides, with the spark plug for the internal combustion engine of the present embodiment, the Ir alloy tip of the center electrode may preferably have side surfaces, each having a corner and a straight portion L, and a width W that lies in the relationship expressed as 0.8×W≦L&lt;W. 
   With such a structure, the Ir alloy tip of the center electrode can have an adequate opposing surface area in opposition to the ground electrode, enabling the spark plug to have increased operating life. 
   In addition, with the spark plug for the internal combustion engine of the present embodiment, the curved portion of the Ir alloy tip of the center electrode may preferably have a depth falling in a value less than 0.3 mm. 
   The presence of the Ir alloy tip having the corners with each depth laying in a value less than 0.3 mm enables the prevention of a reduction in an opposing surface area of the Ir alloy tip of the center electrode, thereby enabling the spark plug to have prolonged operating life. 
   A second aspect of the present invention provides a method of manufacturing a spark plug for an internal combustion engine, comprising the steps of preparing a metal shell having an outer periphery formed with a mounting thread, preparing a porcelain insulator, preparing a center electrode having a substantially columnar shaped center electrode base material body having a base material leading end portion and a substantially square shaped rod-like Ir alloy tip bonded to the base material leading end portion of the center electrode base material body by laser welding, inserting the center electrode through the porcelain insulator in a fixed place, and inserting the porcelain insulator, with which the center electrode is supported, in the metal shell. The base material leading end portion of the center electrode base material body has a columnar shape with a diameter D 2  smaller than a diameter D 1  of the center electrode base material body. The square shaped rod-like Ir alloy tip has a square shape in cross section on a plane perpendicular to an axis of the Ir alloy tip with diagonal lines among which a long diagonal line is supposed to be a diameter A of a circumscribed circle C A  of the Ir alloy tip and an inscribed circle C B  is supposed to internally touch at least two sides of the square shape coaxially with the circumscribed circle C A  of the Ir alloy tip and have a diameter B, with four diameters A, B, D 1  and D 2  lay in the relationship expressed as D 1 &gt;A&gt;D 2 &gt;B. 
   With such a spark plug manufacturing method, the relevant component parts of the spark plug are defined in specified dimensions such that the diameter D 1  of the center electrode base material body, the diameter D 2  of the base material leading end portion of the center electrode base material body, the diameter A of the circumscribed circle C A  of the Ir alloy tip and the diameter B of the inscribed circle C B  lay in the relationship expressed as D 1 &gt;A&gt;D 2 &gt;B. This allows the center electrode base material body and the Ir alloy tip to have increased bonding capability, while providing increase in productivity of the spark plug. 
   That is, satisfying the relationship A&gt;D 2 &gt;B enables unevenness in distance between a circumferential sidewall of the base material leading end portion of the center electrode base material body and a sidewall of the Ir alloy tip to be minimized over an entire circumference of the joint portion. This allows the Ir alloy tip and the center electrode base material body to be welded at the joint portion through the fused layer formed of homogeneously mixed materials resulting from materials of the center electrode base material body and the Ir alloy tip over an entire circumferential area of the fused layer. This results in a reduction in a difference between thermal expansion coefficients over the entire circumferential area of the fused layer. 
   That is, the entire circumferential area of the fused layer can be formed of substantially and homogeneously mixed materials containing materials of the center electrode base material body and the Ir alloy tip. As a result, the fused layer can have a thermal expansion coefficient in the vicinity of an intermediate value between those of the center electrode base material body and the Ir alloy tip. This makes it possible to achieve a reduction in thermal stress acting on between the center electrode base material body and the Ir alloy tip. 
   Accordingly, even under circumstances where the spark plug is exposed in use to thermal shocks in rapid heating and rapid cooling cycles in repeated number of times, no flaking or cracking takes place in the joint portion between the center electrode base material body and the Ir alloy tip, ensuring bonding capability between the center electrode base material body and the Ir alloy tip. 
   With the method of manufacturing the spark plug for the internal combustion engine according to the present invention, the porcelain insulator may have an insulator leading end portion axially extending through the metal shell and ended at a position axially inward of a leading end of the metal shell, and the base material leading end portion of the base material body may be spaced from the insulator leading end portion of the porcelain insulator through an annular space, wherein the joint portion between the Ir alloy tip and the base material leading end portion is exposed to the annular space between the base material leading end portion of the base material body and the insulator leading end portion of the porcelain insulator. 
   With such a manufacturing method, the base material leading end portion of the base material body is spaced from the insulator leading end portion of the porcelain insulator through the annular space and the joint portion between the Ir alloy tip and the base material leading end portion is exposed to the annular space between the base material leading end portion of the base material body and the insulator leading end portion of the porcelain insulator. In such a case, the bore end of the through-bore of the porcelain insulator and the fused layer of the center electrode can be separate from each other through the annular space. Thus, even if remnants of combustion gases accumulate on the bore end of the porcelain insulator, adverse affects of the remnants on the porcelain insulator can be suppressed. This prevents the porcelain insulator from damage due to a stress arising from such remnants. 
   With the method of manufacturing the spark plug for the internal combustion engine according to the present invention, the Ir alloy tip and the base material leading end portion may be preferably bonded to each other at the joint portion through a fused layer having an inward boundary edge exposed to the annular space between the base material leading end portion of the base material body and the insulator leading end portion of the porcelain insulator. 
   With such a manufacturing method, the temperature rise of the fused portion can be avoided, thereby enabling the fused layer to be prevented from thermal stress in excess. 
   With the method of manufacturing the spark plug for the internal combustion engine according to the present invention, the fused layer of the joint portion may preferably have an outer circumferential periphery spaced from the leading end portion of the porcelain insulator through the annular space by a given distance. 
   With such a manufacturing method, the fused layer is separate from the bore end of the porcelain insulator over an entire circumference of the fused layer by a given distance, making it possible to effectively prevent the porcelain insulator from damage. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a half cross-sectional view showing an overall structure of a spark plug of a first embodiment according to the present invention. 
       FIG. 2  is an enlarged cross-sectional view showing a spark discharge portion and its vicinity of the spark plug shown in  FIG. 1 . 
       FIG. 3  is a plan view of the spark plug of the first embodiment, shown in  FIG. 1 , as viewed from a distal end of a center electrode thereof. 
       FIG. 4  is a perspective view showing the center electrode of the spark plug of the first embodiment shown in  FIG. 1 . 
       FIG. 5  is a side view showing the center electrode of the spark plug of the first embodiment shown in  FIG. 1 . 
       FIG. 6  is a perspective view showing a base material body of the center electrode forming part of the spark plug of the first embodiment shown in  FIG. 1 . 
       FIG. 7  is an enlarged illustrative view showing a joint portion between the center electrode base material body and an Ir alloy tip forming the spark plug of the first embodiment shown in  FIG. 1 . 
       FIG. 8  is an enlarged perspective view showing the center electrode under a condition supported with a porcelain insulator. 
       FIG. 9  is an enlarged illustrative view showing a positional relationship between the porcelain insulator and the center electrode. 
       FIG. 10  is a plan view of a center electrode of a modified form of the spark plug of the first embodiment shown in  FIG. 1  with the center electrode formed in a substantially rectangular in cross section. 
       FIG. 11  is a plan view of a center electrode of another modified form of the spark plug of the first embodiment shown in  FIG. 1  with the center electrode formed in a substantially rhombic shape in cross section. 
       FIG. 12  is an illustrative view showing the center electrode, as viewed from a distal end thereof, in which the center electrode base material body and the Ir alloy tip are bonded to each other by a laser welding operation using 16-point equiangular laser beams being irradiated to the joint portion. 
       FIG. 13  is an illustrative view of the center electrode, as viewed from a side area, showing a structure with the center electrode base material body and the Ir alloy tip bonded to each other by the laser welding operation shown in  FIG. 12 . 
       FIG. 14  is a perspective view of the center electrode, forming part of the spark plug of the first embodiment shown in  FIG. 1 , with the center electrode base material body and the Ir alloy tip bonded to each other by laser welding. 
       FIG. 15  is a half cross-sectional view showing an overall structure of a modified form of the spark plug of the first embodiment shown in  FIG. 1  with a ground electrode having a leading end placed in opposition to a distal end of the center electrode. 
       FIG. 16  is an illustrative view showing the positional relationship between an Ir alloy tip of a center electrode and a noble metal tip of a ground electrode forming a spark plug of a second embodiment according to the present invention. 
       FIG. 17  is an illustrative view showing the Ir alloy tip of a center electrode with a substantially quadrate shape in cross section. 
       FIG. 18  is an illustrative view showing the Ir alloy tip of a center electrode with a substantially rectangular shape in cross section. 
       FIG. 19  is an illustrative view showing one positional relationship between the Ir alloy tip of the center electrode and the noble metal tip of the ground electrode, forming the spark plug of the second embodiment shown in  FIG. 16 , under a condition where the Ir alloy tip of the center electrode and the noble metal tip of the ground electrode are displaced from each other within an allowable limit. 
       FIG. 20  is an illustrative view showing another positional relationship between the Ir alloy tip of the center electrode and the noble metal tip of the ground electrode, forming the spark plug of the second embodiment shown in  FIG. 16 , under a condition where the Ir alloy tip of the center electrode and the noble metal tip of the ground electrode are displaced from each other beyond the allowable limit. 
       FIG. 21  is an illustrative view of a spark plug of a third embodiment according to the present invention showing a positional relationship between a corner portion of an Ir alloy tip of the center electrode and a center electrode base material body. 
       FIG. 22A  is a plan view of a center electrode, as viewed from a distal end of an Ir alloy tip bonded to a base material leading end portion with a diameter D 2  of 2.1 mm, on which a first evaluation test was conducted. 
       FIG. 22B  is a cross-sectional view of the center electrode taken on line K-K of FIG.  22 A. 
       FIG. 22C  is a cross-sectional view of the center electrode taken on line N-N of  FIG. 22A . 
       FIG. 23A  is a plan view of a center electrode, as viewed from a distal end of an Ir alloy tip bonded to a base material leading end portion with a diameter D 2  of 2.4 mm, on which a second evaluation test was conducted. 
       FIG. 23B  is a cross-sectional view of the center electrode taken on line K 1 -K 1  of  FIG. 23A . 
       FIG. 23C  is a cross-sectional view of the center electrode taken on line N 1 -N 1  of  FIG. 23A . 
       FIG. 24A  is a plan view of a center electrode, as viewed from a distal end of an Ir alloy tip bonded to a base material leading end portion with a diameter D 2  of 2.9 mm, on which a third evaluation test was conducted. 
       FIG. 24B  is a cross-sectional view of the center electrode taken on line K 2 -K 2  of  FIG. 24A . 
       FIG. 24C  is a cross-sectional view of the center electrode taken on line N 2 -N 2  of  FIG. 24A . 
       FIG. 25  is a graph showing the relationship between a diameter D 2  of a base material leading end portion and an Ir ratio (wt %) of the center electrode. 
       FIG. 26  is a graph showing the relationship between a diameter D 2  of a base material leading end portion and a fusing depth (mm) of the center electrode. 
       FIG. 27  is a graph showing the relationship between an Ir ratio (%) of a fused layer and a thermal expansion coefficient of the fused layer. 
       FIG. 28  is a graph showing the relationship between a flaking incidence rate (%) of an Ir alloy tip from a base material body of a center electrode and a diameter D 2  of a base material leading end portion. 
       FIG. 29  is a perspective view showing a spark plug of the related art. 
       FIG. 30  is a side view showing the spark plug of the related art shown in  FIG. 29 . 
       FIG. 31  is a plan view of the spark plug of the related art, shown in  FIG. 29 , as viewed from a distal end of a center electrode. 
       FIG. 32  is an enlarged illustrative view showing the center electrode encountered with cracking occurred in a fused layer. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Now, spark plugs of various embodiments according to the present invention are described below in detail with reference to the accompanying drawings. However, the present invention is construed not to be limited to such embodiments described below and technical concepts of the present invention may be implemented in combination with other known technologies or the other technology having functions equivalent to such known technologies. 
   In the following description, like reference characters designate like or corresponding parts throughout the several views. Also in the following description, description on the same component parts of one embodiment as those of another embodiment is omitted, but it will be appreciated that like reference numerals designate the same component parts throughout the drawings. 
   The sparks plugs of the various embodiments according to the present invention can be employed as igniting means for internal combustion engines of, for instance, an automotive vehicle, a cogeneration system and a gas-pressure feed pump or the like. 
   In the following description, a distal end of the spark plug inserted to a combustion chamber of the internal combustion engine is referred to as a “leading end” and the opposite side is referred to as a “base end”. 
   First Embodiment 
   A spark plug of a first embodiment according to the present invention is described below in detail with reference to  FIG. 1 to 14  of the accompanying drawings. 
   As shown in  FIGS. 1 to 3 , the spark plug  10  of the present embodiment comprises a cylindrical metal shell  12 , an porcelain insulator  14  fixedly held with the cylindrical metal shell  12  and extending in a central axis thereof, a center electrode  16  fixedly supported with the porcelain insulator  14  in a central axis thereof, and ground electrodes  18  fixedly bonded to a leading end  12   a  of the cylindrical metal shell  12  to provide spark discharge gaps  20  with respect to a leading end  16   a  of the center electrode  16 . 
   The cylindrical metal shell  12  includes an intermediate body  12   b , an upper section  12   c  and a lower section  12   d . The upper section  12   c  has an outer circumferential periphery formed in a hexagonal shape and acting as a tool-fitting section  12   f . The lower section  12   d  of the cylindrical metal shell  12  has an outer circumferential periphery formed with a mounting thread  12   e  to be screwed into an engine block (not shown). 
   As shown in  FIGS. 3 to 5 , the center electrode  16  comprises a substantially columnar shaped center electrode base material body  22 , made of Ni-based alloy, and a substantially square shaped rod-like Ir alloy tip  24  bonded to the center electrode base material body  22 . The center electrode base material body  22  includes a base material body  26  having a downwardly extending base body leading end  26   a.    
   As shown in  FIGS. 1 ,  2  and  6 , the base body leading end  26   a  of the center electrode base material body  22  has a columnar shape with a diameter D 2  smaller than a diameter D 1  of the base material body  26 . 
   Meanwhile, the center electrode base material body  22  and the Ir alloy tip  24  are specified in a particular dimensional relationship and placement relationship as described below in detail. 
   As shown in  FIG. 3 , a substantially square shape representing a cross section of the square shaped rod-like Ir alloy tip  24  and perpendicular to an axis thereof includes a long diagonal line A and a short diagonal line B with a circumscribed circle CA of the Ir alloy tip supposed to have a diameter equal to a length of the long diagonal line A. In addition, the substantially square shape representing the cross section of the square shaped rod-like Ir alloy tip  24  has an inscribed circle C B  with a diameter equal to a length of the short diagonal line B. In such a case, the four diameters A, B, D 1 , D 2  are specified in the relationship expressed as D 1 &gt;A&gt;D 2 &gt;B. In addition, with the spark plug  10  of the present embodiment, two diagonal lines have an equal length and the circumscribed circle C A  of the Ir alloy tip is held in contact with four corners of the substantially square shape. Moreover, the inscribed circle C B  internally touches four sides of the substantially square shape. 
   The spark plug  10  of the present embodiment is applied as a spark plug of a cogeneration gas engine. In installation, the spark plug  10  is screwed into a threaded bore of an engine head (not shown), in which a combustion chamber is defined, and fixedly mounted on the engine head. 
   As set forth above, the spark plug  10  includes the cylindrical metal shell  12 , serving as a housing, which is made of electrically conductive steel (such as, for instance, low carbon steel or the like). The porcelain insulator  14 , made of alumina ceramic (Al 2 O 3 ) or the like, is fixedly supported within the cylindrical metal shell  12 . 
   The center electrode base material body  22  is made of a substantially columnar body composed of inner material, including metallic material such as copper or the like excellent in heat conductivity, and outer material including metallic material such as nickel-based alloy or the like excellent in heat resistance and corrosion resistance. 
   The Ir alloy tip  24  has an upper end (trailing end)  24   a  welded to the base body leading end  26   a  such that the Ir alloy tip  24  extends downward from an insulator leading end portion  14   a  to be exposed in an area around the spark discharge gaps  20  as shown in  FIG. 2 . 
   Meanwhile, each ground electrode  18 , made of Ni-based alloy or the like, has a trailing end  18   b  bonded to the leading end  12   a  of the cylindrical metal shell  12  by welding. In addition, each ground electrode  18  takes the form of a columnar body (such as, for instance, a square column) and has the leading end  18   a  extending toward the center electrode  16  in face-to-face relation with a side face of the center electrode  16 . 
   While the spark plug  10  is shown with reference to the embodiment in which the plural ground electrodes  18  provide a plurality of spark discharge gaps  20 , it will be appreciated that the spark plug  10  may include one ground electrode  18  with one spark discharge gap  20 . In an alternative, the spark plug  10  may be modified in structure so as to allow a leading end of a ground electrode to be placed in face-to-face relation with a leading end of a center electrode as shown in  FIG. 15 . 
   Further, the Ir alloy tip (center electrode tip)  24 , composed of a square shaped rod made of Ir alloy, is bonded to the base body leading end  26   a  of the center electrode base material body  22  by welding as shown in  FIG. 2 . In addition, each of the ground electrode  18  includes a ground electrode base body  18   a  having a ground electrode trailing end  18   b , bonded to the leading end  12   a  of the cylindrical metal shell  12 , and a ground electrode leading end  18   c . An Ir alloy tip (ground electrode tip)  32 , made of square shaped rod-like Ir alloy, is bonded to the ground electrode leading end  18   c  of each ground electrode base body  18   a . The Ir alloy tips  32  are placed in face-to-face relationship with the leading end  24   b  of the Ir alloy tip  24  of the center electrode  16 . More particularly, the Ir alloy tip  32  of each ground electrode  18  is placed in opposition to one side face of the Ir alloy tip  24  of the square shaped rod-like center electrode  16 . 
   In addition, the Ir alloy tip  32  of each ground electrode  18  may have a columnar shape in cross section. 
   In fabricating the center electrode  16 , the square shaped rod-like Ir alloy tip  24  is bonded to the leading end  26   a  of the center electrode base material body  22  by circumferential welding using laser welding as shown in  FIGS. 2 and 7  and  FIGS. 12 to 14 . 
   That is, in bonding the Ir alloy tip  24  to the center electrode base material body  22 , the square shaped rod-like Ir alloy tip  24  is preliminarily unitized with the leading end  26   a  of the center electrode base material body  22  by resistance welding or the like. Thereafter, the square shaped rod-like Ir alloy tip  24  is completely bonded to the leading end  26   a  of the center electrode base material body  22  by laser welding. This laser welding is performed by irradiating a pulse laser beam PL onto a contact area (joint portion)  34  between the leading end  26   a  of the base material body  26  and the trailing end (upper end)  24   a  of the Ir alloy tip  24  at sixteen irradiating points circumferentially and equidistantly spaced from each other while turning the base material body  26  about an axis thereof as shown in  FIGS. 12 and 13 . Thus, performing laser welding on the joint portion  34  allows the center electrode  16  to be fabricated in a structure shown in  FIG. 14 . 
   With such a structure of the center electrode  16  resulting form laser welding, the center electrode base material body  22  and the Ir alloy tip  24  are bonded to each other via a fused layer  30 . 
   Moreover, each Ir alloy tip  32  and each ground electrode base body  18   a  are also bonded to each other through a fused layer  30  formed by laser welding. 
   With the present embodiment, moreover, while the spark plug  10  is shown with reference to a structure including the center electrode  16  and the ground electrodes  18  with the Ir alloy tip  24  and the Ir alloy tips  32  bonded to the center electrode  16  and the ground electrodes  18  to be exposed to the spark discharge gaps  20 , the present invention is not limited to such a structure. That is, the present invention may be implemented even in a modified structure with the Ir alloy tip  24  provided onto only the center electrode  16 . 
   Further, each of the Ir alloy tips  24  and  32  preferably contains Ir of more than 50% by weight and at least one additive with a melting point exceeding a temperature of 2000° C. At least one additive may include material of one kind selected from the group consisting of Pt, Rh, Ni, W, Pd, Ru, Os, Al, Y and Y 2 O 3 . This is because the spark plug  10  has an advantage of Ir with a high melting point higher than 2000° C. while having adequate wear-resistance against spark discharges. 
   Turning back to  FIG. 3 , further, the Ir alloy tip  24  of the center electrode  16  has the substantially square shape in cross section that is shaped in a quadrate. 
     FIG. 10  is a schematic view showing a spark plug  10 A in an alternative form. As shown in  FIG. 10 , the spark plug  10 A in the alternative form includes a center electrode  16 A provided with an Ir alloy tip  24 A having a rectangular shape in cross section. 
   With the spark plug  10 A having the center electrode  16 A provided with the rectangular Ir alloy tip  24 A, a circle internally touching both parallel sides with a short distance corresponds to the inscribed circle C B . 
     FIG. 11  is a schematic view showing a spark plug  10 B in another alternative form. As shown in  FIG. 11 , the spark plug  10 B in another alternative form includes a center electrode  16 B provided with an Ir alloy tip  24 B having a rhombic shape in cross section. In still another alternative, the spark plug  10 B may take the other rectangular form in cross section depending on needs. 
   With the spark plug  10 B having the center electrode  16 B provided with the Ir alloy tip  24 B having a cross section other than the square shape and the rectangular shape, a circumscribed circle with a diameter taking a long diagonal line corresponds to the circumscribed circle CA of the Ir alloy tip as shown in  FIG. 11 . 
   As shown in  FIGS. 3 and 6 , further, the base material body  26  and the base material leading end portion  26   a  have diameters D 1 , D 1  falling in the relationship expressed as 0.5×D 1 ≦D 2 ≦0.95×D 1  and, more preferably, in the relationship expressed as 0.7×D 1 ≦D 2 ≦0.9×D 1 . 
   Furthermore, the base material body  26  of the center electrode base material body  22  is selected to have a diameter D 1  greater than 2.5 mm. 
   In addition, as shown in  FIG. 7 , the fused layer  30 , formed in the joint portion  34  between the base material leading end portion  26   a  of the center electrode base material body  22  and the Ir alloy tip  24 , has a given depth in a radial direction of the center electrode  16  to provide an unfused portion  40  formed in a width F. The width F of the unfused portion  40  and the diameter A (see  FIG. 3 ) of the circumscribed circle CA of the Ir alloy tip lie in the relationship expressed as F≦0.2×A. 
   Further, as shown in  FIGS. 2 ,  8  and  9 , the center electrode base material body  22  extends through the center through-bore  50  of the porcelain insulator  14  in concentric relationship therewith. 
   As best shown in  FIG. 9 , the fused layer  30  includes an inward boundary edge  52 , formed in the base material leading end portion  26   a  of the base material body  26 , and an outward boundary edge  54  formed in the Ir alloy tip  24 , with the inward and outward boundary edges  52 ,  54  being contiguous with each other via a curved outer profile  56 . The base material body  26  of the center electrode base material body  22  has a leading end formed with an annular shoulder  26   b  that is exposed to an annular space  58 , defined between the insulator leading end portion  14   a  and the base material leading end portion  26   a , to which the inward boundary edge  52  of the fused layer  30  is also exposed. 
   As shown in  FIGS. 8 and 9 , the curved outer profile  56  of the fused layer  30  is spaced from the bore end  50   a  of the center through-bore  50  formed in the porcelain insulator  14  by a radial distance H greater than 0.1 mm over an entire circumference of the bore end  50   a  of the center through-bore  50 . 
   In addition, with such a structure of the center electrode  16 , the inward boundary edge  52  of the fused layer  30  faces the annular space  58  defined between the insulator leading end portion  14   a  and the base material leading end portion  26   a.    
   The spark plug  10  of the present embodiment formed in such a structure has advantages effects listed below. 
   As shown in  FIG. 3 , the diameter D 1  of the base material body  26  of the center electrode base material body  22 , the diameter D 2  of the base material leading end portion  26   a , the diameter A of the circumscribed circle CA of the Ir alloy tip  24  and the diameter B of the inscribed circle C B  have the relationship expressed as D 1 &gt;A&gt;D 2 &gt;B. This allows the center electrode base material body  22  and the Ir alloy tip  24  to ensure increased bonding capability, making it possible to provide an increase in productivity of the spark plug  10 . 
   That is, with the dimensional relationship A&gt;D 2 &gt;B, the spark plug  10  can have a minimized unevenness in distance between a circumferential periphery of the base material leading end portion  26   a  of the center electrode base material body  22  and a sidewall of the Ir alloy tip  24 . This allows the fused layer  30  to be formed of substantially and homogeneously mixed materials containing materials of the center electrode base material body  22  and the Ir alloy tip  24  over an entire circumference of the fused layer  30 . 
   As a result, the fused layer  30  becomes possible to have thermal expansion coefficient in agreement with thermal expansion coefficient in the vicinity of an intermediate value between those of the center electrode base material body  22  and the Ir alloy tip  24 . This enables a remarkable reduction in thermal stresses acting on the fused layer  30  between the center electrode base material body  22  and the Ir alloy tip  24 . 
   Accordingly, even under circumstances where the spark plug  10  is frequently exposed to thermal shocks in rapid heating and rapid cooling in repeated cycles, no flaking or cracking occurs on the joint portion  34  between the base material leading end portion  26   a  of the center electrode base material body  22  and the Ir alloy tip  24  to ensure increased bonding capability between the center electrode base material body  22  and the Ir alloy tip  24 . Thus, the spark plug  10  can have highly increased operating life even exposed to thermal shocks in repeated cycles. 
   With the spark plug  10  achieving such a dimensional relationship expressed as A&gt;D 2 &gt;B, furthermore, a distance between an outer circumferential periphery of the joint portion  34  between the base material leading end portion  26   a  of the center electrode base material body  22  and the Ir alloy tip  24  and a center of the Ir alloy tip  24  can be shortened even in an area in a diagonal region of the Ir alloy tip  24 . Therefore, the fused layer  30  is liable to reach the center of the Ir alloy tip  24  upon completion of laser welding, preventing the formation of the unfused region  40  in a widened area of the Ir alloy tip  24 . That is, the base material leading end portion  26   a  of the center electrode base material body  22  and the Ir alloy tip  24  can be bonded to each other at the joint portion  34  with a minimized unfused region  40 . This results in a reduction in thermal stresses acting on an area between the center electrode base material body  22  and the Ir alloy tip  24 . 
   With the diameter D 1  of the base material body  26  of the center electrode base material body  22  and the diameter A of the circumscribed circle C A  of the Ir alloy tip  24  falling in the relationship expressed as D 1 &gt;A, moreover, the Ir alloy tip  24  can be bonded to the center electrode base material body  22  before inserting the center electrode base material body  22  to the through-bore  50  of the porcelain insulator  14  on an assembling stage of the spark plug  10 . That is, since the diameter D 1  of the center electrode base material body  22  is made less than a diameter of the center through-bore  50  of the porcelain insulator  14 , both the Ir alloy tip  24  and the porcelain insulator  14  can be passed through the center through-bore  50  of the porcelain insulator  14 . Therefore, the Ir alloy tip  24  can be bonded to the center electrode base material body  22  before the center electrode base material body  22  is retained in the porcelain insulator  14 , providing increased productivity of the spark plug  10 . In addition, another advantage resides in the fact that a less limitation occurs in a bonding position between the center electrode base material body  22  and the Ir alloy tip  24 . 
   Further, since the Ir alloy tip  24  takes the substantially square pole configuration, a wearing speed of the Ir alloy tip  24  resulting from spark discharge can be maintained at a low level, making it easy to allow the spark plug  10  to have prolonged operating life. 
   Moreover, with the diameter D 1  of the center electrode base material body  22  and the diameter D 2  of the base material leading end portion  26   a  falling in the relationship as expressed as 0.5×D 3 ≦D 2 ≦0.95×D 1 , the Ir alloy tip  24  can be bonded to the center electrode base material body  22  with increased bonding capability, making it possible to increase operating life of the Ir alloy tip  24 . 
   That is, with the diameter D 1  of the center electrode base material body  22  and the diameter D 2  of the base material leading end portion  26   a  satisfying relationship expressed as 0.5×D 1 ≦D 2 , heat developed in the Ir alloy tip  24  can be easily transferred to a base end portion of the center electrode  16  via the base material leading end portion  26   a . This results in an increase in operating life of the Ir alloy tip  24 . 
   In addition, with the relationship established as D 2 ≦0.95×D 1 , a difference between the diameter D 2  of the base material leading end portion  26   a  and the diameter A of the circumscribed circle C A  of the Ir alloy tip  24  becomes possible to be substantially equal to a difference between the diameter D 2  of the base material leading end portion  26   a  and the diameter B of the inscribed circle C B  of the Ir alloy tip  24  while establishing the relationship D 1 &gt;A&gt;D 2 &gt;B. This results in a capability for the Ir alloy tip  24  and the center electrode base material body  22  to be bonded to each other with increased bonding capability. 
   Moreover, with the relationship established as 0.7×D 1 ≦D 2 ≦0.9×D 1 , the Ir alloy tip  24  and the center electrode base material body  22  can be bonded to each other with increased bonding capability. This enables the Ir alloy tip  24  to have increased operating life in a further easy fashion. 
   Further, the diameter D 1  of the center electrode base material body  22  lies in a value greater than 2.5 mm, the spark plug  10  of the present embodiment can exhibit the advantageous effect of the present invention in adequate fashion. That is, in general practice, with a spark plug having a center electrode base material body with a diameter greater than 2.5 mm, the center electrode base material body and an Ir alloy tip suffers from deterioration in bonding capability (see fifth embodiment). Therefore, applying the present invention to such a spark plug results in capability for the spark plug to adequately exhibit the advantageous effect of the present invention. 
   Further, the width F (see  FIG. 7 ) of the unfused region  40  inside the fused layer  30 , formed in the joint portion  34  between the center electrode base material body  22  and the Ir alloy tip  24 , have the relationship expressed as F≦0.2×A. This prevents increased thermal stress from occurring on the unfused region  4 , making it possible to ensure increased bonding capability between the center electrode base material body  22  and the Ir alloy tip  24 . 
   As shown in  FIG. 9 , furthermore, the fused layer  30  has the outer circumferential periphery  56  whose overall circumference is spaced from the bore end  50   a  of the insulator leading end portion  14   a  of the porcelain insulator  14  by the distance H greater than 0.1 mm. This enables an adequate clearance to be provided between the bore end  50   a  of the insulator leading end portion  14   a  and the diffused layer  30 . Therefore, even if residuals of combustion gases accumulate on the bore end  50   a , no adverse affect occurs on the porcelain insulator  14 . That is, adequately ensuring the distance H between the bore end  50   a  and the curved surface  56  of the fused layer  30  results in a capability of preventing the residuals, caused by combustion of fuel gas, from clogging between the bore end  50   a  and the fused layer  30 . Therefore, no cracking takes place in the porcelain insulator  14  with no occurrence of thermal stresses occurring on between the center electrode  16  and the porcelain insulator  14 . 
   Moreover, the fused layer  30  is formed in the joint portion  34  between the center electrode base material body  22  and the Ir alloy tip  24  such that the inward boundary edge  52  is present in the annular space  58  between the insulator leading end portion  14   a  and the fused layer  30  of the center electrode  16 . Thus, this enables a reduction in temperature rise of the fused layer  30 , making it possible to prevent the occurrence of excessive thermal stress in the fused layer  30 . 
   In addition, with the center electrode  16  formed in such a structure, the inward boundary edge  52  of the fused layer  30  is spaced from the bore leading edge  50   a  of the center through-bore  50  of the porcelain insulator  14  by the distance H greater than 0.1 mm over the entire circumference of the bore end  50   a , less damage occurs on the porcelain insulator  14  in an effective fashion. 
   Further, with the Ir alloy tip  24  formed the substantially square shape to be quadrate in cross section, the center electrode base material body  22  and the Ir alloy tip  24  can be positioned with respect to each other in an easy fashion. This provides an ease for the center electrode  16 , including the Ir alloy tip  24  bonded to the center electrode base material body  22 , to be inserted through the porcelain insulator  16 . This results in a capability of obtaining the spark plug  10  with increased productivity. 
   With the spark plug  10  of the present embodiment, as set forth above, it becomes possible to provide a spark plug for an internal combustion engine that has increased bonding capability between a center electrode base material body and an Ir alloy tip while having elongated operating life with increased productivity of the spark plug. 
   Second Embodiment 
   A spark plug  10 C of a second embodiment according to the present invention is described below with reference to  FIGS. 16 to 20 . 
   The spark plug  10 C of the present embodiment differs from the gas sensor  1  of the first embodiment in that the spark plug  10 C includes a center electrode  16 C having an Ir alloy tip  24 C, formed in a substantially square shape in cross section on a plane perpendicular to an axis of the center electrode  16 C, which has flat side surfaces  60  with four corners formed with rounded chamfered portions (curved portions)  60   a.    
   As shown in  FIG. 17 , the Ir alloy tip  24 C has a diagonal line A extending between a pair of the curved portions  60   a  in a diagonal direction. A circumscribed circle with a diameter equal to the diagonal line A corresponds to the circumscribed circle C A  Of the Ir alloy tip expressed in the first embodiment. The spark plug  10 C of the present embodiment satisfies the relationship of the first embodiment expressed as D 1 &gt;A&gt;D 2 &gt;B. 
   As shown in  FIG. 18 , further, the spark plug  10 C may be altered such that a center electrode  16 D includes an Ir alloy tip  24 D that is rectangular in cross section having side surfaces  62  with four corners formed with curved portions  62 D like those of the center electrode  16 C of the second embodiment shown in  FIGS. 16 and 17 . Even with such a structure of the Ir alloy tip  24 D, the relational formula mentioned above can be satisfied. 
   As shown in  FIG. 16 , further, the Ir alloy tip  32 C has the side surface  60  with a straight portion L and a width W that lies in the relationship expressed as 0.8×W≦L&lt;W. 
   Further, the length L of the straight portion of the side surface  60  of the Ir alloy tip  24 C and a width M of an opposing surface  64  of an Ir alloy tip  32 C, forming part of a ground electrode  18 D, facing the side surface  60  of the Ir alloy tip  24 C have the relationship expressed as M&lt;L. 
   Furthermore, the opposing surface  64  of the Ir alloy tip  32 C, placed in face of the lateral side surface  60  of the Ir alloy tip  24 C of the center electrode  16 C, is dimensioned not to protrude from the vertical side surfaces  60  of the Ir alloy tip  24 C. As shown in  FIG. 16 , more preferably, the opposing surface  64  of the Ir alloy tip  32 C is set not to protrude from the straight portion L of the lateral surface  60  of the Ir alloy tip  24 C. However, even if a sidewall  66  of the Ir alloy tip  32 C partly protrudes from the straight portion  60  of the Ir alloy tip  24 C, the Ir alloy tip  32 C of the ground electrode  18 D is set in a position not to cause the sidewall  66  of the Ir alloy tip  32 C from protruding from the side surface  60  of the Ir alloy tip  24 C of the center electrode  16 C as shown in  FIG. 19 . 
   That is, during assembly of the spark plug  10 C, the center electrode  16 C and the ground electrode  18 D are arranged such that the sidewall  66  of the Ir alloy tip  32 C of the ground electrode  18 D does not protrude from the side surface  60  of the Ir alloy tip  24 C as shown in a placement condition shown in  FIG. 20 . 
   As shown in  FIG. 19 , the curved portion  60   a  of the Ir alloy tip  24 C of the center electrode  16 C has a depth J, extending along the side surface  60  placed in face-to-face relationship with the side surface  64  of the Ir alloy tip  32 C of the ground electrode  18 D, which lies in a value less than 0.3 mm. 
   The spark plug  10 C of the second embodiment has the same other structure as that of the spark plug  10  of the first embodiment shown in  FIGS. 1 to 3 . 
   Next, various advantageous effects of the spark plug  10 C of the second embodiment are described below. 
   With the Ir alloy tip  24 C formed in the substantially rectangular shape in cross section with the four corners formed with the curved portions  60   a , a thermal stress is prevented from acting on the corner of the Ir alloy tip  24 C in concentration. This provides an increase in bonding capability between the Ir alloy tip  24 C and the center electrode base material body  22 . 
   In addition, the length L of the straight portion of the side surface  60  and the width W of the Ir alloy tip  24 C have the relationship expressed as 0.8×W≦L&lt;W. This allows the Ir alloy tip  24 C to have adequate opposing surface area in opposition to the ground electrode  18 D, enabling the spark plug  10 C to have increased operating life. 
   Further, the width M of the opposing surface  64  of the Ir alloy tip  32 C of the ground electrode  18 D, placed in face of the side surface  60  of the Ir alloy tip  24 C of the center electrode  16 C, and the length L of the straight portion of the side surface  60  of the Ir alloy tip  24 C have the relationship expressed as M&lt;L. This results in a capability for the Ir alloy tip  32 C of the ground electrode  18 D and the straight portion of the side surface  60  of the Ir alloy tip  24 C of the center electrode  16 C to easily achieve a spark discharge, enabling the Ir alloy tip  24 C to have prolonged life time. 
   Furthermore, the center electrode  16 C and the ground electrode  18 D are set such that the opposing surface  64  of the Ir alloy tip  32 C does not protrude from the side surface  60  of the Ir alloy tip  24 C. Therefore, the Ir alloy tip  24 C of the center electrode  24 C and the Ir alloy tip  32 C of the ground electrode  18 D can face each other in an adequate surface area, thereby achieving prolonged operating life of the sparkplug  10 C. 
   Moreover, the Ir alloy tip  24 C, having the corners each formed with the curved portion  60   a  having the depth J falling in a value less than 0.3 mm, enables the prevention of a reduction in an opposing surface area between the Ir alloy tip  32 C of the ground electrode  18 D and the Ir alloy tip  24 C of the center electrode  16 C. This makes it possible to achieve prolonged operating life of the Ir alloy tip  24 C. 
   That is, under a situation where the depth J of the curved portion  60   a  of the Ir alloy tip  24 C exceeds 0.3 mm, if the curved portion  60   a  of the Ir alloy tip  24 C of the center electrode  16 C is placed in face of the opposing surface  64  of the Ir alloy tip  32 C of the ground electrode  18 D, the side surface  60  of the Ir alloy tip  24 C is worn on an early stage with a fear of the occurrence of a reduction in operating life of the Ir alloy tip  24 C. That is, with the spark discharge gap  20  enlarged to a value greater than 0.3 mm, the misfiring is apt to occur. In this case, with the corner  60   a  of the Ir alloy tip  24 C having the depth J exceeding a value of 0.3 mm, the spark plug  10 C has an increase in a range where the spark discharge gap  20 , defined between the curved portion  60   a  of the Ir alloy tip  24 C of the center electrode  16 C and the Ir alloy tip  32 C of the ground electrode  18 D, exceeds a value of 0.3 mm on an early stage with the resultant fear of an appropriate opposing surface area decreasing on an early stage. 
   Therefore, as set forth above, the presence of the Ir alloy tip  24 C having the corners with each depth J falling in a value less than 0.3 mm enables the prevention of a reduction in an opposing surface area of the Ir alloy tip  24 C of the center electrode  16 C, thereby enabling the spark plug  10 C to have prolonged operating life. 
   In addition, the spark plug  10 C of the present embodiment has the other advantageous effects as those of the spark plug  1  of the first embodiment mentioned above. 
   Third Embodiment 
   A spark plug  10 E of a third embodiment according to the present invention is described with reference to  FIG. 21 . 
   With the spark plug  10 E of the present embodiment, as shown in  FIG. 21 , an Ir alloy tip  24 E of a center electrode  16 E has a substantially square shape in cross section with four corners each formed with a curved portion (rounded chamfered portion)  60   a  that is placed in an area on a contour of the base material leading end portion  26   a  of the center electrode base material body  22  or within the contour of the base material leading end portion  26   a.    
   Even with such a structure of the center electrode  16 E, the Ir alloy tip  24 E and the center electrode base material body  22  are specified in the dimensional relationship and configurations as explained below to obtain the same advantageous effects as those of the spark plug  10  of the first embodiment set forth above. 
   That is, it is supposed that, among diagonal lines of the substantially rectangular shapes of the Ir alloy tip  24 E, a long diagonal line has a length E. In addition, suppose that a square shape is defined with four straight portions  60  of the substantially rectangular shape of the Ir alloy tip  24 E and a virtual circumscribed circle C C  is defined with a diameter, equal to the long diagonal line among the diagonal lines of the square shape, which is designated as “C”. Then, the relationship is established as D 1 &gt;C&gt;D 2 . 
   That is, as shown in  FIG. 21 , even if the diameter D 2  of the base material leading end portion  26   a  and the length of the diagonal line E fall in the relationship expressed as D 2 ≧E, the center electrode  16 E may be suffice to satisfy the relationship expressed as D 1 &gt;C&gt;D 2 . 
   The spark plug  10 E of the third embodiment has the other features as those of the spark plug  10 C of the second embodiment set forth above. 
   With such relationships established, even if the curved portion  60   a  of the Ir alloy tip  24 E is located in an area inside the base material leading end portion  26   a  of the center electrode  16 E, the virtual circumscribed circle C C  is disposed in an area between the contour of the base material body  26  and the contour of the base material leading end portion  26   a . This allows the spark plug  10 E to have the same advantageous effects as those of the spark plug  10  of the first embodiment, enabling the center electrode base material body  22  and the Ir alloy tip  24 E to have increased bonding capability. 
   In addition, the spark plug  10 E of the third embodiment has other advantageous effects as those of the spark plug  10 C of the second embodiment. 
   (First Evaluation Test) 
   Various spark plugs were prepared as test pieces to check states in which the fused layers  30  were formed with the Ir alloy tips  24  bonded to the center electrode base bodies  22  with the base material leading end portions  26   a  having three different diameters D 2 . The base material main bodies  26  had the diameters D 1  of 2.9 mm. In addition, the Ir alloy tips  24  had a square shape in cross section with 2.7 mm on a side. 
     FIGS. 22A ,  23 A and  24 A are views showing the Ir alloy tip  24  and the center electrode base bodies  22  of the center electrode  16  as viewed from the leading end thereof.  FIGS. 22B ,  23 B and  24 B are views of the center electrode  16  taken on line K-K of  FIGS. 22A ,  23 A and  24 A, respectively.  FIGS. 22C ,  23 C and  24 C are views of the center electrode  16  taken on line N-N of  FIGS. 22A ,  23 A and  24 A, respectively. 
   In this Evaluation Test, the base material leading end portions  26   a  were prepared in diameter D 2  of 2.1 mm, 2.4 mm and in diameter of D 1 =D 2 =2.9 mm in the same structure as that of the related art center electrode in which no base material leading end portions  26   a  were prepared under which laser welding was performed in the same condition as that shown in  FIGS. 12 and 13  conducted in the first embodiment. The resulting fused layers  30  are shown in  FIGS. 22B ,  22 C,  FIGS. 23B ,  23 C and  FIGS. 24B and 24C . 
   As shown in  FIGS. 22B and 22C , with the base material leading end portion  26   a  having the diameter D 2  of 2.1 mm, the fused layer  30  had overlapping portions  30   a  formed in a center portion  70  of the joint portion  34  between the Ir alloy tip  24  and the base material leading end portions  26   a  with no formation of the unfused area  40 . As shown in  FIG. 23B , with the base material leading end portion  26   a  having the diameter D 2  of 2.4 mm, the fused layer  30  had abutting portions  30   b  formed in the center portion  70  of the joint portion  34  between the Ir alloy tip  24  and the base material leading end portions  26   a . With the structure of the center electrode  16  shown in  FIG. 23C , the fused layer  30  had the deepest ends  30   c  with the unfused region  40  remained in a slight area. However, the width F of such an unfused area  40  and the diameter A of the circumscribed circle C A  of the Ir alloy tip satisfies the relationship F≦0.2×A (see  FIGS. 1 and 7  of the first embodiment). 
   Meanwhile, as shown in  FIGS. 24B and 24C , with the base material leading end portion  26   a  having the diameter D 2  of 2.9 mm, the fused layer  30  had the deep ends  30   d ,  30   d  with the formation of a comparatively large unfused region  40 . 
   (Second Evaluation Test) 
   Various spark plugs were prepared with the base material leading end portions  26   a  formed in various diameters D 2  to check bonding capabilities of the spark plugs, with the measured results being plotted in  FIGS. 25 to 27 . 
   Moreover, the base material body  26  of the center electrode  16  had a diameter D 1  of 3.0 mm. 
     FIG. 25  shows a graph showing an Ir content (a ratio of Ir content) in the fused layer  30  by weight % plotted in terms of the diameter D 2  of the base material leading end portions  26   a . Analyzed results on the Ir content appearing in cross section taken on line K-K are indicated as Qk and analyzed results on the Ir content appearing in cross section taken on line N-N are indicated as Qn. 
   Further,  FIG. 26  shows the relationship between the diameter D 2  and a fusion depth Rk of the fused layer appearing in cross section taken on line K-K and a fusion depth Rn of the fused layer appearing in cross section taken on line N-N. The fusion depths Rk, Rn represent dimensions shown in  FIG. 24 . 
   Furthermore,  FIG. 27  shows variation in a linear coefficient of expansion in terms of ratio of Iridium (Ir) to Inconel (Ni alloy) forming the center electrode base material body  22 . The center electrode base material body  22  has a linear coefficient of expansion laying at a value nearly intermediate between those of Inconel and Iridium with the Ir ratio remaining in the vicinity of 65% by weight. 
   Moreover,  FIG. 28  shows a flaking incidence rate (%) between the center electrode base material body  22  and the Ir alloy tip  24  on thermal shock endurance tests in terms of the diameter D 2  of the base material leading end portion  26   a . The thermal shock endurance tests were conducted on test pieces that included the center electrodes  16 , each composed of the center electrode base material body  22  and the Ir alloy tip  24 , which were placed in an atmosphere under 900° C. for six minutes and, thereafter, left at room temperature of 25° C. for six minutes. Such thermal shock endurance cycles were conducted in repeated cycles of 400 times. In obtaining the flaking incidence rate, tests were conducted on eight pieces of samples for each level to check a length of flaked area in the joint portion of the center electrode  16  in cross section taken on line K-K and a length of a welded area of the center electrode  16 . These factors were measured and an average value in ratio between these factors was calculated as the flaking incidence rate. 
   As will be apparent from  FIG. 25 , the spark plug of the related art having a center electrode with D 1 =D 2 =3.0 mm with no provision of the base material leading end portion  26   a  undergoes an increased difference in Ir ratio appearing on the joint portion of the center electrode in cross section taken on lines K-K and N-N and the resulting Ir ratios have lower values than an idealistic value of 65% by weight. 
   As will be apparent from  FIG. 26 , further, the center electrode, falling in the relationship of D 1 =D 2 =3.0 mm, have the fusion depths Rk, Rn that are low with respect to D 1 , D 2  in the presence of an increase in the unfused are 40 (see  FIG. 24 ). 
   Therefore, as shown in  FIG. 28 , in case of the center electrode with D 1 =D 2 =3.0 mm, the flaking incidence rate increases to a value of 50%. 
   Further, in case of the center electrode with D 2 =2.7 mm, the Ir ratio exceeds a value of 50% (see  FIG. 25 ) and the unfused area is present only in small area (see  FIG. 23 ) with the flaking incidence rate decreased to be less than a value of 25% (see  FIG. 28 ). 
   Furthermore, in case of the center electrode with D 2  less than 2.4 mm, the Ir ratio is stabilized in a region in the vicinity of 65% (see  FIG. 25 ) and, as shown in  FIGS. 22B and 22C , the fused layer  30  had the overlapping portions  30   a  in the absence of the unfused portion  40 . The flaking incidence rate was found to be less than 15% (see  FIG. 28 ). It is thus confirmed that the sparks plugs of the present invention can have the center electrodes favorably welded conditions. 
   While the specific embodiment of the present invention has been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limited to the scope of the present invention which is to be given the full breadth of the following claims and all equivalents thereof.