Patent Publication Number: US-9837797-B2

Title: Ignition plug

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to Japanese Patent Application No. 2016-052004 filed on Mar. 16, 2016 and Japanese Patent Application No. 2016-202561 filed on Oct. 14, 2016, the disclosures of which are herein incorporated by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present specification relates to an ignition plug for igniting fuel gas in, for example, an internal combustion engine. 
     Description of the Related Art 
     Regarding ignition plugs, a technology of joining an electrode tip to an electrode body in order to increase the durability of the electrode is known (see, for example, PTL 1). The electrode tip is made of a material that is more durable with respect to spark discharge and oxidation than the electrode body. Examples of the material include a noble metal (such as platinum, iridium, ruthenium, and rhodium) and an alloy containing a noble metal as a main component. Since the electrode body and the electrode tip are joined to each other by using various methods, such as laser welding and resistance welding, a welding portion is formed between the electrode body and the electrode tip. 
     When an ignition plug is used in an internal combustion engine, thermal stress occurs in the welding portion due to combustion heat. Therefore, cracks tend to occur at a boundary between the electrode tip and the welding portion and at a boundary between the electrode body and the welding portion. When such cracks occur at these boundaries, the electrode tip may be peeled off from the electrode body. 
     CITATION LIST 
     Patent Literature 
     Patent Document 1 is Japanese Patent Application Laid-Open (kokai) No. 2015-125879. 
     BRIEF SUMMARY OF THE INVENTION 
     Here, since ignition plugs tend to be used under higher temperature environments due to, for example, higher output of internal combustion engines in recent years, the aforementioned thermal stress tends to be large. Therefore, for ignition plugs, a technology of increasing resistance with respect to peeling of the electrode tip from the electrode body (hereunder referred to as “anti-peeling performance”) is required. 
     The present specification discloses a technology that is capable of increasing anti-peeling performance of an electrode tip. 
     The technology that is disclosed in the present specification can be realized in, for example, the following application examples. 
     First Application Example 
     An ignition plug includes an insulator that includes a through hole; a center electrode that includes a first discharge surface and that is held at a front end side of the through hole; a metal shell that is disposed around the insulator in a radial direction and that holds the insulator; a bar-shaped ground electrode body that includes a joining end surface and a free end surface, the joining end surface being joined to a front end of the metal shell, the free end surface being positioned opposite to the joining end surface; a ground electrode tip that, in a vicinity of the free end surface of the ground electrode body, is disposed along a side surface of the ground electrode body opposing the first discharge surface, and that includes a second discharge surface opposing the first discharge surface; and a welding portion that is disposed between the ground electrode tip and the ground electrode body, and that includes a component of the ground electrode tip and a component of the ground electrode body. In a section which extends through a center of gravity of the second discharge surface, which is perpendicular to the second discharge surface, and which is parallel to an axial line of the ground electrode body: 
     when a direction from the center of gravity of the second discharge surface to the free end surface along the second discharge surface is a first direction, and a direction opposite to the first direction is a second direction; 
     when, of an end, located in the first direction, of a boundary between the welding portion and the ground electrode tip and an end, located in the first direction, of a boundary between the welding portion and the ground electrode body, the end that is positioned towards a side in the second direction is a first end; and 
     when an end of the ground electrode tip located in the second direction is a second end; 
     an end of the welding portion located in the first direction is exposed at the free end surface; 
     the welding portion extends along the axial line of the ground electrode body; and 
     in an entire ¼ range, provided at a side of the second end, of a range in the first direction from the first end to the second end, (i.e., over an entire sub-range of a range, the range extending from the first end to the second end, the sub-range being ¼ of the range nearest the second end) a length L 1  of the ground electrode tip in a direction perpendicular to the first direction and a length L 2  of the welding portion in the direction perpendicular to the first direction satisfy (L 2 /L 1 )≧0.25. 
     According to the above-described structure, in the ¼ range, provided at the second end side and where thermal stress tends to occur, of the range in the first direction from the first end to the second end, the length L 2  of the welding portion in the direction perpendicular to the first direction can be made sufficiently large with respect to the length L 1  of the ground electrode tip in the direction perpendicular to the first direction. As a result, thermal stress can be properly reduced by the welding portion, so that it is possible to increase anti-peeling performance of the ground electrode tip. 
     Second Application Example 
     In the ignition plug according to the first application example, in the section, further, in the range in the first direction from the first end to the second end in an entirety thereof, the length L 1  of the ground electrode tip in the direction perpendicular to the first direction and the length L 2  of the welding portion in the direction perpendicular to the first direction satisfy (L 2 /L 1 )≧0.25. 
     According to the above-described structure, in the range in the first direction from the first end to the second end in its entirety, the length L 2  of the welding portion in the direction perpendicular to the first direction can be made sufficiently large with respect to the length L 1  of the ground electrode tip in the direction perpendicular to the first direction. As a result, thermal stress can be further properly reduced by the welding portion, so that it is possible to further increase anti-peeling performance of the ground electrode tip. 
     Third Application Example 
     In the ignition plug according to the first application example or the second application example, in the section, further, a length L 3  from the second end to an end of the welding portion located in the second direction is greater than or equal to 0.1 mm. 
     According to the above-described structure, since the welding portion can more effectively reduce thermal stress in the vicinity of the end of the ground electrode tip located in the second direction, it is possible to further increase anti-peeling performance of the ground electrode tip. 
     Fourth Application Example 
     In the ignition plug according to any one of the first application example to the third application example, in the section, further, in the entire ¼ range, provided at the side of the second end, of the range in the first direction from the first end to the second end, the length L 1  of the ground electrode tip in the direction perpendicular to the first direction and the length L 2  of the welding portion in the direction perpendicular to the first direction satisfy (L 2 /L 1 ) 0.5. 
     According to the above-described structure, it is possible to prevent the occurrence of cracks in the ground electrode tip caused by the length L 1  of the ground electrode tip in the direction perpendicular to the first direction being excessively small with respect to the length L 2  of the welding portion in the direction perpendicular to the first direction in the ¼ range, provided at the second end side. 
     Fifth Application Example 
     In the ignition plug according to any one of the first application example to the fourth application example, an end of the ground electrode tip located in the first direction is positioned towards the side in the second direction than the free end surface of the ground electrode body is (i.e., the free end surface extends in the first direction more than the first end of the ground electrode tip). 
     According to the above-described structure, since the joining area can be made sufficiently large with respect to the size of the ground electrode tip, it is possible to further increase anti-peeling performance of the ground electrode tip. 
     The technology that is disclosed in the present specification can be realized in various forms. For example, the technology can be realized in an ignition plug, an ignition system using the ignition plug, an internal combustion engine in which the ignition plug is installed, and an internal combustion engine in which the ignition system using the ignition plug is installed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Illustrative aspects of the invention will be described in detail with reference to the following figures wherein: 
         FIG. 1  is a sectional view of an ignition plug according to an embodiment; 
         FIGS. 2A and 2B  each illustrate a structure of a vicinity of a ground electrode tip for a ground electrode according to a first embodiment; 
         FIGS. 3A, 3B, and 3C  each illustrate a method of manufacturing the ground electrode; 
         FIG. 4  illustrates a structure of a vicinity of a ground electrode tip for a ground electrode according to a second embodiment; and 
         FIG. 5  illustrates an exemplary modification of the ground electrode. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A. First Embodiment: Structure of Ignition Plug 
       FIG. 1  is a sectional view of an ignition plug  100  according to an embodiment. The alternate long and short dash line in  FIG. 1  indicates an axial line CO of the ignition plug  100  (also called the “axial line CO”). Directions that are parallel to the axial line CO (up-down directions in  FIG. 1 ) are also called axial directions. Radial directions of a circle around the axial line CO are also simply called “radial directions”, and circumferential directions of the circle around the axial line CO are also simply called “circumferential directions”. The downward direction in  FIG. 1  is also called a “front end direction FD”, and the upward direction in  FIG. 2  is also called a “rear end direction BD”. A lower side in  FIG. 1  is called a “front end side” of the ignition plug  100 , and an upper side in  FIG. 1  is called a “rear end side” of the ignition plug  100 . The ignition plug  100  includes an insulator  10 , a center electrode  20 , a ground electrode  30 , a terminal metal shell  40 , and a metal shell  50 . 
     The insulator  10  is formed by sintering, for example, alumina. The insulator  10  is a substantially cylindrical member having a through hole  12  (axial hole) extending along the axial directions and through the insulator  10 . The insulator  10  includes a flange  19 , a rear-end-side body  18 , a front-end-side body  17 , a stepped portion  15 , and an insulator nose length portion  13 . The rear-end-side body  18  is positioned towards the rear end side than the flange  19  is, and has an outside diameter that is smaller than the outside diameter of the flange  19 . The front-end-side body  17  is positioned towards the front end side than the flange  19  is, and has an outside diameter that is smaller than the outside diameter of the flange  19 . The insulator nose length portion  13  is positioned towards the front end side than the front-end-side body  17  is, and has an outside diameter that is smaller than the outside diameter of the front-end-side body  17 . When the ignition plug  100  is installed in an internal combustion engine (not shown), the insulator nose length portion  13  is exposed to a combustion chamber thereof. The stepped portion  15  is disposed between the insulator nose length portion  13  and the front-end-side body  17 . 
     The metal shell  50  is a cylindrical metal shell that is made of a conductive metal material (such as low-carbon steel) and that is provided for securing the ignition plug  100  to an engine head (not shown) of the internal combustion engine. The metal shell  50  has an insertion hole  59  extending therethrough along the axial line CO. The metal shell  50  is disposed around the insulator  10  in a radial direction (that is, is disposed along an outer periphery of the insulator  10 ). In other words, the insulator  10  is inserted and held in the insertion hole  59  in the metal shell  50 . The front end of the insulator  10  protrudes towards the front end side from the front end of the metal shell  50 . The rear end of the insulator  10  protrudes towards the rear end side from the rear end of the metal shell  50 . 
     The metal shell  50  includes a tool engaging portion  51 , a mounting threaded portion  52 , and a flanged seating portion  54 . The tool engaging portion  51  has a hexagonal prism shape, and allows an ignition plug wrench to engage therewith. The mounting threaded portion  52  is provided for being installed in the internal combustion engine. The seating portion  54  is disposed between the tool engaging portion  51  and the mounting threaded portion  52 . The nominal diameter of the mounting threaded portion  52  is, for example, M8 (8 mm), M10, M12, M14, or M18. 
     An annular gasket  5 , which is formed by bending a metal plate, is fitted and inserted in a space between the mounting threaded portion  52  and the seating portion  54  of the metal shell  50 . When the ignition plug  100  is installed in the internal combustion engine, the gasket  5  seals a gap between the ignition plug  100  and the internal combustion engine (engine head). 
     The metal shell  50  further includes a thin crimping portion  53  that is disposed on the rear end side of the tool engaging portion  51 , and a thin compression deformation portion  58  that is disposed between the seating portion  54  and the tool engaging portion  51 . Ring members  6  and  7  are disposed in an annular region that is formed between an inner peripheral surface, extending from the tool engaging portion  51  to the crimping portion  53 , of the metal shell  50  and an outer peripheral surface of the rear-end-side body  18  of the insulator  10 . Talc  9  in the form of powder fills a space between the two ring members  6  and  7  in this region. The rear end of the crimping portion  53  is bent inward in a radial direction, and is fixed to the outer peripheral surface of the insulator  10 . The compression deformation portion  58  of the metal shell  50  is, during manufacturing, compressed and deformed by pressing the crimping portion  53 , which is fixed to the outer peripheral surface of the insulator  10 , towards the front end side. By compressing and deforming the compression deformation portion  58 , the insulator  10  is pressed towards the front end side in the metal shell  50  via the ring members  6  and  7  and the talc  9 . By a stepped portion  56  (metal-shell stepped portion), which is formed at an inner periphery of the mounting threaded portion  52  of the metal shell  50 , the stepped portion  15  (insulator stepped portion) of the insulator  10  is pressed. As a result, gas in the combustion chamber of the internal combustion engine is prevented from leaking to the outside from a gap between the metal shell  50  and the insulator  10  by a plate packing  8 . 
     The center electrode  20  includes a center electrode body  21  that is bar-shaped and that extends in the axial directions, and a center electrode tip  29 . The center electrode body  21  is held in a front-end-side portion in the through hole  12  in the insulator  10 . The center electrode body  21  includes an electrode base material  21 A and a core  21 B that is buried in the electrode base material  21 A. The base material  21 A is composed of, for example, nickel or an alloy whose main component is nickel (such as NCF 600 and NCF 601). The core  21 B is made of copper or an alloy whose main component is copper, the copper and the copper alloy having a thermal conductivity that is higher than that of the alloy of which the electrode base material  21 A is composed. In the embodiment, the core  21 B is made of copper. 
     The center electrode body  21  includes a flange  24  (also called the “flanged portion”) that is disposed in a predetermined location in the axial directions, a head  23  (electrode head) that is disposed towards the rear end side than the flange  24  is, and a leg  25  (electrode leg) that is disposed towards the front end side than the flange  24  is. The flange  24  is supported by the stepped portion  16  of the insulator  10 . A front end portion of the leg  25 , that is, the front end of the center electrode body  21  protrudes towards the front end side from the front end of the insulator  10 . 
     The center electrode tip  29  is a substantially columnar member, and is joined to the front end of the center electrode body  21  (the front end of the leg  25 ) by, for example, laser welding. The front end surface of the center electrode tip  29  is a first discharge surface  295  that forms a spark gap between the front end surface of the center electrode tip  29  and a ground electrode tip  39  (described later). The center electrode tip  29  is made of, for example, a material whose main component is a noble metal having a high melting point. Examples of the material of the center electrode tip  29  are iridium (Ir) or an alloy whose main component is Ir. 
     The ground electrode  30  includes a ground electrode body  31  that is joined to the front end of the metal shell  50 , and the quadrangular-prism-shaped ground electrode tip  39 . The ground electrode body  31  is a bar-shaped member that is curved and that has a square shape in cross section. The ground electrode body  31  includes a free end surface  311  and a joining end surface  312  as two end surfaces. The joining end surface  312  is joined to a front end surface  50 A of the metal shell  50  by, for example, resistance welding. This causes the metal shell  50  and the ground electrode body  31  to be electrically coupled to each other. 
     The ground electrode body  31  is made of, for example, nickel or an alloy whose main component is nickel (such as NCF 600 and NCF 601). The ground electrode body  31  has a two-layer structure including a base material and a core. The base material is composed of a metal having high anti-corrosiveness (such as a nickel alloy). The core is made of a metal having high thermal conductivity (such as copper), and is buried in the base material. 
     The terminal metal shell  40  is a bar-shaped member that extends in the axial directions. The terminal metal shell  40  is made of a conductive metal material (such as low-carbon steel). A metal layer (such as an Ni layer), which is provided for corrosion protection, is formed on a surface of the terminal metal shell  40  by, for example, plating. The terminal metal shell  40  includes a flange  42  (terminal flange) that is disposed in a predetermined location in the axial directions, a cap mounting portion  41  that is positioned towards the rear end side than the flange  42  is, and a leg  43  (terminal leg) that is disposed towards the front end side than the flange  42  is. The cap mounting portion  41  of the terminal metal shell  40  is exposed towards the rear end side from the insulator  10 . The leg  43  of the terminal metal shell  40  is inserted in the through hole  12  in the insulator  10 . A plug gap to which a high-voltage cable (not shown) is connected is mounted on the cap mounting portion  41 . A high voltage for generating a spark discharge is applied to the cap mounting portion  41 . 
     A resistor  70  for reducing radio noise when a spark is generated is disposed between the front end of the terminal metal shell  40  (the front end of the leg  43 ) and the rear end of the center electrode  20  (the rear end of the head  23 ) in the through hole  12  in the insulator  10 . The resistor  70  is made of, for example, a composite material of glass particles as main component, ceramic particles other than glass particles, and a conductive material. In the through hole  12 , a gap between the resistor  70  and the center electrode  20  is filled with a conductive seal  60 . A gap between the resistor  70  and the terminal metal shell  40  is filled with a conductive seal  80 . The conductive seal  60  and the conductive seal  80  are made of a composite material of glass particles (such as B 2 O 3 —SiO 2 -based glass particles) and metal particles (such as Cu particles and Fe particles). 
     Structure of Vicinity of Ground Electrode Tip  39  for Ground Electrode  30   
     The structure of a vicinity of the ground electrode tip  39  for the ground electrode  30  is described in more detail.  FIGS. 2A and 2B  each illustrate the structure of the vicinity of the ground electrode tip  39  for the ground electrode  30  according to a first embodiment.  FIG. 2A  illustrates a section CF of a vicinity of the front end of the ignition plug  100  resulting from cutting through the vicinity by a particular plane. The ground electrode tip  39  has a substantially columnar shape. The rear end surface of the ground electrode tip  39  is a second discharge surface  395  opposing the first discharge surface  295  (see  FIG. 1 ) of the center electrode tip  29 . The section CF in  FIG. 2A  is a plane which extends through a center of gravity GC of the second discharge surface  395 , which is perpendicular to the second discharge surface  395 , and which is parallel to an axial line of the bar-shaped ground electrode body  31 . In the embodiment, a line that extends through the center of gravity GC of the second discharge surface  395  and that is perpendicular to the second discharge surface  395  coincides with the axial line CO of the ignition plug  100 . Therefore, it can be said that the section CF in  FIG. 2A  is a section that extends through the axial line CO of the ignition plug  100  and that is parallel to the axial line of the bar-shaped ground electrode body  31 . 
       FIG. 2B  illustrates a vicinity of the second discharge surface  395  of the ground electrode tip  39  when seen in the front end direction FD from the rear end direction BD. The alternate long and short dash line in  FIG. 2B  indicates the section CF in  FIG. 2A . The direction from the center of gravity GC of the second discharge surface  395  to the free end surface  311  along the second discharge surface  395 , that is, a left direction in  FIGS. 2A and 2B  is a first direction D 1 . The direction away from the free end surface  311  along the second discharge surface  395  from the center of gravity GC of the second discharge surface  395 , that is, a direction opposite to the first direction D 1 , is a second direction D 2 . 
     Of the four side surfaces that cross the free end surface  311  of the ground electrode body  31 , a side surface opposing the first discharge surface  295  is a side surface  315 . Of the four side surfaces of the ground electrode body  31 , two of the side surfaces that cross the side surface  315 , that is, the side surfaces that are located in the up-down directions in  FIG. 2B  are side surfaces  313  and  314 . The direction towards the side surface  313  from the center of gravity GC of the second discharge surface  395 , that is, the downward direction in  FIG. 2B  is a third direction D 3 , and a direction opposite to the third direction D 3  is a fourth direction D 4 . 
     In the vicinity of the free end surface  311  of the ground electrode body  31 , the ground electrode tip  39  is disposed along the side surface  315 . More specifically, a concave portion  316  that is recessed in the front end direction FD from the side surface  315  is formed in the vicinity of the free end surface  311  of the ground electrode body  31 . A portion of the ground electrode tip  39  that is opposite to the second discharge surface  395  (a portion of the ground electrode tip  39  located towards the front end direction FD) is disposed in the concave portion  316 . The second discharge surface  395  of the ground electrode tip  39  protrudes in the rear end direction BD from the side surface  315  of the ground electrode body  31 . As shown in  FIG. 2B , the concave portion  316  has, when seen along the axial directions, a shape that is substantially similar to (square shape in the embodiment) and slightly larger than the shape of the ground electrode tip  39  (square shape in the embodiment) when seen along the axial directions. 
     As illustrated in the section CF in  FIG. 2A , a side surface  391  of the ground electrode tip  39  located in the first direction D 1  is positioned towards the side in the second direction D 2  than the free end surface  311  of the ground electrode body  31  is. 
     The ground electrode tip  39  is joined to the ground electrode body  31  by laser welding. Therefore, a welding portion  35 , formed by the laser welding, is disposed between the ground electrode tip  39  and the ground electrode body  31 . The welding portion  35  is a portion formed by melting and solidifying a portion of the ground electrode tip  39  before the welding and a portion of the ground electrode body  31 . Therefore, the welding portion  35  includes the component of the ground electrode tip  39  and the component of the ground electrode body  31 . The welding portion  35  may also be called a joint where the ground electrode tip  39  and the ground electrode body  31  are joined to each other, or may also be called a bead where the ground electrode tip  39  and the ground electrode body  31  are joined to each other. 
     In  FIG. 2B , the hatched region indicates the welding portion  35 . As can be seen from  FIG. 2B , the welding portion  35  when seen along the axial directions has a shape that is larger than the shape of the ground electrode tip  39  (square shape in the embodiment) when seen along the axial directions, and that is substantially similar to (square shape in the embodiment) and that is slightly larger than the shape of the concave portion  316  when seen along the axial directions. Ends  351  to  354  of the welding portion  35  located in the four directions D 1  to D 4  are positioned outward with respect to the corresponding side surface  391  and corresponding side surfaces  392  to  394  of the ground electrode  39  in the radial directions. A side of the welding portion  35  located in the rear end direction BD contacts the entire surface of the ground electrode tip  39  opposite to the second discharge surface  395  (surface located in the front end direction FD). 
     As illustrated in  FIG. 2A , the end  351  of the welding portion  35  located in the first direction D 1  (also called the “exposed end  351 ”) is exposed at the free end surface  311  of the ground electrode body  31 . The ends  352 ,  353 , and  354  of the welding portion  35  located in the corresponding second direction D 2 , third direction D 3 , and fourth direction D 4  are not exposed at the corresponding surfaces (such as the side surfaces  313  and  314 ) of the ground electrode body  31 . As illustrated in  FIG. 2A , in the section CF, the welding portion  35  extends along the second direction D 2  (the first direction D 1 ). The axial line of the bar-shaped ground electrode body  31  is parallel to the second direction D 2  (the first direction D 1 ) in the vicinity of the free end surface  311 , where the welding portion  35  is formed. Therefore, it can be said that, in the section CF, the welding portion  35  extends along the axial line of the ground electrode body  31 . This is because, as described below, when the welding portion  35  is formed by laser welding, laser beams are applied in the second direction D 2  from the free end surface  311 . 
     Here, a length of the ground electrode tip  39  in directions perpendicular to the first direction D 1  (axial-direction length) is a thickness L 1  of the ground electrode tip  39 , and a length of the welding portion  35  in the directions perpendicular to the first direction D 1  is a thickness L 2  of the welding portion  35 . Although the thickness L 1  of the ground electrode tip  39  is not limited to certain values, the thickness L 1  is, for example, 0.2 mm to 1.0 mm. 
     As illustrated in  FIG. 2A , a portion of the welding portion  35  in the vicinity of the exposed end  351  is an exposure neighboring portion  35 A, a substantially center portion of the welding portion  35  that includes a portion crossing the axial line CO is a center portion  35 B, and a portion of the welding portion  35  that is located in the second direction D 2  from an end of the ground electrode tip  39  located in the second direction D 2  (that is, the side surface  392 ) is a far-side portion  35 C. The thickness L 2  of the welding portion  35  is larger at the exposure neighboring portion  35 A than at the center portion  35 B. At the center portion  35 B, the thickness L 2  of the welding portion  35  does not change greatly, and is substantially uniform. The thickness L 2  of the welding portion  35  is partly large at the far-side portion  35 C because the welding portion is formed between the side surface  392  of the ground electrode tip  39  located in the second direction D 2  and the concave portion  316  of the ground electrode body  31 . 
     Here, in the section CF in  FIG. 2A , an end, located in the first direction D 1 , of a boundary BF 1  between the welding portion  35  and the ground electrode tip  39  is an end P 1 , and an end, located in the first direction D 1 , of a boundary BF 2  between the welding portion  35  and the ground electrode body  31  is an end P 2 . Of the end P 1  and the end P 2 , the end that is positioned towards the side in the second direction D 2  is a first end. In the embodiment in  FIG. 2A , the first end is the end P 1 . The end of the ground electrode tip  39  located in the second direction D 2  (that is, the side surface  392 ) is a second end. 
     Here, a range in the first direction D 1  from the first end to the second end is a range RA 1  (a range having a length W in  FIG. 2A ). A ¼ range, provided at the second end side, of the range RA 1  is a range RA 2  (a range having a length W/4 in  FIG. 2A ). In the embodiment in  FIG. 2 , the length W of the range RA 1  is equal to the width of the ground electrode tip  39  in the second direction D 2 . Although the length W is not limited thereto, the length W is, for example, from 1.0 mm to 2.0 mm, such as 1.3 mm, 1.5 mm, and 1.8 mm. 
     The ¼ range RA 2 , provided at the second end side (the side in the second direction), is, similarly to the first end side (a side in the first direction), situated in the vicinity of the front end of the ignition plug  100 , so that the ¼ range RA 2  is situated near a high-temperature region in the combustion chamber. Therefore, the ¼ range RA 2 , provided at the second end side, tends to become hot. Further, compared to the first end side, the ¼ range RA 2 , provided at the second end side, is close to the joining end surface  312  of the ground electrode body  31 . As a result, the amount of heat conduction is large. Therefore, compared to the first end side, temperature changes are severe in the ¼ range RA 2 , provided at the second end side. Consequently, peeling caused by thermal stress at the boundaries BF 1  and BF 2  tends to occur. 
     As the thickness L 1  of the ground electrode tip  39  increases with respect to the thickness L 2  of the welding portion  35 , thermal stress at the boundaries BF 1  and BF 2  can be reduced. This is because the thermal stress at the boundaries BF 1  and BF 2  occurs due to the difference between the thermal expansion coefficient of the ground electrode tip  39  and that of the ground electrode body  31 , and the welding portion  35  that contains the components of both the ground electrode tip  39  and the ground electrode body  31  has a thermal expansion coefficient that is between that of the ground electrode tip  39  and that of the ground electrode body  31 . In the embodiment, the entire range RA 2  satisfies the condition (L 2 /L 1 )≧0.25. That is, in the entire range RA 2 , the thickness L 2  of the welding portion  35  is greater than or equal to ¼ of the thickness L 1  of the ground electrode tip  39 . As a result, by making the welding portion  35  sufficiently thick, it is possible to properly reduce thermal stress, so that anti-peeling performance of the ground electrode tip  39  can be increased. 
     In the embodiment, further, in the range RA 1  from the first end to the second end in its entirety, the aforementioned condition (L 2 /L 1 )≧0.25 is satisfied. As a result, in the range RA 1  in its entirety, the thickness L 2  of the welding portion  35  can be made sufficiently large with respect to the thickness L 1  of the ground electrode tip  39 . As a result, the welding portion  35  can further properly reduce thermal stress occurring between the ground electrode tip  39  and the ground electrode body  31 , so that it is possible to further increase anti-peeling performance of the ground electrode tip  39 . 
     Here, in the section CF in  FIG. 2A , the length from the side surface  392  (the aforementioned second end) of the ground electrode tip  39  located in the second direction D 2  to the end  352  of the welding portion  35  located in the second direction D 2  is a far-side protruding length L 3 . In the embodiment, the far-side protruding length L 3  is greater than or equal to 0.1 mm. This way, in the vicinity of the side surface  392  at the second-direction side of the ground electrode tip  39 , the far-side portion  35 C of the welding portion  35  can more effectively reduce thermal stress, so that it is possible to further increase anti-peeling performance of the ground electrode tip  39 . 
     If the welding portion  35  is made too thick with respect to the thickness of the ground electrode tip  39 , the ground electrode tip  39  becomes too thin. As a result, the strength of the ground electrode tip  39  is reduced, as a result of which thermal stress causes cracks to occur in the ground electrode tip  39 , and causes the ground electrode tip  39  to break. In the embodiment, in the entire range RA 2 , the condition (L 2 /L 1 )≦0.5 is satisfied. That is, in the entire range RA 2 , the thickness L 2  of the welding portion  35  is less than or equal to half of the thickness L 1  of the ground electrode tip  39 . As a result, it is possible to prevent the occurrence of cracks in the ground electrode tip  39  caused by the thickness L 1  of the ground electrode tip  39  being too small with respect to the thickness L 2  of the welding portion  35 . Therefore, it is possible to increase crack resistant performance of the ground electrode tip  39 . 
     Further, in the embodiment, as described above, the side surface  391  of the ground electrode tip  39  located in the first direction D 1  is positioned towards the side in the second direction D 2  than the free end surface  311  of the ground electrode body  31  is. As a result, the joining area, that is, the area of contact with the welding portion  35  can be made sufficiently large with respect to the size of the ground electrode tip  39 . Therefore, it is possible to further increase anti-peeling performance of the ground electrode tip  39 . 
     Manufacturing Method 
     A method of manufacturing the ignition plug  100  is described while focusing on a method of manufacturing the ground electrode  30 .  FIGS. 3A and 3B  each illustrate the method of manufacturing the ground electrode  30 . First, the bar-shaped ground electrode body  31  that is not yet bent is provided. Then, the ground electrode tip  39  that is not yet welded to the ground electrode body  31  is provided. 
     Next, as shown in  FIG. 3A , by using, for example, a predetermined pressing machine, a pressing member  200  having a shape corresponding to the shape of the concave portion  316  to be formed is pressed into a portion in the vicinity of the free end surface  311  of the side surface  315  of the ground electrode body  31 . This causes the concave portion  316  to be formed in the side surface  315  of the ground electrode body  31  as shown in  FIG. 3B . 
     Next, as shown in  FIG. 3C , the columnar ground electrode tip  39  that is not yet welded is disposed in the concave portion  316  in the ground electrode body  31 . Then, while holding the ground electrode  30  in the front end direction FD (downward direction in  FIG. 3C ) from the side of the second discharge surface  395  by using a jig (not shown), laser welding is performed to form the above-described welding portion  35  (see  FIGS. 2A and 2B ). An arrow LZ in  FIG. 3C  conceptually indicates application of laser for performing the laser welding. As shown by the arrow LZ, a laser beam is applied in the second direction D 2  from the side of the free end surface  311  and along the boundary between the ground electrode tip  39  and the ground electrode body  31 . In the embodiment, a fiber laser is used as the laser. Compared to, for example, a YAG laser, the fiber laser has high light-condensing ability. Therefore, the welding portion  35  that can be formed has high shape flexibility. Consequently, it is possible to form the welding portion  35  having a shape that satisfies the above-described conditions such as the condition (L 2 /L 1 )≧0.25. 
     First Evaluation Test: 
     In a first evaluation test, as shown in Table 1, fourteen Samples 1 to 14 in which at least one of the lengths W, L 1 , L 2 , and L 3  in  FIG. 2A  differed were used to conduct anti-peeling performance tests of the ground electrode tip  39 . 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                 Oxide Scale 
                   
               
               
                   
                   
                   
                   
                   
                   
                 Occurrence  
                   
               
               
                   
                   
                   
                   
                   
                   
                 Rate 
                 Evaluation 
               
               
                 No. 
                 W 
                 L1 
                 L2 
                 L2/L1 
                 L3 
                 [%] 
                 Result 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                  1 
                 1.3 
                 0.43 
                 0.05 
                 0.12 
                 0.2 
                 35 
                 C 
               
               
                  2 
                 1.3 
                 0.4 
                 0.08 
                 0.20 
                 0.0 
                 30 
                 C 
               
               
                  3 
                 1.3 
                 0.4 
                 0.1 
                 0.25 
                 0.2 
                 5 
                 A 
               
               
                  4 
                 1.3 
                 0.4 
                 0.1 
                 0.25 
                 0.1 
                 6 
                 A 
               
               
                  5 
                 1.3 
                 0.4 
                 0.1 
                 0.25 
                 0.0 
                 15 
                 B 
               
               
                  6 
                 1.3 
                 0.35 
                 0.15 
                 0.43 
                 0.2 
                 2 
                 A 
               
               
                  7 
                 1.3 
                 0.35 
                 0.15 
                 0.43 
                 0.1 
                 3 
                 A 
               
               
                  8 
                 1.8 
                 0.42 
                 0.04 
                 0.10 
                 0.2 
                 40 
                 C 
               
               
                  9 
                 1.8 
                 0.4 
                 0.09 
                 0.23 
                 0.0 
                 33 
                 C 
               
               
                 10 
                 1.8 
                 0.39 
                 0.1 
                 0.26 
                 0.2 
                 7 
                 A 
               
               
                 11 
                 1.8 
                 0.39 
                 0.1 
                 0.26 
                 0.1 
                 9 
                 A 
               
               
                 12 
                 1.8 
                 0.39 
                 0.1 
                 0.26 
                 0.0 
                 20 
                 B 
               
               
                 13 
                 1.8 
                 0.37 
                 0.14 
                 0.38 
                 0.2 
                 4 
                 A 
               
               
                 14 
                 1.8 
                 0.37 
                 0.14 
                 0.38 
                 0.1 
                 6 
                 A 
               
               
                   
               
            
           
         
       
     
     The lengths W in the sections CF ( FIG. 2A ) are equal to the widths of the ground electrode tips  39  in the second direction D 2 . Therefore, by varying the widths of the ground electrode tips  39  in the second direction D 2 , the lengths W in the sections CF were adjusted to either one of 1.3 mm and 1.8 mm as shown in Table 1. The widths of the ground-electrode-tip- 39  samples in the third direction D 3  were the same as the widths of the ground-electrode tip- 39  samples in the second direction D 2  (1.3 mm or 1.8 mm). 
     The lengths L 1  to L 3  in the section CF ( FIG. 2A ) were adjusted by varying the lengths of the ground electrode tips  39  before welding in the axial directions and the conditions of laser welding for forming the welding portions  35 . Table 1 shows, for each sample, the values of L 1  ( mm ) and L 2  ( mm ), at a location in the first direction D 1  where the value (L 2 /L 1 ) becomes a minimum, and the value of (L 2 /L 1 ) in the range RA 2  in  FIG. 2A . When the minimum value of (L 2 /L 1 ) in the range RA 2  satisfies the condition (L 2 /L 1 )≧0.25, the condition (L 2 /L 1 ) 0.25 is satisfied over the entire range RA 2 . 
     The minimum value of (L 2 /L 1 ) in the range RA 2  for each of the Samples 1 to 14 is any one of 0.10, 0.12, 0.20, 0.23, 0.25, 0.26, 0.38, and 0.43. 
     The far-side protruding length L 3  of each of the Samples 1 to 14 is any one of 0.0 mm, 0.1 mm, and 0.2 mm. 
     The common materials and dimensions of the samples are as follows: 
     Ground electrode tip  39 : alloy containing platinum (Pt) as main component and 10 mass % of nickel (Ni) 
     Ground electrode body  31 : NCF601 alloy 
     Width H 1  (height) of the ground electrode body  31  in the axial directions in the vicinity of the free end surface  311 : 1.5 mm 
     Width H 2  of the ground electrode body  31  in the third direction D 3  in the vicinity of the free end surface  311 : 2.8 mm 
     Width (height) of the ground electrode tip  39  before welding in the axial directions: 0.45 mm 
     In the first evaluation test, a desk cooling test described below was performed. A cycle of heating and cooling the vicinity of the front end portion of each sample (the vicinity of each ground electrode tip  39 ) was repeated 1000 times. More specifically, in one cycle, the vicinity of the front end portion of each sample was heated for two minutes by using a burner, and was subsequently cooled in air for one minute. The intensity of the burner was adjusted such that, during the two minutes of heating, the temperature of each ground electrode tip  39  reached a temperature of 1100° C. (target temperature) in one minute, and, then, this temperature of 1100° C. was maintained. 
     Thereafter, each ground-electrode- 30  sample was cut to observe the section CF ( FIG. 2A ) of each sample. Then, in each section CF, portions where the joints at the boundaries BF 1  and BF 2  were maintained and any peeled portion at the boundaries BF 1  and BF 2  in the range RA 2  were identified. At the portions where the joints were maintained, oxide scales did not occur, whereas, at the any peeled portion, oxide scales occurred. Therefore, it is possible to identify the portions where the joints are maintained and the any peeled portion by observing the section CF of each sample by using a magnifying glass. The proportion of the range RA 2  occupied by the any peeled portion from the end at the side in the second direction D 2  (that is, the portion where oxide scales occurred) was calculated. (This proportion may hereunder also be called the “oxide scale occurrence rate”.) The oxide scale occurrence rate of each sample is as shown in Table 1. When the oxide scale occurrence rate was less than 10%, the sample evaluation result was “A”; when the oxide scale occurrence rate was 10% to less than 25%, the sample evaluation result was “B”; and when the oxide scale occurrence rate was greater than or equal to 25%, the sample evaluation result was “C”. 
     The evaluation results are as shown in Table 1. The evaluation results of the Samples 3 to 7 and Samples 10 to 14 satisfying the condition (L 2 /L 1 )≧0.25 in the entire range RA 2  were “B” or better regardless of the length W (the width of the corresponding ground electrode tip  39  in the second direction D 2 ) and the far-side protruding length L 3 . The evaluation results of the Samples 1, 2, 8, and 9, where the minimum value of (L 2 /L 1 ) in the range RA 2  was (L 2 /L 1 )&lt;0.25, were “C” or worse. For example, the oxide scale occurrence rates of the samples satisfying the condition (L 2 /L 1 )≧0.25 in the entire range RA 2  was smaller by at least 10% than the oxide scale occurrence rates of the samples whose minimum value of (L 2 /L 1 ) in the entire range RA 2  was (L 2 /L 1 )&lt;0.25. 
     Further, among the evaluation results of the Samples 3 to 7 and 10 to 14 satisfying the condition (L 2 /L 1 )≧0.25 in the entire range RA 2 , the evaluation results of the Samples 3, 4, 6, 7, 10, 11, 13, and 14, whose far-side protruding lengths L 3  were greater than or equal to 0.1 mm, were all “A”. The evaluation results of the Samples 5 and 12, whose far-side protruding lengths L 3  were less than 0.1 mm, were both “B”. For example, the scale occurrence rates of the Samples 3, 4, 6, 7, 10, 11, 13, and 14, whose far-side protruding lengths L 3  were greater than or equal to 0.1 mm, were smaller by at least 9% than the scale occurrence rates of the Samples 5 and 12, whose far-side protruding lengths L 3  were less than 0.1 mm. 
     On the basis of the results of the first evaluation test, it was confirmed that it is desirable to satisfy the condition (L 2 /L 1 )≧0.25 in the entire range RA 2  from the viewpoint of increasing anti-peeling performance. In addition, it was confirmed that it is more desirable that the far-side protruding length L 3  be greater than or equal to 0.1 mm. 
     Second Evaluation Test: 
     In a second evaluation test, as shown in Table 2, six Samples 15 to 20 in which at least one of the length W, L 1 , and L 2  in  FIG. 2A  differed were used to conduct crack resistant performance tests of ground electrode tips  39 . 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 No. 
                   
                   
                   
                   
                   
                 Evaluation 
               
               
                 Result 
                 W 
                 L1 
                 L2 
                 L2/L1 
                 L3 
                 Result 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 15 
                 1.3 
                 0.3 
                 0.15 
                 0.50 
                 0.2 
                 A 
               
               
                 16 
                 1.3 
                 0.3 
                 0.2 
                 0.67 
                 0.2 
                 B 
               
               
                 17 
                 1.3 
                 0.25 
                 0.2 
                 0.80 
                 0.2 
                 B 
               
               
                 18 
                 1.8 
                 0.35 
                 0.15 
                 0.43 
                 0.2 
                 A 
               
               
                 19 
                 1.8 
                 0.32 
                 0.18 
                 0.56 
                 0.2 
                 B 
               
               
                 20 
                 1.8 
                 0.28 
                 0.2 
                 0.71 
                 0.2 
                 B 
               
               
                   
               
            
           
         
       
     
     As in the first evaluation test, by varying the widths of the ground electrode tips  39  in the second direction D 2 , the widths W in the sections CF ( FIG. 2A ) were adjusted to either one of 1.3 mm and 1.8 mm as shown in Table 2. 
     The lengths L 1  to L 3  in the sections CF ( FIG. 2A ) were adjusted by varying the lengths of the ground electrode tips  39  before welding in the axial directions and by using different conditions for laser welding for forming welding portions  35 . Table 2 shows, for each sample, the values of L 1  ( mm ) and L 2  ( mm ), at a location in the first direction where the value (L 2 /L 1 ) becomes a maximum, and the value of (L 2 /L 1 ) in the range RA 2 . When the maximum value of (L 2 /L 1 ) in the range RA 2  satisfies the condition (L 2 /L 1 ) 0.5, the condition (L 2 /L 1 )≦0.5 is satisfied over the entire range RA 2 . 
     The maximum value of (L 2 /L 1 ) in the range RA 2  for each of the Samples 15 to 20 is any one of 0.50, 0.67, 0.80, 0.43, 0.56, and 0.71. The far-side protruding length L 3  of each of the Samples 15 to 20 is 0.2 mm. The material of each sample is the same as the material of each sample in the first evaluation test. 
     In the second evaluation test, a desk cooling test was performed under the same conditions as those in the first evaluation test. Thereafter, each ground-electrode-tip- 39  sample was observed to confirm the occurrence and non-occurrence of cracks. Evaluation results of samples in which cracks were not observed were “A”, and evaluation results of samples in which cracks were observed were “B”. 
     The evaluation results are as shown in Table 2. The evaluation results of the Samples 15 and 18 satisfying the condition (L 2 /L 1 )≦0.5 in the entire range RA 2  were “A” regardless of the length W (the width of each ground electrode tip  39 ). The evaluation results of the Samples 16, 17, 19 and 20, where the maximum value of (L 2 /L 1 ) in the range RA 2  was (L 2 /L 1 )&gt;0.5, were “B”. 
     On the basis of the results of the second evaluation test, it was confirmed that it is desirable to satisfy the condition (L 2 /L 1 )≦0.5 in the entire range RA 2  from the viewpoint of increasing crack resistant performance of the ground electrode tip  39 . 
     B. Second Embodiment 
       FIG. 4  illustrates a structure of a vicinity of a ground electrode tip  39   b  of a ground electrode  30   b  according to a second embodiment. The width of the ground electrode tip  39   b  in  FIG. 4  in the second direction D 2  is greater than that of the ground electrode tip  39  in  FIG. 2 . In addition, a side surface  391   b  of the ground electrode tip  39   b  located in the first direction D 1  protrudes towards the side in the first direction D 1  with respect to a free end surface  311   b  of a ground electrode body  31   b . Therefore, the shape of a welding portion  35   b  of the ground electrode  30   b  in  FIG. 4  differs from the shape of the welding portion  35  of the ground electrode  30  in  FIGS. 2A and 2B . 
     More specifically, the welding portion  35   b  does not contact a portion  396   b , disposed at the side in the first direction D 1 , of a surface of the ground electrode tip  39   b  at the side in the front end direction FD. An end  351   b  of the welding portion  35   b  located in the first direction D 1  is exposed at the free end surface  311   b  of the ground electrode body  31   b  and at the portion  396   b , disposed at the side in the first direction D 1 , of the surface of the ground electrode tip  39   b  at the side in the front end direction FD. 
     The other features of the welding portion  35   b  are similar to those of the welding portion  35  in  FIGS. 2A and 2B . For example, an end  352   b  of the welding portion  35   b  located in the second direction D 2  protrudes towards the second direction D 2  with respect to the side surface  391   b  of the ground electrode tip  39   b . A thickness L 2  of the welding portion  35   b  is larger at an exposed vicinity  35   b  A than at a center portion  35   b  B. The thickness L 2  of the welding portion  35   b  is substantially uniform without changing greatly at the center portion  35   b  B. The thickness L 2  of the welding portion  35  is partly large at a far-side portion  35   b  C. 
     As shown in  FIG. 4 , such welding portion  35   b  is formed by applying a laser beam LZ, used for laser welding, to a boundary between the ground electrode tip  39   b  and the ground electrode body  31   b  from the side of the free end surface  311   b  in a direction that is slightly inclined with respect to the second direction D 2 . 
     Here, as in the first embodiment, in a section CF in  FIG. 4 , an end, located in the first direction D 1 , of a boundary BF 1  between the welding portion  35   b  and the ground electrode tip  39   b  is an end P 1 , and an end, located in the first direction D 1 , of a boundary BF 2  between the welding portion  35   b  and the ground electrode body  31   b  is an end P 2 . Of the end P 1  and the end P 2 , the end that is positioned towards the side in the second direction D 2  is a first end; and a side surface  392   b  of the ground electrode tip  39   b  located in the second direction D 2  is a second end. In the second embodiment in  FIG. 4 , unlike the first embodiment in  FIGS. 2A and 2B , since the end P 2  is positioned towards the side in the second direction D 2  than the end P 1  is, the first end is the end P 2 . 
     Here, a range in the first direction D 1  from the first end to the second end is a range RA 1   b  (a range having a length Wb in  FIG. 4 ). A ¼ range, provided at the second end side, of the range RA 1   b  is a range RA 2   b  (a range having a length Wb/4 in  FIG. 4 ). 
     In the second embodiment, as in the first embodiment, in the section in  FIG. 4 , the following Conditions (A) to (D) are satisfied: 
     (A) In the entire range RA 2   b , (L 2 /L 1 )≧0.25 is satisfied. 
     (B) In the range RAlb in its entirety, (L 2 /L 1 )≧0.25 is satisfied. 
     (C) A far-side protruding length L 3  is greater than or equal to 0.1 mm. 
     (D) The entire range RA 2   b  satisfies (L 2 /L 1 )&lt;0.5. 
     Since the aforementioned Conditions (A) to (C) are satisfied, even in the second embodiment, as in the first embodiment, it is possible to increase anti-peeling performance of the ground electrode tip  39   b . In addition, since the Condition (D) is satisfied, even in the second embodiment, as in the first embodiment, it is possible to increase crack resistant performance of the ground electrode tip  39   b.    
     Modifications 
     (1) In the first embodiment, in the section CF in  FIG. 2A , the following Conditions (A) to (D) are satisfied as already mentioned above: 
     (A) In the entire range RA 2 , (L 2 /L 1 )≧0.25 is satisfied. 
     (B) In the range RA 1  in its entirety, (L 2 /L 1 )≧0.25 is satisfied. 
     (C) The far-side protruding length L 3  is greater than or equal to 0.1 mm. 
     (D) In the entire range RA 2 , (L 2 /L 1 )&lt;0.5 is satisfied. 
     In a modification, not all of the aforementioned Conditions (A) to (D) need to be satisfied. Only at least the aforementioned Condition (A) needs to be satisfied, so that none of the aforementioned Conditions (B) to (D) need to be satisfied, or some of the aforementioned Conditions (B) to (D) need not be satisfied. As can be understood from the results of the first evaluation test, as long as at least the Condition (A) is satisfied, it is possible to increase anti-peeling performance of the ground electrode tip  39 . 
     (2) In the first embodiment, the aforementioned Conditions (A) to (D) are satisfied not only in the section CF in  FIG. 2A , but also in all sections that are parallel to the section CF and that are within a range that extends through the second discharge surface  395  of the ground electrode tip  39 . In a modification, the aforementioned Conditions (A) to (D) need not be satisfied in all of the sections that extend through the discharge surface  395 , so that the aforementioned Conditions (A) to (D) only need to be satisfied in at least the section CF. In addition, of all of the sections that are parallel to the section CF and that are within the range that extends through the second discharge surface  395  of the ground electrode tip  39 , it is desirable that the Conditions (A) to (D) be satisfied in a range that is 50% or greater from the section CF as a center; and it is further desirable that the Conditions (A) to (D) be satisfied in a range that is 80% or greater from the section CF as a center. 
     (3) The specific structure of the ground electrode  30  in  FIGS. 2A and 2B  and the specific structure of the ground electrode  30   b  in  FIG. 4  are examples, so that other specific structures are possible. The specific structure of the ground electrode  30  in  FIGS. 2A and 2B  and the specific structure of the ground electrode  30   b  in  FIG. 4  may be modified as appropriate.  FIG. 5  illustrates an exemplary modification of the ground electrode  30 . 
     As shown in  FIG. 5 , for example, unlike the first embodiment, a gap need not be formed between the side surface  392  of the ground electrode tip  39  located in the second direction D 2  and an inside wall defining the concave portion  316 . In addition, as shown in  FIG. 5 , the position in the second direction D 2  of the end  352  of the welding portion  35  located in the second direction may be aligned with the position in the second direction D 2  of the side surface  392  of the ground electrode tip  39  located in the second direction D 2 . That is, the welding portion  35  need not include the far-side portion  35 C. As shown in  FIG. 5 , the position in the first direction D 1  of the side surface  391  of the ground electrode tip  39  located in the first direction D 1  may be aligned with the position in the first direction D 1  of the free end surface  311  of the ground electrode body  31 . 
     (4) Although in the first and second embodiments, the ground electrode tips  39  and  39   b  each have a substantially quadrangular prism shape, the ground electrode tips  39  and  39   b  may each have other shapes, such as a columnar shape and pentagonal prism shape. 
     (5) In the first and second embodiments, the ground electrode tips  39  and  39   b  are welded to the respective concave portions  316  and  316   b  after forming the concave portions  316  and  316   b  in the respective side surfaces  315  and  315   b  in the vicinity of the free end surfaces  311  and  311   b  of the respective ground electrode bodies  31  and  31   b . However, instead, the ground electrode tips  39  and  39   b  may be welded to the respective side surfaces  315  and  315   b  without forming the concave portions  316  and  316   b  in the respective side surfaces  315  and  315   b  in the vicinity of the respective free end surfaces  311  and  311   b.    
     (6) In the ignition plug  100 , the materials and the dimensions of the ground electrode  30 , the metal shell  50 , the center electrode  20 , the insulator  10 , etc., may be variously changed. For example, the metal shell  50  may be made of low-carbon steel plated with zinc or nickel, or may be made of low-carbon steel that is not plated. The insulator  10  may be made of various types of insulating ceramics in addition to alumina. 
     Although the present invention is described on the basis of the embodiments and modifications, the embodiments according to the present invention described above are described for the sake of facilitating the understanding of the present invention, and do not limit the present invention. The present invention may be changed and modified without departing from the gist thereof and scope of the claims, and includes equivalents of the present invention.