Patent Publication Number: US-8987981-B2

Title: Spark plug

Description:
This application claims the benefit of Japanese Patent Application No. 2013-137463 filed with the Japan Patent Office on Jun. 28, 2013, the entire content of which is hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     This disclosure relates to a spark plug. 
     BACKGROUND OF THE INVENTION 
     Conventionally, a spark plug is employed for an internal combustion engine. The spark plug includes, for example, a center electrode, and a ground electrode. The center electrode and the ground electrode form a gap to generate spark. When the ground electrode absorbs heat, an action to extinguish a flame (also referred to as a flame quenching) occurs. To reduce this, a technique that tapers off a front end portion of the ground electrode has been proposed. 
     Related documents of such spark plug include, for example, Japanese patent application laid-open number 05-159856, Japanese patent application laid-open number 05-159857, and Japanese patent application laid-open number 2001-351761. 
     SUMMARY OF THE INVENTION 
     A sparkplug includes: a center electrode extending in an axial direction; an insulator with an axial hole extending in the axial direction, the center electrode being disposed to be inserted into the axial hole; a metal shell disposed at an outer circumference of the insulator; and a ground electrode that electrically connects to the metal shell, the ground electrode forming a gap with a front end surface of the center electrode. In this spark plug, the ground electrode includes a rod-shaped main body portion, the main body portion including a base material and a core portion, the base material forming at least a part of a surface of the ground electrode, the core portion being buried in the base material and having a higher thermal conductivity than the base material. 
     A front end portion of the main body portion of the ground electrode is disposed at a position facing a front end surface of the center electrode. 
     The front end portion of the main body portion includes a tapered front end portion, the tapered front end portion including an opposed surface and a pair of tapered surfaces, the opposed surface being a surface facing the center electrode, the tapered surfaces being configured to sandwich the opposed surface. 
     When the front end surface of the center electrode is projected along the axial direction, at least a part of the tapered end portion is disposed in a range overlapping the projected front end surface of the center electrode. 
     A shortest distance between the center electrode and a boundary formed by the opposed surface and the tapered surface is equal to or less than 1.2 times a distance of the gap. 
     a perpendicular cross section of the ground electrode includes a front end of the core portion and perpendicular to the axial direction, wherein at least a part of a cross section of the core portion is disposed in a region at a front end side of a straight line that passes a rear end of a line segment corresponding to the tapered surface and is vertical to the line segment, and a shortest distance between the line segment and the cross section of the core portion is 0.2 mm or more to 1.5 mm or less. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein like designations denote like elements in the various views, and wherein: 
         FIG. 1  is a sectional view of a spark plug according to a first embodiment; 
         FIG. 2A  to  FIG. 2D  are schematic diagrams illustrating a constitution of electrodes of the spark plug; 
         FIG. 3A  to  FIG. 3E  are schematic diagrams illustrating a constitution of electrodes of the spark plug of a second embodiment; 
         FIG. 4A  and  FIG. 4B  are schematic diagrams illustrating a constitution of electrodes of a spark plug of a third embodiment; 
         FIG. 5A  and  FIG. 5B  are sectional views illustrating a fusion portion; 
         FIG. 6A  and  FIG. 6B  are sectional views illustrating a fusion portion; and 
         FIG. 7  is a graph illustrating results of evaluation tests. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, for purpose of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawing. 
     When the front end portion of the ground electrode is tapered off, although ignitability is improved, durability of the ground electrode may be degraded. For example, when the front end portion of the ground electrode is tapered off, the front end portion is thinned. In view of this, a volume of the ground electrode is decreased. Therefore, a temperature of the ground electrode is likely to be high. High temperature of the ground electrode may easily cause the ground electrode to be worn due to, for example, oxidation of the surface of ground electrode. 
     An object of this disclosure is to achieve improvement of ignitability and improvement of durability of the ground electrode. 
     This disclosure can be achieved as the following application examples. 
     Application Example 1 
     A sparkplug includes: a center electrode extending in an axial direction; an insulator with an axial hole extending in the axial direction, the center electrode being disposed to be inserted into the axial hole; a metal shell disposed at an outer circumference of the insulator; and a ground electrode that electrically connects to the metal shell, the ground electrode forming a gap with a front end surface of the center electrode, wherein the ground electrode includes a rod-shaped main body portion, the main body portion including a base material and a core portion, the base material forming at least a part of a surface of the ground electrode, the core portion being buried in the base material and having a higher thermal conductivity than the base material, a front end portion of the main body portion of the ground electrode is disposed at a position facing a front end surface of the center electrode, the front end portion of the main body portion includes a tapered front end portion, the tapered front end portion including an opposed surface and a pair of tapered surfaces, the opposed surface being a surface facing the center electrode, the tapered surfaces being configured to sandwich the opposed surface, when the front end surface of the center electrode is projected along the axial direction, at least a part of the tapered end portion is disposed in a range overlapping the projected front end surface of the center electrode, a shortest distance between a boundary between the opposed surface at a surface of the tapered end portion and the tapered surface, and the center electrode is equal to or less than 1.2 times a distance of the gap, and a perpendicular cross section of the ground electrode includes a front end of the core portion and perpendicular to the axial direction, wherein at least a part of a cross section of the core portion is disposed in a region at a front end side with respect to the straight line, the straight line being vertical to the line segment, the straight line passing a rear end of a line segment corresponding to the tapered surface on the perpendicular cross section, and a shortest distance between the line segment corresponding to the tapered surface and the cross section of the core portion is 0.2 mm or more to 1.5 mm or less. 
     With this constitution, on the cross section including the front end of the core portion and is perpendicular to the axial direction, at least a part of the cross section of the core portion is disposed in the region at the front end side with respect to the straight line vertical to the line segment. The straight line passes the rear end of the line segment corresponding to the tapered surface. The shortest distance between the line segment corresponding to the tapered surface and the cross section of the core portion is 0.2 mm or more and 1.5 mm or less. This allows achieving improvement of ignitability and improvement of durability of the ground electrode. 
     Application Example 2 
     The spark plug according to application example 1, wherein at least a part including the front end of the core portion is formed of a material with a melting point of 1350 degrees Celsius or more. 
     This constitution allows reducing damage to the ground electrode. 
     Application Example 3 
     The spark plug according to application example 2, wherein the core portion includes a first core portion and a second core portion, the first core portion having a higher thermal conductivity than the base material, the second core portion being disposed between the base material and the first core portion, the second core portion having a higher thermal conductivity than the first core portion, on the perpendicular cross section, a cross-sectional structure of a front end side of the ground electrode is a two-layered structure of the first core portion and the base material, a cross-sectional structure of a rear end side of the ground electrode being a three-layered structure of the first core portion, the second core portion, and the base material. 
     With this constitution, disposing the first core portion and the second core portion with higher thermal conductivity than the first core portion improves thermal conductivity of the ground electrode. This allows reducing wear of the ground electrode. The second core portion is not disposed at the front end side of the ground electrode. Accordingly, damage to the ground electrode due to a temperature rise of the second core portion can be suppressed. 
     Application Example 4 
     The spark plug according to any one of application example 1 to 3, wherein the ground electrode further includes a noble metal tip, the noble metal tip facing a front end surface of the center electrode. 
     This constitution allows suppressing the gap to be widened. 
     Application Example 5 
     The spark plug according to application example 4, wherein the noble metal tip is secured to the base material by laser beam welding, the main body portion of the ground electrode includes a fusion portion, the fusion portion including a constituent of the base material and a constituent of the noble metal tip, a dividing cross section that is perpendicular to the opposed surface has a line that uniformly divides the opposed surface into two, the line extending on the opposed surface in a cross section of the ground electrode along a longitudinal direction of the ground electrode, wherein among straight lines perpendicular to a direction where the opposed surface extends and overlap a cross section of the fusion portion, a straight line at a most front end side is referred to as a first straight line and a straight line at a rearmost end side is referred to as a second straight line, an area of the cross section of the fusion portion is referred to as a first area S 1 , on a cross section of the main body portion of the ground electrode, an area of a part sandwiched between the first straight line and the second straight line is referred to as a second area S 2 , an area ratio S 1 /S 2  is less than 1/3, a cross section of the core portion extends to a front end side of the ground electrode with respect to the second straight line, and the cross section of the core portion is away from a cross section of the fusion portion. 
     With this constitution, compared with the case where the ratio S 1 /S 2  is 1/3 or more (that is, the area of cross section of the fusion portion is comparatively large), degrade of thermal conductivity of the ground electrode can be suppressed. Since the core portion is away from the fusion portion, degrade of a sealing strength of the noble metal tip and the base material can be suppressed. 
     Application Example 6 
     The spark plug according to application example 4, wherein the noble metal tip is secured to the base material by laser beam welding, the main body portion of the ground electrode includes a fusion portion, the fusion portion including a constituent of the base material and a constituent of the noble metal tip, a dividing cross section that is perpendicular to the opposed surface has a line that uniformly divides the opposed surface into two, the line extending the opposed surface in a cross section of the ground electrode along a longitudinal direction of the ground electrode, wherein among straight lines perpendicular to a direction where the opposed surface extends and overlap a cross section of the fusion portion, a straight line at a most front end side is referred to as a first straight line and a straight line at a rearmost end side is referred to as a second straight line, an area of the cross section of the fusion portion is referred to as a first area S 1 , on a cross section of the main body portion of the ground electrode, an area of a part sandwiched between the first straight line and the second straight line is referred to as a second area S 2 , an area ratio S 1 /S 2  is 1/3 or more, and a cross section of the core portion contacts the cross section of the fusion portion. 
     With this constitution, compared with the case where the ratio S 1 /S 2  is less than 1/3 (that is, the area of cross section of the fusion portion is comparatively small), degrade of the sealing strength of the noble metal tip and the base material can be suppressed. Since the core portion contacts the fusion portion, degrade of thermal conductivity of the ground electrode can be suppressed. 
     This disclosure can be achieved by various aspects. For example, this disclosure can be achieved by an aspect such as a spark plug and an internal combustion engine mounting the spark plug. 
     A. First Embodiment 
     A1. Constitution of Spark Plug 
       FIG. 1  is a sectional view of a spark plug  100  according to a first embodiment. The illustrated line CL indicates a central axis of the spark plug  100 . Hereinafter, a central axis CL is also referred to as an “axis line CL” and a direction parallel to the central axis CL is also referred to as an “axial direction. “A radial direction of a circle placing the central axis CL as a center is also simply referred to as a “radial direction. A circumferential direction of the circle placing the central axis CL as the center is also referred to as a “circumferential direction.” A first direction D1 in the drawing is parallel to the axis line CL. As described later, a center electrode  20  and a ground electrode  30 , which form a spark gap g (also simply referred to as a “gap g”) form an end portion at the first direction D1 side of the spark plug  100 . Hereinafter, such first direction D1 side is also referred to as “a front end side of the spark plug  100  (or simply referred to as a “front end side”)”. The opposite side to the first direction D1 is also referred to as a “rear end side of the spark plug  100  (or simply referred to as a “rear end side”)”. A second direction D2 and a third direction D3 in the drawing are vertical to one another. Both directions are vertical to the first direction D1. Hereinafter, the first direction D1 is also simply referred to as a “+D1 direction” and the direction opposite to the first direction D1 is also simply referred to as a “−D1 direction”. Similarly, “+” or “−” sign is employed for other directions to specify the directions. The +D1 direction side is also simply referred to as a “+D1 side” and the −D1 direction side is also simply referred to as a “−D1 side.” This is similarly applicable to other direction sides. 
     The spark plug  100  includes an insulator  10 , the center electrode  20 , the ground electrode  30 , a terminal metal fitting  40 , a metal shell  50 , a conductive seal  60 , a resistor  70 , a conductive seal  80 , a front end side packing  8 , a talc  9 , a first rear end side packing  6 , and a second rear end side packing  7 . 
     The insulator  10  is an approximately cylindrically-shaped member with a through hole  12  (also referred to as an “axial hole  12 ”). The through hole  12  extends along the central axis CL and passes through the inside of the insulator  10 . The insulator  10  is formed by sintering alumina (other insulating materials can also be employed). The insulator  10  includes a nose portion  13 , a first-outer-diameter-contracted-portion  15 , a tip-end-side trunk portion  17 , a flange portion  19 , a second-outer-diameter-contracted-portion  11 , and a rear-end-side trunk portion  18  that are arranged from the front end side to the rear end side in this order. 
     The flange portion  19  is positioned at an approximately center of the insulator  10  in the axial direction. The outer diameter of the flange portion  19  is the largest among the outer diameter of the insulator  10 . At the front end side of the flange portion  19 , the tip-end-side trunk portion  17  is disposed. At the front end side of the tip-end-side trunk portion  17 , the first-outer-diameter-contracted-portion  15  is disposed. The outer diameter of the first-outer-diameter-contracted-portion  15  gradually decreases from the rear end side to the front end side. At the front end side of the first-outer-diameter-contracted-portion  15 , the nose portion  13  is disposed. If the spark plug  100  is installed to an internal combustion engine (not illustrated), the nose portion  13  is exposed in a combustion chamber. 
     At the rear end side of the flange portion  19 , the second-outer-diameter-contracted-portion  11  is disposed. The outer diameter of the second-outer-diameter-contracted-portion  11  gradually decreases from the front end side to the rear end side. At the rear end side of the second-outer-diameter-contracted-portion  11 , the rear-end-side trunk portion  18  is disposed. 
     Into the front end side of the through hole  12  of the insulator  10 , the center electrode  20  is inserted. The center electrode  20  is a rod-shaped member extending along the central axis CL. The center electrode  20  includes an electrode base material  21  and a core material  22  buried inside of the electrode base material  21 . The electrode base material  21  is, for example, formed using Inconel (“INCONEL” is a registered trademark), which is an alloy containing nickel as a main constituent. The core material  22  is, for example, formed with an alloy containing copper. A part of the rear end side of the center electrode  20  is disposed in the through hole  12  of the insulator  10 . A part of the front end side of the center electrode  20  is exposed to the front end side of the insulator  10 . 
     Into the rear end side of the through hole  12  of the insulator  10 , the terminal metal fitting  40  is inserted. The terminal metal fitting  40  is a rod-shaped member extending along the central axis CL. The terminal metal fitting  40  is formed using a low-carbon steel (however, other conductive materials (for example, metallic materials) can also be employed). The terminal metal fitting  40  includes a flange portion  42 , a plug cap installation portion  41 , and a nose portion  43 . The plug cap installation portion  41  is formed at the rear end side with respect to the flange portion  42 . The nose portion  43  is formed at the front end side with respect to the flange portion  42 . The plug cap installation portion  41  is exposed to the rear end side of the insulator  10 . The nose portion  43  is inserted (press-fitted) into the through hole  12  of the insulator  10 . 
     In the through hole  12  of the insulator  10 , the resistor  70  is disposed between the terminal metal fitting  40  and the center electrode  20 . The resistor  70  reduces radio wave noise during spark generation. The resistor  70  is, for example, formed of a composition containing glass particles such as B 2 O 3 —SiO 2 -based glass particles, ceramic particles such as TiO 2 , and a conductive material such as carbon particles and metal. 
     In the through hole  12 , the conductive seal  60  fills space between the resistor  70  and the center electrode  20 . The conductive seal  80  fills space between the resistor  70  and the terminal metal fitting  40 . As a result, the center electrode  20  is electrically connected to the terminal metal fitting  40  via the resistor  70  and the conductive seals  60  and  80 . The conductive seals  60  and  80  are, for example, formed with above-described various glass particles and metal particles (for example, Cu and Fe). 
     The metal shell  50  is a cylindrically-shaped metal shell to secure the spark plug  100  to an engine head (not illustrated) of the internal combustion engine. The metal shell  50  is formed using a low-carbon steel material (other conductive materials (for example, metallic materials) can also be employed). A through hole  59  is formed at the metal shell  50 . The through hole  59  passes through the inside of the metal shell  50  and extends along the central axis CL. The insulator  10  is inserted into the through hole  59  of the metal shell  50 . The metal shell  50  is secured to the outer circumference of the insulator  10 . The front end of the insulator  10  (namely, the end at the +D1 side) is exposed from the front end of the metal shell  50 . The rear end of the insulator  10  is exposed from the rear end of the metal shell  50 . 
     The metal shell  50  includes a trunk portion  55 , a seal portion  54 , a deformed portion  58 , a tool engagement portion  51 , and a crimp portion  53  that are arranged from the front end side to the rear end side in this order. The seal portion  54  has an approximately cylindrical shape. At the front end side of the seal portion  54 , the trunk portion  55  is disposed. The outer diameter of the trunk portion  55  is smaller than the outer diameter of the seal portion  54 . At the outer peripheral surface of the trunk portion  55 , a thread portion  52  is formed. The thread portion  52  is as to be screwed with a mounting hole of the internal combustion engine. Between the seal portion  54  and the thread portion  52 , an annular-shaped gasket  5  is fitted by insertion. The annular-shaped gasket  5  is formed by folding a metal plate. 
     The trunk portion  55  of the metal shell  50  includes an inner-diameter-contracted-portion  56 . The inner-diameter-contracted-portion  56  is disposed at the front end side with respect to the flange portion  19  of the insulator  10 . The internal diameter of the inner-diameter-contracted-portion  56  gradually decreases from the rear end side to the front end side. The front end side packing  8  is sandwiched between the inner-diameter-contracted-portion  56  of the metal shell  50  and the first-outer-diameter-contracted-portion  15  of the insulator  10 . The front end side packing  8  is an O-ring made of iron. As a material of the front end side packing  8 , other materials (for example, a metallic material such as a copper) can also be employed. 
     At the rear end side of the seal portion  54 , the deformed portion  58  is disposed. The wall thickness of the deformed portion  58  is thinner than the wall thickness of the seal portion  54 . The deformed portion  58  has a deformed center portion protruding toward the outside of the radial direction (the direction away from the central axis CL). At the rear end side of the deformed portion  58 , the tool engagement portion  51  is disposed. The tool engagement portion  51  has a shape with which a spark plug wrench is engaged (for example, a hexagonal prism). At the rear end side of the tool engagement portion  51 , the crimp portion  53  with a wall thickness thinner than the wall thickness of the tool engagement portion  51  is disposed. The crimp portion  53  is disposed at the rear end side with respect to the second-outer-diameter-contracted-portion  11  of the insulator  10  and forms the rear end of the metal shell  50  (namely, the end at the −D1 side). The crimp portion  53  is flexed to radially inside. 
     An annular-shaped space SP is formed between the inner peripheral surface at the rear end side part of the metal shell  50  and the outer peripheral surface of the insulator  10 . The space SP is surrounded by the inner peripheral surface of the metal shell  50  and the outer peripheral surface of the insulator  10 , between the crimp portion  53  and the second-outer-diameter-contracted-portion  11 . At the rear end side in the space SP, the first rear end side packing  6  is disposed. At the front end side in the space SP, the second rear end side packing  7  is disposed. In this embodiment, these rear end side packings  6  and  7  are C-rings made of iron (other materials can also be employed). A powder of talc  9  is filled between the two rear end side packings  6  and  7  in the space SP. 
     The crimp portion  53  is crimped so as to be folded to the inside. Accordingly, the insulator  10  in the metal shell  50  is pressed to the front end side via the rear end side packings  6  and  7  and the talc  9 . Thus, the front end side packing  8  is pressed between the first-outer-diameter-contracted-portion  15  and the inner-diameter-contracted-portion  56 . The front end side packing  8  seals between the metal shell  50  and the insulator  10 . This reduces gas inside of the combustion chamber of the internal combustion engine to leak through between the metal shell  50  and the insulator  10 . 
     The ground electrode  30  is a rod-shaped electrode sealed to the front end of the metal shell  50  (namely, the +D1 side end). The ground electrode  30  extends from the metal shell  50  in the D1 direction, bent to the central axis CL, and reaches a front end portion  31 . The gap g is formed between the front end portion  31  and a front end surface  20   s   1  of the center electrode  20  (the surface  20   s   1  at the +D1 side). The ground electrode  30  is, for example, sealed to the metal shell  50  by laser beam welding. This electrically connects the ground electrode  30  and the metal shell  50 . The ground electrode  30  includes a base material  35  and a core portion  36 . The base material  35  forms the surface of the ground electrode  30 . The core portion  36  is installed by being buried in the base material  35 . The base material  35  is, for example, formed using Inconel. The core portion  36  is formed using a material whose thermal conductivity is higher than the base material  35  (for example, pure copper). 
     A2. Constitution of Electrodes 
       FIG. 2A  to  FIG. 2D  are schematic diagrams illustrating a constitution of the electrodes  20  and  30  of the spark plug  100 .  FIG. 2A  illustrates a sectional view of a part of the spark plug  100  at the first direction D1 side (specifically, the sectional view including the central axis CL).  FIG. 2B  illustrates the cross section of the ground electrode  30  (specifically, the cross section perpendicular to the central axis CL).  FIG. 2C  illustrates a schematic diagram of the ground electrode  30  viewed in the +D1 direction.  FIG. 2D  illustrates a perspective view of the electrodes  20  and  30 .  FIG. 2A  illustrates external views of the center electrode  20  and the insulator  10  viewed facing the +D3 direction at the right side of the central axis CL.  FIG. 2B  is a cross section taken along the line B-B of  FIG. 2A . 
     The ground electrode  30  is formed using a rod-shaped member with a rectangular cross section. As illustrated in  FIG. 2A , the ground electrode  30  includes a nose portion  32  and the front end portion  31 . The nose portion  32  includes a second end  30   e   2  sealed to the metal shell  50 . The front end portion  31  is connected to the nose portion  32 . The front end portion  31  includes a first end  30   e   1  disposed opposite side to the second end  30   e   2 . The nose portion  32  extends from the second end  30   e   2  to the first direction D1 side and then is bent to the central axis CL. The direction from the second end  30   e   2  to the central axis CL is the second direction D2. The front end portion  31  is positioned at the +D1 side with respect to the center electrode  20 . The front end portion  31  extends from the −D2 side to the +D2 side based on the central axis CL. The end portion of the front end portion  31  at the +D2 side is the first end  30   e   1 . The front end portion  31  includes the first end  30   e   1  and an inner surface  31   si . The inner surface  31   si  is a part facing the front end surface  20   s   1  (namely, the surface  20   s   1  at the +D1 side) of the center electrode  20 . 
     As illustrated in  FIG. 2A , the ground electrode  30  includes the base material  35  and the core portion  36 . The base material  35  forms the surface of the ground electrode  30 . The core portion  36  is installed by being buried in the base material  35 . The core portion  36  extends from the second end  30   e   2  to the middle of the front end portion  31 . Here, among both ends of the core portion  36 , an end  36   t  closer to the front end portion  31  of the ground electrode  30  is referred to as a “front end  36   t .”  FIG. 2B  illustrates the cross section of the ground electrode  30  that includes the front end  36   t  of the core portion  36  and is perpendicular to the central axis CL.  FIG. 2B  is a sectional view taken along the line B-B of  FIG. 2A . 
     As illustrated in  FIG. 2A  to  FIG. 2D , the front end portion  31  includes the inner surface  31   si , an outer surface  31   so , a first side surface  31   s   1 , and a second side surface  31   s   2 . The inner surface  31   si  is a surface at the −D1 side of the front end portion  31 . The outer surface  31   so  is a surface at the +D1 side of the front end portion  31 . The first side surface  31   s   1  is a surface at the +D3 side of the front end portion  31 . The second side surface  31   s   2  is a surface at the −D3 side of the front end portion  31 . Both the inner surface  31   si  and the outer surface  31   so  are planes perpendicular to the central axis CL. Both of the two side surfaces  31   s   1  and  31   s   2  are planes vertical to the D3 direction. The inner surface  31   si  faces the front end surface  20   s   1  of the center electrode  20 . The front end surface  20   s   1  is a plane perpendicular to the central axis CL. Between the inner surface  31   si  and the front end surface  20   s   1 , the spark gap g is formed. A distance Dg in  FIG. 2A  indicates a distance of the spark gap g (hereinafter also referred to as the “gap distance Dg”). The gap distance Dg is the shortest distance between the two surfaces  20   s   1  and  31   si , which form the gap g. 
     As illustrated in  FIG. 2A  to  FIG. 2D , the front end portion  31  includes a tapered end portion  31   t . The tapered end portion  31   t  has a tapered shape gradually thinned toward the first end  30   e   1 . The tapered end portion  31   t  includes an opposed surface  31   tsi , an outer surface  31   tso , a first tapered surface  31   ts   1 , a second tapered surface  31   ts   2 , and a front end surface  31   se . The opposed surface  31   tsi  is a surface at the −D1 side of the tapered end portion  31   t . The outer surface  31   tso  is a surface of the +D1 side of the tapered end portion  31   t . The first tapered surface  31   ts   1  is a surface at the +D3 side of the tapered end portion  31   t . The second tapered surface  31   ts   2  is a surface at the −D3 side of the tapered end portion  31   t . The front end surface  31   se  is a surface at the +D2 side of the tapered end portion  31   t . The opposed surface  31   tsi  is a part of the inner surface  31   si  of the front end portion  31  and faces the front end surface  20   s   1  of the center electrode  20 . Between the opposed surface  31   tsi  and the front end surface  20   s   1 , the gap g is formed. The outer surface  31   tso  is a part of the outer surface  31   so  of the ground electrode  30 . The front end surface  31   se  corresponds to the first end  30   e   1  of the ground electrode  30 . The first tapered surface  31   ts   1  connects the first side surface  31   s   1  and the front end surface  31   se . The second tapered surface  31   ts   2  connects the second side surface  31   s   2  and the front end surface  31   se.    
     As illustrated in  FIG. 2C , the opposed surface  31   tsi  has a trapezoidal shape whose width gradually narrows toward the +D2 direction. The outer surface  31   tso  (not illustrated) also has a trapezoidal shape same as the opposed surface  31   tsi . Hereinafter, among two parallel sides Ub and Lb of the trapezoid representing the opposed surface  31   tsi , the comparatively short side Ub is referred to as the “upper bottom Ub”, and the comparatively long side Lb is referred to as the “lower bottom Lb.” The upper bottom Ub is an edge line forming a boundary between the opposed surface  31   tsi  and the front end surface  31   se . A pair of tapered surfaces  31   ts   1  and  31   ts   2  are disposed so as to sandwich the opposed surface  31   tsi . A distance among the two tapered surfaces  31   ts   1  and  31   ts   2  (a distance parallel to the third direction D3) gradually decreases toward the +D2 direction. 
       FIG. 2C  illustrates a symmetric surface CLa. The symmetric surface CLa is a plane that includes the central axis CL and is parallel to the second direction D2. The ground electrode  30  is constituted so as to be symmetry with respect to the symmetric surface CLa. The cross section of the ground electrode  30  illustrated in  FIG. 2A  is a cross section on the symmetric surface CLa. As illustrated in  FIG. 2C , a line Lt extends on the opposed surface  31   tsi  of the tapered end portion  31   t  along the longitudinal direction of the ground electrode  30  (here, in the second direction D2). The line Lt almost uniformly divides the opposed surface  31   tsi  by two. The cross section on the symmetric surface CLa includes this line Lt and is perpendicular to the opposed surface  31   tsi . Hereinafter, the cross section uniformly dividing the opposed surface  31   tsi  by two like this is also referred to as a “halving cross section.” 
       FIG. 2B  illustrates a first line segment L 1  corresponding to the first tapered surface  31   ts   1  and a second line segment L 2  corresponding to the second tapered surface  31   ts   2 . In the drawing, a first rear end E 1  is an end farther from the front end surface  31   se  among both ends of the first line segment L 1 , and a second rear end E 2  is an end farther from the front end surface  31   se  among both ends of a second line segment L 2 . A first vertical line Lo 1  is a straight line that passes the first rear end E 1  and is vertical to the first line segment L 1 . A second vertical line Lo 2  is a straight line that passes the second rear end E 2  and is vertical to the second line segment L 2 .  FIG. 2B  illustrates a part of a region At cut out from the drawing in  FIG. 2B  at the right side. This region At is at the front end side with respect to the first vertical line Lo 1  (the first end  30   e   1  side of the ground electrode  30 ) and a region at the front end side with respect to the second vertical line Lo 2 . A part of the core portion  36  is disposed in the region At in the cross section of  FIG. 2B . Thus, the part of the core portion  36  is disposed in the region At. In view of this, compared with the case where the core portion  36  is not disposed in the region At, during an operation of the internal combustion engine, heat can be easily released from the front end portion  31  to another portion of the ground electrode  30  (here, the nose portion  32 ) with the core portion  36 . Therefore, high temperature of the front end portion  31  and a state where the temperature of the front end portion  31  remains high can be suppressed. This allows suppressing wear of the front end portion  31  (for example, oxidation of the surface of the front end portion  31 ). 
       FIG. 2C  and  FIG. 2D  illustrate a region  20   s   1   p  by dashed line. The region  20   s   1   p  is a region obtained by projecting the front end surface  20   s   1  of the center electrode  20  on the ground electrode  30  along the central axis CL (to the +D1 direction) (hereinafter also referred to as the “projection region  20   s   1   p ”). As illustrated in  FIG. 2C , the projection region  20   s   1   p  (namely, the front end surface  20   s   1 ) has a circular shape. The tapered end portion  31   t  partially overlaps this projection region  20   s   1   p . In the example of  FIG. 2C , the lower bottom Lb of the opposed surface  31   tsi  is disposed at the +D2 side with respect to the central axis CL. However, the lower bottom Lb may be disposed at the −D2 side with respect to the central axis CL. 
       FIG. 2C  and  FIG. 2D  illustrate two edge lines L 11  and L 12 . The first edge line L 11  forms a boundary between the opposed surface  31   tsi  and the first tapered surface  31   ts   1 . The second edge line L 12  forms a boundary between the opposed surface  31   tsi  and the second tapered surface  31   ts   2 . As illustrated in the drawing, both the edge lines L 11  and L 12  do not overlap the projection region  20   s   1   p  and are away from the projection region  20   s   1   p . A distance De illustrated in  FIG. 2D  is the shortest distance between the front end surface  20   s   1  of the center electrode  20  and the second edge line L 12  (hereinafter also referred to as the “edge distance De”). In this embodiment, a length of a line segment connecting the edge of the front end surface  20   s   1  and the second edge line L 12  corresponds to the edge distance De. This edge distance De is longer than the gap distance Dg. Generally, discharge is likely to occur at a pointed part among the surface of the electrode. That is, discharge is likely to occur at the pointed part like the second edge line L 12  rather than a flat surface like the projection region  20   s   1   p . Therefore, even if the edge distance De is longer than the gap distance Dg, discharge can occur between the front end surface  20   s   1  and the second edge line L 12 . As described above, the ground electrode  30  is constituted so as to be symmetry with respect to the symmetric surface CLa. In view of this, the shortest distance between the front end surface  20   s   1  and the first edge line L 11  is also the same as the edge distance De. Therefore, discharge can occur between the front end surface  20   s   1  and the first edge line L 11 . 
     If discharge occurs at the inside of the inner surface  31   si  of the ground electrode  30  (for example, the inside of the projection region  20   s   1   p ), a flame occurred by the discharge spreads to the end of the inner surface  31   si  and then spreads to the outside of the gap g. On the other hand, if discharge occurs at the edge line L 11  and/or L 12 , a flame occurred by the discharge can spread to the outside of the gap g immediately. Therefore, if discharge occurs at the e edge line L 11  and/or L 12 , compared with the case where discharge occurs at the inside of the inner surface  31   si , ignitability can be further improved. 
     A3. First Evaluation Test 
     The following describes the first evaluation test using samples of the spark plugs  100 . The first evaluation test used six pieces of the spark plugs  100  as the samples. The spark plugs  100  differed in a ratio of the edge distance De to the gap distance Dg ( FIG. 2A ), “De/Dg”, (hereinafter referred to as a “gap ratio”) from one another. Then, a ratio of the number of times of discharges occurred between the center electrode  20  and the edge line (the first edge line L 11  or the second edge line L 12 ) to the total number of times of discharges occurred at the spark plug  100  (here, 1000 times) (hereinafter referred to as an “edge discharge ratio”) was measured. All the samples used Inconel as the material of the base material  35  and a pure copper as the material of the core portion  36 . The following Table 1 is measurement results of the first evaluation test. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                 De/Dg 
                 100% 
                 110% 
                 120% 
                 125% 
                 130% 
                 135% 
               
               
                 Discharge ratio at edge 
                  99% 
                  95% 
                  85% 
                  60% 
                  40% 
                  20% 
               
               
                   
               
            
           
         
       
     
     Dimensions common to six pieces of the samples employed for the evaluation test were as follows. 
     1) Width Da of the first end  30   e   1  of the tapered end portion  31   t  in the third direction D3: 1.5 mm 
     This width Da was the same length as the length of the upper bottom Ub of the opposed surface  31   tsi.    
     2) Length Db of the tapered end portion  31   t  in the D2 direction: 1.6 mm 
     3) Width Dc of the front end portion  31  (excluding the tapered end portion  31   t ) in the third direction D3: 3.0 mm 
     This width Dc was the same length as the length of the lower bottom Lb of the opposed surface  31   tsi.    
     4) Thickness Dt of the front end portion  31  in the D1 direction: 1.6 mm 
     5) Diameter Dd of the front end surface  20   s   1  of the center electrode  20 : 1.5 mm 
     6) Gap distance Dg: 1.0 mm 
     The edge distances De of six pieces of the samples differed from one another. The edge distance De was adjusted by adjusting the distance Ds between the lower bottom Lb and the central axis CL of the front end surface  20   s   1  in the second direction D2, and a condition of bending of the nose portion  32  of the ground electrode  30 . 
     The testing method was as follows. The spark plug  100  was disposed in a container for experiment filled with air. The internal pressure of the container was raised to 0.6 MPa. This pressure was determined assuming pressure in the combustion chamber of the internal combustion engine at ignition. With this state, a voltage was applied to the spark plug  100 , thus discharge was conducted. The discharging state was taken with a high-speed camera to confirm whether the discharge occurred at the edge line L 11  and/or L 12 ; or the inside of the inner surface  31   si  on the ground electrode  30 . After 1000 times discharges at 100 Hz, the edge discharge ratio was calculated. 
     As listed in Table 1, the smaller the gap ratio, the higher the edge discharge ratio. This is probably due to the following reason. That is, compared with the case where the gap ratio is large, the edge distance De with respect to the gap distance Dg is short in case the gap ratio is small. In view of this, discharge is likely to occur at the edge lines L 11  and L 12 . Specifically, as listed in Table 1, when the gap ratio was 100% (if the edge line(s) L 11  and/or L 12  overlaps the projection region  20   s   1   p ), the edge discharge ratio was 99%. When the gap ratios was 110%, 120%, 125%, 130%, and 135%, the respective edge discharge ratios were 95%, 85%, 60%, 40%, and 20%. 
     As described above, if discharge occurs at the edge line(s) L 11  and/or L 12 , ignitability can be improved. Therefore, from the aspect of improvement in ignitability, a small gap ratio is preferable. For example, the use of the gap ratio of 120% or less allows achieving the edge discharge ratio of 85% or more. Thus, the gap ratio of 120% or less is preferable, and the gap ratio of 110% or less is particularly preferable, and the gap ratio of 100% is the most preferable. The lower limit of the gap ratio is 100%. 
     The likelihood of discharge at the edge line(s) L 11  and/or L 12  is assumed to change mainly according to the ratio of the edge distance De to the gap distance Dg. Therefore, the above-described preferable upper limits of the gap ratio are presumably applicable regardless of the constitution other than the gap ratio. For example, the preferable upper limits are presumably applicable regardless of a material of a part forming the front end surface  20   s   1  among the center electrode  20 , the area of the front end surface  20   s   1 , and/or a material of a part forming the inner surface  31   si  among the ground electrode  30 . 
     A4. Second Evaluation Test 
     The following describes the second evaluation test using samples of the spark plugs  100 . The second evaluation test measured an amount of gap distance Dg increase after operating the internal combustion engine with the spark plug  100  for 100 hours. The second evaluation test used an internal combustion engine with inline-four engines, a Single OverHead camshaft (SOHC), two valves, and a displacement of 1.3 L. The 100-hour operation repeated an operation of one cycle including one-minute idling operation and one-minute wide open throttle (also referred to as WOT) operation 3000 times. The maximum temperature at the part close to the gap g among the ground electrode  30  was approximately 300 degrees Celsius during the idling operation, and approximately 1000 degrees Celsius during the wide open throttle operation. 
     In the second evaluation test, ten pieces of the spark plugs  100  were prepared as the samples. The positions of the core portions  36  with respect to the tapered surfaces  31   ts   1  and  31   ts   2  of ten pieces of the samples differed from one another. The cross section of  FIG. 2B  illustrates a shortest distance Wm between the first line segment L 1  corresponding to the first tapered surface  31   ts   1  and the core portion  36 . The shortest distances Wm of ten pieces of the samples differed from one another. The shortest distance Wm was adjusted as follows. First, a cup-shaped first member made of Inconel was prepared. Into the first member, a second member made of pure copper was inserted. Then, with the second member inserted, an outer shape of the first member was molded, to manufacture the ground electrode  30 . The first member corresponded to the base material  35 , and the second member corresponded to the core portion  36 . Here, by adjustment of the thickness of the cup-shaped first member before molding, the shortest distance Wm was adjusted. In this embodiment, the shortest distance Wm was shorter than the shortest distance between the core portion  36  and the front end surface  31   se  (that is, a distance between the front end  36   t  and the front end surface  31   se ). As described above, the ground electrode  30  was symmetry with respect to the symmetric surface CLa. In view of this, the shortest distance between the second line segment L 2  corresponding to the second tapered surface  31   ts   2  and the core portion  36  was also the same length as the length of the shortest distance Wm. The ground electrodes  30  of ten pieces the samples had the approximately same outer shape. The following Table 2 indicates the measurement results in the second evaluation test. 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
             
            
               
                 Wm 
                 0.1 
                 0.2 
                 0.3 
                 0.5 
                 0.7 
                 0.9 
                 1.1 
                 1.3 
                 1.5 
                 1.7 
               
               
                 dDg 
                 — 
                 0.13 
                 0.12 
                 0.13 
                 0.14 
                 0.16 
                 0.18 
                 0.21 
                 0.25 
                 0.34 
               
               
                 Eval- 
                 C 
                 A 
                 A 
                 A 
                 A 
                 A 
                 A 
                 A 
                 A 
                 B 
               
               
                 uation 
               
               
                   
               
            
           
         
       
     
     A unit of the shortest distance Wm is a millimeter. An increased amount dDg of the gap distance Dg (hereinafter referred to as an “amount of gap increase dDg”) is a difference (unit is millimeter) subtracting the gap distance Dg before the operation from the gap distance Dg after the 100-hour operation. The evaluation result indicates that Evaluation A suggests that the amount of gap increase dDg is less than 0.3 mm. Evaluation B suggests that the amount of gap increase dDg is 0.3 mm or more. Evaluation C suggests that the base material  35  of the ground electrode  30  was bursted by the 100-hour operation. That is, the core portion  36  protruded out of the base material  35 . 
     Ten pieces of the samples employed for the second evaluation test had the same length Da, Db, Dc, Dt, Dd, Ds, and Dg before the tests (namely, before the 100-hour operation), as the respective length of the samples employed for the first evaluation test. The edge distance De before the test was 1.2 mm. The material of the base material  35  was Inconel, and the material of the core portion  36  was pure copper. 
     As listed in Table 2, the amount of gap increase dDg tends to decrease as the decreasing shortest distance Wm. This is probably due to the following reason. That is, the smaller the shortest distance Wm, the larger the proportion of the core portion  36  to the inside of the front end portion  31  of the ground electrode  30 . In view of this, during the operation of the internal combustion engine, heat can be easily released from the front end portion  31  to another portion of the ground electrode  30  (here, the nose portion  32 ). Therefore, high temperature of the front end portion  31  and a state where the temperature of the front end portion  31  remains high can be suppressed. This allows suppressing wear of the front end portion  31  (for example, oxidation of the surface of the front end portion  31 ). This allows suppressing an increase of the amount of gap increase dDg. 
     Specifically, as listed in Table 2, when the shortest distance Wm was 0.1 mm, the ground electrode  30  bursted during the test. When the shortest distances Wm were 0.2 mm, 0.3 mm, 0.5 mm, 0.7 mm, 0.9 mm, 1.1 mm, 1.3 mm, 1.5 mm, and 1.7 mm, the respective amount of gap increase dDg were 0.13 mm, 0.12 mm, 0.13 mm, 0.14 mm, 0.16 mm, 0.18 mm, 0.21 mm, 0.25 mm, and 0.34 mm. Thus, when the shortest distance Wm was 0.2 mm or more and 1.5 mm or less, the evaluation result was Evaluation A. When the shortest distance Wm was 1.7 mm, the evaluation result was Evaluation B. 
     Thus, setting the shortest distance Wm 1.5 mm or less allows suppressing the amount of gap increase dDg to less than 0.3 mm. Setting the shortest distance Wm to 0.2 mm or more allows suppressing damage (for example, burst) in the ground electrode  30 . Accordingly, to improve durability of the ground electrode  30 , setting the shortest distance Wm to 0.2 mm or more and 1.5 mm or less is preferable. The shortest distances Wm where good evaluation results were obtained were 0.2 mm, 0.3 mm, 0.5 mm, 0.7 mm, 0.9 mm, 1.1 mm, 1.3 mm, and 1.5 mm. Any value among these values can be employed as a preferable upper limit of a range of the shortest distance Wm. Any value among these values equal to or less than the upper limit can be employed as a preferable lower limit of the range of the shortest distance Wm. 
     An effect of cooling the surface of the front end portion  31  (in particular, the tapered surfaces  31   ts   1  and  31   ts   2 ) with the core portion  36  is presumably changed mainly according to the shortest distance Wm. Therefore, the preferable range of the shortest distance Wm is presumably applicable regardless of the constitution other than the shortest distance Wm. For example, the preferable range is applicable regardless of the shape of the ground electrode  30 . 
     A5. Third Evaluation Test 
     The following describes the third evaluation test using samples of the spark plugs  100 . The third evaluation test evaluated durability of the ground electrode  30 . This test employed five pieces of the spark plugs  100  with the core portions  36  whose materials differed from one another as the samples. The following Table 3 lists the evaluation results of the third evaluation test. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Material 
                 Melting point (C. °) 
                 Evaluation 
               
               
                   
                   
               
             
            
               
                   
                 Cu 
                 1083 
                 B 
               
               
                   
                 SUS304 
                 1350 
                 A 
               
               
                   
                 High Ni alloy 
                 1413 
                 A 
               
               
                   
                 Ni 
                 1453 
                 A 
               
               
                   
                 Fe 
                 1536 
                 A 
               
               
                   
                   
               
            
           
         
       
     
     Five pieces of the samples employed for the third evaluation test had the same length Da, Db, Dc, Dt, Dd, Ds, and Dg before the test, as the respective values of the samples employed for the first evaluation test. The material of the base material  35  was Inconel. Before the test, the edge distance De was 1.2 mm, and the shortest distance Wm was 0.2 mm. 
     The third evaluation test repeated a cycle of heating and cooling on the electrodes  20  and  30  of the spark plug  100  by 3000 times. A change in the ground electrode  30  by this was evaluated. Specifically, the one cycle includes heating the electrodes  20  and  30  (in particular, near the gap g) for one minute with burner and subsequently cooling the electrodes  20  and  30  for one minute in the air. The one-minute heating increases the temperature at the part close to the gap g among the ground electrode  30  to 1100 degrees Celsius. This temperature is higher than the temperature in the above-described second evaluation test (approximately 1000 degrees Celsius). That is, the third evaluation test conducted the evaluation under the severe condition compared with the second evaluation test. 
     Table 3 lists materials of the core portions  36 , melting points of the materials, and evaluation results of the third evaluation test. As the materials of the core portion  36 , a pure copper (Cu), a stainless steel (SUS304), a high nickel alloy, a pure nickel (Ni), and a pure iron (Fe) were employed. Evaluation A suggests that no change was seen in the ground electrode  30 . Evaluation B suggests that the ground electrode  30  was bursted. As listed in Table 3, the evaluation result of when the material of the core portion  36  was a pure copper was Evaluation B. This is presumably because the base material  35  was damaged due to thermal expansion of the core portion  36  inside of the base material  35  and a leakage of the core portion  36  melted during heating from the damaged base material  35 . When the material of the core portion  36  was any of a stainless steel (SUS304), a high nickel alloy, a pure nickel, and a pure iron, the evaluation result was Evaluation A. This is presumably that the melting point of the material of the core portion  36  was higher than the temperature of the ground electrode  30  during heating (approximately 1100 degrees Celsius); therefore, the core portion  36  failed to melt. 
     As described above, employing the material with higher melting point than the temperature of the ground electrode  30  during heating as the material of the core portion  36  allows suppressing burst of the ground electrode  30 . The maximum temperature of the ground electrode  30  during operation of the internal combustion engine differs depending on the internal combustion engine. Internal combustion engines widely prevalent generally are designed assuming that the maximum temperature of the ground electrode  30  is less than 1000 degrees Celsius. When using such internal combustion engine, employing various materials with higher melting point than the assumed maximum temperature (here, 1000 degrees Celsius) (for example, various metallic materials containing a pure copper) as the material of the core portion  36  is possible. The internal combustion engine designed assuming the excess of the maximum temperature of the ground electrode  30  over 1000 degrees Celsius is also possibly used. For example, in such internal combustion engines, the assumed maximum temperature of the ground electrode  30  can be 1100 degrees Celsius. In this case, various materials with higher melting point than the assumed maximum temperature can be employed as the material of the core portion  36 . Generally, as evaluated in the evaluation test listed in Table 3, employing the material with the melting point of 1350 degrees Celsius or more to the ground electrode  30  allows providing the spark plug  100  applicable to various internal combustion engines. 
     The third evaluation test was evaluated under the condition of the maximum temperature of the ground electrode  30  being 1100 degrees Celsius. The melting points of the materials where good evaluation result was obtained in the third evaluation test were 1350 degrees Celsius, 1413 degrees Celsius, 1453 degrees Celsius, and 1536 degrees Celsius. Any value among these values can be employed as a preferable lower limit of a range of the melting point. Any value among these values equal to or more than the lower limit can be employed as a preferable upper limit of the range of the melting point. 
     B. Second Embodiment 
     B1. Constitution of Spark Plug 
       FIG. 3A  to  FIG. 3E  are schematic diagrams illustrating a constitution of electrodes  20  and  30   a  of a spark plug  100   a  of a second embodiment.  FIG. 3A  to  FIG. 3C  are schematic diagrams similar to the respective  FIG. 2A  to  FIG. 2C .  FIG. 3D  illustrates a cross section taken along the line A 2 -A 2  of  FIG. 3A .  FIG. 3E  illustrates a cross section taken along the line A 3 -A 3  of  FIG. 3A . The main difference between the spark plug  100  of the first embodiment, which is illustrated in  FIG. 2A  to  FIG. 2D , and the spark plug  100   a  of the second embodiment, which is illustrated in  FIG. 3A  to  FIG. 3D , is the core portion of the ground electrode. That is, in the spark plug  100   a  of the second embodiment, the core portion  36  of the spark plug  100  of the first embodiment is replaced by a core portion  36   a  that includes a first core portion  36   a   1  and a second core portion  36   a   2 . The constitution excluding the ground electrode  30   a  is the same as the constitution of the first embodiment described in  FIG. 1  and  FIG. 2A  to  FIG. 2D . The ground electrode  30   a  is constituted so as to be symmetry with respect to the symmetric surface CLa. Among components of the spark plug  100   a  of the second embodiment, such elements that reference numerals designate identical elements to those of the spark plug  100  of the first embodiment will not be further elaborated. 
     As illustrated in  FIG. 3A , the ground electrode  30   a  includes a base material  35   a  and the core portion  36   a  buried in the base material  35   a . The shape of the ground electrode  30   a  (namely, the outer shape of the base material  35   a ) is the same as the shape of the ground electrode  30  of the first embodiment (namely, the outer shape of the base material  35 ). 
     The core portion  36   a  includes the first core portion  36   a   1  and the second core portion  36   a   2 . The second core portion  36   a   2  is disposed between the base material  35  and the first core portion  36   a   1 . The first core portion  36   a   1  extends from the second end  30   e   2  of the ground electrode  30   a  to a front end  36  at disposed in the middle of the front end portion  31 , similar to the core portion  36  of the first embodiment. The second core portion  36   a   2  is a tube-shaped layer covering the rear end side of the first core portion  36   a   1  (namely, the second end  30   e   2  side). The second core portion  36   a   2  extends from the second end  30   e   2  of the ground electrode  30   a  to the near position with respect to the front end  36  at of the first core portion  36   a   1 . The front end side of the first core portion  36   a   1  (that is, the first end  30   e   1  side) part, is not covered with the second core portion  36   a   2  but contacts the base material  35   a . The front end  36  at of the first core portion  36   a   1  forms the end  36   at  at a part closer to the front end portion  31  of the ground electrode  30   a  among both ends of the core portion  36   a . A part of the first core portion  36   a   1  covered with the second core portion  36   a   2  is thinner than the part not covered with the second core portion  36   a   2 . Accordingly, thickness of the part including the second core portion  36   a   2  among the core portion  36   a  is reduced to be excessively thick. The thickness of the core portion  36   a  smoothly changes from the second end  30   e   2  to the front end  36   at.    
     The first core portion  36   a   1  is formed of a material of higher thermal conductivity than the base material  35   a . The second core portion  36   a   2  is formed of a material of higher thermal conductivity than the first core portion  36   a   1 . For example, the material of the base material  35   a  is Inconel, the material of the first core portion  36   a   1  is a pure nickel, and the material of the second core portion  36   a   2  is a pure copper. Here, as a material of a part including the front end  36  at among the core portion  36   a  (here, the first core portion  36   a   1 ), employing a material with melting point of 1350 degrees Celsius or more is preferable. For example, any material selected from the stainless steel (SUS304), high nickel alloy, the pure nickel, and the pure iron listed in Table 3 may be employed. 
       FIG. 3D  illustrates a cross section of a part including the second core portion  36   a   2  among the ground electrode  30   a  (namely, the rear end side part of the ground electrode  30   a , here, the nose portion  32 ). This cross section is vertical to the direction that the ground electrode  30   a  extends. On the cross section, the second core portion  36   a   2  covers the whole circumference of the first core portion  36   a   1 . The cross-sectional structure is three-layered structure of the first core portion  36   a   1 , the second core portion  36   a   2 , and the base material  35   a.    
       FIG. 3E  illustrates a cross section of a part that includes the first core portion  36   a   1  but does not include the second core portion  36   a   2  (namely, a part at the front end side of the ground electrode  30   a , here, the front end portion  31 ) among the ground electrode  30   a . This cross section is vertical to the direction that the ground electrode  30   a  extends, that is, a cross section vertical to the second direction D2. The cross-sectional structure is two-layered structure of the first core portion  36   a   1  and the base material  35   a.    
       FIG. 3B  illustrates a cross section that includes the front end  36  at of the core portion  36   a  and is perpendicular to the central axis CL. A shortest distance Wma in the drawing is the shortest distance between the first line segment L 1  and the core portion  36   a  (here, the first core portion  36   a   1 ). In this embodiment, the shortest distance Wma is shorter than the shortest distance between the core portion  36  and the front end surface  31   se  of the ground electrode  30   a  (that is, a distance between the front end  36  at of the core portion  36   a  and the front end surface  31   se ). As illustrated in  FIG. 3E , the cross section at the front end side of the ground electrode  30   a  (namely, the first end  30   e   1  side), includes two layers of the first core portion  36   a   1  and the base material  35   a . Here, as the cross-sectional structure of the front end side of the ground electrode  30   a , the cross-sectional structure of the front end side (the first end  30   e   1  side) from a part including the front end  36  at among the contour of the cross section of the first core portion  36   a   1  in the cross section in  FIG. 3B  can be employed. As illustrated in  FIG. 3D , the cross section of the rear end side of the ground electrode  30   a  (namely, the second end  30   e   2  side) includes three layers of the first core portion  36   a   1 , the second core portion  36   a   2 , and the base material  35   a . Here, as the cross-sectional structure of the rear end side of the ground electrode  30   a , the cross-sectional structure of the rear end side from a part including a rear end  36   a   1   b  (the end  36   a   1   b , which is farthest from the first end  30   e   1  of the ground electrode  30   a ) among the contour of the cross section of the first core portion  36   a   1  in the cross section in  FIG. 3B  can be employed. 
     B2. Fourth Evaluation Test 
     The following describes the fourth evaluation test using samples of the spark plugs  100   a  of the second embodiment. The fourth evaluation test measured the amount of gap distance Dg increase after operating the internal combustion engine with the spark plug  100   a  for 100 hours, similarly to the above-described second evaluation test. The difference of the fourth evaluation test from the second evaluation test is that an operation in the internal combustion engine was adjusted such that the maximum temperature at a part close to the gap g in the ground electrode  30  became 1100 degrees Celsius, which is higher than 1000 degrees Celsius, during wide open throttle operation. Thus, the fourth evaluation test conducted the evaluation under the severe condition compared with the second evaluation test. 
     In the fourth evaluation test, two pieces of the spark plugs  100   a  with the second core portions  36   a   2  whose lengths differed from one another were prepared as samples (a first sample and a second sample). The constitution of the first sample was the same as the constitution described in  FIG. 3A  to  FIG. 3D . Meanwhile, the constitution of the second sample (not illustrated) differed from the first sample in the following points. That is, the second core portion  36   a   2  of the second sample extended from the second end  30   e   2  to a position at the cross section taken along the line A 2 -A 2 , which is a midpoint of the nose portion  32 . On the other hand, the second core portion  36   a   2  is not disposed at the front end side with respect to the position at the cross section taken along the line A 2 -A 2 . That is, the second sample did not include the second core portion  36   a   2  at the cross section corresponding to  FIG. 3B . Two pieces of the samples had the same length Da, Db, Dc, Dt, Dd, De, Ds, and Dg as the respective length of the samples employed for the second evaluation test before the test (namely, before the 100-hour operation). The shortest distance Wma (see  FIG. 3B ) of the two samples was 1.3 mm. In the two samples, the materials of the base materials  35   a  were Inconel, the materials of the first core portions  36   a   1  were a pure nickel, and the materials of the second core portions  36   a   2  were a pure copper. 
     Measurements of respective amount of gap distance Dg increase of the two pieces of samples obtained the following results. 
     1) First sample: 0.27 mm 
     2) Second sample: 0.33 mm 
     As described above, the case where the cross section of  FIG. 3B  included the second core portion  36   a   2  (the first sample) succeeded to reduce an amount of gap distance Dg increase compared with the case where the cross section did not include the second core portion  36   a   2  (the second sample). Accordingly, it is inferred that in the case where the cross section including the front end  36  at of the core portion  36   a  includes at least a part of the second core portion  36   a   2 , compared with the case where the cross section does not include the second core portion  36   a   2 , high temperature of the front end portion  31  can be suppressed. 
     In the cross section illustrated in  FIG. 3B , the cross-sectional structure of the front end side of the ground electrode  30   a  (namely, the first end  30   e   1  side) includes two layers of the first core portion  36   a   1  and the base material  35   a . Thus, in  FIG. 3B , the second core portion  36   a   2  is not disposed at the front end side of the core portion  36   a  (that is, a part where a temperature becomes high). As the result, melting of the second core portion  36   a   2  and causing the second core portion  36   a   2  to run out of (burst) the base material  35   a  can be suppressed. 
     In the case where a part of the second core portion  36   a   2  is disposed at the cross section illustrated in  FIG. 3B , regardless of the respective shapes of the first core portion  36   a   1 , the second core portion  36   a   2 , and the base material  35   a , heat can be easily released from the front end portion  31  to another portion of the ground electrode  30   a  (here, for example, the nose portion  32 ) with the second core portion  36   a   2 . In the case where the second core portion  36   a   2  is disposed not at the front end side but the rear end side at the cross section illustrated in  FIG. 3B , regardless of the respective shapes of the first core portion  36   a   1 , the second core portion  36   a   2 , and the base material  35   a , melting of the second core portion  36   a   2  and causing the second core portion  36   a   2  to run out of (burst) the base material  35   a  can be suppressed. 
     C. Third Embodiment 
     C1. Constitution of Spark Plug 
       FIG. 4A  and  FIG. 4B  are schematic diagrams illustrating a constitution of electrodes  20  and  30   b  of a spark plug  100   b  of a third embodiment.  FIG. 4A  and  FIG. 4B  are schematic diagrams similar to respective  FIG. 2A  and  FIG. 2C . The main difference between the spark plug  100  of the first embodiment, which is illustrated in  FIG. 2A  and  FIG. 2C , and the spark plug  100   b  of the third embodiment, which is illustrated in  FIG. 4A  and  FIG. 4B , is in a noble metal tip  38 . That is, the ground electrode  30   b  of the spark plug  100   b  of the third embodiment includes the noble metal tip  38 , facing the front end surface  20   s   1  of the center electrode  20 . Other constitutions of the spark plug  100   b  are the same as the constitutions of the spark plug  100  of the first embodiment, which is described in  FIG. 2A  to  FIG. 2D . Among components of the spark plug  100   b  of the third embodiment, such elements that reference numerals designate identical elements to those of the spark plug  100  of the first embodiment will not be further elaborated. 
     The ground electrode  30   b  includes the ground electrode  30  of the first embodiment as a main body portion (hereinafter also referred to as a “main body portion  30 ”). The ground electrode  30   b  further includes the noble metal tip  38  secured on the inner surface  31   si  of the front end portion  31  of the main body portion  30 . The noble metal tip  38  has a columnar shape placing the central axis CL as its center. Between a surface  38   si  facing the center electrode  20  among the surface of the noble metal tip  38  (here, the surface of  38   si  at the −D1 side) and the front end surface  20   s   1  of the center electrode  20 , the gap g is formed. The noble metal tip  38  is formed using an alloy containing iridium. The noble metal tip  38  is sealed to the base material  35  by laser beam welding. Specifically, a boundary part between the outer peripheral surface of the noble metal tip  38  and the inner surface  31   si  of the front end portion  31  of the main body portion  30  is sealed over the whole circumference by laser beam welding. 
     The schematic diagram of  FIG. 4B  illustrates the noble metal tip  38  welded on the inner surface  31   si . An illustrated distance Dd 8  indicates an outer diameter of the noble metal tip  38 . A distance Dm 8  indicates the shortest distance between the first edge line L 11  and the noble metal tip  38 . The ground electrode  30   b  is constituted so as to be symmetry with respect to the symmetric surface CLa. In the example of  FIG. 4B , the lower bottom Lb of the opposed surface  31   tsi  of the tapered end portion  31   t  is disposed at the −D2 side with respect to the central axis CL. However, the lower bottom Lb may be disposed at the +D2 side with respect to the central axis CL. 
     In the spark plug  100   b  of this embodiment, in addition to between the noble metal tip  38  and the center electrode  20 , discharge can also occur between the edge line(s) L 11  and/or L 12  and the center electrode  20 . If discharge occurs between the edge line(s) L 11  and/or L 12  and the center electrode  20 , the main body portion  30  wears. Wear of the main body portion  30  suppresses cooling of the noble metal tip  38  with the main body portion  30 . In view of this, the temperature of the noble metal tip  38  is likely to be high. As a result, the noble metal tip  38  is likely to wear. Here, to promote cooling the noble metal tip  38  with the core portion  36 , it is considered to increase a proportion of the core portion  36  at the front end portion  31  of the main body portion  30 . However, if the core portion  36  contacts a fusion portion (details will be described later), which is generated by welding of the noble metal tip  38  and the base material  35 , a strength of the welding may be degraded. Therefore, a fifth evaluation test, which will be described later, was conducted, and positions of the fusion portion and the core portion  36 , balancing the wear of the noble metal tip  38  and the strength of welding, were examined. 
     First, the following describes the halving cross section, which will be referred in the description of the fifth evaluation test.  FIG. 5A  and  FIG. 5B  and  FIG. 6A  and  FIG. 6B  are sectional views illustrating the fusion portion generated by laser beam welding. The drawings illustrate a part including the front end portion  31  among the halving cross section of the ground electrode  30   b  illustrated in  FIG. 4A .  FIG. 5A  indicates two fusion portion cross sections Ama and Amb.  FIG. 6A  illustrates one fusion portion cross section Am (the fusion portion cross sections Ama, Amb, and Am are hatched). The fusion portion is a part formed by laser beam welding. The fusion portion is a part including a constituent of the base material  35  and a constituent of the noble metal tip  38 . The fusion portion is formed by mixing the melted base material  35  and the melted noble metal tip  38 .  FIG. 5A  and  FIG. 5B  illustrate an example where the first fusion portion cross section Ama at the front end side (the first end  30   e   1  side) is isolated from the second fusion portion cross section Amb at the rear end side.  FIG. 6A  and  FIG. 6B  illustrate an example where one continuous fusion portion cross section Am is formed at the halving cross section. 
     A first area S 1  of  FIG. 5A  and  FIG. 6A  illustrates an area of the fusion portion cross section in the halving cross section. In the example of  FIG. 5A , the first area S 1  is a sum of an area S 1   a  of the first fusion portion cross section Ama and an area S 1   b  of the second fusion portion cross section Amb. In the example of  FIG. 6A , the first area S 1  is the area of the fusion portion cross section Am. 
     The drawings illustrate three positions Pa, Pb, and Pc. Positions of Pa, Pb, and Pc are configured based on the positions included in the fusion portion cross sections. The first position Pa is configured at the closest position to the first end  30   e   1  in the direction that the opposed surface  31   tsi  extends (here, the second direction D2). The second position Pb is configured at the farthest position from the center electrode  20  (not illustrated) in the first direction D1. The third position Pc is configured at the farthest position from the first end  30   e   1  in the direction that the opposed surface  31   tsi  extends (here, the second direction D2). The following describes the constitution of the halving cross section using these positions Pa, Pb, and Pc. 
       FIG. 5A  and  FIG. 6A  illustrate two straight lines L 31  and L 32 . The first straight line L 31  is positioned at the most front end side (the first end  30   e   1  side) among the straight line perpendicular to the direction that the opposed surface  31   tsi  extends (here, the second direction D2) on the halving cross section and overlapping the fusion portion cross section. In this embodiment, the first straight line L 31  passes the first position Pa and is parallel to the first direction D1. The second straight line L 32  is positioned at the rearmost end side among the straight line perpendicular to the direction that the opposed surface  31   tsi  extends (here, the second direction D2) and overlapping the fusion portion cross section. In this embodiment, the second straight line L 32  passes the third position Pc and is parallel to the first direction D1. A second area S 2  illustrated in  FIG. 5B  and  FIG. 6B  is an area of a part sandwiched between the first straight line L 31  and the second straight line L 32  among the cross section of the main body portion  30  (including the fusion portion). In  FIG. 5B  and  FIG. 6B , the parts corresponding to the second area S 2  are hatched. 
     In the examples of  FIG. 5A  and  FIG. 5B , the cross section of the core portion  36  extends to the front end side (the first end  30   e   1  side) of the ground electrode  30   b  with respect to the second straight line L 32 . The cross section of the core portion  36  (or the first core portion  36   a   1 ) does not contact both the fusion portion cross sections Ama and Amb. The shortest distance Dm in  FIG. 5A  is the shortest distance between the cross section of the core portion  36  and the fusion portion cross section. The cross section of the core portion  36  may contact at least one of the fusion portion cross sections Ama and Amb. In the example of  FIG. 6A , the cross section of the core portion  36  contacts the fusion portion cross section Am. However, the cross section of the core portion  36  may be away from the fusion portion cross section Am. 
     C2. Fifth Evaluation Test 
     The following describes the fifth evaluation test using the spark plugs  100   b  of the third embodiment as samples. The fifth evaluation test measured the amount of gap distance Dg increase and observed a state of the halving cross section after operating the internal combustion engine with the spark plug  100   b  for predetermined time, similarly to the above-described second evaluation test. The ground electrode  30   b  includes the noble metal tip  38 . In view of this, the amount of gap distance Dg increase was suppressed. Therefore, an operating period of the internal combustion engine was set to 300 hours, which was longer than the period in the second evaluation test. The content of the one-cycle operation was the same as the content in the second evaluation test. That is, the one-cycle operation included one-minute idling operation and one-minute wide open throttle operation. The maximum temperature of the ground electrode  30   b  during idling operation was approximately 300 degrees Celsius. The maximum temperature of the ground electrode  30   b  during wide open throttle operation was approximately 1000 degrees Celsius. 
     In the fifth evaluation test, 14 pieces of the spark plugs  100   b  were prepared as samples. The 14 pieces of samples were divided into two groups. The two groups differed in the dimensions of the main body portion  30  and the diameter of the noble metal tip  38  from one another. As described later, the number of samples of the first group was “8” while the number of samples of the second group was “6”. In both samples, the material of the base material  35  was Inconel and the material of the core portion  36  was a pure copper. The following lists dimensions common within the respective groups (for reference numerals of the respective dimensions, see  FIG. 4A  and  FIG. 4B ,  FIG. 5A  and  FIG. 5B , and  FIG. 6A  and  FIG. 6B ). 
     &lt;First Group&gt; 
     1) Width Da of the first end  30   e   1  of the tapered end portion  31   t  in the third direction D3: 1.2 mm 
     This width Da was the same length as the length of the upper bottom Ub of the opposed surface  31   tsi.    
     2) Length Db of the tapered end portion  31   t  in the D2 direction: 2.5 mm 
     3) Width Dc of the front end portion  31  (excluding the tapered end portion  31   t ) in the third direction D3: 2.8 mm 
     This width Dc was the same length as the length of the lower bottom Lb of the opposed surface  31   tsi.    
     4) Thickness Dt of the front end portion  31  in the first direction D1: 1.6 mm 
     5) Outer diameter Ddb of the noble metal tip  38 : 1.0 mm 
     6) Distance DL between the two straight lines L 31  and L 32 : 1.6 mm 
     7) Shortest distance Dm 8  between the noble metal tip  38  and the edge line L 11 : 0.4 mm 
     8) Diameter Dd of the front end surface  20   s   1  of the center electrode  20 : 0.8 mm 
     9) Distance Dsb between the lower bottom Lb and the central axis CL of the front end surface  20   s   1  in the second direction D2: 1.0 mm 
     The lower bottom Lb was disposed at the −D2 side with respect to the central axis CL of the front end surface  20   s   1 . 
     10) Gap distance Dg: 1.0 mm 
     11) Distance corresponding to the edge distance De of  FIG. 2D : 1.2 mm 
     &lt;Second Group&gt; 
     1) Width Da of the first end  30   e   1  of the tapered end portion  31   t  in the third direction D3: 1.0 mm 
     This width Da was the same length as the length of the upper bottom Ub of the opposed surface  31   tsi.    
     2) Length Db of the tapered end portion  31   t  in the D2 direction: 2.0 mm 
     3) Width Dc of the front end portion  31  (excluding the tapered end portion  31   t ) in the third direction D3: 2.2 mm 
     This width Dc was the same length as the length of the lower bottom Lb of the opposed surface  31   tsi.    
     4) Thickness Dt of the front end portion  31  in the first direction D 1: 1.1 mm 
     5) Outer diameter Ddb of the noble metal tip  38 : 1.2 mm 
     6) Distance DL between the two straight lines L 31  and L 32 : 1.8 mm 
     7) Shortest distance Dm 8  between the noble metal tip  38  and the edge line L 11 : 0.3 mm 
     8) Diameter Dd of the front end surface  20   s   1  of the center electrode  20 : 0.6 mm 
     9) Distance Dsb between the lower bottom Lb and the central axis CL of the front end surface  20   s   1  in the second direction D2: 0.5 mm 
     The lower bottom Lb was disposed at the −D2 side with respect to the central axis CL of the front end surface  20   s   1 . 
     10) Gap distance Dg: 1.0 mm 
     11) Distance corresponding to the edge distance De of  FIG. 2D : 1.2 mm 
     The distance corresponding to the shortest distance Wm of  FIG. 2B  was 0.2 mm or more to 1.5 mm or less in all samples. 
     Table 4, which will be illustrated below, lists respective constitutions and evaluation results of the eight pieces of samples (No. 1 to No. 8) in the first group. Table 5 lists respective constitutions and evaluation results of the six pieces of samples (No. 9 to No. 14) in the second group. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Eval- 
               
               
                   
                   
                   
                   
                   
                   
                 Evaluation 
                   
                 uation 
               
               
                   
                   
                   
                 Sr 
                   
                 Core 
                 on  
                   
                 on  
               
               
                 No. 
                 S1 
                 S2 
                 (S1/S2) 
                 Dm 
                 position 
                 peeling 
                 dDg 
                 wear 
               
               
                   
               
             
            
               
                 1 
                 0.5 
                 2.56 
                 0.195 
                 0.2  
                 Between Pb  
                 A 
                 0.15 
                 A 
               
               
                   
                   
                   
                   
                   
                 and Pc 
                   
                   
                   
               
               
                 2 
                 0.5 
                 2.56 
                 0.195 
                 0 
                 Contact 
                 B  
                 — 
                 — 
               
               
                   
                   
                   
                   
                   
                   
                 (peeled) 
                   
                   
               
               
                 3 
                 0.7 
                 2.56 
                 0.273 
                 0.8 
                 Near side  
                 A 
                 0.22 
                 B 
               
               
                   
                   
                   
                   
                   
                 with respect 
                   
                   
                   
               
               
                   
                   
                   
                   
                   
                 to Pc 
                   
                   
                   
               
               
                 4 
                 0.7 
                 2.56 
                 0.273 
                 0.6  
                 Imme- 
                 A 
                 0.18 
                 A 
               
               
                   
                   
                   
                   
                   
                 diately 
                   
                   
                   
               
               
                   
                   
                   
                   
                   
                 below of Pc 
                   
                   
                   
               
               
                 5 
                 0.7 
                 2.56 
                 0.273 
                 0.2 
                 Between Pb 
                 A 
                 0.15 
                 A 
               
               
                   
                   
                   
                   
                   
                 and Pc 
                   
                   
                   
               
               
                 6 
                 0.7 
                 2.56 
                 0.273 
                 0 
                 Contact 
                 B 
                 0.14 
                 A 
               
               
                 7 
                 0.9 
                 2.56 
                 0.352 
                 0.2  
                 Between Pb 
                 A 
                 0.23 
                 B 
               
               
                   
                   
                   
                   
                   
                 and Pc 
                   
                   
                   
               
               
                 8 
                 0.9  
                 2.56 
                 0.352 
                 0 
                 Contact 
                 A 
                 0.17 
                 A 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                   
                   
                   
                 Sr 
                   
                 Core  
                 Evaluation 
                   
                 Evaluation 
               
               
                 No. 
                 S1 
                 S2 
                 (S1/S2) 
                 Dm 
                 position 
                 on peeling 
                 dDg 
                 on wear 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 9 
                 0.5 
                 1.98 
                 0.253 
                 0.2 
                 Between  
                 A 
                 0.15 
                 A 
               
               
                   
                   
                   
                   
                   
                 Pb and Pc 
                   
                   
                   
               
               
                 10 
                 0.5 
                 1.98 
                 0.253 
                 0 
                 Contact 
                 B (peeled) 
                 — 
                 — 
               
               
                 11 
                 0.7 
                 1.98 
                 0.354 
                 0.2 
                 Between 
                 A 
                 0.22 
                 B 
               
               
                   
                   
                   
                   
                   
                 Pb and Pc 
                   
                   
                   
               
               
                 12 
                 0.7 
                 1.98 
                 0.354 
                 0 
                 Contact 
                 A 
                 0.16 
                 A 
               
               
                 13 
                 0.9 
                 1.98 
                 0.455 
                 0.2 
                 Between 
                 A 
                 0.24 
                 B 
               
               
                   
                   
                   
                   
                   
                 Pb and Pc 
                   
                   
                   
               
               
                 14 
                 0.9 
                 1.98 
                 0.455 
                 0 
                 Contact 
                 A 
                 0.17 
                 A 
               
               
                   
               
            
           
         
       
     
     Table 4 and Table 5 list sample numbers, the first areas S 1 , the second areas S 2 , area ratios Sr, the shortest distances Dm, core positions, evaluations on peeling, the amount of gap increase dDg, and evaluations on wear. The area ratio Sr is a ratio dividing the first area S 1  by the second area S 2 . “Core position” indicates the position of the front end  36   t  of the core portion  36  at the halving cross section ( FIG. 5A  and  FIG. 5B , and  FIG. 6A  and  FIG. 6B ). “Between Pb and Pc” indicates that the position of the front end  36   t  in the second direction D2 is at between the second position Pb and the third position Pc (namely, the second straight line L 32 ). “Contact” indicates that the cross section of the core portion  36  contacts the fusion portion cross section. “Near side with respect to Pc” indicates that the position of the front end  36   t  in the second direction D2 was at the −D2 side with respect to the third position Pc (namely, the second straight line L 32 ). “Immediately below of Pc” indicates that the front end  36   t  was disposed on the second straight line L 32 . 
     Regarding the evaluation on peeling, Evaluation A indicates that a length of an oxidized part generated at a boundary line BL between the noble metal tip  38  and the base material  35  at the halving cross section illustrated in  FIG. 5A  and  FIG. 6A  was less than 50% with respect to the length of the boundary line BL, and Evaluation B indicates that the length of the oxidized part was 50% or more with respect to the length of the boundary line BL or the noble metal tip  38  was peeled off from the base material  35 . As listed in Table 4 and Table 5, peeling occurred in the sample No. 2 and the sample No. 10. In the sample No. 6, peeling did not occur but the length of the oxidized part was 50% or more with respect to the length of the boundary line BL. Regarding the evaluation on wear, Evaluation A indicates that the amount of gap increase dDg was less than 0.2 mm, and Evaluation B indicates that the amount of gap increase dDg was 0.2 mm or more. This threshold of 0.2 mm was smaller than a threshold of 0.3 mm of the second evaluation test. That is, the fifth evaluation test conducted the evaluation on wear under the severe condition compared with the second evaluation test. 
     As listed in Table 4 and Table 5, a plurality of the samples in the same group can be different in the first area S 1 , the shortest distance Dm, and the core position. The change in the first area S 1  was achieved by adjusting a condition of laser beam welding (for example, irradiation time of laser light). The changes in the shortest distance Dm and the core position were achieved by adjusting the condition of laser beam welding and a condition of forming the ground electrode  30   b . The constitutions of the halving cross section of the respective samples can be a type illustrated in  FIG. 5A  or a type illustrated in  FIG. 6A  according to the first area S 1  or a similar condition. 
       FIG. 7  is a graph summarizing the results listed in Table 4 and Table 5. The horizontal axis indicates the area ratio Sr (S 1 /S 2 ). The vertical axis indicates the outline of the core position. The circles in the drawing indicate samples where both the evaluation on peeling and the evaluation on wear were Evaluation A. The X marks indicate samples where at least one of the evaluation on peeling and the evaluation on wear was Evaluation B. Numbers attached near the marks indicate sample numbers of the marks. 
     First, the following describes the case where the area ratio Sr was smaller than 1/3. As indicated by the sample No. 2, the sample No. 6, and the sample No. 10, the evaluation on peeling is Evaluation B when the area ratio Sr is smaller than 1/3 and the cross section of the core portion  36  contacts the fusion portion cross section. This is probably due to the following reason. That is, in the case where the area ratio Sr is small, the fusion portion cross section is relatively small. In view of this, the strength of welding becomes weak. Furthermore, by contact of the core portion  36  with the fusion portion, the constituent of the core portion  36  is further contained in the fusion portion. This possibly results in degrade of the strength of the fusion portion. This is likely to promote wear at the boundary part between the noble metal tip  38  and the base material  35  (for example, oxidation). 
     As indicated by the sample No. 3, in the case where the area ratio Sr was smaller than 1/3 and the front end  36   t  of the core portion  36  was disposed at the near side with respect to the third position Pc, the evaluation on wear was Evaluation B. This is probably due to the following reason. That is, the core portion  36  is not disposed at the front end side with respect to the second straight line L 32 . In view of this, the temperature of the front end portion  31  is likely to become high. As a result, the noble metal tip  38  is likely to wear. 
     The sample No. 1, the sample No. 4, the sample No. 5, and the sample No. 9 had the area ratio Sr smaller than 1/3. Moreover, the cross section of the core portion  36  did not contact the fusion portion cross section. Furthermore, the front end  36   t  of the core portion  36  was disposed at the front end side with respect to the third position Pc (that is, the first end  30   e   1  side with respect to the second straight line L 32 ). In this case, both the evaluation on peeling and the evaluation on wear were Evaluation A. Thus, in the case where the area ratio Sr is smaller than 1/3, it is preferred that the cross section of the core portion  36  do not contact the fusion portion cross section and the front end  36   t  of the core portion  36  is disposed at the first end  30   e   1  side with respect to the second straight line L 32 .  FIG. 7  illustrates a preferable constitution range of R 1  by unhatched region. 
     Next, the following describes the case where the area ratio Sr was 1/3 or more. The sample No. 7, the sample No. 11, and the sample No. 13 had the area ratio Sr of 1/3 or more. Furthermore, the front end  36   t  of the core portion  36  was disposed between the second position Pb and the third position Pc (that is, the cross section of the core portion  36  was away from the fusion portion cross section). In this case, the evaluation on wear was Evaluation B. This is probably due to the following reason. That is, the fusion portion contains the constituent of the noble metal tip  38  in addition to the constituent of the base material  35 . Therefore, thermal conductivity of the fusion portion can be lower than thermal conductivity of the base material  35 . In the case where the area ratio Sr is large, the fusion portion cross section become relatively large while the cross section of the base material  35  excluding the fusion portion become relatively small. Therefore, an effect of cooling the front end portion  31  with the base material  35  becomes small. As the results described above, the temperature of the front end portion  31  is likely to become high. In view of this, the noble metal tip  38  is likely to wear. 
     The sample No. 8, the sample No. 12, and the sample No. 14 had the area ratio Sr of 1/3 or more. Furthermore, the cross section of the core portion  36  contacted the fusion portion cross section. In this case, both the evaluation on peeling and the evaluation on wear were Evaluation A. This is probably due to the following reason. That is, large area ratio Sr strengthens the welding strength. Therefore, even if the fusion portion contains the constituent of the core portion  36  due to contact of the cross section of the core portion  36  with the fusion portion cross section, a sufficient welding strength can be achieved between the noble metal tip  38  and the base material  35 . Although the area ratio Sr is large, the cross section of the core portion  36  contacts the fusion portion cross section. This allows improving an effect of cooling the front end portion  31  with the core portion  36 . This allows suppressing wear of the noble metal tip  38 . Thus, in the case where the area ratio Sr is 1/3 or more, it is preferred that the cross section of the core portion  36  contact the fusion portion cross section.  FIG. 7  illustrates a preferable constitution range of R 2  by unhatched region. 
     Generally, in the case where the front end  36   t  of the core portion  36  is disposed at the first end  30   e   1  side with respect to the second straight line L 32 , compared with the different case, an effect of cooling the front end portion  31  with the core portion  36  is high. In the case where the area ratio Sr is comparatively small, compared with the case where the area ratio Sr is comparatively large, degrade of thermal conductivity of the ground electrode  30   b  can be suppressed. Here, isolating the core portion  36  from the fusion portion allows suppressing degrade of the sealing strength between the noble metal tip  38  and the base material  35 . In the case where the area ratio Sr is comparatively large, compared with the case where the area ratio Sr is comparatively small, the sealing strength between the noble metal tip  38  and the base material  35  can be strengthened. Here, contact of the core portion  36  with the fusion portion allows suppressing degrade of thermal conductivity of the ground electrode  30   b . The above-described various characteristics can be achieved regardless of the respective dimensions of various components of the ground electrode  30   b  and/or the constitution of the core portion  36 . Therefore, it is inferred that the preferable constitution of the halving cross section is not limited to the spark plug samples employed in the fifth evaluation test, but is applicable to various spark plugs. For example, the preferable constitution may be applied to a spark plug that includes the ground electrode  30   a  ( FIG. 3A ) and the noble metal tip  38  secured to the ground electrode  30   a . In this case, the ground electrode  30   a  in  FIG. 3A  corresponds to the main body portion of the ground electrode. 
     D. Modification 
     (1) Constitutions of the ground electrode are not limited to the constitutions of the respective embodiments, but can employ various constitutions. For example, at least one of the tapered surfaces  31   ts   1  and  31   ts   2  may not be parallel to but may be inclined with respect to the central axis CL. For example, the two tapered surfaces  31   ts   1  and  31   ts   2  may be inclined with respect to the central axis CL so that a distance between the two tapered surfaces  31   ts   1  and  31   ts   2  (a distance parallel to the third direction D3) gradually increases to the first direction D1. 
     The materials of components of the ground electrodes  30 ,  30   a , and  30   b  are not limited to the above-described materials, but various materials are applicable. For example, the materials of the base materials  35  and  35   a  are not limited to Inconel, but various materials excellent in thermal resistance, such as other nickel alloys or a pure nickel are applicable. 
     (2) The material of the noble metal tip  38  is not limited to an alloy containing an iridium, but a material containing other various noble metals (for example, a platinum) is applicable. The center electrode  20  may include a noble metal tip forming the gap g. 
     (3) Constitutions of the spark plug are not limited to the constitutions of the respective embodiments but various constitutions are applicable. For example, the outer diameter Dd of the front end surface  20   s   1  of the center electrode  20  may be larger than the widths of the ground electrode  30 ,  30   a , and  30   b  (a width in the direction vertical to the direction that the ground electrode extends) when viewed facing the direction parallel to the central axis CL. In any cases, when viewed facing the direction parallel to the central axis CL, a part of the front end surface  20   s   1  of the center electrode  20  may be disposed outside of a range overlapped with the ground electrode  30 ,  30   a , and  30   b . In the respective embodiments, the lower bottom Lb of the opposed surface  31   tsi  of the tapered end portion  31   t  may be disposed at the +D2 side of the central axis CL or may be disposed at the −D2 side of the central axis CL. 
     The embodiments of this disclosure are described above based on the working examples and modifications. The above-described embodiments are for ease of understanding of this disclosure and do not limit this disclosure. This disclosure may be modified or improved without departing from the gist of the invention. This disclosure also includes the equivalents. 
     The foregoing detailed description has been presented for the purposes of illustration and description. Many modifications and variations are possible in light of the above teaching. It is not intended to be exhaustive or to limit the subject matter described herein to the precise form disclosed. Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims appended hereto.