Patent Publication Number: US-9887518-B2

Title: Spark plug

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/JP2015/001961 filed Apr. 7, 2015, claiming priority based on Japanese Patent Application No. 2014-101002, filed May 15, 2014, the contents of all of which are incorporated herein by reference in their entirety. 
     TECHNICAL FIELD 
     The present invention relates to a spark plug. 
     BACKGROUND ART 
     A spark plug generates spark discharge in a gap between a center electrode and a ground electrode to realize ignition of an air-fuel mixture in a combustion chamber of an internal combustion engine. As a ground electrode of a spark plug, ground electrode has been known in which an electrode tip is joined to an electrode base material in order to improve wear resistance of the ground electrode against spark discharge and oxidation (refer to Patent Document 1, for example). The electrode tip of the ground electrode is made of a material whose wear resistance against spark discharge and oxidation is superior to those of the electrode base material. Examples of the material of the electrode tip include a noble metal (e.g., platinum, iridium, ruthenium, or rhodium), nickel, and an alloy composed mainly of any one of these metals. In the ground electrode including the electrode tip joined to the electrode base material, a fusion zone containing the component of the electrode base material and the component of the electrode tip is formed due to welding for joining the electrode tip to the electrode base material. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2006-128076 
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     In recent years, in order to secure durability against high compression and high supercharging in an internal combustion engine, increase in the diameter of an electrode tip of a ground electrode has been studied. However, in the spark plug of Patent Document 1, if the diameter of the electrode tip of the ground electrode is increased, thermal stress that occurs in the fusion zone is increased in relation to the electrode tip having the increased diameter, and thereby a crack is likely to occur in the fusion zone. If the crack excessively progresses in the fusion zone, the electrode tip might peel off from the electrode base material. 
     Means for Solving the Problem 
     The present invention has been made to solve the above problems and can be embodied in the following modes. 
     (1) According to one mode of the present invention, a spark plug is provided which includes: a rod-shaped center electrode; and a ground electrode including an electrode tip which forms a gap with the center electrode, an electrode base material to which the electrode tip is joined, and a fusion zone containing a component of the electrode tip and a component of the electrode base material, wherein the electrode tip projects from a base material surface which extends from a base end portion of the electrode base material to a front end portion thereof, toward the center electrode. In this spark plug, at a cross section of the ground electrode which is orthogonal to a longitudinal direction of the electrode base material extending from the base end portion to the front end portion, and passes an axis of the electrode tip, the base material surface is exposed, and relationships among the following parameters: a length E of a front end surface of the electrode tip; a point Ca at which the fusion zone is in contact with the base material surface, on one side of the axis; a point Cb at which the fusion zone is in contact with the base material surface, on the other side of the axis which is different from the one side; a distance F between the point Ca and the point Cb; a point Ga at which the fusion zone is in contact with a side surface of the electrode tip, on the one side; a point Ha at which a virtual line that passes the point Ga and is parallel to the axis, intersects an interface between the fusion zone and the electrode base material; a depth Da from a virtual line which passes the point Ca and the point Cb, to the point Ha; a point Gb at which the fusion zone is in contact with the side surface of the electrode tip, on the other side; a point Hb at which a virtual line that passes the point Gb and is parallel to the axis, intersects the interface between the fusion zone and the electrode base material; a depth Db from a virtual line which passes the point Ca and the point Cb, to the point Hb; a point I which is, in a portion of the fusion zone closest to the axis, a point most distant from the virtual line which passes the point Ca and the point Cb; and an area J which is a sum of an area of a triangle having the point Ga, the point Ha, and the point I as apexes, and an area of a triangle having the point Gb, the point Hb, and the point I as apexes, satisfy the following conditions: 1.2E≦F≦1.9E; 0.05 mm≦Da≦0.30 mm; 0.05 mm≦Db≦0.30 mm; and 0.20 mm 2 ≦J≦0.70 mm 2 . According to this mode, it is possible to secure sufficient peeling resistance of the ground electrode against peeling of the electrode tip. 
     (2) In the spark plug according to the above mode, the relationships with the area J which is a sum of the area of the triangle having the point Ga, the point Ha, and the point I as apexes, and the area of the triangle having the point Gb, the point Hb, and the point I as apexes, may satisfy the following conditions: 1.2E≦F≦1.8E; 0.05 mm≦Da≦0.25 mm; 0.05 mm≦Db≦0.25 mm; and 0.20 mm 2 ≦J≦0.68 mm 2 . According to this mode, it is possible to secure more sufficient peeling resistance of the ground electrode against peeling of the electrode tip. 
     (3) In the spark plug according to the above mode, a relationship between an area A of a front end surface of the center electrode and an area B of the front end surface of the electrode tip may satisfy a condition of 1.3A≦B≦4.6A. According to this mode, it is possible to secure sufficient wear resistance of the ground electrode against spark discharge and oxidation. 
     (4) In the spark plug according to the above mode, a height K of the electrode tip from the base material surface may satisfy a condition of 0.3 mm≦K≦1.2 mm. According to this mode, it is possible to secure sufficient wear resistance of the ground electrode while securing sufficient ignitability of the spark plug. 
     (5) In the spark plug according to the above mode, the electrode tip may contain at least one element selected from a group consisting of iridium (Ir), platinum (Pt), rhodium (Rh), ruthenium (Ru), and nickel (Ni). According to this mode, it is possible to realize the electrode tip having sufficient wear resistance. 
     The present invention can be implemented in various forms other than the spark plug. For example, the present invention can be implemented as a ground electrode of a spark plug, a spark plug manufacturing method, a spark plug manufacturing apparatus, a computer program for controlling the manufacturing apparatus, and a non-transitory storage medium in which the computer program is stored. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory view showing a partial cross section of a spark plug. 
         FIG. 2(A)  and  FIG. 2(B)  are explanatory views showing a front end side of the spark plug. 
         FIG. 3  is an explanatory view showing an example of a cross section of a ground electrode. 
         FIG. 4  is an explanatory view showing an example of a cross section of a ground electrode according to another embodiment. 
         FIG. 5  is an explanatory view showing an example of a cross section of a ground electrode according to another embodiment. 
         FIG. 6  is an explanatory view showing an example of a cross section of a ground electrode according to another embodiment. 
         FIG. 7  is an explanatory view view showing a ground electrode according to another embodiment. 
         FIG. 8  is a table showing the result of evaluation of peeling resistance of the ground electrode against peeling of an electrode tip. 
         FIG. 9  is a table showing the result of evaluation of peeling resistance of the ground electrode against peeling of the electrode tip. 
         FIG. 10  is a table showing the result of evaluation of peeling resistance of the ground electrode against peeling of the electrode tip. 
         FIG. 11  is a table showing the result of evaluation of peeling resistance of the ground electrode against peeling of the electrode tip. 
         FIG. 12  is a table showing the result of evaluation of peeling resistance of the ground electrode against peeling of the electrode tip. 
         FIG. 13  is an explanatory view showing an example of a cross section of a ground electrode in which cracks occur. 
         FIG. 14  is a table showing the result of evaluation of ignitability of the spark plug. 
         FIG. 15  is a table showing the result of evaluation of wear resistance of the electrode tip. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     A. First Embodiment 
     A-1. Structure of Spark Plug 
       FIG. 1  is an explanatory view showing a partial cross section of a spark plug  10 . In  FIG. 1 , with an axis CA as a center axis of the spark plug  10  being a boundary, an external shape of the spark plug  10  is shown on the left side of the axis CA in the sheet of  FIG. 1 , and a cross-sectional shape of the spark plug  10  is shown on the right side of the axis CA in the sheet of  FIG. 1 . In the description of this embodiment, a lower side of the spark plug  10  in the sheet of  FIG. 1  is referred to as “front end side”, and an upper side thereof in the sheet of  FIG. 1  is referred to as “rear end side”. 
     The spark plug  10  includes a center electrode  100 , an insulator  200 , a metal shell  300 , and a ground electrode  400 . In this embodiment, the axis CA of the spark plug  10  also serves as a center axis of each of the center electrode  100 , the insulator  200 , and the metal shell  300 . 
     The spark plug  10  has, at the front end side, a gap SG formed between the center electrode  100  and the ground electrode  400 . The gap SG of the spark plug  10  is also called a spark gap. The spark plug  10  is configured to be mountable to an internal combustion engine  90 , with the front end side having the gap SG projecting from an inner wall  910  of a combustion chamber  920 . When a high voltage (e.g., 10,000 to 30,000 volts) is applied to the center electrode  100  of the spark plug  10  mounted to the internal combustion engine  90 , spark discharge is generated in the gap SG. The spark discharge generated in the gap SG realizes ignition of an air-fuel mixture in the combustion chamber  920 . 
     In  FIG. 1 , X, Y and Z axes which are orthogonal to one another are shown. The X, Y and Z axes shown in  FIG. 1  correspond to the X, Y and Z axes in other drawings described later. Of the X, Y and Z axes shown in  FIG. 1 , the X axis is an axis orthogonal to the Y axis and the Z axis. Of X axis directions along the X axis, a +X axis direction is a direction from the back side of the sheet of  FIG. 1  to the front side thereof, and a −X axis direction is a direction opposite to the +X axis direction. Of the X, Y and Z axes shown in  FIG. 1 , the Y axis is an axis orthogonal to the X axis and the Z axis. Of Y axis directions along the Y axis, a +Y axis direction is a direction from the right side of the sheet of  FIG. 1  to the left side thereof, and a −Y axis direction is a direction opposite to the +Y axis direction. Of the X, Y and Z axes shown in  FIG. 1 , the Z axis is an axis along the axis CA. Of Z axis directions (axial directions) along the Z axis, a +Z axis direction is a direction from the rear end side of the spark plug  10  to the front end side thereof, and a −Z axis direction is a direction opposite to the +Z axis direction. 
     The center electrode  100  of the spark plug  10  is an electrode having electrical conductivity. The center electrode  100  has a shape of a rod extending around and along the axis CA. An outer surface of the center electrode  100  is electrically insulated from the outside by the insulator  200 . A front end side of the center electrode  100  projects from a front end side of the insulator  200 . A rear end side of the center electrode  100  is electrically connected to a rear end side of the insulator  200 . In the present embodiment, the rear end side of the center electrode  100  is electrically connected to the rear end side of the insulator  200  via a metal terminal  190 . 
     The insulator  200  of the spark plug  10  is an insulator having an electrical insulating property. The insulator  200  has a shape of a tube extending around and along the axis CA. In the present embodiment, the insulator  200  is formed by firing an insulating ceramic material (e.g., alumina). The insulator  200  has an axial hole  290  which is a through-hole extending around and along the axis CA. The center electrode  100  is held in the axial hole  290  of the insulator  200  so as to be located on the axis CA and project from the front end side of the insulator  200 . 
     The metal shell  300  of the spark plug  10  is a metal member having electrical conductivity. The metal shell  300  has a shape of a tube extending around and along the axis CA. In the present embodiment, the metal shell  300  is a member obtained by plating low-carbon steel formed in a tube shape with nickel. In another embodiment, the metal shell  300  may be a member plated with zinc, or a non-plated member. The metal shell  300  is fixed, by means of crimping, to an outer surface of the insulator  200  while being electrically insulated from the center electrode  100 . On a front end side of the metal shell  300 , an end surface  310  is formed. From the center of the end surface  310 , the insulator  200  as well as the center electrode  100  project toward the +Z axis direction. The ground electrode  400  is joined to the end surface  310 . 
     The ground electrode  400  of the spark plug  10  is an electrode having electrical conductivity. The ground electrode  400  includes an electrode base material  410  and an electrode tip  450 . The electrode base material  410  has such a shape that it extends from the end surface  310  of the metal shell  300  in the +Z axis direction and then bends toward the axis CA. A rear end side of the electrode base material  410  is joined to the metal shell  300 . The electrode tip  450  is joined to a front end side of the electrode base material  410 . The electrode tip  450  forms a gap SG with the center electrode  100 . 
     In the present embodiment, the electrode base material  410  is a nickel alloy which contains nickel (Ni) as a main component. In the present embodiment, the electrode tip  450  is made of an alloy which contains platinum (Pt) as a main component, and contains 20% by weight of rhodium (Rh). In another embodiment, the electrode tip  450  may be made of any material as long as the material has excellent wear resistance against spark discharge. For example, the material may be a pure noble metal (e.g., iridium (Ir), platinum (Pt), rhodium (Rh), or ruthenium (Ru)), nickel (Ni), or an alloy composed of at least one of these metals. 
     A-2. Detailed Structure of Ground Electrode 
       FIG. 2(A)  and  FIG. 2(B)  are explanatory views showing the front end side of the spark plug  10 .  FIG. 2(A)  at the upper stage is a partial enlarged view of the center electrode  100  and the ground electrode  400  as viewed from the +X axis direction.  FIG. 2(B)  at the lower stage is a partial enlarged view of a front end side of the ground electrode  400  as viewed from the −Z axis direction. 
     The center electrode  100  has a cylindrical shape. The center electrode  100  has a front end surface  101  and a side surface  107 . The front end surface  101  and the side surface  107  constitute an end portion of the center electrode  100  at the front end side. The front end surface  101  of the center electrode  100  is a plane which is parallel to the X axis and the Y axis and faces in the +Z axis direction. The side surface  107  of the center electrode  100  is a plane which is formed around the axis CA and is parallel to the Z axis. In the present embodiment, among the portions of the center electrode  100 , the front end surface  101  forms a gap SG with the electrode tip  450  of the ground electrode  400 . 
     In the present embodiment, the center electrode  100  is an electrode obtained by joining an electrode tip  150  containing a noble metal as a main component to an electrode base material  110 , and the electrode tip  150  constitutes the front end surface  101  and the side surface  107 . In the present embodiment, the electrode base material  110  is made of a nickel alloy (e.g., INCONEL 600 (“INCONEL” is a registered trademark)) containing nickel (Ni) as a main component, and the electrode tip  150  is made of iridium (Ir). In another embodiment, the center electrode  100  may be an electrode made of the same material as a whole including the front end surface  101  and the side surface  107 . 
     The electrode base material  410  of the ground electrode  400  has base material surfaces  411 ,  412 ,  413 ,  414 ,  415 , and  416 . The base material surface  411  is a plane which is formed extending from the rear end side of the electrode base material  410  to the front end side thereof, and faces in the −Z axis direction at the front end side of the ground electrode  400 . The base material surface  412  is a plane which is formed extending from the rear end side of the electrode base material  410  to the front end side thereof, and faces in the +Z axis direction at the front end side of the ground electrode  400 . The base material surface  413  is a plane which constitutes a front end portion of the ground electrode  400 , and faces in the +Y axis direction. The base material surface  414  is a plane which constitutes a base end portion of the ground electrode  400 , and faces in the −Z axis direction. The base material surface  415  is a plane which is formed extending from the rear end side of the electrode base material  410  to the front end side thereof, and faces in the −X axis direction. The base material surface  416  is a plane which is formed extending from the rear end side of the electrode base material  410  to the front end side thereof, and faces in the +X axis direction. Among the portions of the electrode base material  410 , on a front end side of the base material surface  411  extending from a front end portion (base material surface  413 ) of the electrode base material  410  to a base end portion (base material surface  414 ) thereof, the electrode tip  450  is provided. 
     The electrode tip  450  of the ground electrode  400  is a cylindrical projecting portion which projects from the base material surface  411  of the electrode base material  410  toward the −Z axis direction. In the present embodiment, an axis CAc of the electrode tip  450  is parallel to the Z axis. The electrode tip  450  has tip surfaces  451  and  453 . The tip surface  451  is a front end surface which is parallel to the X axis and the Y axis, and faces in the −Z axis direction. The tip surface  451  forms a gap SG with the front end surface  101  of the center electrode  100 . The tip surface  453  is a side surface which is formed around the axis CAc and is parallel to the Z axis. The electrode tip  450  is joined to the electrode base material  410  at the periphery of the tip surface  453  on the +Z axis direction side. 
     Around the electrode tip  450  on the electrode base material  410 , a fusion zone  430  is formed due to laser welding for joining the electrode tip  450  to the electrode base material  410 . In  FIG. 2(A)  and  FIG. 2(B) , the fusion zone  430  is hatched. The fusion zone  430  is a portion (so-called a weld bead) in which the metals derived from the electrode base material  410  and the electrode tip  450  are fused by laser welding and solidified. The fusion zone  430  contains the component of the electrode base material  410  and the component of the electrode tip  450 . 
       FIG. 3  is an explanatory view showing an example of a cross section of the ground electrode  400 . The cross section shown in  FIG. 3  is a cross section of the ground electrode  400  as viewed from the direction of arrows F 3 -F 3  in  FIG. 2(B) . The line indicated between the arrows F 3 -F 3  is orthogonal to a longitudinal direction (Y axis direction) of the electrode base material  410  extending from the base material surface  413  to the base material surface  414 , and passes the axis CAc of the electrode tip  450 . 
     The electrode base material  410  has a corner portion  419   a  and a corner portion  419   b . The corner portion  419   a  of the electrode base material  410  forms an outwardly convex arc surface which connects the base material surface  411  and the base material surface  415 . The corner portion  419   b  of the electrode base material  410  forms an outwardly convex arc surface which connects the base material surface  411  and the base material surface  416 . 
     In the cross section of the ground electrode  400  as viewed from the direction of the arrows F 3 -F 3 , the fusion zone  430  includes a first portion  430   a  and a second portion  430   b . The first portion  430   a  of the fusion zone  430  is formed of a portion on the −X axis direction side (base material surface  415  side) relative to the axis CAc of the electrode tip  450 . The second portion  430   b  of the fusion zone  430  is formed of a portion on the +X axis direction side (base material surface  416  side) relative to the axis CAc of the electrode tip  450 . 
     In the example of  FIG. 3 , the first portion  430   a  is positioned on the −X axis direction side relative to the axis CAc, and the second portion  430   b  is positioned on the +X axis direction side relative to the axis CAc. In the description of the present specification, this mode of the fusion zone  430  is referred to as a pattern “A”, and the ground electrode  400  which satisfies the pattern “A” is also referred to as a ground electrode  400 A. 
     The fusion zone  430  has an exposed surface  431  and an interface  433 . The exposed surface  431  of the fusion zone  430  is a plane which is formed at a position irradiated with laser during laser welding, and is exposed from the electrode base material  410  and the electrode tip  450 . The interface  433  of the fusion zone  430  is a boundary between the electrode base material  410  and the electrode tip  450 . 
     A length E is a length of the tip surface  451  of the electrode tip  450  at the cross section of the ground electrode  400  as viewed from the direction of the arrows F 3 -F 3 . A point Ca is a point at which the exposed surface  431  of the first portion  430   a  is in contact with the base material surface  411 . A point Cb is a point at which the exposed surface  431  of the second portion  430   b  is in contact with the base material surface  411 . A distance F is a distance between the point Ca and the point Cb. A virtual line VL 3  is a straight line passing between the point Ca and the point Cb. 
     A point Ga is a point at which the exposed surface  431  of the first portion  430   a  is in contact with the tip surface  453  of the electrode tip  450 . A virtual line VL 1  is a straight line which passes the point Ga and is parallel to the axis CAc. A point Ha is a point at which the virtual line VL 1  intersects the interface  433 . A depth Ca is a distance from the virtual line VL 3  to the point Ha. 
     A point Gb is a point at which the exposed surface  431  of the second portion  430   b  is in contact with the tip surface  453  of the electrode tip  450 . A virtual line VL 2  is a straight line which passes the point Gb and is parallel to the axis CAc. A point Hb is a point at which the virtual line VL 2  intersects the interface  433 . A depth Db is a distance from the virtual line VL 3  to the point Hb. 
     A point I is a point which is, in a portion of the fusion zone  430  closest to the axis CAc, most distant from the virtual line VL 3 . An area J 1  is an area of a triangle Gb-Hb-I with the point Ga, the point Ha, and the point I as apexes. An area J 2  is an area of a triangle Ga-Ha-I with the point Gb, the point Hb, and the point I as apexes. 
     In order to secure sufficient peeling resistance of the ground electrode  400  against peeling of the electrode tip  450 , it is preferable that, at the cross section of the ground electrode  400  as viewed from the direction of the arrows F 3 -F 3 , the base material surface  411  is exposed, and the following conditions are satisfied: 
     1.2E≦F≦1.9E; 
     0.05 mm≦Da≦0.30 mm; 
     0.05 mm≦Db≦0.30 mm; and 
     0.20 mm 2 ≦J≦0.70 mm, 
     and it is more preferable that the following conditions are satisfied: 
     1.2E≦F≦1.8E; 
     0.05 mm≦Da≦0.25 mm; 
     0.05 mm≦Db≦0.25 mm; and 
     0.20 mm 2 ≦J≦0.68 mm 2    
     where an area J is a sum of the area J 1  and the area J 2 . Evaluation of the respective parameters regarding the ground electrode  400  will be described later. 
     In order to secure sufficient wear resistance of the ground electrode  400  against spark discharge and oxidation, it is preferable that the relationship between an area A of the front end surface  101  of the center electrode  100  and an area B of the front end surface  451  of the electrode tip  450  satisfies a condition of 1.3A≦B≦4.6A. Evaluation of the areas A and B will be described later. 
     In order to secure sufficient wear resistance of the ground electrode while maintaining sufficient ignitability, it is preferable that a height K of the electrode tip  450  from the base material surface  411  satisfies a condition of 0.3 mm≦K≦1.2 mm. Evaluation of the height K will be described later. 
       FIG. 4  is an explanatory view showing an example of a cross section of a ground electrode  400 B according to another embodiment. The ground electrode  400 B is identical to the ground electrode  400 A shown in  FIG. 3  except the mode of the fusion zone  430 . The cross section shown in  FIG. 4  is a cross section of the ground electrode  400 B as viewed from a position corresponding to the arrows F 3 -F 3  in  FIG. 2(B) . In the example of  FIG. 4 , the first portion  430   a  is formed prior to the second portion  430   b , and the second portion  430   b  is formed so as to partially overlap a front end of the first portion  430   a . In the description of the present specification, this mode of the fusion zone  430  is referred to as a pattern “B”. 
       FIG. 5  is an explanatory view showing an example of a cross section of a ground electrode  400 C according to another embodiment. The ground electrode  400 C is identical to the ground electrode  400 A of  FIG. 3  except the mode of the fusion zone  430 . The cross section shown in  FIG. 5  is a cross section of the around electrode  400 C as viewed from a position corresponding to the arrows F 3 -F 3  in  FIG. 2(B) . In the example of  FIG. 5 , the first portion  430   a  is formed prior to the second portion  430   b , and the second portion  430   b  is formed penetrating through the first portion  430   a . In the description of the present specification, this mode of the fusion zone  430  is referred to as a pattern “C”. 
       FIG. 6  is an explanatory view showing an example of a cross section of a ground electrode  400 D according to another embodiment. The ground electrode  400 D is identical to the ground electrode  400 A of  FIG. 3  except the mode of the fusion zone  430 . The cross section shown in  FIG. 6  is a cross section of the ground electrode  400 D as viewed from a position corresponding to the arrows F 3 -F 3  in  FIG. 2(B) . In the example of  FIG. 6 , the first portion  430   a  is positioned on the −X axis direction side relative to the axis CAc, and the second portion  430   b  is formed at a position apart from the first portion  430   a  so as to extend from the +X axis direction side to the −X axis direction side with respect to the axis CAc. In the description of the present specification, this mode of the fusion zone  430  is referred to as a pattern “D”. 
       FIG. 7  is an explanatory view showing a ground electrode  400 E according to another embodiment. The ground electrode  400 E is identical to the ground electrode  400  of  FIG. 2(A)  and  FIG. 2(B)  except the shape of the electrode base material. The electrode base material  410 E of the ground electrode  400 E is identical to the electrode base material  410  of  FIG. 2(A)  and  FIG. 2(B)  except that the electrode base material  410 E includes a base material surface  417 E and a base material surface  418 E. The base material surface  417 E is a plane facing in the −X axis direction and the +Y axis direction, and connects the base material surface  413  to the base material surface  415 . The base material surface  418 E is a plane facing the +X axis direction and the +Y axis direction, and connects the base material surface  413  to the base material surface  416 . The mode of the fusion zone  430  as viewed from the direction of arrows F 3 -F 3  in  FIG. 7  may be any of the patterns shown in  FIG. 3 ,  FIG. 4 ,  FIG. 5 , and  FIG. 6 . 
     A-3. Evaluation Test 
       FIG. 8 ,  FIG. 9 ,  FIG. 10 ,  FIG. 11 , and  FIG. 12  are tables showing the results of evaluation of peeling resistance of the ground electrode  400  against peeling of the electrode tip  450 . In a peeling resistance evaluation test, a tester evaluated a plurality of spark plugs  10  having different parameters regarding the ground electrode  400 , as samples A1 to A8, B1 to B12, C1 to C16, D1 to D16, and E1 to E16. 
     The specifications of the electrode base material  410  in the samples A1 to A8 are as follows. 
     Material: INCONEL 601 
     Width W along the X axis direction: 1.4 mm (millimeter) 
     Radius of each of the corner portions  419   a ,  419   b:  0.2 mm 
     The specifications of the electrode base material  410  in the samples B1 to B12 are as follows. 
     Material: INCONEL 601 
     Width W along the X axis direction: 1.9 mm 
     Radius of each of the corner portions  419   a ,  419   b:  0.2 mm 
     The specifications of the electrode base material  410  in the samples C1 to C16 are as follows. 
     Material: INCONEL 601 
     Width W along the X axis direction: 2.5 mm 
     Radius of each of the corner portions  419   a ,  419   b:  0.25 mm 
     The specifications of the electrode base material  410  in the samples D1 to D16 are as follows. 
     Material: INCONEL 601 
     Width W along the X axis direction: 3.1 mm 
     Radius of each of the corner portions  419   a ,  419   b:  0.3 mm 
     The specifications of the electrode base material  410  in the samples E1 to E16 are as follows. 
     Material: INCONEL 601 
     Width W along the X axis direction: 3.6 mm 
     Radius of each of the corner portions  419   a ,  419   b:  0.3 mm 
     The specifications of the electrode tip  450  in each sample are as follows. 
     Material: an alloy which contains platinum (Pt) as a main component, and contains 20% by weight of rhodium (Rh) 
     Shape: cylindrical shape 
     Length E (tip diameter): 0.8 mm, 1.0 mm, 1.2 mm, 1.5 mm 
     In the center electrode  100  of each sample, the diameter of the front end surface  101  is 0.7 mm. In each sample, the front end area ratio B/A between the area A and the area B is 1.31 to 4.59. 
     The tester, in a durability test, mounted each sample on an internal combustion engine (engine displacement of 1.5 liters, 4 cylinders), and repeated the following operation states 1 and 2 for 100 hours. 
     Operation state 1: operating the internal combustion engine at 5000 rpm (revolutions per minute) with a full-open throttle for 1 minute. 
     Operation state 2: halting the internal combustion engine for 1 minute. 
     The tester cut each sample subjected to the durability test at a position corresponding to the arrows F 3 -F 3  in  FIG. 2(B) , and then measured the parameters and confirmed progression of cracks in the fusion zone  430 . 
     In measurement of the distance F indicating the outer diameter of the fusion zone  430 , the tester measured, as the distance F, the outer diameter of the fusion zone  430  along the Y axis direction, for the samples A3 to A8, B4, B7, B8, B11, B12, C11, C12, C15, C16, D15, and D16 in which the fusion zone  430  reaches the corner portions  419   a ,  419   b.    
       FIG. 13  is an explanatory view showing an example of a cross section of the ground electrode  400  in which cracks CKa and CKb have occurred. A virtual line VL 4  is a straight line which passes a portion of the electrode tip  450  positioned closest to the +Z axis direction side among the portions of the electrode tip  450 , and is parallel to the X axis. A point P 1  is a point at which the interface  433  of the first portion  430   a  intersects the virtual line VL 4 . A point P 2  is a point at which the interface  433  of the second portion  430   b  intersects the virtual line VL 4 . A point P 3  is a point which is positioned on the +X axis direction side relative to the virtual line VL 1  and on the −Z axis direction side relative to the virtual line VL 4  and is closest to the axis CAc, in a portion of the first portion  430   a  where the crack CKa has occurred. A point P 4  is a point which is positioned on the −X axis direction side relative to the virtual line VL 2  and on the −Z axis direction side relative to the virtual line VL 4  and is closest to the axis CAc, in a portion of the second portion  430   b  where the crack CKb has occurred. 
     A distance Sa is a distance from the virtual line VL 1  to the point P 1 . A distance Sb is a distance from the virtual line VL 2  to the point P 2 . A distance Ta is a distance from the virtual line VL 1  to the point P 3 . A distance Tb is a distance from the virtual line VL 2  to the point P 4 . 
     The tester evaluated peeling resistances of the respective samples based on the following evaluation standard, in accordance with progression of cracks. 
     Excellent (indicated by a double circle): (Ta+Tb)/(Sa+Sb)×100≦50(%) 
     Good (indicated by a circle): 50(%)&lt;(Ta+Tb)/(Sa+Sb)×100&lt;90(%) 
     Poor (indicated by a cross): 90(%)≦(Ta+Tb)/(Sa+Sb)×100 
     According to the results shown in  FIGS. 8 to 12 , in order to secure sufficient peeling resistance, it is preferable that, at the cross section of the ground electrode  400  as viewed from the direction of the arrows F 3 -F 3 , the base material surface  411  is exposed, and the following conditions are satisfied: 
     1.2E≦F≦1.9E; 
     0.05 mm≦Da≦0.30 mm; 
     0.05 mm≦Db≦0.30 mm; and 
     0.20 mm 2 ≦J≦0.70 mm 2    
     and it is more preferable that the following conditions are satisfied: 
     1.2E≦F≦1.8E; 
     0.05 mm≦Da≦0.25 mm; 
     0.05 mm≦Db≦0.25 mm; and 
     0.20 mm 2 ≦J≦0.68 mm 2 . 
       FIG. 14  is a table showing the result of evaluation of ignitability of the spark plug  10 . In an ignitability evaluation test, the tester evaluated, as samples, a plurality of spark plugs  10  including electrode tips  450  having different lengths E and different heights K. In each sample, at the cross section of the ground electrode  400  as viewed from the direction of the arrows F 3 -F 3 , the base material surface  411  is exposed, and the following conditions are satisfied: 
     1.2E≦F≦1.8E; 
     0.05 mm≦Da≦0.25 mm; 
     0.05 mm≦Db≦0.25 mm; and 
     0.20 mm≦J≦0.68 mm 2 . 
     The tester mounted each sample on an internal combustion engine (engine displacement of 1.5 liters, 4 cylinders), confirmed a lean limit of each sample, and compared ignitability of each sample with that of a sample having a height K of 0.8 mm (K=0.8 mm) to evaluate each sample based on the following evaluation standard. 
     Good (indicated by a circle): reduction in ignitability from that of the sample (K=0.8 mm) is less than 2% 
     Poor (indicated by a cross): reduction in ignitability from that of the sample (K=0.8 mm) is 2% or more 
       FIG. 15  is a table showing the result of evaluation of wear resistance of the electrode tip  450 . In a wear resistance evaluation test, the tester evaluated, as samples, a plurality of spark plugs  10  including electrode tips  450  having different lengths E and different heights K. In each sample, at the cross section of the ground electrode  400  as viewed from the direction of the arrows F 3 -F 3 , the base material surface  411  is exposed, and the following conditions are satisfied: 
     1.2E≦F≦1.8E; 
     0.05 mm≦Da≦0.25 mm; 
     0.05 mm≦Db≦0.25 mm; and 
     0.20 mm 2 ≦J≦0.68 mm 2 . 
     The tester mounted each sample on an internal combustion engine (engine displacement of 1.5 liters, 4 cylinders), and operated the internal combustion engine at 5000 rpm with a full-open throttle. Thereafter, the tester confirmed the wearing rate of the electrode tip  450  in each sample, and compared the wearing rate of each sample with that of an electrode tip  450  of a sample having a height K of 0.8 mm (K=0.8 mm), thereby to evaluate each sample based on the following evaluation standard. 
     Excellent (indicated by a double circle): the wearing rate of the electrode tip  450  is less than that of the sample (K=0.8 mm) 
     Good (indicated by a circle): increase in the wearing rate of the electrode tip  450  from that of the sample (K=0.8 mm) is less than 5% 
     Poor (indicated by a cross): increase in the wearing rate of the electrode tip  450  from that of the sample (K=0.8 mm) is 5% or more 
     According to the results of the evaluation tests shown in  FIG. 14  and  FIG. 15 , in order to secure sufficient ignitability and sufficient wear resistance of the ground electrode, it is preferable that the height K of the electrode tip  450  satisfies the condition of 0.3 mm≦K≦1.2 mm. 
     A-4. Effects 
     According to the above-described embodiments, at the cross section of the ground electrode  400  as viewed from the direction of the arrows F 3 -F 3 , the base material surface  411  is exposed, and the conditions of 1.2E≦F≦1.8E, 0.05 mm≦Da≦0.25 mm, 0.05 mm≦Db≦0.25 mm, and 0.20 mm 2 ≦J≦0.68 mm 2  are satisfied, whereby it is possible to secure sufficient peeling resistance of the ground electrode  400  against peeling of the electrode tip  450 . Further, the relationship between the area A of the center electrode  100  and the area B of the ground electrode  400  satisfies the condition of 1.3A≦B≦4.6A, whereby it is possible to secure sufficient wear resistance of the ground electrode  400  against spark discharge and oxidation. 
     Further, the height K of the electrode tip  450  satisfies the condition of 0.3 mm≦K≦1.2 mm, whereby it is possible to secure sufficient wear resistance of the ground electrode  400  while securing sufficient ignitability of the spark plug  10 . Since the electrode tip  450  contains platinum (Pt) and rhodium (Rh), it is possible to realize the electrode tip  450  having sufficient wear resistance. 
     B. Other Embodiments 
     The present invention is not limited to the above-described embodiments, examples, and modifications, and can be realized in various forms without departing from the scope of the invention. For example, the technical features in the embodiments, examples, and modifications which correspond to the technical features in the respective modes described in the “Summary of the Invention” section may be appropriately replaced or combined in order to solve a portion or the entity of the above-described problems or to attain a portion or the entity of the above-described effects. Also, a technical feature(s) may be appropriately omitted if it is not described as an essential feature in the present specification. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
         
           
               10  . . . spark plug 
               90  . . . internal combustion engine 
               100  . . . center electrode 
               101  . . . front end surface 
               107  . . . side surface 
               110  . . . electrode base material 
               150  . . . electrode tip 
               190  . . . metal terminal 
               200  . . . insulator 
               290  . . . axial hole 
               300  . . . metal shell 
               310  . . . end surface 
               351  . . . front end surface 
               400 ,  400 A,  400 B,  400 C,  400 D,  400 E . . . ground electrode 
               410 ,  410 E . . . electrode base material 
               411 ,  412 ,  413 ,  414 ,  415 ,  416 ,  417 E,  418 E . . . base material surface 
               419   a ,  419   b  . . . corner portion 
               430  . . . fusion zone 
               430   a  . . . first portion 
               430   b  . . . second portion 
               431  . . . exposed surface 
               433  . . . interface 
               450  . . . electrode tip 
               451  . . . tip surface 
               453  . . . tip surface 
               910  . . . inner wall 
               920  . . . combustion chamber