Patent Publication Number: US-10777974-B2

Title: Spark plug for internal combustion engine that makes re-discharge less prone to occur

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation application of International Application No. PCT/JP2018/009193, filed Mar. 9, 2018, which claims priority to Japanese Patent Application No. 2017-044932, filed on Mar. 9, 2017. The entire contents of each of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a spark plug for internal combustion engine. 
     Background Art 
     A spark plug in which each of a center electrode and a ground electrode has a base material and a noble metal chip bonded to the base material and a spark discharge gap is formed between the noble metal chip of the center electrode and the noble metal chip of the ground electrode is known. 
     SUMMARY 
     A first aspect of the present disclosure is a spark plug for an internal combustion engine including: a cylindrical housing; a cylindrical insulating glass; a center electrode; and a ground electrode that forms a spark discharge gap between the center electrode and the ground electrode. The ground electrode has a ground protrusion part that forms the spark discharge gap between the center electrode and the ground electrode. An angle between a ground discharge surface and a side surface of the ground protrusion part is a right angle or an acute angle. At least a portion of the side surface of the ground protrusion part and at least a portion of a side surface of the ground base material are flush with each other. 
     A second aspect of the present disclosure is a spark plug for an internal combustion engine including: a cylindrical housing; cylindrical insulating glass; a center electrode; and a ground electrode that forms a spark discharge gap between the center electrode and the ground electrode. The center electrode has a center base material and a center protrusion part. An angle between a center discharge surface of the center protrusion part and a side surface of the center protrusion part is a right angle or an acute angle. At least one of angles between the plurality of side surfaces of the center protrusion part is a center specific angle that is located at an end portion of the center protrusion part opposite to the connection part side of the orthogonal direction. Surfaces forming the center specific angle among the side surfaces of the center protrusion part are flush with a side surface of a center base material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features, and advantages of the present disclosure will be more clarified by the following detailed descriptions with reference to the accompanying drawings. The drawings are as follows: 
         FIG. 1  is a cross-sectional view of a spark plug in a first embodiment; 
         FIG. 2  is a diagram of a tip end portion and its vicinity, in the spark plug in the first embodiment as seen from a vertical direction; 
         FIG. 3  is a diagram of the tip end portion and its vicinity, in the spark plug in the first embodiment as seen from a lateral direction; 
         FIG. 4  is an arrow cross-sectional view of  FIG. 3  taken along line IV-IV; 
         FIG. 5  is an arrow cross-sectional view of  FIG. 4  taken along line V-V; 
         FIG. 6  is a diagram omitting a ground protrusion part from  FIG. 4 ; 
         FIG. 7  is an enlarged front view of the tip end portion and its vicinity, in the spark plug in an ignition device in the first embodiment, illustrating an initial discharge spark; 
         FIG. 8  is an enlarged front view of the tip end portion and its vicinity, in the spark plug in the ignition device in the first embodiment, illustrating a state in which a portion between both starting points of the initial discharge spark is greatly stretched by a gas flow in a combustion chamber; 
         FIG. 9  is an enlarged front view of the tip end portion and its vicinity, in the spark plug in the ignition device in the first embodiment, illustrating a discharge spark immediately before a short-circuit and a discharge spark immediately after the short-circuit; 
         FIG. 10  is a partial cross-sectional view of a ground electrode as seen from a center electrode side in a comparative embodiment; 
         FIG. 11  is an enlarged front view of a tip end portion and its vicinity, in a spark plug in an ignition device in the comparative embodiment, illustrating an initial discharge spark; 
         FIG. 12  is an enlarged front view of the tip end portion and its vicinity, in the spark plug in the ignition device in the comparative embodiment, illustrating a state in which a portion between both starting points of the initial discharge spark is greatly stretched by a gas flow in a combustion chamber; 
         FIG. 13  is an enlarged front view of the tip end portion and its vicinity, in the spark plug in the ignition device in the comparative embodiment, illustrating a discharge spark immediately before a short-circuit and a discharge spark immediately after the short-circuit; 
         FIG. 14  is a diagram illustrating a relationship between a protrusion length L 1  and a ground-side starting point movement ratio in a first experimental example; 
         FIG. 15  is a diagram illustrating a relationship between the rate of ground-side starting point movement and the rate of combustion fluctuation in the first experimental example; 
         FIG. 16  is a diagram of a tip end portion and its vicinity, in spark plug in a second embodiment as seen from a lateral direction; 
         FIG. 17  is a diagram of the tip end portion and its vicinity, in the spark plug in the second embodiment as seen from a vertical direction; 
         FIG. 18  is an arrow cross-sectional view of  FIG. 16  taken along line XVIII-XVIII; 
         FIG. 19  is a diagram of a tip end portion and its vicinity, in a spark plug in a third embodiment as seen from a lateral direction; 
         FIG. 20  is a diagram of the tip end portion and its vicinity, in the spark plug in the third embodiment as seen from a vertical direction; 
         FIG. 21  is an arrow cross-sectional view of  FIG. 19  taken along line XXI-XXI; 
         FIG. 22  is an arrow cross-sectional view of  FIG. 21  taken along line XXII-XXII; 
         FIG. 23  is a diagram of a tip end portion and its vicinity, in a spark plug in a fourth embodiment as seen from a vertical direction; 
         FIG. 24  is a diagram of a tip end portion and its vicinity, in a spark plug in a fifth embodiment as seen from a lateral direction; 
         FIG. 25  is a diagram of the tip end portion and its vicinity, in the spark plug in the fifth embodiment as seen from a vertical direction; 
         FIG. 26  is a diagram of a tip end portion of a center electrode in the fifth embodiment as seen from a ground electrode side in a gap direction; 
         FIG. 27  is an arrow cross-sectional view of  FIG. 26  taken along line XXVII-XXVII; 
         FIG. 28  is an arrow cross-sectional view of  FIG. 26  taken along line XXVIII-XXVIII; 
         FIG. 29  is a diagram of a tip end portion and its vicinity, in a spark plug in a modified embodiment of the fifth embodiment as seen from a vertical direction; 
         FIG. 30  is a diagram illustrating a relationship between a protrusion length L 2  and a center starting point movement ratio in a second experimental example; 
         FIG. 31  is a diagram illustrating the rate of center starting point movement and the rate of combustion fluctuation in the second experimental example; 
         FIG. 32  is a diagram of a tip end portion and its vicinity, in a spark plug in a sixth embodiment as seen from a lateral direction; 
         FIG. 33  is a diagram of the tip end portion and its vicinity, in the spark plug in the sixth embodiment as seen from a vertical direction; 
         FIG. 34  is a diagram of a tip end portion of a center electrode in the sixth embodiment as seen from a ground electrode side in a gap direction; 
         FIG. 35  is a diagram of a tip end portion and its vicinity, in a spark plug in a seventh embodiment as seen from a lateral direction; 
         FIG. 36  is a diagram of the tip end portion and its vicinity, in the spark plug in the seventh embodiment as seen from a vertical direction; 
         FIG. 37  is a diagram of a tip end portion of a center electrode in the seventh embodiment as seen from a ground electrode side in a gap direction; 
         FIG. 38  is an arrow cross-sectional view of  FIG. 37  taken along line XXXVIII-XXXVIII; 
         FIG. 39  is an arrow cross-sectional view of  FIG. 37  taken along line XXXIX-XXXIX; 
         FIG. 40  is a diagram of a tip end portion and its vicinity, in a spark plug in an eighth embodiment as seen from a lateral direction; 
         FIG. 41  is a diagram of the tip end portion and its vicinity, in the spark plug in the eighth embodiment as seen from a vertical direction; 
         FIG. 42  is a diagram of a tip end portion and its vicinity, in a spark plug as seen from a lateral direction in a modification embodiment in which the second embodiment and the fifth embodiment are combined; and 
         FIG. 43  is a diagram of a tip end portion and its vicinity, in a spark plug as seen from a lateral direction in a modification embodiment in which the third embodiment and the fifth embodiment are combined. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A spark plug is used as an ignition means in an internal combustion engine such as an automobile engine. The spark plug is structured such that a center electrode and a ground electrode are axially opposed to each other and a spark discharge gap is formed between these electrodes. A pulse voltage is applied between the center electrode and the ground electrode to cause spark discharge in the spark discharge gap. 
     The inventor of the present disclosure has studied spark plugs for internal combustion engines that help improve ignitability by making re-discharge less prone to occur. 
     In the conventional spark plug described above, the discharge spark generated between the center electrode and the ground electrode is blown off by the air-fuel mixture, and re-discharge is prone to occur such that spark discharge is caused again between the center electrode and the ground electrode. This will be described below. 
     In the conventional spark plug described above, the starting point of the discharge spark generated between the center electrode and the ground electrode may move from the noble metal chip of the center electrode or the noble metal chip of the ground electrode to the base material to which the noble metal chip is bonded. When the starting point of the discharge spark moves from the noble metal chip to the base material, the axial distance between the both starting points of the discharge spark becomes longer in the axial direction. When the axial distance between the both starting points of the discharge spark becomes too long, the portion of the discharge spark between the both end portions tends to be stretched in such a manner as to excessively bulge toward the downstream side of the air-fuel mixture, whereby the discharge spark may be likely to be blown off. When the discharge spark is blown off, re-discharge takes place between the center electrode and the ground electrode in the axial direction. As above, re-discharge is likely to occur in the conventional spark plug described above. 
     With an increase in the frequency of occurrence of re-discharge, the position of the spark discharge may greatly fluctuate and cause variations in overheated portions of the air-fuel mixture, for example, thereby deteriorating ignitability and increase wear of the center electrode and the ground electrode. 
     The present disclosure is intended to provide a spark plug for internal combustion engine that helps improve ignitability by making re-discharge less prone to occur. 
     A first aspect of the present disclosure is a spark plug for an internal combustion engine including: a cylindrical housing; a cylindrical insulating glass held in the housing; a center electrode that is held in the insulating glass such that a tip end portion thereof protrudes; and a ground electrode that has a connection part connected to the housing and forms a spark discharge gap between the center electrode and the ground electrode. The ground electrode has a ground base material that includes the connection part and a ground protrusion part that protrudes from the ground base material toward the center electrode and forms the spark discharge gap between the center electrode and the ground electrode. An angle between a ground discharge surface of the ground protrusion part facing the spark discharge gap and a side surface of the ground protrusion part is a right angle or an acute angle. At least a portion of the side surface of the ground protrusion part and at least a portion of a side surface of the ground base material are flush with each other. 
     A second aspect of the present disclosure is a spark plug for an internal combustion engine including: a cylindrical housing; cylindrical insulating glass held in the housing; a center electrode that is held in the insulating glass such that a tip end portion thereof protrudes; and a ground electrode that has a connection part connected to the housing and forms a spark discharge gap between the center electrode and the ground electrode. The center electrode has a center base material and a center protrusion part that protrudes from the center base material toward the ground electrode and forms the spark discharge gap between the ground electrode and the center electrode. An angle between a center discharge surface of the center protrusion part opposed to the spark discharge gap and a side surface of the center protrusion part is a right angle or an acute angle. The center protrusion part has a plurality of the side surfaces. When, out of planar directions parallel to both an axial direction and a lateral direction orthogonal to the axial direction and in which the connection part of the ground electrode and the center electrode are aligned, a direction orthogonal to a gap direction in which the center electrode, the spark discharge gap, and the ground electrode are aligned is defined as an orthogonal direction, at least one of angles between the plurality of side surfaces of the center protrusion part is a center specific angle that is located at an end portion of the center protrusion part opposite to the connection part side of the orthogonal direction. Surfaces forming the center specific angle among the side surfaces of the center protrusion part are flush with a side surface of the center base material. 
     In the spark plug for internal combustion engine in the first aspect, the angle between the ground discharge surface of the ground protrusion part facing the spark discharge gap and the side surface of the ground protrusion part is a right angle or an acute angle. This makes it easy to ensure the intensity of an electric field around the angle between the ground discharge surface and the side surface of the ground protrusion part. Accordingly, it is easy to keep a ground electrode-side starting point of a discharge spark at the angle between the ground discharge surface and the side surface of the ground protrusion part, thereby suppressing movement of the ground electrode-side starting point of the discharge spark to the ground base material. This suppresses the blow-off and re-discharge of the discharge spark. 
     In addition, at least a portion of the side surface of the ground protrusion part and at least a portion of the side surface of the ground base material are flush with each other. Therefore, it is possible to suppress concentration of an electric field around the portion of the ground base material where the ground protrusion part is disposed. Accordingly, it is possible to suppress movement of the ground electrode-side starting point of the discharge spark from the ground protrusion part to the ground base material. This also suppresses blow-off and re-discharge of the discharge spark. 
     In addition, in the spark plug for internal combustion engine in the second aspect, the angle between the center discharge surface and the side surface of the center protrusion part is a right angle or an acute angle. This makes it easy to keep a center electrode-side starting point of a discharge spark at the angle between the center discharge surface and the side surface of the center protrusion part, thereby suppressing blow-off and re-discharge of the discharge spark. 
     At least one of the angles between the plurality of side surfaces of the center protrusion part is the center specific angle that is located at the end portion of the center protrusion part opposite to the connection part side of the orthogonal direction. That is, the center protrusion part has the angle around which an electric field tends to concentrate, formed at the end portion of the center protrusion part opposite to the connection part side of the orthogonal direction. Therefore, it is easy to keep the center electrode-side starting point of a discharge spark at the end portion of the center protrusion part opposite to the connection part side of the orthogonal direction. This makes it possible to suppress a phenomenon that the heat of a flame generated by ignition of the discharge spark to the air-fuel mixture is lost to the ground electrode (flame-out effect) caused by the discharge spark approaching the portion of the ground electrode on the connection part side of the orthogonal direction of the center protrusion part. 
     As described above, according to the foregoing aspects, it is possible to provide a spark plug for internal combustion engines that helps improve ignitability by making re-discharge less prone to occur. 
     First Embodiment 
     A first embodiment of a spark plug for internal combustion engine will be described with reference to  FIGS. 1 to 9 . 
     As illustrated in  FIG. 1 , the spark plug  1  for internal combustion engines in the present embodiment has a cylindrical housing  11 , cylindrical insulating glass  12  held in the housing  11 , a center electrode  2 , and a ground electrode  3 . The center electrode  2  is held in the insulating glass  12  such that its tip end portion protrudes. The ground electrode  3  has a connection part  331  connected to the housing  11 . The ground electrode  3  forms a spark discharge gap  13  between the center electrode  2  and the ground electrode  3 . 
     As illustrated in  FIG. 2 , the ground electrode  3  has a ground base material  31  that includes the connection part  331  and a ground protrusion part  32  that protrudes from the ground base material  31  toward the center electrode  2  and forms the spark discharge gap  13  between the center electrode  2  and the ground electrode  3 . In a gap direction G in which the center electrode  2 , the spark discharge gap  13 , and the ground electrode  3  are aligned, a protrusion length L 1  of the ground protrusion part  32  from the ground base material  31  is 0.5 mm or more. 
     As illustrated in  FIGS. 2 and 5 , angles between a ground discharge surface  321  of the ground protrusion part  32  facing the spark discharge gap  13  and side surfaces  322 ,  323 , and  324  of the ground protrusion part  32  are right angles or acute angles. In the present embodiment, all the angles between the ground discharge surface  321  and the side surfaces  322 ,  323 , and  324  of the ground protrusion part  32  are right angles. At least part of the side surfaces  322 ,  323 , and  324  of the ground protrusion part  32  and at least part of the side surface of the ground base material  31  are flush with each other. In other words, at least part of the side surfaces  322 ,  323 , and  324  of the ground protrusion part  32  and at least part of the side surface of the ground base material  31  are formed to be smoothly continuous. 
     The spark plug  1  can be used as an ignition means in an internal combustion engine of an automobile, a cogeneration system, or the like, for example. One axial end of the spark plug  1  is connected to an ignition coil which is not illustrated and the other axial end of the spark plug  1  is disposed in the combustion chamber of the internal combustion engine. 
     The simple term “axial direction” here refers to a direction in which the center axis of the spark plug  1  extends unless otherwise specified. 
     A direction orthogonal to the axial direction and in which the connection part  331  of the ground electrode  3  and the center electrode  2  are aligned will be called lateral direction X. A center electrode  2  side of the lateral direction X with respect to the connection part  331  will be called the X 1  side, and the opposite side will be called the X 2  side. A direction orthogonal to both the axial direction and the lateral direction X will be called the vertical direction Y. 
     A ground electrode  3  side of the gap direction G with respect to the center electrode  2  will be called the G 1  side, and the opposite side will be called the G 2  side. As described later, in the present embodiment, the gap direction G is the axial direction. 
     As illustrated in  FIG. 1 , the housing  11  has an attaching screw part  111  for attaching the spark plug  1  to an engine head  101  (see  FIG. 7 ). The insulating glass  12  is held in the housing  11  such that its tip end portion protrudes toward the G 1  side of the housing  11  and its distal end portion protrudes toward the G 2  side of the housing  11 . The center electrode  2  is held at the internal tip end portion of the insulating glass  12 . 
     The center electrode  2  has a center axis approximately aligned with the center axis of the spark plug  1 . The center electrode  2  has an approximately columnar shape as a whole. As illustrated in  FIGS. 2 and 3 , the center electrode  2  has a center base material  21  and a center protrusion part  22  protruding from the center base material  21  toward the ground electrode  3  and forming the spark discharge gap  13  between the ground electrode  3  and the center electrode  2 . In the present embodiment, the center base material  21  and the center protrusion part  22  are separate members. A base material tip end portion  210  as a tip end portion of the center base material  21  is in the shape of a truncated cone that reduces in diameter toward the G 1  side. The center protrusion part  22  is bonded to the tip end surface of the base material tip end portion  210 . The center protrusion part  22  has a columnar shape. The G 1 -side surface of the center protrusion part  22  is a center discharge surface  221  facing the spark discharge gap  13 . 
     The ground base material  31  has an erected part  33  and an inward-facing part  34 . The erected part  33  stands erect from the tip end surface of the housing  11  toward the G 1  side in the gap direction G. As illustrated in  FIG. 2 , the erected part  33  has the connection part  331  at the G 2 -side end portion and is connected at the connection part  331  to the tip end surface of the housing  11 . The erected part  33  has thickness in the lateral direction X. 
     The inward-facing part  34  is extended from the G 1 -side end portion of the erected part  33  toward the X 1  side of the lateral direction X. In the present embodiment, the inward-facing part  34  partially overlaps the center discharge surface  221  of the center protrusion part  22  in the gap direction G. The inward-facing part  34  has thickness in the gap direction G. In  FIG. 4 , the position of the outer shape of the center discharge surface  221  in the planar direction orthogonal to the gap direction G is shown by a dashed line. 
     As illustrated in  FIG. 2 , the ground base material  31  has a ground base material end portion  341  at the end portion opposite to the connection part  331  in the longitudinal direction. As illustrated in  FIG. 6 , the ground base material end portion  341  has the shape of a triangular prism that becomes narrower toward the X 1  side. The ground base material end portion  341  has a triangular cross section orthogonal to the gap direction G. In the present embodiment, the ground base material end portion  341  at least partially overlaps the center discharge surface  221  of the center protrusion part  22  in the gap direction G. 
     The inward-facing part  34  has a tapered portion  342  adjacent to the X 2  side of the ground base material end portion  341 . As seen from the gap direction G, the tapered portion  342  has a trapezoidal shape that becomes narrower toward the X 1  side. 
     As illustrated in  FIG. 6 , each of a side surface  342   s  of the tapered portion  342  and side surfaces  341   a  and  341   b  of the ground base material end portion  341  forms a flat plane inclined with respect to planes parallel to both the gap direction G and the lateral direction X. The side surfaces  341   a  and  341   b  of the ground base material end portion  341  have a larger inclination angle than the side surface  342   s  of the tapered portion  342  with respect to planes parallel to both the gap direction G and the lateral direction X. 
     The ground electrode  3  can be formed, for example, by bending an elongated metal plate material in the thickness direction and then forming the side surface  342   s  of the tapered portion  342  and the side surfaces  341   a  and  341   b  of the ground base material end portion  341  by cutting work. The metal member described above has both width-wise end surfaces swelling outward in the width direction but it is not limited to this structure. 
     As illustrated in  FIGS. 2 and 3 , the ground protrusion part  32  protrudes from the G 2 -side surface of the ground base material end portion  341  toward the G 2  side. In the present embodiment, the ground base material  31  and the ground protrusion part  32  are separate members. The ground protrusion part  32  has the shape of a triangular prism. The ground protrusion part  32  has a triangular cross section orthogonal to the gap direction G. Specifically, the cross section of the ground protrusion part  32  orthogonal to the gap direction G has a triangular shape that becomes narrower toward the X 1  side. 
     The ground protrusion part  32  has the plurality of side surfaces  322 ,  323 , and  324 . In the present embodiment, the ground protrusion part  32  has the three side surfaces  322 ,  323 , and  324 . Each of the three side surfaces  322 ,  323 , and  324  has a right angle with the ground discharge surface  321 . 
     The three side surfaces  322 ,  323 , and  324  include the side surface  322  which is parallel to the gap direction G and the vertical direction Y, and the pair of side surfaces  323  and  324  extending toward the X 1  side from opposite sides of the side surface  322  in the vertical direction Y. The pair of side surfaces  323  and  324  is formed to approach each other toward the X 1  side from the side surface  322  and is inclined with respect to a plane parallel to both the gap direction G and the lateral direction X. The pair of side surfaces  323  and  324  is in contact with each other by the sides opposite to the side surface  322 . 
     Among planar directions parallel to both the lateral direction X and the axial direction, the direction orthogonal to the gap direction G will be defined as an orthogonal direction. As illustrated in  FIGS. 2 to 4 , at least one of the angles between the plurality of side surfaces  322 ,  323 , and  324  of the ground protrusion part  32  is a ground specific angle  32   a  of the ground protrusion part  32  located at the end opposite to the contact portion  331  side of the orthogonal direction. In the present embodiment, since the orthogonal direction is the lateral direction X, the orthogonal direction will be called lateral direction X. In the present embodiment, the ground specific angle  32   a  is an angle between the pair of side surfaces  323  and  324 . 
     As illustrated in  FIGS. 2 to 5 , each of the surfaces forming the ground specific angle  32   a  among the side surfaces  322 ,  323 , and  324  of the ground protrusion part  32  (that is, the pair of side surfaces  322  and  323 ) is flush with the side surface of the ground base material  31 . Out of the pair of side surfaces  323  and  324 , the one entire side surface  323  is flush in a planar form with the one entire side surface  341   a  of the ground base material end portion  341 . The other entire side surface  324  is flush in a planar form with the other entire side surface  341   b  of the ground base material end portion  341 . 
     In the present embodiment, the ground base material end portion  341  entirely overlaps the ground protrusion part  32  in the gap direction G. The cross-sectional shape of the ground base material end portion  341  orthogonal to the gap direction G is the same as the cross-sectional shape of the ground protrusion part  32  orthogonal to the gap direction G. The side surfaces  341   a  and  341   b  of the ground base material end portion  341  are flush with the side surfaces  323  and  324  of the ground protrusion part  32 . The entire side surfaces  341   a  and  341   b  of the ground base material end portion  341  are flush with the side surfaces  323  and  324  of the ground protrusion part  32 . 
     As illustrated in  FIGS. 2 and 3 , the ground specific angle  32   a  is formed in the gap direction G. The angle between the pair of side surfaces  341   a  and  341   b  of the ground base material end portion  341  is also formed in the gap direction G. The angle between the pair of side surfaces  341   a  and  341   b  of the ground base material end portion  341  is smoothly connected to the ground specific angle  32   a  in a linear fashion. 
     As illustrated in  FIG. 2 , the protrusion length L 1  of the ground protrusion part  32  from the inward-facing part  34  is 0.5 mm or more in the gap direction G. That is, the length L 1  of the portion of the ground protrusion part  32  exposed from the inward-facing part  34  in the gap direction G is 0.5 mm or more in the gap direction G. The protrusion length L 1  of the ground protrusion part  32  is preferably 1.0 mm or less from the viewpoint of prevention of pre-ignition. Specifically, when the protrusion length L 1  becomes larger than 1.0 mm, the position of the ground base material  31  is located closer to the G 1  side, that is, closer to the center of the combustion chamber at a relatively high temperature. As a result, when the protrusion length L 1  exceeds 1.0 mm, the ground electrode  3  may become high in temperature and cause pre-ignition. 
     The ground discharge surface  321  of the ground protrusion part  32  is orthogonal to the gap direction G. The ground discharge surface  321  is opposed to the center discharge surface  221  of the center electrode  2  in the gap direction G. The spark discharge gap  13  is formed between the ground discharge surface  321  and the center discharge surface  221  in the gap direction G. 
     The center base material  21  can be a columnar body made of a metal material such as a Ni based alloy, and have therein a metal material excellent in thermal conductivity such as Cu. The center protrusion part  22  can be made of a noble metal such as Ir or Pt, for example. The ground base material  31  can be made of an Ni base alloy having Ni as the main ingredient, for example. The ground protrusion part  32  can be made of a noble metal such as Ir or Pt, for example. 
     As illustrated in  FIG. 1 , inside the insulating glass  12 , a resistor  15  is disposed on the G 2  side of the center electrode  2  with an electrically-conductive glass seal  14  therebetween. The resistor  15  can be formed by heating and sealing a resistor composite including a resistor material such as carbon or ceramic powder and glass powder, or by inserting a cartridge-type resistor body. The glass seal  14  is made of copper glass in which copper powder is mixed into glass. On the G 2  side of the resistor  15 , a stem  16  is disposed with a glass seal  17  made of copper glass between the resistor  15  and the stem  16 . The stem  16  is made of an iron alloy, for example. The spark plug  1  has the stem  16  connected to an ignition coil. 
     Next, an ignition device  10  in which the spark plug  1  of the present embodiment is attached to an internal combustion engine as illustrated in  FIG. 7  will be described. 
     The spark plug  1  is disposed in a posture in which the vertical direction Y is the direction of a gas flow F of air-fuel mixture passing through the spark discharge gap  13 . Hereinafter, the simple term “downstream side” will refer to the downstream side of the gas flow F of air-fuel mixture flowing in the spark discharge gap  13 , and the simple term “upstream side” will refer to the upstream side of the gas flow F of air-fuel mixture flowing in the spark discharge gap  13 . 
     Next, an example of a state in which a discharge spark S between the center electrode  2  and the ground electrode  3  is stretched by the gas flow F will be described with reference to  FIGS. 7 to 9 . 
     As illustrated in  FIG. 7 , applying a predetermined voltage between the center electrode  2  and the ground electrode  3  generates the discharge spark S in the spark discharge gap  13 . The initial discharge spark S is likely to occur on the center discharge surface  221  of the center electrode  2  and at the G 2 -side end portion of the ground specific angle  32   a  of the ground protrusion part  32  as starting points, for example. This is because, in the center electrode  2  and the ground electrode  3 , the distance between the center discharge surface  221  and the G 2 -side end portion of the ground specific angle  32   a  tends to become relatively short and the intensity of an electric field around the G 2 -side end portion of the ground specific angle  32   a  tends to become relatively high. Hereinafter, the starting point on the center electrode  2  side of the discharge spark S will be called “center electrode-side starting point S 2 ”, and the starting point on the ground electrode  3  side of the discharge spark S will be called “ground electrode-side starting point S 1 ”. 
     As illustrated in  FIG. 8 , when the discharge spark S is blown by the gas flow F, the discharge spark S is extended such that the portion between both starting points swells to the downstream side while at least the position of the ground electrode-side starting point S 1  is kept at the G 2 -side end portion of the ground specific angle  32   a . As the portion between the both starting points of the discharge spark S is extended downstream, the curvature of a folded portion St as a most downstream portion of the discharge spark S becomes larger. Accordingly, as the portion between both starting points of the discharge spark S is extended downstream, portions Sa adjacent to both sides of the folded portion St of the discharge spark S approach to each other in the gap direction G, and finally cause a short-circuit as illustrated in  FIG. 9 . This short-circuit makes the discharge spark S slightly shorter in the vertical direction Y. After that, extension of the portion between both starting points of the discharge spark S and short-circuiting are repeated. 
       FIG. 9  shows the discharge spark immediately before the short-circuit by a dashed line, and the discharge spark S immediately after the short-circuit is shown by a solid line. In addition,  FIG. 9  shows a length from the position of the downstream-side end portion of the discharge spark S immediately before the short-circuit of the discharge spark S to the position of the downstream-side end portion of the discharge spark S immediately after the short-circuit of the discharge spark S in the vertical direction Y with the symbol Δy 1 . 
     Next, the actions and effects of the present embodiment will be described. 
     In the spark plug  1  for internal combustion engine, the protrusion length L 1  of the ground protrusion part  32  from the ground base material  31  is 0.5 mm or more in the gap direction G. This makes it possible to suppress the movement of the ground electrode-side starting point S 1  of the discharge spark S generated in the spark discharge gap  13  from the ground protrusion part  32  to the ground base material  31 . This suppresses an increase in the distance between the starting points of the discharge spark in the gap direction G. Accordingly, the portion between both starting points of the discharge spark S is folded at a steep angle at the downstream-side end portion and is extended in a sharp shape toward the downstream side as a whole. Thus, when the portion between the both starting points of the discharge spark S is extended to the downstream side, that portion tends to be partially short-circuited with another portion. This makes blow-off and re-discharge of the discharge spark S less prone to occur. This numerical value will be supported by experimental examples described later. 
     The angles between the ground discharge surface  321  of the ground protrusion part  32  opposed to the spark discharge gap  13  and the side surfaces  322 ,  323 , and  324  of the ground protrusion part  32  are right angles. This makes it easy to ensure the intensity of an electric field around the angles between the ground discharge surface  321  and the side surfaces  322 ,  323 , and  324  of the ground protrusion part  32 . Accordingly, it is possible to easily keep the ground electrode-side starting point S 1  of the discharge spark S at the angles between the ground discharge surface  321  and the side surfaces  322 ,  323 , and  324  of the ground protrusion part  32 , and suppress the movement of the ground electrode-side starting point S 1  of the discharge spark S to the ground base material  31 . This also suppresses the blow-off and re-discharge of the discharge spark S. 
     At least some portions of the side surfaces  322 ,  323 , and  324  of the ground protrusion part  32  and at least a portion of the side surface of the ground base material  31  are flush with each other. This suppresses concentration of an electric field around the portion of the ground base material  31  where the ground protrusion part  32  is disposed. This suppresses movement of the ground electrode-side starting point S 1  of the discharge spark S from the ground protrusion part  32  to the ground base material  31 . This also suppresses the blow-off and re-discharge of the discharge spark S. 
     At least one of the angles between the plurality of side surfaces  322 ,  323 , and  324  of the ground protrusion part  32  is the ground specific angle  32   a  that is located at the end portion of the ground protrusion part opposite to the connection part  331  side of the lateral direction X. That is, the ground protrusion part  32  has the angle around which an electric field tends to concentrate, formed at the end portion of the ground protrusion part  32  opposite to the connection part  331  side of the lateral direction X. This suppresses movement of the ground electrode-side starting point S 1  of the discharge spark S from the ground protrusion part  32  to the portion of the ground base material  31  on the X 2  side of the lateral direction X of the ground protrusion part  32 . This also suppresses a flame-out effect in which the heat of a flame generated by ignition of the discharge spark S to the air-fuel mixture is lost to the ground electrode  3  caused by the discharge spark S approaching the portion of the ground electrode  3  on the X 2  side of the lateral direction X of the ground protrusion part  32 . Each of the side surfaces  323  and  324  forming the ground specific angle  32   a  among the side surfaces  322 ,  323 , and  324  of the ground protrusion part  32  is flush with the side surface of the ground base material  31 . Therefore, the ground electrode-side starting point S 1  of the discharge spark S can be more easily kept at the G 2 -side end portion of the ground specific angle  32   a  formed on the X 1 -side end portion of the lateral direction X. 
     The side surfaces  341   a  and  341   b  of the ground base material end portion  341  are flush with the side surfaces  323  and  324  of the ground protrusion part  32 . This makes it possible to prevent the formation of a portion around which an electric field tends to concentrate between the side surfaces  341   a  and  341   b  of the ground base material end portion  341  and the side surfaces  323  and  324  of the ground protrusion part  32 . Therefore, the ground electrode-side starting point S 1  of the discharge spark S is further easy to keep on the ground discharge surface  321 . 
     The ground protrusion part  32  has a triangular cross section orthogonal to the gap direction G. This makes it easy to form an angle in the ground protrusion part  32  around which an electric field tends to concentrate. This makes it easy to suppress movement of the ground electrode-side starting point S 1  of the discharge spark S from the ground protrusion part  32  to the ground base material  31 . 
     As described above, according to the present embodiment, it is possible to provide a spark plug for internal combustion engine that is less prone to cause re-discharge. 
     Comparative Embodiment 
     The present comparative embodiment is an embodiment in which the first embodiment is modified in the configuration of the ground electrode as illustrated in  FIGS. 10 to 13 . Specifically, the inward-facing part and the ground protrusion part in the first embodiment are modified as illustrated in  FIGS. 10 and 11 . As illustrated in  FIG. 10 , in the present comparative embodiment, an inward-facing part  934  is uniformly formed such that the width in the vertical direction Y becomes constant in the lateral direction X. The inward-facing part  934  has an inside surface  934   a  oriented toward the G 2  side, an outside surface  934   b  oriented toward the G 1  side (see  FIG. 11 ), a pair of first side surfaces  934   c  connecting the inside surface  934   a  and the outside surface  934   b  at both ends in the vertical direction Y, and a second side surface  934   d  connecting the inside surface  934   a  and the outside surface  934   b  on an X 1 -side end portion. The first side surface  934   c  is oriented toward the vertical direction Y, and the second side surface  934   d  is oriented toward the X 1  side of the lateral direction X. 
     A columnar ground chip  932  is disposed on the inside surface  934   a  of the inward-facing part  934 . As illustrated in  FIG. 11 , the ground chip  932  is opposed to the center discharge surface  221  of the center protrusion part  22  in the gap direction G. In the present comparative embodiment, the side surface of the ground chip  932  is not flush with the side surface of a ground base material  931 . As seen from the gap direction G, the outer shape of the ground chip  932  is within the pair of the first side surface  934   c  and the second side surface  934   d  of the inward-facing part  934 . As seen in the gap direction G, the outer shape of the ground chip  932  does not overlap either of the pair of the first side surface  934   c  and the second side surface  934   d . As seen from the gap direction G, the angle between the inside surface  934   a  and the first side surface  934   c  of the inward-facing part  934 , the angle between the inside surface  934   a  and the second side surface  934   d , and the angle between the first side surface  934   c  and the second side surface  934   d  are located on the X 1  side of the ground chip  932 . In the other respects, the present comparative embodiment is the same in basic structure as the first embodiment. 
     Next, an example of a spark plug  9  in the present comparative embodiment in which the discharge spark S is extended by the gas flow F in the combustion chamber will be described with reference to  FIGS. 11 to 13 . 
     As illustrated in  FIG. 11 , the initial discharge spark S is generated between the center discharge surface  221  and the G 2 -side surface of the ground chip  932 . The discharge spark S is blown by the gas flow F and has the portion between the both starting points significantly swelling to the downstream side. As illustrated in  FIGS. 11 and 12 , while the portion between the both starting points of the discharge spark S swells to the downstream side, the ground electrode-side starting point S 1  of the discharge spark S is blown and moved by the gas flow F. 
     First, the ground electrode-side starting point S 1  of the discharge spark S moves from the ground chip  932  to the G 2 -side end portion at the angle between the first side surface  934   c  and the second side surface  934   d  around which an electric field tends to concentrate. 
     Then, the ground electrode-side starting point S 1  of the discharge spark S is further blown by the gas flow F to move to the G 1  side over the angle between the first side surface  934   c  and the second side surface  934   d , and reaches the G 1 -side end portion at the angle between the first side surface  934   c  and the second side surface  934   d  as illustrated in  FIG. 12 . 
     In this manner, the portion between the both starting points of the spark plug S significantly swells to the downstream side with an increase in the distance between the both starting points of the discharge spark S in the gap direction G. Accordingly, as illustrated in  FIG. 12 , even when the portion between the both starting points of the discharge spark S is extended downstream, the curvature of the folded portion St as the most downstream portion of the discharge spark S is less prone to increase. Since portions Sa adjacent to the folded portion St of the discharge spark S are unlikely to approach to each other and cause a short-circuit, the discharge spark S is excessively extended downstream until being blown off. 
     Then, as illustrated in  FIG. 13 , the discharge spark S excessively stretched to the downstream side is finally blown off, and re-discharge occurs between the center discharge surface  221  of the center electrode  2  and the G 2 -side end surface of the ground chip  932 . After that, the stretch of the portion between the both starting points of the discharge spark S, the blow-off, and the re-discharge are repeated. 
       FIG. 13  shows the discharge spark immediately before blow-off by a dashed line and shows the discharge spark S immediately after re-discharge by a solid line.  FIG. 13  also shows a length, in the vertical direction Y, from the position of the downstream-side end portion of the discharge spark S immediately before blow-off to the position of the downstream-side end portion of the discharge spark S immediately after re-discharge with the symbol Δy 2 . 
     In the present comparative embodiment, blow-off and re-discharge of the discharge spark are likely to occur. Therefore, as illustrated in  FIG. 13 , the length Δy 2  in the vertical direction Y from the position of the downstream-side end portion of the discharge spark immediately before the blow-off to the position of the downstream-side end portion of the discharge spark S immediately after the re-discharge tends to be relatively long. That is, in the present comparative embodiment, the position of the downstream-side end portion of the discharge spark S tends to change. Thus, the movement of heat from the discharge spark S to the air-fuel mixture in the combustion chamber does not take place efficiently. This makes it difficult to improve the ignitability of the air-fuel mixture. 
     On the other hand, in the spark plug  1  of the first embodiment, the blow-off and re-discharge are unlikely to occur. As illustrated in  FIG. 9 , the length Δy 1  in the vertical direction Y from the position of the downstream-side end portion of the discharge spark S immediately before the short-circuit of the discharge spark S to the position of the downstream-side end portion of the discharge spark S immediately after the short-circuit of the discharge spark S is unlikely to become long. Therefore, the movement of heat from the discharge spark S to the air-fuel mixture in the combustion chamber efficiently takes place to improve the ignitability in an easy manner. 
     In addition, in the present comparative embodiment, the re-discharge of the discharge spark S tends to occur and wear-out the center electrode and the ground electrode tends to increase. On the other hand, in the spark plug  1  of the first embodiment, the re-discharge is unlikely to occur so that wear of the center electrode  2  and the ground electrode  3  can be suppressed. 
     First Experimental Example 
     The present example is an example of a spark plug similar in basic structure to the first embodiment in which the relationship between the protrusion length L 1  and the rate of ground-side starting point movement described later is evaluated as illustrated in  FIG. 14 . The ground-side starting point movement rate is the rate of movement of the ground electrode-side starting point of the discharge spark from the ground protrusion part  32  to the ground base material  31 , which was obtained by observing the discharge caused 20 times between the center electrode and the ground electrode. 
     In the present example, four samples were prepared, which were similar in basic structure to the spark plug  1  in the first embodiment and had protrusion lengths L 1  of 0 mm, 0.25 mm, 0.5 mm, and 0.75 mm. 
     In the present example, each of the samples was installed in a test device simulating a combustion chamber. Each of the samples was installed in the test device, in a posture in which a flow of air-fuel mixture to pass through the spark discharge gap  13  of the sample is oriented in the vertical direction Y. Then, an air-fuel mixture was supplied at a flow rate of 20 m/s toward the spark discharge gap  13  of each of the samples under a pressure of 0.5 MPa in the device. Discharge was caused 20 times in each of the samples for a discharge time of 1.5 ms to measure the ground-side starting point movement rate.  FIG. 14  shows the results. 
     As can be seen from  FIG. 14 , when the protrusion length L 1  is 0.5 mm or more, the ground-side starting point movement rate becomes as small a value as approximately 0%. On the other hand, when the protrusion length L 1  is 0.25 mm or less, the ground-side starting point movement rate rises sharply as compared to the case in which the protrusion length L 1  is 0.5 mm or more. That is, from the viewpoint of reducing the ground-side starting point movement rate, the protrusion length L 1  of the ground protrusion part  32  from the ground base material  31  in the gap direction G is preferably 0.5 mm or more. 
     Next, as illustrated in  FIG. 15 , the relationship between the rate of ground-side starting point movement and the rate of combustion fluctuation was examined. The rate of combustion fluctuation is represented by indicated mean effective pressure (IMEP) (standard deviation/average)×100. As the ignitability of the spark plug is higher, the value of the combustion fluctuation rate is lower. 
     In the present example, various samples different in the rate of ground-side starting point movement were prepared. Each of the samples was installed in a 2.5-L four-cylinder supercharged engine. Then, the rate of combustion fluctuation was measured under the conditions that the engine speed was 1200 rpm and the brake mean effective pressure (BMEP) was 0.5 MPa.  FIG. 15  shows the results. 
     As can be seen from  FIG. 15 , the lower the rate of ground-side starting point movement, the lower the rate of combustion fluctuation becomes. That is, the lower the rate of ground-side starting point movement, the better the ignitability becomes. 
     As above, it can be seen from  FIG. 15  that, the lower the rate of ground-side starting point movement, the more the ignitability becomes improved, and it can be seen from  FIG. 14  that, from the viewpoint of reducing the rate of ground-side starting point movement, the protrusion length L 1  of the ground protrusion part  32  from the ground base material  31  in the gap direction G is preferably 0.5 mm or more. That is, from the viewpoint of improving the ignitability, the protrusion length L 1  of the ground protrusion part  32  from the ground base material  31  in the gap direction G is preferably 0.5 mm or more. 
     Second Embodiment 
     The present embodiment is an embodiment obtained by modifying the shape of the ground electrode  3  in the first embodiment, as illustrated in  FIGS. 16 to 18 . First, in the present embodiment, the cross section of the ground base material end portion  341  orthogonal to the gap direction G has a square shape. Side surfaces  341   c  and  341   d  of the ground base material end portion  341  on both sides in the vertical direction Y are orthogonal to the vertical direction Y, and a side surface  341   e  of the ground base material end portion  341  on the X 1  side is orthogonal to the lateral direction X. 
     The ground protrusion part  32  has a square prism shape. That is, the cross section of the ground protrusion part  32  orthogonal to the gap direction G has a square shape. As illustrated in  FIG. 17 , the cross section of the ground protrusion part  32  orthogonal to the gap direction G is longer than the ground base material end portion  341  in the lateral direction X. 
     As illustrated in  FIG. 16 , the side surfaces  325   a  and  325   b  of the ground protrusion part  32  on both sides are flush with the side surfaces  341   c  and  341   d  of the ground base material end portion  341  on both sides in the vertical direction Y. Specifically, a portion of the one side surface  325   a  of the ground protrusion part  32  in the vertical direction Y is flush in a planar form with the one entire side surface  341   c  of the ground base material end portion  341  in the vertical direction Y. In addition, a portion of the other side surface  325   b  of the ground protrusion part  32  is flush in a planar form with the other entire side surface  341   d  of the ground base material end portion  341  in the vertical direction Y. 
     The ground protrusion part  32  is more protruded toward the X 1  side of the lateral direction X than the inward-facing part  34  is. That is, the side surface  326  of the ground protrusion part  32  on the X 1  side is located further to the X 1  side than the side surface  341   e  of the ground base material end portion  341  is on the X 1  side.  FIG. 18  shows the side surface  341   e  of the ground base material end portion  341  on the X 1  side by a dashed line. 
     In the present embodiment, at least one of the angles between the plurality of side surfaces  325   a ,  325   b , and  326  of the ground protrusion part  32  is the ground specific angle  32   a  located at the end portion of the ground protrusion part  32  opposite to the connection part  331  side of the lateral direction X. In the present embodiment, there are two ground specific angles  32   a  between the side surface  326  and the pair of side surfaces  325   a  and  325   b  of the ground protrusion part  32 . That is, the present embodiment has the two ground specific angles  32   a . The ground protrusion part  32  has the angles between the side surfaces  325   a  and  325   b , which are on opposite sides in the vertical direction Y, and the side surface  326  on the X 1  side, closer to the X 1  side than to the side surface  341   e  of the ground base material end portion  341  on the X 1  side. 
     In other respects, the second embodiment is similar to the first embodiment 
     Out of the reference signs used in the second and subsequent embodiments, the ones identical to the reference signs used in the embodiment already described represent constituent elements similar to those of the embodiment already described, unless otherwise specified. 
     In the present embodiment, the cross section of the ground protrusion part  32  orthogonal to the gap direction G has a square shape. Therefore, it is easy to provide the ground protrusion part  32  with angles around which an electric field tends to concentrate. This makes it easy to suppress the movement of the ground electrode-side starting point of the discharge spark from the ground protrusion part  32  to the ground base material  31 . 
     The ground protrusion part  32  protrudes more to the X 1  side than the X 1 -side end surface of the inward-facing part  34  does. The ground protrusion part  32  has the angles between the side surfaces  325  on the both sides in the vertical direction Y and the side surface  326  on the X 1  side more protruding to the X 1  side than the X 1 -side end portion of the inward-facing part  34 . Therefore, it is easy to further suppress the movement of the ground electrode-side starting point S 1  of the discharge spark S from the angles of the ground protrusion part  32  between the side surfaces  325  of the ground protrusion part  32  on both sides in the vertical direction Y and the side surface  326  on the X 1  side to the ground base material  31 . 
     In other respects, the second embodiment exhibits actions and effects similar to those of the first embodiment. 
     Third Embodiment 
     The present embodiment is an embodiment obtained by modifying the shape of the ground electrode  3  of the first embodiment, as illustrated in  FIGS. 19 to 22 . In the present embodiment, the angle between the ground discharge surface  321  and at least one of side surfaces  328  and  329  of the ground protrusion part  32  is an acute angle. 
     As illustrated in  FIG. 21 , the cross section of the ground protrusion part  32  orthogonal to the gap direction G has a triangular shape as in the first embodiment, and the ground protrusion part  32  has three side surfaces  327 ,  328 , and  329 . The three side surfaces  327 ,  328 , and  329  include the side surface  327  parallel to the gap direction G and the vertical direction Y and the pair of side surfaces  328  and  329  extended from the opposite sides of the side surface  327  in the vertical direction Y, towards the X 1  side. The pair of side surfaces  328  and  329  come closer to each other toward the X 1  side from the side surface  327 , and are inclined with respect to a plane parallel to both the gap direction G and the lateral direction X. As illustrated in  FIG. 22 , in the present embodiment, the pair of side surfaces  328  and  329  of the ground protrusion part  32  are inclined to approach each other toward the G 1  side. That is, the angles between the ground discharge surface  321  and the side surfaces  328  and  329  of the ground protrusion part  32  are acute angles. In addition, the angle between the ground discharge surface  321  and the side surface  327  of the ground protrusion part  32  is a right angle. 
     As in the first embodiment, the cross section of the ground base material end portion  341  orthogonal to the gap direction G has a triangular shape. A pair of side surfaces  341   f  and  341   g  of the ground base material end portion  341  are inclined to approach each other toward the G 1  side.  FIG. 21  shows G 1 -side end edges of the pair of side surfaces  341   f  and  341   g  of the ground base material end portion  341  by a dashed line. In other words,  FIG. 21  shows the outer shape of the G 1 -side surface of the ground base material end portion  341  by a dashed line. 
     The pair of side surfaces  328  and  329  of the ground protrusion part  32  are flush with the side surfaces of  341   f  and  341   g  of the ground base material end portion  341 . That is, the entire side surface  328  of the ground protrusion part  32  is flush in a planar form with the entire side surface  341   f  of the ground base material end portion  341 , and the other entire side surface  329  is flush in a planar form with the other entire side surface  341   g  of the ground base material end portion  341 . In addition, the side surface  328  of the ground protrusion part  32  and the side surface  341   f  of the ground base material end portion  341  adjacent to each other, and the side surface  329  of the ground protrusion part  32  and the side surface  341   g  of the ground base material end portion  341  adjacent to each other, are flush with each other in a planar form in such a manner as to approach to the inside in the vertical direction Y toward the G 1  side. 
     As illustrated in  FIGS. 19 to 21 , in the present embodiment, the angle between the pair of side surfaces  328  and  329  of the ground protrusion part  32  is the ground specific angle  32   a  located at the end portion of the ground protrusion part  32  opposite to the connection part  331  side of the lateral direction X. As illustrated in  FIG. 20 , the ground specific angle  32   a  is inclined to approach the X 2  side toward the G 1  side. In addition, the angle between the side surfaces  341   f  and  341   g  of the ground base material end portion  341  is also inclined toward the X 2  side toward the G 1  side. The ground specific angle  32   a  and the angle between the side surfaces  341   f  and  341   g  of the ground base material end portion  341  are smoothly connected in a linear fashion. 
     In other respects, the third embodiment is similar to the first embodiment. 
     In the present embodiment, the angle between the ground discharge surface  321  and at least one of the side surfaces  328  and  329  of the ground protrusion part  32  is an acute angle. This makes it easy to concentrate an electric field around the angle between the ground discharge surface  321  and at least one of the side surfaces  328  and  329  of the ground protrusion part  32 . This makes it easy to keep the ground electrode-side starting point of the discharge spark at the angle between the ground discharge surface  321  and at least one of the side surfaces  328  and  329  of the ground protrusion part  32 . 
     In other respects, the third embodiment exhibits actions and effects similar to those of the first embodiment. 
     Fourth Embodiment 
     In the present embodiment, as illustrated in  FIG. 23 , an end edge of the ground electrode  3  opposite to the connection part  331  side in the lateral direction X is located closer to the connection part  331  side of the lateral direction X than an axis ax of the center electrode  2  is. That is, an X 1 -side end portion of the ground protrusion part  32  and an X 1 -side end portion of the inward-facing part  34  are located on an X 2  side of the axis ax in the lateral direction X.  FIG. 23  shows the axis ax of the center electrode  2  from the vertical direction Y by a dot-and-dash line. 
     In other respects, the fourth embodiment is similar to the first embodiment. 
     In the present embodiment, it is easy to generate the ground electrode-side starting point of the discharge spark at the X 1 -side end portion of the ground discharge surface  321 . This makes it easy to suppress the movement of the ground electrode-side starting point of the discharge spark from the ground discharge surface  321  of the ground protrusion part  32  closer to the X 2  side than the ground discharge surface  321  of the inward-facing part  34 . 
     In other respects, the fourth embodiment exhibits actions and effects similar to those of the first embodiment. 
     Fifth Embodiment 
     The present embodiment is similar in basic structure to the first embodiment and is further devised in the shape of the center electrode  2  as illustrated in  FIGS. 24 to 28 . 
     As illustrated in  FIGS. 24, 25, 27, and 28 , a base material tip end portion  210  of the center base material  21  has a base material diameter-reduced portion  211  that is more reduced in diameter toward the G 1  side and a base material extension portion  212  that is extended to the gap direction G from the base material diameter-reduced portion  211  to the G 1  side. The base material extension portion  212  has a square prism shape. That is, the cross section of the base material extension portion  212  orthogonal to the gap direction G has a square shape. In addition, the cross section of the base material diameter-reduced portion  211  orthogonal to the gap direction G has a square shape. Four side surfaces  211   a  of the base material diameter-reduced portion  211  are flush with four side surfaces  212   a  of the base material extension portion  212 . 
     The center protrusion part  22  has a square prism shape. That is, the cross section of the center protrusion part  22  orthogonal to the gap direction G has a square shape. The cross section of the center protrusion part  22  orthogonal to the gap direction G is the same in shape as the cross section of the base material extension portion  212  orthogonal to the gap direction G. 
     As illustrated in  FIG. 26 , the center protrusion part  22  has four side surfaces  222 . As illustrated in  FIG. 28 , the angles between the center discharge surface  221  of the center protrusion part  22  and the side surface  222  of the center protrusion part  22  are right angles. In the present embodiment, the angles between the center discharge surface  221  of the center protrusion part  22  and the four side surfaces  222  of the center protrusion part  22  are right angles. As illustrated in  FIG. 26 , the center protrusion part  22  has four angles formed between the adjacent side surfaces  222 . The four angles between the side surfaces of the center protrusion part  22  are oriented in the vertical direction Y or the lateral direction X. 
     At least one of the angles between the plurality of side surfaces  222  of the center protrusion part  22  is a center specific angle  22   a  located at the end portion of the center protrusion part  22  opposite to the connection part  331  side of the lateral direction X. In the present embodiment, among the angles between the side surfaces  222  of the center protrusion part  22 , the angle oriented to the X 1  side of the lateral direction X is the center specific angle  22   a . The surfaces forming the center specific angle  22   a  among the side surfaces  222  of the center protrusion part  22  are flush with the side surfaces of the center base material  21 . In the present embodiment, the side surfaces of the base material tip end portion  210  are flush with the side surfaces  222  of the center protrusion part  22 . That is, the entire side surfaces of the base material tip end portion  210  are flush with the side surfaces  222  of the center protrusion part  22 . Specifically, all the four side surfaces  222  of the center protrusion part  22  are flush in a planar form with the four side surfaces  212   a  of the base material extension portion  212  of the base material tip end portion  210  of the center base material  21 . As illustrated in  FIG. 27 , the four angles formed between the adjacent side surfaces  222  of the center protrusion part  22  are smoothly connected in a linear fashion to the four angles formed between the adjacent side surfaces  212   a  of the base material extension portion  212  of the center base material  21 . 
     As illustrated in  FIG. 24 , in the gap direction G, the protrusion length L 2  of the center protrusion part  22  from the center base material  21  is 0.5 mm or more. That is, the length L 2  from the tip end surface of the base material tip end portion  210  in the gap direction G to the center discharge surface  221  of the center protrusion part  22  is 0.5 mm or more. The protrusion length L 2  of the center protrusion part  22  from the center base material  21  in the gap direction G is preferably set to 1.0 mm or less from the viewpoint of preventing pre-ignition. Specifically, when the protrusion length L 2  becomes larger than 1.0 mm, the ground electrode  3  is formed closer to the G 1  side, that is, closer to the center of the combustion chamber at a relatively high temperature. As a result, with the protrusion length L 2  of more than 1.0 mm, the ground electrode  3  may come under high temperature conditions, thereby resulting in pre-ignition. Further, when the protrusion length L 2  becomes longer than 1.0 mm, the temperature of the ground electrode  3  become higher and the center protrusion part  22  of the center electrode  2  in the vicinity of the ground electrode  3  may be excessively heated. At a high temperature, the center protrusion part  22  forms an oxide film on its surface to protect the center protrusion part  22 . However, with an excessive rise in temperature, the center protrusion part  22  may not form an oxide film on the center protrusion part  22 . Therefore, the protrusion length L 2  is preferably set to 1.0 mm or less from the viewpoint of ensuring oxidation-resistance of the center protrusion part  22 . 
     In the present embodiment, the shape of the ground electrode  3  is similar to that in the comparative embodiment. That is, the inward-facing part  34  of the ground electrode  3  is uniformly formed such that its width in the vertical direction is constant in the lateral direction X. In addition, the columnar ground protrusion part  32  protrudes from the G 2 -side surface of the inward-facing part  34  to the G 2  side. As seen in the gap direction G, the outer shape of the ground protrusion part  32  is within the outer shape of the inward-facing part  34 . 
     In other respects, the fifth embodiment is similar to the first embodiment. 
     In the present embodiment, the protrusion length of the center protrusion part  22  from the center base material  21  is 0.5 mm or more in the gap direction G. This suppresses an increase in the distance between both starting points of the discharge spark in the gap direction G, and suppresses occurrence of blow-off and re-discharge of the discharge spark S. This numerical value will be supported by experimental examples described later. 
     The angles between the center discharge surface  221  and the side surfaces  222  of the center protrusion part  22  are right angles or acute angles. This makes it easy to keep the center electrode-side starting point of the discharge spark at the angles between the center discharge surface  221  and the side surfaces  222  of the center protrusion part  22 , thereby suppressing the blow-off and re-discharge of the discharge spark. 
     At least one of the angles between the plurality of side surfaces  222  of the center protrusion part  22  is the center specific angle  22   a  that is located at the end portion of the center protrusion part  22  opposite to the connection part  331  side of the lateral direction X. This makes it possible to form a portion of the center protrusion part  22  around which an electric field tends to concentrate, at the end portion on the X 1  side of the lateral direction X. This makes it easy to keep the center electrode-side starting point of the discharge spark at the end portion on the X 1  side of the lateral direction X among the angles between the center discharge surface  221  and the side surfaces  222  of the center protrusion part  22 . The surfaces forming the center specific angle  22   a  among the side surfaces  222  of the center protrusion part  22  are flush with the side surfaces of the center base material  21 . This suppresses the concentration of an electric field around the center specific angle  22   a  and the portion of the center base material  21  in the vicinity of the surfaces forming the center specific angle  22   a . This suppresses the movement of the center electrode-side starting point of the discharge spark from the center protrusion part  22  to the center base material  21 . This also suppresses the blow-off and re-discharge of the discharge spark. 
     The side surfaces of the base material tip end portion  210  are flush with the side surfaces  222  of the center protrusion part  22 . This makes it possible to prevent the formation of portions around which an electric field tends to concentrate between the side surfaces of the base material tip end portion  210  and the side surfaces  222  of the center protrusion part  22 . Therefore, the center electrode-side starting point of the discharge spark is even easier to keep on the center discharge surface  221 . 
     The center protrusion part  22  has a square cross section orthogonal to the gap direction G. This makes it easy to provide the center protrusion part  22  with angles around which an electric field tends to concentrate. This makes it easy to suppress movement of the center electrode-side starting point of the discharge spark from the center protrusion part  22  to the center base material  21 . 
     As described above, according to the present embodiment, it is possible to provide a spark plug for internal combustion engine that is less prone to cause re-discharge. 
     As illustrated in  FIG. 29 , a spark plug identical in basic structure to the present embodiment may be configured such that the end edge of the ground electrode  3  opposite to the connection part  331  side of the lateral direction X is located closer to the connection part  331  side of the lateral direction X than the axis ax of the center electrode  2  as in the fourth embodiment. This makes it easy to suppress the ground electrode-side starting point of the discharge spark from the ground discharge surface  321  of the ground protrusion part  32  from moving further towards the X 2  side than the ground discharge surface  321  of the inward-facing part  34 . 
     Second Experimental Example 
     The present example is an example of a spark plug similar in basic structure to the fifth embodiment in which the relationship between the protrusion length L 2  and the rate of center starting point movement is evaluated as illustrated in  FIG. 30 . The rate of center starting point movement is the rate of movement of the center electrode-side starting point of the discharge spark from the center protrusion part  22  to the center base material  21 , which was obtained by observing discharge that was caused 20 times between the center electrode and the ground electrode. 
     In the present example, four samples were prepared, which were similar in basic structure to the spark plug  1  in the fifth embodiment and had protrusion lengths L 2  of 0 mm, 0.25 mm, 0.5 mm, and 0.75 mm. 
     Each of the samples was measured to determine the rate of center starting point movement.  FIG. 30  shows the results. The test conditions are the same as those in the first experimental example. 
     As seen from  FIG. 30 , when the protrusion length L 2  is 0.5 mm or more, the rate of center starting point movement becomes as small a value as approximately 0%. On the other hand, when the protrusion length L 2  is 0.25 mm or less, the rate of center starting point movement rises sharply as compared to the case in which the protrusion length L 2  is 0.5 mm or more. That is, from the viewpoint of reducing the of ground-side starting point movement, the protrusion length L 2  of the center protrusion part  22  from the center base material  21  in the gap direction G is preferably 0.5 mm or more. 
     Next, as illustrated in  FIG. 31 , the relationship between the rate of center starting point movement and the rate of combustion fluctuation was examined. The test conditions are the same as those in the first experimental example. 
     As can be seen from  FIG. 31 , the lower the rate of center starting point movement, the lower the rate of combustion change rate becomes. That is, the lower the rate of center starting point movement, the better the ignitability becomes. 
     As above, it can be seen from  FIG. 31  that, as the lower the rate of center starting point movement, the more the ignitability becomes improved, and it can be seen from  FIG. 30  that, from the viewpoint of reducing the rate of center starting point movement, the protrusion length L 2  of the center protrusion part  22  from the center base material  21  is preferably 0.5 mm or more in the gap direction G. That is, from the viewpoint of improving the ignitability, the protrusion length L 2  of the center protrusion part  22  from the center base material  21  is preferably 0.5 mm or more in the gap direction G. 
     Sixth Embodiment 
     The present embodiment is an embodiment obtained by modifying the shape of the center electrode  2  in the fifth embodiment as illustrated in  FIGS. 32 to 34 . In the present embodiment, the center protrusion part  22  has the shape of a triangular column. As illustrated in  FIG. 34 , the cross section of the center protrusion part  22  orthogonal to the gap direction G has a triangular shape. Specifically, the cross section of the center protrusion part  22  orthogonal to the gap direction G has a triangular shape that is narrower toward the X 1  side. 
     The center protrusion part  22  has three side surfaces  223 . The center protrusion part  22  has three angles formed between the adjacent side surfaces  223 . One of the three angles is the center specific angle  22   a  located at the X 1 -side end portion of the center protrusion part  22 . The center specific angle  22   a  is oriented to the X 1  side of the lateral direction X. 
     The cross section of the base material extension portion  212  of the base material tip end portion  210  of the center base material  21  orthogonal to the gap direction G has a triangular shape. The cross section of the base material extension portion  212  orthogonal to the gap direction G is the same in shape as the cross section of the center protrusion part  22  orthogonal to the gap direction G. 
     In other respects, the sixth embodiment is similar to the fifth embodiment. 
     In the present embodiment, the center protrusion part  22  has a triangular cross section orthogonal to the gap direction G. This makes it easy to provide the ground protrusion part  32  with angles around which an electric field tends to concentrate. This makes it easy to suppress the movement of the center electrode-side starting point of the discharge spark from the center protrusion part  22  to the center base material  21 . 
     In other respects, the sixth embodiment exhibits actions and effects similar to those of the fifth embodiment. 
     Seventh Embodiment 
     The present embodiment is an embodiment obtained by modifying the shape of the center electrode  2  in the fifth embodiment as illustrated in FIGS.  35  to  39 . As illustrated in  FIGS. 38 and 39 , in the present embodiment, the center protrusion part  22  reduces in diameter toward the G 2  side. The angle between the center discharge surface  221  and at least one side surface  224  of the center protrusion part  22  is an acute angle. 
     In the present embodiment, the cross section of the center protrusion part  22  orthogonal to the gap direction G has a square shape. The four side surfaces  224  of the center protrusion part  22  are inclined towards the inner peripheral side of the center protrusion part  22  toward the G 2  side. That is, in the present embodiment, the angles between the center discharge surface  221  and all the side surfaces  224  of the center protrusion part  22  are acute angles. 
     As illustrated in  FIGS. 35 and 36 , the base material extension portion  212  of the base material tip end portion  210  of the center base material  21  reduces in diameter toward the G 2  side. As illustrated in  FIGS. 38 and 39 , four side surfaces  212   b  of the base material extension portion  212  are inclined toward the inner peripheral side of the base material extension portion  212  toward the G 2  side. In the present embodiment, the four side surfaces  212   b  of the base material extension portion  212  are flush with the four side surfaces  224  of the center protrusion part  22 . The four side surfaces  212   b  of the base material extension portion  212  of the base material tip end portion  210  are flush with side surfaces  211   b  of the base material diameter-reduced portion  211  of the base material tip end portion  210 . 
     In other respects, the seventh embodiment is similar to the fifth embodiment. 
     In the present embodiment, the angles between the center discharge surface  221  and the side surfaces  224  of the center protrusion part  22  are acute angles. This makes it easy to ensure the intensity of an electric field around the angles between the center discharge surface  221  and the side surfaces  224  of the center protrusion part  22 . This makes it easy to keep the center electrode-side starting point of the discharge spark at the angles between the center discharge surface  221  and the side surfaces  224  of the center protrusion part  22 . 
     In other respects, the seventh embodiment exhibits actions and effects similar to those of the fifth embodiment. 
     Eighth Embodiment 
     The present embodiment is similar in the shape of the ground electrode  3  to the first embodiment, and is similar in the shape of the center electrode  2  to the fifth embodiment, as illustrated in  FIGS. 40 and 41 . In other respects, the eighth embodiment is similar in basic structure to the first embodiment. 
     In the present embodiment, it is possible to obtain the actions and effects of the first embodiment and the actions and effects of the fifth embodiment. In the present embodiment, it is further possible to concentrate an electric field around both the angles between the ground discharge surface  321  and the side surfaces  322 ,  323 , and  324  of the ground protrusion part  32  and the angles between the center discharge surface  221  and the side surfaces  222  of the center protrusion part  22 . Accordingly, both starting points of the discharge spark can be further easily kept at the angles between the ground discharge surface  321  and the side surfaces  322  of the ground protrusion part  32  and at the angles between the center discharge surface  221  and the side surfaces  222  of the center protrusion part  22 . 
     The present disclosure has been described so far according to the embodiments, but it is noted that the present disclosure is not limited to the foregoing embodiments or structures. The present disclosure includes various modifications and changes in a range of equivalency. In addition, various combinations and modes, and other combinations and modes including only one element of the foregoing combinations and modes, less or more than the one element are included in the scope and conceptual range of the present disclosure. 
     For example, as illustrated in  FIG. 42 , it is possible to employ a mode in which the ground electrode  3  is similar in shape to that in the second embodiment and the center electrode  2  is similar in shape to that in the fifth embodiment. 
     As illustrated in  FIG. 43 , it is possible to employ a mode in which the ground electrode is similar in shape to that in the third embodiment and the center electrode is similar in shape to that in the fifth embodiment. 
     In the first embodiment and others, the gap direction G is the axial direction, and thus the orthogonal direction described above (that is, the direction orthogonal to the gap direction G among the planar directions parallel to both the lateral direction X and the axial direction) is the lateral direction X. However, when the gap direction G is inclined with respect to the axial direction, the orthogonal direction is a direction inclined with respect to the lateral direction X.