Patent Publication Number: US-2023143447-A1

Title: Spark plug

Description:
FIELD OF INVENTION 
     The present invention relates to a spark plug having a spark gap between a center electrode and a ground electrode. 
     BACKGROUND OF INVENTION 
     Japanese Patent Application Laid-Open (kokai) No. 2019-46660 (FIG. 5) discloses a technique for a spark plug which includes a center electrode, a metallic shell holding the center electrode in an insulated condition, and a ground electrode connected to the metallic shell. According to the technique, a first end portion of a circular columnar ground electrode is held in a penetration hole provided in the metallic shell, a side surface of a second end portion of the ground electrode faces a forward end surface of the center electrode, and the gap between the side surface of the second end portion and the forward end surface of the center electrode is used as a spark gap. 
     However, the above-described technique has the following problem. Since the side surface of the ground electrode which faces the forward end surface of the center electrode via the spark gap is a cylindrical surface, the side surface of the ground electrode may be consumed easily due to discharge, and the spark gap may expand at an early stage of usage. A conceivable measure for solving the problem is, for example, to form a quadrangular penetration hole in the metallic shell and press-fit a ground electrode having the shape of a quadrangular prism into the quadrangular penetration hole. When such a structure is employed, the above-mentioned problem can be solved, because the side surface of the ground electrode facing the forward end surface of the center electrode can be made flat. However, in reality, it is extremely difficult to machine in particular the penetration hole in such a manner that corners of the penetration hole coincide with the shape of the ground electrode. 
     SUMMARY OF INVENTION 
     The present invention has been accomplished so as to solve the above-mentioned problem, and an object of the present invention is to provide a spark plug which can reduce consumption of the ground electrode while facilitating machining of the penetration hole. 
     In order to achieve the object, a spark plug of the present invention comprises a center electrode extending in a direction of an axial line; a tubular metallic shell which holds the center electrode in an insulated condition and which has a penetration hole penetrating the metallic shell in a thickness direction; and a ground electrode which extends in a direction intersecting the direction of the axial line (hereinafter referred to as the axial direction) and which has a first end portion held in the penetration hole, and a second end portion located on a forward end side of the center electrode in the axial direction such that a spark gap is provided between the second end portion and a forward end surface of the center electrode. The penetration hole includes a circular counterbore portion formed on an outer circumferential side of the metallic shell, and a penetrating portion extending from the counterbore portion to an inner circumferential surface of the metallic shell. The ground electrode includes a circular plate-shaped fixing portion which is fixed to the counterbore portion, and an extension portion extending from one surface of the fixing portion to a position which faces the forward end surface of the center electrode in the axial direction. A flat surface which faces the forward end surface of the center electrode in the axial direction is provided on a side surface of the extension portion. The penetrating portion restricts the extension portion such that the flat surface of the extension portion faces toward a rear end side in the axial direction. 
     According to a first mode, the penetration hole penetrating the metallic shell in the thickness direction includes the circular counterbore portion provided on the outer circumferential side of the metallic shell, and the penetrating portion extending from the counterbore portion to the inner circumferential surface of the metallic shell. The circular plate-shaped fixing portion of the ground electrode is fixed to the counterbore portion, and the extension portion extending from the fixing portion faces the forward end surface of the center electrode in the direction of the axial line. Since the counterbore portion to which the fixing portion of the ground electrode is fixed is circular, machining of the penetration hole can be facilitated. The penetrating portion restricts the extension portion such that the flat surface provided on the side of the extension portion faces toward the rear end side in the axial direction, and the spark gap is provided between the flat surface of the extension portion and the forward end surface of the center electrode. Therefore, consumption of the ground electrode due to discharge can be reduced as compared with the case where the side surface of the ground electrode is a cylindrical surface. Therefore, it is possible to prevent expansion of the spark gap at an early stage of usage. 
     According to a second mode, the penetrating portion restricts the orientation of the extension portion such that the angle between the axial line and a plane perpendicular to the flat surface of the extension portion becomes smaller than 90 degrees. By virtue of this configuration, an effect similar to that of the first mode can be yielded. 
     According to a third mode, the penetrating portion restricts the orientation of the extension portion such that the angle between the axial line and the plane perpendicular to the flat surface of the extension portion becomes equal to or smaller than 45 degrees. Since a discharge point (position where discharge occurs) becomes likely to be located on the flat surface of the extension portion, in addition to the effect of the first mode, an effect of reliably enhancing the spark consumption resistance of the ground electrode can be yielded. 
     According to a fourth mode, the penetrating portion restricts the orientation of the extension portion such that the angle between the axial line and the plane perpendicular to the flat surface of the extension portion becomes equal to or smaller than 5 degrees. Since the discharge point becomes more likely to be located on the flat surface of the extension portion, in addition to the effect of the first mode, an effect of more reliably enhancing the spark consumption resistance of the ground electrode can be yielded. 
     According to a fifth mode, the penetrating portion includes a flat surface provided on the rear end side. Since the ground electrode can be disposed in such a manner that the flat surface of the extension portion faces the flat surface of the penetrating portion, in addition to the effect of any one of the first through fourth modes, an effect of simplifying the shape of the extension portion can be yielded. 
     According to a sixth mode, the penetration hole has a recess which is larger in diameter than the counterbore portion and is located on the outer circumferential side of the metallic shell in relation to the counterbore portion. Therefore, in addition to the effect of any one of the first through fifth modes, the following effect can be yielded. Even when the length of the fixing portion of the ground electrode is greater than the depth of the counterbore portion, since the recess is present, the fixing portion is unlikely to project outward from the metallic shell. 
     According to a seventh mode, the metallic shell is a tubular body having a closed bottom on a forward end side in the axial direction. A jetting hole which differs from the penetration hole and penetrates the metallic shell in the thickness direction is provided in the metallic shell. Although the extension portion of the ground electrode located on the inner side of the metallic shell having the shape of a bottomed tube is disposed in an environment in which the extension portion is easily heated and is easily consumed, application of the present invention yields an effect of reducing the consumption of the extension portion of the ground electrode, in addition to the effect of any one of the first through sixth modes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a partial sectional view of a spark plug in a first embodiment. 
         FIG.  2    is a sectional view of the spark plug showing, on an enlarged scale, a portion of  FIG.  1    indicated by II. 
         FIG.  3   , Section (a) is a sectional view of the spark plug taken along line IIIa-IIIa of  FIG.  2   ,  FIG.  3   , Section (b) is a sectional view of the spark plug taken along line IIIb-IIIb of  FIG.  2   , and  FIG.  3   , Section (c) is a sectional view of the spark plug taken along line IIIc-IIIc of  FIG.  2   . 
         FIG.  4    is a sectional view of a spark plug in a second embodiment. 
         FIG.  5   , Section (a) is a sectional view of the spark plug taken along line Va-Va of  FIG.  4   ,  FIG.  5   , Section (b) is a sectional view of the spark plug taken along line Vb-Vb of  FIG.  4   , and  FIG.  5   , Section (c) is a sectional view of the spark plug taken along line Vc-Vc of  FIG.  4   . 
         FIG.  6    is a sectional view of a spark plug in a third embodiment. 
         FIG.  7   , Section (a) is a sectional view of the spark plug taken along line VIIa-VIIa of  FIG.  6   ,  FIG.  7   , Section (b) is a sectional view of the spark plug taken along line VIIb-VIIb of  FIG.  6   , and  FIG.  7   , Section (c) is a sectional view of the spark plug taken along line VIIc-VIIc of  FIG.  6   . 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Preferred embodiments of the present invention will now be described with reference to the attached drawings.  FIG.  1    is a partial sectional view of a spark plug  10  in a first embodiment. In  FIG.  1   , the lower side of the sheet will be referred to as the forward end side of the spark plug  10 , and the upper side of the sheet will be referred to as the rear end side of the spark plug  10  (this also applies to  FIG.  2    and  FIG.  4   ).  FIG.  1    shows a cross section of a forward-end-side portion of the spark plug  10 , the cross section containing an axial line O. As shown in  FIG.  1   , the spark plug  10  includes an insulator  11 , a center electrode  13 , a metallic shell  20 , and a ground electrode  40 . 
     The insulator  11  is an approximately cylindrical tubular member having an axial hole  12  formed therein and extending along the axial line O. The insulator  11  is formed of a ceramic material, such as alumina, which is excellent in mechanical characteristics and insulating performance at high temperatures. The center electrode  13  is disposed in the axial hole  12  of the insulator  11 . 
       FIG.  2    is a sectional view of the spark plug  10 , the sectional view containing the axial line O and showing, on an enlarged scale, a portion of  FIG.  1    indicated by II. The center electrode  13  is a rod-shaped member having electrical conductivity. The center electrode  13  includes a base member  14  in which a core having high thermal conductivity is embedded, and a disk-shaped discharge member  15  joined to the base member  14 . The base member  14  is formed of Ni or an alloy containing Ni as a main component. The core is formed of Cu or an alloy containing Cu as a main component. The core may be omitted. The discharge member  15  is formed of, for example, a noble metal, such as Pt, Ir, Ru, or Rh, which is higher in resistance to consumption caused by spark (hereinafter referred to as “spark consumption resistance”) than the base member  14 , W, or an alloy whose main component is a noble metal or W. 
     Referring back to  FIG.  1   , the center electrode  13  is electrically connected to a metallic terminal member  17  within the axial hole  12 . The metallic terminal member  17  is a rod-shaped member to which a high voltage cable (not shown) is connected. The metallic terminal member  17  is formed of an electrically conductive metallic material (for example, low carbon steel). The metallic terminal member  17  is fixed to the rear end of the insulator  11 . 
     The metallic shell  20  is a bottomed tubular member and formed of an electrically conductive metallic material (for example, low carbon steel). The metallic shell  20  includes a cylindrical tubular portion  21  having a male screw  22  formed on an outer circumferential surface of the cylindrical tubular portion  21 , and a bearing portion  23  located adjacent to and on the rear end side of the cylindrical tubular portion  21 . 
     The male screw  22  of the cylindrical tubular portion  21  is brought into thread engagement with a threaded hole of an engine (not shown). The outer diameter of the bearing portion  23  is larger than the outer diameter of the male screw  22 . The bearing portion  23  bears an axial force produced when the male screw  22  is screwed into the threaded hole of the engine. The metallic shell  20  holds the insulator  11  from the outer circumferential side. 
     A bottom portion  24  is connected to a part of the cylindrical tubular portion  21  of the metallic shell  20 , which part is located on the forward end side of the male screw  22 . The bottom portion  24  is a member having a hemispherical shape or the shape of a bottomed cylindrical tube. The bottom portion  24  is formed of, for example, a metallic material which contains, as a main component(s), one or more metals selected from Fe, Ni, Cu, etc. The bottom portion  24  is substantially a portion of the metallic shell  20 . Since the cylindrical tubular portion  21  is closed by the bottom portion  24 , the metallic shell  20  is a close-bottomed tubular body. In the present embodiment, the bottom portion  24  is a hemispherical member and is joined to the cylindrical tubular portion  21  by a weld portion (not shown). 
     A sub-chamber  25  is defined and surrounded by the cylindrical tubular portion  21  and the bottom portion  24 . A jetting hole  26  penetrating the bottom portion  24  in the thickness direction thereof is formed in the bottom portion  24 . The jetting hole  26  establishes communication between the sub-chamber  25  and a combustion chamber of the engine (not shown). In the present embodiment, a plurality of jetting holes  26  are formed in the metallic shell  20 . The ground electrode  40  is connected to the metallic shell  20 . The ground electrode  40  is a rod-shaped member formed of, for example, a metallic material which contains, as a main component(s), one or more metals selected from Pt, Ni, Ir, etc. 
     As shown in  FIG.  2   , the metallic shell  20  has a penetration hole  29  formed to extend from an inner circumferential surface  27  of the metallic shell  20  to an outer circumferential surface  28  of the metallic shell  20 . In the present embodiment, the penetration hole  29  is formed in the cylindrical tubular portion  21  of the metallic shell  20  to be located at a position corresponding to the male screw  22 . The penetration hole  29  has a recess  30 , a counterbore portion  31 , and a penetrating portion  33 , which are provided in this sequence from the outer circumferential surface  28  toward the inner circumferential surface  27  of the metallic shell  20 . 
     The recess  30  has a circular cross section. The recess  30  has a depth greater than the depth of the groove  22   a  of the male screw  22 . The bottom  30   a  of the recess  30  is an annular flat surface. The counterbore portion  31  is a bottomed cylindrical surface connected to the bottom  30   a  of the recess  30 . The counterbore portion  31  has a diameter smaller than the diameter of the bottom  30   a  of the recess  30 . The penetrating portion  33  extends from the bottom  32  of the counterbore portion  31  to the inner circumferential surface  27  of the metallic shell  20 . The penetrating portion  33  has a cross-sectional area smaller than the cross-sectional area of the counterbore portion  31 . 
     The ground electrode  40  is formed linearly and extends in a direction intersecting the axial direction (in the present embodiment, the ground electrode  40  extends approximately perpendicularly to the axial line O). The ground electrode  40  has a rod-like shape and has a first end portion  41  held in the penetration hole  29  and a second end portion  42  located on the inner side of the metallic shell  20 . The first end portion  41  of the ground electrode  40  is held in the penetration hole  29  of the metallic shell  20 . The first end portion  41  is joined to the metallic shell  20  by a weld portion (not shown). An end surface  41   a  of the first end portion  41  of the ground electrode  40  and the bottom  30   a  of the recess  30  are located on the same plane. 
     The second end portion  42  of the ground electrode  40  is located on the forward end side of the forward end surface  16  of the center electrode  13 . The forward end surface  16  of the center electrode  13  is the same as the forward end surface of the discharge member  15 . The forward end surface  16  has an approximately circular shape. 
     The ground electrode  40  has a fixing portion  43  fixed to the counterbore portion  31 , and an extension portion  44  extending from the fixing portion  43  beyond the inner circumferential surface  27  of the metallic shell  20 . An end portion of the extension portion  44  is the same as the second end portion  42  of the ground electrode  40 . The side surface of the extension portion  44  includes a flat surface  45 . The flat surface  45  faces toward the rear end side in the axial direction. The flat surface  45  faces the forward end surface  16  of the center electrode  13 , whereby a spark gap  46  extending in the axial direction is formed. 
     Section (a) of  FIG.  3    is a sectional view of the spark plug  10  taken along line IIIa-IIIa of  FIG.  2   . The counterbore portion  31  of the penetration hole  29  has a circular cross section. The fixing portion  43  of the ground electrode  40  has the shape of a circular plate (circular column) having a circular cross section and is fitted into the counterbore portion  31 . The fixing portion  43  has rotational symmetry about an axis C which passes through the center of the cross section of the fixing portion  43  and is perpendicular to the axial line O. Since the counterbore portion  31 , to which the circular plate-shaped fixing portion  43  is fixed, has a circular shape, machining of the penetration hole  29  can be facilitated. 
     Section (b) of  FIG.  3    is a sectional view of the spark plug  10  taken along line IIIb-IIIb of  FIG.  2   . The extension portion  44  is partially fitted into the penetrating portion  33  of the penetration hole  29 . In the present embodiment, the penetrating portion  33  has a rectangular cross section having a width greater than its height, and a flat surface  34  is provided at the rear end. The flat surface  34  faces toward the forward end side. In the present embodiment, the flat surface  34  is perpendicular to the axial line O. The cross section of the penetrating portion  33  has 2-fold symmetry about the axis C; i.e., a cross section obtained by rotating 180 degrees the original cross section about the axis C perfectly overlaps the original cross section. 
     In the present embodiment, the extension portion  44  of the ground electrode  40  has a rectangular cross section having a width greater than its height. The flat surface  45  of the extension portion  44  faces the flat surface  34  of the penetrating portion  33 . The extension portion  44  has a size determined such that four corners  44   a  of the cross section of the extension portion  44  are in contact with the outline  43   a  of the cross section of the fixing portion  43 . Notably, the cross section of the penetrating portion  33  differs from the cross section of the extension portion  44  in at least one of size and shape. In the present embodiment, although the shape of the cross section of the penetrating portion  33  is approximately the same as the shape of the cross section of the extension portion  44 , the cross section of the penetrating portion  33  is slightly larger than the cross section of the extension portion  44 . 
     The outline  43   a  of the cross section of the fixing portion  43  refers to the outline of the cross section of a region of the fixing portion  43  where the weld portion (not shown) is not formed. This is for the following reason. Since the fixing portion  43  has been melted into the weld portion, in a region of the fixing portion  43  where the weld portion is formed, the outline  43   a  of the original cross section of the fixing portion  43  cannot be determined. 
     Section (c) of  FIG.  3    is a sectional view of the spark plug  10  taken along line IIIc-IIIc of  FIG.  2    and containing the axial line O. The size and shape of the cross section of the extension portion  44  at the second end portion  42  of the ground electrode  40  (see section (c) of  FIG.  3   ) are identical to those of the cross section of the extension portion  44  at the first end portion  41  of the ground electrode  40  (see section (b) of  FIG.  3   ). Since the extension portion  44  of the ground electrode  40  has the shape of a quadrangular prism, a flat surface  47  whose size is the same as the flat surface  45  is provided on the side opposite the flat surface  45 . The cross section of the extension portion  44  has 2-fold symmetry about the axis C which passes through the center of the cross section of the fixing portion  43  (see section (a) of  FIG.  3   ) and is perpendicular to the axial line O; i.e., a cross section obtained by rotating 180 degrees the original cross section about the axis C perfectly overlaps the original cross section. 
     In a process of manufacturing the spark plug  10 , the ground electrode  40  is inserted into the penetration hole  29  of the metallic shell  20  in such a manner that the second end portion  42  of the extension portion  44  is first inserted into the penetration hole  29 , the first end portion  41  of the extension portion  44  is then fitted into the penetrating portion  33 , and the fixing portion  43  is then fitted into the counterbore portion  31 . Accordingly, the upper limit of the cross-sectional area of the second end portion  42  is equal to the cross-sectional area of the penetrating portion  33 . In the case where the fit between the extension portion  44  of the ground electrode  40  and the penetrating portion  33  of the penetration hole  29  is set to interference fit (press-fit structure), the cross-sectional area of the second end portion  42  becomes approximately equal to the area of the penetrating portion  33 . Notably, the fit between the extension portion  44  of the ground electrode  40  and the penetrating portion  33  may be loose fit or transition fit. In the case where the fit between the extension portion  44  and the penetrating portion  33  is loose fit or transition fit, machining of the extension portion  44  and the penetrating portion  33  can be facilitated. 
     Also, the extension portion  44  has a size determined such that four corners  44   a  of the cross section of the extension portion  44  are in contact with the outline  43   a  of the cross section of the fixing portion  43  (see section (a) of  FIG.  3   ). Therefore, the greater the diameter of the outline  43   a  of the fixing portion  43 , the greater the degree to which the cross-sectional area of the extension portion  44  can be increased. In the case where the fit between the fixing portion  43  of the ground electrode  40  and the counterbore portion  31  is set to interference fit (press-fit structure), the cross-sectional area of the fixing portion  43  becomes approximately equal to the area of the counterbore portion  31 . Since the fixing portion  43  has a circular plate-like shape, the fit between the fixing portion  43  and the counterbore portion  31  whose cross section is circular can be easily set to interference fit. Notably, the fit between the fixing portion  43  of the ground electrode  40  and the counterbore portion  31  may be loose fit or transition fit. 
     After the first end portion  41  of the ground electrode  40  has been fitted into the penetration hole  29 , the fixing portion  43  is welded to the metallic shell  20 . Since both the counterbore portion  31  and the fixing portion  43  have circular outer shapes, it is easy to secure the fit between the counterbore portion  31  and the fixing portion  43 . The weld portion (not shown) where the fixing portion  43  and the metallic shell  20  melt into each other is provided over the entire circumference of the fixing portion  43  in order to secure gastightness. The weld portion extends from the bottom  30   a  of the recess  30  in the thickness direction of the metallic shell  20 . Since the recess  30  is present, it is possible to prevent the thread of the male screw  22  from melting during the welding and to prevent the thread of the male screw  22  from deforming due to heat of the welding. 
     When the ground electrode  40  is inserted into the penetration hole  29  of the metallic shell  20  in such a manner that the second end portion  42  is first inserted into the penetration hole  29 , the extension portion  44  enters the penetrating portion  33 , and the fixing portion  43  enters the counterbore portion  31 . The extension portion  44  cannot enter the penetrating portion  33  unless the extension portion  44  is oriented such that the flat surface  45  or the flat surface  47  of the extension portion  44  faces the flat surface  34  of the penetrating portion  33 . Namely, when the extension portion  44  is disposed in the penetrating portion  33 , the penetrating portion  33  restricts the orientation of the extension portion  44  in such a manner that the flat surface  45  (or the flat surface  47 ) of the extension portion  44  faces toward the rear end side in the axial direction (the upper side in section (c) of  FIG.  3   ). The penetrating portion  33  restricts the orientation of the extension portion  44  in such a manner that the angle between the axial line O and a plane P perpendicular to the flat surface  45  (a plane containing a straight line perpendicular to the flat surface  45 ) becomes smaller than 90 degrees, preferably, equal to or smaller than 45 degrees, more preferably, equal to or smaller than 5 degrees. 
     As a result, the flat surface  45  of the extension portion  44  of the ground electrode  40  is located on the forward end side of the forward end surface  16  of the center electrode  13  in the axial direction, with the spark gap  46  intervening between the flat surface  45  and the forward end surface  16 . Since discharge occurs on the flat surface  45  of the extension portion  44 , consumption of the ground electrode  40  due to discharge can be reduced as compared with the case where the side surface of the ground electrode  40  is cylindrical. Therefore, it is possible to prevent expansion of the spark gap  46  at an early stage of usage. 
     In the case where the penetrating portion  33  restricts the orientation of the extension portion  44  in such a manner that the angle between the axial line O and the plane P perpendicular to the flat surface  45  of the extension portion  44  becomes equal to or smaller than 45 degrees, a discharge point (position where discharge occurs) becomes likely to be located on the flat surface  45  of the extension portion  44 . By virtue of this, the spark consumption resistance of the ground electrode  40  can be enhanced reliably. 
     In the case where the penetrating portion  33  restricts the orientation of the extension portion  44  in such a manner that the angle between the axial line O and the plane P perpendicular to the flat surface  45  of the extension portion  44  becomes equal to or smaller than 5 degrees, the discharge point becomes more likely to be located on the flat surface  45  of the extension portion  44 . Accordingly, the spark consumption resistance of the ground electrode  40  can be enhanced more reliably. 
     Since the penetrating portion  33  includes the flat surface  34  provided on the rear end side, the ground electrode  40  can be disposed in such a manner that the flat surface  45  of the extension portion  44  faces the flat surface  34  of the penetrating portion  33 . Accordingly, the shape of the extension portion  44  can be made simple. Also, since the flat surface  45  of the extension portion  44  continues from the first end portion  41  to the second end portion  42  of the ground electrode  40 , the shape of the extension portion  44  can be made simple. Therefore, machining of the extension portion  44  can be facilitated. 
     Since the extension portion  44  at the second end portion  42  of the ground electrode  40  has 2-fold symmetry about the axis C of the ground electrode  40 , alignment at the time when the ground electrode  40  is disposed in the penetration hole  29  of the metallic shell  20  is easier as compared with an extension portion which is not rotational symmetry (i.e., a cross section obtained by rotating 360 degrees the original cross section of the extension portion about the axis C perfectly overlaps the original cross section). 
     Since the recess  30  is present, even when the length of the fixing portion  43  is greater than the depth of the counterbore portion  31 , the fixing portion  43  is unlikely to project from the outer circumferential surface  28  of the metallic shell  20 . In the case where the fit between the fixing portion  43  and the counterbore portion  31  is interference fit, the first end portion  41  of the ground electrode  40  is firmly fixed to the penetration hole  29  before the ground electrode  40  is welded. 
     The spark plug  10  is attached to the engine (not shown). After that, as a result of operation of a piston and valves of the engine, fuel gas flows from a combustion chamber through the jetting hole  26  into the sub-chamber  25  inside the metallic shell  20 . The spark plug  10  produces a flame kernel at the spark gap  46  through discharge between the center electrode  13  and the ground electrode  40 . When the flame kernel grows, the fuel gas within the sub-chamber  25  is ignited, whereby the fuel gas combusts. Due to expansion pressure produced as a result of the combustion, the spark plug  10  jets a flame-containing gas flow from the jetting hole  26  into the combustion chamber. As a result of the flame jet flow, the fuel gas within the combustion chamber combusts. 
     Since the extension portion  44  of the ground electrode  40  is located in the sub-chamber  25 , the extension portion  44  is disposed in an environment in which the extension portion  44  is easily overheated and is easily consumed. However, since the spark gap  46  is formed between the flat surface  45  at the side of the extension portion  44 , and the forward end surface  16  of the center electrode  13 , consumption of the side surface of the ground electrode  40  caused by discharge can be reduced as compared with the case where the side surface of the ground electrode  40  is a cylindrical surface. 
     Since the first end portion  41  of the ground electrode  40  is held in the penetration hole  29  formed in the cylindrical tubular portion  21  of the metallic shell  20 , where the male screw  22  is provided, heat of the ground electrode  40  is transferred from the cylindrical tubular portion  21  to the engine (not shown) through the male screw  22 , whereby the ground electrode  40  is cooled. Therefore, it is possible to reduce consumption of the ground electrode  40  and occurrence of abnormal combustion (pre-ignition) caused by overheating of the ground electrode  40 . 
     A second embodiment will be described with reference to  FIGS.  4  and  5   . In the first embodiment, the case where the cross section of the extension portion  44  of the ground electrode  40  is rectangular has been described. In the second embodiment, the case where the cross section of an extension portion  64  of a ground electrode  60  is semicircular will be described. Notably, portions identical with the portions described in the first embodiment are denoted by the same reference numerals, and their descriptions will not be repeated.  FIG.  4    is a sectional view of a spark plug  50  in the second embodiment, the sectional view containing the axial line O. Like  FIG.  2   ,  FIG.  4    shows a portion of  FIG.  1    indicated by II (this also applies to  FIG.  6   ). 
     As shown in  FIG.  4   , in the metallic shell  20 , a penetration hole  51  extending from the inner circumferential surface  27  of the metallic shell  20  to the outer circumferential surface  28  of the metallic shell  20  is formed in the cylindrical tubular portion  21  to be located at a position corresponding to the male screw  22 . The penetration hole  51  has a recess  52 , a counterbore portion  53 , and a penetrating portion  55 , which are provided in this sequence from the outer circumferential surface  28  toward the inner circumferential surface  27  of the metallic shell  20 . 
     The recess  52  has a circular cross section. The bottom  52   a  of the recess  52  is an annular flat surface. The counterbore portion  53  communicates with the bottom  52   a  of the recess  52 . The counterbore portion  53  has a diameter smaller than the diameter of the bottom  52   a  of the recess  52 . The penetrating portion  55  extends from the bottom  54  of the counterbore portion  53  to the inner circumferential surface  27  of the metallic shell  20 . The penetrating portion  55  has a cross-sectional area smaller than the cross-sectional area of the counterbore portion  53 . 
     The ground electrode  60  is formed linearly and extends in a direction intersecting the axial direction (in the present embodiment, the ground electrode  60  extends approximately perpendicularly to the axial line O). The ground electrode  60  has a rod-like shape and has a first end portion  61  held in the penetration hole  51  and a second end portion  62  located on the inner side of the metallic shell  20 . The first end portion  61  of the ground electrode  60  is held in the penetration hole  51  of the metallic shell  20 . The second end portion  62  of the ground electrode  60  is located on the forward end side of the forward end surface  16  of the center electrode  13 . The first end portion  61  is joined to the metallic shell  20  by a weld portion (not shown). An end surface  61   a  of the first end portion  61  of the ground electrode  60  and the bottom  52   a  of the recess  52  are located on the same plane. 
     The ground electrode  60  has a fixing portion  63  fixed to the counterbore portion  53 , and an extension portion  64  extending from the fixing portion  63  beyond the inner circumferential surface  27  of the metallic shell  20 . An end portion of the extension portion  64  is the same as the second end portion  62  of the ground electrode  60 . The side surface of the extension portion  64  includes a flat surface  65 . The flat surface  65  faces toward the rear end side in the axial direction. The flat surface  65  faces the forward end surface  16  of the center electrode  13 , whereby a spark gap  66  extending in the axial direction is formed. 
     Section (a) of  FIG.  5    is a sectional view of the spark plug  50  taken along line Va-Va of  FIG.  4   . The counterbore portion  53  of the penetration hole  51  has a circular cross section. The fixing portion  63  of the ground electrode  60  has the shape of a circular plate (circular column) having a circular cross section, and the fixing portion  63  is fitted into the counterbore portion  53 . The fixing portion  63  has rotational symmetry about the axis C which passes through the center of the cross section of the fixing portion  63  and is perpendicular to the axial line O. 
     Section (b) of  FIG.  5    is a sectional view of the spark plug  50  taken along line Vb-Vb of  FIG.  4   . The extension portion  64  is partially fitted into the penetrating portion  55  of the penetration hole  51 . In the present embodiment, the penetrating portion  55  has a semi-circular cross section, and a flat surface  56  is provided at the rear end. The flat surface  56  faces toward the forward end side. In the present embodiment, the flat surface  56  is perpendicular to the axial line O. The cross section of the penetrating portion  55  has line symmetry with respect to a plane containing the axis C and the axial line O. 
     In the present embodiment, the extension portion  64  of the ground electrode  60  has a semi-circular cross section. The flat surface  65  of the extension portion  64  faces the flat surface  56  of the penetrating portion  55 . The extension portion  64  is formed such that the arc  64   a  of the outline of the cross section of the extension portion  64  coincides with the outline  63   a  of the cross section of the fixing portion  63 . The flat surface  65  is located on the rear end side of the arc  64   a . The outline  63   a  of the cross section of the fixing portion  63  refers to the outline of the cross section of a region of the fixing portion  63  where the weld portion (not shown) is not formed (a region where the outline  63   a  of the original cross section of the fixing portion  63  can be determined). 
     In a process of manufacturing the spark plug  50 , the ground electrode  60  enters the penetration hole  51  of the metallic shell  20  in such a manner that the second end portion  62  first enters the penetration hole  51 . If the extension portion  64  is not oriented such that the flat surface  65  of the extension portion  64  faces the flat surface  56  of the penetrating portion  55 , the extension portion  64  cannot enter the penetrating portion  55 . Namely, when the extension portion  64  is disposed in the penetrating portion  55 , the penetrating portion  55  restricts the orientation of the extension portion  64  such that the flat surface  56  of the extension portion  64  faces toward the rear end side. 
     Section (c) of  FIG.  5    is a sectional view of the spark plug  50  taken along line Vc-Vc of  FIG.  4    and containing the axial line O. The size and shape of the cross section of the extension portion  64  at the second end portion  62  of the ground electrode  60  (see section (c) of  FIG.  5   ) are identical to those of the cross section of the extension portion  64  at the first end portion  61  of the ground electrode  60  (see section (b) of  FIG.  5   ). The penetrating portion  55  restricts the orientation of the extension portion  64  in such a manner that the angle between the axial line O and a plane P perpendicular to the flat surface  65  becomes smaller than 90 degrees, preferably, equal to or smaller than 45 degrees, more preferably, equal to or smaller than 5 degrees. 
     As a result, the flat surface  65  of the ground electrode  60  is located at the rear end of the extension portion  64 , and the spark gap  66  is formed between the flat surface  65  and the forward end surface  16  of the center electrode  13 . Consumption of the extension portion  64  due to discharge can be reduced as compared with the case where the cylindrical surface of the extension portion  64  of the ground electrode  60  faces the forward end surface  16  of the center electrode  13 . Therefore, it is possible to prevent expansion of the spark gap  66  at an early stage of usage. 
     The extension portion  64  of the ground electrode  60  has a size determined such that the arc  64   a  of the outline of the cross section of the extension portion  64  coincides with the outline  63   a  of the cross section of the fixing portion  63 . Therefore, it is possible to secure the volume of the extension portion  64  at the second end portion  62  while providing the flat surface  65  on the extension portion  64 . Accordingly, consumption per unit volume of the extension portion  64  caused by discharge can be reduced. Since the flat surface  65  of the extension portion  64  is set to contain the axis C, the width (dimension in the lateral direction in section (c) of  FIG.  5   ) of the flat surface  65  can be maximized. 
     A third embodiment will be described with reference to  FIGS.  6  and  7   . In the first and second embodiments, the case where the flat surface  45  ( 65 ) of the extension portion  44  ( 64 ) continues from the first end portion  41  ( 61 ) to the second end portion  42  ( 62 ) of the ground electrode  40  ( 60 ). In the third embodiment, there will be described the case where a flat surface  86  provided on a second end portion  82  of a ground electrode  80  is interrupted at an extension portion  84  and does not extend to a first end portion  81 . Notably, portions identical with the portions described in the first embodiment are denoted by the same reference numerals, and their descriptions will not be repeated.  FIG.  6    is a sectional view of a spark plug  70  in the third embodiment, the sectional view containing the axial line O. 
     As shown in  FIG.  6   , in the metallic shell  20 , a penetration hole  71  extending from the inner circumferential surface  27  of the metallic shell  20  to the outer circumferential surface  28  of the metallic shell  20  is formed in the cylindrical tubular portion  21  to be located at a position corresponding to the male screw  22 . The penetration hole  71  has a recess  72 , a counterbore portion  73 , and a penetrating portion  74 , which are provided in this sequence from the outer circumferential surface  28  toward the inner circumferential surface  27  of the metallic shell  20 . 
     The recess  72  has a circular cross section. The bottom  72   a  of the recess  72  is an annular flat surface. The counterbore portion  73  is a conical surface connected to the bottom  72   a  of the recess  72 . The counterbore portion  73  has a diameter smaller than the diameter of the bottom  72   a  of the recess  72 . The penetrating portion  74  extends from the counterbore portion  73  to the inner circumferential surface  27  of the metallic shell  20 . The penetrating portion  74  has a cross-sectional area smaller than the cross-sectional area of the counterbore portion  73 . 
     The ground electrode  80  is formed linearly and extends in a direction intersecting the axial direction (in the present embodiment, the ground electrode  80  extends approximately perpendicularly to the axial line O). The ground electrode  80  has a rod-like shape and has a first end portion  81  held in the penetration hole  71  and a second end portion  82  located on the inner side of the metallic shell  20 . The first end portion  81  of the ground electrode  80  is held in the penetration hole  71  of the metallic shell  20 . The second end portion  82  of the ground electrode  80  is located on the forward end side of the forward end surface  16  of the center electrode  13 . The first end portion  81  is joined to the metallic shell  20  by a weld portion (not shown). 
     The ground electrode  80  has a fixing portion  83  fixed to the counterbore portion  73 , and an extension portion  84  extending from the fixing portion  83  beyond the inner circumferential surface  27  of the metallic shell  20 . An end portion of the extension portion  84  is the same as the second end portion  82  of the ground electrode  80 . The side surface of the extension portion  84  includes a flat surface  86 . The flat surface  86  faces toward the rear end side in the axial direction. The flat surface  86  faces the forward end surface  16  of the center electrode  13 , whereby a spark gap  87  extending in the axial direction is formed. 
     Section (a) of  FIG.  7    is a sectional view of the spark plug  70  taken along line VIIa-VIIa of  FIG.  6   . The counterbore portion  73  of the penetration hole  71  has a circular cross section. The fixing portion  83  of the ground electrode  80  has the shape of a circular plate (circular cone) having a circular cross section, and the fixing portion  83  is fitted into the counterbore portion  73 . The fixing portion  83  has rotational symmetry about the axis C which passes through the center of the cross section of the fixing portion  83  and is perpendicular to the axial line O. 
     Section (b) of  FIG.  7    is a sectional view of the spark plug  70  taken along line VIIb-VIIb of  FIG.  6   . The extension portion  84  is partially fitted into the penetrating portion  74  of the penetration hole  71 . In the present embodiment, the penetrating portion  74  is composed of a semi-cylindrical surface  75  whose cross section is a major arc, and a flat surface  76  connecting together opposite side edges of the semi-cylindrical surface  75 . The flat surface  76  is provided at the forward end of the penetrating portion  74 . The flat surface  76  faces toward the rear end side. In the present embodiment, the flat surface  76  is perpendicular to the axial line O. The cross section of the penetrating portion  74  has line symmetry with respect to the plane containing the axis C and the axial line O. 
     One end portion of the extension portion  84  of the ground electrode  80  has a shape obtained by halving a circular column  84   a  and is fitted into the penetrating portion  74 . The extension portion  84  has the flat surface  85  facing toward the forward end side. The flat surface  85  of the extension portion  84  faces the flat surface  76  of the penetrating portion  74 . 
     Section (c) of  FIG.  7    is a sectional view of the spark plug  70  taken along line VIIc-VIIc of  FIG.  6   , the sectional view containing the axial line O and being perpendicular to the axis C. The extension portion  84  has the flat surface  86  which is provided at the second end portion  82  of the ground electrode  80  and intersects the axial line O. The flat surface  86  faces toward the rear end side and is provided on the side opposite the flat surface  85 . The length of the flat surface  86  along the axis C is shorter than the length of the flat surface  85  along the axis C. The length of the flat surface  86  (the length of the chord of a corresponding portion (arc) of the circular column  84   a ) in a cross section perpendicular to the axis C is shorter than the length of the flat surface  85  (the length of the chord of a corresponding portion (arc) of the circular column  84   a ) in the cross section perpendicular to the axis C. 
     In a process of manufacturing the spark plug  70 , the ground electrode  80  enters the penetration hole  71  of the metallic shell  20  in such a manner that the second end portion  82  first enters the penetration hole  71 . If the extension portion  84  is not oriented such that the flat surface  85  of the extension portion  84  faces the flat surface  76  of the penetrating portion  74 , the extension portion  84  cannot enter the penetrating portion  74 . Namely, when the extension portion  84  is disposed in the penetrating portion  74 , the penetrating portion  74  restricts the orientation of the extension portion  84  such that the flat surface  86  of the extension portion  84  faces toward the rear end side. The penetrating portion  74  restricts the orientation of the extension portion  84  in such a manner that the angle between the axial line O and a plane P perpendicular to the flat surface  86  becomes smaller than 90 degrees, preferably, equal to or smaller than 45 degrees, more preferably, equal to or smaller than 5 degrees. 
     As a result, the spark gap  87  is formed between the flat surface  86  of the ground electrode  80  and the forward end surface  16  of the center electrode  13 . Therefore, consumption of the extension portion  84  due to discharge can be reduced as compared with the case where the cylindrical surface of the extension portion  84  of the ground electrode  80  faces the forward end surface  16  of the center electrode  13 . Therefore, it is possible to prevent expansion of the spark gap  87  at an early stage of usage. 
     The present invention has been described on the basis of embodiments. However, it is easily inferred that the present invention is not limited to the above-described embodiments and various improvements and modifications can be made without departing from the spirit of the invention. For example, the shapes, etc. of the bottom portion  24  of the metallic shell  20  and the ground electrodes  40 ,  60 , and  80  can be set appropriately. 
     In the embodiments, the case where the forward end of the metallic shell  20  is closed by the bottom portion  24  has been described. However, the structure of the metallic shell  20  is not limited to such a structure. Of course, the metallic shell  20  can have a structure in which the bottom portion  24  is omitted, so that the sub-chamber  25  is not provided. In this case as well, a flame kernel is produced at the spark gap  46  ( 66 ,  87 ) as a result of discharge between the center electrode  13  and the ground electrode  40  ( 60 ,  80 ). When the flame kernel grows, a fuel gas within the combustion chamber burns. Since the spark gap  46  ( 66 ,  87 ) is formed between the forward end surface  16  of the center electrode  13  and the flat surface  45  ( 65 ,  86 ) of the ground electrode  40  ( 60 ,  80 ), consumption of the extension portion  44  ( 64 ,  84 ) due to discharge can be reduced as compared with the case where the spark gap is provided on the cylindrical surface of the ground electrode  40  ( 60 ,  80 ). 
     In the embodiments, the center electrode  13  including the base member  14  and the discharge member  15  connected thereto has been described. However, the structure of the center electrode  13  is not limited thereto. Of course, the discharge member  15  can be omitted. In the case where the discharge member  15  is omitted, the forward end surface of the center electrode  13  refers to the forward end surface of the base member  14 . 
     In the embodiments, there has been described case where the penetration hole  29  ( 51 ,  71 ) which holds the first end portion  41  ( 61 ,  81 ) of the ground electrode  40  ( 60 ,  80 ) is provided in the metallic shell  20  to be located at a position corresponding to the male screw  22 . However, the position of the penetration hole  29  ( 51 ,  71 ) is not limited thereto. Of course, the penetration hole which holds the first end portion of the ground electrode can be provided in, for example, a region of the cylindrical tubular portion  21 , which region is located on the forward end side of the male screw  22 . Also, in the case where the forward end of the metallic shell  20  is closed by the bottom portion  24 , of course, the penetration hole which holds the first end portion of the ground electrode can be provided in the bottom portion  24 . 
     In the embodiments, there has been described the case where the penetrating portion  33  ( 55 ,  74 ) restricts the orientation of the extension portion  44  ( 64 ,  84 ) of the ground electrode  40  ( 60 ,  80 ) (the angle of the extension portion about the axis C), by utilizing the engagement between the flat surface  34  ( 56 ,  76 ) provided at the penetrating portion  33  ( 55 ,  74 ) and the flat surface  45  ( 65 ,  85 ) provided at the extension portion  44  ( 64 ,  84 ), in such a manner that the flat surface  45  ( 65 ,  86 ) faces toward the rear end side. However, the manner in which the penetrating portion  33  ( 55 ,  74 ) restricts the orientation of the extension portion  44  ( 64 ,  84 ) is not limited thereto. Of course, the penetrating portion can restrict the orientation of the extension portion  44  ( 64 ,  84 ) of the ground electrode  40  ( 60 ,  80 ) by unitizing a recess and a protrusion which are provided at the penetrating portion and the extension portion, respectively, and which engage with each other. 
     In the first embodiment, the extension portion  44  having a rectangular cross section has been described, and in the second embodiment, the extension portion  64  having a semi-circular cross section has been described. However, the cross sectional shapes of the extension portions are not limited thereto. Of course, it is possible to employ an extension portion having a different cross sectional shape so long as the flat surface  45  ( 65 ) for forming the spark gap in cooperation with the forward end surface  16  of the center electrode  13  can be formed on the side surface of the extension portion  44  ( 64 ). An example of the different cross sectional shape of the extension portion is a polygonal shape such as a triangular shape or a pentagonal shape. Of course, it is possible to round or chamfer the edge of the flat surface  45  ( 65 ,  86 ) of the extension portion  44  ( 64 ,  84 ). 
     In the second embodiment, there has been described the case where the extension portion  64  of the ground electrode  60  has a semi-circular cross section, and the flat surface  65  of the extension portion  64  contains the center of the outline  63   a  of the cross section of the fixing portion  63 . However, the cross sectional shape of the extension portion  64  is not limited thereto. Of course, it is possible to set the cross sectional shape of the extension portion  64  such that the arc  64   a  of the outline of the cross section of the extension portion  64  becomes a minor arc or a major arc. 
     In the first and second embodiments, the case where the fixing portion  43  ( 63 ) of the ground electrode  40  ( 60 ) has a circular columnar shape has been described, and in the third embodiment, the case where the fixing portion  83  of the ground electrode  80  has a conical shape has been described. However, the shapes of the fixing portions are not limited thereto. Of course, it is possible to form the fixing portions  43  and  63  of the ground electrodes  40  and  60  into a conical shape and to form the fixing portion  83  of the ground electrode  80  into a circular columnar shape. 
     In the embodiments, the case where the bottom portion  24  of the metallic shell  20  is welded to the cylindrical tubular portion  21  has been described. However, the manner of joining the bottom portion  24  of the metallic shell  20  to the cylindrical tubular portion  21  is not limited thereto. Of course, it is possible to prepare a tubular member having a closed forward end and connect the tubular member to the cylindrical tubular portion  21 , instead of welding the bottom portion  24  to the cylindrical tubular portion  21 , thereby forming the sub-chamber  25 . For example, a female screw which is engaged with the male screw  22  is formed on the inner circumferential surface of the tubular member. A male screw which is engaged with a threaded hole of the engine (not shown) is formed on the outer circumferential surface of the tubular member. As a result of engagement of the female screw of the tubular member with the male screw  22 , the forward end of the metallic shell  20  is closed. The jetting holes  26  are formed in the tubular member. 
     Notably, the means for connecting the tubular member to the cylindrical tubular portion  21  such that the metallic shell  20  becomes a bottomed tubular body is not limited to engaging the female screw of the inner circumferential surface of the tubular member with the male screw  22 . Of cause, it is possible to employ a different means so as to connect the tubular member to the cylindrical tubular portion  21 . An example of the different means is joining the tubular member to the bearing portion  23  by means of, for example, welding. The tubular member may be formed of a metallic material such as a nickel-based alloy or stainless steel, or a ceramic material such as silicon nitride. 
     DESCRIPTION OF REFERENCE NUMERALS AND SYMBOLS 
     
         
           10 ,  50 ,  70 : spark plug 
           13 : center electrode 
           16 : forward end surface of center electrode 
           20 : metallic shell 
           26 : jetting hole 
           27 : inner circumferential surface of metallic shell 
           29 ,  51 ,  71 : penetration hole 
           30 ,  52 ,  72 : recess 
           31 ,  53 ,  73 : counterbore portion 
           33 ,  55 ,  74 : penetrating portion 
           34 ,  56 : flat surface 
           40 ,  60 ,  80 : ground electrode 
           41 ,  61 ,  81 : first end portion 
           42 ,  62 ,  82 : second end portion 
           43 ,  63 ,  83 : fixing portion 
           44 ,  64 ,  84 : extension portion 
           45 ,  65 ,  86 : flat surface 
           46 ,  66 ,  87 : spark gap 
         O: axial line 
         P: perpendicular plane