Patent Publication Number: US-2023155353-A1

Title: Spark plug

Description:
TECHNICAL FIELD 
     The present disclosure relates to a spark plug. 
     BACKGROUND ART 
     As a spark plug for ignition used for a gasoline engine, there is known a spark plug having an insulator in which a penetration hole is formed along an axis direction and a center electrode disposed in the penetration hole (e.g. Patent Document 1). In the spark plug disclosed in Patent Document 1, a step portion formed in the penetration hole of the insulator so that its diameter is reduced toward a top end side supports a brim portion formed at the center electrode so as to protrude outwards in a radial direction. This center electrode of the spark plug does not have, at a rear end side thereof with respect to the brim portion, a portion whose diameter is reduced more than the brim portion, and the brim portion is short in size in the axis direction. 
     Citation List 
     Patent Document 
     Patent Document 1: Japanese Unexamined Patent Application Publication No. 2017-183105 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     The inventors of the present application found that since a surface area, which contacts a seal member, of the center electrode not having, at the rear end side thereof with respect to the brim portion, the portion whose diameter is reduced more than the brim portion is small as compared with that of a structure having the portion whose diameter is reduced, there is a risk that looseness of the center electrode will occur due to vibrations of the engine etc.. Then, the looseness of the center electrode may cause degradation in performance of the spark plug. Therefore, a technique capable of suppressing the occurrence of the looseness of the center electrode has been required. 
     Solution to Problem 
     The present disclosure can be realized as the following embodiment. 
     (1) According to the embodiment of the present disclosure, a spark plug is provided. A spark plug comprises: a center electrode having a leg portion extending in an axis direction along an axis of the spark plug, a brim portion located at an axis direction rear end side with respect to the leg portion and formed so as to protrude outwards in a radial direction with respect to the leg portion and a connecting portion connecting the leg portion and the brim portion; an insulator having a penetration hole formed along the axis direction and supporting the center electrode in the penetration hole; and a seal member filling the penetration hole and fixing the brim portion and the insulator. The insulator has a large diameter portion located at an axis direction rear end side of the insulator; a small diameter portion located at an axis direction top end side with respect to the large diameter portion, a diameter of the penetration hole at the small diameter portion being smaller than a diameter of the penetration hole at the large diameter portion; and a step portion connecting the penetration hole at the large diameter portion and the penetration hole at the small diameter portion and supporting the connecting portion. The center electrode satisfies “(D1-D2)/D1 ≤ 0.06”, where a maximum value of a radius of the brim portion on a cross section including the axis is D1, and a minimum value of the radius of the brim portion on the cross section is D2, and the center electrode also satisfies “L2/L1 ≤ 0.30”, where a size of the center electrode along the axis direction on the cross section is L1, and a size along the axis direction from a boundary between the connecting portion and the leg portion to a center of gravity of the center electrode is L2. 
     According to the spark plug of this embodiment, since the size L1 of the center electrode along the axis direction on the cross section including the axis and the size L2 along the axis direction from a boundary between the connecting portion and the leg portion to the center of gravity of the center electrode satisfy “L2/L1 ≤ 0.30”, in the center electrode having the brim portion whose maximum value D1 and minimum value D2 satisfy “(D1-D2)/D1 ≤ 0.06”, it is possible to prevent a position of the center of gravity of the center electrode from being located at an excessively top end side. Therefore, the position of the center of gravity of the center electrode can be prevented from being excessively separated from a position of the seal member fixing the center electrode and the insulator. This can suppress excessive swing or vibration of the center electrode around the seal member caused by vibrations etc., thereby suppressing an occurrence of looseness of the center electrode which is caused by deformation of the seal member due to the swing or vibration of the brim portion. 
     (2) In the spark plug of the above embodiment, the sizes L 1 and L2 of the center electrode could satisfy “L2/L1 ≤ 0.25”. 
     According to the spark plug of this embodiment, since the sizes L1 and L2 of the center electrode satisfy “L2/L1 ≤ 0.25”, the position of the center of gravity of the center electrode can be brought closer to positions of the connecting portion and the seal member. Therefore, the occurrence of the looseness of the center electrode can be further suppressed. 
     The present invention can be realized with various embodiments. For instance, the present invention can be realized in a manufacturing method of the spark plug and an embodiment of an engine head to which the spark plug is connected, and so on. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a partial sectional view showing a schematic configuration of a spark plug. 
         FIG.  2    is an enlarged sectional view schematically showing a step portion, a brim portion and their vicinities. 
         FIG.  3    is a schematic diagram for explaining a center of gravity of a center electrode. 
         FIG.  4    is a sectional view schematically showing a configuration of a center electrode of a comparative example 2. 
     
    
    
     EMBODIMENTS FOR CARRYING OUT THE INVENTION 
     A. Embodiment 
       FIG.  1    is a partial sectional view showing a schematic configuration of a spark plug  100  as an embodiment of the present disclosure. In  FIG.  1   , an outward appearance of the spark plug  100  is illustrated on a left side of the drawing with an axis CA as an axial center of the spark plug  100  being a boundary, and a cross-sectional shape of the spark plug  100  is illustrated on a right side of the drawing. In the following description, a lower side of  FIG.  1    along the axis CA (a side on which an after-mentioned ground electrode  40  is provided) is called a top end side (or a tip end side), an upper side of  FIG.  1    (a side on which an after-mentioned metal terminal  50  is provided) is called a rear end side, and a direction along the axis CA is called an axis direction AD. In  FIG.  1   , for convenience in description, an engine head  90  to which the spark plug  100  is connected is illustrated by a broken line. 
     The spark plug  100  has an insulator  10 , a center electrode  20 , a metal shell  30 , the ground electrode  40  and the metal terminal  50 . The axis CA of the spark plug  100  is aligned with each axis CA of members of the insulator  10 , the center electrode  20 . the metal shell  30  and the metal terminal  50 . 
     The insulator  10  has a substantially tubular outward appearance having a penetration hole  11  formed along the axis direction AD. A part of the center electrode  20  is accommodated at a top end side in the penetration hole  11 , whereas a part of the metal terminal  50  is accommodated at a rear end side in the penetration hole  11 . Therefore, the insulator  10  supports the center electrode  20  in the penetration hole  11 . Approximately half of a top end side of the insulator  10  is accommodated in an axial hole  38  of the after-mentioned metal shell  30 , and approximately half of a rear end side of the insulator  10  is exposed from the axial hole  38 . The insulator  10  is made of insulating glass formed by burning (or firing) ceramic material such as alumina. 
     The insulator  10  has a large diameter portion  14 , a holding portion  15 , a small diameter portion  16  and a step portion  17 . The large diameter portion  14  is located at the rear end side of the insulator  10  in the axis direction AD. A diameter of the penetration hole  11  at the large diameter portion  14  is formed substantially constant. The holding portion  15  is formed at a top end side of the large diameter portion  14  so that its outside diameter is smaller toward the top end side along the axis direction AD. The small diameter portion  16  is located at the top end side in the axis direction AD with respect to the large diameter portion  14 . A diameter of the penetration hole  11  at the small diameter portion  16  is smaller than the diameter of the penetration hole  11  at the large diameter portion  14 . The penetration hole  11  at the small diameter portion  16  accommodates therein a part of a leg portion  21  of the after-mentioned center electrode  20 . 
       FIG.  2    is an enlarged sectional view schematically showing the step portion  17 , a brim portion  22  and their vicinities.  FIG.  2    shows a cross section including the axis CA. The step portion  17  is located between the large diameter portion  14  and the small diameter portion  16  in the axis direction AD, and connects the large diameter portion  14  and the small diameter portion  16 . The step portion  17  of the present embodiment is formed so that the diameter of the penetration hole  11  is smaller toward the top end side along the axis direction AD. In other words, the step portion  17  is formed so as to protrude or bulge inwards in a radial direction in the penetration hole  11 . The step portion  17  supports a connecting portion  24  of the center electrode  20 . 
     The center electrode  20  shown in  FIGS.  1  and  2    is a rod-shaped electrode extending in the axis direction AD. The center electrode  20  is supported in the penetration hole  11  of the insulator  10 . The center electrode  20  has the leg portion  21 , the brim portion  22  and the connecting portion  24 . 
     As illustrated in  FIG.  1   , the leg portion  21  is formed so as to extend in the axis direction AD, and a part of the leg portion  21  is exposed from the penetration hole  11 . A noble metal chip made of e.g. an iridium alloy etc. could be joined to an end portion of a top end side of the leg portion  21 . 
     As illustrated in  FIG.  2   , the brim portion  22  is located at the rear end side with respect to the leg portion  21 , and is formed so as to protrude outwards in the radial direction with respect to the leg portion  21 . In other words, the brim portion  22  is formed at an end portion of a rear end side of the center electrode  20  so as to protrude or bulge outwards in the radial direction. In the present embodiment, an outside diameter of the brim portion  22  is formed substantially constant. 
     The connecting portion  24  connects the leg portion  21  and the brim portion  22 . The connecting portion  24  abuts on the step portion  17  of the insulator  10 . With this, positioning of the center electrode  20  in the penetration hole  11  of the insulator  10  is made. The connecting portion  24  of the present embodiment has a tapered shape whose outside diameter is gradually reduced toward the top end side. 
     The center electrode  20  of the present embodiment is formed with a core  25 , which is excellent in thermal conductivity, being embedded inside an electrode member  26 . In the present embodiment, the core  25  is made of an alloy containing copper as a main component. The electrode member  26  is made of a nickel alloy containing nickel as a main component. 
     As illustrated in  FIG.  1   , a part of the center electrode  20  is inserted into the penetration hole  11  of the insulator  10  at the top end side of the penetration hole  11 , and a part of the metal terminal  50  is inserted into the penetration hole  11  of the insulator  10  at the rear end side of the penetration hole  11 . In the penetration hole  11  of the insulator  10 , a top end side seal member  61 , a resistor  62  and a rear end side seal member  63  are disposed in an order from the top end side toward the rear end side between the center electrode  20  and the metal terminal  50 . Therefore, the center electrode  20  is electrically connected to the metal terminal  50  at the rear end side of the center electrode  20  through the top end side seal member  61 , the resistor  62  and the rear end side seal member  63 . 
     The resistor  62  is made of ceramic powder, conducting material, glass and adhesive as materials. The resistor  62  functions as an electric resistance between the metal terminal  50  and the center electrode  20 , thereby suppressing an occurrence of noise when spark discharge occurs. The top end side seal member  61  and the rear end side seal member  63  are each made of conductive glass powder as material. In the present embodiment, the top end side seal member  61  and the rear end side seal member  63  are each made of mixed powder of copper powder and calcium borosilicate glass powder as materials. The top end side seal member  61  contacts the brim portion  22 . the insulator  10  and the resistor  62 , and fixes these members to each other. The rear end side seal member  63  contacts the resistor  62 , the insulator  10  and the metal terminal  50 , and fixes these members. 
     As illustrated in  FIG.  1   , the metal shell  30  has a substantially tubular outward appearance having the axial hole  38  formed along the axis direction AD, and supports the insulator  10  in the axial hole  38 . More specifically, the metal shell  30  supports the insulator  10  by surrounding a body part of the insulator  10  from a part of the large diameter portion  14  to the small diameter portion  16 . The metal shell  30  is made of e.g. low-carbon steel, and is subjected to plating treatment such as nickel plating or zinc plating as a whole. 
     The metal shell  30  has a tool engagement portion  31 , a male thread portion  32 , a seat portion  33 , a protruding portion  34 , a caulking portion  35  and a compressive deformation portion  36 . 
     The tool engagement portion  31  is engaged with a tool (not shown) when connecting the spark plug  100  to the engine head  90 . The male thread portion  32  has threads on an outer peripheral surface of a top end portion of the metal shell  30 , and is screwed into a female thread portion  93  of the engine head  90 . The seat portion  33  is located so as to continue to a rear end side of the male thread portion  32 . and is formed into a brim shape. A ring-shaped gasket  65  formed by folding a plate or a sheet is inserted and fitted between the seat portion  33  and the engine head  90  The protruding portion  34  is formed on an inner peripheral surface of the male thread portion  32  so as to protrude inwards in the radial direction. The holding portion  15  of the insulator  10  abuts on the protruding portion  34  from the rear end side. Therefore, the protruding portion  34  supports the insulator  10  inserted into the axial hole  38 . A ring-shaped plate packing (or a ring-shaped sheet packing) (not shown) is provided between the protruding portion  34  and the holding portion  15 . 
     The caulking portion  35  is formed so that a thickness at the rear end side with respect to the tool engagement portion  31  is thinner. The compressive deformation portion  36  is formed so that a thickness between the tool engagement portion  31  and the seat portion  33  is thinner. Annular ring members  66  and  67  are interposed between the axial hole  38  of the metal shell  30  and an outer peripheral surface of the large diameter portion  14  of the insulator  10  from the tool engagement portion  31  to the caulking portion  35  in the axis direction AD, and a space between these ring members  66  and  67  is filled with powder of talc  69 . As described later, the metal shell  30  is fixed to the insulator  10  by caulking the caulking portion  35 . 
     The ground electrode  40  is made of a bent bar-shaped metal member. Like the center electrode  20 . the ground electrode  40  is made of a nickel alloy containing nickel as a main component. One end of the ground electrode  40  is fixed to a top end surface  37  of the metal shell  30 , and the other end of the ground electrode  40  is bent or curved so as to face a top end portion (or a tip) of the center electrode  20 . The ground electrode  40  is provided, at a portion thereof that faces the tip of the center electrode  20 , with an electrode chip  42 . A gap G 1  for the spark discharge is formed between the electrode chip  42  and the tip of the center electrode  20 . The gap G 1  is also called a discharge gap or a spark gap. 
     The metal terminal  50  is provided at an end portion of a rear end side of the spark plug  100 . A top end side of the metal terminal  50  is accommodated in the penetration hole  11  of the insulator  10 , and a rear end side of the metal terminal  50  is exposed from the penetration hole  11 . A high-tension cable (not shown) is connected to the metal terminal  50 , and high voltage is applied to the metal terminal  50 . The spark discharge occurs at the gap G 1  by this high voltage application. The spark discharge occurring at the gap G 1  ignites air-fuel mixture in a combustion chamber  95 . 
     In the present embodiment, the top end side seal member  61  corresponds to a seal member in the present disclosure. The top end side (or the tip end side) corresponds to an axis direction top end side (or an axis direction tip end side) in the present disclosure, and the rear end side corresponds to an axis direction rear end side in the present disclosure. 
     A method of manufacturing the spark plug  100  will be described below. 
     First, the center electrode  20  is inserted into the penetration hole  11  of the insulator  10  from the rear end side. Subsequently, the penetration hole  11  is filled with the material powder of the top end side seal member  61  from the rear end side, and the material powder of the top end side seal member  61  is compressed from the rear end side (hereinafter, also referred to as “seal member filling process”). After that, the penetration hole  11  is filled with the material of the resistor  62  from the rear end side, and the material of the resistor  62  is compressed from the rear end side. Further, the penetration hole  11  is filled with the material powder of the rear end side seal member  63  from the rear end side, and the material powder of the rear end side seal member  63  is compressed from the rear end side. Each compression of the above could be performed, for instance, by inserting a rod-shaped jig (or a rod-shaped tool) into the penetration hole  11 . Afterwards, an end portion of the top end side of the metal terminal  50  is inserted into the penetration hole  11 , and compression is performed by applying a predetermined pressure from the metal terminal  50  side while heating the insulator  10  as a whole (hereinafter, also referred to as “heating compression process”). Each material filling the penetration hole  11  is compressed and burned by the heating compression process. With this, the top end side seal member  61 , the resistor  62  and the rear end side seal member  63  are formed in the penetration hole  11 . In this manner, the center electrode  20  is fixed to the insulator  10 . 
     Further, the insulator  10  to which the center electrode  20  has been fixed is inserted into the axial hole  38  of the metal shell  30  from the rear end side. Subsequently, by caulking the caulking portion  35  of the metal shell  30 , the metal shell  30  and the insulator  10  are fixed together. At this time, by pressing the caulking portion  35  of the metal shell  30  to the top end side so as to fold the caulking portion  35  inwards in the radial direction, the compressive deformation portion  36  is compressed and deformed. By the compressive deformation of the compressive deformation portion  36 , the insulator  10  is pressed toward the top end side in the metal shell  30  through the ring members  66  and  67  and the talc  69 . In this manner, the spark plug  100  is completed. 
     As illustrated in  FIG.  2   . the center electrode  20  of the present embodiment does not have, at a rear end side thereof with respect to the brim portion  22 , a portion whose diameter is reduced more than the brim portion  22 . In the present embodiment, “does not have the portion whose diameter is reduced” means that when a maximum value of a radius of the brim portion  22  on the cross section including the axis CA is D1 and a minimum value of the radius of the brim portion  22  on the cross section including the axis CA is D2, in a case where the maximum value D1 of the radius of the brim portion  22  is 100%, a difference from the minimum value D2 of the radius of the brim portion  22  is within 6%. That is, the center electrode  20  of the present embodiment satisfies the following expression (1). (D1-D2)/D1 ≤ 0.06 expression (1) 
       FIG.  3    is a schematic diagram for explaining a center  29  of gravity of the center electrode  20 .  FIG.  3    schematically illustrates a structure of an outward appearance of the center electrode  20  viewed from a direction perpendicular to the axis CA. In  FIG.  3   , for convenience in description, the axis CA is shown by a dashed line, and the center  29  of gravity of the center electrode  20 , located on the axis CA, is illustrated . In the present embodiment, the center  29  of gravity is positioned at a top end side in the axis direction AD with respect to the brim portion  22  and the connecting portion  24 . Here, regarding the position of the center  29  of gravity, when a string is tied to the leg portion  21  of the center electrode  20  and the center electrode  20  is strung up or suspended from a vertically upper direction by this string, the position of the center  29  of gravity can be determined from a position of the string in the axis direction AD when the axis CA balances parallel with a horizontal direction. 
     When a size (a length) along the axis direction AD of the center electrode  20  on the cross section including the axis CA is L1 and a size (a length) along the axis direction AD from a boundary  28  between the connecting portion  24  and the leg portion  21  to the center  29  of gravity of the center electrode  20  is L2, the center electrode  20  of the present embodiment satisfies the following expression (2). L2/L1 ≤ 0.30 expression (2) 
     In the present embodiment, the boundary  28  between the connecting portion  24  and the leg portion  21  means a boundary between a top end of the connecting portion  24  and a rear end of the leg portion  21 . In a case of a structure in which the connecting portion  24  and the leg portion  21  are connected in a curved shape, the boundary  28  corresponds to a point (a virtual point) of intersection of a line obtained by extending the connecting portion  24  and a line obtained by extending the leg portion  21  on the cross section including the axis CA. 
     The length L1 in the above expression (2) corresponds to an overall length along the axis direction AD of the center electrode  20 . Further, satisfaction of the expression (2) is equivalent to the fact that when the length L1 along the axis direction AD of the center electrode  20  is 100%, the length L2 along the axis direction AD from the boundary  28  to the center  29  of gravity is within 30%. The center electrode  20  of the present embodiment satisfies the above expression (2), thereby preventing the position of the center  29  of gravity with respect to a position of the connecting portion  24  from being excessively separated toward the top end side in the axis direction AD. 
     Here, as illustrated in  FIG.  2   . the center electrode  20  is supported with the connecting portion  24  abutting on the step portion  17  of the insulator  10 , and fixed to the insulator  10  by the top end side seal member  61  filling the penetration hole  11  and contacting the brim portion  22  and the insulator  10 . Besides, as described above, the center electrode  20  of the present embodiment does not have, at the rear end side thereof with respect to the brim portion  22 , the portion whose diameter is reduced more than the brim portion  22 . That is, the above expression (1) is satisfied. Because of this, as compared with a structure having, at the rear end side thereof with respect to the brim portion  22 . the portion whose diameter is reduced more than the brim portion  22 , i.e. a spark plug not satisfying the above expression (1), a surface area, which contacts the top end side seal member  61 , of the brim portion  22  becomes small. Therefore, in a case of a structure in which the center  29  of gravity of the center electrode  20  is excessively separated from forming positions of the connecting portion  24  and the brim portion  22  toward the top end side, when the spark plug  100  is used with the spark plug  100  connected to the engine head  90  as illustrated in  FIG.  1   , since a distance from the center  29  of gravity to the brim portion  22  is long, the brim portion  22  greatly swings or vibrates due to vibrations of the engine etc.. As a consequence, there is a risk that the top end side seal member  61  will be deformed then looseness of the center electrode  20  will occur. However, according to the spark plug  100  of the present embodiment, since the above expression (2) is satisfied, it is possible to prevent the position of the center  29  of gravity of the center electrode  20  from being located at an excessively top end side. This suppresses the excessive swing or vibration of the brim portion  22  caused by the vibrations of the engine etc. As a result, the occurrence of the looseness of the center electrode  20  can be suppressed. 
     A value of L2/L1 is preferably 0.30 or less, more preferably 0.27 or less, and still more preferably 0.25 or less, in terms of suppressing the occurrence of the looseness of the center electrode  20 . When the value of L2/L1 is 0.25 or less, since the position of the center  29  of gravity of the center electrode  20  can be brought closer to the positions of the connecting portion  24  and the top end side seal member  61 , the occurrence of the looseness of the center electrode  20  can be further suppressed. Further, by a method(s) of (i) as material constituting the brim portion  22 , a substance having a higher specific gravity than that of material constituting the leg portion  21  is used. (ii) the size (the length) of the brim portion  22  is set to be large in the axis direction AD and/or (iii) the size of the brim portion  22  is set to be large in the radial direction, the center  29  of gravity is positioned at a further rear end side, then the value of L2/L1 can be smaller. However, if the brim portion  22  is formed as a separate member, the number of manufacturing processes is increased. Also, if the size of the brim portion  22  is set to be large, as a drawback, an electric capacity is increased. Therefore, in terms of reducing the number of manufacturing processes and suppressing the increase in the electric capacity, the value of L2/L1 is preferably 0 or more, more preferably 0.1 or more, and still more preferably 0.2 or more. In terms of suppressing the occurrence of the looseness of the center electrode  20  and suppressing the increase in the electric capacity, the value of L2/L1 could be, for instance, 0.2 or more and 0.27 or less. Here, in the present application, in a case where the center  29  of gravity is positioned at a rear end side along the axis direction AD with respect to the boundary  28  between the connecting portion  24  and the leg portion  21 , a value of L2 becomes a negative value. 
     As illustrated in  FIG.  3   , the size (the length) L1 of the center electrode  20  of the present embodiment may be, for instance, about 10 mm to 30 mm. Further, a size (a length) L3 along the axis direction AD of the brim portion  22  of the center electrode  20  of the present embodiment may be, for instance, about 1.5 mm to 3.0 mm. When the length L3 is formed to be relatively small, since the increase in the electric capacity can be suppressed, it is possible to suppress exhaustion of the center electrode  20 . 
     A method of setting the value of L2/L1 to 0.30 or less is not particularly limited, but the following method can be raised as examples. For instance, at least a part of the brim portion  22  is formed of material having a higher specific gravity than that of material constituting the center electrode  20 . According to this method, since a change in outer dimensions of the center electrode  20  does not occur, it is possible to suppress an occurrence of a design change of the other constituent members of the spark plug  100  other than the center electrode  20 . As other methods, for instance, sizes (lengths) along the axis direction AD of the brim portion  22  and/or the connecting portion  24  are set to large, or sizes along the radial direction of the brim portion  22  and/or the connecting portion  24  are set to large. 
     According to the spark plug  100  of the present embodiment described above, since the above expression (2) is satisfied, in the center electrode  20  satisfying the above expression (1), it is possible to prevent the position of the center  29  of gravity of the center electrode  20  from being located at an excessively top end side. Therefore, since the position of the center  29  of gravity of the center electrode  20  can be prevented from being excessively separated from the position of the top end side seal member  61  fixing the center electrode  20  and the insulator  10 , it is possible to suppress the occurrence of the looseness of the center electrode  20  caused by the vibrations of the engine etc.. Accordingly, in the spark plug  100  having the center electrode  20  satisfying the above expression (1) and not having, at the rear end side thereof with respect to the brim portion  22 , the portion whose diameter is reduced more than the brim portion  22 , it is possible to suppress an occurrence of a crack around the boundary  28  between the connecting portion  24  and the leg portion  21  of the center electrode  20 . Hence, degradation in performance of the spark plug  100  having the center electrode  20  not having, at the rear end side thereof with respect to the brim portion  22 , the portion whose diameter is reduced more than the brim portion  22  can be suppressed. 
     Further, since the above expression (1) is satisfied, i.e. the center electrode  20  does not have, at the rear end side thereof with respect to the brim portion  22 , the portion whose diameter is reduced more than the brim portion  22 , the length L3 along the axis direction AD of the brim portion  22  can be small. This can suppress the increase in the electric capacity, thereby suppressing the exhaustion of the center electrode  20 . Therefore, according to the spark plug  100  of the present embodiment, since the above expression (1) is satisfied and the above expression (2) is satisfied, it is possible to suppress the occurrence of the looseness of the center electrode  20  while suppressing the increase in the electric capacity. 
     B. Example 
     The present invention will be further described below by examples. However, the present invention is not limited to the following examples. 
     Sample 
     As an example 1, the spark plug  100  having the center electrode  20  satisfying the above expression (1) and the above expression (2) was produced. The value of L2/L1 in the above expression (2) of the spark plug  100  of the example 1 was 0.250. As an example 2. the spark plug  100  having the center electrode  20  satisfying the above expression (1) and the above expression (2) was produced. The value of L2/L1 in the above expression (2) of the spark plug  100  of the example 2 was 0.274 
     As a comparative example 1, a spark plug having a center electrode satisfying the above expression (1) but not satisfying the above expression (2) was produced. The value of L2/L1 in the above expression (2) of the spark plug of the comparative example 1 was 0.351. In addition, as comparative examples 2 and 3, spark plugs each having a center electrode not satisfying the above expression (1) were produced. 
       FIG.  4    is a sectional view schematically showing a configuration of a center electrode  120  of the comparative example 2.  FIG.  4    illustrates a cross section like  FIG.  2    with a brim portion  122  and its vicinity being enlarged. The center electrode  120  provided in the spark plug of the comparative example 2 has, at a rear end side thereof with respect to the brim portion  122 , a diameter-reducing portion  126  whose diameter is reduced more than the brim portion  122 . Because of such a configuration (or a structure), the center electrode  120  of the comparative example 2 does not satisfy the above expression (1). Further, as compared with a size (a length) of a rear end side with respect to the boundary  28  between the connecting portion  24  and the leg portion  21  of the center electrode  20  of the examples 1 and 2 as shown in  FIG.  2   , a size (a length) of a rear end side with respect to a boundary  128  between a connecting portion  124  and a leg portion  121  of the center electrode  120  of the comparative example 2 is large. The center electrode of the comparative example 3 has the same structure of an outward appearance as that of the center electrode  120  of the comparative example 2. 
     Impact Resistance Test 
     Impact resistance test was performed on the spark plugs  100  of the examples 1 and 2 and the spark plugs of the comparative examples 1 to 3. The impact resistance test was carried out using four samples for each of the examples and the comparative examples. The impact resistance test was performed in conformity with a method described in “JIS B 8031: 7.4 impact resistance test”, and impact of vibration amplitude of a stroke  22  (+1/0) mm was applied at a rate of 400 (+20/0) times per minute for 10 (+1/0) minutes. A degree of looseness of each of the center electrodes  20  and 120 of the samples after the test was evaluated. Further, impact resistance test was carried out by the same manner except that the test time was changed to 20 to 60 minutes, and a degree of looseness of each of the center electrodes  20  and 120 of the samples after the test was evaluated. Evaluation criteria is shown below. 
     A: extremely good (no occurrence of the looseness)   B: good (few occurrences of the looseness)   C: not good (many occurrences of the looseness)   

     A result of the impact resistance test and an evaluation result are shown in the following table.  
     
       
         
          TABLE 1
           
               
               
               
               
               
               
               
               
               
               
             
               
                   
                 (D1-D2)/D1 
                 test time (the number of looseness / the number of samples ) 
                 L2/L1 
                 evaluation 
               
               
                 10 min. 
                 20 min. 
                 30 min. 
                 40 min. 
                 50 min. 
                 60 min. 
               
             
            
               
                 example 1 
                 ≤ 0.06 
                 0/4 
                 0/4 
                 0/4 
                 0/4 
                 0/4 
                 0/4 
                 0.250 
                 A 
               
               
                 example 2 
                 ≤ 0.06 
                 0/4 
                 0/4 
                 0/4 
                 ¼ 
                 ¼ 
                 ¼ 
                 0.274 
                 B 
               
               
                 comparative example 1 
                 ≤ 0.06 
                 4/4 
                 - 
                 - 
                 - 
                 - 
                 - 
                 0.351 
                 C 
               
               
                 comparative example 2 
                 &gt; 0.06 
                 0/4 
                 0/4 
                 0/4 
                 0/4 
                 0/4 
                 0/4 
                 0.261 
                 A 
               
               
                 comparative example 3 
                 &gt; 0.06 
                 0/4 
                 0/4 
                 0/4 
                 0/4 
                 0/4 
                 0/4 
                 0.190 
                 A 
               
            
           
         
       
     
     From Table 1, the following can be seen. That is, in the cases of the spark plugs  100  of the examples 1 and 2 satisfying the above expression (1) and the above expression (2), the occurrences of the looseness of the center electrode  20  after the impact resistance test are few, then good results were obtained, as compared with the spark plug of the comparative example 1 satisfying the above expression (1) but not satisfying the above expression (2). 
     More specifically, in the case of the spark plug  100  of the example 1, no looseness of the center electrode  20  was observed in the impact resistance test for 60 minutes, and thus its evaluation result was A. Also, in the case of the spark plug  100  of the example 2, no looseness of the center electrode  20  was observed in the impact resistance test for 30 minutes, and only one looseness of the center electrode  20  was observed in the impact resistance test for 60 minutes, and thus its evaluation result was B. From comparison between the examples 1 and 2, it can be seen that as the value of L2/L1 is smaller, the occurrence of the looseness of the center electrode  20  can be suppressed more. 
     In contrast to this, in the case of the spark plug of the comparative example 1, the looseness of the center electrode occurred in all samples in the impact resistance test for 10 minutes, and thus its evaluation result was C. Regarding the spark plugs of the comparative examples 2 and 3 not satisfying the above expression (1), although their evaluation results were each A, as illustrated in  FIG.  4   , since a size (a length) of the brim portion  122  is large as compared with the size (the length) L3 of the brim portions  22  of the examples 1 and 2, the increase in the electric capacity cannot be suppressed. 
     C. Other Embodiment 
     The present invention is not limited to the above embodiment, and can be realized with various configurations without departing from the scope of the present invention. For instance, technical features in the embodiment corresponding to technical features in each embodiment described in the summary of the invention can be replaced or combined as necessary in order to solve some or all of the problems described above or in order to achieve some or all of the effects described above. Further, if the technical features are not described as an essential in the present specification, it is possible to appropriately delete the technical features. 
     The configuration or structure of the spark plug  100  of the above embodiment is merely an example, and can be variously modified . For instance, the connecting portion  24  has the tapered shape whose outside diameter is gradually reduced toward the top end side, but may be formed along a direction substantially perpendicular to the axis direction AD. Further, the step portion  17  is formed so that the diameter of the penetration hole  11  is smaller toward the top end side along the axis direction AD, but may be formed along a direction substantially perpendicular to the axis direction AD. Even with these configurations, the same effects as those of the above embodiment can be obtained. 
     EXPLANATION OF REFERENCE 
     
         
           10  insulator 
           11  penetration hole 
           14  large diameter portion 
           15  holding portion 
           16  small diameter portion 
           17  step portion 
           20  center electrode 
           21  leg portion 
           22  brim portion 
           24  connecting portion 
           25  core 
           26  electrode member 
           28  boundary 
           29  center of gravity 
           30  metal shell 
           31  tool engagement portion 
           32  male thread portion 
           33  seat portion 
           34  protruding portion 
           35  caulking portion 
           36  compressive deformation portion 
           37  top end surface 
           38  axial hole 
           40  ground electrode 
           42  electrode chip 
           50  metal terminal 
           61  top end side seal member (seal member) 
           62  resistor 
           63  rear end side seal member 
           65  gasket 
           66 ,  67  ring members 
           69  talc 
           90  engine head 
           93  female thread portion 
           95  combustion chamber 
           100  spark plug 
           120  center electrode 
           121  leg portion 
           122  brim portion 
           124  connecting portion 
           126  diameter-reducing portion 
           128  boundary 
         AD axis direction 
         CA axis 
         G 1  gap