阅读说明：本技术 火花塞 (Spark plug ) 是由 井笹纯平 黑野启一 山田裕一 吉田治树 平手将太 于 2019-06-28 设计创作，主要内容包括：本发明提供一种火花塞,可抑制火花塞的不良情况。火花塞具备绝缘体、主体金属件、中心电极及接地电极。绝缘体具有贯通孔、大径部、前端侧主干部及后端侧主干部。中心电极形成位于最前端侧的端点即前端点。在与绝缘中心线垂直的绝缘体的第一剖面上,绝缘体的内周面的中心即第一中心配置于远离绝缘体的外周面的中心即第二中心的位置。在将中心电极的前端点投影到第一剖面上的情况下,投影在第一剖面上的前端点配置成与第一中心相比更靠近第二中心侧。在将穿过第一剖面上的第一中心和第二中心的直线上的绝缘体的壁厚中厚一方的壁厚设为A1、薄一方的壁厚设为A2的情况下,满足5.1≤(A1-A2)/((A1+A2)/2)×100≤28.1。(The invention provides a spark plug, which can inhibit the defect of the spark plug. The spark plug includes an insulator, a main metal member, a center electrode, and a ground electrode. The insulator has a through hole, a large diameter portion, a front end side trunk portion, and a rear end side trunk portion. The center electrode forms an end point located on the most front end side, i.e., a front end point. In a first cross section of the insulator perpendicular to the insulating center line, a first center that is a center of an inner peripheral surface of the insulator is disposed at a position distant from a second center that is a center of an outer peripheral surface of the insulator. In a case where the front end point of the center electrode is projected onto the first cross section, the front end point projected onto the first cross section is arranged closer to the second center side than the first center. When the thicker wall thickness of the insulator is A1 and the thinner wall thickness is A2 on a straight line passing through the first center and the second center on the first cross section, 5.1 ≦ (A1-A2)/((A1+ A2)/2) × 100 ≦ 28.1 is satisfied.)

1. A spark plug is provided with:

a cylindrical insulator having a through hole extending from a rear end side toward a front end side, a large diameter portion which is a portion having the largest outer diameter, a front end side trunk portion connected to the front end side of the large diameter portion and having an outer diameter smaller than the large diameter portion, and a rear end side trunk portion connected to the rear end side of the large diameter portion and having an outer diameter smaller than the large diameter portion;

a cylindrical main body metal member disposed on an outer periphery of the insulator;

a center electrode inserted through a front end side of the through hole; and

a ground electrode connected to the body metal member and forming a discharge gap with the center electrode,

the center electrode forms an end point located on the most front end side that is a front end point,

a first center that is a center of an inner peripheral surface of the insulator is disposed at a position distant from a second center that is a center of an outer peripheral surface of the insulator in a first cross section that is a cross section of the insulator perpendicular to an insulation center line passing through a center axis of an outer peripheral surface of the rear end side trunk portion and a center axis of an outer peripheral surface of the front end side trunk portion and is at a distance of 1mm from a front end of the insulator,

in a case where the front end point of the center electrode is projected onto the first cross section, the front end point projected onto the first cross section is arranged closer to the second center side than the first center,

when the thicker wall thickness of the insulator is A1 and the thinner wall thickness is A2 on a straight line passing through the first center and the second center on the first cross section,

satisfies the conditions of (A1-A2)/((A1+ A2)/2) × 100 ≤ 28.1.

2. The spark plug of claim 1,

the front end face of the center electrode is inclined obliquely with respect to the insulation center line,

the front end point of the center electrode is a portion located on the most front end side on the front end face.

3. The spark plug according to claim 1 or 2,

when the spark plug is projected on a projection plane perpendicular to the insulation center line, when a direction from the insulation center line to a position of a center of gravity of a connection region between the main body metal fitting and the ground electrode on the projection plane is a first direction and a direction from the insulation center line to the first center is a second direction, an angle formed between the first direction and the second direction is 20 degrees or less.

4. The spark plug according to any one of claims 1 to 3,

a third center that is a center of an inner peripheral surface of the insulator is disposed at a position distant from a fourth center that is a center of an outer peripheral surface of the insulator in a second cross section that is a cross section of the insulator perpendicular to the insulation center line and that is a cross section at a position of a center of the large diameter portion,

when the thicker one of the thicknesses of the insulator on a straight line passing through the third center and the fourth center on the second cross section is B1 and the thinner one thereof is B2,

satisfies 0.3 ≦ (B1-B2)/((B1+ B2)/2) × 100 ≦ 9.6.

Technical Field

The present description relates to a spark plug.

Background

Spark plugs have been used for ignition in devices that combust fuel (e.g., internal combustion engines). As the spark plug, the following spark plugs are used: the cylindrical insulator includes a through hole extending in the axial direction, a cylindrical main metal fitting disposed on the outer periphery of the insulator, and a center electrode inserted through the front end side of the through hole. Further, the following techniques have been proposed for providing a spark plug excellent in fouling resistance and preignition resistance. That is, by shifting the axis of the center electrode from the axis of the insulator, the minimum gap x and the maximum gap y are formed between the through hole of the insulator and the center electrode. Furthermore, the gap x and y satisfy the relationship of 0. ltoreq. x/y. ltoreq.0.43, thereby improving the anti-preignition property and the fouling resistance.

Patent document 1: japanese patent laid-open publication No. 2011-34959

Disclosure of Invention

In recent years, a spark plug is required to have a smaller diameter. As the diameter of the spark plug is reduced, the diameter of the insulator may be reduced. When the diameter of the insulator is reduced, various problems may occur. For example, the insulator has a small thickness due to the reduction in diameter. When the insulator has a small wall thickness, a problem may occur. For example, an unexpected discharge may occur that penetrates the insulator.

The present specification discloses a technique capable of suppressing a failure of a spark plug.

The technique disclosed in the present specification can be implemented as the following application example.

[ application example 1]

A spark plug is provided with: a cylindrical insulator having a through hole extending from a rear end side toward a front end side, a large diameter portion which is a portion having the largest outer diameter, a front end side trunk portion connected to the front end side of the large diameter portion and having an outer diameter smaller than the large diameter portion, and a rear end side trunk portion connected to the rear end side of the large diameter portion and having an outer diameter smaller than the large diameter portion; a cylindrical main body metal member disposed on an outer periphery of the insulator; a center electrode inserted through a front end side of the through hole; and a ground electrode connected to the metal body and forming a discharge gap with the center electrode, wherein the center electrode forms a tip point which is an end point located at the most tip end side, the center electrode is arranged such that a first center which is a center of an inner peripheral surface of the insulator is arranged at a position distant from a second center which is a center of an outer peripheral surface of the insulator on a first cross section which is a cross section of the insulator perpendicular to an insulation center line and which is a distance of 1mm from a tip end of the insulator, the insulation center line passes through a center axis of an outer peripheral surface of the rear end side trunk portion of the insulator and a center axis of an outer peripheral surface of the tip side trunk portion, and when the tip point of the center electrode is projected on the first cross section, the tip point projected on the first cross section is arranged closer to the second center side than the first center, when the thicker wall thickness of the insulator and the thinner wall thickness of the insulator are A1 and A2 on a straight line passing through the first center and the second center on the first cross section, 5.1 ≦ (A1-A2)/((A1+ A2)/2) × 100 ≦ 28.1 is satisfied.

According to this configuration, since the center electrode forms the tip point located at the tip end side, discharge is likely to occur at the tip point of the center electrode. This can improve the ignitability. Further, since the first center of the inner peripheral surface of the insulator is disposed at a position distant from the second center of the outer peripheral surface of the insulator in the first cross section, the thickness of the insulator in the first cross section varies depending on the position in the circumferential direction. The tip end point of the center electrode projected on the first cross section is disposed closer to the second center side of the outer peripheral surface of the insulator than the first center of the inner peripheral surface of the insulator. That is, the tip of the center electrode, at which discharge is likely to occur, is disposed in the vicinity of a thick portion of the insulator. When the temperature of the insulator is high, discharge that penetrates the insulator is likely to occur. In the above structure, since the discharge is generated in the vicinity of the thick portion in the insulator, the portion in the insulator where the temperature is likely to rise is the thick portion. Therefore, compared to a case where the temperature of a thin portion is likely to increase, the discharge through the insulator can be suppressed. In particular, when the thickness of the first cross section is A1 for the thicker side and A2 for the thinner side, it satisfies 5.1. ltoreq. A1-A2/((A1 + A2)/2). times.100. ltoreq.28.1. Therefore, the discharge through the insulator can be appropriately suppressed.

[ application example 2]

In the spark plug according to application example 1, a tip end surface of the center electrode is inclined obliquely with respect to the insulating center line, and the tip end point of the center electrode is a portion located on a most tip end side on the tip end surface.

According to this configuration, it is possible to suppress the shape of the center electrode from becoming complicated and to suppress the defects of the insulator.

[ application example 3]

In the spark plug according to application example 1 or 2, when the spark plug is projected on a projection plane perpendicular to the insulated center line, when a direction from the insulated center line toward a position of a center of gravity of a connection region between the body metal fitting and the ground electrode on the projection plane is a first direction and a direction from the insulated center line toward the first center is a second direction, an angle formed between the first direction and the second direction is 20 degrees or less.

According to this configuration, since the portion that is easily discharged on the ground electrode is prevented from being dispersed, the decrease in ignitability can be suppressed.

[ application example 4]

In the spark plug according to any one of application examples 1 to 3, a third center, which is a center of the inner peripheral surface of the insulator, is located at a position distant from a fourth center, which is a center of the outer peripheral surface of the insulator, in a second cross section, which is a cross section of the insulator perpendicular to the insulation center line and is a center of the large diameter portion, and when a thicker wall of the insulator on a straight line passing through the third center and the fourth center in the second cross section is B1 and a thinner wall thereof is B2, 0.3 ≦ B1-B2/(B1 + B2)/2) × 100 ≦ 9.6 is satisfied.

According to this configuration, the large diameter portion can be suppressed from being damaged, compared to a case where the through hole in the large diameter portion of the insulator is largely displaced in position.

The technology disclosed in the present specification can be implemented in various forms, for example, in a spark plug, an ignition device using a spark plug, an internal combustion engine equipped with the spark plug, an internal combustion engine equipped with an ignition device using the spark plug, and the like.

Drawings

Fig. 1 is a sectional view of a spark plug 100 as an embodiment.

Fig. 2(a) is a partial sectional view of the spark plug 100. (B) A cross-sectional view of the insulator 10.

Fig. 3 is a sectional view of the insulator 10.

Fig. 4(a) and (B) are schematic diagrams of the spark plug 100 projected on the projection plane Sp.

Fig. 5 is a table showing the correspondence between the structure of the sample of the spark plug 100 and the evaluation result.

Detailed Description

A. The implementation mode is as follows:

A1. structure of spark plug:

fig. 1 is a sectional view of a spark plug 100 as an embodiment. The figure shows a center axis CL (also referred to as "axis CL") of the spark plug 100 and a flat cross section including the center axis CL of the spark plug 100. Hereinafter, the direction parallel to the center axis CL is also referred to as "the direction of the axis CL", or simply as "the axial direction". The radial direction of the circle centered on the axis CL is also referred to as "radial direction". The radial direction is a direction perpendicular to the axis CL. The circumferential direction of a circle centered on the axis CL is also referred to as "circumferential direction". The downward direction in fig. 1 in the direction parallel to the center axis CL is referred to as the front end direction Df or the forward direction Df, and the upward direction is also referred to as the rear end direction Dfr or the rear direction Dfr. The distal end direction Df is a direction from the terminal fitting 40 described later toward the center electrode 20. The front end direction Df in fig. 1 is referred to as the front end side of the spark plug 100, and the rear end direction Dfr in fig. 1 is referred to as the rear end side of the spark plug 100.

The spark plug 100 includes: a cylindrical insulator 10 having a through hole 12 (also referred to as a shaft hole 12) extending from the rear Dfr side toward the front Df side; a center electrode 20 held at the tip end side of the through hole 12; a terminal fitting 40 held at the rear end side of the through hole 12; a resistor 73 disposed between the center electrode 20 and the terminal fitting 40 in the through hole 12; a conductive first sealing portion 72 that is in contact with and electrically connected to the center electrode 20 and the resistor 73; a conductive second sealing portion 74 which is in contact with and electrically connected to the resistor 73 and the terminal fitting 40; a cylindrical body fitting 50 fixed to the outer peripheral side of the insulator 10; and a ground electrode 30 having one end joined to the annular front end surface 55 of the main metal fitting 50 and the other end facing the center electrode 20 with a discharge gap g therebetween.

A large diameter portion 14, which is a portion having the largest outer diameter, is formed at the center of the insulator 10. A rear end side trunk portion 13 having an outer diameter smaller than that of the large diameter portion 14 is connected to the rear direction Dfr side of the large diameter portion 14. At the connecting portion 18 of the large diameter portion 14 and the rear end side trunk portion 13, the outer diameter gradually becomes smaller toward the rear direction Dfr (the connecting portion 18 is also referred to as a reduced diameter portion 18).

A distal end side trunk portion 15 having an outer diameter smaller than that of the large diameter portion 14 is connected to the front Df side of the large diameter portion 14. A leg portion 19 having an outer diameter smaller than the outer diameter of the distal end side trunk portion 15 is connected to the distal end side trunk portion 15 in the front direction Df. The leg 19 is a portion including the front end of the insulator 10. At a connecting portion 16 of the leading end side trunk portion 15 and the leg portion 19, the outer diameter gradually becomes smaller toward the front direction Df (the connecting portion 16 is also referred to as a reduced diameter portion 16 or a stepped portion 16). Further, the distal-end-side barrel portion 15 is provided with a reduced inner diameter portion 11. The inner diameter of the reduced inner diameter portion 11 is gradually reduced toward the front direction Df.

It is preferable that the insulator 10 is formed in consideration of mechanical strength, thermal strength, and electrical strength. The insulator 10 is formed by firing alumina, for example (other insulating materials may be used).

In the present embodiment, the outer peripheral surface 13o of the portion of the rear end-side barrel portion 13 of the insulator 10 connected to the large diameter portion 14 and the outer peripheral surface 15o of the distal end-side barrel portion 15 are each substantially cylindrical. The center line CL10 in the drawing is a center line (hereinafter referred to as an insulation center line CL10) passing through the center axis of the outer peripheral surface 13o of the rear end side barrel portion 13 and the center axis of the outer peripheral surface 15o of the front end side barrel portion 15. In the present embodiment, the insulation center line CL10 is the same as the axis line CL of the spark plug 100. In the present embodiment, the through hole 12 of the insulator 10 is formed at a position offset from the insulation center line CL 10. In the cross-sectional view of fig. 1, the through-hole 12 is formed at a position shifted to the right. The through-hole 12 is inclined obliquely with respect to the insulation center line CL 10. The reason why the insulator 10 has such a shaft hole 12 will be described later.

The center electrode 20 is a metal member and is disposed at an end portion on the Df side in the front direction in the through hole 12 of the insulator 10. The center electrode 20 has a rod portion 28. The rod 28 has a head 24, which is a portion on the rear Dfr side, and a shaft 27 connected to the front Df side of the head 24. The shaft portion 27 has a substantially cylindrical shape extending in the forward direction Df. The portion of the head portion 24 on the front Df side is formed with a flange portion 23 having an outer diameter larger than the outer diameter of the shaft portion 27. The front Df side surface of the flange 23 is supported by the reduced diameter portion 11 of the insulator 10. The shaft portion 27 is connected to the front Df side of the flange portion 23.

The rod 28 has an outer layer 21 and a core 22 disposed on the inner periphery side of the outer layer 21. The outer layer 21 is formed of a material (for example, an alloy containing nickel as a main component) having better oxidation resistance than the core 22. Here, the main component means a component having the highest content (weight percent). The core 22 is formed of a material having higher thermal conductivity than the outer layer 21 (for example, pure copper, an alloy containing copper as a main component, or the like). A portion of the center electrode 20 on the rear Dfr side is disposed in the axial hole 12. A portion of the center electrode 20 on the front Df side is exposed from the axial hole 12 of the insulator 10 on the front Df side. Thus, the center electrode 20 is inserted to the tip side of the through hole 12 of the insulator 10. Further, a part of the center electrode 20 on the front direction Df side protrudes further toward the front direction Df than the front end of the insulator 10. Further, the core 22 may be omitted.

The terminal fitting 40 is a rod-shaped member extending from the rear Dfr side toward the front Df side. The terminal fitting 40 is formed using a conductive material (for example, a metal containing iron as a main component). The rod-like portion 41 on the front Df side of the terminal fitting 40 is inserted into the rear Dfr side portion of the axial hole 12 of the insulator 10.

The resistor 73 in the through hole 12 of the insulator 10 is a member for suppressing electrical noise. The resistor 73 is formed using a mixture of glass, a conductive material (for example, carbon particles), and ceramic particles, for example. The sealing portions 72 and 74 are formed using a mixture of a conductive material (for example, metal particles such as copper and iron) and glass. The center electrode 20 is electrically connected to the terminal fitting 40 through the first seal portion 72, the resistor 73, and the second seal portion 74.

The body metal 50 is a cylindrical member having a through hole 59 extending along the axis CL. In the present embodiment, the central axis of the main body metal fitting 50 is the same as the axis CL. The insulator 10 is inserted into the through hole 59 of the metal shell 50, and the metal shell 50 is fixed to the outer periphery of the insulator 10. The main metal 50 is formed using an electrically conductive material (for example, a metal such as carbon steel containing iron as a main component). A portion of the insulator 10 on the front Df side is exposed out of the through hole 59. Further, a part of the insulator 10 on the rear Dfr side is exposed to the outside of the through hole 59.

The body metal fitting 50 includes a tool engagement portion 51, a middle trunk portion 54, and a distal-side trunk portion 52. The tool engagement portion 51 is a portion into which a wrench (not shown) for a spark plug is fitted. The intermediate trunk portion 54 is a flange-like portion that is disposed on the front side Df of the tool engagement portion 51 and extends radially outward. A front Df side surface 54f of the middle trunk 54 is a seating surface, and forms a seal with a mounting portion (for example, an engine cover) that is a portion of the internal combustion engine where the mounting hole is formed. The distal-side trunk portion 52 is a portion connected to the front Df side of the middle trunk portion 54, and includes a distal end face 55 of the body metal 50. A screw portion 57 is provided on the outer peripheral surface of the distal-side trunk portion 52, and the screw portion 57 is a portion in which a male screw for screwing with a mounting hole of an internal combustion engine, not shown, is formed.

A support portion 56 that extends radially inward is formed on the distal end side trunk portion 52 of the main body metal fitting 50. The inner diameter of the surface 56r (also referred to as a rear surface 56r) on the rear Dfr side of the support portion 56 gradually decreases in the forward direction Df. The front end side seal 8 is interposed between the rear surface 56r of the support portion 56 and the reduced diameter portion 16 of the insulator 10. The support portion 56 indirectly supports the step portion 16 of the insulator 10 via the seal 8.

A rear end portion 53 is formed on the rear end side of the tool engagement portion 51 of the main metal fitting 50, and the rear end portion 53 forms the rear end of the main metal fitting 50 and is a portion having a smaller thickness than the tool engagement portion 51. Further, a connecting portion 58 for connecting the intermediate stem portion 54 and the tool engagement portion 51 is formed between the intermediate stem portion 54 and the tool engagement portion 51. The connecting portion 58 is thinner than the thickness of each of the middle trunk portion 54 and the tool engagement portion 51. Annular ring members 61 and 62 are inserted between the inner peripheral surface of the body metal 50 from the tool engagement portion 51 to the rear end portion 53 and the outer peripheral surface of the portion of the insulator 10 on the rear side Dfr side of the reduced diameter portion 18. Further, talc 70 powder is filled between the ring members 61 and 62. In the manufacturing process of the spark plug 100, when the rear end portion 53 is bent inward and clamped, the connecting portion 58 is deformed, and as a result, the main metal fitting 50 and the insulator 10 are fixed. The talc 70 is compressed during the clamping process, and improves the airtightness between the metal shell 50 and the insulator 10. The seal 8 is pressed between the reduced diameter portion 16 of the insulator 10 and the support portion 56 of the metal shell 50, and seals between the metal shell 50 and the insulator 10.

The ground electrode 30 is a metal member and has a rod-shaped body 37. An end portion 33 (also referred to as a base end portion 33) of the body portion 37 is joined to a tip end surface 55 of the metal body 50 by resistance welding. The body 37 extends from the base end 33 joined to the body metal fitting 50 in the distal direction Df, is bent toward the central axis CL, extends in a direction intersecting the axis CL, and reaches the distal end 34. The front end 34 of the ground electrode 30 forms a discharge gap g with the center electrode 20.

The body portion 37 includes an outer layer 31 and an inner layer 32 disposed on the inner peripheral side of the outer layer 31. The outer layer 31 is formed of a material (for example, an alloy containing nickel as a main component) having better oxidation resistance than the inner layer 32. The inner layer 32 is formed of a material having higher thermal conductivity than the outer layer 31 (for example, pure copper, an alloy containing copper as a main component, or the like). Further, the inner layer 32 may be omitted.

Fig. 2(a) is a partial cross-sectional view of a portion of the spark plug 100 on the front Df side, including the insulation center line CL 10. In the partial sectional view, the illustration of the internal structure of the ground electrode 30 and the illustration of the internal structure of the center electrode 20 are omitted. In the figure, the insulation center line CL10 is indicated by a vertical line. In the present embodiment, the through-hole 12 is inclined obliquely with respect to the insulation center line CL10 in the leg portion 19 of the insulator 10. The deviation of the through-hole 12 from the insulation center line CL10 gradually increases in the forward direction Df. In the cross-sectional view of fig. 2(a), the portion of the through-hole 12 on the front Df side is located on the right side of the portion on the rear Dfr side.

The front end point 20f of the center electrode 20 is an end point located on the most front direction Df side in the center electrode 20. In the present embodiment, the center electrode 20 has a substantially cylindrical shape. The center electrode 20 is disposed to extend along the through hole 12. Like the through hole 12, the center electrode 20 is inclined obliquely with respect to the insulation center line CL 10. The end surface 20fs on the front direction Df side of the center electrode 20 is not perpendicular to the insulation center line CL10, but is inclined obliquely with respect to the insulation center line CL 10. In the cross-sectional view of fig. 2(a), the left side portion of the front end surface 20fs is located on the front direction Df side of the right side portion. The front end point 20f is a portion on the front end surface 20fs located on the most forward direction Df side. Such a front end point 20f is located at the edge of the front end face 20 fs. In the present embodiment, the front end portion 34 of the ground electrode 30, which forms the discharge gap g, is disposed on the front side Df of the center electrode 20. Therefore, the portion of the center electrode 20 closest to the ground electrode 30 is the leading end point 20 f. The discharge is easily generated on a path (for example, path PT) passing through the front end point 20 f. In this way, the portion of the center electrode 20 where the end of the discharge path is easily formed is not a surface, but a small portion, that is, the front end point 20 f. Therefore, discharge is more likely to occur than in the case where the portion of the center electrode 20 where the end portion of the discharge path is more likely to be formed is dispersed over a wider surface. As a result, the ignitability can be improved.

In the cross-sectional view of fig. 2(a), the rear surface 30rs, which is the surface of the front end portion 34 of the ground electrode 30 on the rear Dfr side, is disposed on the front Df side of the front end surface 20fs of the center electrode 20 and faces the front end surface 20 fs. In the present embodiment, the rear surface 30rs is a plane perpendicular to the insulation center line CL 10. The end surface 30e of the ground electrode 30 is an end surface opposite to the end surface connected to the metal shell 50, out of both end surfaces of the rod-shaped ground electrode 30. The portion 30r is an angle formed by the rear surface 30rs and the end surface 30e, and is an edge of the rear surface 30rs (also referred to as an edge portion 30 r). In the present embodiment, in the cross-sectional view of fig. 2 a, the edge portion 30r of the ground electrode 30 is located on the same side as the leading end point 20f of the center electrode 20 with respect to the insulation center line CL10 (on the left side of the insulation center line CL10 in fig. 2 a). Thus, the edge portion 30r of the ground electrode 30 is disposed near the front end point 20f of the center electrode 20. Generally, discharge is easily generated in the tapered portion of the electrode. Therefore, in the present embodiment, the discharge is likely to occur on the path PT connecting the front end point 20f of the center electrode 20 and the edge portion 30r of the ground electrode 30.

Fig. 2(B) is a sectional view of the insulator 10. A first cross section CS1 of the insulator 10 is shown in the figure. The first cross section CS1 is a cross section perpendicular to the insulation center line CL 10. The first cross section CS1 (fig. 2 a) is a cross section at a position where the distance D1 from the front end 10f of the insulator 10 toward the rear direction Dfr side is 1 mm.

In the first cross section CS1 of fig. 2(B), the inner circumferential surface S1 is the inner circumferential surface of the insulator 10 and is the surface on which the through-hole 12 is formed. The outer peripheral surface S2 is the outer peripheral surface of the insulator 10. In the first cross section CS1, the inner circumferential surface S1 and the outer circumferential surface S2 have a substantially circular shape. The first center C1 is the center of the inner peripheral surface S1. The second center C2 is the center of the outer circumferential surface S2. As shown, on the first cross-section CS1, the first center C1 is disposed away from the second center C2.

The first straight line L1 in the figure is a straight line passing through the centers C1, C2 on the first section CS 1. The wall thicknesses a1, a2 are the wall thicknesses of the insulator 10 on the first straight line L1. Specifically, the thicknesses a1 and a2 are lengths of the portions of the first straight line L1 that overlap the insulator 10. The first wall thickness a1 is a thicker wall thickness and the second wall thickness a2 is a thinner wall thickness. The first center C1 is located away from the second center C2, and thus the first wall thickness a1 is different from the second wall thickness a 2(a 1 > a 2). The degree of such unevenness in wall thickness, that is, the degree of deviation of the centers C1 and C2 can be evaluated using, for example, the following first degree of deviation Ax.

Ax＝(A1-A2)/((A1+A2)/2)×100

The first degree of deviation Ax is the ratio of the difference in wall thickness (A1-A2) to the average wall thickness ((A1+ A2)/2) (in%). The greater the first deviation Ax, the greater the degree of unevenness in wall thickness.

The front end point 20f in fig. 2(B) represents the position of the front end point 20f when the front end point 20f is projected on the first cross section CS1 by being moved parallel to the insulation center line CL 10. The front end point 20f projected on the first section CS1 is located at a position closer to the second center C2 side than the first center C1. A vertical line L1x in the drawing is a straight line passing through the first center C1 and perpendicular to the first straight line L1. When the plane including the first cross-section CS1 is divided into two regions AR1 and AR2 by the vertical line L1x, the front end point 20f is located closer to the second center C2 side than the first center C1 when the front end point 20f is included in the region AR1 including the second center C2. The direction Db in the drawing is a direction from the insulation center line CL10 toward the first center C1 on the first cross section CS 1. In the example of fig. 2(B), the tip end point 20f of the center electrode 20 is located on the opposite side of the direction Db with respect to the insulation center line CL 10.

The temperature of the insulator 10 is raised by heat from the fire generated by the discharge. When the temperature of the insulator 10 is high, discharge penetrating the insulator 10 is likely to occur. For example, as shown by the virtual path PTx in fig. 2(a), discharge is likely to occur in a path that penetrates the leg 19 of the insulator 10 and connects the main body metal 50 and the center electrode 20. In the present embodiment, since the discharge is generated in the vicinity of the front end point 20f of the center electrode 20, the inflammation is likely to spread in the vicinity of the front end point 20 f. Moreover, the temperature of the portion of the insulator 10 near the front end point 20f is easily increased. For example, in the embodiment of fig. 2(a), the temperature of the portion of the insulator 10 on the left side close to the front end point 20f is likely to be higher than the temperature of the portion on the right side far from the front end point 20 f.

In the cross-sectional view of fig. 2(B), when the first center C1 and the second center C2 are assumed to be the same, the wall thickness of the insulator 10 is substantially the same regardless of the circumferential position, specifically, the average value ((a1+ a2)/2) of the wall thicknesses a1 and a 2. In the present embodiment, the wall thickness a1 of the portion of the insulator 10 near the front end point 20f is thicker than the wall thickness assumed in the case where the first center C1 and the second center C2 are the same. Therefore, even in the case where the temperature of the portion of the insulator 10 near the front end point 20f is high, the discharge penetrating the insulator 10 is suppressed.

As described above, the front end surface 20fs of the center electrode 20 is inclined obliquely with respect to the insulation center line CL 10. Therefore, the center electrode 20 can be prevented from becoming complicated in shape and the center electrode 20 can form the front end point 20 f. As a result, the center electrode 20 having a simple structure can be used to suppress the discharge that penetrates the insulator 10.

Fig. 3 is a cross-sectional view of insulator 10 including an insulation centerline CL 10. The outer peripheral surface 14o in the figure is the outer peripheral surface of the large diameter portion 14. The outer peripheral surface 14o of the large diameter portion 14 has a substantially circular shape in a cross section perpendicular to the insulation center line CL 10. In such a cross section, the outer diameter of the large diameter portion 14 is larger than the outer diameter of the other portion of the insulator 10.

The front end 14f and the rear end 14r of the large diameter portion 14 are shown in the figure. The front end 14f is an end of the large diameter portion 14 on the front direction Df side, and the rear end 14r is an end of the large diameter portion 14 on the rear direction Dfr side. The center position 14m is a position at the center of the large diameter portion 14 and bisects the range from the rear end 14r to the front end 14 f. A second cross section CS2 of the insulator 10 is shown on the right in fig. 3. The second cross section CS2 is a cross section of the insulator 10 perpendicular to the insulation center line CL10 and is a cross section at a position 14m at the center of the large diameter portion 14.

In the second cross section CS2, the inner peripheral surface 14i is the inner peripheral surface of the insulator 10 and is the surface on which the through-hole 12 is formed. The outer peripheral surface 14o is the outer peripheral surface of the insulator 10. In the second cross section CS2, the inner peripheral surface 14i and the outer peripheral surface 14o each have a substantially circular shape. The third center C3 is the center of the inner peripheral surface 14 i. The fourth center C4 is the center of the outer peripheral surface 14 o. As shown, on the second cross-section CS2, the third center C3 is disposed away from the fourth center C4.

The second straight line L2 is a straight line passing through the centers C3, C4 on the second section CS 2. The wall thicknesses B1, B2 are the wall thicknesses of the insulator 10 on the second straight line L2. That is, the thicknesses B1 and B2 are lengths of portions of the second straight line L2 that overlap the insulator 10. The first wall thickness B1 is a thicker wall thickness and the second wall thickness B2 is a thinner wall thickness. Since the third center C3 is disposed at a position distant from the fourth center C4, the first wall thickness B1 is different from the second wall thickness B2 (B1 > B2). The degree of such unevenness in wall thickness, that is, the degree of displacement of the centers C3 and C4 can be evaluated using, for example, the following second degree of displacement Bx.

Bx＝(B1-B2)/((B1+B2)/2)×100

The second degree of offset Bx is the ratio (in%) of the difference in wall thickness (B1-B2) to the average wall thickness ((B1+ B2)/2). The larger the second deviation Bx, the larger the degree of unevenness in wall thickness.

Fig. 4(a) and 4(B) are schematic diagrams of the spark plug 100 projected on the projection plane Sp. The projection plane Sp is a plane perpendicular to the insulation center line CL 10. Each of the drawings shows a part of the spark plug 100 projected when the spark plug 100 is projected onto the projection plane Sp by being moved parallel to the insulation center line CL 10. Specifically, the front end face 55 of the body metal 50, the first cross section CS1 (fig. 2(B)) of the insulator 10, and the ground electrode 30 are shown. The connection region 350 shown in hatched lines shows a portion where the base end portion 33 of the ground electrode 30 and the front end surface 55 of the main metal piece 50 are connected to each other. In the present embodiment, the ground electrode 30 has a rectangular cross-sectional shape. The shape of the connection region 350 is substantially the same as the shape of the cross section of the ground electrode 30. The center of gravity 350c in the figure is the center of gravity of the connection region 350. The center of gravity of the region is a position of the center of gravity in the case where the mass is assumed to be distributed uniformly in the region.

The first direction Da is a direction from the insulation center line CL10 toward the center of gravity 350c on the projection plane Sp. The second direction Db is a direction from the insulation center line CL10 toward the first center C1 on the projection plane Sp. The angle Ang is an angle formed by the first direction Da and the second direction Db. Fig. 4(a) shows a case where the angle Ang is small, and fig. 4(B) shows a case where the angle Ang is large.

The ground electrode 30 extends from the connection region 350 in a direction opposite to the first direction Da. As shown in fig. 2(a), the ground electrode 30 has a rear surface 30rs which is a surface facing the front end surface 20fs of the center electrode 20. As shown in fig. 4(a) and the like, the surface 30rs is a surface intersecting the insulation center line CL 10. Therefore, the edge portion 30r of the ground electrode 30 is located on the opposite side of the first direction Da with respect to the insulation center line CL 10. The leading end 20f of the center electrode 20 is located on the opposite side of the second direction Db with respect to the insulation center line CL 10. Therefore, when the angle Ang is small as shown in fig. 4(a), the edge portion 30r of the ground electrode 30 is disposed in the vicinity of the front end point 20f of the center electrode 20. When the angle Ang is large as shown in fig. 4(B), the edge 30r of the ground electrode 30 is disposed at a position away from the front end point 20f of the center electrode 20, and the rear surface 30rs faces the front end point 20 f.

When the angle Ang is small as shown in fig. 4(a), the portion of the ground electrode 30 where the end of the discharge path is likely to be formed is a small edge 30 r. On the other hand, in the case where the angle Ang is large as in fig. 4(B), a portion at which the end of the discharge path is easily formed on the ground electrode 30 may be scattered on the rear surface 30 rs. As described above, the smaller the angle Ang, the more the portion of the ground electrode 30 that is likely to discharge is prevented from being dispersed, and thus the reduction in ignitability can be prevented.

As a method for manufacturing the insulator 10, various methods can be employed. For example, the insulator 10 may be manufactured by molding an unfired molded body using a molding die and then firing the molded body. As the molding die, a first molding die for forming the outer peripheral surface of the insulator 10 and a rod-shaped second molding die for forming the through-hole 12 may be used. Further, the unfired insulator 10 can be molded by offsetting the position of the second molding die with respect to the first molding die from a position centered on the axis of the first molding die corresponding to the insulation center line CL 10. For example, the second molding die may be configured to be inclined with respect to an axis corresponding to the insulation center line CL 10. In this way, the insulator 10 having the uneven wall thickness on the first cross section CS1 shown in fig. 2(B) can be molded by using a molding die. In the case where the molding die is used in this manner, the insulator 10 may have an uneven wall thickness even in the second cross section CS2 shown in fig. 3. As a method for manufacturing the other part of the spark plug 100, a known method can be adopted.

B. Evaluation test:

fig. 5 is a table TB showing the correspondence between the structure of the sample of the spark plug 100 and the evaluation result. The table TB shows the correspondence among the sample numbers, the first deviation degree Ax, the second deviation degree Bx, the angle Ang (in degrees (fig. 4)), the evaluation result R1 of withstand voltage, the evaluation result R2 of compressive strength, and the evaluation result R3 of ignitability. In this evaluation test, 15 samples in which combinations of the first degree of deviation Ax, the second degree of deviation Bx, and the angle Ang are different from each other were evaluated. The threaded portion 57 of the body metal 50 of each sample had a nominal diameter of M8.

The outline of the withstand voltage test is as follows. After 10 specimens of nos. 1 to 15 were prepared, each of the prepared specimens was assembled in a four-cylinder DOHC engine having an exhaust gas amount of 0.66L, and then the engine was operated at a rotation speed of 3200rpm for 10 minutes. Then, the total number of samples, from among the 10 samples, for which penetration due to discharge was not confirmed at the leading end portion (here, the leg portion 19) of the insulator 10, was used as the number of evaluation results R1 of withstand voltage. For example, when 3 out of 10 specimens were confirmed to have passed through, the evaluation result R1 was "7". Thus, the larger the number of the evaluation results R1, the better the withstand voltage performance.

The outline of the compressive strength test is as follows. The large diameter portion 14 of each sample was compressed via hard fibers. Then, the large diameter portion 14 was visually observed, and the compression load (in MPa) when breakage or cracking was confirmed was measured. This compressive load was used as an evaluation result R2 of compressive strength. Thus, the strength is improved as the evaluation result R2 (compressive load) is larger. As the hard fibers, hard fibers having a hardness of 2.0t were used, and the compression rate was set to 5 mm/min.

The outline of the ignitability test is as follows. Samples nos. 1 to 15 were prepared in 12 pieces each. In addition, a test automobile having a six-cylinder DOHC gasoline engine with an exhaust gas volume of 2000cc was prepared. 6 of the 12 samples were assembled with the engine of the test automobile. Then, the engine was operated in an operating state (specifically, at a rotational speed of 2000rpm) at a speed corresponding to 60km/h under the condition that the air-fuel ratio (A/F) was 23.6. The number of misfires for 1000 applications of the voltage for discharge was counted for each sample. 12 specimens were tested by performing 2 such tests. Then, the total number of samples having the misfire count of 10 or more out of the 12 samples, that is, the misfire sample number Ns, is determined. The number of the results of the evaluation of ignitability R3 is determined as follows using the number of misfire samples Ns.

Ns：R3

0：10

1：9

2：8

3：7

4：6

5：5

6：4

7：3

8：2

9：1

10：1

11：1

12：1

As described above, the larger the number of the evaluation results R3, the better the ignitability.

In the spark plug 100 used in the evaluation test, the outer surface of the insulator 10 was not coated with a glaze. Further, the outer surface of the insulator may be coated with a glaze. In this case, the thicknesses a1, a2, B1, and B2 (or the degrees of displacement Ax and Bx) are determined by the insulator from which the glaze has been removed.

The evaluation result R1 of the withstand voltage of the distal end portion of the insulator 10 is greatly influenced from a first deviation degree Ax indicating the degree of deviation of the wall thickness of the distal end portion of the insulator 10. The first degree of deflection Ax of the samples tested was, in order from small to large, 4.5, 5.1, 10.5, 12.3, 13.2, 16.4, 20.9, 25.6, 26.0, 28.1, 30.2. The relationship between the first shift degree Ax, the evaluation result R1, and the sample number is as follows if the first shift degree Ax is arranged in descending order of magnitude.

Ax 4.5: r1 ═ 5: no. 7

Ax 5.1: r1 ═ 6: number 5

Regarding nos. 12, 13, 4, 11, 3, 10, 2, 9, 14, and 8, Ax is 10.5 or more and 26.0 or less, and R1 is 6 or more and 10 or less.

Ax 28.1: r1 ═ 6: no. 1, No. 15

Ax is 30.2: r1 ═ 5: number 6

When the first deviation Ax is 4.5, the evaluation result R1 is 5 or less. The reason why the withstand voltage performance is reduced when the deviation of the wall thickness of the insulator 10 in the first cross section CS1 (fig. 2(B)) is small is estimated as follows. That is, when the variation in thickness is small, the thickness a1 of the thick portion is insufficient, and therefore, the discharge penetrating the leg portion 19 of the insulator 10 is likely to occur.

In the case where the first deviation degree Ax is 30.2, the evaluation result R1 is 5. The reason why the withstand voltage performance is lowered when the thickness variation is large is presumed as follows. That is, when the variation in thickness is large, the thickness a2 of the thin portion is too thin, and therefore, the discharge that penetrates the thin portion of the insulator 10 is likely to occur.

The first degree of deviation Ax of the R1 was 5.1, 10.5, 12.3, 13.2, 16.4, 20.9, 25.6, 26.0, 28.1, which achieved a good evaluation result of 6 or more. The above-mentioned 9 values can also be used to determine a preferred range of the first degree of deviation Ax. Specifically, any of the 9 values may be used as the lower limit of the preferable range of the first degree of displacement Ax. For example, the first degree of offset Ax may be 5.1 or more. Any value of these values that is not lower than the lower limit may be used as the upper limit of the first shift degree Ax. For example, the first degree of offset Ax may be 28.1 or less. When the first deviation Ax is within the preferable range, the discharge penetrating the insulator 10 can be appropriately suppressed.

In the spark plug 100 of fig. 1, 2(a), and 2(B), the center electrode 20 forms a tip point 20f, which is an end point located on the most tip side. Therefore, the center electrode 20 is likely to generate discharge as compared with a case where the portion where the end portion of the discharge path is likely to be formed is dispersed over a wide surface. As a result, the ignitability can be improved. Further, in the first cross section CS1 (fig. 2B), the first center C1 of the inner peripheral surface S1 of the insulator 10 is located at a position distant from the second center C2 of the outer peripheral surface S2 of the insulator 10, and therefore the thickness of the insulator 10 in the first cross section CS1 changes depending on the position in the circumferential direction. Further, the front end point 20f of the center electrode 20 projected on the first cross section CS1 is disposed closer to the second center C2 side of the outer circumferential surface S2 of the insulator 10 than the first center C1 of the inner circumferential surface S1 of the insulator 10. That is, the front end point 20f of the center electrode 20, at which discharge is likely to occur, is disposed in the vicinity of a thick portion of the insulator 10. When the temperature of the insulator 10 is high, discharge penetrating the insulator 10 is likely to occur. In the above structure, since the discharge is generated in the vicinity of the thick portion in the insulator, the portion in the insulator where the temperature is likely to rise is the thick portion. Therefore, as compared with a case where the temperature of the thin portion is likely to increase, the discharge penetrating through the insulator 10 can be suppressed. In particular, in the case where the first deviation Ax is within the above-described preferable range (for example, 5.1 ≦ (a1-a2)/((a1+ a2)/2) × 100 ≦ 28.1), the electric discharge penetrating through the insulator can be appropriately suppressed.

The evaluation result R2 of the compressive strength of the large diameter portion 14 of the insulator 10 is greatly influenced from the second deviation degree Bx indicating the degree of deviation of the wall thickness of the large diameter portion 14 of the insulator 10. The second degree of deviation Bx of the tested sample is 0.3, 4.5, 4.6, 4.7, 4.8, 4.9, 9.6, 12.5 from small to large. As shown in fig. 5, the smaller the second deviation degree Bx, the better the evaluation result R2. For example, the evaluation result R2 No. 12 with the smallest second degree of deviation Bx (0.3) is the largest 10. The evaluation No. 14R 2 with the largest second degree of deviation Bx (12.5) was the smallest 5. The reason why the evaluation result R2 is better as the second deviation Bx is smaller is because the thinner portion of the large diameter portion 14 is easily damaged because the thickness of the large diameter portion is greatly deviated when the second deviation Bx is larger.

The second degree of deviation Bx of R2 was 0.3, 4.5, 4.6, 4.7, 4.8, 4.9, 9.6, which achieved good evaluation results of 6 or more. The preferred range of the second degree of offset Bx can also be determined using the above-mentioned 7 values. Specifically, any of 7 values may be used as the upper limit of the preferable range of the second shift degree Bx. For example, the second degree of offset Bx may be 9.6 or less. Any value of these values that is not higher than the upper limit may be used as the lower limit of the second degree of deviation Bx. For example, the second shift degree Bx may be 0.3 or more. It is assumed that the smaller the second deviation Bx, the stronger the large diameter portion 14. Therefore, the second shift degree Bx may be 0 or more.

When the second deviation Bx is within the above-described preferable range, the large diameter portion 14 of the insulator 10 can be prevented from being damaged when the spark plug 100 is manufactured. In addition, it is possible to suppress the insulator 10 from being damaged by the operation of the internal combustion engine to which the spark plug 100 is attached (for example, by vibration). In an actual manufacturing process of the spark plug 100, the load applied to the large diameter portion 14 of the insulator 10 may be smaller than the load in the compressive strength test. In an actual internal combustion engine, the load applied to the large diameter portion 14 of the insulator 10 may be smaller than the load in the compressive strength test. In the case where the spark plug is manufactured and used under the condition that the insulator is not easily broken, the second deviation Bx may be outside the above-described preferred range. For example, the second offset Bx may also exceed 9.6.

The evaluation result of the ignitability R3 is greatly influenced from an angle Ang indicating the arrangement of the ground electrode 30. The angle Ang of the sample tested is within any of the following ranges.

1) Below 5 degree

2)6 to 10 degrees inclusive

3)11 degrees or more and 15 degrees or less

4)16 to 20 degrees inclusive

5) Over 21 DEG

Further, the smaller the angle Ang, the better the evaluation result R3. The reason for this is as follows. As described with reference to fig. 4(a) and 4(B), when the angle Ang is small, the edge portion 30r of the end portion of the ground electrode 30 where the discharge path is likely to be formed is disposed in the vicinity of the tip end 20f of the center electrode 20. As a result, discharge is easily generated, and ignitability can be improved.

The range of the angle Ang at which the evaluation result R3 of 6 or more was achieved was "5 degrees or less", "6 degrees or more and 10 degrees or less", "11 degrees or more and 15 degrees or less", and "16 degrees or more and 20 degrees or less". The four ranges may also be used to determine a preferred range for the angle Ang. Specifically, any of the boundary values (i.e., the upper limit and the lower limit) of each of the four ranges may be used as the upper limit of the preferred range of the angle Ang. For example, the angle Ang may be 20 degrees or less. In addition, any value not more than the upper limit of the above boundary values may be used as the lower limit of the angle Ang. For example, the angle Ang may be 5 degrees or more. It is assumed that the smaller the angle Ang is, the better the ignitability is. Therefore, the angle Ang may be 0 or more.

Further, the actual operating conditions of the internal combustion engine may be conditions that are prone to ignition, as compared with the operating conditions in the ignition test described above. In the case of using the spark plug under conditions where ignition is easy, the angle Ang may be outside the above-described preferable range. For example, the angle Ang may exceed 20 degrees.

B. Modification example:

(1) the structure of the center electrode 20 may be other various structures instead of the structures illustrated in fig. 2(a), 2(B), and the like. For example, in the first cross section CS1 of fig. 2(B), the distal end point 20 may be located closer to the second direction Db than the second center C2. In this case, if the front end point 20f is disposed closer to the second center C2 side than the first center C1, the discharge penetrating through the insulator 10 can be suppressed. In order to suppress the discharge penetrating through the insulator 10, the front end point 20f is preferably located on the opposite side of the second direction Db with respect to the second center C2 in the first cross section CS 1.

In addition, the tip of the center electrode may be tapered like the tip of the needle. In this case, the tapered tip of the center electrode forms a tip point located on the most forward direction Df side. A tip formed of a material having a higher durability against electric discharge than the rod portion 28 (for example, a noble metal such as iridium (Ir) or platinum (Pt)) may be joined to the end portion of the rod portion 28 of the center electrode 20 on the front Df side. In this case, the end portion of the tip on the front Df side forms the end point of the center electrode located on the frontmost Df side, that is, the front end point.

(2) The structure of the ground electrode 30 may be other various structures instead of the structure illustrated in fig. 2(a) and the like. For example, the entire ground electrode 30 may be disposed at a position not intersecting the insulation center line CL 10. In addition, the rear surface 30rs may not be perpendicular to the insulation center line CL10, but may be inclined obliquely with respect to the insulation center line CL 10. In this case, it is preferable that a portion of the rear surface 30rs that is closer to the edge portion 30r be located on the rear direction Dfr side than a portion that is farther from the edge portion 30 r. According to this structure, the edge portion 30r may be the most rearward direction Dfr-side portion of the rear surface 30 rs. Such a rim portion 30r may be a portion of the ground electrode 30 closest to the front end point 20f of the center electrode 20. Therefore, the discharge path PT connecting the front end point 20f of the center electrode 20 and the edge portion 30r of the ground electrode 30 may be the shortest path connecting the center electrode 20 and the ground electrode 30. As a result, discharge is likely to occur on the path PT connecting the front end point 20f of the center electrode 20 and the edge 30r of the ground electrode 30. In this way, the portion of the ground electrode 30 where the end of the discharge path is easily formed is not a surface, but a smaller portion, i.e., the edge portion 30 r. Therefore, the ground electrode 30 is likely to generate discharge as compared with a case where a portion of the end portion where the discharge path is likely to be formed is dispersed over a wide surface. As a result, the ignitability can be improved. A tip formed of a material having a higher durability against discharge than that of the main body portion 37 (for example, a noble metal such as iridium (Ir) or platinum (Pt)) may be joined to the rear surface 30rs of the main body portion 37. Also, the tip of the ground electrode and the center electrode 20 may form a discharge gap.

(3) The structure of the insulator 10 may be other various structures instead of the structures illustrated in fig. 2(a), 2(B), 3, and the like. For example, the insulation centerline CL10 of the insulator 10 may be different from the center axis CL of the spark plug 100. For example, the insulation centerline CL10 may also be located away from the center axis CL between the front and rear ends of the insulator 10. In addition, in the second cross section CS2 (fig. 3), the third center C3 of the inner peripheral surface 14i may be disposed at the same position as the fourth center C4 of the outer peripheral surface 14 o. As the structure of the insulator having the first degree of displacement Ax within the above-described preferable range and the second degree of displacement Bx, which is smaller within the above-described preferable range, various structures can be adopted. For example, the insulator may form a through hole inclined with respect to the insulating center line. When a first molding die for forming an outer peripheral surface of an insulator and a rod-shaped second molding die for forming a through hole are used for forming the insulator, the insulator may be molded by arranging the second molding die so as to be inclined with respect to an axis of the first molding die corresponding to an insulating center line.

(4) The structure of the spark plug may be other various structures instead of the structures of the above-described embodiments and modifications. For example, the front-end side seal 8 (fig. 1) may be omitted. That is, the support portion 56 of the body metal 50 may directly support the step portion 16 by contacting the step portion 16 of the insulator 10. Further, a magnetic body may be disposed between the terminal fitting 40 and the center electrode 20 in the axial hole 12 of the insulator 10. In addition, the entire center electrode may be disposed in the through hole of the insulator. In addition, the nominal diameter of the threaded portion 57 of the body metal 50 may be other diameters instead of M8. As described with reference to fig. 5, in the case of using a spark plug having a small diameter such that the nominal diameter of the threaded portion 57 is M8, if the first deviation Ax is within the above-described preferable range, the electric discharge penetrating the insulator 10 can be appropriately suppressed. When the nominal diameter of the threaded portion 57 is M8 or more (for example, M8 or more and M18 or less), since the wall thickness of the insulator can be increased, if the first deviation Ax is within the above-described preferable range, the discharge penetrating through the insulator can be appropriately suppressed. In addition, the actual operating condition of the internal combustion engine may be a condition in which appropriate discharge can be performed. Therefore, the preferred range of the first degree of offset Ax may be applied to spark plugs of nominal diameter smaller than M8.

The present invention has been described above based on the embodiments and the modified examples, but the embodiments of the present invention described above are for facilitating the understanding of the present invention and are not intended to limit the present invention. The present invention can be modified and improved without departing from the gist thereof, and the present invention includes equivalents thereof.

Description of the reference numerals

8 … front end side seal, 10 … insulator, 10f … front end, 11 … reduced inner diameter portion, 12 … through hole (shaft hole), 13 … rear end side trunk portion, 13o … outer peripheral surface, 14 … large diameter portion, 14f … front end, 14i … inner peripheral surface, 14m … center position, 14o … outer peripheral surface, 14r … rear end, 15 … front end side trunk portion, 15o … outer peripheral surface, 16 … connecting portion (reduced diameter portion, step portion), 18 … connecting portion (reduced diameter portion), 19 … leg portion, 20 … center electrode, 20f … front end point, 20fs … front end face, 21 … outer layer, 22 … core portion, 23 … flange portion, 24 … head portion, 27 …, 28 … rod portion, 30 … ground electrode, 30e … end face, 30r … edge portion, 30rs … rear surface, 31 …, … inner layer, 3633 outer layer, … main body portion, … front end portion, … terminal …, … terminal …, 41, 50 body metal, 51 tool engagement portion, 52 front end side barrel portion, 53 rear end portion, 54 middle barrel portion, 54f surface, 55 front end surface, 56 support portion, 56r rear surface, 57 screw portion, 58 connection portion, 59 through hole, 61 ring member, 70 talc, 72 first seal portion, 73 resistor, 74 second seal portion, 100 spark plug, 350 connection region, 350C center of gravity, g discharge gap, CL center axis (axis), CL insulation center line, S inner peripheral surface, S outer peripheral surface, C first center, C second center, L first straight line, L1 vertical line, a first wall thickness, a second wall thickness, C third center, C fourth center, L second straight line, B first wall thickness, B second wall thickness, Da first direction, Db … second direction, Df … front end direction (front direction), Dfr … rear end direction (rear direction), CS1 … first cross section, CS2 … second cross section, Sp … projection plane, AR1 … region, PT … discharge path, PTx … imaginary path, Ang … angle.