Spark plug

Abstract

To provide a spark plug that can ensure the strength of a molten portion.SOLUTION: A spark plug includes a center electrode, a metal shell that insulates and holds the center electrode on the inside, and a ground electrode that protrudes from the metal shell toward the center electrode. The metal shell includes a cylindrical portion with a male thread on its outer periphery and a hole that penetrates the cylindrical portion in the radial direction. The ground electrode is connected to the cylindrical portion in the hole via a fusion portion. In a cross section including the center line of the hole and the fusion portion, the portion of the fusion portion that contacts the cylindrical portion is located radially outward from the ground electrode.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a spark plug in which a ground electrode is connected to a main fitting via a melting part.

Background Art

[0002] Prior art related to a spark plug including a center electrode, a cylindrical main fitting that insulatively holds the center electrode inside, and a ground electrode protruding from the main fitting toward the center electrode is disclosed in Patent Document 1. In the prior art, inside a hole penetrating the main fitting, the ground electrode is connected to the main fitting via a melting part.

Prior Art Document

Patent Document

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the prior art, since the bonding area of the melting part is restricted by the thickness of the main fitting through which the hole penetrates and the size of the hole, the strength of the melting part may be reduced.

[0005] The present invention has been made to solve this problem, and an object thereof is to provide a spark plug capable of ensuring the strength of the melting part.

Means for Solving the Problems

[0006] To achieve this object, a first aspect of the present invention provides a spark plug including a center electrode, a main fitting that insulates and holds the center electrode inside, and a ground electrode that protrudes from the main fitting toward the center electrode. The main fitting includes a cylindrical portion provided with an external thread on its outer periphery, and a hole penetrating the cylindrical portion in the radial direction. The ground electrode is connected to the cylindrical portion through a molten portion in the hole. In a cross section including the center line of the hole and the molten portion, the portion of the molten portion in contact with the cylindrical portion exists outside the ground electrode in the radial direction.

[0007] A second aspect is that, in the first aspect, in a cross section including the center line of the hole and the molten portion, when the size of the hole at the intersection of the portion of the molten portion in contact with the cylindrical portion and the inner surface of the hole is A (mm), and the distance along the center line of the portion of the molten portion in contact with the cylindrical portion is B (mm), 2 < A / B ≦ 15.

[0008] A third aspect is that, in the first or second aspect, the hole has a constant size in the radial direction of the cylindrical portion.

[0009] A fourth aspect is that, in any one of the first to third aspects, a cap for closing the tip side of the main fitting is provided, and a through hole for communicating the inside and outside of the space closed by the cap is provided in the cap.

Advantages of the Invention

[0010] According to the spark plug of the present invention, in a cross section including the center line of the hole provided in the cylindrical portion and the molten portion, the portion of the molten portion in contact with the cylindrical portion exists outside the ground electrode in the radial direction. Since the portion of the molten portion in contact with the cylindrical portion can be extended outside the cylindrical portion in the radial direction, the joint area of the molten portion can be increased as compared with the case where there is no molten portion outside the ground electrode in the radial direction. Therefore, the strength of the molten portion can be ensured.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Mode for Carrying Out the Invention

[0012] Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a partial cross-sectional view of a spark plug 10 in one embodiment. FIG. 1 shows a cross-section including the axis O of the tip side portion of the spark plug 10. In FIG. 1, the lower side of the paper is the tip side of the spark plug 10, and the upper side of the paper is the rear end side of the spark plug 10 (the same applies to FIG. 2). As shown in FIG. 1, the spark plug 10 includes an insulator 11, a center electrode 15, a main body fitting 20, and a ground electrode 25.

[0013] The insulator 11 is a substantially cylindrical member having an axial hole 12 extending along the axis O, and is formed of ceramics such as alumina which is excellent in mechanical properties and insulation properties at high temperatures. The insulator 11 includes a locking portion 13 and a tip portion 14 adjacent to the tip side of the locking portion 13. The outer diameter of the tip portion 14 is smaller than the outer diameter of the locking portion 13.

[0014] The center electrode 15 is disposed in the axial hole 12 along the axis O at least from the locking portion 13 to the tip portion 14 of the insulator 11. The center electrode 15 includes a rod-shaped base material 16 mainly composed of Ni, a chip 17 mainly composed of one or more of noble metals such as Pt, Rh, Ru, and Ir disposed at the tip of the base material 16, and a molten portion 18 that joins the chip 17 and the base material 16. The chip 17 and the molten portion 18 can be omitted.

[0015] The tip of the central electrode 15 protrudes from the insulator 11 toward the tip side. The central electrode 15 is electrically connected to the terminal fitting 19 within the axial hole 12. The terminal fitting 19 is a rod-shaped member to which a high-voltage cable (not shown) is connected, and is formed of a conductive metal material (such as low-carbon steel, etc.). The terminal fitting 19 is fixed to the rear end of the insulator 11.

[0016] The main body fitting 20 is a substantially cylindrical member formed of a conductive metal material (such as copper, copper alloy, low-carbon steel, etc.). The main body fitting 20 is disposed on the outer periphery of the insulator 11. The main body fitting 20 includes a cylindrical portion 21 located on the outer peripheral side of at least the locking portion 13 and the tip portion 14 of the insulator 11. The cylindrical portion 21 is a cylindrical part provided with an external thread 22 on the outer periphery, and a shelf portion 23 is provided on the inner periphery. The external thread 22 engages with an internal thread provided in a plug hole of an engine (not shown). The shelf portion 23 is located on the tip side of the locking portion 13 of the insulator 11 and locks the locking portion 13.

[0017] A hole 24 that penetrates the cylindrical portion 21 in the radial direction is provided in a portion of the cylindrical portion 21 on the tip side of the shelf portion 23. In the present embodiment, the hole 24 is provided at the position of the external thread 22, and the cross section of the hole 24 is circular. A ground electrode 25 is disposed within the hole 24 of the cylindrical portion 21. The ground electrode 25 disposed in the hole 24 protrudes from the cylindrical portion 21 toward the central electrode 15.

[0018] A space 29 is provided inside the cap 26 by the cap 26 that closes the tip side of the main body fitting 20. Examples of the material of the cap 26 include metal materials mainly composed of one or more of Fe, Ni, Cu, etc. In the present embodiment, the cap 26 is connected to the tip side of the cylindrical portion 21 via a melted portion 28. A through hole 27 that communicates the inside and the outside of the space 29 is provided in the cap 26.

[0019] Figure 2 is a cross-sectional view of the spark plug 10 with the portion shown by II in Figure 1 enlarged. The inner surface 30 of the hole 24 provided in the cylindrical portion 21 is connected to the outer peripheral surface 31 of the cylindrical portion 21, and a ground electrode 25 is provided between the inner peripheral surface 32 of the cylindrical portion 21 and the inner surface 30 of the hole 24. The ground electrode 25 includes, for example, a base material 33 mainly composed of Ni, a chip 34 mainly composed of one or more of noble metals such as Pt, Rh, Ru, and Ir, and a molten portion 35 that joins the chip 34 and the base material 33. The chip 34 and the molten portion 35 can be omitted. The end face 36 of the base material 33 is located inside the hole 24. The hole 24 provided in the cylindrical portion 21 is blocked by the ground electrode 25.

[0020] In this embodiment, the portion of the base material 33 inside the hole 24 is cylindrical, and the portion of the base material 33 inside the hole 24 is thicker than the portion where the molten portion 35 is provided. The chip 34 of the ground electrode 25 faces the side surface of the chip 17 of the center electrode 15, and a spark gap G is provided between the tip of the ground electrode 25 and the side surface of the center electrode 15.

[0021] The base material 33 of the ground electrode 25 is connected to the cylindrical portion 21 through a molten portion 37 inside the hole 24. The molten portion 37 is formed by irradiating a laser beam onto the end face 36 of the base material 33 of the ground electrode 25 arranged inside the hole 24. The molten portion 37 is formed by melting a portion including a part of the end face 36 of the base material 33 and a part of the inner surface 30 of the hole 24 of the cylindrical portion 21.

[0022] The spark plug 10 attached to an engine (not shown) allows fuel gas to flow into the space 29 from the combustion chamber of the engine through the through-hole 27 by the valve operation of the engine. The spark plug 10 generates a flame kernel by discharge between the center electrode 15 and the ground electrode 25. When the flame kernel grows, it ignites the fuel gas in the space 29 and the fuel gas burns. Due to the expansion pressure generated by the combustion of the fuel gas, a gas flow containing a flame is generated, and the gas containing the flame is injected from the through-hole 27 into the combustion chamber. The fuel gas in the combustion chamber burns due to the jet of the flame. That is, the space 29 inside the cap 26 functions as a sub-combustion chamber provided inside the combustion chamber of the engine.

[0023] FIG. 3 is a cross-sectional view of the spark plug 10 taken along line III-III of FIG. 2. In FIG. 3, a part of the cross-section of the cylindrical portion 21 and the ground electrode 25 including the center line C of the hole 24 is shown. The lower side of the paper surface is the inner side in the radial direction of the cylindrical portion 21, and the upper side of the paper surface is the outer side in the radial direction of the cylindrical portion 21 (the same applies in FIG. 4). The center line C of the hole 24 is a straight line passing through the geometric centers of a plurality of cross-sections obtained by cutting the inner surface 30 of the hole 24 such that the cuts form a ring as a plurality of planar figures. In the present embodiment, since the melting portion 37 is provided over the entire length around the center line C of the hole 24, in FIG. 3 which is a cross-sectional view including the center line C, the melting portions 37 and 38 appear on both sides of the center line C, respectively.

[0024] A portion 39 of one melting portion 37 that contacts the cylindrical portion 21 is a portion (the interface between the melting portion 37 and the cylindrical portion 21) connecting the intersection 41 between the outer surface 40 of the melting portion 37 on the outer side in the radial direction of the cylindrical portion 21 and the inner surface 30 of the hole 24, and the intersection 42 where the inner peripheral surface 32 of the cylindrical portion 21 intersects the melting portion 37. A part of the portion 39 exists on the outer side in the radial direction of the cylindrical portion 21 than the base material 33 (the end surface 36 in the present embodiment).

[0025] Since the portion 39 is longer than when the melting portion 37 does not exist on the outer side in the radial direction than the ground electrode 25, the bonding area related to the strength of the melting portion 37 can be increased. Therefore, the strength of the melting portion 37 can be ensured.

[0026] In the present embodiment, the outermost position in the radial direction of the surface 40 of the melting portion 37 is the intersection 41. Since the excess of the melting portion 37 can be prevented from becoming excessive, the stress generated at the intersection 41 can be reduced. As a result, the fatigue strength of the cylindrical portion 21 and the melting portion 37 can be prevented from decreasing.

[0027] The portion 43 of the other melting part 38 that contacts the cylindrical part 21 is a portion (interface between the melting part 38 and the cylindrical part 21) connecting the intersection point 45 between the outer surface 44 in the radial direction of the cylindrical part 21 and the inner surface 30 of the hole 24 in the melting part 38 and the intersection point 46 where the inner peripheral surface 32 of the cylindrical part 21 intersects the melting part 38. A part of the portion 43 exists outside the cylindrical part 21 in the radial direction from the base material 33 (the end surface 36 in this embodiment). Since the intersection points 41 and 45 exist radially inside the trough of the male thread 22 (see FIG. 1) on the inner surface 30 of the hole 24, the male thread 22 can be fitted into the female thread provided in the plug hole.

[0028] The portion 43 exists outside the cylindrical part 21 in the radial direction from the base material 33. Since the portion 43 becomes longer compared to the case where the melting part 38 does not exist outside the grounding electrode 25 in the radial direction, the bonding area related to the strength of the melting part 38 can be increased. Therefore, the strength of the melting part 38 can be ensured.

[0029] In this embodiment, the outermost position in the radial direction of the surface 44 of the melting part 38 is the intersection point 45. Since the excess of the melting part 38 can be prevented from becoming excessive, the stress generated at the intersection point 45 can be reduced. As a result, the fatigue strength of the cylindrical part 21 and the melting part 38 can be prevented from decreasing.

[0030] Both the portions 39 and 43 exist outside the cylindrical part 21 in the radial direction from the base material 33. Thereby, the strength of the melting parts 37 and 38 can be increased compared to the case where one of the portions 39 and 43 exists outside the cylindrical part 21 in the radial direction from the base material 33.

[0031] When the portion including the end face 36 of the base material 33 melts and the molten portions 37 and 38 extend toward the inner side in the radial direction of the cylindrical portion 21, the entire portions 39 and 43 come to exist outside the cylindrical portion 21 in the radial direction rather than the base material 33. At this time, compared with the case where a part of the portions 39 and 43 exists outside the cylindrical portion 21 in the radial direction rather than the base material 33, a large amount of thermal energy for melting the base material 33 and the cylindrical portion 21 is applied to the base material 33 and the cylindrical portion 21, so there is a risk that the base material 33 and the cylindrical portion 21 may be deformed. Since a part of the portions 39 and 43 exists outside the cylindrical portion 21 in the radial direction rather than the base material 33 as in the present embodiment, the strength of the molten portions 37 and 38 can be ensured and the deformation of the base material 33 and the cylindrical portion 21, particularly the deformation of the male screw 22, can be reduced.

[0032] In the present embodiment, the hole 24 has a constant size in the radial direction of the cylindrical portion 21. This makes it easier to control the conditions of laser welding for forming the molten portions 37 and 38. The fact that the size of the hole 24 is constant in the radial direction of the cylindrical portion 21 means that the difference between the maximum value and the minimum value when the dimension (size of the hole 24) of the inner surface 30 of the hole 24 is measured at a plurality of positions over the entire length in the radial direction of the cylindrical portion 21 is 0.20 mm or less. The size of the hole 24 is measured up to the third decimal place to obtain the maximum value and the minimum value, and the third decimal place of the difference between the maximum value and the minimum value is rounded off.

[0033] The intersection point 41 of the molten portion 37 is located outside the intersection point 45 of the molten portion 38 in the radial direction of the cylindrical portion 21. For the spark plug 10, when the size of the hole 24 at the intersection point 41 located outside in the radial direction among the intersection points 41 and 45 of the molten portions 37 and 38 is A (mm) and the distance along the center line C of the portion 39 including the intersection point 41 is B (mm), it is preferable to satisfy 2 < A / B ≤ 15. A is the length of the line segment formed by a straight line perpendicular to the center line C being intercepted by the inner surface 30 of the hole 24.

[0034] It is preferable to satisfy 2 < A / B ≤ 15 for the following reasons. As A increases, the radial length of the inner surface 30 of the hole 24 excluding the portion of the male screw 22 becomes shorter, so it tends to be difficult to form the melted portion 37 on the inner surface 30 without deforming the male screw 22 while ensuring the joining strength of the melted portion 37. As B increases, the male screw 22 of the cylindrical portion 21 tends to be easily deformed by the thermal energy when forming the melted portion 37. As A and B decrease, the joining area of the melted portion 37 becomes smaller, so the joining strength of the melted portion 37 tends to become smaller. When 2 < A / B ≤ 15, the joining strength of the melted portion 37 can be ensured without deforming the male screw 22.

[0035] In this embodiment, when the distance along the center line C of the portion 43 including the intersection 45 located radially inside among the intersections 41 and 45 of the melted portions 37 and 38 is defined as B (mm), 2 < A / B ≤ 15 is also satisfied. Since the portions 39 and 43 located on both sides of the center line C satisfy 2 < A / B ≤ 15, the strength of the melted portions 37 and 38 can be improved.

[0036] The size A of the hole 24 is preferably 1 mm < A ≤ 3 mm. This is because when A is 1 mm or less, it becomes difficult to irradiate the laser beam into the hole 24 to form the melted portions 37 and 38. When A exceeds 3 mm, it becomes difficult to form the melted portions 37 and 38 on the inner surface 30 of the hole 24 excluding the portion of the male screw 22.

[0037] The tip side of the main metal fitting 20 of the spark plug 10 is closed by the cap 26 (see FIG. 1), fuel gas burns in the space 29 inside the cap 26, and the expansion pressure is applied to the cap 26, the cylindrical portion 21, and the melted portions 37 and 38. The melted portions 37 and 38 require mechanical strength capable of withstanding the pressure. Since the portions 39 and 43 of the melted portions 37 and 38 exist radially outside the cylindrical portion 21 from the base material 33 in the spark plug 10, the strength of the melted portions 37 and 38 can be ensured. Therefore, it is suitable for the spark plug 10 provided with the cap 26.

[0038] A second embodiment will be described with reference to FIG. 4. In the first embodiment, the case where the portions 39 and 43 of the melting portions 37 and 38 intersect the inner peripheral surface 32 of the cylindrical portion 21 was described. In contrast, in the second embodiment, the case where the portions 54 and 59 of the melting portions 52 and 53 that are in contact with the cylindrical portion 21 are accommodated between the outer peripheral surface 31 and the inner peripheral surface 32 of the cylindrical portion 21 will be described. In the second embodiment, the same parts as those in the first embodiment are denoted by the same reference numerals, and the following description thereof will be omitted.

[0039] FIG. 4 is a cross-sectional view including the center line C of the hole 47 provided in the cylindrical portion 21 of the spark plug 10 in the second embodiment. The hole 47 is provided at the position of the male thread 22 (see FIG. 1) of the cylindrical portion 21. The inner surface 48 of the hole 47 includes a first portion 49 that intersects the outer peripheral surface 31 of the cylindrical portion 21, a second portion 50 that is adjacent to the inside in the radial direction of the first portion 49, and a third portion 51 that is adjacent to the inside of the second portion 50. The size of the hole 47 in the first portion 49 is larger than the size of the hole 47 in the third portion 51. The second portion 50 is provided near the groove of the male thread 22.

[0040] In the present embodiment, the first portion 49 and the third portion 51 are cylindrical surfaces, and the second portion 50 is a plane perpendicular to the center line C. A melting portion is provided in the third portion 51 over the entire length around the center line C. Therefore, in FIG. 4, the melting portions 52 and 53 appear on both sides of the center line C of the third portion 51, respectively. The third portion 51 of the hole 47 has a constant size in the radial direction of the cylindrical portion 21. This makes it easier to control the conditions for laser welding to form the melting portions 52 and 53.

[0041] In the second portion 50 and the first portion 49 connected to the third portion 51, since the hole 47 expands with respect to the third portion 51, the male thread 22 is less likely to be affected by the heat of laser welding for providing the melting portions 52 and 53 by irradiating a laser beam into the hole 47. Therefore, deformation of the male thread 22 can be reduced.

[0042] Of the melting part 52 of the chip, the part 54 in contact with the cylindrical part 21 is a part connecting the intersection 56 between the outer surface 55 in the radial direction of the cylindrical part 21 and the inner surface 48 (third part 51) of the hole 47 in the melting part 52 and the intersection 58 between the inner surface 57 in the radial direction of the melting part 52 and the inner surface 48 of the hole 47 (the interface between the melting part 52 and the cylindrical part 21). A part of the part 54 exists outside the cylindrical part 21 in the radial direction from the base material 33 (end face 36 in this embodiment). The intersection 56 is located at the outermost position in the radial direction on the surface 55 of the melting part 52.

[0043] Since the part 54 is longer than when the melting part 52 does not exist outside the grounding electrode 25 in the radial direction, the joint area related to the strength of the melting part 52 can be increased. Therefore, the strength of the melting part 52 can be ensured.

[0044] Of the other melting part 53, the part 59 in contact with the cylindrical part 21 is a part connecting the intersection 61 between the outer surface 60 in the radial direction of the cylindrical part 21 and the inner surface 48 of the hole 47 in the melting part 53 and the intersection 63 between the inner surface 62 in the radial direction of the melting part 53 and the inner surface 48 of the hole 47 (the interface between the melting part 53 and the cylindrical part 21). A part of the part 59 exists outside the cylindrical part 21 in the radial direction from the base material 33 (end face 36 in this embodiment). The intersection 61 is located at the outermost position in the radial direction on the surface 60 of the melting part 53. The intersections 56 and 61 exist inside the groove of the male screw 22 (see FIG. 1) in the inner surface 48 of the hole 47 in the radial direction.

[0045] Since the part 59 is longer than when the melting part 53 does not exist outside the grounding electrode 25 in the radial direction, the joint area related to the strength of the melting part 53 can be increased. Therefore, the strength of the melting part 53 can be ensured.

[0046] The intersection point 61 of the melting part 53 is located on the outer side in the radial direction of the cylindrical part 21 than the intersection point 56 of the melting part 52. When the size of the hole 47 at the intersection point 61 located on the outer side in the radial direction of the melting parts 52 and 53 is A (mm) and the distance along the center line C of the part 59 including the intersection point 61 is B (mm), 2 < A / B ≤ 15 is satisfied. Thereby, the joining strength of the melting part 53 can be ensured without deforming the male screw 22.

[0047] In the present embodiment, 2 < A / B ≤ 15 is also satisfied when the distance along the center line C of the part 54 including the intersection point 56 located on the inner side in the radial direction of the intersection points 56 and 61 is B (mm). Since the parts 54 and 59 located on both sides of the center line C satisfy 2 < A / B ≤ 15, the strength of the melting parts 52 and 53 can be improved.

Example

[0048] The present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

[0049] The tester provided a male screw on the outer periphery of a carbon steel cylinder with a thickness of 1.5 mm having the same shape as the cylindrical part 21 of the spark plug 10 in the first embodiment, and prepared a sample in which holes with diameters from 1 mm to 4 mm penetrated the male screw part. The nominal diameter of the male screw was 10 mm, and the male screw conformed to JIS B8031:2006. The holes were circular with a constant size in the radial direction of the cylinder, and the center line of the hole intersected perpendicularly with the center line of the cylinder.

[0050] After inserting the cylinder into the hole so that the end face of the cylinder was located on the inner side in the radial direction than the groove of the male screw, a laser beam was irradiated on the end face of the cylinder, and the entire circumference of the cylinder was laser welded to the cylinder. The material of the cylinder was Ni-based alloy (NCF601), the length of the cylinder was 3 mm, and the diameter of the cylinder was 0.1 mm smaller than the diameter of the hole. Thereby, the samples No. 1-10 shown in Table 1 were produced.

[0051]

Table 1

[0052] Next, each sample was cut, and an image of the melted part appearing on the cross-section including the center line of the cylinder was obtained with an optical microscope. By image analysis, the size A (mm) of the hole at the intersection of the melted parts and the distance B (mm) in the melted part including the intersection where the size A of the hole was measured were measured. The value obtained by multiplying the size A (diameter of the circle) by the pi and the distance B was defined as the joint area (mm 2 ) of the melted part, and the stress (N / mm 2 ) obtained by dividing the maximum load by the joint area was determined.

[0053] Samples with a stress less than or equal to 60 N / mm obtained by adding a safety factor to the pressure at which destructive abnormal combustion (so-called mega knock) occurred were determined to be good (G), and samples with a stress exceeding 60 N / mm 2 were determined to be inferior (P). Samples No. 1 and 10 with deformed threads were also determined to be inferior (P). The A, B, A / B, stress, and determination of each sample are shown in Table 1. 2 As shown in Table 1, Samples No. 3 - 8 that satisfied 2.0 < A / B ≤ 15.0 were determined to be G, while Samples No. 1, 2 with A / B ≤ 2.0 and Samples No. 9, 10 with A / B > 15.0 were determined to be P. According to the examples, it was clarified that by satisfying 2.0 < A / B ≤ 15.0, deformation of the thread can be reduced and the strength of the melted part can be ensured.

[0054] As described above, the present invention has been described based on the embodiments. However, it can be easily inferred that the present invention is not limited to the above embodiments at all, and various improvements and modifications can be made without departing from the spirit of the present invention.

[0055]

[0056] ​In the embodiment, the case where the holes 24 and 47 are circular has been described, but it is not necessarily limited to this. Other shapes of the holes 24 and 47 include an ellipse, a semi-circle, polygons such as a triangle, a square, a hexagon, and a polygon with rounded corners. The shape of the cross-section of the ground electrode 25 disposed in the holes 24 and 47 is appropriately set according to the shape of the holes 24 and 47.

[0057] In the embodiment, the case where the melting portions are continuously provided around the center line C of the holes 24 and 47 has been described, but it is not necessarily limited to this. It is of course possible to intermittently provide the melting portions around the center line C of the holes 24 and 47. When the melting portions are intermittently provided in the holes 24 and 47, in the cross-section including the center line C of the holes 24 and 47 and the melting portions, the melting portions do not necessarily appear on both sides of the center line C as in the embodiment, and the melting portions may appear only on one side of the center line C.

[0058] In the embodiment, the case where a part of the end face 36 of the base material 33 of the ground electrode 25 remains unmelted has been described, but it is not necessarily limited to this. It is possible that the entire end face 36 of the base material 33 has melted into the melting portions 37, 38, 52, and 53 and the end face 36 has disappeared. When the end face 36 of the base material 33 has disappeared, it can be said that the portions 39, 43, 54, and 59 of the melting portions 37, 38, 52, and 53 that are in contact with the cylindrical portion 21 exist on the outer side in the radial direction of the ground electrode 25, provided that at least a part of the portions 39, 43, 54, and 59 is located on the outer side in the radial direction of the cylindrical portion 21 compared to the portion located most on the outer side in the radial direction among the interfaces between the melting portions and the base material 33.

[0059] In the embodiment, the case where the size of the inner surface 30 of the hole 24 and the third portion 51 of the hole 47 is constant in the radial direction of the cylindrical portion 21 has been described, but it is not necessarily limited to this. It is of course possible to provide a so-called tapered hole in the cylindrical portion 21 whose size decreases from the outer side to the inner side in the radial direction of the cylindrical portion 21, and dispose the ground electrode 25 in that hole.

[0060] In the embodiment, the case where holes 24 and 47 are provided in the male screw 22 portion of the cylindrical portion 21 has been described, but it is not necessarily limited to this. For example, it is naturally possible to provide a cylindrical portion without a male screw 22 at the tip of the cylindrical portion 21 and to provide a ground electrode 25 by making a hole in the tip portion (cylindrical portion) of the cylindrical portion 21.

[0061] In the embodiment, the case where the portion of the ground electrode 25 inside the holes 24 and 47 is thicker than the portion closer to the axis O has been described, but it is not necessarily limited to this. It is naturally possible to make the thickness of the portion of the ground electrode 25 inside the holes 24 and 47 the same as the thickness of the portion closer to the axis O.

[0062] In the embodiment, the case where a spark gap G is provided between the side surface of the center electrode 15 and the tip of the ground electrode 25 has been described, but it is not necessarily limited to this. While shifting the positions of the holes 24 and 47 provided in the cylindrical portion 21 toward the tip side and making the ground electrode 25 slightly longer, a spark gap G may be provided between the tip of the center electrode 15 and the side surface of the ground electrode 25.

[0063] In the embodiment, the case where a cap 26 is arranged on the tip side of the main fitting 20 has been described, but it is not necessarily limited to this. It is naturally possible to omit the cap 26.

[0064] In the embodiment, the case where a hemispherical cap 26 is arranged on the main fitting 20 has been described, but it is not necessarily limited to this. The shape of the cap 26 can be set as appropriate. Other shapes of the cap 26 include a bottomed cylindrical shape and a disk shape as examples.

[0065] In the embodiment, the case where the cap 26 is welded to the main fitting 20 has been described, but it is not necessarily limited to this. It is of course possible to prepare a cylindrical member provided with a cap at its tip and connect this to the main fitting 20 to form the space 29. The cylindrical member is a cylindrical member with its tip closed by a cap, and a female screw for coupling to the male screw 22 of the main fitting 20 is provided on the inner peripheral surface. A male screw for coupling to the female screw of the engine's spark plug hole is provided on the outer peripheral surface of the cylindrical member. By coupling the female screw of the cylindrical member to the male screw 22 of the main fitting 20, a cap is disposed on the tip side of the main fitting 20. A through hole 27 is provided in this cap.

[0066] The means for connecting the cylindrical member to the main fitting 20 and disposing a cap on the tip side of the main fitting 20 is not limited to coupling the female screw on the inner peripheral surface of the cylindrical member to the male screw 22 of the main fitting 20. It is of course possible to connect the cylindrical member to the main fitting by other means. Examples of other means include joining the cylindrical member and the main fitting by welding or the like. Examples of the material of the cylindrical member include metal materials such as Ni-based alloys and stainless steels, and ceramics such as silicon nitride.

Explanation of Reference Numerals

[0067] 10 Spark plug 15 Center electrode 20 Main fitting 21 Cylindrical portion 22 Male screw 24, 47 Holes 25 Ground electrode 26 Cap 27 Through hole 29 Space 30 Inner surface 37, 38, 52, 53 Molten part 39, 43, 54, 59 Portion of the molten part in contact with the cylindrical portion 41, 61 Intersection point C Center line of the hole

Claims

1. A central electrode, a main fitting that insulatively holds the central electrode inside, and a ground electrode that protrudes from the main fitting toward the central electrode, and the main fitting includes a cylindrical portion provided with an external thread on the outer periphery, and a hole that penetrates the cylindrical portion in the radial direction, a spark plug in which the ground electrode is connected to the cylindrical portion through a molten portion in the hole, in a cross section including the center line of the hole and the molten portion, a spark plug in which a portion of the molten portion that contacts the cylindrical portion exists outside the ground electrode in the radial direction.

2. In a cross section including the center line of the hole and the molten portion, when the size of the hole at the intersection of the portion and the inner surface of the hole is A (mm) and the distance along the center line of the portion is B (mm), the spark plug according to claim 1, wherein 2 < A / B ≤ 15.

3. The spark plug according to claim 1 or 2, wherein the hole has a constant size in the radial direction of the cylindrical portion.

4. including a cap that closes the tip side of the main fitting, the spark plug according to claim 1 or 2, wherein a through hole that communicates the inside and the outside of the space closed by the cap is provided in the cap.

Images