Spark plug

Abstract

The present invention provides a spark plug capable of reducing the occurrence of premature ignition. The spark plug includes: a center electrode; a main body fitting that insulates and holds the center electrode; and a columnar ground electrode that is electrically connected to the main body fitting and has one end facing the center electrode. The main body fitting includes a cylindrical front end portion in which one end portion of the ground electrode is located inside. The front end portion has a hole into which the other end portion of the ground electrode is inserted. Among them, the value obtained by dividing the surface area of the portion of the ground electrode located on the inner peripheral side of the front end portion by the side area of the portion of the ground electrode located inside the hole is 13.1 or less. This value is preferably 6.0 or less, and more preferably 5.5 or less.

Description

Technical Field

[0001] The present invention relates to a spark plug in which a ground electrode is connected to a main body fitting. Background Art

[0002] In Patent Document 1, there is disclosed a prior art in which a ground electrode is disposed in a hole provided in a cylindrical front end portion of a main body fitting in a spark plug including a center electrode, a main body fitting that insulates and holds the center electrode, and a ground electrode connected to the main body fitting.

[0003] Prior Art Documents

[0004] Patent Documents

[0005] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2020-145018 Summary of the Invention

[0006] Problems to be Solved by the Invention

[0007] In the prior art, the ground electrode surrounded by the front end portion is difficult to be cooled by fuel and intake air, so the overheated ground electrode may become an ignition source and cause pre-ignition (early combustion).

[0008] The present invention has been completed to solve this problem, and an object thereof is to provide a spark plug capable of reducing the occurrence of pre-ignition.

[0009] Means for Solving the Problems

[0010] To achieve this object, a mode of the present invention is a spark plug including: a center electrode; a main body fitting that insulates and holds the center electrode; and a columnar ground electrode that is electrically connected to the main body fitting and has one end facing the center electrode. The main body fitting includes a cylindrical front end portion in which one end of the ground electrode is located inside, and the front end portion has a hole into which the other end of the ground electrode is inserted. Among them, the value obtained by dividing the surface area of the portion of the ground electrode located on the inner peripheral side of the front end portion by the side area of the portion of the ground electrode located in the hole is 13.1 or less. This value is preferably 6.0 or less, and more preferably 5.5 or less.

[0011] Advantages of the Invention

[0012] According to the present invention, the value obtained by dividing the surface area of the portion of the ground electrode located on the inner peripheral side of the front end portion, that is, the portion receiving the heat of the combustion gas, by the side area of the portion of the ground electrode located in the hole, that is, the portion releasing heat to the main body fitting, is 13.1 or less. Therefore, overheating of the ground electrode can be prevented, and the occurrence of pre-ignition can be reduced. Brief Description of the Drawings

[0013] Figure 1 It is a partial cross-sectional view of the spark plug of the first embodiment.

[0014] Figure 2 is a cross-sectional view of a spark plug in which the portion shown in II of Figure 1 is enlarged.

[0015] Figure 3 is a cross-sectional view of a spark plug according to a second embodiment. Detailed Embodiment

[0016] Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. Figure 1 is a partial cross-sectional view of a spark plug 10 according to a first embodiment. In Figure 1 is shown a cross-section including the axis O of a portion on the front end side of the spark plug 10. In Figure 1 , the lower side of the paper surface is referred to as the front end side of the spark plug, and the upper side of the paper surface is referred to as the rear end side of the spark plug 10 (the same applies in Figure 2 and Figure 3 ). As shown in Figure 1 , the spark plug 10 includes an insulator 11, a center electrode 15, a main body fitting 20, and a ground electrode 25.

[0017] The insulator 11 is a substantially cylindrical member having an axial hole 12 extending along the axis O. The insulator 11 is formed of a ceramic such as alumina having excellent mechanical properties and insulation properties at high temperatures. The insulator 11 includes a locking portion 13 and a front end portion 14 adjacent to the front end side of the locking portion 13. The outer diameter of the front end portion 14 is smaller than the outer diameter of the locking portion 13.

[0018] The center electrode 15 is disposed in the axial hole 12 along the axis O from at least the locking portion 13 to the front end portion 14 of the insulator 11. The center electrode 15 includes: a rod-shaped base material 16 mainly composed of Ni; a tip 17 mainly composed of one or more noble metals such as Pt, Rh, Ru, Ir, etc. disposed at the front end of the base material 16; and a molten portion 18 that joins the tip 17 and the base material 16. The tip 17 and the molten portion 18 can be omitted.

[0019] The front end of the center electrode 15 projects from the insulator 11 toward the front end side. The center electrode 15 is electrically connected to a terminal fitting 19 within the axial hole 12. The terminal fitting 19 is a rod-shaped member that connects to an ignition device (not shown) 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.

[0020] 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 front end portion 21 located on the outer peripheral side of at least the locking portion 13 and the front end portion 14 of the insulator 11. The front end portion 21 is provided with an external thread 22 on the outer periphery and a support portion 23 on the inner periphery. The external thread 22 is fitted into the internal thread of a plug hole of an engine (not shown). The support portion 23 is located on the front end side of the locking portion 13 of the insulator 11 and locks the locking portion 13. The front end portion 21 extends forward from the front end portion 14 of the insulator 11 towards the front end side.

[0021] A hole 24 is provided in a portion of the front end portion 21 on the front end side of the support portion 23, and a part of the inner periphery of the front end portion 21 is recessed. In the present embodiment, the hole 24 penetrates the front end portion 21 in the radial direction, has a circular cross-section, and a part of the external thread 22 is missing due to the hole 24. A part of the ground electrode 25 is disposed in the hole 24. The ground electrode 25 having a part disposed in the hole 24 protrudes from the front end portion 21 towards the center electrode 15.

[0022] By closing the cover 26 on the front end side of the main body fitting 20, a space 29 is provided inside the cover 26. Examples of the material of the cover 26 include metal materials mainly composed of one or more of Fe, Ni, Cu, etc. In the present embodiment, the cover 26 is connected to the front end side of the front end portion 21 via a melting portion 28. A through hole 27 for communicating the inside and outside of the space 29 is provided in the cover 26.

[0023] Figure 2 It is a cross-sectional view of the spark plug 10 magnifying the portion shown in II of Figure 1 . The ground electrode 25 has a columnar shape and includes an end portion 32 that forms a spark gap facing the center electrode 15 (refer to Figure 1 ) and the other end portion 33 disposed in the hole 24. The end portion 32 is located inside the inner periphery 30 of the front end portion 21. The end portion 32 is cylindrical, and the cross-section of the end portion 32 is circular. The other end portion 33 is located inside the outer periphery 31 of the front end portion 21 and at a position closer to the inside than the bottom 22a of the external thread 22. The other end portion 33 is cylindrical, and the cross-section of the other end portion 33 is circular and fits into the hole 24. The hole 24 penetrating the front end portion 21 is blocked by the other end portion 33.

[0024] The ground electrode 25 includes, for example, a base material 34 mainly composed of Ni, a tip 35 mainly composed of one or more of noble metals such as Pt, Rh, Ru, Ir, etc., and a melting portion 36 that joins the tip 35 and the base material 34. One end portion 32 of the ground electrode 25 includes a part of the tip 35, and the other end portion 33 includes a part of the base material 34. The tip 35 and the melting portion 36 can be omitted.

[0025] The ground electrode 25 is connected to the front end portion 21 via the melting portion 37 in the hole 24. The melting portion 37 is formed by irradiating a laser beam onto the bottom surface 38 of the ground electrode 25 disposed in the hole 24. The melting portion 37 is formed by melting a portion including a part of the bottom surface 38 of the ground electrode 25 and a portion including a part of the front end portion 21. In the present embodiment, the melting portion 37 is continuous over the entire circumference of the bottom surface 38 of the ground electrode 25, and the radially inner end of the melting portion 37 reaches the inner circumference 30 of the front end portion 21.

[0026] The heat of the ground electrode 25 is transferred to the front end portion 21 through the interface 39 between the melting portion 37 and the front end portion 21. Thereby, the ground electrode 25 is cooled. The area of the interface 39 has a great influence on heat transfer. The melting portion 37 is a part of the ground electrode 25. On the other hand, the side surface 40 and the bottom surface 41 of the ground electrode 25 in the portion of the ground electrode 25 located inside the inner circumference 30 of the front end portion 21 (the portion located in the space 29) are heat receiving surfaces that receive the heat generated by the combustion of the fuel in the space 29.

[0027] The spark plug 10 (refer to Figure 1 ) installed in an engine (not shown) is such that, through the valve operation of the engine, fuel flows from the combustion chamber of the engine into the space 29 through the through hole 27. The spark plug 10 generates a flame kernel by the discharge between the center electrode 15 and the ground electrode 25. When the flame kernel grows, the fuel in the space 29 is ignited and the fuel burns. Due to the expansion pressure generated by the combustion of the fuel, an air flow containing a flame is generated, and the gas containing the flame is ejected from the through hole 27 into the combustion chamber. Through the jet of this flame, the fuel in the combustion chamber burns. That is, the space 29 inside the cover 26 functions as a sub-combustion chamber provided in the combustion chamber of the engine.

[0028] The value S / E obtained by dividing the surface area S obtained by adding the area of the side surface 40 and the area of the bottom surface 41 of the portion of the ground electrode 25 of the spark plug 10 located on the inner circumference 30 side of the front end portion 21 by the side area E of the portion of the ground electrode 25 located inside the hole 24 (the area of the interface 39 in the present embodiment) is 13.1 or less. This is to prevent overheating of the ground electrode 25 by the balance between the heat reception of the ground electrode 25 and the heat transfer from the ground electrode 25 to the front end portion 21, and to reduce the occurrence of premature ignition. The value S / E is preferably 6.0 or less, and more preferably 5.5 or less.

[0029] The portion of the ground electrode 25 of the spark plug 10 located on the inner circumferential 30 side of the front end portion 21 is covered by the cover 26. The space 29 on the inner circumferential 30 side of the front end portion 21 is connected to the combustion chamber (not shown) through the through hole 27 of the cover 26. Therefore, the ground electrode 25 is difficult to be cooled by the fuel and intake air supplied to the combustion chamber, and the ground electrode 25 is likely to overheat. However, the value S / E of the spark plug 10 is set to 13.1 or less, so that overheating of the ground electrode 25 can be prevented and the occurrence of pre-ignition can be reduced. Therefore, it is suitable for the spark plug 10 provided with the cover 26.

[0030] Refer to Figure 3 The second embodiment will be described. In the first embodiment, the case where the molten portion 37 is continuous over the entire circumference of the bottom surface 38 of the ground electrode 25 and the molten portion 37 reaches the inner circumference 30 of the front end portion 21 has been described. In contrast, in the second embodiment, the case where the molten portion 59 is intermittently provided along the edge of the bottom surface 60 of the ground electrode 53 and there is a gap between the inner circumference 30 of the front end portion 21 and the molten portion 59 will be described. In the second embodiment, the same parts as those described in the first embodiment are denoted by the same reference numerals, and the following description will be omitted.

[0031] Figure 3 It is a cross-sectional view of the spark plug 50 of the second embodiment. Figure 3 The vicinity of the hole 51 provided in the front end portion 21 of the main body fitting 20 is enlarged and shown, and other illustrations are omitted. Through the hole 51, a part of the inner circumference 30 of the front end portion 21 is recessed. In the present embodiment, the hole 51 penetrates the front end portion 21 in the radial direction and has a circular cross-section. The hole 51 is provided with a countersink 52 on the outer circumference 31 of the front end portion 21. The countersink 52 is located at a position closer to the inside than the bottom 22a of the external thread 22.

[0032] The ground electrode 53 includes, for example, a base material 54 mainly composed of Ni, a tip 55 mainly composed of one or more noble metals such as Pt, Rh, Ru, and Ir, and a molten portion 56 that joins the tip 55 and the base material 54. The ground electrode 53 has a columnar shape and includes an end portion 57 that forms a spark gap facing the center electrode 15 (refer to Figure 1 ) and another end portion 58 disposed in the hole 51.

[0033] One end portion 57 is cylindrical, and the cross-section of the one end portion 57 is circular. The other end portion 58 is cylindrical, and the cross-section of the other end portion 58 is circular and fits into the hole 51. The hole 51 penetrating the front end portion ills>21 is blocked by the other end portion 58. The fit between the other end portion 58 and the hole 51 can be any one of an interference fit, a clearance fit, and a transition fit.

[0034] The ground electrode 53 is connected to the front end portion 21 via the fusion portion 59 in the hole 51. The bottom surface 60 of the ground electrode 53 is located on substantially the same plane as the counterbore 52. The fusion portion 59 is formed by irradiating a laser beam onto the bottom surface 60 of the ground electrode 53 disposed in the hole 51. The fusion portion 59 is formed by melting a portion including a part of the bottom surface 60 of the ground electrode 53 and a portion including a part of the counterbore 52 of the front end portion 21. In the present embodiment, the fusion portion 59 is intermittently provided along the edge of the bottom surface 60 of the ground electrode 53, and a gap exists between the inner end portion in the radial direction of the fusion portion 59 and the inner periphery 30 of the front end portion 21.

[0035] The heat of the ground electrode 53 is transferred to the front end portion 21 through the interface 61 between the fusion portion 59 and the front end portion 21 and the side surface 62 in the hole 51 of the ground electrode 53. Thereby, the ground electrode 53 is cooled. The area of the interface 61 and the side surface 62 has a great influence on heat transfer. The fusion portion 59 is a part of the ground electrode 53. On the other hand, the side surface 63 and the bottom surface 64 of the ground electrode 53 in the portion of the ground electrode 53 located inside the inner periphery 30 of the front end portion 21 are heat receiving surfaces that receive the heat generated by fuel combustion.

[0036] The value S / E obtained by dividing the surface area S obtained by adding the area of the side surface 63 and the area of the bottom surface 64 of the portion of the ground electrode 53 of the spark plug 50 located on the inner periphery 30 side of the front end portion 21 by the side area E of the portion of the ground electrode 53 located in the hole 51 (in the present embodiment, the area obtained by adding the area of the interface 61 and the area of the side surface 62) is 13.1 or less. This is to prevent overheating of the ground electrode 53 and reduce the occurrence of premature ignition through the balance between the heat reception of the ground electrode 53 and the heat transfer from the ground electrode 53 to the front end portion 21.

[0037] [Embodiment]

[0038] The present invention will be described in more detail by way of examples, but the present invention is not limited to these examples.

[0039] The tester prepared components such as a main body fitting and a ground electrode for manufacturing a spark plug. The tester prepared main body fittings with a radial thickness of the front end portion of 0.3 mm to 1.8 mm and main body fittings with a diameter of a circular hole penetrating the front end portion in the radial direction of 0.8 mm to 2.5 mm. After inserting ground electrodes of various lengths having a thickness that fits into the holes of the main body fittings into the holes, the tester irradiated a laser beam onto the bottom surfaces of the ground electrodes and laser-welded the entire circumference of the bottom surfaces of the ground electrodes to the main body fittings to manufacture samples No. 1-12 having the same shape as the spark plug of the first embodiment.

[0040] A three-dimensional image was constructed from the tomographic images of Samples No. 1 - 12 obtained using an X-ray CT scanner, and the surface area S (mm 2 ) of the portion on the inner peripheral side at the front end of the grounding electrode and the lateral area E of the portion of the grounding electrode within the hole (the area of the interface of the melting part (mm 2 )) were determined, and the value S / E obtained by dividing the surface area S by the lateral area E was calculated. S / E is the value obtained by dividing the surface area S by the lateral area E, rounded to the second decimal place. The surface areas S, lateral areas E of Samples No. 1 - 12 are different, but other dimensions and materials such as the size of the spark gap and the size of the bottom surface of one end of the grounding electrode are the same. The surface areas S, lateral areas E, and S / E of Samples No. 1 - 12 are shown in Table 1.

[0041] [Table 1]

[0042]

[0043] (Pre-ignition test)

[0044] The tester measured the advance angle of the ignition timing of the test engine relative to the original spark plug according to the pre-ignition test specified in JIS D1606:2020. The larger the crank angle at which pre-ignition occurs, the more difficult it is for pre-ignition to occur. Samples with a crank angle at which pre-ignition occurs more than 7° ahead of the crank angle of the original spark plug are judged as A, samples with an advance of more than 5° and less than 7° are judged as B, samples with an advance of more than 2° and less than 5° are judged as C, and samples with an advance of less than 2° are judged as D. The results are shown in the pre-ignition resistance column of Table 1.

[0045] (Bench durability test)

[0046] The tester investigated the durability distance of Samples No. 1 - 12 according to the bench durability test specified in JIS D1606:2020. The durability distance is the distance equivalent to when the spark gap between the center electrode and the grounding electrode is 0.2 mm larger than before the test. Samples with a durability distance of 100,000 km or more are judged as A, and samples with a durability distance of more than 50,000 km and less than 100,000 km are judged as D. The results are shown in the wear resistance column of Table 1.

[0047] As shown in Table 1, the pre-ignition resistance of Sample No. 1 with S / E greater than 13.1 is judged as D, but the pre-ignition resistances of Samples No. 2 - 12 with S / E of 13.1 or less are judged as A - C. Therefore, it can be seen that if S / E is 13.1 or less, pre-ignition can be reduced.

[0048] The drought resistance of sample No. 2 with S / E greater than 6.0 and less than or equal to 13.1 was judged as C, but the drought resistance of samples No. 3 - 12 with S / E less than or equal to 6.0 was judged as A or B. Therefore, it can be seen that if S / E is less than or equal to 6.0, premature ignition can be further reduced.

[0049] The drought resistance of sample No. 3 with S / E greater than 5.5 and less than or equal to 6.0 was judged as B, but the drought resistance of samples No. 4 - 12 with S / E less than or equal to 5.5 was judged as A. Therefore, it can be seen that if S / E is less than or equal to 5.5, premature ignition can be further reduced.

[0050] The drought resistance of sample No. 12 with S / E of 0.5 was judged as A, but the wear resistance was judged as D. This is because if S / E is less than or equal to 0.5, the volume of the part that functions as the ground electrode where discharge occurs between the center electrode becomes smaller, so the time until it no longer functions as the ground electrode becomes shorter. Therefore, considering wear resistance, it is desirable that S / E is greater than 0.5.

[0051] As described above, the present invention has been described based on the embodiments, but the present invention is not limited by any of the above - mentioned embodiments, and it can be easily inferred that various improvements and modifications can be made without departing from the gist of the present invention.

[0052] In the embodiment, the case where the holes 24 and 51 are circular has been described, but it is not necessarily limited thereto. Other shapes of the holes 24 and 51 can be exemplified by polygons such as ellipse, semi - circle, triangle, quadrilateral, hexagon, and polygons with rounded corners. The cross - sectional shapes of the ground electrodes 25 and 53 disposed in the holes 24 and 51 are appropriately set to shapes that fit the holes 24 and 51 according to the shapes of the holes 24 and 51.

[0053] In the first embodiment, the case where the melting part 37 is continuously provided around the center line of the hole 24 has been described, and in the second embodiment, the case where the melting part 59 is intermittently provided around the center line of the hole 51 has been described, but it is not necessarily limited thereto. Of course, the melting part 37 can be intermittently provided around the center line of the hole 24 in the first embodiment, or the melting part 59 can be continuously provided around the center line of the hole 51 in the second embodiment.

[0054] In the embodiment, the case where the grounding electrodes 25 and 53 are joined to the front end portion 21 via the melting portions 37 and 59 has been described, but it is not necessarily limited thereto. Of course, instead of welding, the grounding electrodes 25 and 53 may be fitted and fixed in the holes 24 and 51. The fit between the holes 24 and 51 and the grounding electrodes 25 and 53 is an interference fit or a transition fit. In this case, since there is no interface of the melting portion, the side area E is the area of the side surfaces of the portions of the grounding electrodes 25 and 53 located within the holes 24 and 51.

[0055] In the embodiment, the case where a part of the bottom surfaces 38 and 60 of the grounding electrodes 25 and 53 remains has been described, but it is not necessarily limited thereto. Sometimes, the entire bottom surfaces 38 and 60 of the grounding electrodes 25 and 53 melt into the melting portions 37 and 59, causing the bottom surfaces 38 and 60 to disappear. In the case where the entire bottom surfaces 38 and 60 melt into the melting portions 37 and 59, the areas of the interfaces 39 and 61 between the melting portions 37 and 59 and the front end portion 21 are also included in the side area E.

[0056] In the first embodiment, the case where the size of the hole 24 is fixed in the radial direction of the front end portion 21 has been described, but it is not necessarily limited thereto. For example, of course, a so-called tapered hole whose size decreases as it goes from the outer side to the inner side in the radial direction of the front end portion 21 may be provided in the front end portion 21, and the grounding electrode 25 may be arranged in this hole.

[0057] In the embodiment, the case where the holes 24 and 51 are provided in the portion of the external thread 22 of the front end portion 21 has been described, but it is not necessarily limited thereto. For example, of course, a cylindrical portion without the external thread 22 may be provided in the front end portion 21, and the grounding electrodes 25 and 53 may be provided by opening holes in the cylindrical portion.

[0058] In the embodiment, the case where the holes 24 and 51 penetrate the front end portion 21 has been described, but it is not necessarily limited thereto. This is because, even if the holes do not penetrate the front end portion 21, as long as a part of the inner periphery 30 of the front end portion 21 is recessed, the other end portions 33 and 58 of the grounding electrodes 25 and 53 can be arranged in the holes.

[0059] In the embodiment, the case where the thicknesses of the other end portions 33 and 58 of the grounding electrodes 25 and 53 are substantially the same as the thicknesses of the one end portions 32 and 57 of the grounding electrodes 25 and 53 has been described, but it is not necessarily limited thereto. Of course, the thicknesses of the one end portions 32 and 57 of the grounding electrodes 25 and 53 may be different from the thicknesses of the other end portions 33 and 58.

[0060] In the present embodiment, the case where the cross-sectional shapes of one ends 32 and 57 of the grounding electrodes 25 and 53 are the same as those of the other ends 33 and 58 has been described, but it is not necessarily limited thereto. Of course, the cross-sectional shapes of one ends 32 and 57 can be different from those of the other ends 33 and 58.

[0061] In the embodiment, the case where a spark gap is provided between the side surface of the center electrode 15 and one ends 32 and 57 of the grounding electrodes 25 and 53 has been described, but it is not necessarily limited thereto. It is also possible to shift the positions of the holes 24 and 51 provided at the front end portion 21 toward the front end side, and to make the grounding electrodes 25 and 53 slightly longer, and provide a spark gap between the front end of the center electrode 15 and the side surfaces of the grounding electrodes 25 and 53.

[0062] In the embodiment, the case where the cover 26 is disposed on the front end side of the front end portion 21 of the main body fitting 20 has been described, but it is not necessarily limited thereto. Of course, the cover 26 can also be omitted. This is because, even without the cover 26, as long as one ends 32 and 57 of the grounding electrodes 25 and 53 are surrounded by the front end portion 21 of the main body fitting 20, compared with a spark plug having a grounding electrode provided outside the main body fitting, the cooling by fuel and intake air is insufficient.

[0063] In the embodiment, the case where the hemispherical cover 26 is disposed on the main body fitting 20 has been described, but it is not necessarily limited thereto. The shape of the cover 26 can be appropriately set. Other examples of the shape of the cover 26 include a bottomed cylindrical shape and a circular plate shape.

[0064] In the embodiment, the case where the cover 26 is welded to the main body fitting 20 has been described, but it is not necessarily limited thereto. Of course, it is possible to prepare a cylindrical member having a cover at the front end, and connect it to the main body fitting 20 to form the space 29. The cylindrical member is a cylindrical member whose front end is closed by a cover, and an internal thread that engages with the external thread 22 of the main body fitting 20 is provided on the inner peripheral surface. An external thread that engages with the internal thread of the engine's spark plug hole is provided on the outer peripheral surface of the cylindrical member. By engaging the internal thread of the cylindrical member with the external thread 22 of the main body fitting 20, the cover is disposed on the front end side of the main body fitting 20. A through hole 27 is provided in this cover.

[0065] The means of connecting the cylindrical member to the main body fitting 20 to dispose the cover on the front end side of the main body fitting 20 is not limited to the means of engaging the internal thread on the inner peripheral surface of the cylindrical member with the external thread 22 of the main body fitting 20. Of course, the cylindrical member can also be connected to the main body fitting by other means. As other means, for example, a means of joining the cylindrical member to the main body fitting by welding or the like can be cited. 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.

[0066] Description of Reference Numerals

[0067] 10, 50 Spark Plug

[0068] 15 Center Electrode

[0069] 20 Body Fitting

[0070] 21 Front End

[0071] 24, 51 Hole

[0072] 25, 53 Ground Electrode

[0073] 32, 57 One End

[0074] 33, 58 The Other End

Claims

1. A spark plug, comprising: A center electrode; A main body fitting that insulatively holds the center electrode; and A columnar ground electrode that is electrically connected to the main body fitting and has one end facing the center electrode, The main body fitting includes a tubular front end portion in which the one end of the ground electrode is located inside, The front end portion has a hole into which the other end of the ground electrode is inserted, wherein, The value obtained by dividing the surface area of the portion of the ground electrode located on the inner peripheral side of the front end portion by the side area of the portion of the ground electrode located in the hole is 13.1 or less.

2. The spark plug according to claim 1, wherein, The value is 6.0 or less.

3. The spark plug according to claim 1, wherein, The value is 5.5 or less.

Images