spark plug

Abstract

Spark plug (100), showing: a central electrode (20, 20c); and a ground electrode (30) which forms a gap (G) between the ground electrode (30) and the center electrode (20, 20c), wherein the ground electrode (30) has a tip holding section (31, 31b, 52c) with a hole (34, 34b, 34c) and an electrode tip (39, 39b, 39c) which has a discharge surface (395, 395b, 395c) which forms the gap (G) and of which at least one section is arranged in the hole (34, 34b, 34c) of the tip holding section (31, 31b, 52c), wherein the section of the electrode tip (39, 39b, 39c) which is arranged in the hole (34, 34b, 34c) has a press-fit section (394, 394b, 394c) which is in direct contact with the tip holding section (31, 31b, 52c) over its entire circumference, and wherein a section of the electrode tip (39, 39b, 39c) from the discharge surface (395, 395b, 395c) to the press-fit section (394, 394b, 394c) is formed as a single element, where the hole (34, 34b, 34c) penetrates the tip retention section (31, 31b, 52c), and wherein a section (391, 392, 391b, 392b, 391c, 392c) of the electrode tip (39, 39b, 39c) on one side relative to the press-in section (394, 394b, 394c) closer to the discharge surface (395, 395b, 395c) has a length in a direction parallel to the discharge surface (395, 395b, 395c), wherein the length is less than a length of the press-in section (394, 394b, 394c) in the direction parallel to the discharge surface (395, 395b, 395c).

Description

TECHNICAL FIELD

[0001] The present invention relates to a spark plug for igniting fuel gas in an internal combustion engine or the like. STATE OF THE ART

[0002] A spark plug used for an internal combustion engine, for example, causes a spark discharge to occur in a gap between a center electrode and a ground electrode, thereby igniting fuel gas in the internal combustion engine or the like. A spark plug is known in which, for the purpose of improving wear resistance, an electrode tip made of a precious metal such as iridium is bonded by resistance welding or laser welding to a portion of a center electrode or a ground electrode that forms a gap where spark discharge occurs. When the electrode tip is bonded to the portion by welding, the welding creates a weld portion between the electrode body and the electrode tip.

[0003] JP 2005 - 135 783 A discloses a spark plug. EP 3 358 686 A1 discloses a spark plug and a method for its manufacture. DE 10 2004 044 397 A1 discloses a spark plug for a gas engine. DE 697 04 598 T2 discloses a spark plug and its manufacturing method.

[0004] In recent years, the development of internal combustion engines with higher power and lower fuel consumption has been accelerated. Along with this, spark plugs tend to be exposed to higher temperatures. The weld portion is relatively easily oxidized and has relatively low thermal conductivity. Therefore, with the aforementioned technology, when the spark plug is exposed to high temperatures, oxidation of the weld portion may cause the electrode tip to peel off or fall off, or the heat conduction performance may deteriorate due to the low thermal conductivity of the weld portion.

[0005] The present specification discloses a technology capable of preventing peeling and falling off of an electrode tip in a spark plug and improving the heat conduction performance of the spark plug. SUMMARY OF THE INVENTION

[0006] The technology disclosed here can be presented according to claim 1 taking into account the following application examples. APPLICATION EXAMPLE 1

[0007] A spark plug comprising: a center electrode; and a ground electrode forming a gap between the ground electrode and the center electrode, wherein the ground electrode comprises a tip holding portion having a hole, and an electrode tip having a discharge surface forming the gap and at least a portion of which is arranged in the hole of the tip holding portion, wherein the portion of the electrode tip disposed in the hole has a press-in portion directly in contact with the tip holding portion over an entire circumference thereof, and a portion of the electrode tip from the discharge surface to the press-in portion is formed as a single element, wherein the hole penetrates the tip holding portion, forming, for example, a through hole, and wherein a portion of the electrode tip on a side closer to the discharge surface relative to the press-in portion has a length in a direction parallel to the discharge surface, the length being smaller than a length of the press-in portion in the direction parallel to the discharge surface.

[0008] According to the above configuration, the portion of the electrode tip disposed in the hole has the press-fit portion directly in contact with the tip holding portion over the entire circumference thereof. In such a press-fit portion, the tip holding portion and the electrode tip are joined together without a welding portion between them, and therefore, they hardly oxidize even when exposed to high temperatures. Thus, peeling and falling off of the electrode tip can be prevented. Furthermore, since the portion from the discharge surface to the press-fit portion is formed as a single member, a reduction in thermal conductivity and a reduction in thermal conduction performance can be prevented compared to a case where a welding portion is provided in the portion from the discharge surface to the press-fit portion.Thus, the peeling and falling off of the electrode tip in the spark plug can be prevented according to the above configuration and the heat conduction performance can be improved.

[0009] According to the above configuration, the electrode tip can be press-fitted into the hole from a side opposite to the side facing the other electrode. Consequently, deformation of the discharge surface during press-fit can be prevented. According to one embodiment, the hole, i.e., a through-hole, penetrates the ground electrode, so that the tip holding portion extends through the ground electrode and the tip penetrates the ground electrode.

[0010] According to the above configuration, when the electrode tip is press-fitted into the hole from the side opposite to the side facing the other electrode, the portion of the electrode tip on the side closer to the discharge surface relative to the press-fit portion can be prevented from being damaged. APPLICATION EXAMPLE 2

[0011] The spark plug according to application example 1, where if a surface of the electrode tip opposite the discharge surface is called an opposite surface, the spark plug further comprises a welding portion connecting the tip holding portion to a portion of the electrode tip on a side closer to the opposite surface relative to the press-fit portion.

[0012] According to the above configuration, peeling and falling off of the electrode tip can be further prevented because the welding portion is provided at a portion on the side opposite to the discharge surface which is hardly damaged. APPLICATION EXAMPLE 3

[0013] The spark plug according to any one of application examples 1 or 2, wherein when a length of the press-fit portion in a thickness direction of the electrode tip is L, and a length of the press-fit portion in a direction perpendicular to the thickness direction is D, (D / L) ≤ 1.33 is met.

[0014] According to the above configuration, since the length L of the press-fitting portion in the press-fitting direction is not excessively short with respect to the length D of the press-fitting portion in a direction perpendicular to the press-fitting direction, the electrode tip can be fixed to the tip holding portion with sufficient strength. APPLICATION EXAMPLE 4

[0015] The spark plug according to application example 3, where (D / L) ≤ 1 is met.

[0016] According to the above configuration, since the length L of the press-fitting portion in the press-fitting direction is sufficiently long with respect to the length D of the press-fitting portion in a direction perpendicular to the press-fitting direction, the electrode tip can be fixed to the tip holding portion with higher strength.

[0017] The present invention can be embodied in various forms. For example, the present invention can be embodied in the form of a spark plug, an ignition device using the spark plug, an internal combustion engine equipped with the spark plug, an internal combustion engine equipped with the ignition device using the spark plug, and an electrode of a spark plug. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Embodiments of the inventions are described with reference to the accompanying drawings, without being limited thereto. Fig. 1 shows a cross-sectional view of a spark plug 100 according to the present embodiment. Fig. 2A, Fig. 2B, Fig. 2C show explanatory diagrams for an area around a front end of the spark plug 100. Fig. 3 shows an explanatory diagram for a manufacturing method of a ground electrode 30. Fig. 4A, Fig. 4B show explanatory diagrams for a spark plug according to a second embodiment. Fig. 5 shows a cross-sectional view of a region around a gap G of a spark plug according to a third embodiment. Fig. 6 shows a cross-sectional view of an area around a center electrode tip 29d of a spark plug according to an exemplary alternative embodiment. EMBODIMENTS OF THE INVENTION A. FIRST EMBODIMENT: A-1. STRUCTURE OF THE SPARK PLUG:

[0019] Fig. 1 shows a cross-sectional view of a spark plug 100 according to the present embodiment. Fig. 2A, Fig. 2B and Fig. 2C show explanatory diagrams for a region near a front end of the spark plug 100. In Fig. 1 and Fig. 2A, Fig. 2B and Fig. 2C, alternating long and short dashed lines each represent an axis AX of the spark plug 100. A direction parallel to the axis AX (top-bottom direction in Fig. 1 and Fig. 2A, Fig. 2B and Fig. 2C) is also called an axial direction. The radial direction of a circle around the axis AX on a plane perpendicular to the axis AX is simply called the "radial direction", and the circumferential direction of the circle is simply called the "circumferential direction". The downward direction in Fig. 1 is called “forward direction FD”, and the upward direction in Fig. 1 is called “reverse direction BD”. The lower side in Fig. 1 and Fig. 2A, Fig. 2B and Fig. 2C is called the “front side” of the spark plug 100, and the upper side in Fig. 1 and Fig. 2A, Fig. 2B and Fig. 2C is referred to as the “rear side” of the spark plug 100.

[0020] The spark plug 100 is attached to an internal combustion engine and is used for igniting fuel gas in a combustion chamber of the internal combustion engine. The spark plug 100 includes an insulator 10, a center electrode 20, a ground electrode 30, a terminal electrode 40, a metal housing 50, a resistor 70, and conductive sealing elements 60 and 80.

[0021] The insulator 10 is a substantially cylindrical member having an axial hole 12, which is a through-hole extending along the axis AX and penetrating the insulator 10. The insulator 10 is formed from a ceramic material, such as alumina. The insulator 10 includes a flange portion 19, a rear body portion 18, a front body portion 17, a reduced outer diameter portion 15, and a leg portion 13.

[0022] The flange portion 19 is a portion of the insulator 10 located substantially at the center in the axial direction. The rear trunk portion 18 is located at the rear side relative to the flange portion 19 and has a smaller outer diameter than the flange portion 19. The front trunk portion 17 is located at the front side relative to the flange portion 19 and has a smaller outer diameter than the rear trunk portion 18. The leg portion 13 is located at the front side relative to the front trunk portion 17 and has a smaller outer diameter than the front trunk portion 17. The outer diameter of the leg portion 13 is reduced toward the front side. When the spark plug 100 is mounted on an internal combustion engine (not shown), the leg portion 13 is exposed in the combustion chamber of the internal combustion engine.The reduced outer diameter portion 15 is formed between the leg portion 13 and the front body portion 17. The reduced outer diameter portion 15 has an outer diameter that is reduced from the rear side toward the front side.

[0023] With regard to the structure of its inner peripheral side, the insulator 10 includes: a large inner diameter portion 12L located on the rear side; a small inner diameter portion 12S located on the front side relative to the large inner diameter portion 12L and having a smaller inner diameter than the large inner diameter portion 12L; and a reduced inner diameter portion 16. The reduced inner diameter portion 16 is a portion formed between the large inner diameter portion 12L and the small inner diameter portion 12S, and whose inner diameter is reduced from the rear side to the front side. In the present embodiment, the axial position of the reduced inner diameter portion 16 is the position of a front side portion of the front body portion 17.

[0024] The metal shell 50 is formed of a conductive metal material (e.g., a low-carbon steel material) and is a cylindrical metal member for fixing the spark plug 100 to the engine head (not shown) of the internal combustion engine. The metal shell 50 has a through-hole 59 formed to penetrate the metal shell 50 along the axis AX. The metal shell 50 is disposed on the radial periphery (i.e., the outer periphery) of the insulator 10. That is, the insulator 10 is inserted into the through-hole 59 of the metal shell 50 and held there. The front end of the insulator 10 protrudes to the front side relative to the front end of the metal shell 50. The rear end of the insulator 10 protrudes to the rear side relative to the rear end of the metal shell 50.

[0025] The metal housing 50 includes: a tool engagement portion 51 having a hexagonal column shape and with which a spark plug wrench is to be engaged; a fastening screw portion 52 for fastening the spark plug 100 to the internal combustion engine; and a flange-shaped seat portion 54 formed between the tool engagement portion 51 and the fastening screw portion 52. The nominal diameter of the fastening screw portion 52 is, for example, M8 to M14.

[0026] A gasket 5, made of metal and annular in shape, is inserted and fitted between the fastening screw portion 52 and the seat portion 54 of the metal shell 50. When the spark plug 100 is attached to the internal combustion engine, the gasket 5 seals the gap between the spark plug 100 and the internal combustion engine (the engine head).

[0027] The metal shell 50 further includes: a thin crimping portion 53 provided at the rear of the tool engaging portion 51; and a thin compression deformation portion 58 provided between the seat portion 54 and the tool engaging portion 51. Annular lead seals 6 and 7 are arranged in an annular region formed between: the inner peripheral surface of a portion of the metal shell 50 from the tool engaging portion 51 to the crimping portion 53; and the outer peripheral surface of the rear body portion 18 of the insulator 10. The space between the two lead seals 6 and 7 in this region is filled with powder of a talc 9. The rear end of the crimping portion 53 is bent radially inward and fixed to the outer peripheral surface of the insulator 10.The compression-deforming portion 58 of the metal shell 50 is compression-deformed by the crimping portion 53 fixed to the outer peripheral surface of the insulator 10, being pressed toward the front side during manufacturing. The insulator 10 is pressed into the metal shell 50 toward the front side via the lead seals 6 and 7 and the talc 9, as well as the compression deformation of the compression-deforming portion 58. The outer diameter reduced portion 15 (insulator-side step portion) of the insulator 10 is pressed by a step portion 56 (metal shell-side step portion) formed at the position of the fastening screw portion 52 on the inner periphery of the metal shell 50, via an annular disc gasket 8. Consequently, the disc gasket 8 prevents gas in the combustion chamber of the internal combustion engine from leaking to the outside through the gap between the metal shell 50 and the insulator 10.

[0028] The center electrode 20 includes a rod-shaped center electrode body 21 extending along the axis AX and a center electrode tip 29. The center electrode body 21 is held in a front portion in the axial hole 12 of the insulator 10. That is, the rear side of the center electrode 20 (the rear side of the center electrode body 21) is disposed in the axial hole 12. The center electrode body 21 is formed of a metal, such as nickel (Ni), which has high corrosion resistance and high heat resistance, or an alloy containing Ni as a main component (e.g., NCF 600, NCF 601). The center electrode body 21 may have a two-layer structure including a base material made of Ni or Ni alloy; and a core portion embedded in the base material.In this case, the core section is made of, for example, copper, which has a higher thermal conductivity than the base material, or an alloy containing copper as the main component.

[0029] The center electrode body 21 includes: a flange portion 24 provided at a predetermined position in the axial direction; a head portion 23 located on the rear side relative to the flange portion 24; and a leg portion 25 located on the front side relative to the flange portion 24. The flange portion 24 is supported by the reduced inner diameter portion 16 of the insulator 10. That is, the center electrode body 21 engages with the reduced inner diameter portion 16. The front side of the leg portion 25, that is, the front side of the center electrode body 21, protrudes to the front side relative to the front end of the insulator 10.

[0030] The center electrode tip 29 is a member having, for example, a substantially cylindrical shape and is joined to the front end of the center electrode body 21 (the front end of the leg portion 25) by, for example, laser welding. The front end surface of the center electrode tip 29 is a first discharge surface 295, which forms a spark gap with a ground electrode tip 39, which will be described later. The center electrode tip 29 is formed, for example, from a refractory noble metal such as iridium (Ir) or platinum (Pt), or an alloy containing the noble metal as a main component.

[0031] The terminal electrode 40 is a rod-shaped member extending in the axial direction. The terminal electrode 40 is inserted into the axial hole 12 of the insulator 10 from the rear side and is arranged on the rear side relative to the center electrode 20 in the axial hole 12. The terminal electrode 40 is formed of a conductive metal material (e.g., low-carbon steel), and a plating layer of Ni or the like is formed on the surface of the terminal electrode 40 for the purpose of corrosion protection, for example.

[0032] The terminal electrode 40 includes: a flange portion 42 formed at a predetermined position in the axial direction; a cap fixing portion 41 located on the rear side relative to the flange portion 42; and a leg portion 43 located on the front side relative to the flange portion 42. The cap fixing portion 41 of the terminal electrode 40 is exposed to the rear side relative to the insulator 10. The leg portion 43 of the terminal electrode 40 is inserted into the axial hole 12 of the insulator 10. A spark plug cap, to which a high-voltage cable (not shown) is attached, is attached to the cap fixing portion 41, and a high voltage for inducing discharge is applied to the cap fixing portion 41.

[0033] Resistor 70 is disposed in the axial hole 12 of insulator 10 between the front end of terminal electrode 40 and the rear end of center electrode 20. Resistor 70 has a resistance of 1 kΩ or more (e.g., 5 kΩ) and functions to reduce electrical noise during ignition. Resistor 70 is formed, for example, from a composition containing glass particles, non-glass ceramic particles, and conductive material as a main component.

[0034] The gap between the resistor 70 and the center electrode 20 in the axial hole 12 is filled with the conductive sealing member 60. The gap between the resistor 70 and the terminal electrode 40 is filled with the sealing member 80. That is, the sealing member 60 contacts the center electrode 20 and the resistor 70, separating the center electrode 20 and the resistor 70 from each other. The sealing member 80 contacts the resistor 70 and the terminal electrode 40, separating the resistor 70 and the terminal electrode 40 from each other. Thus, the sealing members 60 and 80 electrically and physically connect the center electrode 20 to the terminal electrode 40 via the resistor 70. The sealing members 60 and 80 are formed of a conductive material, for example, a composition containing glass particles, a B2O3-SiO2-based material, or the like, and metal particles (Cu, Fe, etc.).

[0035] The ground electrode 30 (the ground electrode body 31) is a rod-shaped body with a rectangular cross section, as shown in an enlarged cross-sectional view in Fig. 2A. The ground electrode body 31 has, as end portions, a connecting end portion 312 and a free-end portion 311 located on the opposite side to the connecting end portion 312. The connecting end portion 312 is connected to a front end 50t of the metal shell 50 by, for example, resistance welding. Accordingly, the metal shell 50 and the ground electrode body 31 are electrically and physically connected to each other. A portion of the ground electrode body 31 near the connecting end portion 312 extends in the direction of the axis AX, while a portion thereof near the free-end portion 311 extends in the direction perpendicular to the axis AX. The rod-shaped ground electrode body 31 is inclined by approximately 90° at a central portion thereof.

[0036] The ground electrode body 31 is formed of a metal, such as Ni, which has high corrosion resistance and high heat resistance, or an alloy containing Ni as a main component (e.g., NCF 600, NCF 601). Like the center electrode body 21, the ground electrode body 31 may have a two-layer structure including a base material; and a core portion formed of a metal (e.g., copper) having higher thermal conductivity than the base material and embedded in the base material.

[0037] The ground electrode tip 39 is attached to the free-end portion 311. The ground electrode tip 39 has a second discharge surface 395 opposite the first discharge surface 295 of the center electrode 20, with the gap G formed therebetween. The gap G between the first discharge surface 295 and the second discharge surface 395 is a so-called spark discharge gap in which a spark discharge occurs. Like the center electrode tip 29, the ground electrode tip 39 is formed, for example, from a noble metal or an alloy containing the noble metal as a main component.

[0038] Fig. Figure 2B shows an enlarged view of a rectangular area AA from Fig. 2A. Fig. 2C shows the section that Fig. 2B, viewed from the front side toward the rear direction BD along the axis AX. The ground electrode tip 39 has a substantially cylindrical shape. A surface of the ground electrode tip 39 opposite the second discharge surface 395 is referred to as an opposing surface 396. A direction (axial direction) along the axis AX of the ground electrode tip 39 is also referred to as a thickness direction of the ground electrode tip 39. A direction perpendicular to the axis AX of the ground electrode tip 39, that is, a direction perpendicular to the thickness direction, is also referred to as a width direction.

[0039] The ground electrode tip 39 includes: a small outer diameter portion 391 on the second discharge surface 395 side (the rear side); a large outer diameter portion 393 on the opposite surface 396 side (the front side); and an enlarged outer diameter portion 392 located between the small outer diameter portion 391 and the large outer diameter portion 393. The outer diameter of the large outer diameter portion 393 is larger than that of the small outer diameter portion 391. The enlarged outer diameter portion 392 is a portion whose outer diameter is enlarged from the second discharge surface 395 side (the rear side) toward the opposite surface 396 side (the front side).

[0040] A hole 34 with a size corresponding to the large outer diameter portion 393 of the ground electrode tip 39 is formed in the free-end portion 311 of the ground electrode body 31. The hole 34 is a substantially cylindrical hole penetrating the ground electrode body 31 in the axial direction. The hole 34 has a uniform inner diameter portion 341 and an enlarged inner diameter portion 342. The enlarged inner diameter portion 342 is a portion whose inner diameter is enlarged from the second discharge surface 395 side (the rear side) toward the opposite surface 396 side (the front side).

[0041] The axial length of the large outer diameter portion 393 of the ground electrode tip 39 is substantially equal to the axial length of the hole 34, i.e., the axial length of the free-end portion 311. The small outer diameter portion 391 and the enlarged outer diameter portion 392 each protrude to the rear side relative to a rear surface 311B of the free-end portion 311. The opposite surface 396 of the ground electrode tip 39 is substantially flush with a front surface 311F of the free-end portion 311.

[0042] The ground electrode tip 39 is fixed to the free-end portion 311 by press-fitting into the hole 34. Therefore, a portion of the large outer diameter portion 393 located in the uniform inner diameter portion 341 of the hole 34 is in direct contact with an inner peripheral surface 34i of the hole 34 over the entire circumference thereof in the circumferential direction. The portion of the large outer diameter portion 393 that is in direct contact with the inner peripheral surface 34i of the hole 34 over the entire circumference thereof in the circumferential direction is also referred to as a press-fit portion 394.For example, when the ground electrode 30 is cut along any plane including the axis of the ground electrode tip 39 (in this embodiment, the axis AX of the spark plug 100), when an outer peripheral surface of the ground electrode tip 39 is in close and gap-free contact with the inner peripheral surface 34i of the ground electrode body 31 (the free end portion 311) forming the hole 34, this close contact portion can be regarded as the press-fit portion 394.

[0043] The length of the press-fit portion 394 in the thickness direction (the length in the axial direction) is referred to as a press-fit length L. The length (the outer diameter of the press-fit portion 394) of the press-fit portion 394 in the direction perpendicular to the thickness direction is referred to as a press-fit diameter D. The press-fit length L can be regarded as the length of the ground electrode tip 39 in the press-fit direction.

[0044] The ground electrode tip 39 is a single element formed from one type of material (e.g., noble metal). That is, the portion from the second discharge surface 395 to the press-fit portion 394 is formed as a single element. A-2. SPARK PLUG MANUFACTURING PROCESS:

[0045] A manufacturing method of the spark plug 100 is described mainly for a manufacturing method of the ground electrode 30. Fig. Figure 3 shows the manufacturing process of the ground electrode 30. First, a ground electrode body 31 having a cuboid rod shape and a ground electrode tip 39 having a substantially cylindrical shape are manufactured. Before the ground electrode tip 39 is press-fitted into the hole 34, an inner diameter D2 of the uniform inner diameter portion 341 of the hole 34 is slightly smaller than an outer diameter D1 of the large outer diameter portion 393 of the ground electrode 30. In the case where the outer diameter D1 of the large outer diameter portion 393 before press-fitting is Φ1 mm (tolerance r6), the inner diameter D2 of the uniform inner diameter portion 341 of the hole 34 before press-fitting is Φ1 mm (tolerance H7). The tolerances r6 and H7 are defined in JIS B0401 (1998).

[0046] The ground electrode tip 39 is press-fitted from the front surface 311F to the rear surface 311B into the hole 34 of the ground electrode body 31, which is fixed by a jig (not shown). Therefore, in the present embodiment, the press-fit direction of the ground electrode tip 39 is a direction along the axis of the ground electrode tip 39 (in this embodiment, along the axis AX of the spark plug 100). When the ground electrode tip 39 is press-fitted into the hole 34, the opposite surface 396 is pressed by a pressing member (not shown). When the ground electrode tip 39 is pressed into the hole 34 from the opposite direction, that is, from the rear surface 311B side toward the front surface 311F, it is necessary to press the second discharge surface 395.In this case, the small outer diameter portion 391 including the second discharge surface 395 is likely to be damaged. Such a defect may cause, for example, breakage of the ground electrode tip 39. Furthermore, in this case, the small outer diameter portion 391 is likely to be deformed during press-fitting. Such deformation may cause the length of the gap G, the area of ​​the second discharge surface 395, and the like to have values ​​other than the desired values. In the present embodiment, these disadvantages are prevented to improve product accuracy.

[0047] The connecting end portion 312 of the thus formed rod-shaped ground electrode 30 is resistance-welded to the front end 50t of the metal shell 50. Next, an assembly obtained by attaching the terminal electrode 40, the center electrode 20, the resistor 70, and the like to the insulator 10 is prepared, and this assembly is fixed to the metal shell 50. Specifically, the disc seal 8, the assembly, the lead seals 6 and 7, and the talc 9 are placed in the through-hole 59 of the metal shell 50. The disc seal 8 is placed between the outer diameter reduced portion 15 of the insulator 10 and the stepped portion 56 of the metal shell 50. The crimp portion 53 of the metal shell 50 is crimped to be bent inward, thereby fixing the metal shell 50 to the insulator 10. Subsequently, the rod-shaped ground electrode 30 is bent to form the gap G.This completes the spark plug 100.

[0048] As described above, in the spark plug 100 according to the present embodiment, the large outer diameter portion 393, which is a portion of the ground electrode tip 39 disposed in the hole 34, is provided with the press-fit portion 394 that is in direct contact with the ground electrode body 31 over the entire circumference thereof. In the press-fit portion 394, the ground electrode body 31 and the ground electrode tip 39 are joined together without a welding portion between them and therefore hardly oxidize even when exposed to high temperatures. Consequently, peeling and falling off of the ground electrode tip 39 can be prevented. In general, a welding portion formed by welding is more likely to oxidize than the base material.In the case where the ground electrode body 31 and the ground electrode tip 39 are joined to each other with such a welding portion between them, when heating and cooling are repeated during use of the spark plug 100, a difference in the linear expansion coefficient between the center electrode tip 29 and the ground electrode body 31 may cause thermal stress at an interface between the center electrode tip 29 and the welding portion and at an interface between the welding portion and the ground electrode body 31, which may lead to peeling.In contrast, in the press-fitting portion 394, even if thermal stress occurs between the ground electrode tip 39 and the inner peripheral surface 34i of the hole 34 in the direction parallel to the inner peripheral surface 34i, the connection is not reduced by the press-fitting, thereby not causing peeling or falling off of the ground electrode tip 39 due to the thermal stress.

[0049] In the present embodiment, since the portion from the second discharge surface 395 to the press-fit portion 394 is formed as a single member, a reduction in thermal conductivity and a reduction in thermal conduction performance can be prevented compared with, for example, the case where a welded portion is provided in the portion from the second discharge surface 395 to the press-fit portion 394. The reason for this is as follows. Since a welded portion includes the constituents of two base materials bonded to each other via the welded portion, the thermal conductivity of the welded portion is generally lower than the thermal conductivities of the base materials. As can be seen from the above description, according to the spark plug 100 of the present invention, peeling and falling off of the ground electrode tip 39 can be prevented, and the thermal conduction performance of the spark plug 100 can be improved.

[0050] According to the present invention, the hole 34 penetrates the ground electrode body 31. Therefore, the ground electrode tip 39 can be press-fitted into the hole 34 from the side (front end surface 311F side) opposite the side facing the center electrode 20. Consequently, during press-fitting, the small outer diameter portion 391 on the second discharge surface 395 side can be prevented from being damaged, and deformation of the small outer diameter portion 391 can be prevented.

[0051] According to the present embodiment, the length of the portion of the ground electrode tip 39 in the direction parallel to the second discharge surface 395 on the second discharge surface 395 side relative to the press-fit portion 394 (in the present embodiment, each of the outer diameters of the enlarged outer diameter portion 392 and the small outer diameter portion 391) is smaller than the length of the press-fit portion 394 in the direction parallel to the first discharge surface 295 (in the present embodiment, the outer diameter D of the large outer diameter portion 393).Consequently, when the ground electrode tip 39 is press-fitted into the hole 34 from the side (front end surface 311F side) opposite the side facing the center electrode 20, the portion of the ground electrode tip 39 on the second discharge surface 395 side opposite the press-fit portion 394 (in the present embodiment, each of the larger outer diameter portion 392 and the small outer diameter portion 391) can be prevented from coming into contact with the hole 34. Therefore, the portion of the ground electrode tip 39 on the second discharge surface 395 side relative to the press-fit portion 394 can be prevented from being damaged and deformed during press-fitting. B. ASSESSMENT TEST

[0052] In an evaluation test regarding the spark plug 100, 13 samples 1 to 13, which differed from each other in at least one of the press-in length L and the press-in diameter D, were prepared. Specifically, the press-in diameter D of each of the samples 1 to 13 is one of 0.6 mm, 0.8 mm, 1 mm, and 1.2 mm. The press-in length L of samples 1 to 3, each having the press-in diameter D of 0.6 mm, is set to 0.4 mm, 0.6 mm, and 0.8 mm, whereby the values ​​of (D / L) thereof are set to 1.5, 1.00, and 0.75, respectively. The press-in lengths L of samples 4 to 6, each having the press-in diameter D of 0.8 mm, are set to 0.6 mm, 0.8 mm and 1.2 mm, whereby the values ​​of (D / L) thereof are set to 1.33, 1.00 and 0.67, respectively.The offset lengths L of samples 7 to 10, each having the offset diameter D of 1 mm, are set to 0.6 mm, 1 mm, 1.4 mm, and 1.7 mm, whereby the values ​​of (D / L) thereof are set to 1.67, 1.00, 0.71, and 0.59, respectively. The offset lengths L of samples 11 to 13, each having the offset diameter D of 1.2 mm, are set to 0.6 mm, 0.9 mm, and 1.6 mm, whereby the values ​​of (D / L) thereof are set to 2.00, 1.33, and 0.75, respectively.

[0053] The press-in length L was adjusted by changing the axial length of the large outer diameter portion 393 of the ground electrode tip 39 and the axial length of the hole 34 (the axial length of the free-end portion 311 of the ground electrode body 31). The press-in diameter D was adjusted by changing the outer diameter of the large outer diameter portion 393 and the inner diameter of the hole 34.

[0054] The elements common to each sample are as follows.

[0055] The material of the ground electrode tip 39: platinum alloy

[0056] The material of the ground electrode body 31: NCF600

[0057] In a vibration test conducted as an evaluation test, each specimen was mounted on a vibration testing machine and vibrated in the axial direction at an acceleration of 50 G and a frequency of 100 Hz for 100 hours. The vibration test was conducted in a state where an area around the ground electrode tip 39 of each specimen was heated to 1000 degrees Celsius with a torch.

[0058] A sample in which the ground electrode tip 39 fell off during the test was rated as "C." A sample in which no falling off of the ground electrode tip 39 was observed but a displacement of the ground electrode 39 in the axial direction was observed at the end of the test was rated as "B." A sample in which neither falling off nor displacement of the ground electrode tip 39 was observed at the end of the test was rated as "A." The results of the evaluation test are shown in Table 1 below. Table Nr. Einpressdurchmesser D Einpresslänge L D / L Beurteilung 1 0,6 0,4 1,50 C 2 0,6 0,6 1,00 A 3 0,6 0,8 0,75 A 4 0,8 0,6 1,33 B 5 0,8 0,8 1,00 A 6 0,8 1,2 0,67 A 7 1 0,6 1,67 C 8 1 1 1,00 A 9 1 1,4 0,71 A 10 1 1,7 0,59 A 11 1,2 0,6 2,00 C 12 1,2 0,9 1,33 B 13 1,2 1,6 0,75 A

[0059] Samples 1, 7, and 11, for which the ratio (D / L) was greater than 1.33, were rated as "C" regardless of the offset diameter D. While samples 2 to 6, 8 to 10, 12, and 13, for which the ratio (D / L) was not greater than 1.33, were rated as "B" or better regardless of the offset diameter D.

[0060] The reason for this is assumed to be as follows. With a larger press-in diameter D, that is, with a larger size of the ground electrode tip 39, the weight of the ground electrode tip 39 increases, and thus the force applied during vibration increases. Meanwhile, due to the longer press-in length L, the joining force between the ground electrode tip 39 and the ground electrode body 31 increases. Therefore, if the press-in length L is too short compared to the press-in diameter D, sufficient joining force cannot be achieved, while sufficient joining force can be achieved if the press-in length L is long enough compared to the press-in diameter D.

[0061] In the case where the value of the ratio (D / L) is larger than 1.33, since the press-in length L is excessively short with respect to the press-in diameter D, sufficient joining force cannot be obtained and the ground electrode tip 39 falls off. In the case where the value of the ratio (D / L) is not larger than 1.33, since the press-in length L is not excessively short with respect to the press-in diameter D, the ground electrode tip 39 can be fixed to the ground electrode body 31 with sufficient strength.

[0062] As described above, the assessment test determines that (D / L) ≤ 1.33 is preferable.

[0063] Among samples 2 to 6, 8 to 10, 12, and 13 with the ratio (D / L) not exceeding 1.33, samples 4 and 12 with the ratio (D / L) exceeding 1 were rated as "B." On the other hand, samples 2, 3, 5, 6, 8 to 10, and 13 with the ratio (D / L) not exceeding 1 were rated as "A."

[0064] The reason for this is as follows. In the case where the value of the ratio (D / L) is not greater than 1, by making the press-fit length L sufficiently longer than the press-fit diameter D, the ground electrode tip 39 can be fixed to the ground electrode body 31 with more sufficient strength than in the case where the value of the ratio (D / L) is greater than 1. As described above, it is determined through the evaluation test that (D / L) ≤ 1 is particularly satisfied. C. SECOND EMBODIMENT

[0065] Fig. 4A and Fig. 4B show explanatory diagrams for a spark plug according to a second embodiment. Fig. Figure 4A shows a cross-sectional view of an area AAb corresponding to the rectangular area AA shown in Fig. 2B of the first embodiment. Fig. Figure 4B shows the section that Fig. 4A, viewed from the front side in the rearward direction BD along the axis AX.

[0066] A ground electrode tip 39b of the second embodiment, like the ground electrode tip 39 of the first embodiment, includes: a small outer diameter portion 391b; a large outer diameter portion 393b located on the front side relative to the small outer diameter portion 391b; and an enlarged outer diameter portion 392b between the small outer diameter portion 391b and the large outer diameter portion 393b. The small outer diameter portion 391b is a portion having a second discharge surface 395b. The large outer diameter portion 393b is a portion having a larger outer diameter than that of the small outer diameter portion 391b and having an opposing surface 396b.

[0067] The large outer diameter portion 393b includes a press-fit portion 394b and a welding target portion 397b located on the opposite surface 396b side relative to the press-fit portion 394b. Like the press-fit portion 394 of the first embodiment, the press-fit portion 394b is a portion that is in direct contact with an inner peripheral surface 34bi of a hole 34b over the entire circumference thereof in the circumferential direction. The welding target portion 397b is a portion connected to a ground electrode body 31b (a free-end portion 311b) via a welding portion WP. The welding portion WP is formed by laser welding, so that the constituent part of the ground electrode body 31b and the constituent part of the ground electrode tip 39b are melted and solidified.

[0068] As in Fig. As shown in Figure 4C, the welding portion WP is formed over the entire circumference along a contact surface between the ground electrode tip 39b and the hole 34b. In a modified example, the welding portion WP may not be formed over the entire circumference. For example, a plurality of welding portions WP may be formed at a distance from each other along the contact surface between the ground electrode tip 39b and the hole 34b.

[0069] The ground electrode of the second embodiment is manufactured as follows. First, as with the ground electrode 30 of the first embodiment, the substantially cylindrical ground electrode tip 39b is press-fitted into the hole 34b of the ground electrode body 31b (the free-end portion 311b). Then, laser cutting is performed over the entire circumference along the contact surface between the ground electrode tip 39b and the hole 34b from the opposite surface 396b side in a direction perpendicular to the opposite surface 396b. This forms the weld portion WP.

[0070] According to the aforementioned second embodiment, as in the first embodiment, the press-fit portion 394b is provided, and the portion from the second discharge surface 395b to the press-fit portion 394b is formed as a single element. Thus, peeling and falling off of the ground electrode tip 39b can be prevented, and the heat conduction performance of the spark plug can be improved.

[0071] According to the second embodiment, by providing the welding portion WP connecting the ground electrode body 31b to the welding target portion 397b, peeling or falling off of the ground electrode tip 39b is further prevented. For example, even if the welding portion is provided on the second discharge surface 395b side relative to the press-fit portion 394b, the welding portion comes into contact with the spark discharge and is damaged by the impact of the discharge, and therefore, wear due to oxidation is more likely. However, in this embodiment, by providing the welding portion WP on a portion opposite the second discharge surface 395b that is unlikely to be damaged, peeling and falling off of the ground electrode tip 39b can be further prevented. D. THIRD EMBODIMENT

[0072] In the aforementioned first and second embodiments, the ground electrode bodies 31 and 31b each function as a tip holding portion for holding the ground electrode tip, respectively. Alternatively, the tip holding portion may be another member. An example in which the tip holding portion is a metal shell will be described as a third embodiment.

[0073] Fig. 5 shows a cross-sectional view of a region around a gap G of a spark plug according to the third embodiment. A front end of a metal shell 50c (a front end of a fastening screw portion 52c) of the third embodiment is located on the front side relative to a front end of an insulator 10c (a front end of a leg portion 13c). The front end of the metal shell 50c is located on the front side relative to a front end of a center electrode 20c, that is, relative to a first discharge surface 295c of a center electrode tip 29c connected to a front end of a leg portion 25c. A portion of the fastening screw portion 52c on the front side relative to the first discharge surface 295c has a hole 34c penetrating the fastening screw portion 52c in the direction perpendicular to the axis AX of the spark plug.

[0074] The hole 34c has a large inner diameter portion 342c on the outer peripheral surface side of the fixing screw portion 52c, and a small inner diameter portion 341c on the inner peripheral surface side of the fixing screw portion 52c.

[0075] A ground electrode tip 39c of the third embodiment has a substantially cylindrical shape extending in the direction perpendicular to the axis AX. That is, in the third embodiment, the axis AX of the spark plug and an axis AXc of the ground electrode tip 39c are perpendicular to each other. The ground electrode tip 39c includes: a small outer diameter portion 391c; a large outer diameter portion 393c located on the outer peripheral side relative to the small outer diameter portion 391c; and an enlarged outer diameter portion 392c located between the small outer diameter portion 391c and the large outer diameter portion 393c. The small outer diameter portion 391c is a portion having a second discharge surface 395c.The large outer diameter portion 393c is a portion having a larger outer diameter than that of the small outer diameter portion 391c and an opposing surface 396c. The outer diameter of the large outer diameter portion 393c is approximately equal to the inner diameter of the small inner diameter portion 341c of the fastening screw portion 52c. In the third embodiment, the second discharge surface 395c is a side surface of the substantially cylindrical ground electrode tip 39c.

[0076] As in Fig. 5, the ground electrode tip is press-fitted into the hole 34c of the metal shell 50c from the outer peripheral side toward the inner peripheral side. Therefore, the larger outer diameter portion 393c of the ground electrode tip 39c has a press-fit portion 394c that comes into direct contact with the small inner diameter portion 341c of the hole 34c over the entire circumference thereof in the circumferential direction about the axis AXc.

[0077] In the aforementioned third embodiment, the fastening screw portion 52c of the metal shell 50c has the hole 34c and functions as a ground electrode tip holding portion for holding the electrode tip 39c. According to the third embodiment, as in the first embodiment, the press-fit portion 394c is provided, and the portion from the second discharge surface 395c to the press-fit portion 394c is formed as a single member. Therefore, peeling and falling off of the ground electrode tip 39c can be prevented, and the heat conduction performance of the spark plug can be improved. E. FOURTH EMBODIMENT

[0078] In the aforementioned first to third embodiments, the ground electrode tip 39, 39b, or 39c is press-fitted to the tip holding portion (the ground electrode body 31 or 31b or the metal shell 50c). Instead of or in conjunction with the ground electrode tip, the center electrode tip may be press-fitted to the tip holding portion (e.g., the center electrode body).

[0079] Fig. 6 shows a cross-sectional view of a region around a center electrode tip 29d of a spark plug according to an exemplary alternative embodiment. A center electrode 20d of this embodiment has a hole 251d extending along the axis AX at a front end of a leg portion 25d of a center electrode body 21d. The hole 251d has a bottom 252d. That is, the hole 251d does not penetrate the center electrode body 21d and is open only at the front side.

[0080] The center electrode tip 29d has a small outer diameter portion 293d on the rear side and a large outer diameter portion 291d on the front side. The outer diameter of the small outer diameter portion 293d is approximately equal to the inner diameter of the hole 251d of the center electrode body 21d. The outer diameter of the large outer diameter portion 291d is larger than the outer diameter of the small outer diameter portion 293d. A front surface of the large outer diameter portion 291d is a first discharge surface 295d, which forms a gap for discharging between itself and a second discharge surface of a ground electrode (not shown).

[0081] As in Fig. 6, the center electrode tip 29d is press-fitted into the hole 251d of the center electrode body 21d from the first discharge surface 295d side (front side) toward a side of an opposite surface 296d (rear side). Therefore, the small outer diameter portion 293d of the center electrode tip 29d has a press-fit portion 294d that comes into direct contact with the hole 251d over the entire circumference thereof in the circumferential direction about the axis AX.

[0082] According to the aforementioned exemplary embodiment, the press-fit portion 294d is provided, and a portion from the first discharge surface 295d to the press-fit portion 294d is formed as a single element. Therefore, peeling and falling off of the center electrode tip 29d can be prevented, and the heat conduction performance of the spark plug can be improved. F. MODIFICATIONS:

[0083] (1) In the first embodiment, the hole 34 penetrates the ground electrode body 31 in the direction of the axis AX ( Fig. 2). Alternatively, the hole 34 may be opened only at the rear side (side of the rear surface 311B) without penetrating the ground electrode body 31. In this case, for example, the ground electrode tip 39 may be press-fitted into the hole 34 from the rear side toward the front side. Likewise, in the second embodiment, the hole 34b does not need to penetrate the ground electrode body 31b. In the third embodiment, the hole 34c does not need to penetrate the fixing screw portion 52c.

[0084] (2) In the respective embodiments described above, the shapes of the ground electrode tips 39, 39b, and 39c and the shape of the center electrode tip 29d are merely examples, and these tips may have other shapes and / or dimensions. For example, these tips may have shapes and / or dimensions that do not satisfy (D / L) ≤ 1.33. Each ground electrode tip may have a shape other than a cylindrical shape, such as a quadrangular prism shape.

[0085] The spark plug may be one in which a discharge-enabling gap is formed between a cylindrical ground electrode tip extending in the direction perpendicular to the axis AX and a side surface of a cylindrical center electrode tip. In this case, for example, a hole penetrating the ground electrode body in the direction perpendicular to the axis AX may be formed near the front end of the ground electrode body extending in the direction parallel to the axis AX, and a cylindrical ground electrode tip extending in the direction perpendicular to the axis AX may be press-fitted into the hole.

[0086] (3) Although the structures of the ground electrode tips 39, 39b, and 39c and the center electrode tip 29d have been substantially described in the above embodiments, the materials, dimensions, and the like of other elements, such as the center electrode 20, the terminal electrode 40, and the ground electrode 30, may be variously changed. For example, in the first embodiment, the center electrode may be configured without a noble metal tip. In the fourth embodiment, the ground electrode may be configured without a noble metal tip. Also, different structures and materials may be adopted with respect to the structures and materials of the metal shells 50 and 50c. For example, the material of the metal shell 50 may be a low-carbon steel plated with zinc, nickel, or the like, or may be a low-carbon steel not plated therewith.

[0087] Although the present invention has been described above based on the embodiments and modified embodiments, the above-described embodiments of the invention are intended to facilitate understanding of the present invention, but not to limit the present invention. The present invention may be changed and modified without departing from its spirit, and the scope of the claims and equivalents thereof are included in the present invention. DESCRIPTION OF REFERENCE SYMBOLS 5 Seal 6, 7 Cable seal 8 Window seal 9 Talc 10, 10c insulator 12 axial hole 12L section with large inner diameter 12S section with small inner diameter 13, 13c thigh section 15 Section with reduced outer diameter 16 Section with reduced inner diameter 17 front fuselage section 18 rear fuselage section 19 Flange section 20, 20c, 20d center electrode 21, 21d center electrode body 23 Head section 24 Flange section 25, 25c, 25d leg section 29, 29c, 29d center electrode tip 30 Ground electrode 31, 31b Ground electrode body 34, 34b, 34c holes 39, 39b, 39c ground electrode tip 40 connection electrode 41 Cap mounting section 42 Flange section 43 thigh section 50, 50c metal case 51 Tool engagement section 52, 52c fastening screw section 53 Crimping section 54 seating section 56 step section 58 compression deformation section 59 Through hole 60 sealing element 70 resistance 80 sealing element 100 spark plugs 251d hole 252d Floor 291d Large outer diameter section 293d Section with small outer diameter 294d press-fit section 295, 295c, 295d first discharge area 296d opposite surface 311, 311b Section with free end 312 connecting end section 341 Section with uniform inner diameter 341c Small inner diameter section 342 section with enlarged inner diameter 342c Large inner diameter section 391, 391b, 391c Small outer diameter section 392, 392b, 392c Section with enlarged outer diameter 393, 393b, 393c Large outer diameter section 394, 394b, 394c press-fit section 395, 395b, 395c second discharge area 396, 396b, 396c opposite surface 397b Welding target section L Offset length D offset diameter D1 outer diameter FD forward direction BD Reverse direction WP welding section AX axis

Claims

[1] Spark plug (100), comprising: a central electrode (20, 20c); and a ground electrode (30) forming a gap (G) between the ground electrode (30) and the center electrode (20, 20c), wherein the ground electrode (30) has a tip holding portion (31, 31b, 52c) with a hole (34, 34b, 34c) and an electrode tip (39, 39b, 39c) having a discharge surface (395, 395b, 395c) forming the gap (G) and at least a portion of which is arranged in the hole (34, 34b, 34c) of the tip holding portion (31, 31b, 52c), wherein the portion of the electrode tip (39, 39b, 39c) disposed in the hole (34, 34b, 34c) has a press-in portion (394, 394b, 394c) directly in contact with the tip holding portion (31, 31b, 52c) over an entire circumference thereof, and wherein a portion of the electrode tip (39, 39b, 39c) from the discharge surface (395, 395b, 395c) to the press-in portion (394, 394b, 394c) is formed as a single element, wherein the hole (34, 34b, 34c) penetrates the tip holding portion (31, 31b, 52c), and wherein a portion (391, 392, 391b, 392b, 391c, 392c) of the electrode tip (39, 39b, 39c) on a side relative to the press-in portion (394, 394b, 394c) closer to the discharge surface (395, 395b, 395c) has a length in a direction parallel to the discharge surface (395, 395b, 395c), the length being smaller than a length of the press-in portion (394, 394b, 394c) in the direction parallel to the discharge surface (395, 395b, 395c). [2] Spark plug (100) according to claim 1, wherein when a surface of the electrode tip (39b) opposite the discharge surface (395b) is referred to as an opposite surface (396b), the spark plug (100) further comprises a welding portion (WP) connecting the tip holding portion (31, 31b, 52c) to a portion of the electrode tip (39, 39b, 39c) on a side closer to the opposing surface (396b) relative to the press-fit portion (394, 394b, 394c). [3] The spark plug (100) according to claim 1 or 2, wherein when a length of the press-fit portion (394, 394b, 394c) in a thickness direction of the electrode tip (39, 39b, 39c) is L, and a length of the press-fit portion (394, 394b, 394c) in a direction perpendicular to the thickness direction is D, (D / L) ≤ 1.33 is satisfied. [4] Spark plug (100) according to claim 3, wherein (D / L) ≤ 1 is satisfied. [5] The spark plug (100) according to any one of claims 1 to 4, further comprising a metal shell (50c) having a fixing screw portion (52c), wherein the ground electrode having the tip holding portion (34c) is formed by a portion of the fixing screw portion (52c). [6] Spark plug (100) according to any one of claims 1 to 4, further comprising a metal shell (50), wherein the ground electrode (30) is connected to a front end (50t) of the metal shell (50).

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