Spark plugs for internal combustion engines

Abstract

To provide a spark plug which can ensure bonding strength between a ground electrode and a housing.SOLUTION: A spark plug 1 for an internal combustion engine has a cylindrical insulator 3, a center electrode 4, a cylindrical housing 2, and a ground electrode 6. The ground electrode 6 defines a discharge gap G between itself and the center electrode 4, and is bonded to a tip end of the housing 2. The ground electrode 6 has a main part 61 which is mainly made of platinum. The ground electrode 6 is fixed to the housing 2 via the main part 61 directly bonded to the housing 2. The spark plug 1 also has a metal coating layer 5 which substantially does not contain platinum. The metal coating layer 5 at least covers a surface of the main part 61, and a surface of a bonding part 11 between the main part 61 and the housing 2.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a spark plug for an internal combustion engine.

Background Art

[0002] For example, as disclosed in Patent Document 1, there is known a spark plug provided with a ground electrode welded to the tip of a housing and having a main part mainly composed of platinum. In the spark plug described in Patent Document 1, the main part of the ground electrode is covered with a nickel alloy layer. This is intended to suppress the occurrence of a so-called sweating phenomenon caused by factors such as the diffusion of sputtering atoms at high temperatures, and to achieve a longer life of the spark plug.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in the spark plug described in Patent Document 1, a nickel alloy layer is interposed at the joint between the housing and the ground electrode. Therefore, there is a possibility that the joint strength between the housing and the ground electrode cannot be sufficiently ensured.

[0005] The present invention has been made in view of such problems, and an object thereof is to provide a spark plug capable of ensuring the joint strength between a ground electrode and a housing.

Means for Solving the Problems

[0006] One aspect of the present invention is a cylindrical insulator (3), a center electrode (4) held on the inner peripheral side of the insulator and exposed from the insulator to the tip side (Z1), A cylindrical housing (2) that holds the aforementioned insulating insulator on its inner circumference, It has a discharge gap (G) between it and the central electrode, and a ground electrode (6) joined to the tip of the housing, The ground electrode has a main part (61) whose main component is platinum, The ground electrode is fixed to the housing by the main part being directly joined to the housing. The spark plug (1) for an internal combustion engine has a metal coating layer (5) that covers at least the surface of the main part and the surface of the joint (11) between the main part and the housing, and which is substantially free of platinum. [Effects of the Invention]

[0007] In the spark plug described above, the main part of the ground electrode, which is primarily composed of platinum, is directly bonded to the housing. Therefore, the ground electrode can be firmly fixed to the housing. As a result, the bonding strength between the ground electrode and the housing can be ensured.

[0008] As described above, according to the above embodiment, it is possible to provide a spark plug that can ensure the bonding strength between the ground electrode and the housing. The symbols in parentheses in the claims and the means for solving the problem indicate the correspondence with the specific means described in the embodiments later, and do not limit the technical scope of the present invention. [Brief explanation of the drawing]

[0009] [Figure 1] A half-cross-sectional view of the spark plug in Embodiment 1. [Figure 2] A half-cross-sectional view of the area near the tip of the spark plug in Embodiment 1. [Figure 3] This is a view of the spark plug in Embodiment 1, seen from the tip side, as shown by arrow III in Figure 2. [Figure 4] View from arrow IV in Figure 2. [Figure 5]A half-cross-sectional view showing the shortest distance from the ground electrode to the plug's central axis in Embodiment 1. [Figure 6] A half-cross-sectional view showing how the main part is brought into contact with the housing in Embodiment 1. [Figure 7] A half-section view of the housing with the main parts joined together, according to Embodiment 1. [Figure 8] A half-cross-sectional view of a housing in which a metal coating layer has been formed by carrying out a barrel plating process, according to Embodiment 1. [Modes for carrying out the invention]

[0010] (Embodiment 1) Embodiments relating to spark plugs for internal combustion engines will be described with reference to Figures 1 to 8. The spark plug 1 for an internal combustion engine in this embodiment, as shown in Figures 1 to 3, comprises a cylindrical insulator 3, a center electrode 4, a cylindrical housing 2, and a ground electrode 6. The center electrode 4 is held on the inner circumference side of the insulator 3 and is exposed from the insulator 3 to the tip side Z1. The housing 2 holds the insulator 3 on the inner circumference side. The ground electrode 6 forms a discharge gap G with the center electrode 4 and is joined to the tip of the housing 2.

[0011] As shown in Figure 2, the ground electrode 6 has a main portion 61 that is mainly composed of platinum. The ground electrode 6 is fixed to the housing 2 by the main portion 61 being directly joined to the housing 2.

[0012] Furthermore, the spark plug 1 has a metal coating layer 5 that is substantially free of platinum. The metal coating layer 5 covers at least the surface of the main part 61 and the surface of the joint 11 between the main part 61 and the housing 2. Note that "substantially free of platinum" includes cases where platinum is present as an unavoidable impurity.

[0013] The spark plug 1 of this embodiment can be used as an ignition means in an internal combustion engine such as an automobile. The spark plug 1 of this embodiment is provided in an internal combustion engine (not shown) that uses hydrogen gas as fuel. The threaded portion 21 of the housing 2 is screwed into the female threaded portion of the plug hole of the cylinder head (not shown), and the spark plug 1 is attached to the internal combustion engine. And one end in the axial direction Z of the spark plug 1 is arranged in the main combustion chamber of the internal combustion engine. In the axial direction Z of the spark plug 1, the side exposed to the main combustion chamber is referred to as the tip side Z1, and the opposite side is referred to as the base end side Z2. Also, the axial direction Z of the spark plug 1 is also appropriately referred to as the plug axial direction Z. Note that the plug central axis C means the central axis C of the spark plug 1. Also, the plug radial direction means the radial direction of a circle centered on the plug central axis C on a plane orthogonal to the plug central axis C. Also, the plug circumferential direction is the direction along the circumference centered on the plug central axis C. Also, the plug central axis C is also the central axis of the center electrode 4 and the housing 2 in this embodiment.

[0014] The center electrode 4 has a long shape in the plug axial direction Z. The center electrode 4 is inserted and held on the inner peripheral side of the insulator 3. As shown in FIG. 2, the tip surface 41 of the center electrode 4 is located on the tip side Z1 with respect to the tip of the insulator 3 and the tip of the housing 2.

[0015] In this embodiment, a chip 42 mainly composed of iridium and substantially containing no platinum is provided at the tip portion of the center electrode 4. The chip 42 has a tip surface 41 and forms a discharge gap G with the ground electrode 6. The tip surface 41 formed on the chip 42 is formed so as to be orthogonal to the plug axial direction Z and is a flat surface. Also, the portion of the center electrode 4 on the base end side Z2 with respect to the chip 42 can be made of, for example, a metal material mainly composed of nickel. In this embodiment, the portion of the center electrode 4 on the base end side Z2 with respect to the chip 42 also substantially contains no platinum.

[0016] As shown in FIG. 1, a first glass seal layer 12, a resistor 13, a second glass seal layer 14, and a terminal fitting 15 are arranged on the proximal end side Z2 of the center electrode 4 inside the insulator 3. The first glass seal layer 12 and the second glass seal layer 14 can be made of, for example, glass containing copper powder. The resistor 13 can be made of, for example, an aggregate in which a conductive material such as a carbon material is dispersed in a base material including a glass material and an aggregate. The end of the proximal end side Z2 of the terminal fitting 15 is exposed from the insulator 3. The spark plug 1 is electrically connected to an ignition coil (not shown) at the terminal fitting 15. The terminal fitting 15 can be made of, for example, an iron alloy.

[0017] The insulator 3 is locked to the housing 2 in the plug axial direction Z. As shown in FIG. 2, a metal packing 16 for ensuring the sealing property between the insulator 3 and the housing 2 is provided at the locking portion between the insulator 3 and the housing 2. In this embodiment, the packing 16 does not substantially contain platinum. Further, the insulator 3 is made of a ceramic such as alumina.

[0018] In this embodiment, the tip of the housing 2 has an inner protruding portion 25 protruding inward in the plug diameter direction. The inner protruding portion 25 protrudes toward the plug central axis C and is formed in an annular shape. The insulator 3 is locked to the inner protruding portion 25 in the plug axial direction Z via the packing 16. The housing 2 can be made of, for example, low-carbon steel, stainless steel, or the like. In this embodiment, the housing 2 is made of low-carbon steel.

[0019] As shown in Figures 4 and 6, a recess 22 is formed at the tip of the housing 2. The recess 22 is formed by a part of the tip side surface of the housing 2 receding toward the base end Z2. The recess 22 opens toward the tip end Z1. A part of the main portion 61 of the ground electrode 6 is positioned inside the recess 22 and fixed to the recess-forming surface 221 that forms the recess 22. The recess-forming surface 221 has two circumferentially opposing surfaces 222 that face each other in the circumferential direction of the plug, and one axially opposing surface 223 that faces the main portion 61 in the plug axial direction Z. In this embodiment, the main portion 61 is joined to the two circumferentially opposing surfaces 222 and the axially opposing surface 223, respectively.

[0020] As shown in Figures 2 and 5, the grounding electrode 6 is positioned to be inclined with respect to the plug axis direction Z. In other words, the central axis of the grounding electrode 6 is inclined with respect to the plug axis direction Z. Furthermore, the grounding electrode 6 protrudes toward the plug central axis C without bending. The grounding electrode 6 is formed to move toward the tip side Z1 as it approaches the plug central axis C.

[0021] The base end side surface 62 of the ground electrode 6 is formed to curve toward the tip side Z1 as it approaches the plug's central axis C. The base end side surface 62 forms a discharge gap G with the central electrode 4 and is flat overall.

[0022] As shown in Figures 2 and 5, in a cross-section including the plug's central axis C and along the protruding direction of the ground electrode 6, the base end side surface 62 is formed in a straight line and is inclined with respect to the inner circumferential surface 24 of the inner protrusion 25. As shown in Figure 5, in a cross-section including the plug's central axis C and along the protruding direction of the ground electrode 6, the angle θ1 between the base end side surface 62 and the inner circumferential surface 24 of the inner protrusion 25 can be, for example, 110° or more.

[0023] Furthermore, the base side surface 62 is formed to be inclined with respect to the tip surface 41 of the central electrode 4. As shown in Figure 3, when viewed from the plug axis direction Z, a part of the base side surface 62 and a part of the tip surface 41 of the central electrode 4 overlap each other. Also, the plug central axis C does not pass through the ground electrode 6. As shown in Figure 5, the angle θ2 between the base side surface 62 and the tip surface 41 can be, for example, 20° or more.

[0024] In this embodiment, the shortest distance L from the ground electrode 6 to the plug's central axis C is smaller than the radius R of the outer diameter at the tip of the central electrode 4. In other words, the shortest distance L is smaller than the radius R of the outer diameter of the tip 42.

[0025] Furthermore, the spark plug 1 has two or more ground electrodes 6. In this embodiment, it has two ground electrodes 6 and two discharge gaps G. The shortest distance from one ground electrode 6 to the other ground electrode 6 is twice the distance L. As shown in Figure 3, when viewed from the plug axis direction Z, the protruding ends of the two ground electrodes 6 are arranged to face each other in the plug radial direction. Also, when viewed from the plug axis direction Z, the two ground electrodes 6 are arranged in a straight line, and one ground electrode 6 protrudes toward the other ground electrode 6. When viewed from the plug axis direction Z, the two ground electrodes 6 are arranged to be symmetrical with respect to a predetermined straight line passing through the plug central axis C.

[0026] Furthermore, as shown in Figures 2 and 5, the main part 61 of the ground electrode 6 can contain, for example, 50% by mass or more of platinum. In addition to platinum, the main part 61 can also contain, for example, iridium, rhodium, etc.

[0027] The main part 61 is joined to the tip of the housing 2 by welding. The joint 11 between the main part 61 and the housing 2 has a molten portion 111 formed by welding. In this embodiment, the molten portion 111 contains platinum.

[0028] Furthermore, the ground electrode 6 has a metal coating layer 5 that covers the surface of the main part 61. A base end side surface 62 is formed on the metal coating layer 5. The discharge gap G is formed between the tip 42 of the central electrode 4 and the metal coating layer 5 with the base end side surface 62.

[0029] In this embodiment, the metal coating layer 5 covers the entire surface of the main part 61 and the entire surface of the joint 11. In other words, the metal coating layer 5 covers the main part 61 and the joint 11 so that they are not exposed.

[0030] The metal coating layer 5 is made of a metal mainly composed of nickel, zinc, etc. In this embodiment, the metal coating layer 5 is made of nickel or an alloy mainly composed of nickel. The metal coating layer 5 may contain, for example, 50% by mass or more of nickel.

[0031] The metal coating layer 5 further covers the surface of the housing 2. The metal coating layer 5 covering the surface of the housing 2 and the metal coating layer 5 covering the surface of the main part 61 have the same composition and are formed continuously with respect to each other. In other words, the metal coating layer 5 covering the surface of the joint 11, the metal coating layer 5 covering the surface of the housing 2, and the metal coating layer 5 covering the surface of the main part 61 have the same composition and are formed continuously with respect to each other. In this embodiment, the metal coating layer 5 covers the entire surface of the housing 2.

[0032] Furthermore, the thickness of the metal coating layer 5 covering the surface of the joint 11, the thickness of the metal coating layer 5 covering the surface of the housing 2, and the thickness of the metal coating layer 5 covering the surface of the main part 61 are all equal. The thickness of the metal coating layer 5 can be, for example, 2 to 40 μm.

[0033] Next, a method for manufacturing the spark plug 1 of this embodiment will be described. First, as shown in Figure 6, the main part 61 is welded to the housing 2 before the metal coating layer 5 is applied and before the center electrode 4, insulator 3, etc. are assembled. Specifically, as shown by arrow M in Figure 6, the base end side Z2 surface of the main part 61 is brought into contact with the recess-forming surface 221 of the housing 2 in the plug axial direction Z. Then, resistance welding is performed with the main part 61 in contact with the recess-forming surface 221 to join the main part 61 to the housing 2. This produces a housing 2 with the main part 61 joined, as shown in Figure 7. After joining one main part 61 to the housing 2, the other main part 61 is joined to the housing 2 in the same manner. Furthermore, by performing resistance welding, a molten portion 111 is formed at the joint 11 between the main part 61 and the housing 2, where a part of the main part 61 and a part of the housing 2 are melted and solidified. In addition, at the joint 11 before the metal coating layer 5 is applied, a part of the molten portion 111 is exposed.

[0034] Next, a barrel plating process is performed to form a metal coating layer 5 on the surfaces of the main part 61, the joint 11, and the housing 2. In the barrel plating process, the housing 2 to which the main part 61 is joined is placed in a barrel (not shown) containing nickel plating solution, and nickel plating is applied to the surfaces of the main part 61, the joint 11, and the housing 2 while the barrel is rotated. As a result, a metal coating layer 5 is formed on the surfaces of the main part 61, the joint 11, and the housing 2, as shown in Figure 8. Note that in addition to this barrel plating process, known methods can also be used to form the metal coating layer 5 on the surfaces of the main part 61, etc.

[0035] Next, the insulator 3 and other components are assembled to the housing 2 on which the metal coating layer 5 is formed. Specifically, the insulator 3, which has a center electrode 4, a first glass seal layer 12, a resistor 13, a second glass seal layer 14, and terminal fittings 15 arranged inside, is inserted into the housing 2 from the base end side Z2. Then, the base end of the housing 2 is deformed toward the inner circumference and pressed toward the tip side Z1 to crimp it to the insulator 3. This allows the spark plug 1 of this form to be manufactured.

[0036] Next, we will explain the effects and benefits of this embodiment. In the spark plug 1 described above, the main part 61 of the ground electrode 6, which is mainly composed of platinum, is directly bonded to the housing 2. Therefore, the ground electrode 6 can be firmly fixed to the housing 2. As a result, the bonding strength between the ground electrode 6 and the housing 2 can be ensured.

[0037] Furthermore, let's consider a spark plug that does not have a metal coating layer, with the main part, which is mainly composed of platinum, exposed. If this spark plug is installed in an internal combustion engine that uses hydrogen as fuel, which tends to generate a relatively large amount of water, there is a risk that water may directly adhere to the main part, and corrosion may occur in the parts of the main part where water adheres. Therefore, in the present embodiment of the spark plug 1, the metal coating layer 5 covers at least the surface of the main part 61, which is mainly composed of platinum, and the surface of the joint 11. Therefore, when the internal combustion engine is stopped, etc., it is possible to suppress water generated by the combustion of hydrogen from directly adhering to the main part 61 and the joint 11. Therefore, even when the spark plug 1 is installed in an internal combustion engine that uses hydrogen as fuel, which tends to generate a relatively large amount of water, corrosion of the main part 61 and the joint 11 can be suppressed. As a result, the corrosion resistance of the ground electrode 6 and the joint 11 can be improved, and the lifespan can be extended.

[0038] Furthermore, the metal coating layer 5 covers the entire surface of the main part 61 and the entire surface of the joint 11. Therefore, direct adhesion of water to the main part 61 and the joint 11 can be reliably suppressed. As a result, the corrosion resistance of the ground electrode 6 and the joint 11 can be reliably improved.

[0039] The metal coating layer 5 is made of nickel or an alloy mainly composed of nickel. Therefore, the corrosion resistance of the ground electrode 6 and the joint 11 can be further improved.

[0040] The metal coating layer 5 further covers the surface of the housing 2. Therefore, the corrosion resistance of the housing 2 can be improved.

[0041] Furthermore, the metal coating layer 5 covering the surface of the housing 2 and the metal coating layer 5 covering the surface of the main part 61 have the same composition and are formed continuously with respect to each other. Therefore, the metal coating layer 5 can be stably maintained on the surfaces of the housing 2 and the main part 61, and corrosion resistance can be further improved.

[0042] The base end side surface 62 of the ground electrode 6 is formed to curve toward the tip side Z1 as it approaches the plug's central axis C. Furthermore, the base end side surface 62 forms a discharge gap G between itself and the central electrode 4. Therefore, when a discharge occurs in the discharge gap G, the area in which the discharge originates on the base end side surface 62 can be relatively narrowed. Consequently, even if a portion of the metal coating layer 5 having the base end side surface 62 is worn away by the discharge in the discharge gap G, the extent of wear on the metal coating layer 5 can be suppressed. As a result, exposure of the main part 61 can be reduced.

[0043] The base end side surface 62 is entirely flat. Therefore, the distance from the protruding end of the ground electrode 6 to the fixed end of the ground electrode 6 fixed to the housing 2 tends to be short. Therefore, the heat dissipation path from the protruding end of the ground electrode 6 to the housing 2 tends to be short, and the ground electrode 6 can dissipate heat easily. As a result, overheating of the ground electrode 6 can be suppressed, and pre-ignition and the like can be suppressed. In addition, since the discharge gap G can be formed without bending the ground electrode 6, cracking and peeling of the metal coating layer 5 can be reliably suppressed. Therefore, exposure of the main part 61 can be further suppressed. As a result, the corrosion resistance of the ground electrode 6 can be further improved.

[0044] The base end side surface 62 is formed to be inclined with respect to the tip surface 41 of the central electrode 4. Therefore, when a discharge is generated in the discharge gap G, the area in which the discharge originates on the base end side surface 62 can be made relatively narrow. As a result, even if a part of the metal coating layer 5 having the base end side surface 62 is worn away by the discharge generated in the discharge gap G, the area of ​​wear of the metal coating layer 5 can be suppressed.

[0045] Furthermore, when viewed from the plug axial direction Z, a portion of the base end side surface 62 and a portion of the tip surface 41 of the center electrode 4 overlap each other. Also, the plug's central axis C does not pass through the ground electrode 6. Therefore, the ground electrode 6 can be shortened while forming a discharge gap G between the base end side surface 62 and the center electrode 4. As a result, the heat dissipation path from the protruding end of the ground electrode 6 to the housing 2 can be shortened, and multiple ground electrodes 6 can be easily fixed to the housing 2.

[0046] The tip of the central electrode 4 is provided with a tip 42 that is mainly composed of iridium and substantially free of platinum. Therefore, wear on the tip of the central electrode 4 can be suppressed, and corrosion of the tip of the central electrode 4 due to water adhesion can be suppressed. As a result, the lifespan of the central electrode 4 can be extended.

[0047] The spark plug 1 has two or more ground electrodes 6. Therefore, a discharge can be generated between the two or more ground electrodes 6 and the center electrode 4. As a result, wear of the ground electrodes 6 can be suppressed.

[0048] When viewed from the plug axis direction Z, the two ground electrodes 6 are arranged in a straight line. Therefore, it is easy to install the two ground electrodes 6 in the housing 2. In other words, when fixing one ground electrode 6 to the housing 2 and then fixing the other ground electrode 6 to the housing 2, it is possible to avoid interference between the protruding ends of the ground electrodes 6. As a result, productivity can be improved.

[0049] When viewed from the spark plug axial direction Z, the protruding ends of the two ground electrodes 6 are positioned to face each other in the radial direction of the spark plug. Therefore, even when the airflow in the main combustion chamber of an internal combustion engine is relatively fast, the discharge formed in the discharge gap G is less likely to short-circuit. In other words, when the discharge is extended by the airflow, the starting point of the discharge on the ground electrode 6 side tends to move from one ground electrode 6 to the other ground electrode 6. Therefore, excessive extension of the discharge by the airflow can be suppressed, and short-circuiting of the discharge can be easily suppressed. Therefore, ignition performance can be improved. As a result, fuels such as hydrogen can be burned stably.

[0050] The grounding electrode 6 has a main part 61 whose main component is platinum. Therefore, the heat resistance of the grounding electrode 6 can be improved.

[0051] The main part 61 is positioned so as to be inclined with respect to the plug axis direction Z and is joined to the recess 22 of the housing 2. Therefore, the joint 11 can be positioned away from the tip of the spark plug 1 in the plug axis direction Z. As a result, water generated by the combustion of hydrogen fuel is less likely to adhere to the vicinity of the joint 11. Consequently, even if a part of the joint 11 is exposed, corrosion of the joint 11 can be suppressed.

[0052] In this embodiment, a metal coating layer 5 is also formed on the surface of the housing 2. Therefore, the corrosion resistance of the housing 2 can be improved.

[0053] The metal coating layer 5 is formed after the main part 61 is joined to the housing 2. Therefore, the entire surface of the main part 61 and the entire surface of the joint 11 can be easily covered by the metal coating layer 5. In other words, if the main part of the ground electrode is covered with the metal coating layer and then the ground electrode is welded to the housing, the metal coating layer near the joint between the ground electrode and the housing may melt into the molten part formed during welding, making it easy for a part of the main part to be exposed. Therefore, in this embodiment, the metal coating layer 5 is formed after the main part 61 is joined to the housing 2. This makes it possible to fix the ground electrode 6 to the housing 2 without exposing the main part 61.

[0054] In this embodiment, the metal coating layer 5 is formed by a barrel plating process. Therefore, the metal coating layer 5 can be formed on multiple housings 2 simultaneously. As a result, productivity can be improved.

[0055] As described above, this embodiment provides a spark plug 1 that can ensure the bonding strength between the ground electrode 6 and the housing 2.

[0056] In Embodiment 1, the housing 2 is made of low-carbon steel. However, the housing may be made of, for example, stainless steel. In this case, the corrosion resistance of the housing can be further improved.

[0057] In Embodiment 1, the metal coating layer 5 is formed by a barrel plating process. However, the metal coating layer can also be formed, for example, by passing the housing with the main parts joined into molten nickel, or by vapor deposition.

[0058] The present invention is not limited to the embodiments described above, and can be applied to various embodiments without departing from its spirit. [Explanation of symbols]

[0059] 1...Spark plug, 2...Housing, 3...Insulator, 4...Center electrode, 5...Metal coating layer, 6...Ground electrode, 11...Joint, 61...Main part, G...Discharge gap, Z1...Tip side

Claims

1. A cylindrical insulator (3), A central electrode (4) is held on the inner circumference side of the insulator and is exposed from the insulator toward the tip side (Z1), A cylindrical housing (2) that holds the insulator on its inner circumference, It has a discharge gap (G) formed between the central electrode and a ground electrode (6) joined to the tip of the housing, The ground electrode has a main part (61) whose main component is platinum, The ground electrode is fixed to the housing by the main part being directly joined to the housing. A spark plug (1) for an internal combustion engine, having a metal coating layer (5) that substantially does not contain platinum, covering at least the surface of the main part and the surface of the joint (11) between the main part and the housing.

2. The spark plug for an internal combustion engine according to claim 1, wherein the metal coating layer is made of nickel or an alloy mainly composed of nickel.

3. The spark plug for an internal combustion engine according to claim 1 or 2, wherein the metal coating layer covers the entire surface of the main part and the entire surface of the joint.

4. The spark plug for an internal combustion engine according to claim 1 or 2, wherein the metal coating layer further covers the surface of the housing, and the metal coating layer covering the surface of the housing and the metal coating layer covering the surface of the main part have the same composition and are formed continuously with respect to each other.

5. The base end side surface (62) of the ground electrode is formed to curve toward the tip as it approaches the plug central axis (C), and the base end side surface forms the discharge gap with the central electrode, and the entire surface is flat, as described in claim 1 or 2 for a spark plug for an internal combustion engine.

6. The base end side surface is formed to be inclined with respect to the tip surface (41) of the central electrode, and when viewed from the plug axis direction (Z), a part of the base end side surface and a part of the tip surface of the central electrode overlap each other, and the plug central axis does not pass through the ground electrode, as described in claim 5 for a spark plug for an internal combustion engine.

7. A spark plug for an internal combustion engine according to claim 1 or 2, having two or more of the aforementioned ground electrodes.

8. The spark plug for an internal combustion engine according to claim 1 or 2, wherein the tip of the central electrode is provided with a tip (42) that is mainly composed of iridium and substantially free of platinum.

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