JP7430490B2 - spark plug - Google Patents

Description

The present specification relates to a spark plug for igniting fuel gas in an internal combustion engine or the like.

In the electrode of a spark plug, an electrode tip made of a noble metal with excellent spark abrasion resistance has conventionally been used in a portion that forms a gap where a spark is generated. As a method for joining the electrode tip to the electrode body, for example, a method using laser welding is known (for example, Patent Document 1).

As shown in Patent Document 1, various shapes of the fused portion formed by laser welding on the contact surface between the electrode tip and the electrode body have been proposed. For example, in order to increase the bonding strength between the electrode tip and the electrode body, it is known to bond the entire contact surface between the electrode tip and the electrode body. In this spark plug, a laminar fused portion is formed between the electrode tip and the electrode body.

JP2012-74271

In such a spark plug, the electrode tip and the electrode body are made of different materials, so thermal stress is applied to the fused portion due to the difference in linear expansion coefficient between the two materials. The thicker the melted part is, the more the thermal stress can be alleviated, so the resistance to peeling of the electrode tip from the electrode body due to the thermal stress (peeling resistance) is improved. On the other hand, in order to thicken the molten zone, it is necessary to increase the energy required for welding during laser welding, which may cause problems such as part of the fused zone adhering to the discharge surface of the electrode tip or near the discharge surface ( So-called melt sagging (rising melt sag) is likely to occur. When melt sagging occurs, the melted part is exposed to electric discharge, which deteriorates the melted part and may reduce peeling resistance.

This specification discloses a technique that can improve the peeling resistance of an electrode tip in a spark plug in which an electrode tip and an electrode body are joined via a molten part.

The technology disclosed in this specification can be realized as the following aspects and application examples.
[Mode]
a center electrode;
a metal shell disposed around the center electrode and insulatingly holding the center electrode;
a rod-shaped electrode body whose one end is a free end and whose other end is connected to the metal shell; an electrode tip having a discharge surface and disposed on a specific side of the electrode body; the electrode body and the electrode; a ground electrode comprising a melted part formed between the chip and the ground electrode;
A spark plug comprising:
A direction along the specific side surface toward the one end from the center of gravity of the discharge surface is a first direction, and a direction opposite to the first direction is a second direction;
A third direction perpendicular to the discharge surface of the molten part in a first cross-section perpendicular to the first direction passing through a first portion of the ground electrode on the first direction side relative to the center of gravity of the discharge surface. Let H3 be the minimum value of the length of
The maximum length of the molten part in the third direction in a second section of the ground electrode that is perpendicular to the second direction and passes through a second portion on the second direction side of the center of gravity of the discharge surface. When the value is H2,
A spark plug characterized by satisfying H3>H2.

[Application example 1] Center electrode,
a metal shell disposed around the center electrode and insulatingly holding the center electrode;
a rod-shaped electrode body whose one end is a free end and whose other end is connected to the metal shell; an electrode tip having a discharge surface and disposed on a specific side of the electrode body; the electrode body and the electrode; a ground electrode comprising a melted part formed between the chip and the ground electrode;
A spark plug comprising:
A direction along the specific side surface toward the one end from the center of gravity of the discharge surface is a first direction, and a direction opposite to the first direction is a second direction;
A third direction perpendicular to the discharge surface of the molten part in a first cross section perpendicular to the first direction passing through a first portion of the discharge surface on the first direction side relative to the center of gravity of the discharge surface. Let H1 be the maximum value of the length of
The maximum length of the melted portion in the third direction in a second cross section that passes through a second portion of the discharge surface on the second direction side relative to the center of gravity of the discharge surface and is perpendicular to the second direction. When the value is H2,
A spark plug characterized by satisfying H1>H2.

The first direction side of the center of gravity is farther from the connection end connected to the metal shell compared to the second direction side of the center of gravity. For this reason, the heat removal efficiency on the first direction side of the center of gravity is lower than on the second direction side of the center of gravity, so the temperature becomes higher than that on the second direction side of the center of gravity. Therefore, the thermal stress generated on the first direction side of the center of gravity is greater than that on the second direction side of the center of gravity. In this embodiment, as described above, since H1>H2 is satisfied, the center of gravity can be The thickness of the melted portion on the first direction D1 side can be ensured more than the thickness of the melted portion on the first direction D1 side. As a result, thermal stress can be alleviated while suppressing the rise of melt sag, thereby improving the peeling resistance of the ground electrode chip.

[Application Example 2] The spark plug according to Application Example 1,
In the first cross section,
The spark plug is characterized in that a position where the length in the third direction is maximum is different from both ends in a fourth direction perpendicular to the third direction.

In the first cross section, both ends in the fourth direction are exposed to the outside air, but a portion different from both ends in the fourth direction is not exposed to the outside air. For this reason, a portion different from both ends in the fourth direction has a lower heat removal efficiency than both ends in the fourth direction, so thermal stress tends to increase, and it is highly necessary to ensure the thickness of the molten part. According to the above configuration, the position where the length of the welded part in the third direction is the maximum is a position different from both ends in the fourth direction, so the position where the length of the welded part in the third direction is the maximum is the position where the length in the third direction is the maximum. It is possible to ensure the thickness of the molten part at a position different from both ends in the fourth direction without excessively increasing the energy required for welding than when the welding part is located at both ends in the fourth direction. Further, if the molten portion is excessively thick at both ends in the fourth direction, melt sag tends to rise at both ends. According to the above configuration, it is possible to suppress the molten portion from becoming excessively thick at both ends in the fourth direction, and to suppress the rise of melt sag at both ends. Therefore, the peeling resistance of the electrode chip can be further improved.

[Application Example 3] The spark plug according to Application Example 2,
In the first cross section,
When the range located at the center in the fourth direction is defined as a central range among three ranges obtained by equally dividing the entire range in the fourth direction where the melting part and the electrode tip contact,
The spark plug, wherein the position where the length in the third direction is maximum is in the central range.

According to the above configuration, since the position where the length in the third direction is maximum is located in the central range, the thickness of the molten part in the central range can be ensured, and the position where the length in the third direction is maximum is located at both ends. It is possible to suppress the melted portion from becoming excessively thick near both ends than when the temperature is within this range. As a result, thermal stress can be alleviated while further suppressing the rise of melt sag.

[Application example 4] The spark plug according to application example 3,
Among the three ranges, the range in which the average length of the fused portion in the third direction is maximum is the central range.

According to the above configuration, it is possible to ensure the thickness of the fused portion in the central range, and it is possible to suppress the fused portion from becoming excessively thick in both end ranges. As a result, thermal stress can be alleviated while further suppressing the rise of melt sag.

[Application Example 5] The spark plug according to Application Example 2,
In the first cross section,
When the range located at both ends in the fourth direction is defined as an end range among three ranges obtained by dividing the entire range in the fourth direction in which the melting part and the electrode tip contact into three equal parts,
The spark plug, wherein the position where the length in the third direction is maximum is located in either of the two end ranges.

[Application Example 6] The spark plug according to Application Example 5,
Among the three ranges, the range where the average length of the melted part in the third direction is maximum is the position where the length in the third direction is maximum among the two end ranges. A spark plug, characterized in that it is a range including.

According to the above configuration, it is possible to ensure the thickness of the fused portion in one of the end ranges, and it is possible to suppress the fused portion from becoming excessively thick in the other end range. Therefore, it is possible to further suppress the rise of melt sag in one of the end ranges.

[Application example 7] The spark plug according to any one of application examples 1 to 6,
In the first cross section, when the minimum value of the length of the molten part in the third direction is H3,
A spark plug characterized by satisfying H3>H2.

According to the above configuration, since H3>H2 is satisfied, the thickness of the fused portion in the first portion can be further ensured without excessively increasing the energy required for welding. As a result, thermal stress can be further alleviated while suppressing the rise of melt sag, so that the peeling resistance of the electrode tip can be further improved.

Note that the technology disclosed in this specification can be realized in various aspects, such as a spark plug, an ignition device using a spark plug, an internal combustion engine equipped with the spark plug, and a spark plug electrode. , a method of welding a spark plug electrode and an electrode tip, a method of manufacturing a spark plug electrode, and the like.

FIG. 1 is a cross-sectional view of a spark plug 100 according to the present embodiment. FIG. 3 is a diagram showing a configuration near a ground electrode tip 39 of a ground electrode 30 according to the embodiment. FIG. 3 is a cross-sectional view of a portion including a ground electrode chip 39 taken along a plane perpendicular to the second direction D2. 5 is a flowchart of a method for manufacturing the ground electrode 30. 1 is a first explanatory diagram of a method for manufacturing a ground electrode 30. FIG. FIG. 3 is a second explanatory diagram of the method for manufacturing the ground electrode 30; A graph conceptually showing thermal energy input in the first welding process. The figure which shows the AA cross section of the state where the melted part 35A was formed in the 1st welding process. An explanatory diagram of a ground electrode 30B of a modified example. An explanatory diagram of a ground electrode 30C of a modified example.

A. Embodiment:
A-1. Spark plug configuration:
FIG. 1 is a cross-sectional view of a spark plug 100 of this embodiment. A dotted line in FIG. 1 indicates the axis CO of the spark plug 100. The direction parallel to the axis CO (vertical direction in FIG. 1) is also referred to as the axial direction. The radial direction of a circle centered on the axis CO is also simply referred to as the "radial direction," and the circumferential direction of the circle centered on the axis CO is also simply referred to as the "circumferential direction." The downward direction in FIG. 1 is also referred to as the distal end direction FD, and the upward direction is also referred to as the rear end direction BD. The lower side in FIG. 1 is called the front end side of the spark plug 100, and the upper side in FIG. 1 is called the rear end side of the spark plug 100.

The spark plug 100 generates a spark discharge in a gap (spark gap) formed between a center electrode 20 and a ground electrode 30, the details of which will be described later. The spark plug 100 is attached to an internal combustion engine and is used to ignite fuel gas within a combustion chamber of the internal combustion engine. The spark plug 100 includes an insulator 10 as an insulator, a center electrode 20, a ground electrode 30, a terminal fitting 40, and a metal shell 50.

The insulator 10 is formed by firing alumina or the like. The insulator 10 is a substantially cylindrical member that extends along the axial direction and has an axial hole 12 that is a through hole passing through the insulator 10 . The insulator 10 includes a flange 19 , a rear trunk 18 , a front trunk 17 , a step 15 , and a long leg 13 . The rear body portion 18 is located on the rear side of the flange 19 and has an outer diameter smaller than the outer diameter of the flange 19 . The distal end body portion 17 is located on the distal side of the flange 19 and has an outer diameter smaller than the outer diameter of the flange 19 . The long leg portion 13 is located on the distal side of the distal body portion 17 and has an outer diameter smaller than the outer diameter of the distal body portion 17 . The leg portion 13 is exposed to the combustion chamber of the internal combustion engine (not shown) when the spark plug 100 is installed in the internal combustion engine (not shown). The stepped portion 15 is formed between the leg length portion 13 and the distal end body portion 17.

The metal shell 50 is a cylindrical metal fitting that is made of a conductive metal material (eg, low carbon steel) and is used to fix the spark plug 100 to an engine head (not shown) of an internal combustion engine. The metal shell 50 has a through hole 59 formed therethrough along the axis CO. The metal shell 50 is arranged around the insulator 10 in the radial direction (ie, on the outer periphery). That is, the insulator 10 is inserted and held in the through hole 59 of the metal shell 50. In other words, the metal shell 50 is arranged around the center electrode 20 and holds the center electrode 20 insulated via the insulator 10. The tip of the center electrode projects further toward the tip than the tip of the metal shell 50. The rear end of the insulator 10 projects further toward the rear end than the rear end of the metal shell 50 .

The metal shell 50 is formed between a hexagonal prism-shaped tool engaging portion 51 that engages with a spark plug wrench, a mounting screw portion 52 for attaching to an internal combustion engine, and the tool engaging portion 51 and the mounting screw portion 52. A flange-shaped seat portion 54 is provided. The nominal diameter of the mounting screw portion 52 is, for example, one of M8 (8 mm (millimeters)), M10, M12, M14, and M18.

An annular gasket 5 formed by bending a metal plate is fitted between the mounting screw portion 52 of the metal shell 50 and the seat portion 54 . The gasket 5 seals a gap between the spark plug 100 and the internal combustion engine (engine head) when the spark plug 100 is attached to the internal combustion engine.

The metal shell 50 further includes a thin caulking portion 53 provided on the rear end side of the tool engaging portion 51 and a thin compression deforming portion 58 provided between the seat portion 54 and the tool engaging portion 51. It is equipped with. An annular region is formed between the inner circumferential surface of the metal shell 50 from the tool engaging portion 51 to the caulking portion 53 and the outer circumferential surface of the rear end body portion 18 of the insulator 10. Ring members 6 and 7 are arranged. A powder of talc (talc) 9 is filled between the two ring members 6 and 7 in this region. The rear end of the caulking portion 53 is bent radially inward and fixed to the outer peripheral surface of the insulator 10. During manufacture, the compressively deformable portion 58 of the metal shell 50 is compressively deformed by the caulking portion 53 fixed to the outer peripheral surface of the insulator 10 being pressed toward the distal end side. Due to the compression deformation of the compression deformation portion 58, the insulator 10 is pressed toward the distal end side within the metal shell 50 via the ring members 6, 7 and talc 9. As a result, the stepped portion 15 (insulating body side step) is pressed. As a result, the plate packing 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.

The center electrode 20 includes a rod-shaped center electrode body 21 extending in the axial direction and a center electrode tip 29. The center electrode main body 21 is held at a distal end portion inside the shaft hole 12 of the insulator 10 . The center electrode body 21 has a structure including an electrode base material 21A and a core portion 21B buried inside the electrode base material 21A. The electrode base material 21A is formed using, for example, nickel or an alloy containing nickel as a main component (for example, NCF600, NCF601). The core portion 21B is made of copper or an alloy containing copper as a main component, which has higher thermal conductivity than the alloy forming the electrode base material 21A, and in this embodiment is made of copper.

The center electrode main body 21 also includes a flange 24 (also referred to as a flange) provided at a predetermined position in the axial direction, and a head 23 (electrode head) that is a portion on the rear end side of the flange 24. and a leg portion 25 (electrode leg portion) which is a portion on the distal side of the flange portion 24. The collar portion 24 is supported by the stepped portion 16 of the insulator 10. The distal end portion of the leg portion 25, that is, the distal end of the center electrode body 21 projects further toward the distal end than the distal end of the insulator 10.

The center electrode tip 29 is a member having a substantially cylindrical shape, and is joined to the tip of the center electrode body 21 (the tip of the leg portion 25) using, for example, laser welding. The distal end surface of the center electrode tip 29 is a first discharge surface 295 that forms a spark gap with a ground electrode tip 39, which will be described later. The center electrode tip 29 is made of a material whose main component is a noble metal with a high melting point. The center electrode tip 29 is, for example, a noble metal tip formed using a noble metal such as iridium (Ir) or Ir, or an alloy containing the noble metal as a main component.

The ground electrode 30 includes a ground electrode main body 31 joined to the tip of the metal shell 50 and a ground electrode tip 39 in the shape of a square prism. The ground electrode main body 31 is a rod-shaped body with a square cross section. One end of the ground electrode body 31 is a free end 311, and the other end is a connecting end 312. The connecting end 312 is joined to the distal end surface 50A of the metal shell 50 by, for example, resistance welding. Thereby, the metal shell 50 and the ground electrode main body 31 are electrically and physically connected. The ground electrode main body 31 includes a tip portion 31a that includes a free end 311 and extends in a direction perpendicular to the axis CO, and a rear end portion 31b that includes a connection end 312 and extends in the axial direction. A portion between the leading end 31a and the rear end 31b is curved.

The ground electrode main body 31 is formed using, for example, nickel or an alloy containing nickel as a main component (eg, NCF600, NCF601). The ground electrode main body 31 is formed using a base material made of a metal with high corrosion resistance (e.g., nickel alloy) and a metal with high thermal conductivity (e.g., copper), and has a core embedded in the base material. It may have a two-layer structure including: The ground electrode tip 39, like the center electrode tip 29, is a noble metal tip formed using a noble metal such as iridium (Ir) or Ir, or an alloy containing the noble metal as a main component.

The terminal fitting 40 is a rod-shaped member extending in the axial direction. The terminal fitting 40 is made of a conductive metal material (for example, low carbon steel), and a metal layer (for example, a Ni layer) for corrosion prevention is formed on the surface of the terminal fitting 40 by plating or the like. The terminal fitting 40 includes a flange 42 (terminal jaw) formed at a predetermined position in the axial direction, a cap mounting portion 41 located on the rear end side of the flange 42, and a leg portion 43 on the distal end side of the flange 42. (terminal leg) and. The cap mounting portion 41 of the terminal fitting 40 is exposed on the rear end side of the insulator 10. The leg portion 43 of the terminal fitting 40 is inserted into the shaft hole 12 of the insulator 10. A plug cap to which a high voltage cable (not shown) is connected is attached to the cap attachment part 41, and a high voltage for generating spark discharge is applied.

In the shaft hole 12 of the insulator 10, there is a gap between the tip of the terminal fitting 40 (the tip of the leg 43) and the rear end of the center electrode 20 (the rear end of the head 23) to prevent radio noise when sparks are generated. A resistor 70 is arranged to reduce the amount of power. The resistor 70 is formed of, for example, a composition containing glass particles as main components, ceramic particles other than glass, and a conductive material. In the shaft hole 12, the gap between the resistor 70 and the center electrode 20 is filled with a conductive seal 60. The gap between the resistor 70 and the terminal fitting 40 is filled with a conductive seal 80. The conductive seals 60 and 80 are made of a composition containing glass particles such as B ₂ O ₃ --SiO ₂ and metal particles (Cu, Fe, etc.), for example.

A-2. Configuration of the ground electrode 30 near the ground electrode tip 39:
The configuration of the ground electrode 30 near the ground electrode tip 39 will be described in further detail. FIG. 2 is a diagram showing a configuration near the ground electrode tip 39 of the ground electrode 30 of the embodiment. FIG. 2A shows a view of the vicinity of the second discharge surface 393 of the ground electrode tip 39 as viewed from the rear end direction BD toward the front end direction FD along the axial direction. FIG. 2(B) shows a cross section CF obtained by cutting the vicinity of the tip of the spark plug 100 along a specific plane. The dotted line in FIG. 2(A) indicates the cutting position of cross section CF in FIG. 2(B).

The rear end surface of the ground electrode tip 39 is a second discharge surface 393 that faces the first discharge surface 295 (FIG. 1) of the center electrode tip 29. A cross section CF in FIG. 2B is a cross section passing through the center of gravity GC of the second discharge surface 393, perpendicular to the second discharge surface 393, and parallel to the axis of the rod-shaped ground electrode body 31. In this embodiment, a line passing through the center of gravity GC of the second discharge surface 393 and perpendicular to the second discharge surface 393 coincides with the axis CO of the spark plug 100.

Note that the center of gravity of the second discharge surface 393 is the center of gravity when it is assumed that the mass is evenly distributed on the second discharge surface 393. The second discharge surface 393 is determined as follows, for example. A digital image is obtained by photographing the second discharge surface 393 from the rear end direction BD side toward the front end direction FD. In the digital image, the average coordinates of the plurality of pixels constituting the second discharge surface 393 are calculated. The position indicated by the average coordinates is defined as the center of gravity of the second discharge surface 393.

The direction from the center of gravity GC of the second discharge surface 393 toward the free end 311 along the second discharge surface 393, that is, the left direction in FIGS. 2(A) and 2(B) is defined as a first direction D1. A direction from the center of gravity GC of the second discharge surface 393 away from the free end 311 along the second discharge surface 393, that is, a direction opposite to the first direction D1, is a second direction D2.

Furthermore, the thickness direction of the ground electrode chip 39, that is, the direction perpendicular to the second discharge surface 393, and the direction from the ground electrode main body 31 to the ground electrode chip 39 (in this embodiment, the rear end direction BD) is 3 directions D3.

Among the four side surfaces connected to the free end 311 (free end surface) of the ground electrode main body 31, the side surface facing the first discharge surface 295 is defined as the inner side surface 313. Among the four side surfaces of the ground electrode main body 31, the two side surfaces connected to the inner surface 313, that is, the side surfaces located in the vertical direction in FIG. 2(A) are referred to as side surfaces 314 and 315. The direction from the center of gravity GC of the second discharge surface 393 toward the side surface 314, that is, the upward direction in FIG. 2A, is defined as a fourth direction D4, and the direction opposite to the fourth direction D4 is defined as a fifth direction D5.

The ground electrode chip 39 is a square columnar member. That is, the ground electrode chip 39 includes a quadrangular (square in this embodiment) second discharge surface 393 and four side surfaces 391, 392, 394, and 395 connected to the second discharge surface 393. Among these four side surfaces, the side surface 391 faces the free end 311 side (first direction D1), and the side surface 392 faces the connecting end 312 side (second direction D2). The length W of one side of the square (rectangular) second discharge surface 393 of the ground electrode chip 39, that is, the length of the ground electrode chip 39 in the first direction D1 and the length in the fourth direction D4 are as follows. For example, it is 1.5 mm to 3.0 mm. In particular, in the present embodiment, the length W of one side of the square (rectangular) second discharge surface 393 is preferably 2.5 mm or more. The average thickness (average length in the axial direction) of the ground electrode tip 39 is, for example, 0.2 mm to 1.0 mm.

The ground electrode tip 39 is arranged along the inner surface 313 at the tip 31a of the ground electrode main body 31. The ground electrode tip 39 is joined to the ground electrode main body 31 by laser welding. For this purpose, a fused portion 35 formed by laser welding is disposed between the ground electrode tip 39 and the ground electrode main body 31. The melted portion 35 is a portion where a portion of the ground electrode tip 39 and a portion of the ground electrode body 31 before welding are melted and solidified. For this purpose, the melted portion 35 includes components of the ground electrode tip 39 and components of the ground electrode body 31. It can be said that the ground electrode tip 39 is joined to the inner surface 313 of the ground electrode main body 31 via the fused portion 35 .

As can be seen from FIG. 2A, the shape of the melted part 35 when viewed along the axial direction is approximately similar (in this embodiment, the shape is slightly larger than the shape of the ground electrode tip 39 when viewed along the axial direction). square). The side surfaces 351, 352, 354, and 355 of the melted portion 35 in the four directions D1, D2, D4, and D5 are located radially outward from the corresponding side surfaces 391, 392, 394, and 395 of the ground electrode tip 39. exposed to the outside.

The contact surface 353 (FIG. 2B) on the rear end direction BD side of the melting part 35 is the contact surface with the surface on the opposite side of the second discharge surface 393 of the ground electrode tip 39 (the surface in the distal direction FD). . The entire surface of the ground electrode tip 39 opposite to the second discharge surface 393 (the surface in the distal direction FD) is in contact with the melting portion 35 . The entire surface 356 (FIG. 2(B)) on the FD side in the distal direction of the melted portion 35 is in contact with the ground electrode main body 31.

The melted portion 35 has a melted sag 35n that covers the FD side portion of the side surfaces 391, 392, 394, and 395 of the ground electrode tip 39 (FIG. 2(B)). The melting sag 35n is formed over the entire circumference of the ground electrode chip 39.

As shown in FIG. 2(B), in the cross section CF, the thickness of the molten part 35 (the length in the third direction) excluding the part of the molten sag 35n is the minimum at the end in the second direction D2, and the thickness in the first direction It is maximum at the end of D1. In the cross section CF, the thickness of the fused portion 35 excluding the portion of the fused sag 35n continuously increases from the second direction D2 to the first direction D1. Hereinafter, the thickness of the melted portion 35 shall mean the thickness at a portion excluding the melted sag 35n.

FIG. 3 is a cross-sectional view of a portion including the ground electrode chip 39 taken along a plane perpendicular to the second direction D2. 3(A) shows the AA cross section in FIGS. 2(A) and (B), and FIG. 3(B) shows the BB cross section in FIGS. 2(A) and (B). (C) shows the CC cross section of FIGS. 2(A) and 2(B).

The AA cross section is a cross section that is perpendicular to the second direction D2 and passes through the point C1. The point C1 is the midpoint between the end point T1 in the second direction D2 of the line segment where the cross section CF and the second discharge surface 393 intersect, and the center of gravity GC. The AA cross section is perpendicular to the second direction D2 and passes through the first portion 39A (the single hatched portion in FIG. 2A) on the first direction D1 side with respect to the center of gravity GC of the second discharge surface 393. This is an example of a cross section.

As shown in the AA cross section and in FIG. 2(A), the melted portion 35 includes a thick portion 35M that protrudes in the third direction D3. The portion of the melted portion 35 excluding the thick portion 35M and the melted sag 35n is a thin portion 35T. The position of the thick portion 35M in the fourth direction D4 is the central portion of the melted portion 35 in the fourth direction D4. The thick portion 35M extends from the end of the fusion portion 35 in the first direction D1 to the center in the first direction D1 to the second direction D2.

In the AA cross section, the maximum value (also referred to as maximum thickness) of the thickness (length in the third direction D3) of the melted portion 35 is defined as H1. The position in the fourth direction D4 where the thickness of the melted part 35 becomes the maximum thickness H1 is the position of the center of the thick part 35M in the fourth direction D4. Here, in FIG. 3(A), three ranges LR, CR, and RR, which are obtained by equally dividing the entire range in the fourth direction D4 where the molten part 35 and the ground electrode tip 39 are in contact in the AA cross section, are shown. It is shown. In the AA cross section, the position in the fourth direction D4 where the thickness of the melted portion 35 reaches the maximum thickness H1 is in the central range CR of the three ranges. In the AA cross section, the thickness of the thick portion 35M is thicker than the thickness of the thin portion 35T.

In the AA cross section, the minimum value (also called minimum thickness) of the thickness of the molten part 35 is defined as H3. The position in the fourth direction D4 where the thickness of the melted part 35 is the minimum thickness H3 is in the range where the thin part 35T is located, and is in the end ranges LR and RR of the three ranges described above.

In addition, in the AA cross section, among the three ranges LR, CR, and RR of the melted part 35, the range where the average thickness (average value of the length in the third direction D3) is the maximum is the thickness of the melted part 35. This range includes the position where the maximum thickness H1 is reached. In this embodiment, the position where the thickness of the melted portion 35 reaches the maximum thickness H1 is in the central range CR, so the range where the average thickness is maximum is the central range CR. Note that the average thickness HaL of the range LR of the fusion zone 35 is measured as follows, for example. First, the AA cross section obtained by cutting the ground electrode tip 39 and the ground electrode body 31 of the spark plug 100 to be measured is polished to obtain a mirror surface. Then, an enlarged photograph of the mirror surface is taken using a metallurgical microscope. In the enlarged photograph, ten or more measurement positions are determined at equal intervals within the range LR, and the thickness of the melted portion 35 at the measurement positions is measured. The average value of the thickness of the melted portion 35 at ten or more measurement positions is defined as the average thickness of the range LR. The same applies to the average thicknesses HaC and HaR of the other ranges CR and RR.

The BB cross section is a cross section that is perpendicular to the second direction D2 and passes through the center of gravity GC (FIG. 2(A)). In the BB cross section, the maximum thickness of the melted portion 35 is assumed to be H0. The position in the fourth direction D4 where the thickness of the melted part 35 becomes the maximum thickness H0 is the position of the center of the thick part 35M in the fourth direction D4. Here, in FIG. 3(B), as in FIG. 3(A), the entire range in the fourth direction D4 where the melted part 35 and the ground electrode tip 39 are in contact is divided into three in the BB cross section. Three ranges LR, CR, RR are shown. In the BB cross section, the position in the fourth direction D4 where the thickness of the melted portion 35 becomes the maximum thickness H0 is in the central range CR of the three ranges. In the BB cross section, the thickness of the thick portion 35M is thicker than the thickness of the thin portion 35T. In the BB cross section, the minimum thickness of the melted part 35 is assumed to be Hm. In the BB cross section, the position in the fourth direction D4 where the thickness of the molten part 35 is the minimum thickness Hm is in the range where the thin part 35T is located, and the end range LR of the above-mentioned three ranges, It's in RR.

In addition, in the BB cross section, among the three ranges LR, CR, and RR of the fusion zone 35, the range where the average thickness (average value of the length in the third direction D3) is the maximum is the AA cross section. Similarly, the range includes the position where the thickness of the melted portion 35 is the maximum thickness H0. In this embodiment, the position where the thickness of the melted portion 35 reaches the maximum thickness H0 is in the central range CR, so the range where the average thickness is maximum is the central range CR. Note that the method for measuring the average thickness of each of the three ranges LR, CR, and RR of the fusion zone 35 is the same as described for the AA cross section.

The CC cross section is a cross section that is perpendicular to the second direction D2 and passes through point C2 (FIG. 2(A)). As shown in FIG. 2(A), the point C2 is the midpoint between the end point T2 of the second direction D2 of the line segment where the cross section CF and the second discharge surface 393 intersect, and the center of gravity GC. The CC cross section is perpendicular to the second direction D2 and passes through the second portion 39B (the cross-hatched portion in FIG. 2A) on the second direction D2 side with respect to the center of gravity GC of the second discharge surface 393. This is an example of a cross section. In the CC cross section, the entire melted part 35 is a thin part 35T, and no thick part 35M exists. Here, in the CC cross section, the maximum thickness of the fused portion 35 is assumed to be H2. In the CC cross section, the thickness of the melted portion 35 is almost uniform, but actually varies depending on the position in the fourth direction D4.

The maximum thickness H1 of the fused portion 35 in the AA cross section is thicker than the maximum thickness H0 of the fused portion 35 in the BB cross section. The maximum thickness H0 of the fused portion 35 in the BB cross section is thicker than the maximum thickness H2 of the fused portion 35 in the CC cross section. That is, H1>H0>H2 is satisfied.

Further, the minimum thickness H3 of the fused portion 35 in the AA cross section and the minimum thickness H3 of the fused portion 35 in the BB cross section are thicker than the maximum thickness H1 in the CC cross section. That is, H3>H2 and Hm>H2 are satisfied.

A-3: Manufacturing method The method for manufacturing the spark plug 100 will be explained with a focus on the method for manufacturing the ground electrode 30. FIG. 4 is a flowchart of a method for manufacturing the ground electrode 30. 5 and 6 are explanatory diagrams of a method of manufacturing the ground electrode 30. First, a rod-shaped ground electrode main body 31 is prepared before being bent. Then, the ground electrode tip 39 is prepared before being welded to the ground electrode main body 31.

In S10, a square prism-shaped ground electrode tip 39 before welding is placed on the inner surface 313 of the ground electrode main body 31, as shown in FIGS. 5(A) and 6. In this state, the front end side surface 39S of the ground electrode tip 39 and the inner surface 313 are in contact with each other.

In S20, the ground electrode chip 39 is fixed to the ground electrode main body 31 by the holding member 500. Specifically, as shown in FIG. 6, the ground electrode tip 39 is pressed in the distal direction FD (downward in FIG. 6) from the second discharge surface 393 side by the pressing member 500. As a result, the ground electrode tip 39 and the ground electrode main body 31 are fixed with the distal side surface 39S of the ground electrode tip 39 and the inner surface 313 of the ground electrode main body 31 in contact with each other. The contact surface between the tip side surface 39S and the inner surface 313 of the ground electrode main body 31, that is, the surface to be joined between the ground electrode tip 39 and the ground electrode main body 31 is also referred to as the tip bonding surface BS.

In S30, a laser for laser welding is scanned and output, and approximately half of the chip joint surface BS on the fourth direction D4 side is welded. Note that in this embodiment, a fiber laser is used as the laser. For example, compared to a YAG laser, a fiber laser has a higher light focusing ability, and therefore has a higher degree of freedom in the shape of the fused portion 35 that can be formed.As shown in FIG. 2, the fiber laser is relatively thin and , it is possible to form the melted portion 35 having a relatively long length in the direction perpendicular to the axis (for example, the first direction D1).

Laser LZ1 in FIG. 5A shows the laser at the start of the welding process (also referred to as the first welding process) in S30, and laser LZ2 shows the laser at the end of the first welding process. As shown in FIG. 5(A), the laser is irradiated from the side surface 394 in the fourth direction D4 to the fifth direction D5. As shown in FIGS. 5A and 6, in the first welding process, a portion of the side surface 394 slightly on the rear end direction BD side from the chip joint surface BS is The laser is continuously irradiated while moving the irradiation position in the first direction D1 to the end point P2.

FIG. 7 is a graph conceptually showing the thermal energy input in the first welding process. This graph is obtained by plotting the laser irradiation position in the first welding process on the horizontal axis and the thermal energy at the irradiation position on the vertical axis. As shown in FIG. 7, the energy LE of the laser irradiated in the first welding process is constant from the end point P1 in the second direction D2 to the end point P2 in the first direction D1. Here, the laser energy applied to the ground electrode chip 39 is transmitted to the ground electrode body 31 as thermal energy. The heat transferred to the ground electrode body 31 is transmitted through the ground electrode body 31 in the first direction D1 and the second direction D2. As shown in FIG. 6, the end of the ground electrode main body 31 in the first direction D1 is a free end 311 and is in contact with air. On the other hand, the end of the ground electrode main body 31 in the second direction D2 is a connection end 312 (FIG. 1), and is connected to the metal shell 50. Since air has a significantly lower thermal conductivity than the metal shell 50, the amount of heat removed in the first direction D1 is significantly smaller than the amount of heat removed in the second direction D2. For this reason, when the laser is irradiated while moving the irradiation position in the first direction D1, a part of the laser energy applied to the ground electrode chip 39 does not escape in the first direction D1 as thermal energy and is transferred to the ground electrode chip 39. It is accumulated in the chip 39. The hatched portion in FIG. 7 indicates the thermal energy accumulated in the ground electrode chip 39. In FIG. In the first welding process, the total energy TE input into each position from the end point P1 to the end point P2 is the sum of the thermal energy accumulated in the ground electrode tip 39 and the newly input laser energy. Become. As a result, as shown in FIG. 7, in the first welding process, the total TE of energy input at each position from end point P1 to end point P2 increases as it goes from end point P1 to end point P2. As a result, the thickness of the fused portion 35 that is formed increases from the end point P1 toward the end point P2.

In FIG. 5(B), the hatched fusion part 35A is the fusion part formed in the first welding process. In the first welding step, approximately half of the chip joint surface BS on the fourth direction D4 side is welded. FIG. 8 is a diagram illustrating a cross section taken along line AA in a state where the fused portion 35A is formed in the first welding step. As shown in this figure, the fused portion 35A includes a thick portion 35AM located at the center of the ground electrode chip 39 in the fourth direction D4, and a thin portion 35AT on the fourth direction D4 side with respect to the thick portion 35AM. Contains. The thick portion 35AM is formed because the central portion of the ground electrode chip 39 in the fourth direction D4 is away from the surface that comes into contact with the outside air and is particularly prone to accumulating heat.

In S40 after the first welding step, the laser is scanned and output again to weld the remaining approximately half of the chip joint surface BS, that is, approximately half on the fifth direction D5 side.

Laser LZ3 in FIG. 5(B) indicates the laser at the start of the welding process of S40 (also referred to as the second welding process), and laser LZ4 indicates the laser at the end of the second welding process. As shown in FIG. 5(B), the laser is irradiated from the side surface 395 in the fourth direction D4 from the fifth direction D5 side. In the second welding process, the irradiation position is moved in the second direction D2 from the end point P3 in the first direction D1 to the end point P4 in the second direction D2 at a portion of the side surface 395 that is slightly on the rear end direction BD side from the chip joint surface BS. Meanwhile, the laser is continuously irradiated. From the end point P3 in the first direction D1 to the end point P4 in the second direction D2, the moving speed of the irradiation position is constant, and the energy of laser irradiation is constant. In addition, in the second welding process, unlike the second welding process, heat is smoothly drawn from the ground electrode tip 39 to the ground electrode body 31 because the fused part 35A is formed, and the connection end 312 Since heat is efficiently removed in the second direction D2, thermal energy is difficult to accumulate in the ground electrode chip 39.

Through the two steps of the first welding step and the second welding step explained above, the fused portion 35 explained with reference to FIGS. 2 and 3 is formed, and the ground electrode body 31 and the ground electrode tip 39 are welded together. is completed.

Note that welding between the ground electrode body 31 and the ground electrode tip 39 may be performed, for example, after the rod-shaped ground electrode body 31 is welded to the metal shell 50. Further, the ground electrode main body 31 may be welded to the metal shell 50 after the ground electrode main body 31 and the ground electrode tip 39 are welded.

Furthermore, an assembly including the insulator 10, the center electrode 20, the conductive seal 60, the resistor 70, the conductive seal 80, and the terminal fitting 40 is produced by a known method. For example, the center electrode 20, the material for the conductive seal 60, the material for the resistor 70, and the material for the conductive seal 80 are inserted into the shaft hole 12 of the insulator 10 in this order from the rear end direction BD side. Then, the assembly is manufactured by inserting the terminal fitting 40 into the shaft hole 12 from the rear end direction BD side while the insulator 10 is heated.

Thereafter, the assembly is fixed to the metal shell 50. Specifically, the assembly, talc 9, and ring members 6 and 7 are arranged in the through hole 59 of the metal shell 50. A plate packing 8 is interposed between the stepped portion 15 of the insulator 10 and the stepped portion 56 of the metal shell 50. Then, the metal shell 50 and the insulator 10 are assembled by crimping the crimping portion 53 of the metal shell 50 so as to bend it inward. Then, the rod-shaped ground electrode 30 is bent to form a gap between the center electrode tip 29 and the ground electrode tip 39. Through the above steps, the spark plug 100 is completed.

According to the spark plug 100 of the embodiment described above, the first section (specifically, 3(A), the maximum length (maximum thickness) of the molten part 35 in the third direction D3 is defined as H1, and the second In a second cross section (specifically, the CC cross section in FIG. 3C) passing through the second portion 39B on the direction D2 side and perpendicular to the second direction D2, the maximum thickness of the molten part 35 is defined as H2. Then, H1>H2 is satisfied.

The side closer to the first direction D1 than the center of gravity GC is farther from the connection end 312 connected to the metal shell 50 than the side closer to the second direction D2 than the center of gravity GC. For this reason, the heat removal efficiency on the first direction D1 side relative to the center of gravity GC is lower than on the second direction D2 side relative to the center of gravity GC, so the temperature becomes higher than that on the second direction D2 side relative to the center of gravity GC. Therefore, the thermal stress generated is greater on the first direction D1 side than the gravity center GC than on the second direction D2 side than the gravity center GC. In this embodiment, as described above, since H1>H2 is satisfied, the energy input during laser welding is not excessively increased, compared to, for example, the case where the fused portion 35 has a uniform thickness. The thickness of the melted portion 35 on the first direction D1 side relative to the center of gravity GC can be ensured. As a result, thermal stress can be alleviated while suppressing the rise of melt sag, so that the peeling resistance of the ground electrode chip 39 can be improved.

The rising melt sag is a problem in which the molten sag 35n adheres to the second discharge surface 393 during laser welding. When the molten sag rises, the discharge concentrates on the molten sag, so that the molten sag 35n and the entire molten part 35 are exposed to the discharge and easily damaged. As a result, the ground electrode tip 39 is likely to peel off or fall off due to damage to the melted portion 35. If the overall thickness of the fusion zone 35 is increased, the thermal stress will be more easily relaxed, but the energy input during welding will increase, making it easier for molten sag to rise. In this embodiment, as described above, the occurrence of such rising melt sag can be suppressed.

Further, according to the present embodiment, in the AA cross section (FIG. 3(A)), the position where the thickness of the melted portion 35 is maximum is different from both ends in the fourth direction D4. In the AA cross section, both ends in the fourth direction D4 are exposed to the outside air, but a portion different from both ends in the fourth direction D4 is not exposed to the outside air. For this reason, a portion different from both ends in the fourth direction D4 has a lower heat removal efficiency than both ends in the fourth direction D4, and therefore thermal stress tends to increase. For this reason, it is highly necessary to ensure the thickness of the melted portion 35 in a portion different from both ends in the fourth direction D4. According to the present embodiment, the position where the thickness of the melted part 35 is maximum is a position different from both ends in the fourth direction D4, so the position where the thickness of the melted part 35 is maximum is at both ends in the fourth direction D4. The thickness of the fused portion 35 at a position different from both ends in the fourth direction D4 can be ensured without excessively increasing the energy required for laser welding. Furthermore, if the melted portion 35 is excessively thick at both ends in the fourth direction D4, melt sag tends to rise at both ends. According to the present embodiment, it is possible to prevent the melted portion 35 from becoming excessively thick at both ends in the fourth direction D4, and to suppress the rise of melt sag at both ends. Therefore, the peeling resistance of the ground electrode chip 39 can be further improved.

More specifically, according to the present embodiment, in the AA cross section, the entire range in the fourth direction D4 where the melted part 35 and the ground electrode tip 39 are in contact is divided into three equal parts, LR, CR, When the range located in the center of the fourth direction D4 in RR is defined as the center range CR, the position where the thickness of the melted portion 35 is maximum is in the center range CR (FIG. 3(A)). As a result, the melted portion 35 is prevented from becoming excessively thick near both ends in the fourth direction D4 than when the position where the thickness of the melted portion 35 is maximum is in the end ranges LR and RR at both ends. can. Therefore, rising of melt sag can be further suppressed.

Furthermore, according to the present embodiment, among the three ranges LR, CR, and RR, the range where the average value of the thickness of the melted portion 35 is the maximum is the central range CR (FIG. 3(A)). As a result, the thickness of the fused portion 35 in the central range CR can be ensured, and it is possible to prevent the fused portion 35 from becoming excessively thick in the ranges at both ends in the fourth direction D4. Therefore, thermal stress can be alleviated while further suppressing the rise of melt sag.

Further, according to the present embodiment, in the AA cross section, when the minimum value of the thickness of the fused portion 35 is H3, H3>H2 is satisfied. As a result, the thickness of the fused portion 35 on the first direction D1 side relative to the center of gravity GC can be further ensured without excessively increasing the energy required for laser welding. As a result, thermal stress can be further alleviated while suppressing the rise of melt sag, so that the peeling resistance of the ground electrode chip 39 can be further improved.

B. Variant:
(1) FIG. 9 is an explanatory diagram of a ground electrode 30B of a modified example. The ground electrode 30B of the modified example differs from the above embodiment in the configuration of the AA cross section and the BB cross section. FIG. 9(A) shows an AA cross section of the ground electrode 30B. FIG. 9(B) shows a BB cross section of the ground electrode 30B.

In the AA cross section (FIG. 3A) of the ground electrode 30 of the above embodiment, the thick portion 35M is located in the central range CR of the three ranges LR, CR, and RR. Instead, in the AA cross section (FIG. 9A) of the ground electrode 30B of the modified example, the thick portion 35M is located in the end range LR of the three ranges LR, CR, and RR. There is. In the AA cross section of the modified example, the position in the fourth direction D4 where the thickness of the melted part 35B reaches the maximum thickness H1 is the position of the center of the thick part 35M in the fourth direction D4, as in the above embodiment. It is. Therefore, in the AA cross section of the modified example, the position in the fourth direction D4 where the thickness of the fused portion 35B reaches the maximum thickness H1 is in the end range LR of the three ranges. The other configuration of the AA cross section of the modified example is the same as that of the above embodiment.

In the BB cross section (FIG. 3(B)) of the ground electrode 30 of the above embodiment, the thick portion 35M is located in the central range CR of the three ranges LR, CR, and RR. Instead, in the BB cross section (FIG. 9(B)) of the ground electrode 30B of the modified example, the thick portion 35M is located in the end range LR of the three ranges LR, CR, and RR. There is. In the BB cross section of the modified example, the position in the fourth direction D4 where the thickness of the melted part 35B reaches the maximum thickness H1 is the position of the center of the thick part 35M in the fourth direction D4, as in the above embodiment. It is. Therefore, in the BB cross section of the modified example, the position in the fourth direction D4 where the thickness of the fused portion 35B reaches the maximum thickness H1 is in the end range LR of the three ranges. The other configuration of the BB cross section of the modified example is the same as that of the above embodiment.

The configuration of the CC cross section of the ground electrode 30B of the modified example is the same as the configuration of the CC cross section of the ground electrode 30 of the above embodiment (not shown).

In the ground electrode 30B of the modified example described above, the position where the maximum thickness H1 is maximum is in the end range LR. Further, the range where the average thickness of the melted portion 35B is maximum is the end range LR including the position where the maximum thickness H1 is obtained. As a result, the thickness of the fused portion 35B in the end range can be ensured, and it is possible to prevent the fused portion 35B from becoming excessively thick in the end range RR. Therefore, it is possible to suppress the rise of melt sag in the end range LR.

Note that the position where the maximum thickness H1 is maximum may be in the other end range RR, and the range where the average thickness of the fused portion 35B is maximum is the end including the position where the maximum thickness H1 is the maximum. It may be a partial range RR. In this case, it is possible to suppress the rise of melt sag in the end range RR.

(2) FIG. 10 is an explanatory diagram of a ground electrode 30C of a modified example. The ground electrode 30 of the above embodiment includes a thick portion 35M and a thin portion 35T. Instead, the ground electrode 30C of the modified example does not include the thick portion 35M. FIG. 10(A) shows an AA cross section of the ground electrode 30C. FIG. 10(B) shows a BB cross section of the ground electrode 30C.

In the AA cross section of the modified example in FIG. 10(A), the maximum thickness of the fused portion 35C is designated as H1, and the minimum thickness is designated as H3. In the AA cross section of the ground electrode 30C, the thickness of the melted portion 35C is almost uniform, but actually varies depending on the position in the fourth direction D4.

In the BB cross section of the modified example in FIG. 10(B), the maximum thickness of the melted portion 35C is set to H0, and the minimum thickness is set to Hm. In the BB cross section of the ground electrode 30C, the thickness of the melted portion 35C is almost uniform, but actually varies depending on the position in the fourth direction D4.

The configuration of the CC cross section of the ground electrode 30B of the modified example is the same as the configuration of the CC cross section of the ground electrode 30 of the above embodiment (not shown).

Also in this modification described above, the maximum thickness H1 of the fused portion 35C in the AA cross section of FIG. 10(A) and the maximum thickness H2 of the fused portion 35C in the CC cross section (FIG. 3(C)). satisfies H1>H2. As a result, thermal stress can be alleviated while suppressing the rise of melt sag, so that the peeling resistance of the ground electrode chip 39 can be improved.

Also in this modification, when the minimum value of the thickness of the fused portion 35C in the AA cross section is H3, H3>H2 is satisfied. As a result, the peeling resistance of the ground electrode chip 39 can be further improved.

(3) In the above embodiment, the minimum thickness H3 of the fused portion 35 in the AA cross section is larger than the maximum thickness H2 of the fused portion 35 in the CC cross section (H3>H2). Instead of this,
The minimum thickness H3 of the fused portion 35 in the AA cross section may be less than or equal to the maximum thickness H2 of the fused portion 35 in the CC cross section (H3≦H2). Even if H3≦H2, if at least H1>H2 is satisfied, the peeling resistance of the ground electrode chip 39 can be improved.

(4) The specific configuration of the spark plug 100 of the above embodiment is merely an example, and may be modified as appropriate. For example, the ground electrode tip 39 may have other shapes such as a pentagonal prism shape. Further, the ground electrode tip 39 may be formed using an alloy containing platinum (Pt) as a main component, for example, an alloy containing 50% by weight or more of Pt. Further, the materials, dimensions, shapes, etc. of the ground electrode 30, the metal shell 50, the center electrode 20, the insulator 10, etc. can be changed in various ways. For example, the material of the metal shell 50 may be zinc-plated or nickel-plated low carbon steel, or unplated low carbon steel. Further, the material of the insulator 10 may be various insulating ceramics other than alumina.

Although the present invention has been described above based on the embodiments and modifications, the embodiments of the invention described above are for facilitating understanding of the present invention, and do not limit the present invention. The present invention may be modified and improved without departing from the spirit and scope of the claims, and the present invention includes equivalents thereof.

5...Gasket, 6...Ring member, 8...Plate packing, 9...Talc, 10...Insulator, 12...Shaft hole, 13...Leg length part, 15, 16...Step part, 17...Tip side body part, 18...Rear End side trunk, 19...flange, 20...center electrode, 21...center electrode body, 21A...electrode base material, 21B...core, 23...head, 24...flange, 25...leg, 29...center Electrode tip, 30, 30B, 30C...ground electrode, 31...ground electrode body, 31a...tip part, 31b...rear end part, 35, 35B, 35C...melted part, 35M, 35AM...thick part, 35T, 35AT... Thin wall part, 35n... Melting sag, 39... Ground electrode chip, 39A... First part, 39B... Second part, 39S... Surface, 40... Terminal fitting, 41... Cap attachment part, 42... Flange part, 43... Leg part , 50... Metal shell, 50... Melted part, 50A... Tip surface, 51... Tool engaging part, 52... Mounting screw part, 53... Caulking part, 54... Seat part, 56... Step part, 58... Compression deformation part , 59... Through hole, 60... Conductive seal, 70... Resistor, 80... Conductive seal, 100... Spark plug, 500... Holding member

Claims (5)

a center electrode;
a metal shell disposed around the center electrode and insulatingly holding the center electrode;
a rod-shaped electrode body whose one end is a free end and whose other end is connected to the metal shell; an electrode tip having a discharge surface and disposed on a specific side of the electrode body; the electrode body and the electrode; a ground electrode comprising a melted part formed between the chip and the ground electrode;
A spark plug comprising:
A direction along the specific side surface toward the one end from the center of gravity of the discharge surface is a first direction, and a direction opposite to the first direction is a second direction;
A third direction perpendicular to the discharge surface of the molten part in a first cross-section perpendicular to the first direction passing through a first portion of the ground electrode on the first direction side relative to the center of gravity of the discharge surface. Let H3 be the minimum value of the length of
The maximum length of the molten part in the third direction in a second section of the ground electrode that is perpendicular to the second direction and passes through a second portion on the second direction side of the center of gravity of the discharge surface. When the value is H2,
Satisfy H3>H2,
In the first cross section, a position where the length in the third direction is maximum is different from both ends in a fourth direction perpendicular to the third direction . The spark plug according to claim 1 ,
In the first cross section,
When the range located at the center in the fourth direction is defined as a central range among three ranges obtained by equally dividing the entire range in the fourth direction where the melting part and the electrode tip contact,
The spark plug, wherein the position where the length in the third direction is maximum is in the central range. The spark plug according to claim 2 ,
Among the three ranges, the range in which the average length of the fused portion in the third direction is maximum is the central range. The spark plug according to claim 1 ,
In the first cross section,
When the range located at both ends in the fourth direction is defined as an end range among three ranges obtained by dividing the entire range in the fourth direction in which the melting part and the electrode tip contact into three equal parts,
The spark plug, wherein the position where the length in the third direction is maximum is located in either of the two end ranges. The spark plug according to claim 4 ,
Among the three ranges, the range where the average length of the melted part in the third direction is maximum is the position where the length in the third direction is maximum among the two end ranges. A spark plug, characterized in that it is a range including.

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