JP7490507B2 - Spark plug - Google Patents

Description

The technology disclosed in this specification relates to spark plugs.

The spark plug comprises an insulator having an axial hole, and a center electrode held inside the insulator. The center electrode comprises a flange supported on a step provided in the axial hole, a shaft extending from the flange toward the tip end, and a head extending from the flange toward the rear end. Inside the axial hole, a seal material is filled around the head and flange, and the center electrode is fixed inside the axial hole by this seal material. In this type of spark plug, the head of the center electrode may have an expanding shape in which the diameter increases the further it is from the flange (see Patent Document 1).

When the air-fuel mixture burns in the combustion chamber, the combustion pressure exerts axial pressure (the direction in which the axis extends) on the center electrode. This can reduce the adhesive strength of the sealing member and cause the center electrode to wobble (loosen). If the head has an expanded diameter shape as described above, the center electrode is prevented from moving axially toward the rear end, which can prevent the center electrode from loosening.

Japanese Patent Application Laid-Open No. 5-343157

When the head of the center electrode has an expanded diameter shape as described above, the corner formed by the surface of the flange and the surface of the head forms an acute angle. For this reason, there is a concern that during the manufacturing process of the spark plug, the sealing material will not spread sufficiently to this corner, resulting in a gap around the head. In addition, there is a concern that the sealing material will be destroyed because stress is likely to concentrate at the tip of the expanded diameter shape. If such a situation occurs, there is a risk that the effect of suppressing loosening of the center electrode will be limited.

The spark plug disclosed in this specification comprises an insulator having an axial hole extending in the axial direction, and a center electrode disposed on the axial tip side of the axial hole, the axial hole having, in order from the axial tip side, a first inner diameter portion, a step portion whose inner diameter increases toward the axial rear end side, and a second inner diameter portion whose inner diameter is larger than that of the first inner diameter portion, and the center electrode has a flange portion whose tip is supported by the step portion, and a second inner diameter portion which extends from the flange portion to the axial rear end side and is supported by the flange portion. A spark plug having a head portion disposed in the second inner diameter portion, and a shaft portion extending from the flange portion toward the tip side in the axial direction and disposed in the first inner diameter portion, and a conductive seal material filled around the flange portion and the head portion inside the shaft hole, the head portion having a cylindrical column portion connected to the flange portion, and an enlarged diameter portion connected to the cylindrical portion and having an outer diameter that increases with distance from the flange portion, and the center electrode has a smooth connection between the surface of the flange portion and the surface of the cylindrical portion.

The spark plug disclosed in this specification can suppress loosening of the center electrode and avoid a decrease in ignition performance.

1 is a cross-sectional view of a spark plug according to an embodiment. 1 is a partially enlarged cross-sectional view of a spark plug according to an embodiment of the present invention. 4 is a partially enlarged cross-sectional view showing a state before an enlarged diameter portion is formed in a manufacturing process of the spark plug according to the embodiment. FIG.

[Overview of the embodiment]
(1) A spark plug disclosed in this specification includes an insulator having an axial hole extending in an axial direction, and a center electrode disposed on a tip side of the axial hole in the axial direction, the axial hole having, in order from the tip side in the axial direction, a first inner diameter portion, a step portion whose inner diameter becomes larger toward a rear end side in the axial direction, and a second inner diameter portion whose inner diameter is larger than that of the first inner diameter portion, and the center electrode has a flange portion whose tip is supported by the step portion, and a front end portion extending from the flange portion to the rear end side in the axial direction. a spark plug having a head disposed in the second inner diameter portion, and a shaft portion extending from the flange portion toward the tip side in the axial direction and disposed in the first inner diameter portion, and having a conductive sealing material filled around the flange portion and the head inside the axial hole, wherein the head has a cylindrical portion connected to the flange portion, and an enlarged diameter portion connected to the cylindrical portion and having an outer diameter that increases with increasing distance from the flange portion, and the center electrode has a surface of the flange portion and a surface of the cylindrical portion that are smoothly connected to each other.

Compared to conventional configurations, the above configuration allows the conductive seal material to be spread sufficiently around the flange and head, and in particular prevents gaps from occurring at the corner formed by the surface of the flange and the outer circumferential surface of the head. Furthermore, in the cylindrical portion, the force caused by thermal expansion is transmitted only in the radial direction and is evenly distributed around the cylindrical portion, so stress concentration at the rear end of the enlarged diameter portion can be mitigated. As a result, loosening of the center electrode can be suppressed and a decrease in ignition performance can be avoided.

(2) In the spark plug of (1) above, the inner diameter of the second inner diameter portion may be 3.1 mm or less. Also, in the spark plug of (1) above, the length of the cylindrical portion in the axial direction may be 1.9 mm or less.

In recent years, there has been a demand for vehicle internal combustion engines to be smaller and have smaller displacements. This has resulted in a demand for smaller spark plugs, and the center electrodes have also become smaller and smaller in diameter. This has resulted in a smaller surface area of the center electrode, which is one of the important factors for fastening the center electrode, and the problem of loosening of the center electrode has become more pronounced. The above configuration can be suitably applied to spark plugs that use such small, thin-diameter center electrodes.

[Details of the embodiment]
Specific examples of the technology disclosed in this specification will be described below with reference to the drawings. Note that the present invention is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims.

<Overall configuration of spark plug 1>
A first embodiment will be described with reference to Figs. 1 to 3. The spark plug 1 is attached to a cylinder head of an internal combustion engine and is used to ignite a mixture gas in a combustion chamber of the internal combustion engine. As shown in Fig. 1, the spark plug 1 includes an insulator 10, a metal shell 20, a center electrode 30, a terminal metal fitting 40, a resistor 50, conductive seal members (hereinafter abbreviated as "seal members") 60, 70, and a ground electrode 80. The dashed line in Fig. 1 indicates the axis AX of the spark plug 1. In the following description, the direction parallel to the axis AX (the vertical direction in Fig. 1) is referred to as the "axial direction". The lower side in Fig. 1 is referred to as the leading end side in the axial direction, and the upper side in Fig. 1 is referred to as the rear end side in the axial direction.

<Insulator 10>
1, the insulator 10 is a cylindrical member extending along the axis AX and has an axial hole 11 extending in the axial direction. The insulator 10 is formed using ceramics such as alumina. The axial hole 11 has, in order from the tip end side in the axial direction, a first inner diameter portion 12, a step portion 13 connected to the first inner diameter portion 12 and having an inner diameter increasing toward the rear end side in the axial direction, and a second inner diameter portion 14 connected to the step portion 13 and having an inner diameter larger than that of the first inner diameter portion 12.

<Metal shell 20>
The metal shell 20 is a member used when mounting the spark plug 1 to a cylinder head. As shown in Fig. 1, the metal shell 20 has an overall cylindrical shape extending in the axial direction, and is made of a conductive metal material (e.g., low carbon steel).

As shown in FIG. 1, the metal shell 20 has a through hole 21 that penetrates the inside in the axial direction, and the insulator 10 is held inside the metal shell 20 by being inserted into the through hole 21. The rear end of the insulator 10 protrudes outward (upper side in FIG. 1) from the rear end of the metal shell 20. The front end of the insulator 10 protrudes outward (lower side in FIG. 1) from the front end of the metal shell 20.

<Center electrode 30>
As shown in FIG. 1, the center electrode 30 includes a rod-shaped center electrode body 31 extending in the axial direction and a cylindrical tip 32 attached to the tip of the center electrode body 31. The center electrode body 31 is held on the tip side of the axial hole 11 of the insulator 10 so that its tip is exposed to the outside of the insulator 10. The center electrode body 31 is made of nickel (Ni) or a nickel-based alloy containing the most nickel (e.g., NCF600, NCF601, etc.). The center electrode body 31 may have a two-layer structure including an outer layer (base material) made of nickel or a nickel-based alloy and a core embedded in the outer layer. In that case, the core is preferably made of copper (Cu) or a copper-based alloy containing the most copper, which has better thermal conductivity than the outer layer. The tip 32 is mainly composed of a precious metal such as platinum or iridium. The tip 32 can be omitted.

As shown in FIG. 2, the center electrode body 31 includes a flange 33, a head 34, and a shaft 35 extending from the flange 33. The flange 33 is generally disc-shaped with an outer diameter larger than the inner diameter of the first inner diameter portion 12 and smaller than the inner diameter of the second inner diameter portion 14. The head 34 extends from the flange 33 to the rear end side in the axial direction. The shaft 35 is an elongated round rod-shaped with an outer diameter smaller than the outer diameter of the flange 33, and extends from the flange 33 to the tip side in the axial direction. The flange 33 is supported by the surface on the tip side (lower side in FIGS. 1 and 2) abutting against the inner peripheral surface of the step portion 13. A part of the flange 33 on the tip side is disposed in the step portion 13, and the remaining majority is disposed in the second inner diameter portion 14. The head 34 is disposed in the second inner diameter portion 14. Most of the shaft 35 is disposed within the first inner diameter portion 12, except for the tip portion that is exposed to the outside of the insulator 10.

The head 34 has a cylindrical section 36 connected to the flange 33, and an expanded diameter section 37 connected to the cylindrical section 36. The cylindrical section 36 is cylindrical with a constant outer diameter over its entire length, and extends from the flange 33 to the rear end in the axial direction. The expanded diameter section 37 extends from the cylindrical section 36 to the rear end in the axial direction, and has a truncated cone shape with an outer diameter that increases the farther it is from the flange 33. The connection portion 38 between the surface of the flange 33 and the surface of the cylindrical section 36 is smoothly curved over the entire circumference. In other words, the surface of the flange 33 and the surface of the cylindrical section 36 are smoothly connected.

<Terminal metal fitting 40>
As shown in FIG. 1, the terminal fitting 40 is a rod-shaped member extending in the axial direction, and is held at the rear end side of the axial hole 11 of the insulator 10 so that the rear end is exposed to the outside of the insulator 10. The terminal fitting 40 is arranged in the axial hole 11 on the rear end side of the center electrode 30. The terminal fitting 40 is made of a conductive metal material (e.g., low carbon steel). The surface of the terminal fitting 40 may be plated with nickel or the like for the purpose of corrosion prevention or the like. The terminal fitting 40 includes a flange portion 41 formed at a predetermined position in the axial direction, a terminal connection portion 42 located on the rear end side of the flange portion 41, and a leg portion 43 located on the tip side of the flange portion 41. The leg portion 43 is inserted into the axial hole 11 of the insulator 10. The terminal connection portion 42 is exposed on the rear end side of the insulator 10. A plug cap to which a high-voltage cable (not shown) is connected is attached to the terminal connection portion 42, and a high voltage for generating a discharge is applied thereto.

<Resistor 50>
1, the resistor 50 is disposed in the axial hole 11 of the insulator 10 between the tip of the terminal fitting 40 and the rear end of the center electrode 30. The resistor 50 has a resistance value of, for example, 1 kΩ or more (for example, 5 kΩ) and has a function of reducing radio noise when a spark occurs.

<Sealing members 60, 70>
A conductive seal member 60 is disposed between the tip of the resistor 50 and the rear end of the center electrode 30 in the axial hole 11. The seal member 60 fills the periphery of the flange 33 and the head 34 inside the axial hole 11, and the entire head 34 and a part of the flange 33 on the head 34 side are embedded in the seal member 60. A conductive seal member 70 is disposed between the rear end of the resistor 50 and the tip of the terminal metal fitting 40 in the axial hole 11. The seal members 60, 70 are formed of a conductive material, for example, a composition containing metal particles (Cu, Fe, etc.) and glass particles such as B ₂ O ₃ -SiO ₂ .

<Ground electrode 80>
The ground electrode 80 has an overall shape that is bent into a substantially L-shape, and its rear end is joined to the tip of the metallic shell 20. The tip is disposed so as to face the tip 32 at the tip of the center electrode 30 while maintaining a gap therebetween. The ground electrode 80 and the metallic shell 20 are joined to each other by, for example, resistance welding, laser welding, or the like. As a result, the ground electrode 80 and the metallic shell 20 are electrically connected to each other. The ground electrode 80 is made of, for example, nickel or a nickel-based alloy.

There is a gap between the tip 32 at the tip of the center electrode 30 and the tip of the ground electrode 80, and when a high voltage is applied between the center electrode 30 and the ground electrode 80, a spark discharge occurs in the gap, generally along the axis AX.

<Manufacturing process of spark plug 1>
An example of a manufacturing process of the spark plug 1 having the above-mentioned configuration will be described below. First, an electrode base material 100 that will become the center electrode body 31 is prepared. As shown in FIG. 3, the electrode base material 100 includes a flange portion 33 and a shaft portion 35, similar to the center electrode body 31, and also includes a straight portion 101. The straight portion 101 has a cylindrical shape with a constant outer diameter over its entire length, and extends from the flange portion 33 in the opposite direction to the shaft portion 35. A recess is formed in the surface on the rear end side of the straight portion 101. The electrode base material 100 is inserted into the axial hole 11 from the second inner diameter portion 14 side (upper side in FIG. 3). The flange portion 33 is supported by the inner peripheral surface of the step portion 13, the straight portion 101 is disposed in the second inner diameter portion 14, and the shaft portion 35 is disposed in the first inner diameter portion 12.

Next, a press pin (not shown) is inserted into the axial hole 11 to press the rear end of the straight portion 101 disposed in the axial hole 11. The rear end of the straight portion 101 has a recess formed in the rear end face, so it is crushed and deformed by pressing with an appropriate load, becoming the enlarged diameter portion 37. The portion of the straight portion 101 on the flange portion 33 side that has not been deformed becomes the cylindrical portion 36. In this manner, the center electrode body 31 is formed.

After the center electrode body 31 is formed, the raw material powder of the sealing member 60 is poured into the inside of the axial hole 11 from the second inner diameter portion 14 side and filled around the flange portion 33 and the head portion 34. Next, a press pin is used to pre-compress the filled raw material powder of the sealing member 60.

The raw powder of the resistor 50 is filled on top of the raw powder of the pre-compressed sealing member 60, and pre-compressed. Next, the raw powder of the sealing member 70 is filled on top of the raw powder of the pre-compressed resistor 50, and pre-compressed.

Next, the terminal fitting 40 is inserted into the axial hole 11 from the second inner diameter portion 14 side. The insulator 10 with the terminal fitting 40 inserted is placed in an electric furnace, and the raw material powders of the sealing members 60, 70 and resistor 50 are compressed by the terminal fitting 40 while being heated. Each raw material powder is compressed and sintered to form the sealing members 60, 70 and resistor 50.

After that, the necessary steps such as assembling the metal shell 20 and machining the ground electrode 80 are carried out, and the spark plug 1 is completed.

In the above manufacturing process, the electrode base material 100 is inserted into the axial hole 11, pressed to form the enlarged diameter portion 37, and then filled with the seal member 60. However, other manufacturing methods can be used. For example, the center electrode 30 with the enlarged diameter portion 37 formed in advance may be inserted into the axial hole 11, and then filled with the seal member 60. Alternatively, the electrode base material 100 may be inserted into the axial hole 11, and then a portion of the seal member 60 may be filled in such a manner that the rear end of the electrode base material 100 is exposed, and then pressed to form the enlarged diameter portion 37, and then the remaining seal member 60 may be filled.

<Action and effect>
When a high voltage is applied between the center electrode 30 and the ground electrode 80, a spark discharge occurs in the gap generally along the axis AX. The seal member 60 fixes the center electrode 30 in the axial hole 11 and provides electrical continuity between the center electrode 30 and the terminal fitting 40 via the resistor 50 and the seal member 70.

When the air-fuel mixture burns in the combustion chamber, the combustion pressure exerts axial pressure on the center electrode 30. This reduces the adhesive strength of the seal member 60, which may cause the center electrode 30 to become loose.

If the head of the center electrode has an expanded diameter shape in which the diameter increases as it moves away from the flange, the center electrode is prevented from moving toward the rear end in the axial direction, and the loosening of the center electrode can be prevented. This is because the expanded diameter portion is fixed from both sides in the axial direction by the seal member. In addition, the contact area between the rear end face of the center electrode and the seal member increases, so that when the center electrode is subjected to combustion pressure, the pressure acting from the center electrode to the seal member decreases, making it difficult for the seal member to be compressed. However, on the other hand, in the conventional configuration, the entire head has an expanded diameter shape, so the corner formed by the surface of the flange and the outer peripheral surface of the head forms an acute angle. Therefore, during the manufacturing process, the seal member may not be sufficiently distributed at this corner, and a gap may occur. If such a gap occurs, the adhesive strength between the center electrode and the seal member decreases. In addition, the center electrode thermally expands due to heat received from the internal combustion engine. If the entire head has an expanded diameter shape, stress is likely to concentrate at the tip of the expanded diameter portion, and the seal member may be destroyed near the tip. As a result, there is concern that the expanded diameter shape will have limited effect in preventing the center electrode from loosening.

In this embodiment, the head 34 includes a cylindrical portion 36 that is connected to the flange 33 and has a constant outer diameter, and an expanded diameter portion 37 that is connected to the cylindrical portion 36. In addition, the surface of the flange 33 and the surface of the cylindrical portion 36 are smoothly connected. With this configuration, compared to the conventional configuration, the seal member 60 can be sufficiently spread around the flange 33 and the head 34 during the manufacturing process, and in particular, the seal member 60 can be sufficiently spread around the connection portion 38 between the surface of the flange 33 and the surface of the head 34, ensuring the adhesive force between the center electrode 30 and the seal member 60.

In addition, in the cylindrical portion 36, the force due to thermal expansion is transmitted only in the radial direction (perpendicular to the axial direction) and is evenly distributed around the cylindrical portion 36. This makes it possible to reduce the concentration of stress on the rear end of the enlarged diameter portion 37.

This makes it possible to prevent the center electrode 30 from loosening and avoid a decrease in ignition performance.

In particular, in recent years, there has been a demand for vehicle internal combustion engines to be more compact and have smaller displacements, and to meet this demand, supercharged engines have come to be used. In supercharged engines, the combustion pressure tends to be higher than in the past, and the axial pressure on the center electrode 30 also increases.

On the other hand, as internal combustion engines become smaller and smaller in displacement, the components constituting the spark plug 1 are becoming smaller and smaller in diameter, and the center electrode 30 is also becoming smaller and smaller in diameter. This reduces the surface area of the center electrode 30, and the contact area between the center electrode 30 and the seal member 60. This reduces the fixing force of the seal member 60, and the seal member 60 becomes more likely to become loose. The above configuration can be suitably applied to such a small-sized, small-diameter spark plug 1. More specifically, the above configuration can be suitably applied when the inner diameter of the second inner diameter portion 14 is 3.1 mm or less. Alternatively, the above configuration can be suitably applied when the axial length of the cylindrical portion 36 is 1.9 mm or less. The axial length of the cylindrical portion 36 refers to the length of the portion with a constant outer diameter.

1: Spark plug 10: Insulator 11: Shaft hole 12: First inner diameter portion 13: Step portion 14: Second inner diameter portion 30: Center electrode 33: Flange portion 34: Head portion 35: Shaft portion 36: Cylindrical portion 37: Enlarged diameter portion 60: Conductive seal member

Claims (3)

an insulator having an axial hole extending in an axial direction;
a center electrode disposed on a tip side of the axial hole in the axial direction,
the axial hole has, in order from a tip end side in the axial direction, a first inner diameter portion, a step portion whose inner diameter becomes larger toward a rear end side in the axial direction, and a second inner diameter portion whose inner diameter is larger than that of the first inner diameter portion,
the center electrode has a flange portion whose front end side is supported by the step portion and whose rear end side is one step , a head portion extending from the rear end side of the flange portion to the rear end side in the axial direction and disposed in the second inner diameter portion, and a shaft portion extending from the front end side of the flange portion to the front end side in the axial direction and disposed in the first inner diameter portion,
A spark plug including a conductive seal member that is filled around the flange and the head inside the axial hole,
The head portion includes a cylindrical portion connected to the flange portion and having a constant outer diameter , and an expanded diameter portion connected to the cylindrical portion and having an outer diameter that increases as it moves away from the flange portion,
The center electrode has a surface of the flange portion and a surface of the cylindrical portion that are smoothly connected to each other. The spark plug according to claim 1, wherein the inner diameter of the second inner diameter portion is 3.1 mm or less. The spark plug according to claim 1 or 2, wherein the length of the cylindrical portion in the axial direction is 1.9 mm or less.

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