JPH0837082A - Spark plug for internal combustion engine - Google Patents

Abstract

PURPOSE:To reduce the manufacturing cost, prevent the peeling and dropping of platinum electrodes, and prolong the life by providing platinum electrodes made of a prescribed platinum alloy and having stratified crystal grain structures parallel with the discharge faces of the platinum electrodes. CONSTITUTION:A spark gap is provided between a central electrode 1 and earth electrodes 21, 22, and the base section 11 of the central electrode 1 is buried in an insulator 4. Platinum electrodes 31, 32 made of a platinum alloy are connected to the discharge section of the electrode 1, and the central protrusion length from the tip face of the electrode 1 to the tip face of the insulator 4 is set to 3mm or above. The electrodes 31, 32 are made of the platinum alloy constituted of 10-300 wt.% of iridium, 5.5-10wt.% of nickel, and the remainder of platinum. The electrodes 31, 32 have stratified crystal grain structures parallel with the discharge faces of the electrodes 31, 32, and the crystal grain size satisfies the equations T<=5mum and B/T>=2, where B is the average grain size on the cross section parallel with the discharge face, and T is the average grain size on the longitudinal section perpendicular to the discharge face. The manufacturing cost can be reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spark plug for an internal combustion engine, and more particularly to a spark plug having a center electrode protruding from an insulator by 3 mm or more.

[0002]

2. Description of the Related Art In recent years, it has become difficult to attach and detach a spark plug in an internal combustion engine for an automobile as the number of auxiliary devices increases. Therefore, the spark plug is required to have a long life which does not need to be attached or detached or has a small number of attachments and detachments. Further, from the standpoint of environmental protection, it is required that the internal combustion engine maintain a good combustion state during the entire usage period. In other words, spark plugs are always required to have high ignitability.

In order to meet these requirements, spark plugs in which a platinum electrode made of a platinum alloy is arranged on the discharge surface of the electrode have been widely used. That is, the platinum electrode suppresses the consumption of the electrode and prevents the spark gap from expanding, thereby extending the life of the spark plug.

In order to further improve the ignitability, there has been proposed a projecting type spark plug in which a spark gap formed between electrodes is largely projected into a combustion chamber of an internal combustion engine. Furthermore, in order to extend the life, a spark plug of a side facing type has been proposed in which a ground electrode is placed opposite to a platinum electrode placed on the side surface of the tip of the center electrode, and a spark gap is formed between them. There is.

In the spark plug of the side facing type, the distance between the lower end surface of the ground electrode and the tip surface of the insulator (see FIG.
If the reference sign b) is not sufficiently larger than the spark gap (refer to the reference sign g in FIG. 2,), a spark will occur between the lower end surface of the ground electrode and the tip end surface of the insulator. When a flying fire occurs, not only the ignitability deteriorates but also the insulator is damaged (channeling).
Will be given.

For this reason, in the above-described side-facing spark plug, the distance between the lower end surface of the ground electrode and the tip of the insulator (see FIG. 2, symbol b) must be sufficiently large. Therefore, in the center electrode, the amount of protrusion (hereinafter referred to as the center protrusion length) from the tip end surface of the insulator in which the base portion is embedded to the tip end surface of the center electrode is inevitably large.

[0007]

However, the above-mentioned conventional spark plug for an internal combustion engine has the following problems. That is, when the center protrusion length of the center electrode increases, the center electrode receives a large amount of heat due to the combustion of the internal combustion engine, and receives a large cooling effect when the air-fuel mixture flows in. Therefore, the temperature of the center electrode when the center protrusion length is large is higher during combustion and lower when the air-fuel mixture flows in than when the center protrusion length is small. In other words, the increase in the center protrusion length increases the temperature fluctuation of the center electrode.

The platinum electrode is joined to the center electrode by resistance welding or the like. In addition, the linear expansion coefficient of the nickel-based alloy for the platinum electrode and the center electrode is very different. Therefore, due to the above-mentioned temperature fluctuation, a large thermal stress is generated at the joint between the two. This thermal stress may cause oxidative cracks in the joint, and in extreme cases, may cause peeling or falling of the platinum electrode.

In order to reduce the thermal stress, an invention in which nickel is added to a platinum alloy of a platinum electrode to improve the bondability is disclosed in Japanese Patent Publication Nos. 59-4835 and 61-30014.
Proposed in the issue. The former is shown to add 0.5 to 2% by weight of nickel to the platinum alloy, and the latter is shown to add 0.1 to 5% by weight of nickel to the platinum alloy.

However, when the center protrusion length is 3 mm or more, a sufficient long life cannot be achieved only by the effect of improving the bondability by adding nickel to the platinum alloy. Also, if the amount of nickel added is simply increased to 5% or more, the amount of wear of the platinum electrode will increase sharply,
The longevity effect is lost.

Also, in order to reduce thermal stress, an invention in which a stress relaxation layer made of a platinum alloy having a linear expansion characteristic intermediate between the platinum electrode and the center electrode is provided, Japanese Patent Publication No. 3-220.
No. 33 is proposed. However, in the present invention, the number of man-hours is increased by increasing the number of joining steps by increasing the number of stress relaxation layers. Also, since the stress relaxation layer is a platinum alloy,
The amount of platinum used increases, leading to higher costs.

The present invention has been made in view of the above conventional problems, and provides a spark plug for an internal combustion engine which has a low manufacturing cost, prevents peeling and dropping of the platinum electrode, and has a long life. It is what

[0013]

The present invention is for an internal combustion engine in which a center electrode made of a nickel-based alloy, a ground electrode, and a spark gap are provided between the two, and the base of the center electrode is embedded in an insulator. In the spark plug, the center electrode is formed by joining a platinum electrode made of a platinum alloy to the discharge part, and the center protrusion length from the tip surface of the center electrode to the tip surface of the insulator is 3 mm or more, The platinum electrode is made of a platinum alloy composed of 10 to 30% by weight of iridium, 5.5 to 10% by weight of nickel, and the balance of platinum, and the crystal grain structure of the platinum electrode is the same as that of the platinum electrode. It has a layered structure parallel to the discharge surface, and the crystal grain size of the platinum electrode is B, the average grain size of the cross section parallel to the discharge face of the platinum electrode is B, and the vertical cross section is perpendicular to the discharge face of the platinum electrode. The average grain size of the surface When, in the spark plug for an internal combustion engine, characterized in that T ≦ 5 [mu] m, a B / T ≧ 2.

Most notable in the present invention is that the platinum electrode is made of a platinum alloy composed of 10 to 30% by weight of iridium, 5.5 to 10% by weight of nickel, and the balance of platinum. The crystal grain structure of the electrode has a layered structure parallel to the discharge surface of the platinum electrode. The crystal grain size of the platinum electrode has a relationship of T ≦ 5 μm and B / T ≧ 2 between the average grain size B of the transverse section and the average grain size T of the longitudinal section. .

The platinum electrode is joined to the center electrode by resistance welding or the like. And in a spark plug with a center protrusion length of 3 mm or more,
When the amount of nickel added to the platinum electrode is less than 5.5% by weight, regardless of the amount of iridium added,
Oxidation cracks tend to occur on the joint surface between the platinum electrode and the center electrode (see Fig. 4).

When nickel is added in an amount of 5.5% by weight or more, the platinum electrode consumption increases sharply and the spark gap widens (see FIG. 5). This phenomenon is considered to be due to the following reasons. That is, first, 4-ethyllead contained in the fuel of the internal combustion engine is converted into P by combustion.
Change to bO (lead monoxide). And PbO adheres to the platinum electrode.

In addition, Ni (nickel) + O (oxygen) → N
According to the reaction formula of iO (nickel monoxide), Pb (lead) + O → PbO, NiO has a lower standard free energy of formation for producing NiO and PbO. Therefore, PbO adhering to the platinum electrode is mainly due to Ni existing in the grain boundaries.
Is reduced by. That is, in the grain boundary part, PbO + N
i → Pb + NiO. The reduced Pb easily forms a solid solution in Pt (platinum), so that it penetrates into the crystal near the grain boundaries to form an alloy. The alloy of Pb and Pt has a very low melting point and reaches about 300 ° C. Therefore, the melting point is lowered near the grain boundary where Pb penetrates.

When NiO is produced at the grain boundary, Ni
Due to the volume of O, the grain boundaries tend to be expanded. That is, the grain boundary loosens. If the grain boundaries become loose, the heat conduction between the crystal grains deteriorates. Therefore, the temperature of the crystal grains is greatly increased by the heat received by the combustion of the internal combustion engine and the heat received by the discharge.

Further, Pb penetrates into the crystal grains and has a low melting point. Therefore, when the temperature of the crystal grains rises, the crystal grains melt or scatter, or become molten spheres and fall off from the discharge surface of the platinum electrode. And, these phenomena become more remarkable as the amount of nickel added to the platinum alloy increases. That is, the greater the amount of nickel added, the more the platinum electrode is consumed.

As described above, in the spark plug containing a large amount of nickel, the platinum electrode is consumed before the effect of preventing the above-mentioned oxidation crack is exhibited. In other words, the life of the spark plug cannot be extended simply by increasing the amount of nickel added to the platinum alloy.

Therefore, in the present invention, a method of increasing the amount of nickel added to the platinum alloy and suppressing consumption of the platinum electrode has been found. That is, by making the crystal grain structure of the platinum electrode a layered structure parallel to the discharge surface of the platinum electrode, Pb is prevented from entering the grain boundaries of the platinum electrode.

Further, the crystal grain size of the platinum electrode is
The average particle size of the cross section parallel to the discharge surface of the platinum electrode is B,
Assuming that the average particle diameter of the vertical section perpendicular to the discharge surface of the platinum electrode is T, it is necessary to satisfy the conditions of T ≦ 5 μm and B / T ≧ 2. The average grain size is represented by 100 / n μm, where n is the number of crystal grains crossed by a line segment having a length of 100 μm in the crystal structure on each cross section.

When B / T <2, even if T = 5 μm, the effect of suppressing the platinum electrode consumption by the layered structure is not so remarkable. That is, the platinum electrode added with 5.5 wt% or more of nickel becomes heavily consumed and the spark gap increases (see FIG. 11). When T> 5 μm,
Even if B / T = 2, the effect of suppressing the platinum electrode consumption by the layered structure is not so great. That is, 5.5% by weight
The platinum electrode with nickel added as described above is heavily consumed and the spark gap increases (see FIG. 12).

When the crystal grain size of the platinum electrode is within the above-mentioned optimum range, the amount of nickel added is 5.
If it is in the range of 5 to 10% by weight, it is possible to suppress an increase in the spark gap (see comparison between FIG. 5 and FIG. 13). However, when nickel is added in an amount of 10% by weight or more, the platinum electrode is consumed much. That is, the addition amount of nickel is 1
When the content is 0% by weight or more, the effect of suppressing the wear of the platinum electrode by the layered structure is not seen so much, and the spark gap becomes large (see FIG. 13).

This is for the following reason. That is, when the amount of nickel added exceeds 10% by weight, the recrystallization temperature becomes low. Therefore, the heat load (welding process, etc.) during the manufacture of the spark plug and the high temperature (about 70
At 0 ° C., the platinum electrode recrystallizes and the crystal grains become coarse. Therefore, if the added amount of nickel exceeds 10% by weight, the optimum crystal grain size cannot be maintained. Therefore, the nickel content is 5.5-10% by weight.
It is necessary to

The amount of iridium in the platinum electrode is 10 to 30% by weight. If the amount is less than 10% by weight, the strength is lowered, and there is a problem that the platinum electrode itself cracks due to thermal stress. On the other hand, if it exceeds 30% by weight, the hardness becomes high,
It becomes difficult to process, linear expansion decreases, thermal stress increases, and oxidation cracks occur on the contact surface.

Further, the center electrode has one or two or more platinum electrodes on the side surface of its tip portion, and one or two or more ground electrodes are arranged so as to face the platinum electrode. It is preferable that a spark gap be formed between them. As a result, more reliable ignitability can be secured.

[0028]

In the spark plug for an internal combustion engine according to the present invention, the central protruding length of the center electrode is 3 mm or more. In other words, the spark gap largely projects into the combustion chamber of the internal combustion engine. Therefore, it has excellent ignitability. Further, iridium and nickel are added to the platinum electrode in the above specific range. Therefore, it is possible to prevent the generation of oxidative cracks at the joint surface between the center electrode and the platinum electrode.

The crystal structure of the platinum electrode has a layered structure parallel to the discharge surface. The crystal grain size B,
The relationship of T is T ≦ 5 μm and B / T ≧ 2. Therefore, even if the amount of nickel added is as large as 5.5 to 10% by weight, it is possible to prevent PbO generated by combustion from penetrating into the crystal grains and prevent the platinum electrode from peeling or falling off. it can. In addition, it is not necessary to increase the relaxation layer, etc., and the cost increase due to the increase in the amount of platinum can be avoided.

As described above, according to the present invention, it is possible to provide a spark plug for an internal combustion engine which has a low manufacturing cost, can prevent the platinum electrode from peeling off, and has a long life.

[0031]

【Example】

Embodiment 1 A spark plug for an internal combustion engine according to an embodiment of the present invention will be described with reference to FIGS. As shown in FIG. 1, the spark plug for an internal combustion engine of the present example comprises a center electrode 1 made of a nickel-based alloy, ground electrodes 21, 22 and a spark gap provided between the center electrode 1 and the center electrode 1.
The base 11 is embedded in the insulator 4.

As shown in FIG. 2, the center electrode 1 is formed by joining platinum electrodes 31 and 32 made of a platinum alloy to the discharge part, and the center electrode 1 has a front end surface 101 to a front end surface of the insulator 4. The central protrusion length a up to 41 is 3 mm. The platinum electrodes 31 and 32 are made of iridium 10
.About.30% by weight, 5.5 to 10% by weight of nickel, and platinum as the balance.

The crystal grain structure of the platinum electrodes 31, 32 has a layered structure parallel to the discharge surfaces of the platinum electrodes 31, 32. The crystal grain size of the platinum electrodes 31, 32 is the average grain size B (FIG. 8) of the cross section (FIG. 7) parallel to the discharge surface of the platinum electrodes 31, 32, and the platinum electrode 31,
The average particle size of the vertical section (Fig. 9) perpendicular to the discharge surface of 32 is T
(FIG. 10), T ≦ 5 μm and B / T ≧ 2.

The center electrode 1 is a nickel-based alloy [Ni
-Fe-Cr, Inco 600 (trademark MA600: manufactured by Mitsubishi Materials Corporation)]. As shown in FIGS. 1 and 2, platinum electrodes 31 and 32 are joined by resistance welding to two side surfaces of the tip portion 10 of the center electrode 1.

The platinum electrodes 31 and 32 are manufactured by rolling an ingot having a predetermined amount of components to form a thin plate and punching the thin plate. As shown in FIGS. 7 and 9, the dimension and shape thereof are disk-shaped and have a diameter of 1.2 mm and a thickness of 0.3 mm. Further, when changing the crystal structure, the rolling reduction and the heat treatment conditions during the rolling are changed.

The ground electrodes 21 and 22 are the same as those shown in FIGS.
As shown in A and B, the tips are platinum electrodes 31, 32.
It is arranged so as to face. And the ground electrode 2
1 m between the tips of the electrodes 1, 22 and the platinum electrodes 31, 32
A spark gap g (FIG. 2) of m is formed. The contact electrodes 21, 22 are also made of the same nickel-based alloy as the center electrode. The distance b between the upper surfaces 210 and 220 of the ground electrodes 21 and 22 and the tip surface 41 of the insulator 4 is 1.
It is set to 5 mm.

Further, as shown in FIG. 1, the insulator 4 is inserted and fixed in the inner hole 50 of the metal housing 5 and is integrated by caulking the abdomen 51 of the metal housing 5. The center electrode 1 is made of conductive glass 134,
It is connected to the terminal 136 via the built-in resistor 135.
Further, gaskets 131 and 132 are assembled in order to ensure airtightness between the insulator 4 and the metal housing 5.

Next, the function and effect of this example will be described with reference to the results of comparative evaluation. First, the platinum electrode 31,
32 and the central electrode 1, the platinum electrode 31,
It shows how the material of 32 influences. As the material of the platinum electrodes 31 and 32, iridium is 10 to 30.
The amount was changed in the range of wt%, the amount of nickel was changed in the range of 2 to 10% by weight, and the balance was platinum.

The evaluation engine is a water-cooled 4-cycle 6-cylinder 2
A 000 cc engine was used. The evaluation conditions were such that idling for 1 minute and full load 5000 rpm for 1 minute were repeated and the operation was performed for 50 hours. As the characteristic value of the evaluation, the oxidation rate of the above-mentioned joint surface defined below was used. As shown in FIG. 3, the oxidation rate of the above-mentioned bonding surface is determined by setting the length of the oxidation crack 39 as the upper crack e and the lower crack f, and determining the platinum electrodes 31, 3
When the diameter of 2 is d (FIG. 7), (e + f) / d × 10
It was decided to express it as 0%.

As shown in FIG. 4, the results of the evaluation show that, when the amount of iridium added is within the range of 10 to 30% by weight, good bonding reliability can be ensured if the amount of nickel added is 5.5% by weight or more. Is shown. That is, within the material range of the platinum electrodes 31 and 32 of the present invention, the bondability of the bonding surface is sufficiently reliable.

Next, as a comparative example, platinum electrodes 31, 32
The results of evaluation of the consumability of the platinum electrodes 31 and 32 in the case where the amount of nickel added is simply increased while the crystal structure of No. 1 is maintained as it is, are shown below. That is, first, as the material of the platinum electrodes 31 and 32, the amount of iridium added was fixed to 20% by weight, the amount of nickel added was changed within the range of 2 to 10% by weight, and the rest was platinum. Evaluation engine is water-cooled 4-cycle 6-cylinder 20
A 00cc engine was used. The evaluation condition is that the load is Road-Load (load equivalent to running on a flat road),
60 km / H running for 30 minutes and 100 km / H running for 30 minutes were repeated, and the running was performed for 50,000 km. Then, the increase amount (mm) of the spark gap after running was measured.

The fuel used was the one to which 0.01 g / usgal of 4-ethyllead, which is the upper limit of the amount of lead added in the US market, was added. Further, as the polarity of voltage applied to the spark plug, a general one in which the center electrode 1, that is, the platinum electrodes 31 and 32 are negative electrodes, and a special one in which the positive electrodes are positive electrodes were evaluated.

As a result, when the platinum electrodes 31 and 32 were of the negative polarity, the platinum electrodes consumed more than those of the positive electrode. This is because when the platinum electrodes 31 and 32 have a negative polarity, positive ions generated by the discharge are platinum electrodes 31 and 32.
This is due to the fact that the electrode collides with and heats the electrode to a high temperature, and the consumption increases due to sputtering.

FIG. 5 shows the evaluation results when the platinum electrodes 31, 32 are of negative polarity, that is, the relationship between the Ni addition amount (wt%) and the gap increase amount (mm). As is known from the figure, when the amount of nickel added exceeds 5% by weight, the platinum electrodes 31 and 32 wear rapidly, and the spark gap g (FIG. 2) rapidly expands. Further, FIG. 6 shows a result of observing a cross section near the discharge surface of the platinum electrodes 31 and 32 after the consumption evaluation. The observed platinum electrodes 31 and 32 are
20% by weight of iridium and 10% by weight of nickel.

As shown in FIG. 6, the discharge surfaces of the platinum electrodes 31 and 32 are once melted and become spherical due to surface tension.
After that, a large amount of solidified molten spheres 301 was observed. A loose part 305 of the crystal grain boundary was observed near the discharge surface. As a result of elemental analysis (using EDS manufactured by JEOL) of the components of the molten sphere, Pb was detected in addition to Pt, Ir, and Ni. The detected amount of Pb was larger as the amount of nickel added was larger.

Also, as a result of the elemental analysis of the grain boundary loosened portion 305, a large amount of Pb and Ni were detected. The depth L (FIG. 6) of the grain boundary loosened portion 305 from the discharge surface tended to become deeper as the amount of nickel added increased.

Next, the platinum electrode 31 according to the present invention,
32 shows the evaluation result of the correlation between the layered crystal structure and the consumable property. First, the material of the platinum electrodes 31, 32 is
The amount of iridium added was fixed at 20% by weight, the amount of nickel added was also fixed at 7% by weight, and the balance was platinum.

Regarding the characteristics of the crystal structure, as shown in FIGS. 7 to 10, the crystal grain size B (FIG. 8) at the cross section 37 (FIG. 7) parallel to the discharge surfaces of the platinum electrodes 31, 32 is shown. , The crystal grain size T (FIG. 9) in the vertical section 38 (FIG. 9) perpendicular to the platinum electrodes 31, 32.

Further, as described above, the crystal grain size was calculated as 100 / n μm when a line segment of 100 μm was drawn on each cross section and the number of crystal grains crossed by the line segment was n. . In addition, the evaluation method of the engine used and the like was the same as the evaluation method of the consumability of the platinum electrodes 31 and 32 (the above comparative example) without adjusting the grain structure.

As a result, as shown in FIG. 11, T = 5 μ
In the case of m, when the B / T is 2 or more, the increase in the spark gap decreases. That is, as B increases, the total length (mm) of grain boundaries exposed on the discharge surface decreases. Therefore, the penetration of PbO into the grain boundaries is reduced, and the platinum electrodes 31 and 32 are less consumed. Further, as shown in FIG. 12, when B / T = 2, the increase of the spark gap becomes extremely small when T becomes 5 μm or less. That is,
The thinner the crystal grains, the more difficult it is for PbO to penetrate deep inside the grain boundaries. Therefore, the platinum electrodes 31, 32 are less consumed.

Next, FIG. 13 shows the increase amount of the spark gap g (FIG. 2) when B / T = 2 and T = 5 μm and the addition amount of nickel was changed from 2 to 12% by weight. FIG.
3 and the above-mentioned FIG. 5 (comparative example), the amount of nickel added was 5 to 10
It can be seen that the effect of suppressing the increase of the spark gap g (FIG. 2) is high in the range of wt%.

As described above, in the spark plug for an internal combustion engine of this example, the center projection length a of the center electrode 1 is a.
(FIG. 2) is 3 mm. In other words, the spark gap largely projects into the combustion chamber of the internal combustion engine. Therefore, it has excellent ignitability. Further, iridium and nickel are added to the platinum electrodes 31 and 32 joined to the center electrode 1 in the above range. Therefore, it is possible to prevent the generation of oxidative cracks at the joint surface between the center electrode 1 and the platinum electrodes 31, 32.

The crystal structure of the platinum electrode has a layered structure parallel to the discharge surface. The crystal grain size B,
At T, T ≦ 5 μm and B / T ≧ 2. Therefore, even if the amount of nickel added is increased to 5.5 to 10% by weight, it is possible to prevent the PbO generated by combustion from entering the crystal grains and prevent the platinum electrode from peeling or falling off. You can Further, unlike the conventional example, it is not necessary to separately increase the relaxation layer and the like, and the cost increase due to the increase in the amount of platinum can be avoided.

Example 2 In the spark plug for an internal combustion engine of this example, as shown in FIG. 14, a platinum electrode 9 is arranged at the tip of the center electrode 7, and one ground electrode 8 is connected to the discharge surface of the platinum electrode 9. Are arranged opposite to each other. The center protrusion length a is 3 mm. Others are the same as in the first embodiment. Also in this example, Example 1
The same effect as can be obtained.

[Brief description of drawings]

FIG. 1 is a partial cross-sectional side view of a spark plug for an internal combustion engine according to a first embodiment.

FIG. 2A is a side view and FIG. 2B is a bottom view of a main part of the spark plug for an internal combustion engine according to the first embodiment.

FIG. 3 is an explanatory diagram showing cracks in the center electrode of the spark plug for an internal combustion engine according to the first embodiment.

FIG. 4 is an explanatory diagram of a correlation between a nickel addition amount of a platinum electrode and a bonding surface oxidation rate in a spark plug for an internal combustion engine of Example 1.

5 is an explanatory diagram of a correlation between the amount of nickel added to the platinum electrode and the amount of increase in the spark gap in the comparative example of Example 1. FIG.

FIG. 6 is an explanatory diagram of a state of crystal grains after the platinum electrode is consumed in the comparative example of Example 1.

FIG. 7 is an explanatory view of a cross section of a platinum electrode in the spark plug for an internal combustion engine of Example 1.

8 is an explanatory diagram of a crystal grain size on a cross section of a platinum electrode in the spark plug for an internal combustion engine of Example 1. FIG.

9 is an explanatory view of a vertical cross section of a platinum electrode in the spark plug for an internal combustion engine of Example 1. FIG.

10 is an explanatory diagram of a crystal grain size on a vertical section of a platinum electrode in the spark plug for an internal combustion engine of Example 1. FIG.

FIG. 11 shows the crystal grain size (B /
Explanatory drawing of the correlation between T) and the spark gap increase amount.

FIG. 12: Crystal grain size (T) of platinum electrode in Example 1
Explanatory diagram of the correlation between the spark gap increase amount and.

13 is an explanatory view of the correlation between the amount of nickel added to the platinum electrode and the amount of increase in the spark gap in Example 1. FIG.

FIG. 14 is an enlarged side view of a main part of an internal combustion engine spark plug according to a second embodiment.

[Explanation of symbols]

 1. ． ． Center electrode, 101. ． ． Tip surface, 21, 22. ． ． Ground electrode, 31, 32. ． ． Platinum electrode, 4. ． ． Insulator, 41. ． ． Tip surface, 5. ． ． A metal housing, b. ． ． Center protrusion length,

Claims (2)

[Claims] 1. A spark plug for an internal combustion engine, comprising: a center electrode made of a nickel-based alloy; a ground electrode; and a spark gap provided between the center electrode and the ground electrode, and the base of the center electrode being embedded in an insulator. The center electrode is formed by joining a platinum electrode made of a platinum alloy to the discharge part, and the center projection length from the tip surface of the center electrode to the tip surface of the insulator is 3 mm or more. Iridium 10-30% by weight, nickel 5.5-10
The platinum electrode has a layered structure parallel to the discharge surface of the platinum electrode, and has a crystal grain size of the platinum electrode. Let B be the average particle size of the cross section parallel to the discharge surface of the platinum electrode and T be the average particle size of the vertical cross section of the platinum electrode, then T ≤ 5 μm, B / T ≥ 2 A spark plug for an internal combustion engine, characterized by being present. 2. The center electrode according to claim 1, wherein the center electrode has one or more platinum electrodes on a side surface of a tip portion thereof, and the platinum electrode has one or more ground electrodes. A spark plug for an internal combustion engine, wherein the spark plugs are arranged opposite to each other and a spark gap is formed between them.