JPH071711B2 - Spark plug for internal combustion engine - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a spark plug for an internal combustion engine, and particularly to a thermal stress caused by a difference in linear expansion coefficient value between an insulator and a center electrode. The present invention relates to a spark plug for an internal combustion engine, which reliably prevents cracks or cracks at the tip of the insulator.

(Prior Art) As one of conventional methods for holding a center electrode on an insulator of a spark plug for an internal combustion engine (hereinafter referred to as a plug),
There is a method disclosed in JP-A-58-40789. In this method, (a) a center electrode chip made of a noble metal molded into a thin nail shape is introduced into the insulating hole, and (b) both the molded body for an insulator and the center electrode chip are passed through a firing furnace to form this molded body. The center electrode chip is fixed to the tip of the insulator by utilizing the shrinkage of the molded body when the body is sintered to form the insulator, and (c) the center electrode chip and the center electrode at the inner hole of the insulator. There is a step of connecting the base material through a seal glass or joining them by welding to bring the center electrode chip and the center electrode base material into an electrically conductive state.

(Problems to be Solved by the Invention) However, in the method according to the prior art, since there is a relatively large difference in linear expansion coefficient between the ceramic insulator and the center electrode base material made of metal, the center electrode chip is There is a risk that the tip of the insulator fixing the element is subject to thermal stress in the axial direction due to a large temperature difference between when the internal combustion engine is operating and when the engine is stopped, and the insulator is cracked. In order to prevent this insulator cracking, in the conventional technology, the center electrode base material is made of Kovar having a linear expansion coefficient substantially equal to that of the insulator, and the center electrode chip and the Kovar center electrode base material are welded together. To prevent the above-mentioned insulator cracking.

However, since Kovar is an expensive material, the cost of the plug becomes high, and since the center electrode chip and the Kovar center electrode base material are joined by welding, there is a problem in that the mass productivity of the plug is poor.

(Means for Solving the Problems) An object of the present invention is to obtain a spark plug for an internal combustion engine, which solves the above-mentioned drawbacks of the prior art.

The spark plug for an internal combustion engine of the present invention comprises an insulator having an inner hole extending in the axial direction, and a center electrode arranged in the inner hole, the center electrode being the insulator of the insulator. A center electrode tip fixed to the tip and having one end protruding outward from the insulator tip and the other end protruding from the insulator tip into the inner hole; and disposed in the inner hole. An end portion having a center electrode base material axially opposed to the other end portion of the center electrode tip, and the other end portion of the center electrode tip and the end of the center electrode base material The portion is a distance enough to absorb the elongation of the center electrode base material based on the difference in the coefficient of linear expansion between the center electrode base material and the insulator, and is separated from each other, Characterized by being set in an electrically conducting state through an electrically conducting means And

(Function) When the plug is used, the temperature of the plug becomes considerably high, and the coefficient of linear expansion of the base material of the center electrode is usually larger than that of the insulator, so the axial expansion of the center electrode is the elongation of the insulator. Will be larger than. Due to this difference in elongation, stress is applied to the tip of the center electrode chip fixed insulator, and cracks or cracks are likely to occur at this tip. In the present invention, the tip portion of the center electrode base material is separated from the inner end portion of the center electrode chip fixed to the insulator tip portion by a predetermined distance. Only when it occurs, no stress is applied to the tip portion of the insulator, so that cracks or cracks can be reliably prevented. Further, since the difference in shrinkage value caused by the temperature change after the internal combustion engine is stopped also occurs in the separated voids, it is possible to prevent the tip portion of the insulator from cracking.

(Embodiment) An embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a sectional view showing a first embodiment of a spark plug for an internal combustion engine of the present invention, and the features of the first embodiment are shown in FIG. In FIG. 2, 1 is an insulator made of ceramic, 2 is a platinum center electrode chip with a diameter of 0.5 to 3 mm fixed to the insulator tip, and the outer end 2a of this center electrode chip is from the insulator tip 1a. The inner end 2b of the center electrode chip, which projects outwardly, has a ceramic insulator tip 1a.
To project into the inner hole 6 of the insulator.

Reference numeral 3 denotes a center electrode base material made of copper or Ni or having a diameter of 1.5 to 3 mm and made of Ni or Cu coated with Ni, which is disposed in the inner hole 6 of the insulator 1. The end 3a of the center electrode base material 3 has a center electrode. It is separated from the inner end 2b of the chip 2 by a predetermined distance (L) in the axial direction.

Reference numeral 4 is a conductive coating containing a refractory metal such as platinum, which is baked on the surface of the inner hole 6 portion of the insulator 2 corresponding to the center electrode chip 2 and the center electrode base material end portion 3a facing the center electrode chip 2. Since the center electrode chip 2 is in contact with the conductive coating 4 at the inner end 2b in the axial direction, the center electrode chip 2 and the conductive coating 4 are electrically connected. There is a gap of about 0.1 mm between the end of the center electrode base material 3 facing the center electrode chip 2 and the conductive coating 4. However, since a high voltage of 10 to 30 KV is applied to the plug when the plug is operated, this end portion 3a of the center electrode base material 3 and the conductive film 4 are electrically connected to each other when the plug is operated.

There is about 1 mm of gear at the ignition part of the plug.
Since sparks are generated in the gap of mm when the plug is activated,
Therefore, the end portion 3a of the center electrode base material 3 and the conductive coating 4
It is proved that the above-mentioned gap between and becomes electrically conductive when the plug is operated.

In order to determine the range of the distance (L) between the inner end 2b of the center electrode chip and the end 3a of the center electrode base material, pre-ignition occurs when the water-cooled gasoline engine is operated at full load and maximum rotation speed. The value of the advance angle of the ignition timing was calculated. The result is shown in FIG.

In FIG. 5, the horizontal axis is the above distance (L), and the vertical axis is the advance value of the ignition timing at which the pre-ignition occurs (ignition advance angle).
That is, the larger the ignition advance value, the less likely preignition occurs. The test was carried out n = 6 times, and the maximum value, average value, and minimum value are shown in the figure. From the results shown in FIG. 5, it can be seen that preignition is likely to occur when the internal gear is 1.5 mm <L. That is, the gear distance (L) for preventing pre-ignition from occurring early must be set to 1.5 mm or less, and L ≦ 1.5 mm. However, L ≧ 0.4 mm is preferable in order to compensate for the difference in the coefficient of linear expansion between the center electrode and the insulator and in consideration of the tolerance during roughening.

The thickness of the conductive coating 4 is 0.15 mm or less, and the material must be selected in consideration of the operating temperature of the plug. That is, in a material having a melting point lower than the maximum operating temperature of the plug, the coating metal will melt during use. Therefore, the conductive coating is
It is necessary to select from high melting point metals such as platinum with a melting point of 1000 ° C or higher. Then, in order to form the film, the metal powder and the glass powder may be applied in a liquid form or a paste form with an organic melting material or the like.

In this example, the film made of metal was described, but if the material has conductivity instead of metal, SiC, MoS
Carbides, silicides and nitrides typified by i ₂ , TiN, ZrSi and the like may be used.

In the first embodiment, the shape of the tip portion 3a of the center electrode base material is cylindrical, but in addition to this, the shape of the tip portion 3a may be the conical shape or the truncated cone shape shown in FIG. 3 or 4. Good.

FIG. 6 shows a second embodiment of the spark plug for an internal combustion engine of the present invention. The structure and manufacturing method of the insulator 1, the center electrode chip 2 fixed to the tip of the insulator, and the conductive coating film 4 provided on the surface of the inner hole 6 of the insulator 1 are the same as those of the first embodiment. It is the same as the case shown in. In the second embodiment, the diameter of the tip 3a of the center electrode base material, which is opposed to and spaced from the inner end 2a of the center electrode chip, is smaller than the diameter of the tip of the center electrode base material in the case of the first embodiment. Therefore, the gap between the tip portion 3a and the conductive coating 4 is larger than that in the first embodiment. In order to ensure electrical continuity in this gap, in the second embodiment, the center electrode base metal tip 3a is used.
Further, a plate spring 5 having a disc shape is fixed by resistance welding, argon welding, or the like. Since the leaf spring 5 has such a size as to come into contact with the conductive coating 4 when the center electrode base material 3 is arranged in the inner hole 6 of the insulator 1, the center electrode chip 2 and the center electrode base material 3 are Becomes electrically conductive.

FIG. 7 is a perspective view of the frustoconical leaf spring 5 fixed to the tip 3a of the center electrode base material. In addition to the shape shown in FIG. 7, the leaf spring 5 may have a substantially square shape with the four corners of the square bent as shown in FIG.

FIG. 9 is a sectional view of a main part of a plug showing a third embodiment of the present invention. In the third embodiment, the structure and manufacturing method of the insulator 1 and the center electrode chip 2 are the same as those of the first embodiment. Same as the example. In the third embodiment, no conductive coating is provided, and a cylindrical hole 7 is provided in the tip portion 3a of the center electrode base material concentrically with the tip portion 3a. It differs from the first embodiment in that the tip portion 2b of the center electrode chip is inserted so as to provide a gap (L) in the axial direction. The tip portion 2b of the center electrode chip located in the hole 7 has a radial gap of about 0.1 mm between itself and the inner wall of the hole 7. However, since a high voltage of 10 to 30 KV is applied to the plug when the plug is activated, it is electrically connected to the center electrode base material 3 of the center electrode chip 2 when the plug is activated as described in the first embodiment. Become.

(Effect of the Invention) In the spark plug for an internal combustion engine of the present invention, the inner end portion 2b of the center electrode chip and the end portion 3a of the center electrode base material are separated by a predetermined distance (L). Relative expansion or contraction of the central electrode 3 with respect to the insulator 1 caused by the difference between the linear expansion coefficient value and the linear expansion coefficient value of the central electrode base material 3 occurs in the gap of this distance (L). The elongation or contraction of the base material does not give any stress to the tip of the insulator. Therefore, based on the difference in the linear expansion coefficient values, it is possible to surely prevent the cracks or cracks at the tip of the insulator, which have occurred conventionally, without using expensive metal such as Kovar. Further, since the step of welding the center electrode chip to the tip of the center electrode base material, which was required in the conventional technique, is not necessary, the productivity of the plug can be increased.

[Brief description of drawings]

1 is a sectional view showing a first embodiment of a spark plug for an internal combustion engine of the present invention, FIG. 2 is a sectional view of a main portion showing a characteristic portion of the present invention in the plug of FIG. 1, FIG. 3 and FIG. Fig. 4 is a cross-sectional view of the main part of the plug showing a modification of the inner end of the center electrode chip and the end of the center electrode base material, and Fig. 5 is the center electrode chip inner end 2b and center electrode base material end.
3a is a graph showing the correlation between the separation distance (L) from 3a and the ignition advance angle, FIG. 6 is a sectional view of the main part of the plug according to the second embodiment of the present invention, and FIG. 6 is a perspective view of the center electrode base material end portion 3a shown in FIG. 6, and FIG. 8 is a perspective view showing a modification of the leaf spring 5 fixed to the center electrode base material tip portion of the plug shown in FIG. FIG. 9 and FIG. 9 are sectional views of the main part of the plug according to the third embodiment of the present invention. 1 ... Insulator 2 ... Center electrode chip 3 ... Center electrode base material 4 ... Conductive coating 6 ... Inner hole

Claims (1)

[Claims] 1. A spark plug for an internal combustion engine, comprising: an insulator having an inner hole extending in an axial direction; and a center electrode arranged in the inner hole, the center electrode comprising: A center electrode tip fixed to the tip of the insulator and having one end protruding outward from the insulator tip and the other end protruding from the insulator tip into the inner hole; And a center electrode base material having an end portion axially opposed to the other end portion of the center electrode tip, the other end portion of the center electrode tip and the center electrode mother body. The end portion of the material, the distance to the extent that can absorb the elongation of the center electrode base material based on the difference in the coefficient of linear expansion between the center electrode base material and the insulator, and while being spaced apart, Both ends are electrically connected to each other via an electrically conductive means. Spark plug for an internal combustion engine, wherein the door.