JPH031472A - Ignition plug - Google Patents

Abstract

PURPOSE:To preclude drop of the insulation resistance due to formation of a dielectric breakdown path and prevent misfire by coating the body piece with insulative substance consisting of ceramics at least in its internal circumferential portion mating with the longer leg part in the neighborhood of a shelf. CONSTITUTION:When an ignition plug 1 is used in serve service condition to raise the temp. of its longer leg part 5 made of ceramics of nitride type, the insulation resistance value of this leg part 5 will drop. However, insulating substance 11 coated over the inner wall of the body piece 4 holds the insulations between the body piece 4 and longer leg part 5, in particular between a shelf 10 and that portion of the longer leg part 5 which is mating the shelf 10. This relieves concentrated electric field between the shelf 10 and that portion of the longer leg part 5 which is mating with the shelf 10. Accordingly formation of the dielectric breakdown path can be suppressed even though the insulating substance 3 is formed from ceramic of nitride type.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to measures against treeing deterioration of spark plugs.

[Conventional technology] Nitride ceramics have very good thermal conductivity.

For this reason, it is known that if nitride ceramics such as aluminum nitride are used as the insulator of the ignition plug, the range of the ignition plug can be expanded.

[Problems to be Solved by the Invention] However, compared to alumina and the like that have been conventionally used as insulators, nitride-based ceramics have a greater degree of decrease in insulation resistance when the temperature becomes high.

On the other hand, as shown in FIG.
A step 102 formed around the metal shell 103 contacts a shelf 104 formed on the inner periphery of the metal shell 103, thereby restricting movement in the axial direction. A shelf portion 105 is formed on the inner periphery of the metal shell 103 to form a shelf 104 that protrudes from the inner periphery. Since the shelf portion 105 has a shape that protrudes from the inner periphery of the metal shell 103, when a high voltage is applied to the center electrode,
Electric field concentration tends to occur between the shelf portion 105 and the long leg portion 106 facing the shelf portion 105 (the shaded area in the figure).

Therefore, when a high voltage is applied to the center electrode in a state where the temperature of the long leg portion 106 is high and the insulation resistance value of the long leg portion 106 is decreased, the concentration of the electric field near the shelf portion 105 causes the long leg portion 106 to Corona discharge occurs on the surface of the long leg part 1.
A dielectric breakdown path A is formed on the surface of 06 (treeing deterioration). When this dielectric breakdown path A is formed, an ignition error occurs and the dielectric breakdown path A grows, eventually resulting in a decrease in the insulation resistance of the insulator 101 and a misfire.

The present invention has been made in view of the above circumstances, and its purpose is to suppress the occurrence of dielectric breakdown paths to eliminate ignition errors, and to prevent the decrease in insulation resistance of the insulator due to the occurrence of dielectric breakdown paths. On offer.

Means for Solving the Problem] In order to achieve the above object, the spark plug of the present invention employs the following technical means.

The ignition plug of the present invention includes a center electrode, a cylindrical metal shell with a shelf on the inner periphery, and a stage that contacts the shelf is biased, and
The insulator is made of a nitride-based ceramic material and has a small diameter through this step and has a long leg portion facing the metal shell with a gap therebetween.

The metal shell is coated with an insulator made of ceramic at least on an inner circumferential portion near the shelf and facing the long leg portion.

[Function] The spark plug configured as described above has improved insulation performance at least near the shelf between the long leg portion and the metal shell. Therefore, even if the insulation resistance value of the long leg portion decreases due to an increase in the temperature of the leg rest, the insulation between the insulator and the metal shell is maintained by the insulator applied to the metal shell.

As a result, the concentration of the electric field near the shelf of the long leg portion is alleviated, and the formation of a dielectric breakdown path is prevented.

[Effects of the Invention] As explained above, the present invention prevents the formation of a dielectric breakdown path even if the temperature of the long leg portion increases. As a result, ignition errors due to the formation of dielectric breakdown paths are eliminated, and a decrease in insulation resistance of the insulator due to the formation of dielectric breakdown paths can be prevented, thereby preventing misfires.

[Example] Next, a spark plug of the present invention will be described based on an example shown in the drawings.

(Configuration of Example) FIG. 1 is a sectional view of a main part of an ignition plug disposed in a combustion chamber of an internal combustion engine.

The ignition plug 1 is roughly divided into a center electrode 2, an insulator 3, and a metal shell 4.

The center electrode 2 is an electrode body that passes through the axis of an insulator 3 to which a high voltage is applied from an ignition device (not shown).

The insulator 3 is a nitride-based ceramic made of aluminum nitride, silicon nitride, etc., and is a cylindrical body that covers the center electrode 2 and holds the center electrode 2 insulated. The insulator 3 includes a long leg portion 5 that faces the metal shell 4 with a gap therebetween, and the long leg portion 5 has a tapered shape that becomes thinner toward the end.

The insulator 3 has a slightly larger diameter with a step 6 interposed between it and the leg length portion 5. Note that the step 6 comes into contact with a shelf 7 (described above) of the metal shell 4 via a metal packing 8, and movement in the axial direction is restricted.

The metal shell 4 holds the insulator 3 when the internal combustion engine (
It is a cylindrical metal fitting that is screwed into the metal fitting (not shown), and the outer electrode 9 is joined to the end by welding or the like. This outer electrode 9 has a rod shape and is bent so that its end faces the center electrode 2 with a spark gap 1 in between.

On the other hand, the metal shell 4 has a shelf 7 on its inner periphery, with which the step 6 of the insulator 3 abuts via a metal plate 8.

This shelf 7 is for holding the insulator 3 as described above. An annular shelf portion 10 is formed on the inner periphery of the metal shell 4 so as to form a shelf 7 .

Further, the entire inner wall of the metal shell 4 is coated with insulating ceramic powder made of alumina or the like by thermal spraying or the like. As a result, the inner peripheral portion of the metal shell 4 facing the long leg portion 5 near the shelf 7 is also coated with the insulator 11 made of ceramic.

(Experimental Data) Experimental data regarding treeing deterioration of the spark plug 1 of the above example is shown in FIG.

The comparative product shown in the figure is a conventional product in which the inner wall of the metal shell 4 is not coated with ceramics, and has the same heat value as the ignition plug 1 of the example.

Note that the experimental data used a 2000cc 6-cylinder engine. And one cycle shown in the experimental data is
After driving at 5,500 rpm (full throttle) for 5 minutes, the test was performed with 5 accelerations and decelerations per minute, and the center type f! The voltage applied to 2 is up to 2
It was set to 5kV.

Treeing deterioration was determined when a normal spark discharge did not occur (ignition error occurred) in the spark gap g (marked with Δ in the figure).

(Effects of the Embodiment) When the spark plug 1 is used under harsh conditions and the temperature of the long leg portion 5 increases, the insulation resistance value of the long leg portion 5 formed of nitride ceramics (ANN) decreases. However, the insulator 11 coated on the inner wall of the metal shell 4 maintains an insulated state between the metal shell 4 and the long leg portion 5, particularly between the shelf portion 10 and the long leg portion 5 facing the shelf portion 10. Therefore, concentration of the electric field between the shelf 10 and the long leg portion 5 facing the shelf 10 is alleviated. As a result, insulator 3
Even if it is made of nitride-based ceramics, the formation of dielectric breakdown paths can be suppressed.

(Effects of Example) As shown in Figure 2, the comparative product has a
Treeing deterioration occurs between approximately 3,800 and 4,500 cycles, whereas treeing deterioration occurs between approximately 3,800 and 4,500 cycles in the spark plug 1 of this embodiment. In other words, compared to conventional products, the formation of dielectric breakdown paths can be dramatically suppressed.

This makes it possible to substantially eliminate the formation of dielectric breakdown paths under normal internal combustion engine operating conditions. As a result, ignition errors due to the formation of a dielectric breakdown path are eliminated, and a reduction in insulation resistance of the insulator 3 due to the formation of a dielectric breakdown path is prevented, misfires are prevented, and widening is achieved.

(Modified Example) In this embodiment, the entire inner wall of the metal shell 4 is coated with the ceramic insulator 11, but the coating may be applied only to the part facing the long leg part 5, or to the part facing the long leg part 5 near the step 6. It is also possible to coat only a portion.

It is a sectional view of the main part of the major examination.

In the diagram 2... Center type fi 3... Insulator 4... Metal shell 5... Long leg portion 6... Step
7...Shelf

Claims (1)

[Scope of Claims] 1) A center electrode, a cylindrical metal shell with a shelf on its inner periphery, and a step that abuts the shelf, and a small diameter through this step, which connects the metal shell with the center electrode. and an insulator having long leg portions facing each other with a gap therebetween, wherein the insulator is made of a nitride-based ceramic material, wherein the metal shell faces at least the long leg portion near the shelf. A spark plug characterized by having an inner circumference coated with an insulator made of ceramics.