Spark plug with specific gap between insulator and electrodes

Details Spark plug with specific gap between insulator and electrodes Spark plug with specific gap between insulator and electrodes

1999-06-10

2001-05-08

Patel, Vip (Department: 2879)

Electric lamp and discharge devices

Spark plugs

Particular electrode structure or spacing

C313S143000

Reexamination Certificate

active

06229253

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a spark plug used for the igniter of an internal combustion engine.
2. Description of the Related Art
Generally, a conventional type spark plug is provided with a central electrode and a parallel ground electrode. The central electrode is protruded downward from the lower end face of insulator. The parallel ground electrode is arranged opposite to the central electrode and one end of which is bonded to main metal shell for igniting fuel mixed gas by spark discharge in an air gap between the central electrode and the parallel ground electrode.
To enhance ignitability in the air gap, a spark plug provided with auxiliary ground electrodes opposite to the side face of a central electrode in addition to a parallel ground electrode opposite to the end face of the central electrode is proposed in Unexamined Japanese Patent Publication (kokai) No. Hei. 5-326107, U.S. Pat. No. 5,581,145 and EP 0 774 812A. These auxiliary ground electrodes are not provided to fly sparks in a gap between the auxiliary ground electrode and the central electrode. However, these auxiliary ground electrodes are provided to improve the distribution of electric fields between the parallel ground electrode and the central electrode by the existence of the auxiliary ground electrodes. Accordingly, ignitability is enhanced by flying a spark in the gap between the parallel ground electrode and the central electrode at lower discharge voltage. Therefore, in the structure of these spark plugs, the edge of the end face of the auxiliary ground electrode is not necessarily positioned in the vicinity of the lower end face of the insulator.
Further, in U.S. patent application Ser. No. 08/749,309 and EP 0 774,812 A, a spark plug provided with an auxiliary ground electrode in the vicinity of the lower end face of insulator in addition to a parallel ground electrode opposite to the end face of a central electrode is proposed.
However, there is a problem that both conventional type spark plug disclosed in Unexamined Japanese Patent Publication No. Hei5-326107 and U.S. Pat. No. 5,581,145 are weak in a so-called carbon fouling. At the time of regular operation in which an internal combustion engine is rotated at engine speed equal to or faster than predetermined engine speed at predetermined temperature, a leg portion which is a lower part of the insulator of the spark plug is suitably burned and surface temperature in the vicinity of the lower end face of the insulator inside a combustion chamber rises up to approximately 500° C. Therefore, carbon which adheres to the surface of the insulator is burned and the surface of the insulator is kept clean. Therefore, no carbon fouling is caused. However, in the case of a low load in which the temperature of the internal combustion engine is extremely low and the engine speed is also low, the surface temperature of the insulator does not rise, carbon by the combustion adheres to the surface of the insulator and is accumulated to be a so-called carbon fouling state. When this further progresses, insulation between the central electrode and the ground electrode is deteriorated, spark discharge is disabled and an engine stall is caused.
As for the conventional type spark plug disclosed in U.S. patent application Ser. No. 08/749,309, relationship between distance (a main air gap or a semi-surface gap) from the parallel ground electrode or the auxiliary ground electrode to the central electrode and distance (a gap between a semi-surface and the insulator) from the end face of the auxiliary ground electrode to the side face of the insulator is not disclosed.
Further, in Unexamined Japanese Patent Publication (kokai) No. Sho. 59-71279, a semi-surface spark plug in which an ground electrode is arranged opposite to the side face of insulator is disclosed. In the above spark plug, as sparks fly along the surface of the insulator, carbon which adheres to the surface of the insulator is burned off and the problem of a carbon fouling is hardly caused. However, as sparks always fly along the surface of the insulator, the problem of so-called channeling that the surface of the insulator is damaged by sparks is caused. Therefore, there is a problem that the life of the spark plug is short.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a spark plug strong in a carbon fouling, the life of which is long and also excellent in ignitability.
A spark plug according to the present invention comprises an insulator, a central electrode, a main metal shell, a parallel ground electrode and at least one semi-surface ground electrode. The insulator has a central through hole. The central electrode is held in said central through hole and protruded from the lower end face of said insulator downward. The main metal shell for holding said insulator. The parallel ground electrode having one end which is bonded to the main metal shell and the other end which is arranged so that the other end is opposite to the end face of said central electrode for holding said insulator and a parallel ground electrode arranged. A main air gap (A) is formed by said parallel ground electrode and the end face of said central electrode. The at least one semi-surface ground electrode has one end which is bonded to said main metal shell and the other end which is arranged so that the other end is opposite to the side face of said central electrode or the side face of said insulator. A semi-surface gap (B) is formed between the end face of the other end of said semi-surface ground electrode and the side face of said central electrode opposite to the end face. A gap (C) between a semi-surface ground electrode and the insulator (hereinafter, referred to “a semi-surface air gap (C)) is formed between the end face of said semi-surface ground electrode and the side face of said insulator opposite to the end face. In the spark plug, difference in a level E between the height of the lower end face of said insulator and the height of the upper edge of the end face of said semi-surface ground electrode is equal to or smaller than +0.7 mm where ‘+’ means a direction in which the upper edge of the end face of the semi-surface ground electrode separates downward from the lower end face of the insulator. The distance B of said semi-surface gap (B) is longer than the distance A of said main air gap (A). If a first extended line acquired by extending a line showing said lower end face of said insulator outward, a second extended line acquired by extending a line showing the side face in the vicinity of the semi-surface gap (B) of said insulator in the direction of said lower end face and a third extended line acquired by extending a line showing the end face of said semi-surface ground electrode downward are drawn in case the end face of said semi-surface ground electrode and said insulator are cut along the central axis of said insulator, distance that is a distance C of a semi-surface air gap (C) between the intersection of said first and second extended lines and the intersection of said first and third extended lines is shorter than the distance A of said main air gap (A).


REFERENCES:
patent: 5581145 (1996-12-01), Kato et al.
patent: 0 774 812 A1 (1997-05-01), None
patent: 49-120932 (1973-02-01), None
patent: 58-59580 (1983-04-01), None
patent: 59-71279 (1984-04-01), None
patent: 60-81784 (1985-05-01), None
patent: 60-180082 (1985-09-01), None
patent: 5-326107 (1993-12-01), None

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