Semi-creeping discharge type spark plug

Details Semi-creeping discharge type spark plug Semi-creeping discharge type spark plug

1998-03-05

2001-03-27

Patel, Ashok (Department: 2879)

Electric lamp and discharge devices

Spark plugs

Particular electrode structure or spacing

C313S142000, C313S143000

Reexamination Certificate

active

06208066

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a semi-creeping discharge type spark plug in which a spark discharge gap is formed by an air-gap and a creeping spark discharge gap through which spark discharges run along a front end surface of an insulator.
2. Description of Prior Art
As shown in
FIG. 6
, a semi-creeping discharge type spark plug (J) has been known in which a cylindrical metal shell
1
and an insulator
2
are provided, the latter of which has an axial bore
22
and is placed in the metal shell
1
so that a front end of the insulator
2
extends from a front end surface
11
of the metal shell
1
. Within the axial bore
22
, a center electrode
3
is placed, a front end surface
31
of which is located at a level substantially the same as the front end surface
23
of the insulator
2
. L-shaped ground electrodes are provided which is welded to the front end surface
11
of the metal shell
1
as designated at numeral
4
. In this situation, the front end surface
31
of the center electrode
3
is generally in flush with a forward edge portion
42
of a front end surface
41
of the ground electrode
4
. Upon applying a high voltage across the electrodes
3
,
4
, spark discharges creep along the front end surface
23
of the insulator
2
.
In a provisional publication No. 0765017 published on Mar. 26, 1997 under EPO, a semi-creeping discharge type spark plug similar to that of
FIG. 6
has been disclosed which however remains silent about a geometrical dimensional relationship between the front end surface of the insulator and the forward edge portion of the front end surface of the ground electrode. Upon considering the purposes of the invention disclosed in the provisional publication No. 0765017, the publication puts an emphasis on a prevention of the channeling phenomenon rather than an avoidance of the soot fouling to insure an extended service life. On the contrary, the present invention makes much of preventing the soot fouling even though permitting the channeling phenomenon in a tolerable degree.
As well known for those versed in the art, this type of the spark plugs are, in fact, superior to a general air-gap type spark plug in the point of fouling resistance because the formers are to burningly evaporate the carbon-related deposit collected on the front end surface of the insulator.
In those semi-creeping discharge type spark plugs, it is, however, recognized that the insulation resistance reduces due to the carbon-related deposit (
FIG. 9
) when the fouling resistance experimental test was carried out under very cold conditions (−15° C.) in conformity with a predelivery pattern in
FIG. 4
as described in detail hereinafter. Besides insuring a desirable fouling resistant property, it has generally been demanded to impart a good heat resistant property to a semi-creeping discharge type spark plug without inviting unfavorable channeling phenomenon.
Therefore, it is a main object of the invention to provide a semi-creeping discharge type spark plug which is capable of concurrently insuring a good heat resistance and fouling resistance so as to maintain a desirable insulation resistance for an extended period of time.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a semi-creeping discharge type spark plug having a ground electrode, one end of which is connected to a front end of the metal shell, and the other end of which is bent to oppose an outer surface of the insulator so as to form an air-gap therebetween, a foward edge portion of a front end surface of the ground electrode extending by 0.0~1.0 mm from the front end surface of the insulator. A spark gap between a front end surface of the ground electrode and a front end surafce of the center electrode, is formed by the air-gap and a creeping spark discharge gap through which spark discharges creep along the front end surface of the insulator. The center electrode is placed within the axial bore of the insulator so that a front end surface edge of the center electrode retracts inward by 0.1~0.6 mm from the front end surface of the insulator. The front end surface edge of the center electrode acts as an emitting segment or receiving segment of the spark discharges.
When the forward edge portion of the ground electrode is located behind the front end surface of the insulator, the heat resistant property is likely to reduce which is especially important upon running an internal combustion engine consecutively at high speed. This is because the spark discharges are supposed to occur across the air-gap between the ground electrode and insulator in order to ignite the air-fuel mixture injected into a combustion chamber. At the time of igniting the air-fuel mixture, the combustion spreads into a cylinder of the internal combustion engine to expose the insulator directly to the combustion flames. This may result in an excessive temperature rise of the front end of the insulator to reduce the heat resistance of the insulator to an unacceptable degree.
When the forward edge portion of the ground electrode is located forward by 1.0 mm or more from the front end surface of the insulator, the spark discharges is likely to converge into a steady path without colliding agaist the outer surface of the insulator. This reduces the fouling resistance which affects particularly on the cold starting capability of the engine, and at the same time, inducing the channeling at the front end surface of the insulator which adversely influences the heat resistant property upon running the engine continuously at high speed. By way of illustration, a heat resistance exprimental test result data are shown in
FIG. 11
in which an insulator nose is 13 mm, and a diameter of the front end of the insulator is 4.0 mm while a diameter of the center electrode is 2.0 mm, and a distance between the forward edge portion of the ground electrode and the front end surface of the insulator is 0.0~0.5 mm.
With the front end edge of the center electrode retracted by 0.1 mm or more behind from the front end surface of the insulator, it is possible to creep the spark discharges appropriately along the front end surface of the insulator when permitting the spark discharge between the front end surface of the center electrode and the ground electrode. This facilitates the self-cleaning action to burningly evaporate the carbon-related deposit collected on the front end surface of the insulator. When the front end edge of the center electrode is located by more than 0.6 mm behind from the front end surface of the insulator, it supposedly quickens the progress of the channeling.
With the front end edge of the center electrode retracted by 0.1~0.6 mm behind from the front end surface of the insulator, and the forward edge portion of the ground electrode located by 0.0~1.0 mm forward from the front end surface of the insulator, it is possible to insure the good heat resistance and fouling resistance at once without sacrificing the channeling resistance.
With the diameter of the front end of the center electrode thinned to 2.0 mm or less, it is possible to induce the spark discharges with a relatively low discharge voltage so as to meliorate the ignitability and fouling resistance by facilitating the self-cleaning action. From a point of preventing the spark erosion of the center electrode, it is necessary to increase the diameter of the front end of the center electrode to 1.0 mm or more (preferably 1.6 mm or more).
With an inner edge portion of the front end surface of the insulator beveled by 0.1~1.0 mm (preferably 0.2~0.8 mm) in terms of chamfer length (C) or rounded by 0.1~1.0 1/mm (preferably 0.2~0.8 1/mm) in terms of radius of curvature (R), it is possible to weaken an attraction of the spark discharges against the beveled or rounded surface so as to effectively reduce the channeling with the least damage done thereon. When the chamfer length (C) or the radius of curvature (R) exceeds 1.0 mm (1.0 mm 1/mm), it reduces the fouling resistance while deteriorating the physical

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