Electric lamp and discharge devices – Spark plugs – Particular electrode structure or spacing
Reexamination Certificate
1999-12-02
2001-11-13
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
Spark plugs
Particular electrode structure or spacing
C313S123000
Reexamination Certificate
active
06316868
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a spark plug for internal combustion engine having a better self-cleaning function in use of surface creeping spark discharges.
2. Description of Related Art
Recently, as the environmental preservation has been watched with more keen interest, stratified charge internal combustion engines with lower fuel consumption have been widely noticed as environment-friendly engines.
However, when stratified fuel mixtures are burned in a combustion chamber, rich fuel mixtures are concentrated near a spark slug so that the spark plug may tend to be smoldered or fouled by carbon. The carbon-fouling makes an insulation property of an insulator surrounding a center electrode worse so that a spark discharge may not occur across a regular discharge gap provided between center and ground electrodes but occur between the insulator, on a surface of which carbon is deposited, and an inside of a metal housing for mounting at a portion deep into the metal housing from a front end surface of the insulator.
To cope with this problem, there are known self-cleaning spark plugs as disclosed in JP-Y2-53-41629 or JP-A-4719236.
According to JP-Y2-53-41629, the spark plug has a plurality of electrodes constituting first and second ground electrodes. A first discharge gap is formed between the first ground electrode and the center electrode and a second discharge gap is formed between the second ground electrode and the center electrode. A regular spark discharge occurs through the first discharge gap and, when the insulator is fouled by carbon deposit, a spark discharge occurs through the second discharge gap, not through the portion deep into the metal housing, so that carbon may be burned without decreasing ignitability of the spark plug.
Further, according to JP-A-47-19236, there are provided with the regular first discharge gap and the second discharge gap through which sparks are discharged when the insulator is fouled. It is characterized, in this case, that a front end of the center electrode is nearly equal in height to a front end of the insulator.
Therefore, as the spark discharge at the first discharge gap occurs at a position nearly same in height as the second discharge gap, it is contemplated, therefore, that the respective ignitability characteristics at both first and second discharge gaps do not have much difference.
However, the spark plug according to JP-Y2-4719236 has a drawback that there exists a big difference of ignitability between the respective spark discharges at the first and second discharge gaps, since the second discharge gap formed at a leading end of the metal housing is arranged at a position far away from the first discharge gap, so that drivability is adversely affected, in particular, in the stratified fuel combustion.
Further, as the spark discharge at the second discharge gap occurs at a place deep from the leading end of the insulator into an insulator base, channeling is likely to occur. on the other hand, according to the spark plug disclosed in JP-A-4719236, there is also a problem that ignitability is not good, as the front end of the first electrode is obliged to be almost same in height as the front end of the insulator so that the front end of the insulator may operate to cool flame cores generated by spark discharge at the first discharge gap.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above mentioned problem, and an object of the present invention is to provide a spark plug for internal combustion engines in which a remarkably Longer life time of fouling resistance is secured in such a manner that an air-gap spark discharge with a good ignitabilty usually occurs at a first discharge gap and, when the insulator is fouled, a surface creeping spark discharge occurs at a second discharge gap to burn carbon deposited on the surface of the insulator.
To achieve the above object, the spark plug has a center electrode, first and second ground electrodes, an insulator and a metal housing. The first discharge gap is constituted between a front end of the center electrode and a front side of the first ground electrode and the second discharge gap is constituted between a front end of the second electrode and a front side of the center electrode. In this case, dimensional relationships of the center electrode, the first and second ground electrodes, the insulator and the metal housing are respectively in ranges of,
0.7 mm≦A≦1.3 mm,
0.3 mm≦B≦A−0.1 mm
1.0 mm≦C≦4.0 mm,
0.5 mm≦H≦3.0 mm,
1.0 mm≦F≦+0.5 mm, and
1.0 mm≦L
1
≦C+0.5 mm,
where A is a distance of the first discharge gap,
B is a shortest distance between the front end of the second ground electrode and the insulator,
C is an axial length from a leading end of the metal housing and a front end of the insulator,
H is an axial length from the front end of the insulator and the front end of the center electrode,
F is an axial length from the front end of the insulator to the front end edge of the second electrode, and
L
1
is a shortest axial length from the leading end of the metal housing to the front end of the second ground electrode.
Further it is preferable that the center electrode is shaped as a column having a base electrode portion and a diametrically reduced electrode portion whose diameter is smaller than a diameter of the base electrode portion. A base point where the diametrically reduced electrode portion starts is located inside by 0.1 to 0.8 mm from the, front end of the insulator. The spark discharge starting from the base point at the second discharge gap hits at first inner periphery surfaces of the insulator, then, proceeds so as to surround and creep along surfaces of the leading end of the insulator and, finally, reaches the second ground electrode. In this case, the spark discharge runs into carbon deposited on the leading end of the insulator so that the carbon may be burned or scattered by spark energy, thus cleaning the carbon fouling on the insulator. It is preferable to have a small gap between the base point of the center electrode and the inner surface of the insulator in order to cause the spark discharge through the small gap.
Furthermore, when a formula, B+D≧A, is satisfied, where D is a radial thickness of the front end of the insulator, the spark discharge position may be effectively changed between the usual spark discharge at the first discharge gap and the carbon-fouling spark discharge at the second discharge gap.
Further, the insulator is, preferably, provided at a vicinity of the front end thereof with a diametrically reduced insulator portion whose diameter is nearly uniform in an axial direction and is smaller than a diameter of the base insulator portion. A shortest axial length E from the front end of the second ground electrode to a point where the diametrically reduced insulator portion starts should be in a range of E ≧B+0.1 mm for preventing the spark discharge from occurring deep into the metal housing.
To realize a spark plug having a longer consumption life time, it is preferable that at least one of the front side of the first ground electrode and the front end of the center electrode is provided with a noble metal chip preferably made of any one material of pure Pt, pure Ir, Pt alloy and Ir alloy.
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patent: 47-19236 (1972-09-01), None
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patent: 60-81784 (1985-05-01), None
patent: 3-50396 (1991-08-01), None
Ishino Yasutake
Osamura Hironori
Yatou Makoto
Denso Corporation
Patel Ashok
Pillsbury & Winthrop LLP
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