| 摘要 |
A spark plug in use for an internal combustion engine, includes an insulator placed in a metal shell in a manner to extend its front end beyond the metal shell, a center electrode is placed within an axial bore provided by the insulator, an outer electrode which is welded to a front end of the metal shell is so bent that a firing end of the outer electrode faces a front end of the insulator with a space therebetween to cause surface-spark discharges along a front end surface of the insulator and to cause a semi-surface spark discharge between the center electrode and the outer electrode, the front end of the center electrode is protroacted from a front end of the insulator by at most than 0.5 mm or retracted from the front end of the insulator by at most than 1.0 mm, and the firing end of the outer electrode is in line with the front end surface of the insulator, thereby optimizing a carbon-deposit cleaning performance of the spark plug with the semi-surface spark discharge, according to the invention. In addition, a firing end portion of the center electrode has a slender noble metal tip in a preferred embodiment of the spark plugs of the invention.
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