Patent ID: 6337533
Filing Date: 2002-01-08
Classification: H01T

Abstract:
A spark plug comprising:a center electrode having a leading end; a housing holding, but insulated with, the center electrode; a ground electrode fixed to the housing and having a leading end which faces the leading end of the center electrode with a gap space, wherein at least one of the leading ends of the ground and center electrodes is a subject leading end and the other one of the leading ends of the ground and center electrodes is an opponent leading end, a shortest gap between the subject and opponent leading ends defining a spark discharge gap; a noble metal chip partly buried into and bonded to the subject leading end so as to define a junction between contacting surfaces of the chip and the subject leading end, and wherein a first surface part of the chip is a spark discharge surface exposed directly to the spark discharge gap; and a molten portion made of a molten mixture of materials of the chip and the subject leading end and extending from a second surface part of the chip through an inside of the chip, through the junction, and into an inside of the subject leading end in such a manner that a cross-sectional area of the molten portion is gradually narrower from said second surface part of said chip toward the subject leading end, wherein the molten portion is not formed on the first, spark discharge surface part of the chip, and wherein a ratio of B/A is not less than 0.5, where A is a maximum length from the end of the junction on the side of the opponent leading end to the end of the molten portion on the junction on the side opposite to the opponent leading end and, within the length A, B is a length of both ends of the molten portion, and a ratio of d/t is not less than 2, but not more than 4, where t is a length of the chip perpendicularly extending from the junction and d is a sum of the length t and a length of the molten portion protruding from the junction into the subject leading end.