Patent ID: 7723906
Filing Date: 2010-05-25
Classification: H01T

Abstract:
1. A spark plug for an internal combustion engine comprising: a hollow metal shell having an inner wall and an inner shoulder formed on the inner wall thereof; a porcelain insulator having an outer wall and an outer shoulder formed on the outer wall thereof, said porcelain insulator being disposed inside said metal shell with the outer shoulder of the porcelain insulator in abutment with the inner shoulder of said metal shell so as to extend in a longitudinal direction of the spark plug; a center electrode disposed inside said porcelain insulator; a ground electrode joined to said metal shell to define a spark gap between itself and said center electrode; and an insulator nose that is a portion of said porcelain insulator and continues from the outer shoulder to a top end of said porcelain insulator, said insulator nose including an upright portion extending from the outer shoulder and a tapered portion extending from the upright portion to the top end of said porcelain insulator, the upright portion having an outer peripheral surface extending substantially parallel to a longitudinal center line of the spark plug, the tapered portion being shaped to have a diameter decreasing from the upright portion toward the top end of said porcelain insulator, the tapered portion having an outer peripheral surface shaped to have an outline one of extending straight and being curved outwardly on a place, as defined to extend through the longitudinal center line of the spark plug; wherein said upright portion has a length of 1.5 mm to 6 mm which is selected to lie in a range of 7% a to 40% of a total length that is the sum of lengths of the outer shoulder and said insulator nose, wherein said insulator nose has a boundary between the outer peripheral surface of the upright portion and the outer peripheral surface of the tapered portion, said boundary being so located as to face an inner face of said metal shell in a radial direction of said spark plug.