Patent ID: 8258686

Claim:
A spark plug for an internal combustion engine comprising: a tubular metal shell; an insulator retained in said metal shell such that an end thereof protrudes from an end of said metal shell; a cylindrical center electrode secured in said insulator, said center electrode having a protruding portion that protrudes from the end of said insulator; at least one ground electrode having an inclined portion, a straight portion, and a bend between the inclined and straight portions, the inclined portion extending obliquely with respect to an axial direction of said center electrode from a base end of said ground electrode, which is joined to the end of said metal shell, to the bend that is positioned closer to said center electrode in a radial direction of said center electrode than the base end, the straight portion extending substantially parallel to the axial direction of said center electrode from the bend to a tip end of said ground electrode, the straight portion having an inner side surface that faces a side surface of an end portion of the protruding portion of said center electrode through a spark gap in the radial direction of said center electrode; and a noble metal chip that has a length with a base end joined to the inner side surface of the straight portion of said ground electrode and a tip end facing the side surface of the end portion of the protruding portion of said center electrode through the spark gap in the radial direction of said center electrode, wherein the end portion of the protruding portion of said center electrode has a cross-sectional area perpendicular to the axial direction of said center electrode in a range of 0.79 to 4.9 mm 2 , the protruding portion of said center electrode has a surface area in a rage of 10 to 30 mm 2 , said noble metal chip has a cross-sectional area perpendicular to a lengthwise direction of said noble metal chip in a range of 0.12 to 1.13 mm 2 and a protruding length, which represents a distance from the inner side surface of the straight portion of said ground electrode to the tip end of said noble metal chip in the radial direction of said center electrode, in a range of 0.3 to 1.5 mm, the inner side surface of the straight portion of said ground electrode has a length in the axial direction of said center electrode in a range of 1.8 to 3.0 mm, and said noble metal chip is cylindrical and wherein the following dimensional relationship is defined: 0<( W−d )<1.5 mm, where W is a width of the inner side surface of the straight portion of said around electrode perpendicular to the axial direction of said center electrode, and d is a diameter of said noble metal chip.