Patent ID: 8072125

Claim:
A spark plug for use in an internal-combustion engine, comprising: a center electrode; an insulator at least partially surrounding the center electrode; a cylindrical metal shell at least partially surrounding the insulator; a ground electrode so disposed that one end thereof is joined to a front-end face of the metal shell and the other end thereof faces the front-end face of the center electrode; and a spark discharge gap formed between the front-end face of the center electrode and a front-end portion of the ground electrode, wherein the ground electrode has a width between 1.3 mm and 1.7 mm at least in the front-end portion thereof located forward of a center of the spark discharge gap, wherein at least a portion of the ground electrode where sparks are discharged has an outer layer made of a nickel alloy selected from the group consisting of a nickel alloy having the following chemical composition: a minimum of 72% nickel (plus cobalt); 14.0% to 17.0% chromium; 6.00% to 10.00% iron; a maximum of 0.15% carbon; a maximum of 1.0% manganese; a maximum of 0.015% sulfur; a maximum of 0.50% silicon; and a maximum of 0.50% copper or a nickel alloy having the following chemical composition: 58.0% to 63.0% nickel; 15.0% to 21.0% chromium; 1.0% to 1.7% aluminum; a maximum of 0.10% carbon; a maximum of 1.0% manganese; a maximum of 0.015% sulfur; a maximum of 0.50% silicon; a maximum of 1.0% copper; and the remainder iron, and an inner layer made of pure copper or a copper alloy containing 98 wt% or more of copper having a better thermal conductivity than that of the outer layer, and wherein a ratio of a cross-sectional area of the inner layer to the entire cross-sectional area of the ground electrode is 10% or more to 28.4% or less, and said ratio is selected such that spring-back of said ground electrode during use of said spark plug is prevented.