Patent ID: 8217561

Claim:
A spark plug, comprising: a center electrode; a ceramic insulator having an axial hole extending in an axial direction and retaining the center electrode in the axial hole; a metal shell surrounding a radial outer circumference of the ceramic insulator and retaining therein the ceramic insulator; a ground electrode joined at one end thereof to a front end face of the metal shell and having a bent portion formed between the one end and the other end thereof in such a manner that the other end of the ground electrode faces a front end of the center electrode; and an electrode tip joined to the other end of the ground electrode at a position facing the front end of the center electrode and having a protrusion length of 0.5 mm or larger from the other end of the ground electrode and a cross sectional area of 0.20 to 1.13 mm 2 , the ground electrode consisting of a first structural member extending from the one end toward the other end of the ground electrode and at least one ith structural member (i=2, 3, 4, 5) laminated to cover an outer surface of the first structural member; the minimum curvature radius of a side surface of the bent portion facing the center electrode being 2.3 mm or smaller; the length of protrusion of a point of the other end of the ground electrode protruding most in the axial direction from the front end face of the metal shell being 4.5 mm or larger; the metal shell having a mounting thread formed with a nominal diameter of M12 or smaller based on JIS standard; and the total thermal conductivity X of the ground electrode as expressed by the formula [1] being 35 W/(m·K) or higher at 20° C. [ Formula ⁢ ⁢ 4 ] X = volume ⁢ ⁢ of ⁢ ⁢ first ⁢ ⁢ structual ⁢ ⁢ member volume ⁢ ⁢ of ⁢ ⁢ ground ⁢ ⁢ electrode × thermal ⁢ ⁢ conductivity ⁢ ⁢ of ⁢ ⁢ first ⁢ ⁢ structural ⁢ ⁢ member + ∑ i = 2 n ⁢ ( volume ⁢ ⁢ of ⁢ ⁢ ith ⁢ ⁢ structural ⁢ ⁢ member volume ⁢ ⁢ of ⁢ ⁢ ground ⁢ ⁢ electrode × ⁢ thermal ⁢ ⁢ ⁢ conductivity ⁢ ⁢ ⁢ of ⁢ ⁢ ith ⁢ ⁢ structural ⁢ ⁢ member ) ( 1 ) where n is an integer of 2 to 5 indicating the maximum number of the structural members of the ground electrode.