Patent ID: 8531094

Claim:
A spark plug, comprising: a center electrode extending in an axial direction and having a front end portion; a ceramic insulator having an axial hole formed in the axial direction to retain the center electrode in a front side of the axial hole and thereby form an assembly unit of the center electrode and the ceramic insulator; a metal shell surrounding an outer circumference of the ceramic insulator to retain therein the assembly unit; and a ground electrode having one end portion joined to a front end face of the metal shell and one side of the other end portion facing the front end portion of the center electrode to define a spark gap therebetween, wherein the ceramic insulator has a front end portion formed with no step; and wherein the spark plug satisfies the following conditions: H≧1 mm, Vc≦17 mm 3 and Ra≧1.0×10 3 K/(m·W) where H is a length by which the ceramic insulator protrudes toward the front from the front end face of the metal shell in the axial direction; Vc is a volume of part of the ceramic insulator extending within a range of 2 mm from a front end of the ceramic insulator toward the rear in the axial direction; and Ra is a thermal resistance per unit length, excluding air space, at 20° C. at a cross section of the assembly unit taken perpendicular to the axial direction at a position 2 mm away from the front end of the ceramic insulator.