Patent ID: 8203258

Claim:
A spark plug comprising: a rod-like center electrode extending in a direction of an axis; an approximately cylindrical tubular insulator provided on the periphery of the center electrode; a tubular metallic shell provided on the periphery of the insulator; and a main ground electrode and at least two auxiliary ground electrodes having respective proximal end portions joined to a front end portion of the metallic shell, the spark plug being characterized in that the main ground electrode is disposed so that its distal end portion faces a side surface of a front end portion of the center electrode and forms a main spark discharge gap between the distal end portion and the front end portion of the center electrode; each of the auxiliary ground electrodes is disposed so that a portion of its distal end portion end surface faces an outer circumferential surface of a front end portion of the insulator; and a total area S (mm 2 ) satisfies an expression S/Av<1.3, which represents a relation between the total area S and an average gap distance Av (mm) of the main spark discharge gap, where the total area S is the sum of a projection area C (mm 2 ) of a portion of the distal end portion of the main ground electrode which overlaps with a projected region of the front end portion of the center electrode when the distal end portion of the main ground electrode and the front end portion of the center electrode are projected along a radial direction of the center electrode, and a projection area D (mm 2 ) of a portion of the front end portion of the center electrode which overlaps with a projected region of the distal end portion of the main ground electrode when the distal end portion of the main ground electrode and the front end portion of the center electrode are projected along the radial direction of the center electrode.