Patent ID: 8141532

Claim:
A spark plug comprising: a center electrode; an insulator having an axial bore that extends in an axis direction and holding the center electrode at a front end side of the axial bore; a metal shell having a cylindrical hole that extends in the axis direction and accommodates the insulator therein, wherein the metal shell includes a peripheral wall extending portion at a front end side thereof which is formed so as to radially surround a perimeter of the front end portion of the insulator when the metal shell is mounted on an engine head of a combustion engine, and wherein the peripheral wall extending portion has a projecting portion that extends in the axis direction along with the front end of the insulator towards the combustion chamber side with respect to the inner wall surface of the combustion chamber; and a ground electrode having one end joined to the peripheral wall extending portion and the other end forming a spark discharge gap with a front end side of the center electrode, and wherein the peripheral wall extending portion is comprised of a plurality of projecting portions intermittently formed in the metal shell in a circumference direction thereof and extending in the axis direction towards the combustion chamber side with respect to the inner wall surface, and a connecting portion connecting between the plurality of projecting portions of the metal shell in the circumference direction and positioned so that a front end of the connecting portion is disposed relatively rearward with respect to the projecting portions, wherein a center of a line segment bounded by a center of a front end portion of the center electrode and an intersection, which is formed by a straight line passing through the center of the center electrode and parallel to an axis of the center electrode and the ground electrode, is regarded as a center of the spark discharge gap, wherein a direction of the spark plug is adjusted when the spark plug is mounted on the engine head, wherein, in the projecting portions of the peripheral wall extending portion, a first projecting portion where the one end of the ground electrode is joined to a projecting front end thereof is located in a position where a first virtual straight line connecting an opening center of the injection orifice and the center of the spark discharge gap is interrupted when the injection orifice of the fuel, the peripheral wall extending portion and the spark discharge gap, those of which are exposed to the combustion chamber, are projected on a first virtual plane perpendicular to the axis direction, and wherein, when a direction that connects both ends of the first projecting portion in a peripheral direction of the metal shell is regarded as a width direction, a length of the first projecting portion in the width direction is longer than a length between ends of the ground electrode in the width direction.