Patent ID: 8432092

Claim:
A spark plug comprising: an insulator having an axial hole which extends along an axis; a center electrode which is inserted into a front end portion of the axial hole and which has a front end placed further forwards than a front end of the insulator; and a cylindrical shell which is provided around an outer circumference of the insulator; the center electrode including a shoulder portion which increases in diameter as it extends from a rear end of a front end portion of the center electrode towards a rear end side and a main body portion which extends from a rear end of the shoulder portion towards the rear end side along the axis, the center electrode having a multi-layer construction configured of an outer layer and an inner layer which is provided in an interior of the outer layer and which contains a material having higher thermal conductivity than that of the outer layer, characterized in that: a front end face, which is connected to an outer circumferential surface of the insulator and the axial hole and slopes towards the rear end side, is formed at a front end portion of the insulator, the front end of the insulator is placed further forwards than a boundary between the shoulder portion and the main body portion of the center electrode, a front end portion of the inner layer is placed further forwards along the axis than the boundary between the shoulder portion and the main body portion of the center electrode, and regarding a cross-section which includes the axis, when a straight line which is obtained by extending an outline of the axial hole towards the front end side is referred to as a straight line L 1 , a straight line which is obtained by extending an outline of an outer surface of the front end portion of the insulator towards the front end side is referred to as a straight line L 2 , a straight line which is obtained by extending an outline of the front end face of the insulator is referred to as a straight line L 3 , a bisector of an angle formed by an outline of the shoulder portion and an outline of the main body portion is referred to as a straight line L 4 , and a straight line which intersects the axis at right angles is referred to as a straight line L 5 , angles A 1 , A 2 , A 3 , A 4 and A 5 which are described below satisfy the following expressions (1), (2), (3) and (4): A1>90° (1); A2<90° (2); A4>A5 (3); A3>A1 (4), where, the angle A 1 : an angle placed on a side where the insulator exists in angles which are formed by the straight line L 1 and the straight line L 3 ; the angle A 2 : an angle placed on a side where the insulator exists in angles which are formed by the straight line L 2 and the straight line L 3 ; the angle A 3 : an angle which is formed by the outline of the shoulder portion and the outline of the main body portion; the angle A 4 : an acute angle in angles which are formed by the straight line L 3 and the straight line L 5 ; the angle A 5 : an acute angle in angles which are formed by the straight line L 4 and the straight line L 5 .