Abstract: A spark plug for an internal combustion engine includes insulating glass having a tubular shape, a central electrode including a tip protruding portion, a housing having a tubular shape, and a plug cover provided over an auxiliary combustion chamber. The plug cover is provided with injection holes through which the auxiliary combustion chamber communicates with the exterior. The central electrode includes a base material and a core material disposed inside the base material and having a higher thermal conductivity than the base material. The core material includes a large diameter portion and a small diameter portion continuously formed on a tip side of the large diameter portion. The small diameter portion has a smaller diameter than the large diameter portion, and includes, in a part of the small diameter portion in an axial direction, a small-diameter columnar portion having a constant diameter.

Abstract: A spark plug for an internal combustion engine includes a ground electrode, an insulator held inside the ground electrode, and a center electrode held inside the insulator. When a segment of a line extending in a plug radial direction to connect an arbitrary start point on a surface of the ground electrode and an outer peripheral surface of the insulator is a line segment H, a point of intersection between the line segment H and the outer peripheral surface of the insulator is an intersection point K, a length of the line segment H is L1, and an axial distance between the intersection point K and the distal end of the insulator is L2, the ground electrode is provided on the surface thereof with a shortest discharge forming portion as the start point along a plug circumferential direction at which a value of (L1+L2) becomes minimum.

Abstract: A spark plug for an internal combustion engine includes a ground electrode, an insulator held inside the ground electrode, and a center electrode held inside the insulator. When a segment of a line extending in a plug radial direction to connect an arbitrary start point on a surface of the ground electrode and an outer peripheral surface of the insulator is a line segment H, a point of intersection between the line segment H and the outer peripheral surface of the insulator is an intersection point K, a length of the line segment H is L1, and an axial distance between the intersection point K and the distal end of the insulator is L2, the ground electrode is provided on the surface thereof with a shortest discharge forming portion as the start point along a plug circumferential direction at which a value of (L1+L2) becomes minimum.