Patent ID: 8884504

Claim:
A spark plug for an internal combustion engine, the spark plug comprising a cylindrical housing, a cylindrical insulation porcelain held inside the housing, a center electrode held inside the insulation porcelain, with a tip portion thereof being projected, and a ground electrode connected to the housing and having an opposing portion opposed to the center electrode in an axial direction of the plug to form a spark discharge gap between the center electrode and the ground electrode, wherein at least one of the tip portion of the center electrode and the opposing portion of the ground electrode has a projection portion projected toward the spark discharge gap; and at least one of the projection portions: i) has a cross section perpendicular to the axial direction of the spark plug, the cross section including a minimum curvature radius portion having a smallest curvature radius in a contour of the cross section; and ii) is in a specific shape that meets a requirement, the requirement being that, when a first straight line is supposed to connect between the minimum curvature radius portion and a geometric centroid in the cross section, a first line segment is supposed to connect between two intersections at which the first straight line intersects the contour of the cross section, and a second straight line is supposed to be perpendicular to the first line segment at a midpoint in the first line segment, and when the cross section is divided by the second straight line into a first region that includes the minimum curvature radius portion and a second region that does not include the minimum curvature radius portion, the second region has an area larger than an area of the first region.