Patent ID: 7541724

Claim:
A spark plug for an internal combustion engine comprising: a metal shell; an insulator retained in the metal shell, the insulator having an end, which protrudes from the metal shell, and a bore that extends in a longitudinal direction of the insulator and opens at the end of the insulator to form an inner edge of the insulator; a center electrode including a base member and a thin member, the base member being fitted in the bore of the insulator and having an end that is positioned inside the bore of the insulator, the thin member being thinner than the base member and joined to the end of the base member, the thin member having an axis and an end edge that is a given distance away from the end of the base member in an axial direction of the thin member; and a ground electrode including a base member fixed to the metal shell and a protruding member joined to the base member, the protruding member protruding from a surface of the base member and having an end face that faces the thin member of the center electrode through a spark gap, the end face of the protruding member having an inner and an outer edge, the inner edge being positioned closer to the axis of the thin member of the center electrode than the outer edge, wherein, 0.7 ×R 3 ≦R 1, 0.5 ×R 4 ≦R 2≦1.2 ×R 4, L≦G 1≦1.2 ×L, G 1 <G 2 +H , and G 2 /G 1≦1.5 where, R1 is a distance between the axis of the thin member of the center electrode and the inner edge of the protruding member of the ground electrode in a radial direction of the thin member, R2 is a distance between the axis of the thin member of the center electrode and the outer edge of the protruding member of the ground electrode in the radial direction of the thin member, R3 is a radius of the thin member of the center electrode at the end edge of the thin member, R4 is a radius of the bore of the insulator at the end of the insulator, G1 is a minimum distance between the end edge of the thin member of the center electrode and the inner edge of the protruding member of the ground electrode, G2 is a minimum distance between the inner edge of the insulator and the outer edge of the protruding member of the ground electrode, L is a distance between the thin member of the center electrode and the protruding member of the ground electrode in the axial direction of the thin member, and H is a distance between the end of the base member of the center electrode and the end of the insulator in the axial direction of the thin member of the center electrode, wherein the protruding member of the ground electrode has a tubular shape to form the inner edge at an inner circumference of the end face of the protruding member and the outer edge at an outer circumference of the same.