Patent ID: 6573641
Filing Date: 2003-06-03
Classification: H01T

Abstract:
A spark plug comprising:a metallic shell having an externally threaded portion; an insulator disposed within the metallic shell and having an axial through hole; a center electrode disposed within the axial through hole of the insulator; and a ground electrode having a joining end portion joined to the metallic shell and a free end portion having a discharge surface which is opposite to an axial end surface of the center electrode to provide therebetween a spark gap; wherein the center electrode has a center electrode tip forming the axial end surface thereof, the center electrode tip being made of a material containing Ir as a major constituent; and wherein G&lE;2A+0.5 where G is the spark gap in millimeter(s) and A is the distance in millimeter(s) between a first imaginary line and a second marginal line, the first imaginary line being parallel to an imaginary axis of the spark plug and in a position where it touches a first marginal line for the first time when moved toward the spark gap from a side of the spark gap opposite to the joining end portion along a plane including the imaginary axis and crossing the second marginal line at right angles, the imaginary axis being determined on the basis of an axis of the externally threaded portion, the first marginal line being formed by the axial end surface and a side surface of the center electrode tip, the second marginal line being formed by the discharge surface and an end surface of the free end portion of the ground electrode; and wherein the spark gap G ranges from 0.7 to 1.5 mm.