Patent ID: 8242673
Filing Date: 2012-08-14
Classification: H01T

Abstract:
1. A spark plug generating a spark discharge including a surface discharge, comprising: a center electrode; a ceramic insulator having an axial hole that is formed in an axial center of the ceramic insulator in an axis direction to support the center electrode therein with a top end portion of the center electrode protruding from a top end surface of the ceramic insulator, the ceramic insulator provided with recesses at a first edge portion between the top end surface of the ceramic insulator and an inner circumferential surface of the axial hole; a metal shell which has a plug attachment portion provided with screw thread for installation to an internal combustion engine and holds the ceramic insulator with an outer circumference of the ceramic insulator covered with the metal shell; and a ground electrode, one end portion of which is fixedly connected with the metal shell, and the other end portion of which is located apart from an outer circumferential surface of the top end portion of the center electrode for defining a spark discharge gap therebetween, the spark discharge including the surface discharge that appears along the top end surface of the ceramic insulator occurring in the spark discharge gap, and when defining a first imaginary circle with the axis being a center as a circle that passes through a portion of the recess whose radial distance from the axis is a maximum and defining a second imaginary circle with the axis being the center as a circle that passes through a portion of the recess whose radial distance from the axis is a minimum, from among the recesses, and further expressing a difference of diameters of the first and second imaginary circles as a diameter difference X, the diameter difference X being less than or equal to 0.08 mm, and greater than or equal to 0.004 mm.