Patent ID: 8536770
Filing Date: 2013-09-17
Classification: H01T

Abstract:
1. A plasma jet spark plug comprising: a cylindrical insulator having an axial bore extending in an axis direction; a rod-like center electrode accommodated in the axial bore of the insulator; a plate-like ground electrode disposed on a front end of the insulator, wherein the center electrode has a body portion, a forward portion that has an outer diameter smaller than that of the body portion and that is located on a front end side with respect to the body portion, and a frontmost portion having an outer diameter smaller than that of the forward portion and located on a front end side with respect to the forward portion, wherein a portion of the insulator where the axial bore is formed has: wherein the frontmost portion of the center electrode is accommodated in the small diameter portion of the insulator such that a front end of the center electrode is located in the small diameter portion of the insulator toward a rear side such that the front end of the center electrode is spaced from a front end of the insulator wherein the front end of the center electrode forms a cavity with an inner circumference of the small diameter portion of the insulator, wherein the ground electrode has an opening for use in communicating the cavity and an outside air, wherein the small diameter portion of the insulator in the axis direction assumes a linear shape, and wherein the portion of the insulator where the axial bore is formed further includes a first step portion located between the accommodating portion and the small diameter portion, wherein the following relationship is satisfied: where “a” represents a distance between a first intersection and a second intersection, the first intersection being an intersection of a first straight line drawn from an inner circumference of the opening of the ground electrode in the axis direction and the front end of the insulator, and a second intersection being an intersection of the first straight line and the first step portion of the insulator, when the inner diameter of the opening of the ground electrode is smaller than the outer diameter of the forward portion of the center electrode and is larger than the inner diameter of the small diameter portion of the insulator, where “a” represents a length of an inner circumferential surface of the small diameter portion of the insulator in the axis direction when the inner diameter of the opening of the ground electrode is smaller than the outer diameter of the forward portion of the center electrode and is smaller than the inner diameter of the small diameter portion of the insulator, where “b” represents a distance between a third intersection and a fourth intersection, the third intersection being an intersection of a second straight line drawn from the outer circumference of the forward portion of the center electrode in the axis direction and the first step portion of the insulator, and the fourth intersection being an intersection of the second straight line and the front end of the insulator, and where “c” represents an overlapping area of the ground electrode with the forward portion of the center electrode when the ground electrode and the forward portion of the center electrode are projected in the axis direction.