Patent ID: 8082897

Claim:
A plasma jet ignition plug comprising: a center electrode; an insulator which has an axial bore extending along an axial direction for holding the center electrode while accommodating a front end face of the center electrode within the axial bore, and a recess formed on the front-end side of the axial bore as a cavity, the recess having a wall surface defined by an inner circumferential surface of the axial bore and the front end face of the center electrode and having a volume less than 15 mm 3 ; a metallic shell surrounding and holding a radially outer circumference of the insulator; and a ground electrode electrically connected to the metallic shell and provided on the front end side of the insulator, plasma being generated within the recess as a result of discharge between the center electrode and the ground electrode, wherein the recess of the insulator is composed of a constricted portion which has at least a portion extending along the axial direction while maintaining a certain diameter and communicates with an opening provided at the front end of the insulator, and a diameter-increased portion which communicates with the constricted portion, which is larger in diameter than the constricted portion, and in which the front end face of the center electrode is exposed; the recess satisfies a relation X≦Y where X represents a length of the diameter-increased portion as measured along the axial direction, and Y represents a length of the constricted portion as measured along the axial direction; and the ground electrode is a plate-shaped electrode which has a communication hole for establishing communication between the recess and the atmosphere, and satisfies a relation Z<X+Y≦3.0 mm, where Z represents a thickness of the ground electrode as measured along the axial direction.