Patent ID: 8441177

Claim:
A plasma jet ignition plug, comprising: an elongated insulating body having a first end and a second end; an elongated axial hole extending along an axis through said insulating body from said first end to said second end; said elongated axial hole having a decreasing diameter portion near said first end of said insulating body that decrease in diameter in the axial direction toward said first end, a metal shell disposed on the outer periphery of the insulating body, said metal shell having a first end that extends beyond said first end of said insulating body; a ground electrode fixed to a first end portion of said metal shell adjacent said first end of said insulating body, said ground electrode having a hole therethrough in registry with said cavity portion of said elongated hole; a center electrode disposed in said axial elongated hole, said center electrode having a first end having an end face of a predetermined diameter, said end face spaced from and facing said decreasing diameter portion of said elongated hole, and a cavity portion formed by being surrounded by the insulating body and the center electrode with the leading end of the axial hole as an opening, such that the following relationship is satisfied: A≦B, where “A” is a shortest distance, taken in a direction parallel to said axis, between said end face of said center electrode and said decreasing diameter portion of said elongated hole in said insulating body, and where “B” is a shortest distance, taken perpendicular to said axis, between the outer circumference of said leading edge of the center electrode and a peripheral surface of said elongated hole in said insulating body.