Patent ID: 7305954

Claim:
A plasma-jet spark plug, comprising: a metal shell; an electrical insulator retained in the metal shell and formed with an axial hole; a center electrode held in the axial hole of the electrical insulator so as to define a discharge cavity by a front end face of the center electrode and an inner circumferential surface of the axial hole in a front end part of the electrical insulator; and a ground electrode formed in a plate shape, arranged on a front end of the electric insulator and connected electrically with the metal shell, the ground electrode having an opening defining portion defining therein an opening for communication between the discharge cavity and the outside of the spark plug; said opening defining portion being located radially inside of or in contact with a first imaginary circular conical surface and including a section projecting radially inwardly from a second imaginary circular conical surface with the proviso that: the first imaginary circular conical surface has an axis coinciding with an axis of the spark plug and a vertex angle of 120° opening toward a front of the spark plug and passing through a front edge of the axial hole of the electrical insulator; and the second imaginary circular conical surface has an axis coinciding with the axis of the spark plug and a vertex angle of 60° opening toward the front of the spark plug and passing through the front edge of the axial hole of the electrical insulator; and said radially inwardly projecting section having a volume of 0 mm 3 to less than 1.5 mm 3 .