Abstract: A plasma-jet spark plug includes a metal shell, an electrical insulator retained in the metal shell, a center electrode held in an axial hole of the electrical insulator to define a cavity by a front end face of the center electrode and an inner circumferential surface of the insulator axial hole and a ground electrode fitted to a front end face of the electrical insulator and formed with an opening for communication between the cavity and the outside of the spark plug and satisfies the following dimensional relationships: d?D?3d; 0.5 mm?d?1.5 mm; L1?1.5 mm; 2d?L2?3.5 mm; and L2+{(D?d)/2}?3.5 mm where D is a diameter of the ground electrode opening; L1 is a thickness of the ground electrode; d is a diameter of the cavity; and L2 is a depth of the cavity.

Abstract: A plasma-jet spark plug includes a metal shell, an electrical insulator retained in the metal shell, a center electrode held in an axial hole of the electrical insulator to define a cavity by a front end face of the center electrode and an inner circumferential surface of the insulator axial hole and a ground electrode arranged on a front end of the electrical insulator. The ground electrode has an opening defining portion defining an opening for communication between the cavity and the outside of the spark plug. The opening defining portion is located radially inside of or in contact with a first imaginary circular conical surface where 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 end of the spark plug and passing through a front edge of the insulator axial hole.

Abstract: A plasma-jet spark plug includes a metal shell, an electrical insulator retained in the metal shell, a center electrode held in an axial hole of the electrical insulator to define a cavity by a front end face of the center electrode and an inner circumferential surface of the insulator axial hole and a ground electrode fitted to a front end face of the electrical insulator and formed with an opening for communication between the cavity and the outside of the spark plug and satisfies the following dimensional relationships: d?D?3d; 0.5 mm?d?1.5 mm; L1?1.5 mm; 2d?L2?3.5 mm; and L2+{(D?d)/2}?3.5 mm where D is a diameter of the ground electrode opening; L1 is a thickness of the ground electrode; d is a diameter of the cavity; and L2 is a depth of the cavity.

Abstract: A plasma-jet spark plug includes a metal shell, an electrical insulator retained in the metal shell, a center electrode held in an axial hole of the electrical insulator to define a cavity by a front end face of the center electrode and an inner circumferential surface of the insulator axial hole and a ground electrode arranged on a front end of the electrical insulator. The ground electrode has an opening defining portion defining an opening for communication between the cavity and the outside of the spark plug. The opening defining portion is located radially inside of or in contact with a first imaginary circular conical surface where 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 end of the spark plug and passing through a front edge of the insulator axial hole.