Abstract: There is provided a method for controlling a plasma-jet spark plug in a four-stroke internal combustion engine, including the steps of: causing the spark plug to generate a primary discharge during either a compression stroke or an expansion stroke of the engine in such a manner as to produce a plasma by the primary discharge; and causing the spark plug to generate a secondary discharge during a time after the primary discharge and before the completion of a subsequent intake stroke of the engine.

Abstract: A plasma jet spark plug provides improved ignitability and durability by forming a part of a spark discharge gap outside the electric discharge space which generates plasma. An ignition system for the plasma jet spark plug is also disclosed. The plasma jet spark plug includes a center electrode, an insulator defining an axial bore which partially surrounds the center electrode, a cavity surrounded by an inner circumferential face of the axial bore which extends from an opening portion of a front end of the axial bore of the insulator and wherein a front end face of the center electrode is formed. A ground electrode is bent towards a front end portion of the insulator.

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.

Abstract: A plasma jet spark plug provides improved ignitability and durability by forming a part of a spark discharge gap outside the electric discharge space which generates plasma. An ignition system for the plasma jet spark plug is also disclosed. The plasma jet spark plug includes a center electrode, an insulator defining an axial bore which partially surrounds the center electrode, a cavity surrounded by an inner circumferential face of the axial bore which extends from an opening portion of a front end of the axial bore of the insulator and wherein a front end face of the center electrode is formed. A ground electrode is bent towards a front end portion of the insulator.

Abstract: There is provided a method for controlling a plasma-jet spark plug in a four-stroke internal combustion engine, including the steps of: causing the spark plug to generate a primary discharge during either a compression stroke or an expansion stroke of the engine in such a manner as to produce a plasma by the primary discharge; and causing the spark plug to generate a secondary discharge during a time after the primary discharge and before the completion of a subsequent intake stroke of the engine.