Plasma-jet spark plug and ignition system

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.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a half section view of a plasma-jet spark plug according to a first embodiment of the present invention.

FIG. 2 is an enlarged section view of a front side of the plasma-jet spark plug according to the first embodiment of the present invention.

FIG. 3 is a circuit diagram of a power supply unit of an ignition system according to the first embodiment of the present invention.

FIG. 4 is an enlarged section view of a ground electrode of the plasma-jet spark plug, in the case where the ground electrode has an opening defining portion projecting radially inwardly from a first imaginary circular conical surface, according to the first embodiment of the present invention.

FIG. 5 is an enlarged section view of the ground electrode of the plasma-jet spark plug, in the case where the opening defining portion of the ground electrode is in contact with the first imaginary circular conical surface, according to the first embodiment of the present invention.

FIGS. 6 to 10 are graphs showing experimental data on ignition probability, electrode consumption and discharge voltage of the plasma-jet spark plug according the first embodiment of the present invention.

FIG. 11 is an enlarged section view of a front side of a plasma-jet spark plug according to a second embodiment of the present invention.

FIG. 12 is an enlarged section view of a front side of a plasma-jet spark plug according to a third embodiment of the present invention.

FIG. 13 is an enlarged section view of a front side of a plasma-jet spark plug according to a fourth embodiment of the present invention.

FIG. 14 is an enlarged section view of a front side of a plasma-jet spark plug according to a fifth embodiment of the present invention.

Claims

1. 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; anda 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; andsaid radially inwardly projecting section having a volume of 0 mm3 to less than 1.5 mm3.

2. A plasma-jet spark plug according to claim 1, wherein said opening defining portion is kept from contact with a third imaginary circular conical surface with the proviso that the third imaginary circular conical surface has an axis coinciding with the axis of the spark plug and a vertex angle of 30° opening toward the front of the spark plug and passing through the front edge of the axial hole of the electrical insulator.

3. A plasma-jet spark plug according to claim 1, wherein the ground electrode satisfies a dimensional relationship of D≧T where D is a minimum diameter of the opening of the ground electrode; and T is an axial thickness of the ground electrode.

4. An ignition system, comprising: a plasma-jet spark plug according to claim 1; anda power source having a capacity to supply 50 to 200 mJ of energy to the spark plug.