Patent ID: 7948158

Claim:
A plasma ignition system for an internal combustion engine, comprising: an ignition plug attached to the engine; and a high-energy supply that supplies electrical energy to the ignition plug, wherein the ignition plug includes: a center electrode; a ground electrode; and an insulating member that insulates the center electrode from the ground electrode and defines a discharge space therein, wherein: the center electrode and the ground electrode are disposed such that at least a part of a surface of the center electrode faces the discharge space and that at least a part of a surface of the ground electrode faces the discharge space; the ignition plug is configured to release the electrical energy, which is supplied to the ignition plug by the high-energy supply, into a combustion chamber of the engine so as to perform ignition in the engine; the center electrode is configured to serve as a positive pole; the ground electrode is configured to serve as a negative pole; the center electrode has a recess portion, which is opposed to the discharge space and recessed in a direction opposite to an injection direction in which the gas is injected into the engine; the insulating member has a discharge space defining portion, which defines the discharge space; and a relationship among: a recess portion opening diameter, which is an opening diameter of the recess portion at an axial end portion thereof facing the discharge space, and a center electrode outer diameter, which is an outer diameter of the center electrode of the recess portion thereof, satisfies the following expressions: φ D 1=φ D 2; and φ D 2<φ D 3, provided that φD 1 is the recess portion opening diameter; φD 2 is the insulating member inner diameter; and φD 3 is the center electrode outer diameter.