US Patent 7328677 Plasma-jet spark plug and ignition system

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; andL2+{(D−d)/2}≦3.5 mmwhere 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.