Plasma-jet spark plug and ignition system

摘要

[Objective] To maintain excellent ignition performance over a long period of time. [Means for Solution] A plasma jet ignition plug 1 includes an insulator 2 having an axial hole 4 extending therethrough in the direction of an axis CL1, a center electrode 5 inserted into the axial hole 4, and a ground electrode 27 disposed frontward of the front end of the insulator 2. The insulator 2 has a cavity 31 which is defined by the wall surface of the axial hole 4 and the front end surface of the center electrode 5 and opens frontward. The ground electrode 27 has a through hole 28 for establishing communication between the cavity 31 and an ambient atmosphere. A front end portion of the center electrode 5 is formed into a taper portion 51 whose diameter reduces frontward with respect to the direction of the axis CL1. The outside diameter of a front end of the taper portion 51 is smaller than the diameter of the through hole 28. The cavity 31 has a substantially constant diameter with respect to the direction of the axis CL1. The diameter DC (mm) of the cavity 31 and the length LC (mm) of the cavity 31 along the direction of the axis CL1 satisfy the dimensional relation 0.5 ‰¤ LC/DC < 1.0.