Patent ID: 6819032
Filing Date: 2004-11-16
Classification: H01T

Abstract:
A spark plug comprising an insulator having a center through-hole formed therein; a center electrode held in the center through-hole and disposed at an end portion of the insulator; a metallic shell for holding the insulator such that an end portion of the insulator projects from an end face thereof; a parallel ground electrode disposed such that one end thereof is joined to a front end face of the metallic shell, and a side face of the other end faces, in parallel, an end face of the center electrode; and a plurality of semi-creeping discharge ground electrodes disposed such that one end of each of the electrodes is joined to the metallic shell, and the other end of each of the electrodes faces a circumferential side surface of the center electrode and/or a circumferential side surface of the insulator; and the spark plug being characterized in that an air gap (&agr;) is formed between the parallel around electrode and the end face of the center electrode; a semi-creepage gap (&bgr;) is formed between the end face of each of the semi-creeping discharge around electrodes and the circumferential side surface of the center electrode; an insulator gap (&ggr;) is formed between the end face of each of the semi-creeping discharge around electrodes and the circumferential side surface of the insulator; a distance &agr; (unit: mm) across the air gap (&agr;) and a distance &bgr; (unit: mm) across the semi-creepage gap (&bgr;) satisfy a relationship &agr;&ggr;, andwherein the air gap (&agr;) is not greater than 1.1 mm; the insulator gap (&ggr;) falls within a range of 0.5 mm to 0.7 mm; and a diametral difference &dgr; (unit: mm) between the insulator and the metallic shell as measured along the front end face of the metallic shell is not less than 3.6 mm.