| 摘要 |
An object of the present invention is to offer a spark plug for internal-combustion engines, which has excellent ignitability and sufficient anti-fouling characteristics. A spark plug 1 is comprised of a ceramic insulator 2 having an axial bore 4 extending in an axis CL1 direction, a metal shell 3 provided on an circumference of the ceramic insulator 2, a center electrode 5 inserted in the axial bore 4, and a ground electrode 27 provided in a front end portion 26 of the metal shell 3. A spark discharge gap 33 is formed between the ground electrode 27 and the center electrode 5. The ground electrode 27 is positioned such that the front end portion thereof is outside of a virtual outer circumferential face KG and is positioned on the front end side with respect to a virtual face KS in the axis CL1 direction. The virtual outer circumferential face KG is defined by extending a front end outer circumferential face 5G of the center electrode 5 in the axis CL1 direction, and the virtual face KS includes the front end of the center electrode 5. The present invention satisfies an equation of: 1.1 < = b / a < = 1.6 ,
where "a" (mm) represents a first minimal distance between the front end portion of the center electrode 5 and the front end portion of the ground electrode 27,
where "b" (mm) represents a second minimal distance between the front end portion of the ceramic insulator 2 and the front end portion of the ground electrode. |
