| 摘要 |
A spark plug has an axially extending center electrode, an insulator having an axially extending shaft hole, holding the center electrode on the front end side in the shaft hole, and forming an assembly with the central electrode, a main fitting for holding the insulator in the assembly while surrounding the periphery of the insulator, and a ground electrode having one end joined to the front end surface of the main fitting and the other end forming a spark discharge gap between the ground electrode and the center electrode. The spark plug satisfies the relationships of H >= 1 mm, Vc <= 17 mm, and Ra >= 1.0 × 10m K/W, where, in the axial direction of the spark plug, H is the length of that portion of the insulator which projects from the front end surface of the main fitting toward the front end, Vc is the volume of that portion of the insulator which is present in the range of 2 mm from the front end of the insulator toward the rear end, and Ra is the thermal resistance of the assembly, excluding an air layer, per unit length at 20°C, the thermal resistance being resistance at a cross-section that is perpendicular to the direction of the axis of the assembly and is located at the 2 mm position. |
