| 摘要 |
<p>An object is to prevent breakage at a rear end portion of an insulator even in the case where a rear trunk portion of the insulator is long and/or the area of a flat portion of the insulator is small. An ignition plug 1 includes a ceramic insulator 2 having an axial bore 4, a metallic shell 3 provided around the ceramic insulator 2, and a terminal electrode 6 having a leg portion 6A inserted into a rear end portion of the axial bore 4 and a head portion 6B projecting from the rear end of the ceramic insulator 2. The length L of a portion of the ceramic insulator 2 projecting from the rear end of the metallic shell 3 is not less than 28 mm but not greater than 34 mm, and/or the flat portion 2F of the ceramic insulator 2 is 25 mm 2 or less. In such an ignition plug 1, the thickness t of the thinnest portion 2A of the ceramic insulator 2 is 1.0 mm or greater within a region which extends 5.5 mm from the rear end of the ceramic insulator 2 toward the forward end side with respect to the direction of the axis CL1. Further, the area D of a region AR where the projected area PA1 of the flat portion 2F overlaps with the projected area PA2 of a flat portion 6F is set to 8 mm 2 or greater.</p> |
