| 摘要 |
An ignition plug (1) includes a ceramic insulator (2) having an axial bore (4), a center electrode (5) inserted into the axial bore (4) in such a manner that a forward end surface thereof is located rearward of the forward end of the insulator (2), a metallic shell (3), and a ground electrode (27) fixed to the metallic shell (3), and has a cavity (29) defined by an inner circumferential surface of the axial bore (4) and the forward end surface of the center electrode (5). The axial bore (4) includes a first straight portion (41) extending forward with respect to the direction of the axis (CL1) from the forward end surface of the center electrode (5), and a diameter-reducing portion (43) whose diameter reduces forward with respect to the direction of the axis (CL1) from the forward end of the first straight portion (41). As viewed on a section which contains the axis (CL1), a relational expression ± ‰¥ 10 is satisfied, where ± (°) is an acute angle formed by a straight line orthogonal to the axis (CL1) and the outline of the diameter-reducing portion (43). By virtue of these features, excellent ignition performance can be maintained over a long period of time through restraint of channeling while ignition performance is improved. |
