| 摘要 |
<p>Breakage of a center electrode, etc., is more reliably prevented, and the effect of improving ignition performance and durability associated with provision of a noble metal tip is sufficiently exhibited. A spark plug 1 includes a ceramic insulator 2, a center electrode 5, a metallic shell 3, and a ground electrode 27. The center electrode 5 has a shoulder portion 51 at a forward end portion thereof. The shoulder portion 51 tapers forward with respect to the direction of an axis CL1. A noble metal tip 31 is joined to the forward end portion of the center electrode 5 through a fusion zone 35 and forms a spark discharge gap 33 in cooperation with the ground electrode 27. The shortest distance between the fusion zone 35 and a distal end surface of the noble metal tip 31 is 0.8 mm to 1.2 mm inclusive as measured on an outer side surface of the noble metal tip 31 along the axis CL1. The outside diameter of the fusion zone 35 as measured at a distal end of the fusion zone 35 is smaller than that as measured at a proximal end of the fusion zone 35. An acute angle ¸1 formed by a straight line L1 and a straight line L2 satisfies the relational expression ¸1 ‰¤ 72°.</p> |
