Patent ID: 8841826
Filing Date: 2014-09-23
Classification: F02P,H01T

Abstract:
1. A spark plug comprising: a tubular insulator having an axial bore extending through the insulator in a direction of an axis; a center electrode inserted into a forward end portion of the axial bore; a tubular metallic shell provided externally of an outer circumference of the insulator; a ground electrode disposed at a forward end portion of the metallic shell; and a noble metal tip joined to a forward end portion of the center electrode, a gap being formed between said metal tip and the ground electrode; wherein the center electrode has a shoulder portion at a forward end portion of the center electrode, the shoulder portion tapering forward with respect to the direction of the axis; the noble metal tip is jointed to the center electrode via a fusion zone provided at least partially at a rear end portion of the noble metal tip; and a shortest distance between the fusion zone and a forward end surface of the noble metal tip is 0.8 mm to 1.2 mm inclusive as measured on an outer side surface of the noble metal tip along the axis, wherein; an outside diameter of the fusion zone as measured at a forward end of the fusion zone is smaller than that as measured at a rear end of the fusion zone, and relational expressions θ where θ θ where the straight line L the straight line L the straight line L the straight line L