| 摘要 |
PROBLEM TO BE SOLVED: To provide a spark plug that has an insulator having a high voltage resistance characteristics under such a high temperature as 700 deg.C or the proximity and that satisfies small size. SOLUTION: The spark plug 10 has a small size (the thickness of the insulator 2 having been thinned) to satisfy the following formula, B-A<=2.0 mm, and 0.4 mm<=(A-C)/2<=2.0 mm, wherein, A is the outside diameter of the base of the long leg portion 2i of the insulator 2, B is the minimum inner diameter of the fitting engagement portion 4c of the main fitting 4 and C is the inner diameter of the penetrating hole 6a inside the long leg portion 2i for arranging a center electrode 3 therein. The insulator 2 is composed mainly of alumina (Al2O3) and contains at least one or more kinds of rare earth elements (as expressed RE hereafter). Thereby the spark plug 100 has an excellent voltage resistance characteristics at a high temperature of 700 deg.C or so suppressing occurrence of insulation destruction by the effect of lower melting point glass phase and has an insulator 2 which is suited and preferable for thinness. |
