Patent ID: 7652413

Claim:
A spark plug, comprising: a center electrode; an insulator disposed on an outer circumferential surface of the center electrode, the insulator having an end portion protruded from an end surface of a cylinder head of an internal combustion engine into a combustion chamber of the engine facing the end surface of the cylinder head; a metallic housing disposed on an outer circumferential surface of the insulator and fixed to the cylinder head, the housing having an end portion facing the combustion chamber, a pocket bore being formed between the insulator and the housing so as to face the combustion chamber; a ground electrode attached to the housing; a tip portion disposed on the center electrode so as to be placed in the combustion chamber; and another tip portion disposed on the ground electrode to form a spark discharging gap between the tip portions, wherein a spark position length H between the end surface of the cylinder head and the tip portion of the center electrode is set within a range from 6.5 mm to 10 mm, a spark discharging gap length G between the tip portions is set within a range from 1.1 mm to 2.0 mm, a housing position length J between the end surface of the cylinder head and the end portion of the housing, the spark position length H and an insulator position length F between the end surface of the cylinder head and the end portion of the insulator are set to satisfy a relation of J≦F≦H−1.0 mm, a sectional area S 1 of the tip portion of the center electrode on a plane perpendicular to a center axis of the center electrode is set within a range from 0.07 mm 2 to 0.95 mm 2 , the tip portion of the center electrode is made of a first noble metal having a melting point equal to or higher than 2000° C. or is made of a first alloy containing the first noble metal, the tip portion of the ground electrode is made of a second noble metal having a melting point equal to or higher than 1700° C. or is made of a second alloy containing the second noble metal, and the spark discharging gap length G, the sectional area S 1 of the tip portion of the center electrode, a sectional area S 2 of the tip portion of the ground electrode on a plane perpendicular to a center axis of the ground electrode chip, and a pocket clearance P of the pocket bore denoting a half of a difference between an inner diameter of the end portion of the housing and an outer diameter of the end portion of the insulator are set to satisfy a relation of P≦ 1.1×( G+ 0.0345 ×S 1 −1.2418 +0.0327 ×S 2 −1.2418 ) when the areas S 1 and S 2 are expressed in mm 2 while the length G and the clearance P are expressed in mm.