Patent ID: 7234429

Claim:
An ignition device comprising: a spark plug to be installed in a head of an internal combustion engine, said spark plug having a structure including: a metal shell having a base end and a top end opposed to the base end in a lengthwise direction of the spark plug; a hollow cylindrical porcelain insulator having a length which includes a body and an insulator head, the body being retained within said metal shell, the insulator head extending from the base end of said metal shell in a lengthwise direction of said porcelain insulator, said porcelain insulator having an inner chamber extending in the body and the insulator head; a center electrode retained within the inner chamber of said porcelain insulator in alignment with the length of said porcelain insulator; and a ground electrode extending from the top end of said metal shell to define a spark gap between itself and said center electrode, wherein the insulator head of said porcelain insulator has a length made up of a major body leading to the body of said porcelain insulator and an end portion lying far away from the body of said porcelain insulator, the major body having an outer diameter D 1 , an inner diameter D 2 , and a section modulus Z, where Z=π (D 1 4 −D 2 4 )/(32 D 1 ) at a smallest-outer diameter portion thereof which meet relations below, 7.1 mm≦D1≦8.8 mm D2≧2.8 mm Z≧33 mm 3 ; and an ignition coil to be installed in a head cover covering the head of the engine in engagement with the insulator head of the spark plug.