Conventionally, a spark plug for ignition is used in an internal combustion engine. A spark plug in general includes: a center electrode; an insulator for holding the center electrode in an axial hole; a metal shell for holding the insulator by surrounding its radial periphery; and a ground electrode having one end portion joined to the metal shell and the other end portion, a spark discharge gap being formed between the other end portion and the center electrode. Further, as spark discharge is generated in the spark discharge gap, the ignition of an air-fuel mixture is ignited.
In recent years, it has become necessary to enlarge valve diameters of an intake valve and an exhaust valve which are provided in an engine for a higher engine output, and to secure a larger water jacket for the engine to improve a water cooling system. Since the mounting space for the spark plug which is mounted on the engine becomes small, the spark plug is required to smaller in diameter. However, if the spark plug becomes smaller in diameter, the insulation distance between the insulator and the metal shell becomes narrow. As a result, the spark plug fails to discharge sparks in a regular spark discharge gap, and side sparks are prone to be generated from the center electrode to the metal shell along the insulator. Further, in a dry fouling state, flashover are likely to occur. This is due to the fact that electrically conductive carbon and the like deposited on the surface of the insulator causes a deterioration in the insulation properties between the insulator and the metal shell. In this case, it is necessary to ensure insulation properties on each occasion by burning off the carbon adhered on the insulator by increasing the front end temperature of the insulator.
Accordingly, for example, a spark plug has been proposed in which the following formulae are satisfied: (X+0.3Y+Z)/G≧2, Y1 (mm)≧1, W/Z≧4, and 1.25≦Z (mm)≦1.55 where X is the distance between the insulator and the center electrode at a front end portion of the insulator, Y is a creeping distance of the surface of the insulator outside the metal shell, Y1 is an amount of protrusion of the insulator from the metal shell, Z is a pocket gap, G is the distance of the spark discharge gap, and W is the length on the surface of the insulator up to a portion where the distance between the insulator and the metal shell becomes G or less inside the metal shell (e.g., see patent document 1). This spark plug excels in that, by respectively defining the aforementioned various distances among the component parts, even a spark plug with its diameter reduced is able to allow sparks to be discharged stably to a regular spark discharge gap when the spark plug is not dry fouling, and is able to ensure ignitability even in cases where the spark plug has dry-fouled and creeping discharge such as side sparks and flashover have occurred.    Patent Document 1: JP-A-2005-116513