After a new motor vehicle (powered by a gasoline engine) is assembled in an automobile factory, it is often moved (for transport to a motor pool or for loading into a truck or a cargo boat for exportation before it is delivered to a user) by running the gasoline engine. The running mode for this movement is not uniform, but the speed of the vehicle is ordinarily at from approximately 30 to 40 km per hour, and the running distance is about 500 meters for each movement. The condition of driving is low speed--short distance. When the gasoline engine is driven at such low speed over a short distance, each spark plug (mounted on the engine of the vehicle) tends to smolder. Smoldering reduces the insulation resistance of the spark plug and may lead to misfiring and defective running. Such a smoldering phenomenon is especially frequent when ambient air is cold, for example in winter. Furthermore, such smoldering occurs not only in a new car, such as in the previously-described case, but also in used cars in general in the wintertime.
Such smoldering results from deposition of unburnt carbon, such as soot, caused by combustion of gasoline within an engine, on the surface of a taper-nose portion 61 of an insulator 6 of a spark plug 1, as shown in FIG. 1, when that surface is exposed to combustion gases. [In the drawing, the reference numeral 2 represents a housing; 3, a grounded electrode; 4, a washer; and 5, a central electrode.]
To solve the instantly-noted problem, the grounded electrode could be specially designed to use a creeping discharge to remove any deposit of unburnt carbon. However, the structure between the taper-nose portion 61 of insulator 6 and the grounded electrode 3 is complex for such a solution, and difficulties are encountered in producing spark plugs of this type.