1. Field of the Invention
The present invention relates to a spark plug to be installed in an internal combustion engine.
2. Description of the Related Art
Conventionally, in order to prevent flashover between a terminal connected to a terminal electrode and a metallic shell, the surface of an insulator has been coated with glaze in an area extending from the head portion to the trunk portion of the insulator.
Since glost firing has been performed at a temperature between 900.degree. C. and 1100.degree. C., there has been glaze whose Pb content is equal to or greater than 20 wt. % in terms of PbO (PbO contributes toward a decrease in glost firing temperature).
The present inventors have found that when a corona discharge occurs on the exposed portion (the surface of a head portion not covered with a plug cap) of an insulator, glaze discolors to red or reddish yellow, and the flashover voltage of a spark plug reduces.
As a result of investigation, the inventors have identified the cause of this discoloration and the accompanying reduction in the flashover voltage, as described below.
In an ordinary traveling state (city driving or highway driving), there arises no problem with a spark plug having an insulator coated with glaze whose Pb content is rather high at 20 wt. % to 40 wt. % in terms of PbO. However, during travel over a long period of time under special conditions, such as along a road having many uphill and downhill sections, a strong electric field is induced, mainly on the trunk portion of an insulator in the vicinity of the caulked portion of a metallic shell, resulting in the occurrence of a corona discharge. Energy of the corona discharge causes Pb contained in glaze to change the Pb.sub.3 O.sub.4 (red) or Pb.sub.2 O.sub.3 (reddish yellow) and causes the insulation resistance of the insulator to decrease drastically.
As a result, there is a reduction in the flashover voltage between the terminal electrode and the metallic shell, resulting in a high likelihood of the occurrence of flashover.