PURPOSE: To prevent attachment and accumulation of carbon onto an insulator by installing a step portion on the external circumference of a center electrode and making a leg portion to which an insulator is facing thin-walled.
CONSTITUTION: A plurality of step portions 13, two or more are installed on the external circumference of a center electrode 4 which forms a spark plug, and a portion facing to the step portion 13 of the insulator leg portion 11 of the insulator which holds the electrode 4 is made small in its diameter and thin-walled so that a small-diameter portion 14 may be formed. According to this constitution, the carbon produced due to incomplete combustion of an air/fuel mixture during a slow-speed travelling time is washed and removed by a spark generated between the step portion 13 and the insulator opening end 12. Moreover, the temperature of the small-diameter portion 14 rises up due to the combustion of air-fuel mixture so that the removal of the carbon is promoted and the attachment and accumulation of carbon onto the insulator can thereby be prevented and resultantly, a good operating condition may be kept over a broad range.
YOSHIDA MITSUTAKA
GOTOU EIGO
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