This research visualizes the process of initial fire spread as the electrode shape of the spark plug is altered for the purpose of determining the characteristics of discharge kernel and initial flame to contribute to improving combustion performance. The experiment devices were a closed vessel combustion chamber of about 450cc in volume, and 4 types of preconfigured spark plugs and the experiment conditions were set at l (lambda) = 1.0, 1.2, and 1.4 as results were observed. As a result, it was observed that as the air and fuel mixture state approached the theoretical air-fuel ratio, the initial flame characteristics from all spark plugs showed increase and it was shown B type spark plug increases the connectivity with gas mixture for improved growth efficiency of discharge kernel. And as the mixture ratio approached the rarefied air-fuel ratio, the effect of quenching action increased and it was observed that the flame efficiency increases in C type spark plug which miniaturized the center electrode and ground electrode.

discharge kernel, quenching action, flame efficiency,theoretical air-fuel ratio,rarefied air-fuel ratio.