To improve thermal efficiency of internal combustion engine, dilution combustion system, such as lean burn and Exhaust Gas Recirculation (EGR) system, have been developed with spark ignition coils generating large discharge current and energy. Several researches have clarified that large discharge current increases discharge channel stretch and decreases possibility of discharge channel blow-off and misfire. However, these investigations don\u27t mention effect of discharge current profile on combustion speed and discharge channel behaviour enough. Purpose of this research is to investigate relation among dilution rate, combustion speed, discharge channel behaviour and discharge current. To achieve this purpose, five coils having different current profiles were evaluated by combustion test and in-cylinder optical measurement test with research single cylinder engine. The combustion test results showed a correlation between dilution limit and initial combustion period. And optical measurement test results showed a correlation of initial combustion period with discharge channel stretch. Moreover, saturation of discharge channel stretch were observed from a certain discharge current value on up. Based on these results, adequate coil for dilution system was selected. Finally, the coil was equipped on a vehicle and performance test was conducted. The vehicle with LP-EGR system was stably driven with 18% of EGR rate, and 2.3% of fuel reduction rate were verified in the WLTC mode. The selected high energy ignition coil was contributing reduction of misfire during EGR operation
