An investigation has been done on the relationship between spark ignition characteristics and combustion stability in a gasoline engine. The spark discharge parameters examined were the spark current, energy, and duration. It has been found that lengthening the spark discharge duration is particularly effective in achieving stabilized combustion.A longer spark duration provides a continued supply of electric energy as kinetic energy to the mixture around the spark gap. The analytical results of a constant volume combustion chamber test verify that a longer spark duration promotes flame initiation and makes reliable flame propagation possible.The length of the spark duration is regarded as the period from ignition to the onset of combustion pressure rise. The results of a combustion pressure analysis reveal that the spark duration must be longer than the heat release delay. The reason is that a long-duration spark shortens the initial combustion period, thereby making it possible to reduce the fluctuations that occur during that period. This is particularly important because such fluctuations are known to be the main cause of the cycle-to-cycle fluctuations in engine combustion.It has been found that the spark duration should be three to four times longer than the conventional spark discharge. A longer duration causes fuel economy to deteriorate because of the increase in electric power consumption.Experimental results confirm that a longer spark duration stabilizes engine combustion under such severe operating conditions as idling, heavy EGR rates and lean mixture operation.