Influence of spark discharge characteristics on ignition and combustion process and the lean operation limit in a spark ignition engine

Lean combustion technologies have been investigated to decrease the heat loss of spark ignition engines. However, as the excess air ratio approaches the lean operation limit, the cycle-to-cycle variation of combustion becomes an obstacle to improving thermal efficiency. This paper discusses the influences of spark discharge characteristics, such as discharge current and spark-shortening phenomena, on the ignition and combustion process under lean conditions (excess air ratio (lambda) of 1.8-2.3) to suppress the cycle-to-cycle variation of combustion and extend the lean operation limit. In this study, a customized inductive ignition system equipped with 20 conventional ignition coils was applied to enhance the ignition energy. The discharge interval between each coil unit was controlled to change the discharge current and duration. The results show that the in-cylinder discharged energy increased with the discharge interval. The cycle-to-cycle variation of combustion was minimized when the discharge interval was 0.4 ms, and consequently, the lean operation limit was extended to an excess air ratio (lambda) of 2.1. The discharge waveforms indicated that the longer discharge interval could promote spark-shortening phenomena such as re-strike due to the lower discharge current. Finally, in-cylinder photographs of ignition and combustion process showed that the flame kernel formation could be promoted by the repetition of spark-shortening phenomena such as re-strike, as well as by the high elongation of the spark channel.