Influences of spark discharge characteristics on ignition and combustion process in lean-burn SI engine

Lean-burn SI engines have been investigated to decrease the heat loss, such as the cooling loss and the pumping loss. However, as excess air ratio approaches the lean operation limit, the cycle-to-cycle variation (CCV) of SI combustion becomes an obstacle to stable operation. This study reports the effects of spark discharge characteristics, such as discharge current and spark-shortening phenomena, on the CCV of SI combustion under lean operation (excess air ratio = 1.8 - 2.3). An enhanced ignition system equipped with 20 ignition coils was utilized in this study. In the experiment, the discharge interval between each coil unit was modified to change the discharge current and duration. The results showed that the CCV of combustion was suppressed with the discharge interval, and the lean operation limit was extended to = 2.1. Observations of in-cylinder photographs suggest that the repetition of spark-shortening phenomena promoted the flame kernel formation.