Impact of Spark Plasma Length on Flame Kernel Development under Flow Condition 2020-01-1114

Advanced ignition systems with enhanced discharge current have been extensively investigated in research, since they are highly regarded as having the potential to overcome challenges that arise when spark-ignition engines are running under lean or EGR diluted conditions. Local flow field is also of particular importance to improve the ignitability of the air-fuel mixture in SI engines as the spark plasma channel can be stretched by the flow across the spark gap, leading to longer plasma length, thus more thermal spark energy distributed to the air-fuel mixture in the vicinity of the spark plug. Research results have shown that a constantly high discharge current is effective to maintain a stable spark plasma channel with less restrikes and longer plasma holding period. However, with the further increase in discharge current, plasma channel becomes thicker, and the stretched plasma length becomes shorter under a constant flow speed, which may suppress the advantages of the enhanced discharge current.

In this work, the interaction between discharge current level and plasma length under flow conditions is investigated. Whether a thick but shorter plasma channel or a thin but stretched farther plasma channel is more effective for the flame kernel formation is discussed to provide an insight into the influence of the plasma length on flame initiation. An optical combustion chamber platform along with a cross-flow generation system was used to study the plasma channel behavior; an in-house developed spark energy management module was employed to boost the discharge current level up to 3 A; in order to decouple the effect of discharge duration on flame initiation, the total discharge process was controlled within 0.8 ms.