Experiments and analysis of high cyclic variability at the operational limits of spark-assisted HCCI combustion

During combustion mode switches, between homogeneous charge compression ignition (HCCI) and spark ignition (SI) combustion, the engine will operate at throttled and stoichiometric conditions where high cyclic variability (CV) is typically observed. To analyze and eventually model the engine behavior at the high CV condition we perform measurements on a four-cylinder HCCI engine with negative valve overlap and describe a cycle-resolved analysis method that enables the characterization of cycle-to-cycle variations at such conditions. The dynamic behavior observed is characterized by the recycling of thermal and chemical energy between cycles. We quantify the cyclic exchange and relate it to the dynamic patterns that emerge from this high CV condition. We also clarify the contributions of the spark at these conditions, where advancing the spark can significantly reduce the variability. It is our conclusion that the dynamic patterns observed can be characterized by the cycle-resolved combustion efficiency as it is an essential non-linearity in the dynamic evolution.