Researchers at the University of Wisconsin-Madison have used computational fluid dynamics modeling to investigate cycle-to-cycle instability of homogeneous charge compression ignition (HCCI) and reactivity-controlled compression ignition (RCCI) engines—two approaches to low-temperature combustion. Low-temperature combustion engines offer the promise of simultaneously increasing engine efficiency and reduce NOx and PM emissions, but have not been widely implemented due to difficulties controlling combustion phasing, combustion duration, and cycle-to-cycle variation.
A paper on their study is published in the International Journal of Engine Research.
They performed a large design of experiment with small perturbations to the intake and fueling conditions, then fit a response surface model to the design of experiment results to predict the combustion characteristics and to determine the main sources of cycle-to-cycle variation.
Results showed that:
Reactivity-controlled compression ignition and homogeneous charge compression ignition have significantly more variation than conventional diesel combustion.
Reactivity-controlled compression ignition combustion phasing (CA50—crank angle when 50% of fuel is burned) is most sensitive to variations in diesel fuel mass, level of exhaust gas recirculation, and charge gas temperature. The peak pressure rise rate of reactivity-controlled compression ignition combustion is most sensitive to variations in gasoline fuel mass.
Sources of variation for homogeneous charge compression ignition are similar to those of reactivity-controlled compression ignition combustion; however, trapped gas pressure and cylinder liner temperature become significant factors.
Because of the late combustion phasing required for homogeneous charge compression ignition to maintain acceptable pressure rise rate, its cycle-to-cycle variation was found to be higher than that of reactivity-controlled compression ignition combustion for the same input variations.
David Klos, Sage L Kokjohn (2014) “Investigation of the sources of combustion instability in low-temperature combustion engines using response surface models,” International Journal of Engine Research doi: 10.1177/1468087414556135