UW Madison team investigates cycle-to-cycle combustion instability in HCCI and RCCI

8 December 2014

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 NO_x and PM emissions, but have not been widely implemented due to difficulties controlling combustion phasing, combustion duration, and cycle-to-cycle variation.

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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.

Resources

• 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