Flamelet-based modeling of auto-ignition with thermal inhomogeneities for application to HCCI engines

Homogeneous-charge compression ignition (HCCI) engines have been shown to have higher thermal efficiencies and lower NOx and soot emissions than spark ignition engines. However, HCCI engines experience very large heat release rates which can cause too rapid an increase in pressure. One method of reducing the maximum heat release rate is to introduce thermal inhomogeneities, thereby spreading the heat release over several crank angle degrees. Direct numerical simulations (DNS) with complex H-2/air chemistry by Cook and Pitsch [D.J. Cook, H. Pitsch, Western States Section of the Combustion Institute, Boise, Idaho 06S-08, 2006] showed that both ignition fronts and deflagration-like fronts may be present in systems with such inhomogeneities. Here, an enthalpy-based flamelet model is presented and applied to the four cases of varying initial temperature variance presented in Cook and Pitsch [D.J. Cook, H. Pitsch, Western States Section of the Combustion Institute, Boise, Idaho 06S-08, 2006]. This model uses a mean scalar dissipation rate to model mixing between regions of higher and lower enthalpies. The predicted heat release rates agree well with the heat release rates of the four DNS cases. The model is shown to be capable of capturing the combustion characteristics for the case in which combustion occurs primarily in the form of spontaneous ignition fronts, for the case dominated by deflagration-type burning, and for the mixed mode cases. The enthalpy-based flamelet model shows considerably improved agreement with the DNS results over the popular multi-zone model, particularly, where both deflagrative and spontaneous ignition are occurring, that is, where diffusive transport is important. (C) 2006 Published by Elsevier Inc. on behalf of The Combustion Institute.