A New Reduced Chemical Kinetic Model for Autoignition and Oxidation of Lean n-heptane/Air Mixtures in HCCI Engines

The paper has presented a new reduced chemical kinetic model for the Homogeneous Charge Compression Ignition (HCCI) combustion of n-heptane in an engine, which contains 41 species and 63 reactions. The new model includes three sub-models: the first is the low-temperature reaction sub-model, which is established by determining particular aldehydes and small hydrocarbons in the model developed by Li et al. The second is the sub-model for large molecules decomposing directly into small molecules that is developed for linking the low-temperature reaction with high-temperature reaction. The third is used for high-temperature reaction, which is derived by several modifications to the model developed by Griffiths et al., eliminating several reactions, adding two oxidization reactions related to CO and CH₃O. In addition the kinetic parameters of the key reactions in the new model were adjusted by using a genetic algorithm optimization methodology to improve predicted ignition timings in wide operation conditions. N-heptane as a fuel surrogate is used to simulate HCCI combustion behaviors because of similar cetane number to that of diesel fuel. Both the new model and the optimized new model were validated under engine conditions. The results showed the well-known two-stage ignition characteristics of n-heptane, which involve low and high temperature regimes followed by a branched chain explosion, and that the optimized new model agreed well with those of the detailed chemical kinetic model proposed by Curran et al. The CPU time of consuming in the new model is less 1/1,000 that of the detailed model. Thus, the computational efficiency of diesel HCCI engine simulation is significantly improved by using the reduced model.