Development and Validation of a Reduced Toluene/N-Heptane/N-Butanol Mechanism for Combustion and Emission Prediction in IC Engine

The present study proposed a reduced mechanism for a fuel blend of toluene reference fuel (TRF, toluene/n-heptane) and n-butanol for modeling the combustion and soot formation processes of n-butanol/diesel blend fuel. A detailed reaction mechanism for n-butanol, consisting of 243 species and 1446 reactions, and a reduced TRF mechanism, containing 158 species and 468 reactions, were reduced separately and then combined to create a new TRF/n-butanol mechanism. The new TRF/n-butanol mechanism contained 107 species and 413 reactions. A multi-technique reduction methodology was used which included directed relation graph with error propagation and sensitivity analysis (DRGEPSA), unimportant reaction elimination, reaction pathway analysis, and sensitivity analysis. In addition, a reduced 12-step NO_x mechanism was combined with the TRF/n-butanol mechanism to predict NO_x emissions. The proposed mechanism was also coupled with a multi-step soot model to predict the combustion and soot formation processes. The proposed mechanism was validated using available ignition delay times, laminar flame speeds and species concentration profiles from shock tubes, flat flame burner and jet stirred reactors. Good agreements were found for the above comparisons and with results from detailed mechanisms. Furthermore, multi-dimensional CFD simulations were conducted by using the KIVA-3V R2 code coupled with the preconditioned Krylov method. The effects of exhaust gas recirculation (EGR), injection timing and blending ratio of n-butanol on combustion and NO_x formation were analyzed and validated experimental data. The pressure, heat release rate, NO_x, and soot emissions with respect to fuel blends, EGR rates and start of injection (SOI) timings agreed well with the experimental results. With increasing n-butanol content, both experimental and calculated soot emission decreased, demonstrating that butanol additive was capable of reducing soot emission compared to pure diesel. Both experiments and models revealed that soot emissions peak occurred at SOI close to TDC. The proposed mechanism can readily be used to predict the combustion and soot formation processes of butanol-diesel blends fuel in combustion CFD simulations.