A reduced mechanism for biodiesel surrogates for compression ignition engine applications

A skeletal mechanism with 115 species and 460 reactions for a tri-component biodiesel surrogate, which consists of methyl decanoate, methyl 9-decenoate and n-heptane, was developed to reduce computational costs for 3-D engine simulations. The detailed mechanism for biodiesel developed by Lawrence Livermore National Laboratory (LLNL) was employed as the starting mechanism. The rate constants for the n-heptane and larger alkane subcomponents in the detailed mechanism were first updated. The detailed mechanism was then reduced with direct relation graph (DRG), isomer lumping, and DRG-aided sensitivity analysis (DRGASA), which was improved to achieve a larger extent of reduction. The reduction was performed for pressures from 1 to 100 atm and equivalence ratios from 0.5 to 2 for both extinction and ignition applications. The initial temperatures for ignition were from 700 to 1800 K, covering the compression ignition (CI) engine conditions. Extensive validations were performed against 0-D simulations with the detailed mechanism and experimental data for spatially homogeneous systems, 1-D flames and 3D-turbulent spray combustion. The skeletal mechanism was able to predict various combustion characteristics accurately such as ignition delay, flame lift-off length, and equivalence ratio at flame lift-off location under different ambient conditions. Compared with the detailed mechanism that consists of 3299 species and 10806 reactions, the skeletal mechanism features a reduction by a factor of around 30 in size while still retaining good accuracy and comprehensiveness.