Proposing a chemical kinetic mechanism for biodiesel/NG blend in RCCI engine condition

Abstract

Abundance of natural gas (NG), its clean combustion characteristics and relatively low cost among various fuels, make it a suitable alternative for gasoline. In use of NG instead of gasoline especially in RCCI engines, due to lower cetane number of NG, using diesel similar fuel with higher cetane number, such as biodiesel, is recommended. Numerical study of biodiesel-NG oxidation needs to have a special chemical kinetic mechanism which predicts characteristics of combustion and emission accurately. There is no compact, accurate and reliable chemical mechanism which predict combustion specification of biodiesel-natural gas blend for using in CFD simulations, especially in RCCI engine conditions. For this purpose, a compact skeletal Methyl decanoate (MD), Methyl-5-Decanoate (MD5D), n-decane, and NG mechanism including 66 species and 229 reactions is constructed in this work for biodiesel-natural gas CFD simulations. The GRI-mech3.0 NG mechanism, consist of 53 species and 325 reactions, is reduced first by DRGEP and FSSA methods in Chemkin Pro 0-Dimensional homogeneous reactor, and then the reduced mechanism containing H2/CO/C1-C3 is merged with the biodiesel sub-mechanisms of MD, MD5D, and n-decane which consist of 60 species and 172 reactions. In following, the coupled mechanism is optimized by reaction rate adjustment and reduced with QSSA method. Ignition delay and flame speed are validated with primary mechanisms results and experimental data. Comparing ignition delay times of based and developed mechanisms revealed that arithmetical mean error (AME) is 1.2% for 0%-NG and 6.34% for 0%-biodiesel, respectively. In 1-D simulation, flame speed is calculated and the AME for 0%-biodiesel and 0%-NG is 7.2% and 4.7%, respectively. To ensure the applicability and accuracy of the developed mechanism, this mechanism was used in the CFD simulation of an RCCI engine and the obtained results such as in-cylinder pressure and the heat release rate, were in appropriate agreement with the experimental results.