Simulation of biodiesel combustion in a light-duty diesel engine using integrated compact biodiesel–diesel reaction mechanism

Abstract

This computational fluid dynamics (CFD) study is performed to investigate the combustion characteristics and emissions formation processes of biodiesel fuels in a light-duty diesel engine. A compact reaction mechanism with 80 species and 303 reactions is used to account for the effects of chemical kinetics. Here, the mechanism is capable of emulating biodiesel–diesel mixture of different blending levels and biodiesel produced from different feedstock. The integrated CFD-kinetic model was validated against a test matrix which covers the entire saturated–unsaturated methyl ester range typical of biodiesel fuels, as well as the biodiesel–diesel blending levels. The simulated cases were then validated for in-cylinder pressure profiles and peak pressure values/timings. Errors in the peak pressure values did not exceed 1%, while the variations in peak pressure timings were kept within 1.5 crank angle degree (CAD). For this reported work, in-cylinder formation mechanisms of nitrogen monoxide (NO) and soot from the combustion of biodiesel fuels such as coconut methyl ester (CME), palm methyl ester (PME) and soybean methyl ester (SME) were simulated and compared against that of the baseline diesel fuel. It is established here that an increase in the degree of unsaturation in biodiesel fuels can have detrimental effects on engine-out NO and soot concentrations. Biodiesel fuel with greater unsaturation level such as SME has the predisposition to produce greater thermal NO due to larger regions of high temperature achieved during combustion. Besides, the highly unsaturated SME has greater tendencies to produce acetylene which are precursors for prompt NO and soot, leading to higher engine-out emissions of these pollutants. In all, this study has demonstrated the feasibility of integrating a compact multi-component surrogate fuel mechanism with CFD to elucidate the in-cylinder combustion and emissions characteristics of biodiesel fuels. The knowledge acquired here is useful for the development of biodiesel usage strategies in light-duty diesel engines for mass adoption.