Impact of chemical structure of individual fatty acid esters on combustion and emission characteristics of diesel engine

• Molecular structure of esters has great influence in combustion and emission.
• Effects of molecular structure are different on nucleation and accumulation mode.
• Ethyl ester has more accumulated particles and larger diameter of primary particle.
• Optimization of biodiesel composition could improve combustion and emission.

The effects of fatty acid chain length, number of double bonds and alcohol moiety chain length on combustion and emission characteristics were investigated. With the increase of carbon chain length of methyl ester, HC (hydrocarbon), CO and smoke emissions increase. The particle number concentration of nucleation mode decrease and accumulation mode increases with carbon chain length. The diameter of primary particle increases with carbon chain length. The double bonds of methyl ester are strongly correlated with CO, HC, nucleation mode and accumulation mode particles. The BSFC of unsaturated methyl ester is slightly higher. With increase of alcohol moiety chain length, the start of combustion is advanced, leading to lower premixed combustion, and thus resulting in the higher HC, CO and smoke and lower NOx emissions. Ethyl ester generates lower number concentration in nucleation mode, while it has higher number concentration in accumulation mode. The primary particle size of ethyl ester is larger than that of methyl ester. These results indicate that molecular structure of ester has great influence in combustion and emission characteristics. The composition of biodiesel could be modified and optimized during production process through new techniques or various feedstocks in order to improve combustion and emission of biodiesel.