Chemical feature of the soot emissions from a diesel engine fueled with methanol-diesel blends

Abstract

The chemistry of soot particles has high affinity with the concentration and accessibility of potential radical sites for the reactions regarding to soot growth and oxidation behaviors during in-cylinder combustion, and thus affects the particle emissions of the engines. The addition of methanol in diesel fuel is widely believed to transform the soot formation and oxidation behaviors. However, the instinctive explanation on these transformation by soot chemistry is not well understood. In this study, the chemical feature of soot emissions from a methanol-diesel blend fueled diesel engine was characterized, using X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR), respectively. Results showed that epoxy, ether and C vacancy as well as ester groups were the dominant surface species, while the carboxyl and carbonyl groups exerted less contribution amongst the carbon–oxygen groups on soot surfaces for both of pure diesel fuel and methanol-diesel blends. At low engine load conditions, the increase in methanol blending content led to an increase–decrease-increase behavior of C-O epoxy concentration, which was opposite with the C vacancy contribution. At the higher engine load, increasing methanol content in fuel blends led to higher increment in the hydroxyl species with respect to the epoxy groups on soot surfaces, especially for the 5% and 10% methanol addition. The predominant groups within the bulk of soot particles consisted of aliphatic C–H (methyl and methylene), carbonyl-like CO, aromatic C–H groups and C–O bonds in phenols, anhydrides, esters and ether-like groups for both diesel and methanol blending fuels. Relative to the aromatic CC, the concentration of aliphatic C–H reduced with the increasing methanol content in the fuel blends. The lower-volume-proportion addition of methanol at low engine load led to an increase in the aromatic C–H with respect to aromatic CC. While different, the addition of methanol exerted no significant influence on the aromatic C–H concentration within the soot particles at high engine load condition.