Aliphatic C-H and oxygenated surface functional groups of diesel in-cylinder soot: Characterizations and impact on soot oxidation behavior

This paper reports the characteristics of surface functional groups (SFGs) and oxidation reactivity of in-cylinder soot during the diesel combustion process and discusses the correlation between SFGs and the oxidation reactivity of soot. Test soot samples were obtained from a total cylinder sampling system. SFGs and the apparent activation energy for the soot oxidation were determined by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and thermogravimetric analysis. Results reveal that the concentrations of aliphatic C-H, C-OH and C=O groups are dependent on the combustion stages. The ratio of aromatic C=C to aliphatic C-H groups rapidly increases in the premixed combustion phase, indicating significant loss of aliphatic C-H groups from the soot surface. As the combustion proceeds, the ratio of the two groups gradually increases, following an earlier drop during the diffusion combustion phase. In contrast, the concentration of both C-OH and C=O groups shows a bimodal distribution during the combustion process, with the concentrations peaking in the premixed and diffusion combustion stages, respectively. The apparent activation energies for the oxidation of soot collected under the applied engine operating conditions are observed in the range of 122.12-158.2 kJ mol(-1) at various crank angles, revealing a similar variation to that of aliphatic C-H groups. Aliphatic C-H groups are a more important factor in governing soot oxidation reactivity than the oxygenated SFGs. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.