Surface functional groups and sp3/sp2 hybridization ratios of in-cylinder soot from a diesel engine fueled with n-heptane and n-heptane/toluene

• In-cylinder soot from n-heptane and n-heptane/toluene fueled-diesel engine was studied.
• Surface functional groups (SFGs) and sp3/sp2 hybridization ratios were assessed.
• Adding toluene to n-heptane increased the concentrations of SFGs.
• The sp3/sp2 hybridization ratio was obviously increased by toluene addition.
• Aliphatic CH groups had a correlation with sp3/sp2 hybridization ratios.

This work compared the surface functional group (SFG) types and concentrations and sp3/sp2 hybridization ratios of in-cylinder soot samples generated by a heavy-duty diesel engine when employing n-heptane and a toluene/n-heptane mixture (20% toluene by volume) as the fuels. In-cylinder soot samples were obtained from a total cylinder sampling system, and the SFGs and sp3/sp2 hybridization ratios were analyzed using Fourier transform infrared and X-ray photoelectron spectroscopy. Despite the differences in fuel formulation, both n-heptane and n-heptane/toluene soot exhibited similar trends in terms of changes in the SFGs concentrations and sp3/sp2 hybridization ratios during the combustion process. However, the addition of toluene to the n-heptane was found to increase the concentrations of all SFGs as well as the sp3/sp2 hybridization ratio. The COH and CO group concentrations exhibited a bimodal distribution for both the n-heptane and n-heptane/toluene soot throughout the combustion process, with the concentrations peaking in the premixed and diffusion combustion phases, respectively. In contrast, the relative amounts of aliphatic CH groups decreased in the premixed combustion phase, increased in the early diffusion combustion phase, and then decreased in the subsequent combustion phase. The sp3/sp2 hybridization ratios obtained from both fuel soot were observed to initially decrease, then to increase before a decrease during the combustion process. There was a definite correlation between the sp3/sp2 hybridization ratio and the relative concentration of aliphatic CH groups.