Experimental and numerical study on suitable diesel fuel surrogates in low temperature combustion conditions

Abstract

Motivated by the lack of information regarding an appropriate diesel fuel surrogate under low temperature combustion conditions, an experimental work had been carried out in a single-cylinder common-rail diesel engine running with diesel fuel and several diesel fuel surrogates containing the blend of n-heptane/toluene and n-heptane/toluene/1-hexene. The combustion and emission characteristics of the diesel fuel and its surrogates had been investigated in a wide range of intake oxygen concentration ranging from 21% to approximately 10%, covering both conventional diesel combustion and low temperature combustion conditions. The chemical kinetics mechanism had been used to analyze the experimental phenomenon. Results demonstrated that the ignition delay was longer with the increase of toluene in the pure n-heptane. The toluene had larger effects on ignition delay at lower intake oxygen concentrations because less oxidation process resulted in a lower HO2 emission at lower oxygen concentrations and more termination reactions occurred with more benzyl radical. The ignition delay and smoke emission of TRF20 (80% n-heptane/20% toluene in volume) were closer to that of diesel fuel as compared with the pure n-heptane and TRF30 (70% n-heptane/30% toluene in volume) fuel. However, there were still some disparities between TRF20 and diesel fuel in smoke emission at lower intake oxygen concentrations ([O2]in<16%). Therefore, the further study of fueling TRF20/1-hexene mixture (95/5vv) was conducted. Results showed that the addition of 1-hexene retarded the ignition delay slightly than that of TRF20 and the smoke emission of TRF20/1-hexene mixture was closer to that of diesel fuel. Both experiments and chemical kinetics analysis showed that the acetylene as the soot precursor had a higher emission at lower intake oxygen concentrations for TRF20/1-hexene mixture and thus led to a higher smoke emission compared to TRF20. In addition, other emissions studies demonstrated that NOx emissions were almost identical for diesel and its surrogates, while HC and CO emissions had some disparities at very low intake oxygen concentrations. Finally, it can be concluded that the TRF20/1-hexene mixture had a better match in auto-ignition delay and emissions at both conventional combustion and low temperature combustion processes in the current study.