A comparison of spray and combustion characteristics of biodiesel (soy methyl ester, rapeseed methyl ester) with diesel

Physical properties are known to play a pivotal role in the different characteristics of spray, combustion, and emission between diesel and biodiesel. This paper reports the development and application of physical properties of diesel and biodiesel for fuel spray and combustion modeling under various conditions. An integrated numerical model has been developed based on the General Transport Equation Analysis code. The effect of turbulence in the nozzle was considered by the hybrid breakup model in the simulation, and the skeletal mechanism of diesel surrogate fuel and biodiesel surrogate fuel was used to simulate fuel oxidation. The results indicated that liquid lengths and droplet sizes are always higher for biodiesel because of its bigger surface tension and worse vaporization characteristics caused by higher critical temperature and lower vapor pressure. This phenomenon has strong influence on fuel evaporation process and results in slow evaporation rate, and this is not helpful for the mixture preparation process. The effects of ambient density, ambient temperature, and oxygen concentration on ignition delay and lift-off length of diesel and biodiesel are also analyzed and discussed. This analysis revealed that longer ignition delay usually results in longer lift-off length and biodiesel always has longer ignition delay and lift-off length compared to diesel. The flame propagation was observed to be similar for both fuels. In addition, diesel and biodiesel were employed to simulate the combustion and emission characteristics of a low-temperature combustion engine. The different combustion and NOx emission characteristics between diesel and biodiesel were observed, and the different physical properties may be the reason for these discrepancies.