Influence of the addition of titanium oxide nanoparticles to Fischer-Tropsch diesel synthesised from coal on the combustion characteristics and particulate emission of a diesel engine

Abstract

The use of Fischer–Tropsch (F–T) diesel synthesised from coal in automobiles can alleviate petroleum shortages and promote clean utilisation of coal. Because F–T diesel does not contain oxygen, in this study, we added TiO2 nanoparticles and n-octanol to the F–T diesel to serve as oxygenated enhancers to increase the oxygen content and reduce the particulate matter emission of F–T diesel. To achieve better combustion characteristics and emission performance, TiO2 nanoparticles with different concentrations (20, 50, and 100 ppm) were dispersed to the fuel blends of F–T diesel and n-octanol to determine the optimum amount of TiO2 nanoparticles. The brake thermal efficiency (BTE), combustion stability, number concentration, and size distribution of the ultrafine particulate (UFP) emission of the three nano-emulsion fuels were investigated on a turbocharged heavy-duty diesel engine. The experimental results indicated that the BTE of the F–T diesel and that of the nano-emulsion fuel T50 increased by 0.75 % and 2.26 %, respectively, compared with petro-diesel. The nano-emulsion fuels had higher peak cylinder pressure and peak heat release rate owing to the faster combustion rate caused by the micro-explosion of fuel droplets and higher thermal conductivity caused by the high surface-to-volume ratio of TiO2 nanoparticles. Moreover, the nano-emulsion fuels exhibited higher cyclic variations of peak cylinder pressures and more dispersed corresponding crank angles with the increase in the concentration of TiO2 nanoparticles. Compared with petro-diesel, the soot emission of the F–T diesel was reduced by an average of 27.32 % at various loads, whereas that of the optimal T50 decreased by an average of 43.61 %. Additionally, the number concentration of UFPs of T50 was reduced by an average of 21.2 % compared to the F–T diesel. At low loads, the three nano-emulsion fuels exhibited greater geometric mean diameters (GMDs) of UFPs and lower ratios of nucleation mode particulates (NMPs) owing to the higher fuel viscosity in the pre-injection stage at a lower in-cylinder temperature. At medium and high loads, the nano-emulsion fuels exhibited smaller GMDs of UFPs and higher ratios of NMPs owing to micro-explosion and secondary atomisation at higher temperatures.