The aim of this work is to investigate the effect of mixing ratio and pilot injection on spray and combustion characteristics of biodiesel fuel and compared with those of diesel fuel in a direct injection common-rail diesel engine. In order to study the influence factors of biodiesel fuel on the spray and combustion characteristics, the experiments were conducted at various mixing ratios and injection conditions of the biodiesel and engine operating conditions. The macroscopic and microscopic characteristics such as injection rate, split injection effect, spray tip penetration, droplet diameter, and axial velocity distribution of biodiesel fuel were compared with the results of conventional diesel fuel by using spray visualization system composed of Ar-ion laser, ICCD camera and phase Doppler particle analyzer (PDPA) system. The combustion and exhaust emission characteristics of biodiesel fuel were studies using common-rail diesel engine with four cylinders. For the biodiesel blended fuel, it was revealed that higher injection pressure is needed to achieve the same injection rate at the higher mixing ratio. The spray tip penetration of biodiesel fuel was much the same with those of diesel. The atomization characteristics of biodiesel were inferior to conventional diesel fuel due to high viscosity and surface tension. The peak combustion pressures of both fuels were increased with advanced injection timing and the combustion pressure of biodiesel fuel is higher than that of diesel fuel. As the pilot injection timing is advanced to the TDC, the dissimilarities of both fuels combustion pressure are reduced. It can be also founded that the pilot injection can enhance the deteriorated spray and combustion characteristics of biodiesel fuel caused by physical fuel properties.

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