Performance and emissions of a direct-injection diesel engine operating on emulsions of corn stalk bio-oil in diesel

The experimental bio-oil produced from corn stalk through a fast pyrolysis process is mainly composed of oxygenated organics and water, which makes it unsuitable for direct use as fuel. However, the use of bio-oil in diesel engines can be realized by developing stable emulsions of bio-oil in diesel. In this paper, two emulsions with 10% and 20% bio-oil by mass fraction in No. 0 diesel, represented by BO10 and BO20 respectively, were prepared using an ultrasonic emulsification method after determining the optimum hydrophilic and lipophilic balance value of the emulsifier for the emulsions. Then, the performance and emissions of an unmodified direct-injection diesel engine operating with the two emulsions were studied. The results show that the two emulsions display longer ignition delays, exhibit higher peak values of the premixed burning rate and pressure rise rate, demonstrate lower peak values of the in-cylinder pressure and combustion temperature and have a shorter combustion duration than No. 0 diesel does. In comparison with BO10, BO20 has a longer ignition delay, while it exhibits lower peak values of the premixed burning rate, the pressure rise rate, the in-cylinder pressure and the combustion temperature. In addition, the fuel economy of BO10 is comparable with that of No. 0 diesel, while the fuel economy of BO20 is poorer than that of No. 0 diesel. Nitrogen oxide emissions of the emulsions are lower but carbon monoxide emissions are higher than those of No. 0 diesel, and these trends are more marked with increasing bio-oil mass fraction. The hydrocarbon and smoke emissions of BO10 are lower than those of No. 0 diesel, but the hydrocarbon and smoke emissions of BO20 are higher.