**3.1 Direct use or blending**

In beginning of 1980, there was a considerable discussion about the use of vegetable oil as a fuel. The concept of using food as a fuel was explained in 1981 by Bartholomew, demonstrating that petroleum should be the alternative fuel for combustion rather than the vegetable oil. Direct use of vegetable oils has been considered impractical and not satisfactory for both direct and indirect diesel engines. The high viscosity, free fatty acid content, acid composition and the formation of gum due to polymerization and oxidation during storage and combustion are the obvious problems.

Ma et al. [13] highlighted two severe problems such as incomplete combustion and oil deterioration associated with the direct use of vegetable oil as a fuels. Therefore, it will be significant to dilute the vegetable oils with some materials such as diesel fuels, ethanol or solvents to reduce the density and viscosity of vegetable oils.

Bilgin et al. [14] reported that 4% ethanol addition to diesel fuel increased the brake torque, brake thermal efficiency and brake power while decreasing the consumption of brake specific fuel. As the ethanol boiling point is less than the diesel fuel, ethanol could assist the process of combustion through an unburned blend spray.
