Fig. 3. Possible results of the collision of two drops at ambient temperature T  = 300 K and pressure p = 1 atm \cite{Qia97}.
From the point of view of biofuels, it is important the size of the drop during spraying depends mostly on their physical and chemical properties, density, surface tension, viscosity. These properties of fuels are the main causes why the different fuels form different properties of air-fuel mixtures in engine cylinder and why the combustion properties are different. The fuel drop size is crucial during the combustion of air-fuel mixture by affecting directly the combustion efficiency and engine exhaust gas.