Fig. 10. The dependency of the relative diameterdc/d of a drop that formed after the collision and the impact parameter B at two different colliding drop diameter ratios Δ (equation 4) (Δ1 = 0.5,Δ2 = 1.0) in case of four different fuel types.
Fig. 10 was obtained with the parameters v1 =v2 = 200 m/s and the physical parameters of the fuels correspond to the temperature of 90 °C. Fig. 10 shows that the drop size can differ in the air-fuel mixture at various values ofΔ . It can be concluded that the air-fuel mixture of fuels with high viscosity (for example, HVO) contains somewhat larger drops than the mixture of low viscosity fuel. At the same time, the drops of FAME fuel are in the same magnitude as gasoline and diesel fuel. In addition to viscosity, another important influencing factor is surface tension. At low values of B , the ratio dc/d is mostly determined by the fuel’s surface tension forces. If the value ofB is greater, then the interaction volume remains smaller, which means that the ratio dc/d is also greater.
Table 4 was prepared to illustrate better the breakout of fuel drops. The table gives the drop sizes of various fuels at various values of the impact parameter B . Table 4 exemplifies also a situation in which the colliding drops are equal. The physical parameters of fuels in Table 4 correspond to the temperature of 90 °C. In the first case (1), the value of Cv is negative for stretching separation, which means that the stretching separation does not occur. The positive value of Cv shows that reflexive separation takes place.
In the other cases (2-4) the reflexive and stretching separation of drops occurs. The main difference between the different cases is that when the impact parameter’s value is B  = 0.1, then the drop size of diesel fuel and gasoline is ~5 % smaller than that of HVO and FAME fuels. It can be deduced from here that the drop size in the air-fuel mixture of HVO and FAME fuels is somewhat greater than that of diesel fuel. Here the air-fuel mixture corresponds to the general knowledge, according to which the drop size in air-fuel mixtures of high viscosity fuels is greater. At the same time, it is not sure why the soot level of emission gas of biofuels is lower. If we presume that the use of FAME fuels results in lower soot levels in the exhaust gas mostly due to the oxygen content in the fuel, then what is the reason for the lower soot level of HVO fuel? In conclusion, it can be claimed that the drop size of biofuels in air-fuel mixture is somewhat larger. The approach of this article does not give the answer why the soot level in the engine’s emission gas decreases when HVO and FAME fuels are used. At the same time, the results illustrate that there are no important differences in the quality of air-fuel mixture. In order to account for the reduced soot level, it is necessary to study experimentally the breakout of drops in the fuel spray, the effect of oxygen content on the combustion of fuel and the effect of various fuel fractions to the combustion process.
Table 4. Drop’s diameter dc after collision and the value of separation coefficientCv in case of a collision of two drops with equal diameters.