## 3 Results and Discussion
### Enhancement of Fuel Ignition
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \multirow{2}{*}{**Composition No.**} & \multicolumn{6}{c}{**1 st Group**} & \multicolumn{6}{c}{**2 nd Group**} \\ \cline{2-11}  & **21** & **22** & **23** & **24** & **25** & **31** & **32** & **33** & **34** & **35** \\ \hline \multicolumn{11}{c}{Basic composition} \\ Coal & 40\% & 40\% & 40\% & 40\% & 40\% & 40\% & 40\% & 40\% & 40\% & 40\% & 40\% \\ Water & 45\% & 45\% & 45\% & 45\% & 45\% & 20\% & 20\% & 20\% & 20\% & 20\% \\ Rapeseed oil & 15\% & 15\% & 15\% & 15\% & 15\% & 40\% & 40\% & 40\% & 40\% & 40\% \\ \hline \multicolumn{11}{c}{Addition of HFL to the basic composition} \\ Gasoline & – & 5\% & – & – & – & – & 5\% & – & – & – \\ Kerosene & – & – & 5\% & – & – & – & 5\% & – & – \\ Diesel fuel & – & – & – & 5\% & – & – & – & 5\% & – \\ Formic acid & – & – & – & 5\% & – & – & – & – & 5\% \\ \hline \end{tabular}
\end{table} 
Table 5: Compositions of composite fuels with the addition of flammable liquids. Figure 7: _Cont._  
Figure 7: Typical video frames of ignition and combustion of composite fuel droplets at \(T_{0}\) = 20 \({}^{\circ}\)C, \(T_{g}\) = 900 \({}^{\circ}\)C.  
Meanwhile, for the compositions of the 1 st group that contain 15% of the main combustible liquid, the addition of even 5% HFL reduces the ignition delay times by 35-65%. The greatest decrease is found for the composition containing 5% diesel fuel due to its physico-chemical characteristics. Diesel fuel, in comparison with other flammable liquids, has the lowest vaporization heat (Table 6). Therefore, a vapor-gas mixture in the vicinity of a fuel droplet forms faster, which reduces the ignition delay time. The addition of formic acid (No. 25) to the initial fuel composition of the 1 st group containing 15% rapeseed oil and 45% water has a less significant effect (compared to other HFL)--the ignition delay time is reduced by only 1.5 times. This is due to the fact that formic acid has the highest vaporization heat (Table 6) compared to other HFL, but its value is still less than that of rapeseed oil. As a result, the formation of the vapor-gas mixture is faster than that of composition No. 21, but slower than that of compositions No. 22-No. 24, which is the reason for the difference in ignition delay times. The burnout times of composite fuels (Figure 9) depend on the combustion mechanisms. Figure 8: Ignition delay times of composite fuel droplets with different HFL additives. Figure 9: Burnout times of composite fuel droplets with different HFL additives. Dispersion of composite fuel droplets significantly reduces the burnout time, since exothermic reactions are more intense and proceed in a larger region (Figure 10). Previous studies [37] show that composite liquid fuels containing 40% oil in their composition are guaranteed to disperse during burnout. HFL were added to the fuel composition to intensify this phenomenon. However, the addition of 5% HFL to the compositions of the 2 nd group containing a relatively large amount (40%) of the main combustible liquid (No. 31-No. 35) does not have a significant effect on enhancing the fuel droplet dispersion. The burnout times change by 5-15% (Figure 9) and the size of the burnout region by 1-5% (Figure 10). Meanwhile, the addition of HFL to the compositions of the 1 st group (No. 21-No. 25) with a low content (15%) of rapeseed oil has a significant effect on the dispersion intensification. The addition of 5% diesel fuel reduces the burnout time by 40% due to an increase in the average diameter of the burnout region by a factor of 1.7 (Figure 10). This is due to the fact that most of the used HFL (gasoline, diesel, kerosene) is dispersed (Table 6) and, therefore, increases the dispersed component of the fuel mixture and enhances the dispersion mechanism [37].