## 2 Materials and Methods  
### Description of the Experimental Test  
Laboratory equipment for single suspended droplet research, developed by the Odessa National University (Ukraine), was used as shown in Figure 1. Experiments were conducted in an optically accessible constant-volume combustion furnace. The radiation-supported heating conditions of a suspended droplet in a tubular furnace were used for the experiment. If too many experiments were carried out, the vapor pressure of the fuel in the furnace increased and had a negative effect on combustion. To avoid this effect, the air in the furnace was properly changed after every second experiment.  
As documented in Ref. [40], during the ignition delay phase, diesel fuel is subjected to an in-cylinder temperature ranging from 960 to 1000 K. The upper limit of this interval for biofuel may reach 1017 K. During the period of physical ignition delay at the afore  
Figure 1: Droplet combustion stand: 1—furnace, 2—thermocouple head assembly equipped with transmitter, 3—integrated computer imaging system, 4—compressed air unit, 5—temperature monitoring unit, 6—suspended droplet.  
mentioned in-cylinder temperature values, the fuel is atomized to form smaller droplets, then vaporized and mixed with the atmospheric air [40]. For carrying out an experiment, droplets with 2.2 mm diameter of pure and blended fuels were used. The alteration of a droplet radius over time was monitored using the integrated computer imaging system (see Figure 2). Depending on the surface tension characteristics of the tested fuel and the unique in-cylinder temperature range of the diesel engine, the most suitable temperature range for diesel fuel and RME drops was 930-1100 K. The surrounding conditions used in the laboratory during droplet combustion experiments were atmospheric pressure 1005 hPa, ambient temperature 20 \({}^{\circ}\)C and relative air humidity 47%.  
The compound of the B20 blend provided for laboratory droplet combustion experiments was prepared by applying graduated Blaubrand 5.0 mL pipettes (subdivision 0.05 mL, error limit 0.03 mL, Class AS) [28].  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Diesel Fuel** & **RME** \\ \hline Density at 15 \({}^{\circ}\)C, kg/m\({}^{3}\) & 835 & 884.3 \\ \hline Viscosity at 40 \({}^{\circ}\)C, mm\({}^{2}\)/s & 4.2 & 4.7 \\ \hline Cetane number & 51.0 & 55.0 \\ \hline Water content, mg/kg & 200 & 223 \\ \hline Methanol content, \% mass & – & 0.05 \\ \hline Total glycerol, \% mass & – & 0.51 \\ \hline Sulphur content, mg/kg & 6.9 (negligible) & negligible \\ \hline \hline \end{tabular}
\end{table}
Table 1: Properties of test fuels [28].  
Figure 2: Screenshots of the tracking results on the alteration radius of the DF droplet (\(d_{0}=2.2\) mm) in a two-dimensional space over time.  
The composition of the diesel and biodiesel fuels was determined at the Lithuanian Energy Institute using the standardized gas chromatography technique. The fatty acid composition in % of the tested samples was determined using Clarus 500 (Perkin Elmer, Waltham, MA, USA) gas chromatography (ISO 12966-4:2015 [43]). For chromatographic separation of compounds, capillary column Alltech AT-FAME (30 m long, 0.25 mm-0.25 \(\upmu\)m) was used.