## 2 Materials and Methods  
This section presents a description of the experimental setup and procedures performed in the experiments. The following methodology, equipment and test procedures have been used to achieve the purpose of the research. The measurements of the injection and spray characteristics and data analysis were performed for normal diesel as the reference fuel (DF), straight rapeseed oil (RO) and (DF75% + RO25%) RO25, (DF50% + RO50%) RO50 and (DF25% + RO75%) RO75 blends (by volume). The normal automotive diesel fuel (class 1) used in the study was produced by the refinery \"Orlen Lietuva\" and met the quality parameters specified in standard EN-590:2009 + A1. The composition of the diesel fuel was C/H = 0.8608/0.1299, with a residue 0.0093 containing traces of sulfur and water detected at the refinery's laboratory. The rapeseed oil used in the experiments was sourced from the RME production factory 'Rapsoila' in Mazeikaiai and met the quality requirements specified in standard DIN 51605:2020-11 for use in agricultural machinery [2]. The elementary composition of RO was as follows: 77.2% carbons, 11.9% hydrogen and 10.9% oxygen. RO includes complicated long-side chains of fatty acids. Its density is 10% and surface tension 60.5% higher, respectively, than those of diesel fuel. The viscosity of RO at 40 \({}^{\circ}\)C is even 17.5 times higher than that of diesel fuel. The main properties of RO, diesel fuel and their blends are listed in Table 2.  
The miscibility of RO with the diesel fuel is good to produce a stable mixture [32]; therefore, no co-solvents were used to avoid any contributing effect on the macroscopic spray characteristics. The fuel mixtures were prepared by mechanically mixing diesel fuel and rapeseed oil in the appropriate proportions. Figure 1 shows a schematic diagram and photo of the injection rate measuring system consisting of the fuel injection system and the injection rate measuring system. The fuel injection system consisted an electrically driven high-pressure pump, a rail, an injector, and an injection pressure control unit. A standard Bosch solenoid injector (0 445 120 141) with 0.24 mm 6 hole nozzle (DLA140P1790) was used for this study. The NI PXIe 1062Q system with NI-9751 C series module was used as a driver to control the injector's energizing duration. The injector was controlled by a peak current of 26.0 A and a hold current of 14.0 A. The fuel injection rates were measured and analyzed using an injection rate measuring system based on the Bosch method [33]. The measurement concept is based on measuring a change in pressure by the fuel injection into a long measuring tube filled with fuel. This pressure change is proportional to the fuel injection rate:  
\[\dot{m}=Const\cdot p(t), \tag{1}\]  
\[Const=\frac{A_{tube}}{a}=\frac{m}{\int p(t)dt} \tag{2}\]  
where  
\(\dot{m}\)--mass injection rate;  
\(A_{tube}\)--cross-sectional area of the measuring tube;  
\(a\)--sound velocity in the fuel;  
\(m\)--injected fuel mass;  
\(p(t)\)--pressure variation. \begin{table}
\begin{tabular}{l l l l l l} \hline
**Fuel Properties** & **Diesel** & **RO25** & **RO50** & **RO75** & **RO** \\ \hline Density at 15 \({}^{\circ}\)C, kg/m\({}^{3}\) & 831.1 & 851.8 & 872.5 & 893.3 & 914.0 \\ \hline Kinematic viscosity at 40 \({}^{\circ}\)C, mm\({}^{2}\)/s & 2.72 & 5.58 & 10.62 & 22.22 & 47.58 \\ \hline Surface tension, mN/m & 36.0 & 41.2 & 46.7 & 52.4 & 57.8 \\ \hline \end{tabular}
\end{table} 
Table 2: The main properties of the tested fuels. The pressure variation in the tube was measured using a Kistler type 6052C piezoelectric pressure sensor (Kistler) (measuring range 0-150 bar, \(<\)\(\pm\)0.5%) coupled to a Kistler charge amplifier module 5064. The fuel pressure at the injector inlet was measured using a high-pressure Kistler Inc. piezoresistive sensor 4067A2000 (measuring range 0-2000 bar, \(<\)\(\pm\)0.5%) the signal of which was being amplified by an amplifier module 4665. Both amplifier modules were mounted on the signals conditioning platform compact 2854A.