## 3 Results and Discussion
### Injection Rate Characteristics  
The fuel injection process in a diesel engine plays a crucial role in the in-cylinder combustible mixture formation, ignition, combustion, and formation of harmful emissions. Figure 3a,b present the volume (a) and mass (b) injection rates as a function of time after injector energizing for diesel fuel (DF), fuel blends RO25, RO50, RO75 and pure (100%) rapeseed oil (RO). The flow rates of the fuels are recorded for injection pressures of 24.0, 40.0, 60.0 and 80.0 MPa at a constant back pressure of 4.0 MPa and the respective energizing times selected within an interval of 3.30-1.10 ms to ensure the same fuel volume injected per cycle. The peak volumetric injection rates of RO and its blends with DF were lower compared to diesel fuel. By injection of the same amount of fuel, the peak injection rates of RO were 22.0, 20.6, 11.3 and 9.1% lower with a group of fuel blends RO25, RO50 and RO75 positioned in decreasing flow rate order between DF and RO for the injection pressures of 24.0, 40.0, 60.0 and 80.0 MPa, correspondently (Figure 3a). The changes in flow rate, caused by the widely dissimilar properties of RO, impact the entire injection process. As the injection pressure increases, the maximum injection rate also increases, and the time to reach the maximum injection rate was advanced for all fuel tested. A similar effect on the injection rate of such fuel properties as density and viscosity was also found by other researchers who studied the characteristics of FAME injection [17; 18; 21]. As shown in Figure 3, the injection rate of diesel fuel exhibits the steepest rising slope at the beginning of injection, while RO shows a smoother injection rate shape, potentially due to their viscosities slowing down the needle lift process, as also observed by [PERSON] et al. [6]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & \multicolumn{4}{c}{**Standard Deviation, mm**} \\ \hline
**Injection Pressure** & **DF** & **RO25** & **RO50** & **RO75** & **RO** \\ \hline
24.0 MPa & \(\pm\)1.145 & \(\pm\)0.960 & \(\pm\)0.665 & \(\pm\)0.698 & \(\pm\)0.716 \\ \hline
40.0 MPa & \(\pm\)1.196 & \(\pm\)0.864 & \(\pm\)1.121 & \(\pm\)0.743 & \(\pm\)0.785 \\ \hline
60.0 MPa & \(\pm\)0.826 & \(\pm\)0.718 & \(\pm\)0.839 & \(\pm\)1.094 & \(\pm\)0.873 \\ \hline
80.0 MPa & \(\pm\)0.735 & \(\pm\)1.124 & \(\pm\)0.824 & \(\pm\)0.677 & \(\pm\)0.670 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The overaged standard deviation of the spray tip penetration. Figure 3: The fuel volumetric (**a**) and mass (**b**) flow rates as a function of time after injector energizing for injection pressures of 24.0, 40.0, 60.0, and 80.0 MPa. All injection delay data fall within a range between 0.3 ms and 0.47 ms. The injection delay is defined as the time interval between the start of the energizing signal and the start of fuel injection from the nozzle. It can be seen in the graphs, the diesel fuel portions appear at first, followed in increasing order by fuel blends RO25, RO50, RO75 and RO (100%). A trivial decrease is observed with increased injection pressure, as higher fuel pressure provides a greater force to lift the needle. The actual injection durations for all tested fuels were approximately 30, 39, 48 and 54% longer than the energizing durations for the injection pressures of 24.0, 40.0, 60.0 and 80.0 MPa, correspondently. The injection lasts longer than the control pulse because, after the control valve closes, it takes some time for the fuel to fill the control chamber and generate enough force to press the nozzle needle to the seat. This delay is influenced by the fuel's density and viscosity as well as the injection pressure. It is important to stress that the impact of cavitation on internal flow and fuel flow rate may be less damaging when using dense and viscous vegetable oil rather than diesel fuel. [PERSON] et al. [35] reviewed literature from the past fifty years and highlighted the positive role of biofuels as a solution against the cavitation phenomenon.