## 3 Results and Discussion
### Spray Characteristics  
Figure 4 shows the spray tip penetration of diesel fuel, rapeseed oil and the three DF-RO blends for different injection pressures of 24.0, 40.0, 60.0 and 80.0 MPa at a constant (4.0 MPa) back pressure in the spray chamber (not all points are plotted for better distinction of the curves). The time scale represents the time after the start of energizing, as such a technique allows for superimposing the start of injection (SOI) of all the fuels, regardless of whether the hydraulic delay of each fluid is dissimilar [26]. As the study intends to show the differences in spray tip penetration (STP) and cone angle caused by the vastly different physical properties of the fuels, the spray macroscopic characteristics for all the injection pressures start at the SOI point. As a result, the small hydraulic delay between the energizing impulse and the onset of spray tip penetration is not depicted on the graphs. Because all characteristics are time-adjusted and the first droplets of each fuel appear simultaneously, applying such a technique simplifies the data analysis. Analysis of the data showed that at the beginning of injection, the spray tip penetration of all tested fuels is nearly identical. [PERSON] et al. [11] tested the penetration of diesel and biodiesel and also revealed that during the early stages of spray formation, the penetration is quite similar for all fuels. One millisecond after the start of injection, the spray tip penetration was 26.9, 25.5, 14.0 and 7.4% longer for pure RO than for diesel fuel at the respective injection pressures of 24.0, 40.0, 60.0 and 80.0 MPa. The RO spray develops faster than that of the diesel because RO is dense and viscous, resulting in a larger droplet diameter and higher momentum to penetrate the compressed ambient gas. The spray tip penetrations of fuel blends RO25, RO50 and RO75 were in decreasing order between DF and RO (Figure 4). Whereas, as both the injection rate and the spray development increased at an injection pressure of 80.0 MPa, the changes in the spray tip penetrations diminished, making it difficult to detect significant differences in the spray development of various fuel blends. Figure 5 illustrated the spray evolution process of all tested fuels at two injection pressures. It can be seen that the difference in spray tip penetrations of the tested fuel jets is more noticeable at lower pressure. Additionally, it can be observed that the spray cone angle decreases with increasing rapeseed oil content in the fuel. The test results indicated that rapeseed oil, having the highest density among the tested fuels, achieved the longest spray tip penetration. However, as a trade-off, it produced the smallest spray cone angle, which may result in a more unevenly distributed air-oil mixture in the chamber (Figure 6). Figure 4: The spray tip penetration as a function of time after the start of injection. Figure 5: Spray evolution of diesel fuel, rapeseed oil and its blend: (**a**) at 40.0 MPa injection pressure and 2 ms after the start of injection; (**b**) at 80 MPa injection pressure and 1 ms after the start of injection. Figure 6: Development of the spray cone angle as a function of time after the start of injection. The spray cone angle of rapeseed oil at 1 ms after the start of injection was 17.6\({}^{\circ}\), which is 1.6 times smaller than the 28.2\({}^{\circ}\) diesel fuel produces for the injection pressure of 24.0 MPa (Figure 6). Spray angles developed within 0.5 to 1 ms after the SOI for fuel blends RO25, RO50 and RO75 were also in increasing order 6.4%, 13.5% and 20.2% lower than the diesel fuel develops under identical injection conditions. The smaller spray angle of RO with larger (viscosity, surface tension) and heavier (density) droplets experienced lower deceleration and produced higher momentum to overcome back pressure (Figure 4). Therefore, penetration length increased while the spray cone angle decreased with increasing RO content due to the higher density and viscosity of fuel blends causing poor atomization. [PERSON] et al.