## 3 Results and Discussion
### Spray Characteristics
[18] study on spray characteristics of hydrotreated vegetable oil and vegetable oil methyl esters shows a similar effect of fuel density on spray cone angle. After reaching the maximum during the early injection stage, the spray cone angle of all tested fuels slowly decreases and then remains stable. Even though the differences in the spray macroscopic characteristics diminish with the injection pressure increased to 40.0, 60.0 and 80.0 MPa, the spray cone angles of diesel fuel were high enough while the respective angles of viscous RO were 50.1%, 37.3% and 31.7% lower in a time interval from 0.5 to 1 ms after the start of injection, with spray cone angles of fuel blends distributed between them. A study on the effect of physical properties of residual fuel oil (RFO) on the spray parameters also showed that the relationship between the viscosity and spray cone angle is inversely proportional at higher viscosity values and increases monotonously within the range of diesel fuel at lower viscosity values [15]. The spray angles become smaller for a time longer than 1 ms and the differences in the cone angles between the fuels of various origins diminish to some extent, along with the disintegration of the spray occurring earlier after the SOI at the higher injection pressures. In the steady-state spray angle, the differences diminish and when the injection pressure is sufficiently high, the spray angle no longer depends on the injection pressure [11]. Figure 7 presents the projected spray area as a function of the spray length of diesel fuel (DF) and rapeseed oil (RO) for all injection pressures of 24.0, 40.0, 60.0 and 80.0 MPa at a back pressure of 4.0 MPa in the spray chamber. The larger the spray area of the fuel, the better the air-fuel mixing quality would be expected with a more widely distributed plume of fuel droplets and thus fuel-efficient and clean combustion. Although the spray area generally demonstrates a linear trend as a function of time for all the fuels [19], to emphasize the difference, the spray area per injected mass is consistently lower for rapeseed oil than for lighter diesel fuel produces at the same spray tip penetration length. Consequently, the test results confirm that rapeseed oil has poorer air-fuel mixing quality than conventional diesel under the same injection conditions. The spray area difference for the same spray length between diesel fuel and rapeseed oil is greater at higher injection pressure at constant back pressure (4.0 MPa) and the temperature (293 K) in the chamber. The spray area for diesel fuel is less dependent on the  
Figure 7: Spray area as a function of the spray length. injection pressure difference and remains within a narrow variation range of 460-500 mm\({}^{2}\) for the same (50 mm) penetration length, while the spray plume area for RO is consistently (31.8-20.0%) lower and changes within a wider range of 300-400 mm\({}^{2}\) under identical test conditions. The results obtained confirm that the spray cone angle is a crucial factor since, for diesel fuel, it is consistently higher, and the spray area is also larger than that of RO for the same spray tip penetration developed under non-evaporating conditions. Therefore, it can be assumed that rapeseed oil and its blends with diesel tend to produce poor initial air-fuel mixture distribution, resulting in a lower fuel portion premixed for ignition and rapid premixed combustion compared to conventional diesel fuel.