## 3 Research Results and Discussion
### Combustion Indicators  
When using the BURN sub-software of AVL BOOST software, the use of the parameters measured during the experiment (operating cycle pressure in the cylinder \(p\) (Figure 10) and others) allowed for the determination of key combustion parameters (Figures 11-13). With the engine operating in the mode \(n\) = 2000 rpm, brake torque \(M_{B}\) = 45 Nm and SOI = 6 CAD BTDC, the maximum combustion pressure in the cylinder (72.8 bar) is reached using conventional diesel (Figure 10). The maximum combustion pressure of HVO fuels decreased by 3.3%, D40M60 by 6.8%, HVO40M60 by 10.9%, D40 BM60 by 7.6%, and HVO40 BM60 by 10.1%. All of the fuels reached the maximum pressure at 6-7 CAD ATDC.  
The maximum rate of heat release (37.2 J/CAD at 1 CAD ATDC) was achieved using conventional diesel (Figure 11). The maximum ROHR of HVO fuel decreased by 31% at 1 CAD ATDC, D40M60 17% at 1 CAD ATDC, HVO40M60 49% at 8 CAD ATDC, D40 BM60 19% at 1 CAD ATDC, and HVO40 BM60 48% at 8 CAD ATDC. Reduced heat release in the kinetic (premixed) combustion phase decreased the temperature and the intensity of NO\({}_{\rm x}\) formation. The combustion of HVO fuels can be characterized by a more intense fuel release in the diffusion phase. Methane gases contained in dual fuel burn at a lower intensity and significantly increase the ROHR in the diffusion phase, which in turn improves the combustion of soot. This is confirmed by the parameter describing the intensity (combustion shape parameter \(m_{p}\)) (Table 4).  
Figure 11: Rate of heat release in the cylinder depending on fuel composition.  
Figure 12: Temperature rise in the cylinder depending on fuel composition.  
Figure 13: Pressure rise in the cylinder depending on fuel composition.  
The maximum temperature rise for all types of fuel was reached at 1 CAD ATDC. The values of the temperature rise indicator correlate with ROHR. The maximum value was reached (73 \({}^{\circ}\)C/CAD) using conventional diesel (Figure 12). The maximum temperature rise of HVO declined by 29%, D40M60 by 24%, HVO40M60 by 56%, D40 BM60 by 22%, and HVO40 BM60 by 45%. However, HVO fuels create a higher temperature rise in the diffusion phase, while when using dual fuel, this effect of a longer combustion process is even more obvious.  
The maximum pressure rise (3.4 bar/CAD) was reached using conventional diesel (Figure 13). The maximum pressure rise of HVO fuels decreased by 31%, D40M60 by 22%, HVO40M60 by 46%, D40 BM60 by 23%, and HVO40 BM60 by 41%.  
The maximum pressure rise for all fuels was reached at TDC or 1 CAD ATDC. The value of the maximum pressure rise was also significantly affected by the replacement of conventional diesel with HVO biofuels and the use of dual fuel. Having reduced the maximum pressure rise, the load on engine parts and engine noise decreased.  
ROHR schedule (Figure 11) shows that the SOC (Start Of Combustion) timing (\(q_{SOC}\)) of all the analysed fuels differs very slightly and is 2-3 CAD BTDC. Combustion (\(\Delta q_{CD}\)) is the shortest using conventional diesel (50.7 CAD). When using HVO fuel, combustion duration increased by 1.8%, D40M60 by 4.5%, HVO40M60 by 7.7%, D40 BM60 by 10.1%, and HVO40 BM60 by 9.9% (Table 5).  
The ROHR and the combustion shape parameter (\(m_{v}\)) determined by numerical modelling revealed that having replaced conventional diesel fuel with HVO fuels, the maximum heat release shifts towards the end of combustion. The analysis of all the analyzed dual fuels (D40M60, HVO40M60, D40 BM60, and HVO40 BM60) ROHR and the combustion shape parameters determine that gas combustion is slower, and a significant share of heat is released closer to the end of combustion.