## 3 Results
### In-Cylinder Temperature and NO Emission  
The temperature distributions inside the engine cylinder at the TDC in all simulation cases are shown in Figure 11. From the figure, it is very easy to observe and visualize the effect of SA and IP on the target injection region mentioned in Section 3.1. This figure helps the readers to observe and visualize more clearly the positional relationship between the radial fuel injection regions, as well as between the fuel injection regions and the piston  
Figure 9: In-cylinder pressure and RoHR diagrams in the IP = 0.01 m case (**a**), IP = 0.015 m case (**b**), IP = 0.02 m case (**c**), and peak pressure in all cases (**d**). surface when adjusting the SA and IP in this study. Figure 11 also clearly shows regions of high temperature (\(\geq\)1800 K--red color regions) where NOx is normally formed. Figure 11: _Cont_. Figure 10: IMEPs of the engine in all simulation cases. Figure 12 shows the in-cylinder temperature diagrams, while Figure 13 shows a summary of the in-cylinder peak average temperatures in all simulation cases. The simulation results showed a lower peak temperature in the DF modes compared to the diesel modes. Moreover, the in-cylinder temperature during the expansion process of the engine in the DF modes was lower than that compared to the diesel modes. The lower cylinder temperature in the DF mode compared to the diesel mode may be due to the more homogeneous quality of the fuel-air mixture within the engine combustion chamber in the DF mode. As is known, the viscosity and molecular size of gaseous fuels are many times smaller than those of liquid fuel droplets. Therefore, when injected into engine cylinders, the quality of mixing with the air in the engine cylinders of gaseous fuels is also better than that of liquid fuels. The higher homogeneous quality within the engine combustion chamber helped to distribute the temperature from the combustion throughout the engine cylinder more uniformly. As a result, this consequently resulted in a reduction in both the peak temperature and in-cylinder temperature during the expansion process in the DF mode compared to those in the diesel mode, as shown in Figures 12 and 13. At each SA, the simulation results showed an increasing tendency in peak temperature as the IP increased in both diesel and DF modes. As mentioned above, as the IP increased, the target injection region shifted closer to the piston surface. Due to the U-shaped piston surface, the displacement of the target injection region closer to the piston surface may have increased the degree of swirl inside the engine combustion chamber. The increased vortex level increased the mixing quality of the air-fuel mixture, leading to an increase in the quality of combustion inside the engine combustion chamber. As a result, the peak temperature in the cylinder increased, as can be observed in Figure 13. Meanwhile, among SAs, the SA of 45 degrees produced the highest peak temperature in both diesel and DF modes. Figure 11: Temperature distributions inside the engine cylinder at the TDC in all simulation cases: (**a**) Diesel mode, (**b**) DF modes. Additionally, as is widely known, for both ideal and real gases, under the same volumetric conditions, the pressure and temperature of gases are linearly proportional to each other. This explains why the changes in peak pressure and peak temperature in the engine cylinder according to the changes in SA and IP had the same trend, as can be compared between Figures 9d and 13. Figure 14a,b show the NO mass fraction and specific NO emission in all simulation cases, respectively. Figure 14a shows that, at each SA, the NO mass fraction tended to increase as the IP increased. Meanwhile, among SAs, the SA of 45 degrees also produced the highest NO mass fraction with the same trend as in the in-cylinder peak temperature. In comparison to the diesel modes, the DF modes produced lower NO mass fractions due to the lower peak temperatures inside the engine cylinder. This trend was also reported in studies in [7,11,12].