## 3 Results
### In-Cylinder Temperature  
The mean temperatures inside the cylinder of the engine in all simulations are shown in Figure 9. The results showed a lower in-cylinder peak temperature in the DF mode, compared to the diesel mode. However, the in-cylinder mean temperature during the late stage of combustion in the DF mode was higher than that, in comparison to the diesel mode. The lower peak temperature and higher mean temperature during the late-stage combustion are interesting characteristics of the port-injection premixed combustion, in comparison to the direct-injection combustion. In the port-injection approach, gaseous fuels are injected into the intake port of the engine during the intake stroke and mixed with the charge, to form a premixed mixture inside the engine cylinder. As a result, this fuel-air premixed mixture is compressed during the compression stroke of the engine. Because of this feature, there is always enough time for the gaseous fuel and charge air to mix with each other to form a gas-air premixed mixture, prior to an ignition source being supplied for the ignition. Owing to the fuel-air premixed mixture being perfectly prepared, the combustion in the cylinder of the port-injection engines occurs more uniformly than in the direct-injection engines. The more uniform temperature distribution within the combustion chamber of the engine reduced the peak mean temperature, as shown in Figure 9. As the same as the in-cylinder temperature, the reduction of the in-cylinder mean temperature  
Figure 8: Cylinder power in all operating modes.  
Figure 7: In-cylinder peak pressure in all operating modes.  
in the DF mode of the NG-diesel DF engines, compared to the diesel mode has also been reported in previous studies [8,9].  
Figure 10 presents the temperature distributions inside the engine cylinder at the TDC in all simulation cases. The temperature contours point out the targeting point of the injection, the diffusion flame shape, and reaction zones inside the engine combustion chamber. The figure showed that a high-temperature combustion region appeared in the flame kernel caused by the fuel injector of the engine. This is because, in ICEs, the temperature of the fuel-rich zone is the highest in the entire engine cylinder, as reported in [40]. In addition, the figure pointed out that the injector SA of 155\({}^{\circ}\) and 150\({}^{\circ}\) produced the highest local peak temperatures in the diesel and DF modes, respectively.  
Figure 10: Temperature contour at the TDC in all simulation cases.  
Figure 9: In-Cylinder temperature in all operating modes.  
It is very interesting to observe the change in the diesel flame kernel size when changing the injector SA in the diesel mode. Similarly, the change in the flame curvature when changing the injector SA in the DF mode is also notable. In the diesel mode, when the engine ran with diesel oil solely, the diesel flame kernel tended to become bigger as the injector SA increased from 145\({}^{\circ}\) to 160\({}^{\circ}\). Meanwhile, in the DF mode, the curvature of the flame decreased when the injector SA increased from 145\({}^{\circ}\) to 150\({}^{\circ}\), and then reversed the curvature as the SA continued to increase to 155\({}^{\circ}\) and 160\({}^{\circ}\). These changes in the flame shape might relate to the spatial relationship between the injection axis and the piston and the cylinder head surfaces as the injector SA changed. It is clear that there will always be an interaction between the fuel injection beam and the combustion chamber wall when the fuel is injected into the engine combustion chamber. This interaction will obviously be affected by the relative position between the fuel injection beam inside the combustion chamber and the combustion chamber wall surface. Therefore, when the axis of the fuel injection beam changes, the shape of the flame is also changed, accordingly.