## 3 Results
### In-Cylinder Pressure
The combustion stages of both the diesel and DF combustions are presented in Figure 6.  
As shown in Figure 6, the ID time (stage A-B) in both the diesel and DF modes were almost the same. This implies that introducing CH\({}_{4}\) gas to the engine cylinder during the intake stroke of the engine has no significant influence on the in-cylinder temperature at the end of the compression stroke, the moment when liquid fuel (diesel oil) is injected. This was also confirmed by the in-cylinder temperature diagram shown in Section 3.2. At the start of the combustion (S.O.C), the RoHRs in the diesel and DF modes were almost the same (stage B-C). This is because the combustions inside the engine cylinder in both diesel and DF modes were controlled by the premixed combustion phase of diesel fuel. However, after this phase of combustion, the RoHRs in the diesel and DF modes were totally different. Following the premixed combustion phase (stage B-C), the combustion inside the engine cylinder in the diesel mode was transferred to the diffusion combustion phase (stage C-D), followed by the late combustion stage (stage D-E). Due to the fuel injection process ending at 380 CAD, the combustion was almost finished at around 400 CAD. Most of the energy was released in a shorter duration, increasing the peak pressure in the diesel mode. In another manner, in the DF mode, after the premixed combustion process of the diesel pilot fuel (stage B-C), the combustion inside the engine cylinder was transferred to the premixed combustion of the NG-air premixed mixture within the engine cylinder, due to the spread of the pilot flame (stage C-D'). Since the fuel and air were premixed, the local concentration of the reactant species in a premixed combustion system was lower than that in a diffusion combustion system. This led to a slower reaction rate or a lower RoHR, as can be observed in Figure 6. The slower reaction rate led to a longer combustion duration. The longer combustion duration creates a higher in-cylinder pressure in the combustion late stage in the DF mode, in comparison to the diesel mode, as shown in Figure 6.  
Figure 7 presents the peak pressure in the engine cylinder in all simulation cases. The simulation result showed a slight reduction in the peak pressure as the SA of the injector increased from 145 to 160\({}^{\circ}\) in both the diesel and DF modes. This may be due to the increase in the fuel-air mixing quality when the injector SA increases. As the injector SA increases the interaction region between the injected fuel and the charge air increases, accordingly. This increased the fuel-air mixing quality. The increase in the fuel-air mixing quality reduced the ID stage and thus the peak pressure in the engine cylinder when the fuel was burnt. [PERSON] et al. [20] and [PERSON] et al. [23] also revealed in their study that the smaller SAs (narrow SAs) produced a higher maximum combustion pressure and heat release rate than the wider SAs. The reduction of the in-cylinder peak pressure in the DF mode of the NG-diesel DF engines, compared to the diesel mode has also been reported in previous studies [8; 9]. The powers of the engine cylinder in all simulation cases are presented in Figure 8. As a result of the slight increase in the peak pressure as the SA of the injector decreased from 160 to 145\({}^{\circ}\), the engine cylinder power also had the same increase trend in both the diesel and DF modes. Compared to the originally designed injector SA of 155\({}^{\circ}\), the SA of 150 and 145\({}^{\circ}\) increased the cylinder power by approximately 0.062 and 0.144%, respectively, while the SA of 160\({}^{\circ}\) decreased the cylinder power by approximately 0.123% in the diesel mode. In the DF mode, the SA of 150 and 145\({}^{\circ}\) increased the cylinder power by approximately 0.165 and 0.288%, respectively, while the SA of 160\({}^{\circ}\) decreased the cylinder power by approximately 0.226%, in comparison to the originally designed injector SA of 155\({}^{\circ}\).