## 3 Results  
### In-Cylinder Pressure  
The in-cylinder mean pressure and rate of heat release (RoHR) in all simulations are shown in Figure 6. The simulation results showed a slightly lower in-cylinder peak pressure in the DF mode, compared to the diesel mode. The lower peak pressure in the DF mode  
Figure 4: Comparison between the simulation and experimental results: (**a**) Cylinder power, (**b**) In-cylinder peak pressure, (**c**) NO emission, and (**d**) CO\({}_{2}\) emission. Figure 5: Mesh independence analysis results: (**a**) In-cylinder pressure, (**b**) In-cylinder temperature, and (**e**) CO\({}_{2}\) emission. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Mesh Resolution** & **Coarse** & **Medium** & **Fine** \\ \hline No. of faces of the 2D mesh at the TDC & 12,949 & 17,715 & 39,307 \\ No. of cells of the 3D mesh at the BDC & 586,796 & 882,620 & 1,593,732 \\ Calculation time & 24 h & 36 h & 92 h \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Mesh metrics and the calculation times. is owing to the lower amount of pilot diesel fuel (approximately 5% of the total supplied energy) injected directly into the engine cylinders to provide the ignition source for the NG-air premixed mixture. Typically, the combustion of diesel DIEs (working with diesel only) is divided into four stages: (1) ignition delay (ID) stage; (2) premixed combustion stage; (3) diffusion combustion stage; and (4) late stage of combustion. In these four stages of combustion, stages 1 and 2 play a critical role in the pressure rise rate (PRR) and thus the in-cylinder peak pressure of the engine. The longer the ID stage and premixed combustion, the higher the PRR and peak pressure. The combustion process of the engine in the DF mode, however, typically occurs in three stages [36]. The first stage begins at the time the diesel pilot fuel is injected directly into the engine cylinder, which already contained a gaseous fuel-air premixed mixture. Since the injected diesel droplets need time to receive heat from the surrounding premixed gas-air mixture for the heating and vaporization processes, the diesel pilot fuel will not ignite immediately but after a few seconds. This period is the ID time of the DF mode. During the ID time, the injected diesel oil droplets will mix with air inside the engine cylinder to form a diesel-air pre-mixed mixture. The ID stage finishes when the premixed diesel-air mixture auto-ignites under a high-enough temperature condition inside the engine cylinder. This stage of combustion, therefore, includes the premixed combustion of the premixed direct-injection-diesel air mixture and a small portion of the premixed port-injection-gas air mixture. However, because the gaseous fuel (CH\({}_{4}\)) has a higher activation energy, leading to a higher auto-ignition temperature, the premixed combustion of the premixed diesel air mixture is the main contributor. During this stage, the in-cylinder pressure and temperature rapidly rise due to the initiation of the diesel premixed combustion. The second stage begins immediately after this initial combustion of diesel pilot fuel owing to an ignition source available inside the engine cylinder. During this stage, the continuing injected diesel pilot fuel is immediately ignited. Thus, this stage involves the diffusion combustion of the direct-injection-diesel pilot fuel along with the initiation and flame propagation of the gaseous fuel due to the presence of the ignition source. In this stage, the diffusion combustion of the diesel fuel is mixing-controlled, while the premixed gas-air flame propagates throughout the engine cylinder from the ignition kernels. Lastly, the diffusion combustion of the remaining diesel fuel and the late combustion phase of the port-injection-gas fuel is completed in the 3 rd stage of combustion [37; 38; 39]. In the DF mode, only approximately 5% of diesel oil energy share was employed for the ignition, the ID stage and premixed combustion stages were thus very short. These reduced the peak pressure inside the engine cylinder.