## 3 Results and Discussion
### Effects of Different DMG Substitution Ratios on the Dual-Fuel Engine
#### 3.2.2 Combustion Characteristics  
The effects of different DMG substitution ratios on in-cylinder combustion are depicted in Figure 4.  
Figure 4a illustrates the impact of the DMG substitution ratio on the in-cylinder pressure of the dual-fuel engine. As shown in the figure, the peak in-cylinder pressure and the rate of pressure rise exhibit an increase with an increase in the DMG substitution ratio. Specifically, the original engine records a peak in-cylinder pressure of 16.32 MPa, occurring at 8.8\({}^{\circ}\) crank angle (CA). However, for a 20% DMG substitution ratio, the peak cylinder pressure rises to 19.74 MPa, marking a 21.96% increase. The corresponding crank angle is also delayed to 10.4\({}^{\circ}\) CA.  
Figure 4b demonstrates the effects of different DMG substitution ratios on the in-cylinder average temperature of the blend combustion. In the figure, the peak temperature and the temperature rise rate increase as the DMG substitution ratio increases. The in-cylinder peak temperatures for the 0%, 10%, 15%, and 20% DMG substitution ratios are 1710.5 K, 1747.0 K, 17183.2 K, and 1824.9 K, respectively, and their corresponding crank angles are 41.7\({}^{\circ}\) CA, 37.1\({}^{\circ}\) CA, 34.6\({}^{\circ}\) CA, and 30.5\({}^{\circ}\) CA, respectively.  
The increased in-cylinder pressure and temperature result from the main components of the DMG, i.e., hydrogen, CO, and gaseous methanol, which all have faster burning speeds and higher combustion temperatures than diesel. When the DMG substitution ratio increased, the premixed combustion was enhanced and promoted the isochoric combustion rate.  
The impact of the DMG substitution ratio on combustion can be better understood by simulating the distribution of in-cylinder temperatures (the simulated temperature distribution for the crank angle is shown in Table A1 in Appendix A). The simulation results regarding temperature indicated that the incorporation of DMG had a significant positive impact on achieving more homogeneous combustion.  
Figure 5a shows the heat release rate against crank angle for different DMG substitution ratios. It is observed that the maximum HRR increases as the DMG substitution ratio increases.  
The combustion quality within the cylinder can be characterized by the parameters CA10-90 and CA50. CA10-90 represents the crank angle duration between 10% and 90% of the total fuel's heat release, while CA50 denotes the crank angle at which 50% of the  
Figure 3: Effects of different substitution ratios on ITE and IMEP.  
total fuel's heat release occurs. In Figure 5b, variations in CA10-90 and CA50 with the crank angle are depicted for different DMG substitution ratios. The figure illustrates that the addition of DMG leads to a reduction in both CA10-90 and CA50. This can be attributed to the enhanced premixing ratio resulting from the increased DMG content. The improved premixing promotes a higher combustion rate, ultimately leading to a shorter combustion duration.  
Figure 4: Effects of different DMG substitution ratios on in-cylinder combustion: (**a**) Pressure; (**b**) temperature.