## 3 Results and Discussion
### Effects of Diesel Pilot Injection  
Figure 8a presents the variation of in-cylinder pressure with crank angles under the effect of diesel injection. The engine was fueled with biogas M7C3-20% DME, operating at 2400 rpm. The calculation was performed with three value of \(\phi_{\text{die}}\): 0, 0.10 and 0.20 with the same global equivalence ratio \(\phi=0.97\). \(\phi_{\text{die}}=0\) corresponds to the biogas-DME PCCI engine. The maximum in-cylinder pressure increased with the decrease of \(\phi_{\text{die}}\) (i.e., with the increase of \(\phi_{\text{gas}}\)). This was because of the rise in flame speed with gaseous fuel mixture compared to diesel fuel, which improved heat release rate, as shown in Figure 8b. In the crucial case when the amount of diesel injection drops to 0, the biogas DME hybrid dual-fuel engine can be considered a biogas-DME PCCI engine. Figure 8a shows that as it switched from dual-fuel mode to PCCI mode with the same global equivalence ratio, the in-cylinder pressure increased sharply, and the pressure peak tended to be closer to the TDC. This led to a loss in indicative engine cycle work due to the rise in pressure at the end of the compression process. As can be observed from Figure 9a, with a given global equivalence ratio \(\phi=0.97\), as \(\phi_{\text{die}}\) decreased from 0.20 to 0.10, the gas temperature increased by 80 \({}^{\circ}\)K. This was due to the laminar flame speed of mixture biogas-DME being higher than that of diesel, which made the heat release rate near the TDC increase. The increase in temperature resulted in a rise of NO\({}_{\text{x}}\), as shown in Figure 9b. NO\({}_{\text{x}}\) concentration increased by 55%, with a decrease of 10% in \(\phi_{\text{die}}\). However, the growth of biogas-DME content improved the homogeneity of the mixture leading to a reduction in CO concentration in the exhaust gas. Figure 9c shows that CO concentration dropped by 30% when switching from \(\phi\) 0.77/0.97 fuel mixture to \(\phi\) 0.87/0.97 fuel mixture. As mentioned above, soot concentration depends mainly on diesel injection. Thus, the reduction of diesel injection resulted in a reduction in soot volume fraction. Figure 9d shows that the soot volume fraction decreased by 75% as \(\phi_{\text{die}}\) dropped from 0.20 to 0.15. Figure 8: Effects of diesel injection on variations of in-cylinder pressure (**a**) and heat release rate (**b**) (Biogas M7C3-20% DME, \(n=2400\) rpm). As previously stated, although the amount of diesel injection for pilot ignition is small, it significantly affects the formation of pollutants due to the diffusion combustion characteristics of diesel fuel. The result in Figure 9a shows that the combustion temperature decreased by 80 \({}^{\circ}\)K as \(\phi_{\text{die}}\) increased by 0.1, leading to a reduction in NO\({}_{\text{x}}\) concentration. Figure 9b shows that NO\({}_{\text{x}}\) concentration increased by 20% when switching from hybrid dual-fuel mode with \(\phi\) 0.87/0.97 to PCII mode with the same global equivalence ratio \(\phi\) = 0.97. Figure 9c shows that when switching from dual-fuel mode with \(\phi\) 0.77/0.97 to PCII mode with the same global equivalence coefficient \(\phi\) = 0.97, the CO concentration in the exhaust gas decreased from 1.6% down to 0.2%, meaning a decrease of 80%. This can be attributed to the fact that without diesel injection, the gaseous fuel mixture in the combustion chamber was homogeneous; thus, the combustion took place completely, resulting in a reduction of CO emission. Similarly, without a pilot jet, the diffusion combustion does not exist in the combustion chamber; therefore, the conditions of soot nuclei formation are eliminated. As a result, the soot concentration in the exhaust gas of the biogas-DME PCCI engine was almost negligible compared to soot volume fraction \(f_{\text{v}}\) = 0.041 ppm in the case of a biogas-DME hybrid dual-fuel engine with \(\phi_{\text{die}}\) = 0.20 (Figure 9d).