## 3 Results and Discussion
### Effects of Diesel Pilot Injection
Figure 9: Effects of diesel injection on variations of temperature (**a**), NO\({}_{\text{x}}\) concentration (**b**), CO concentration (**c**), and soot volume fraction (**d**) with crank angle (Biogas M7C3-20% DME, \(n\) = 2400 rpm). Figure 10a,b present the effects of DME content on the variations of W\({}_{i}\), CO, NO\({}_{x}\), and soot concentrations with \(\phi_{\text{gas}}\). It can be seen from these figures, generally, at a fixed global equivalence ratio \(\phi\) when \(\phi_{\text{gas}}\) increased (i.e., \(\phi_{\text{die}}\) decreases) W\({}_{i}\) and NO\({}_{x}\) concentration went up while CO concentration and soot volume fraction decreased. Otherwise, the effects of DME content on the combustion properties and pollutant emissions were more significant at high \(\phi_{\text{gas}}\). This means that the addition of DME clearly improves the combustion characteristics of biogas over those of diesel because the properties of DME and diesel were not significantly different. Figure 10a shows that as the engine was fueled with biogas M7C3-40% DME, W\({}_{i}\) increased by 6% at \(\phi_{\text{gas}}\) = 0.77 but increased by 11% at \(\phi_{\text{gas}}\) = 0.97 as compared to biogas fueling mode. The reduction of CO emission was 15% and 25% at \(\phi_{\text{gas}}\) = 0.77 and \(\phi_{\text{gas}}\) = 0.97, respectively, at switching from biogas dual-fuel mode to biogas-DME hybrid dual-fuel mode with 40% DME in the mixture with biogas (Figure 10a). As mentioned above, the combustion temperature was in line with DME content or/and \(\phi_{\text{gas}}\), and it affected the NO\({}_{x}\) formation. It can be seen in Figure 10b, the NO\({}_{x}\) concentration increased by 15% at \(\phi_{\text{gas}}\) = 0.77 and 30% at \(\phi_{\text{gas}}\) = 0.97 as there was switching from biogas dual-fueling mode to biogas-40% DME hybrid dual-fueling mode. This is because DME effects on biogas combustion properties were more substantial than those on diesel fuel. The effects of DME on the variation of soot volume fraction with \(\phi_{\text{gas}}\) were significant, as shown in Figure 10b. The addition of 40% DME to biogas M7C3 led to a reduction of 60% in soot volume fraction at \(\phi_{\text{gas}}\) = 0.77. The soot volume fraction decreased sharply with the increase of \(\phi_{\text{gas}}\). It reduced by 65% at \(\phi_{\text{gas}}\) = 0.77 as it shifted from biogas M7C3 fueling mode to biogas M7C3-40% DME fueling mode. The soot emission practically vanished in the exhaust gas of the biogas-DME PCCI engine. This confirms the experimental results of [PERSON] et al. [42]. The above results show that adding DME into biogas could improve the combustion characteristics of the biogas-DME hybrid dual-fuel engine. The reduction of diesel pilot injection is particularly interesting for CO and soot emissions control. Theoretically, the biogas-DME PCCI engine (without diesel pilot injection) is ideal. However, controlling ignition timing and heat release characteristics is still a challenge, as mentioned by [PERSON] et al. [12]. Thus, the substitution of diesel pilot ignition by DME pilot ignition in a biogas-DME hybrid dual engine is the optimal solution for both performance improvement and emissions reduction. The present simulation research results will be beneficial for practical application of biogas and DME on internal combustion engines. They are also essential to orient the experimental research of our future works on the topic. Figure 10: Variation of W\({}_{i}\), CO concentration (**a**); NOx concentration and soot volume fraction (**b**) with \(\phi_{\text{gas}}\) under effects of DME content in biogas M7C3 (\(n\) = 2400 rpm).