## 3 Results and Discussion  
### Mixture Preparation and Combustion Analysis  
Figure 2a illustrates the contours of HC, oxygen concentrations, and airflow speed field in the biogas-DME hybrid dual engine at four positions of crankshaft angle 50\({}^{\circ}\), 330\({}^{\circ}\), 350\({}^{\circ}\) and 355 \({}^{\circ}\)CA without combustion. Biogas-DME in the gaseous state after injection diffused quickly into the airflow. The fuel distribution was not homogenous in the intake manifold. The rich mixture area was on the opposite side of the biogas-DME nozzle, resulting in a slightly higher fuel concentration in the cylinder towards the intake port. During the compression process, the fuel-air mixture distribution in the cylinder depended on the gas speed field, which was very strong at the top of the combustion chamber (Figure 2a). Therefore, at the end of the compression process, the mixture in the half-space of the combustion chamber toward the inlet port was slightly richer than the other side. After the closure of the intake valve, the amount of fuel and air introduced into the cylinder was unchanged; thus, the global equivalence ratio of the mixture remained stable until the starting of diesel injection for ignition (Figure 2b). When diesel fuel was injected, the area with high fuel concentration was concentrated locally in the jet axis (Figure 2a). Figure 2b shows that the density of diesel droplets (DPD) appears during the diesel injection phase and disappears at the end of the injection process. This indicates that diesel droplets evaporate rapidly due to the high gas temperature at the end of the compression process. When the evaporation of liquid fuel droplets is complete, the global equivalence ratio of the mixture reaches a new stable value. For dual-fuel mode combustion, the main combustion mixture was prepared prior to ignition; therefore, the remaining oxygen content in the combustion chamber was reduced compared to the CI diesel engine. This affected the ignition properties of the pilot injection in the dual-fuel combustion mode. Figure 2a shows that in the case of the biogas-DME hybrid dual-fuel engine, in the region of diesel jet, the oxygen concentration decreased by more than 5% compared to the remaining area and more than 15% compared to the oxygen component present in the original air. This is an essential factor that needs to be considered in the organization of the combustion process of a biogas-DME hybrid dual-fuel engine. The combustion chamber should be appropriately designed to ensure that the oxygen concentration around the diesel jet is enough for ignition. Figure 3a presents the contours of temperature, fuel concentrations, soot volume fraction, CO, and NOx concentrations at top dead center (TDC) during the combustion when the engine was fueled with M7C3-20 DME operating at 2000 rpm. At this moment, the diesel injection was completed, and diesel was mostly burnt. There was a small amount of unburnt diesel near the nozzle leading to a slightly rich mixture in this area (Figure 3a). Figure 2: Fuel-air mixing process in the biogas-DME hybrid dual-fuel engine (M7C3-30% DME, \(n=2000\) rpm, \(\phi\) 0.83/1.00, without combustion). (**a**) Contours of HC, O\({}_{2}\) concentrations and velocity field at 50, 330, 350, and 355 \({}^{\circ}\)CA. (**b**) Variation of global equivalence ratio \(\phi\) and diesel particles density (DPD) with crank angle. Figure 3b shows that the combustion process of the biogas-DME hybrid dual-fuel engine can be divided into two phases. During the first phase, the rate of fuel consumption is very high, and the diesel concentration sharply decreases, leading to a significant rise in the heat release rate curve, as shown in Figure 3c. This is due to the combustion taking place in the main space of the combustion chamber with considerable ignition energy of the pilot jet. During the second phase, the rate of fuel consumption reduces because the flame front propagates in the narrow kernel between the cylinder head and the piston head with a lean mixture.