## 3 Results and Discussion
### Mixture Preparation and Combustion Analysis
The gas temperature reaches the maximum value as the combustion is complete. Figure 3: Variation of temperature and pollutant concentrations with crank angle during the combustion and expansion process (M7C3-30% DME, \(n=2000\) rpm, \(\phi\) 0.83/1.00). **(a)** contours of temperature, HC, Diesel, CO, NOx concentrations and soot volume fraction at TDC; **(b)** variation of total fuel concentration and diesel concentration; **(c)** variation of in-cylinder pressure and heat release rate; **(d)** variation of T, NOx, CO, soot volume fraction with crank angle. The results of Figure 3d reveals that the CO was generated at the beginning of the combustion process. The concentration of CO increased gradually and reached its maximum value at about 380 \({}^{\circ}\)CA then decreased due to CO oxidation. It can be seen in Figure 3a that CO was concentrated mainly in the burnt mixture, behind the flame front. CO concentration reached a stable equilibrium value, which depended on the equivalence ratio and temperature. The formation of NOx was recorded later when the piston passed TDC, and the combustion temperature reached about 1250 \({}^{\circ}\)K, as shown in Figure 3d. The formation of NOx mainly occurred in the reaction region with a high temperature (Figure 3a). NOx concentration reached the maximum value at approximatively the same position with the peak of temperature. It then remained almost a stable value  
Figure 5a shows the effects of DME content in the mixture with biogas on in-cylinder pressure variation when the engine operated at 1200 rpm with the equivalence ratio of \(\phi\) 0.77/0.94. It can be observed that with a given equivalence ratio, the maximum pressure increased with the DME content. This confirmed the results of [PERSON] et al. [42]. DME substitution improved the heat value of the fuel and combustion velocity leading to an increase in engine efficiency. The simulation results showed that with the same fuel supply conditions and the same equivalence ratio, the engine cycle work increased by 6% and 10%, respectively, with 20% and 40% DME addition in mixture with biogas M7C3 at an engine speed of 1200 rpm. Under the same operating conditions, the increase of DME content led to a rise in temperature, which resulted in an increase in \(\text{NO}_{\text{x}}\) concentration in the exhaust gas (Figure 5b). The addition of 40% DME into M7C3 biogas led to an increase of \(\text{NO}_{\text{x}}\) concentration in the exhaust gas by 40% as compared to biogas only fueling mode. The concentration of soot depended mainly on the diffusion combustion of the diesel jet. The high temperature of the combustion products as there was increasing DME content in the mixture with biogas increased the soot production rate. However, in this case, the soot oxidation reaction rate became more intensive. Figure 5c shows that, as compared to M7C3  
Figure 5: The effects of DME content in the mixture with biogas on combustion characteristics and pollutants formation (M7C3, \(n=1200\) rpm, \(\phi\) 0.77/0.97). The effects of DME content on variation with crank angle of pressure (**a**), temperature and \(\text{NO}_{\text{x}}\) concentration (**b**), diesel concentration and soot volume fraction (**c**), and CO concentration (**d**). biogas fueling mode, M7C3-40% DME mixture fueling mode produced almost the same soot maximum concentration but only produced a 15% value of soot concentration in the exhaust gas. Since CO concentration was in the equilibrium state of the gas-water reaction, although the poor mixture \(\phi\) 0.77/0.94 was used, there was an amount of CO in the exhaust gas. Figure 5d compares the variation of CO concentration with respect to the crankshaft angle when the engine was fueled with neat biogas M7C3 and with mixtures M7C3-20% DME, M7C3-40% DME. In the case of the M7C3-40% DME mixture, the CO concentration in the exhaust decreased by 30% as compared to the neat biogas case. This was due to the increase in the combustion rate with DME content in the fuel mixture, which resulted in complete combustion.