## 3 Results and Discussion
### Environmental Indicators  
Increase of the CO\({}_{2}\) concentration in biogas increased the total CO\({}_{2}\) emission. At a constant ST of 26 CAD BTDC, CO\({}_{2}\) emission increased by 24.5%, 89.4% and 141.3% at 20 vol%, 40 vol% and 50 vol% of CO\({}_{2}\), respectively, in biogas (Figure 5). ST optimization reduces BSEC and requires less fuel. As a result, the CO\({}_{2}\) concentration in the exhaust gas decreased by 15.9% and 37.1% when the engine was operated using BG40 and BG50, respectively. Biogas is considered to be a short carbon cycle fuel (during the combustion of biogas, CO\({}_{2}\) is released into the atmosphere and then recaptured by newly growing biomass) [51]; therefore, the CO\({}_{2}\) content in biogas was not assessed, and only the CO\({}_{2}\) formed from combustion of methane was assessed (Figure 5, column chart, CH4 Constant ST--CO\({}_{2}\) emission from methane at constant ST, CH4 Optimal ST--CO\({}_{2}\) emission from methane at optimal ST). In this case, CO\({}_{2}\) emission increased by just 3.1, 11.8 and 13.8% when the engine works on BG20, BG40 and GB50, respectively, and ST changed to the optimal value. Figure 4: Energy distribution. A—with optimal ST; B—with constant ST.  
Figure 5: Energy distribution. A—with optimal ST; B—with constant ST. The concentration of nitrogen oxides (Figure 6) in the exhaust gas decreased for all measuring points. Lower \(LHV\) of biogas decreased the in-cylinder pressure, while a decreased burning speed of biogas and air mixture reduced the in-cylinder temperature [39]. Consequently, the NO\({}_{\text{x}}\) concentration reduced by 31.2%, 86.5% and 93.6% with BG20, BG40 and BG50, respectively, when ST was 26 CAD BTDC. The constant ST led to a decrease in the temperature and pressure in the cylinder, as the combustion rate of biogas with a higher concentration of CO\({}_{2}\) decreased and shifted the peak pressure in the cylinder further away from the TDC. This is also visible from the exhaust gas temperature. The temperature increased by 0.22%, 1.01% and 2.50% (when comparing BG20, BG40 and BG50, respectively, with BG0) because of retarded ST and a higher amount of BG burning in the exhaust. However, optimal ST reduced the exhaust gas temperature by 0.80%, 2.05% and 4.08% as compared with the same measurement points (BG20, BG40 and BG50, respectively) and at 26 CAD BTDC. Reduced exhaust gas temperature indicates increased in-cylinder temperature and shifted the in-cylinder pressure closer to TDC. This is also confirmed by an increase in the NO\({}_{\text{x}}\) concentration (by 13.5%, 26.2% and 16.3% with BG20, BG40 and BG50, respectively, and optimal ST as compared with 26 CAD BTDC). Harmful carbon monoxide emissions (Figure 7) were reduced by 2.8%, 9.6% and 34.4% with BG20, BG40 and BG50, respectively, when changing the constant ST of 26 CAD BTDC to the optimal ST. The optimal ST increases the engine BTE, and less fuel is needed; therefore, more air could get inside the cylinder and a larger amount of biogas could burn completely [52]. Comparing BG0 with BG20 and BG40, the CO emission decreased by 12.6% and 10.1%, respectively (optimal ST). CO emission decreased due to the reduced volumetric efficiency of the engine, because the addition of CO\({}_{2}\) to an air-methane mixture limits the amount of methane that can get into the cylinder. Therefore, the amount of carbon involved in the combustion process decreases, and at the same time, less CO forms. Hydrocarbons (HC) emission increased by 6.7%, 16.7% and 43.3% at optimal ST when using BG20, BG40 and BG50, respectively. With a constant ST of 26 CAD BTDC, the HC emissions were reduced by 16.3%, 29.3% and 36.0% (with BG20, BG40 and BG50, respectively) compared with the same measurement points and optimal ST. Due to the constant ST (26 CAD BTDC), the exhaust gas temperature rose and reached the autoignition temperature of methane, resulting in post oxidation of HC [53].