## 4 Conclusions  
In this study, the effect of different biogas compositions on spark ignition engine energy, environmental and combustions parameters was investigated under different spark timings with a 15% opened throttle, at 2000 rpm and with a stoichiometric air and biogas mixture. The following conclusions can be made from the collected and analyzed experiment results:  
1. BSEC rises as the CO\({}_{2}\) concentration in biogas increases, because the carbon dioxide reduces the biogas lower heat value (BG40--by 64.8% and BG50--by 73.6%) and reduces the combustion and flame propagation speed. Optimal ST compensates for the low combustion speed of biogas and BSEC increased by 6.39% and 16.6% for BG40 and BG50, respectively, compared to constant ST.
2. Engine brake thermal efficiency decreases when adding CO\({}_{2}\). Additionally, BG40 and BG50 increase the amount of fuel flow, and therefore, they reduce the volumetric efficiency of the engine. Carbon dioxide increases the combustion duration and flame propagation time; therefore, it increases the exhaust losses. For a constant ST of 26 CAD BTDC, exhaust losses increase by 1.6%, 3.7% and 8.7% with BG20, BG40 and BG50, respectively, compared to BG0. At the same time, heat losses increase due to the increased heat transfer time to the cylinder walls. Optimal ST shortened the combustion duration and increased the combustion speed, therefore, the time for heat transfer is reduced. In this case, heat losses are reduced by 8.6% and exhaust losses by 5.4% when comparing BG50 with different ST cases. 3. Biogas is considered a short carbon cycle fuel; therefore, only CO\({}_{2}\) formed from the combustion of methane is assessed. In the optimized ST case using BG20, BG40 and BG50, CO\({}_{2}\) emission increases just by 3.1%, 11.8% and 13.8%, respectively, compared to BG0. The emission raised due to increased BSEC and fuel consumption, because a larger amount of fuel with lower LHV is needed. 4. The concentration of NO\({}_{x}\) decreases in all measuring points. Lower \(LHV\) of biogas decreases the in-cylinder pressure, fuel burning speed and in-cylinder temperature. Constant ST reduced the NO\({}_{X}\) emissions by 13.5%, 26.2% and 16.3% more than optimal ST (using BG20, BG40 and BG50), because the peak in-cylinder pressure moves further away from TDC and more fuel burns in the exhaust. This is confirmed by the increase in the exhaust gas temperature (0.22%, 1.01% and 2.50% with BG20, BG40 and BG50, respectively, compared with BG0). 5. Increasing the CO\({}_{2}\) concentration in biogas decreases the combustion speed and increases the combustion delay. Therefore, the peak in-cylinder pressure moves further from TDC and the maximum in-cylinder pressure drops by 9.6%, 30.3% and 40.7% when comparing BG20, BG40 and BG50, respectively, to BG0 at 26 CAD BTDC. The optimal ST moves the peak in-cylinder pressure in the range of 12-14 CAD ATDC and increases the maximum value of the in-cylinder pressure. 6. The optimal ST increases the 10% of MBF time due to the combustion at a higher cylinder volume and distance from air and fuel molecules; therefore, the combustion speed decreases. In case of 90% MBF time, the combustion duration is reduced by 8.4% and 10.6% (BG40 and BG50, respectively, compared to constant ST), because the main fraction of combustion occurs closer to TDC and fuel burns at a lower volume and at a higher speed. 7. The pressure rise decreases at a constant spark timing for all CO\({}_{2}\) concentrations, because the flame formation phase extends and prolongs the combustion duration of BG. The maximum pressure rise value shifts further from TDC and the main fraction of biogas burns in a larger cylinder volume. The optimal ST increases the BTE of the engine and the combustion parameters; therefore, the PR increases by 40.1% and 48.4% compared to BG40 and BG50, respectively, at 26 CAD BTDC. 8.