## 3 Results and Discussion
### Energy Indicators  
Compressed natural gas (marked as BG0) was selected as a baseline for comparison of the experiment results. A change of the brake-specific energy consumption (BSEC) and brake thermal efficiency (BTE), depending on the changing biogas composition, is presented in Figure 3. As can be seen, the low concentration of CO\({}_{2}\) in biogas increased the BSEC by just 0.81% with constant ST and by 0.60% with an ST optimal for the mixture. Increasing the CO\({}_{2}\) concentration in biogas (40 and 50 vol%) increased the BSEC by 9.78 and 21.8% with a constant ST. Using biogas increased the BSEC, because of the low calorific value of the fuel mixtures with CO\({}_{2}\)[43]. The value of the biogas lower heat value decreased in the mixture with 40 vol% CO\({}_{2}\) by 64.8% and in 50 vol% CO\({}_{2}\) by 73.6%. However, at the same biogas concentrations (40% and 50% CO\({}_{2}\) by volume) and spark timing optimization, the BSEC increased by only 3.39% and 5.27% (which is 6.39% and 16.6% less than when using a constant ST) compared to the values for BG0. The optimized ST provides sufficient time for biogas to be burned, as biogas has a low combustion [44] and flame propagation \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Marking**} & **CO\({}_{2}\)**, **vol\%** & **CH\({}_{4}\)**, **vol\%** & **LHV\({}_{7}\)**, **MJ/kg** & **Methane** & **Stoichiometric** & **ST, CAD BTDC 1** \\  & **vol\%** & **vol\%** & **MJ/kg** & **Number** & **A/F Ratio** & **Constant** & **Optimal** \\ \hline BG0 & 0 & 100 & 50.0 & 100 & 17.2 & 26 & 26 \\ BG20 & 20 & 80 & 29.7 & 118 & 10.2 & 26 & 30 \\ BG40 & 40 & 60 & 17.7 & 140 & 6.1 & 26 & 35 \\ BG50 & 50 & 50 & 13.4 & 151 & 4.6 & 26 & 40 \\ \hline \hline \end{tabular} \({}^{1}\)CAD BTDC—crank angle degrees (CAD) before top dead center (BTDC).  
\end{table} 
Table 4: Biogas properties and ST cases. speed [45]. ST also ensures higher engine efficiency and an increase in the engine power (by 5.5% for BG40 and by 11.2% for BG50 compared with the same measurement points at 26 CAD BTDC) [46]. Engine brake thermal efficiency (BTE) decreased while adding CO\({}_{2}\). Comparing BG20 and BG0, BTE decreased by 0.8% and 0.6% in cases of constant ST and optimal ST, respectively. A further increasing CO\({}_{2}\) concentration in biogas lowered the BTE by 8.9% and 17.9% at 40 vol% and 50 vol% of CO\({}_{2}\), respectively, in case of 26 CAD BTDC ST. BTE decreased due to the increased combustion duration (CD) of biogas [47], and shifted maximal in-cylinder pressure further from top dead center (TDC). The end of combustion also shifts further from the TDC, leading to higher exhaust losses. Optimization of ST and shifting maximum in-cylinder pressure closer to TDC ensured a lower drop of BTE (comparing to BG0), by 3.27% and 5.00% (5.63% and 12.9% lower, respectively, as compared to the constant ST) at 40 vol% and 50 vol% of CO\({}_{2}\), respectively. At optimized ST, a decrease of BTE occurred due to the increased fuel flow (BG40-165%, BG50-246%) and deteriorated volumetric efficiency of the engine [48]. The changes in the energy distribution of the SI engine working on biogas are presented the Figure 4. While engine working on BG0, the largest energy part is used to generate power (35.5%), adding CO\({}_{2}\) prolonged the combustion duration and increased the exhaust losses [49]. A total of 20 vol% of CO\({}_{2}\) increased the exhaust losses by 0.7% and 1.6% in case of optimal ST and constant ST, respectively. Further increasing the CO\({}_{2}\) concentration in biogas and keeping the ST constant (26 CAD BTDC) increased the exhaust losses by 3.7% and 8.7% at 40 vol% and 50 vol% of CO\({}_{2}\), respectively. In this ST case, the heat losses rose by 5.9% and 10.9% at 40 vol% and 50 vol% of CO\({}_{2}\), respectively; because more carbon dioxide needed to be heated, combustion occurred in larger cylinder volume and heat of the combustion was transferred to a greater surface area of the cylinder [50]. Adjusting the ST reduced the head and exhaust energy losses.