## 1 Introduction
However, it may contain traces of hydrogen sulfide (H2S), hydrogen (H2), ammonia (NH3), oxygen (O2), carbon monoxide (CO), nitrogen (N2) and water vapor (H2O) [28]. Before biogas can be used as alternative fuel [29], harmful and toxic compounds must be segregated. Purification technologies, such as physical and chemical absorption, biological desulfurization or membrane separation, are widely used to separate impurities from biogas. The methane volumetric percentage of ready-to-use biogas varies from 50 to 80 vol%. The characteristics of different biogas compositions are given in Table 1, based on data from [14; 30]. Changing SI engine fuel to gaseous fuels (liquefied petroleum gas (LPG), natural gas or biogas) increases ignition delay, which can lead to overheating of the exhaust valves. Without an adjustment of the engine spark timing, the temperature of exhaust gasses increases due to the subsequent combustion phase. However, this could be avoided if the engine ST is adjusted accordingly for each specific biogas composition [31]. CO2, contained in biogas, decreases biogas _LHV_ or the calorific value of the fuel, resulting in a reduction in the total amount of heat released from the complete combustion of biogas [32]; however, at the same time, it increases the BG methane number. The methane number (MN) of biogas indicates a resistance to knocking. For example, methane has an MN of 100, while hydrogen has an MN of 0 and a mixture of 70% CH4 and 30% hydrogen has an MN of 70. Hydrogen is a carbon-free fuel; therefore, using it as an additive reduces CO2 and HC emissions, and increases flame speed, rate of heat release (_ROHR_) and engine brake thermal efficiency [12], but at the same time, it reduces the critical compression ratio and knocking resistance [33]. Adding CO2 to methane increases the MN of the mixture. The MN is defined as 100 plus the percentage of CO2 added to the blend [34]; for example, a biogas of 60% methane and 40% carbon dioxide has an MN of 140 [35]. The CO2 concentration increases the resistance to knocking; therefore, biogas could be used in an engine with a higher compression ratio. Biogas with 40% of CO2 could be used in engines with a compression ratio up to 17.6:1, while biogas with 40% of methane could be used in engines with a compression ratio up to 14.4:1 [36]. Carbon dioxide reduces the energy density and combustion speed of biogas. Biogas containing 41 vol% of CO2 has a combustion speed of 0.25 m/s when LPG-0.38 m/s and CNG ~0.34 m/s [11]. As the CO2 concentration in BG increases, not only does the combustion speed decrease, but also the calorific value and flammability range, compared to natural gas or LPG. However, the CO2 raises the self-ignition temperature and MN of BG, which allows the use of biogas in engines with a higher compression ratio spark ignition (SI) without detonation [37]. Using biogas with a lower CO2 concentration increases \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Property** & **BG41** & **BG34** & **BG13** \\ \hline \multirow{4}{*}{Composition (vol\%)} & CH4–57\% & CH4–65\% & CH4–87\% \\  & CO2–41\% & CO2–34\% & CO2–13\% \\  & CO–0.18\% & CO–traces & CO–traces \\  & H2–0.18\% & H2–traces & H2–traces \\ Lower heating value (_LHV_) at 1 atm and 15 \({}^{\circ}\)C (MJ/kg) & 17.0 & 21.7 & 35.5 \\  & Density at 1 atm and 15 \({}^{\circ}\)C (kg/m\({}^{3}\)) & 1.20 & 1.15 & 0.68 \\  & Stoichiometric A/F (kg of air/kg of fuel) & 5.8 & 9.5 & 12.2 \\  & Leamer flammability limits (vol\% in air) & 7.5 & 7.5 & 7.5 \\  & Richer flammability limits (vol\% in air) & 14 & 14 & 14 \\  & Methane number & 142 & 135 & 110 \\  & Autotigation temperature (\({}^{\circ}\)C) & 650 & 650 & 650 \\  & Carbon/total mass ratio & 0.43 & 0.47 & 0.61 \\  & Oxygen/total mass ratio & 0.48 & 0.43 & 0.21 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Biogas properties at different compositions. the peak in-cylinder pressure and reduces the brake-specific fuel consumption, because of the increased \(LHV\) of biogas [14].