## 1 Introduction
Due to the higher compression ratios of CI engines and high antiknock index of biogas, CI engines are often converted into an SI biogas engine by replacing its injectors with spark plugs and modifying the pistons [42; 43]. In this case, increased CR leads to higher engine performance and brake thermal efficiency (\(BTE\)). It was also found by [PERSON] and his team [44] that at higher compression ratios, engines operated with biogas do not lose power in their operating CR range, and at CR 13 a brake thermal efficiency of 25% was reported in the full load and wide open throttle (WOT) condition. Increasing the compression ratio for an SI engine running on biogas at WOT conditions improves brake power output and brake thermal efficiency, and reduces brake-specific fuel consumption (\(BSFC\)) [41]. Based on the experimental results, where SI biogas engines with different CR values between 10 and 14 were tested, the optimally achievable engine performance, \(BSFC\), and \(BTE\) under WOT conditions were observed at a compression ratio of 12 [45]. Due to different biogas feedstock and anaerobic process condition, biogas composition varies and has a different effect on the tested engine. Experiments showed that for the SI biogas engine, increasing methane concentration from 55 to 80 vol% increases the burning rate and temperature in the cylinder [46]. Higher compression ratios of 13 and the decreasing CO\({}_{2}\) concentration from 41 to 20 vol% confirmed that a low CO\({}_{2}\) concentration increases the pressure in the cylinder and flame speed, thereby increasing the thermal efficiency and performance of the engine [47]. Increasing CO\({}_{2}\) concentration by more than 50 vol% significantly retards ignition timing and reduces burning rate [48]. To increase the efficiency of the combustion process, various additives or fuel blends are used [49; 50]. Researchers found that adding titanium dioxide (TiO\({}_{2}\)) nanoparticles could increase engine \(BTE\) by 17%. However, before using TiO\({}_{2}\), more detailed studies of the long-term impact on engine component life are needed [51]. Therefore, often liquid or gaseous biofuels are used to increase \(BTE\) and reduce exhaust emissions. Researchers from various research institutes tested compressed natural gas (CNG), CH\({}_{4}\)[52], methanol, ethanol, butanol [53], acetone-n-butanol-ethanol, isopropanol-n-butanol-ethanol [54] and other alternative fuels. All of the fuels that contain carbon or hydrocarbons (HC) in a molecular composition will form CO\({}_{2}\) after combustion; therefore, the use of hydrogen is investigated. As stated by [PERSON] [55] and [PERSON] [56], adding hydrogen to a biogas engine increases the burning rate and extends the flammability range, and 10 vol% hydrogen causes a significant increase in burning stability. However, further increase in hydrogen concentration does not have such a noticeable effect on burning stability [57]. The addition of hydrogen increases the total heat transfer of the fuel by up to 20-30% when adding 30 vol% H\({}_{2}\)[58]. Testing biogas and hydrogen mixtures showed that H\({}_{2}\) reduces combustion duration but, at the same time, increases the possibility of detonation, and the ignition advance angle should be retarded [59]. Biogas, which contains inert CO\({}_{2}\), featured lower \(BTE\), a decrease in engine power, and extended combustion duration when compared to an identical SI engine running on natural gas [60]. Nevertheless, biogas is an eligible fuel, especially for stationary engines, because it is made from renewable sores. Moreover, hydrogen addition could increase the lower heat value (\(LHV\)) of the biogas and possibly the \(BTE\) of the engine. Therefore, the purpose of this paper is to investigate the influence of hydrogen addition on the energy and environmental performance of an SI biogas engine. In this study, a spark ignition engine running on a biogas-hydrogen mixture was tested. The biogas contained of 62 vol% CH\({}_{4}\) and 38 vol% CO\({}_{2}\).