Combustion characteristics and NO_X emissions of biogas fuels with various CO₂ contents in a micro co-generation spark-ignition engine

Biogas is a renewable alternative fuel for internal combustion engines that has several advantages over conventional fuels, including lower costs and reduced levels of harmful emissions. In particular, it exhibits a neutral recirculation loop for carbon dioxide (CO₂), which is one of the main causes of global warming. In this study, we investigated biogas fuels with various compositions using a micro co-generation engine system. The ratio of methane to CO₂ and engine load were varied, and the intake air and fuel flow rates were controlled to change the equivalence ratio. The results show that for a given engine load, the ignition delay and combustion period increased with CO₂ content, and the combustion speed decreased. The fuel consumption increased slightly with CO₂ content; however, the thermal efficiency improved using a lean burn strategy, resulting in lower nitrogen oxide (NO_X) emission, and moreover, the use of biogas with the stoichiometric air–fuel ratio appears effective in reducing NO_X emissions and can improve the fuel economy at higher loads.