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

- Simulated biogas using CH4 and CO2 is supplied to a micro co-generation SI engine.
- Combustion period and methane consumption increase with CO2 content generally.
- CO2 content at higher load with stoichiometric mixture improves engine efficiency.
- CO2 content appears effective in reducing NOX emissions in general.
- Thermal efficiency improves and NOX reduces with using a lean burn strategy.

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 (NOX) emission, and moreover, the use of biogas with the stoichiometric air-fuel ratio appears effective in reducing NOX emissions and can improve the fuel economy at higher loads.