Optical diagnostic analysis of methane combustion performance and emissions while incorporating ammonia and multiple flames in a spark-ignition engine

The current state of internal combustion engine technology necessitates the use of cleaner fuels with lower carbon content in order to mitigate CO2 release into the atmosphere. Ammonia and methane show potential as alternatives to traditional hydrocarbon fuels due to their ability to reduce carbon emissions. This study investigated the use of different energy fractions of ammonia in a methane mixture to reduce carbon-based emissions. However, ammonia, being a less reactive fuel with a lower flame speed than methane, may negatively impact engine performance. To address this, the study employed a multiple flame generation approach to accelerate the combustion rate. The experiment was carried out on a single-cylinder four-stroke optical research engine. To initiate multiple flame fronts, a customized metal liner with four equi-spaced spark plugs was utilized. To compare the results with conventional spark-ignition conditions, one spark plug was placed at the center of the cylinder head. The findings indicated that increasing the ammonia fraction in the methane mixture resulted in decreased engine performance and unstable combustion with a single spark plug. However, the use of a multiple flame front strategy showed a significant reduction in engine instability, as well as higher engine power output and efficiency. Additionally, increasing the ammonia fraction led to a decrease in CO2, CO, and THC emissions. However, introducing ammonia into methane resulted in a notable increase in NOx emissions. Further increasing the ammonia fraction showed a decreasing trend in NOx emissions. Furthermore, high-speed natural-flame-luminosity imaging was used to observe flame propagation in various methane-ammonia dual-fuel combustion cases.