Improving the combustion performance of ammonia in a spark-ignition engine by mixing methane coupled with multiple spark ignition: An optical study

Ammonia (NH3) has recently emerged as a promising alternative fuel for both internal and external combustion engines. The combustion of pure ammonia does not produce unwanted pollutant emissions such as oxides of carbon, total unburned hydrocarbon (THC), and particulates. Additionally, ammonia contains 17.8% by mass of hydrogen (H2), making it a potential fuel for hydrogen storage. However, its narrow flammability range, low reactivity, and low flame propagation speed have limited its engine performance. To address these issues, this study was conducted to explore the use of multiple flame generation and ammonia-methane blend approaches to enhance the flame propagation speed of ammonia and improve engine performance. The study utilized a novel metal liner to generate multiple flames inside the combustion chamber and installed four spark plugs, as well as an additional spark plug at the top of the cylinder head, similar to a spark-ignition engine. Furthermore, the study investigated the effect of adding methane to ammonia combustion, with methane fractions ranging from 30% to 60% in terms of energy fraction. The combustion process was initiated with various spark ignition cases, and a high-speed natural flame-luminosity (NFL) technique was utilized to capture various combustion cases. The results showed that multiple flames inside the chamber led to stable ammonia combustion, resulting in better engine efficiency due to a faster flame propagation rate. Additionally, the study found that increasing the methane fraction in the ammonia fuel mixture increased the in-cylinder pressure and heat release rate (HRR), improving engine performance significantly. However, higher NOx emissions were observed with increasing methane fraction in the fuel mixture, resulting from both fuel-bound and thermal NOx caused by higher in-cylinder temperatures. The study also found that higher THC and CO2 emissions were observed while increasing the methane fraction in the fuel mixture. Moreover, multiple spark ignition cases produced lower THC than the single spark plug case because it burned the air-fuel mixture rapidly.