Understanding the difference in combustion and flame propagation characteristics between ammonia and methane using an optical SI engine

As a carbon-free hydrogen-carrier fuel with a high content of hydrogen, ammonia (NH₃) exhibits unfavorable combustion properties such as low flammability and large cyclic variations, requiring further investigation in actual engines. In this work, for the first time, using a single-cylinder optical spark ignition (SI) engine with a high compression ratio, engine performance as well as combustion and flame propagation characteristics were investigated. Synchronization measurement of in-cylinder pressure and high-speed photography was performed, and the combustion characteristics were compared between ammonia and methane under the same operating conditions. The results show that the combustion stability and power capability of ammonia are significantly lower than that of methane. Further analysis combining combustion phasing and flame propagation characteristics indicates that the burning time loss mainly comes from the initial flame development process rather than the main combustion duration which results in a deficit in engine performance. The average probability maps of the flame location indicate that the flame front propagation of the two fuels emerges with obviously different characteristics. Quantitative flame solving shows that NH₃ flame propagation features a higher flame stretch sensitivity which is macroscopically reflected in the cyclic variations. Furthermore, the quantification of flame response to turbulence was solved and observed much higher effects for NH₃ flame than methane, which indicates NH₃ flame propagation is mainly driven by the flame response to turbulence while the flame propagation of methane exhibits higher sensitivity to temperature than ammonia. The current study shall provide insights into the application of ammonia in the internal combustion engine.