Visualization Study on Combustion Characteristics of Ammonia/Methane/Air Mixtures Ignited by Diesel Pilot in Rapid Compression and Expansion Machine

The application of ammonia fuel in engines can significantly reduce carbon emissions, serving as a crucial method for achieving carbon neutrality. However, its potential is hindered by the challenges of ammonia's difficulty in ignition and slow combustion rate. An effective solution to these drawbacks is to blend methane into ammonia mixtures and use a small amount of diesel for ignition. This study investigates the effects of mixture equivalence ratio and gas composition on the combustion characteristics of diesel-ignited NH₃/CH₄/Air mixtures. Pressure measurements and visual observations were conducted using a rapid compression expansion machine (RCEM). Experimental results reveal that the combustion process exhibits two distinct stages: initial intense diesel combustion followed by mixture combustion. Higher equivalence ratios prolong ignition delay while accelerate secondary combustion. Pure ammonia mixtures show incomplete lean combustion, while richer mixtures achieve more vigorous and complete burning. Notably, the two phases demonstrate an inverse relationship in the trends of flame luminance variation with respect to equivalence ratio changes. Methane addition (40% energy ratio) significantly improves overall performance, enhancing heat release rates, chamber pressure, flame characteristics, and heat release efficiency while reducing ignition delays. The research provides a systematic analysis of combustion phasing, flame development, and efficiency parameters under varying mixture conditions, highlighting methane's role in optimizing ammonia-based combustion systems.