Spectroscopic investigation of diesel-piloted ammonia spray combustion

This work investigates the flame emissions of diesel-piloted ammonia spray combustion under conditions relevant to internal combustion engines. Due to ammonia's carbon-free structure, its flame emissions differ significantly from carbon-containing fuels. This work provides new insights into the combustion process by distinguishing pilot and main fuel combustion by analyzing spectrally resolved flame emission footprints between 275nm and 750nm in a rapid compression expansion machine (RCEM). Additional results based on Shadowgraphy (SG), OH*-chemiluminescence (CL), visible natural flame luminosity (NL) imaging, and heat release rate (HRR) analysis describe the combustion process in detail. OH*, NH*, NH₂*, as well as broadband CL from NH₂*, NO₂*, and H₂O* contribute to ammonia flame luminosity. The results show that the combustion process encompasses a pilot ignition phase, a flame transition phase, and an ammonia combustion phase, during which only ammonia-related flame emissions occur and the main share of heat is released. Background corrected OH*, NH*, and NH₂* intensities reveal the evolution of individual contributions to the flame emissions. NH₂*-CL is the main contributor to visible ammonia flame emissions under the conditions investigated. Therefore, visible flame emissions are a suitable indicator for the flame front during the ammonia combustion phase.