NO<i>x</i> and N2O Formation During the Combustion of CH4–NH3 Blends: A Numerical Study Validated by Experimental Data

Abstract Ammonia (NH3) combustion introduces new pathways for nitrogen oxides (NOx) and nitrous oxide (N2O) production. This study used the spark ignition (SI) Engine Zonal Simulator in ansys chemkin pro to investigate emissions inside a heavy-duty compression ignition (CI) engine converted to SI. The engine was fueled with methane (CH4)–NH3 blends characterized by the energy substitution ratio (ESR; the NH3 energy fraction in the blend). ESR effects were evaluated at constant energy per cycle from ESR = 0 to ESR = 1.0, under lean conditions, medium speed, and similar CA50. The goal was to evaluate the contribution of fuel NOx and then, if NH3 presence in the fuel affected emissions formation and destruction compared to CH4, find and quantify the source of these differences. The zero-dimensional (0D) model overpredicted NOx by up to 76% for ESR = 0 and grossly underpredicted N2O, while producing a different trend of NOx versus ESR compared to the experiment. A secondary chemkin reactor that simulated the chemistry inside the unburned mixture previously trapped inside crevices predicted an additional 250 ppm of NO at ESR = 0.6, which would further increase the differences between the simulation and experiments. A third chemkin reactor that simulated the thermodynamic behavior of in-cylinder gases during the exhaust blowdown at the same ESR predicted up to 4000 ppm decrease in NOx and important N2O production, which was so large that it could explain the differences between the 0D engine simulation and experiments even when accounting for real mixture inhomogeneities that will substantially decrease predicted reaction rates.