Predicting NOx emissions from ammonia engines – Fuel and thermal effects

Ammonia, a promising zero-carbon fuel, faces engine application challenges from high NOx and ammonia slip. A key knowledge gap remains in predicting NOx and ammonia slip with chemical kinetic mechanisms within complex engine environments, beyond simple metrics. This research evaluates 14 ammonia combustion mechanisms in a spark-ignition (SI) engine model, using a two-zone thermodynamic approach. Experimental data from stoichiometric pure ammonia combustion in a research engine validate NOx predictions. The analysis details NOx formation, NH₃ slip, NO production rates, and differentiates thermal-NOx from fuel-NOx. While most mechanisms predict NOx within 20 % error, those by Otomo, Stagni, and Nakamura show superior accuracy. Furthermore, a significant divergence in N₂O predictions was found; only the Konnov mechanism yielded plausible concentrations (14–24 ppm), exposing a common limitation in other models. This study identifies thermal-NOx as ∼75 % of total NOx, offering vital insights for targeted emission control and guiding mechanism selection for engine development.