Ultra-high compression ratio as a key enabler for pure ammonia spark-ignition engines: Flame propagation and emission formation

The pure ammonia spark-ignition (SI) strategy has emerged as a promising solution for ammonia-fueled engines owing to its simplified system architecture and reduced control complexity. This study investigates flame propagation and pollutant formation of pure ammonia SI mode in a modern multi-cylinder heavy-duty engine equipped with an ultra-high compression ratio (CR) of 24:1 through combined experiments, computational fluid dynamics (CFD) and chemical kinetic simulations. Stable pure ammonia combustion is experimentally achieved over a broad load range at a constant engine speed of 1500 rpm. A pronounced two-stage heat-release behavior is observed, indicating that the late combustion process is increasingly influenced by enhanced chemical reactivity under ultra-high-CR conditions. The first-stage heat release is dominated by turbulent flame propagation, whereas the second-stage heat release is associated with accelerated oxidation of the residual NH 3 /air mixture, with a possible contribution from localized autoignition. Owing to ammonia’s intrinsically low reactivity, early flame kernel development exhibits strong sensitivity to local charge motion. Increasing engine load shortens the combustion duration, enabling the brake thermal efficiency (BTE) to exceed 43%. Although NH 3 slip decreases with increasing load, it remains relatively high (∼8000 ppm), mainly due to incomplete oxidation and near-wall quenching. NOx emissions remain at a moderate level (∼2000 ppm), while N 2 O emissions (∼10 ppm) decrease as elevated in-cylinder temperatures promote more complete oxidation. Simulation-based parametric analyses further indicate that increasing CR enhances combustion performance through improved in-cylinder thermodynamic conditions, but may aggravate NH 3 slip and N 2 O formation through intensified near-wall and crevice effects. Furthermore, increasing ignition energy facilitates early flame development while exerting only a limited influence on the overall combustion duration and thermal efficiency. Overall, this work provides new insights into flame development and nitrogen chemistry in pure ammonia SI engines, offering guidance for the development of next-generation carbon-free heavy-duty engines.