Nitrogen-Based Emissions from Ammonia Combustion in a Heavy-Duty Spark Ignition Engine

Ammonia (NH₃) is a carbon-free fuel with strong potential for spark-ignition (SI) engine applications. However, the engine can produce complex nitrogen-based emissions not adequately captured by conventional engine models. This study consolidated the results of experimental and numerical studies on the use of neat NH₃ combustion in a heavy-duty compression-ignition engine converted to spark-ignition operation, first for a sweep of equivalence ratios (ϕ) from 0.7 to 1.0, and another from varying the energy substitution ratio of methane (CH₄)– NH₃ blends from neat CH₄ to neat NH₃ at constant ϕ = 0.8. Two 0-D two-zone SI engine models with detailed chemistry (called “original” and “extended”) predicted engine thermodynamics and emissions. While the original model reproduced in-cylinder pressure and combustion phasing, it failed to capture the effect of fuel composition or operating condition on NO trends, both under- and over-predicting them for neat NH₃ and CH₄-rich operations. An extension of the model incorporating a burned-zone batch reactor and two more reactors simulating the post-combustion oxidation of the mixture exiting crevices and the DeNO_x processes during exhaust blowdown were implemented to address these limitations. Analysis of NO formation pathways highlighted the differences between modeling approaches. The equilibrium assumptions in the original model restricted NO formation primarily to thermal (Zeldovich) mechanisms. In contrast, the kinetics-driven model showed that non-thermal pathways dominate NO formation for all NH₃-containing cases, which shows the limitations of conventional SI models developed for hydrocarbons when applied to nitrogen-containing fuels. Post-combustion homogeneous reactors for crevice-based oxidation and exhaust blowdown revealed significant NO and N₂O formation after the end of combustion at moderate temperatures (850–1200 K), suggesting that N₂O formation was dominated by secondary thermal processes. Therefore, the inclusion of post-combustion chemistry and more consistent models are essential for accurate emission prediction in NH₃-fueled SI engines.