Chemical mechanism development for ammonia/n-heptane blends in dual fuel engines

Abstract

Ammonia (NH₃) is a carbon-free energy carrier with significant potential for sustainable transportation, particularly in heavy-duty applications such as trucks, construction machinery, agricultural equipment, locomotives, and ships. To enable the use of ammonia/diesel dual-fuel engines in these demanding applications, this study develops a reduced NH₃/n-heptane chemical kinetic mechanism, with n-heptane serving as a single-component surrogate for diesel, designed for multi-dimensional computational fluid dynamics (CFD) simulations. The mechanism incorporates advanced sub-models for ammonia oxidation, n-heptane oxidation, and carbon-nitrogen interactions, improving predictions for both low- and high-temperature combustion phenomena. Validation against fundamental combustion data, including ignition delays and laminar flame speeds, confirms its accuracy and reliability. A key feature of this study is the further validation of the kinetic mechanism in CFD simulations using experimental engine data from ammonia port fuel injection and diesel direct injection compression ignition operation, effectively bridging fundamental research and practical applications. The simulations confirm the ability of the mechanism to predict primary engine combustion behaviors, including cylinder pressure, heat release rate, and key combustion characteristics such as ignition delay, premixed/diffusion combustion proportions, and nitrogen-based emissions trends (including unburned NH₃, nitrogen oxides (NOx), and nitrous oxide (N₂O)) across varying ammonia substitution levels. Additionally, the mechanism accurately captures the de-NOx effects of NH₃, which modulate NOx and N₂O concentrations during the late oxidation stage, with predicted emission levels closely matching experimental data. Overall, this work provides a robust and reliable tool to advance the development of high-efficiency, low-emission ammonia/diesel engine systems, thereby paving the way for cleaner and more sustainable solutions in heavy-duty transportation.