Ammonia/diesel dual-fuel combustion offers a promising pathway toward low-carbon engine technologies, yet compact and accurate chemical mechanisms suitable for engine simulations remain limited. In this study, a four-component diesel surrogate—n-hexadecane, iso-hexadecane, cyclohexane, and toluene—was formulated to better capture the representative molecular features of diesel fuel. Based on this surrogate, a reduced ammonia/diesel mechanism with 135 species and 697 reactions was developed using a combination of mechanism decoupling, skeletal reduction, and targeted optimization, effectively addressing the compositional and kinetic limitations in previous models. The mechanism was validated using ignition delay times under high-pressure and intermediate-to-high-temperature conditions, together with laminar flame speeds and emission-related species for ammonia and alkanes. Reaction sensitivity and rate-of-production analyses identified several key pathways controlling ammonia/diesel combustion, including the dominant contribution of H2O2 thermal decomposition to the rapid rise of OH radicals, the influence of alkane–NH2 reactions on ignition delay, and the combined roles of NH2–NOx conversion, alkane–NOx interactions, and primary alkane oxidation in shaping NOx and N2O formation. Finally, when implemented into a three-dimensional RCCI engine CFD model, the mechanism accurately reproduced the in-cylinder pressure, heat release rate, and emissions across different ammonia energy ratios, demonstrating strong predictive capability under engine-relevant conditions. This study provides a validated and computationally efficient mechanism that supports detailed analysis and simulation of ammonia/diesel dual-fuel combustion.
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