Experimental and Kinetic-Modeling Study of Ammonia/n-Heptane Combustion

In recent years, researchers have increasingly focused on ammonia–diesel dual-fuel engines as a means of reducing CO₂ emissions. Analyzing in-cylinder combustion processes is essential for optimizing the performance of ammonia–diesel dual-fuel engines. However, there is currently a lack of suitable reaction kinetics models for ammonia–diesel engine conditions. In this study, the ignition delay of ammonia/n-heptane mixtures was measured, and a reduced chemical mechanism was developed. Using rapid compression machine (RCM) experiments, the ignition delays of ammonia/n-heptane mixtures with different ammonia energy fractions (AEFs) (40%, 60%, and 80%) were measured. The test pressure ranged from 1.5 to 3.0 MPa, while the temperature ranged from 667 to 919 K, with an equivalence ratio of 1. The results showed that as the AEFs increased, the ignition delay of the premixed mixture also increased. When the AEF was 40%, the ammonia/n-heptane premixed mixture exhibited the negative temperature coefficient (NTC) phenomenon in the temperature range of 690 to 830 K. This phenomenon weakened as the AEF increased. Based on the experimental results, a reduced chemical mechanism for ammonia/n-heptane was developed, consisting of 162 species and 755 reactions. This model was able to accurately predict the ignition delay and laminar flame speed of ammonia/n-heptane mixtures, while reducing computational time by 94% compared to the detailed mechanism. When applied in three-dimensional simulations, this model effectively predicted the combustion and emission trends of ammonia–diesel engines. Advancing the first injection timing resulted in a decrease in NH₃ concentration near the wall. The fuel injected during the first combustion injection increased the temperature near the wall, promoting the thermal decomposition of ammonia.