Reaction-level exergy destruction mechanisms and injection optimization in ammonia-diesel dual-fuel engines

Ammonia-diesel dual-fuel combustion offers a pivotal decarbonization strategy for heavy-duty engines. However, at high ammonia energy ratios (RAE), the low reactivity of ammonia compromises thermal efficiency and exacerbates unburned ammonia slip. Moreover, the underlying mechanisms of thermodynamic irreversibility remain obscure. Herein, this study integrates engine bench testing, three-dimensional simulations, and reaction-level non-equilibrium exergy analysis to investigate the impacts of RAE and pre-injection strategies on combustion characteristics, microscopic exergy destruction, and emissions. Results reveal that in pure diesel mode, chemical exergy destruction originates primarily from fuel pyrolysis and intermediate conversion within diffusion flames. As the RAE increases to 70%, the dominant microscopic mechanism shifts toward highly irreversible N-H-O pathways. Key reactions exhibit significant negative exergy-to-heat ratios, macroscopically manifesting as combustion delay and substantial unburned losses, which reduce the indicated thermal efficiency to 46.9%. Implementing a 50% pre-injection strategy reshapes the in-cylinder active atmosphere via the low-temperature oxidation of the pre-injected diesel. This effectively overcomes chemical inertness of ammonia and fosters a synergistic enhancement of the carbon-nitrogen reaction system. The pre-injection strategy substantially reduces the exergy-to-heat ratio of critical reactions, thereby improving thermodynamic quality at the microscopic level. As a result, the thermal efficiency recovers to 50.6%, accompanied by a 59.6% reduction in greenhouse gas emissions. These results demonstrate that injection strategies can optimize macroscopic thermodynamic performance by reshaping microscopic chemical pathways, providing a theoretical basis for achieving high-efficiency and clean combustion in ammonia-fueled engines.