Research on Combustion Characteristics of Ammonia/N-Heptane Dual-Fuel Marine Compression Ignition Direct-Injection Engine

To address the decarbonization requirements of the shipping industry, this study establishes an in-cylinder combustion simulation model for a medium–high speed four-stroke ammonia-fueled marine engine based on the CONVERGE v3.0 platform. A diesel combustion model was first developed and validated against experimental data. Building on this validated model, an ammonia/n-heptane dual-fuel combustion model was further developed by coupling a chemical kinetic mechanism for ammonia/n-heptane. To overcome the challenge of igniting pure ammonia, a combustion strategy employing intake port injection of n-heptane and direct in-cylinder injection of ammonia fuel was adopted, leveraging thermal compression ignition. The results indicate that under initial cylinder conditions of 1 bar and 350 K, misfire occurs when the ammonia energy proportion (AEP) reaches 70%, preventing stable ignition and combustion of ammonia. Based on an analysis of intake boundary conditions, the influence of intake supercharging coupled with intake heating on ammonia combustion characteristics was investigated. As the AEP increases further, the combustion of n-heptane deteriorates significantly. At a 90% AEP, the combustion efficiency of n-heptane is approximately 67% at an initial temperature of 350 K but drops to about 28% at 400 K.