Numerical investigation on impacts of ammonia injection protocols on NO, NO2 and N2O emissions from ammonia/n-heptane dual-fuel marine engines

Abstract

In recent years, global warming caused by greenhouse gas emissions has attracted widespread attention, and carbon emission reduction has become a key issue for the global shipping industry. As a promising zero-carbon fuel, ammonia is expected to significantly reduce carbon emissions from marine internal combustion engines. This paper presents a numerical investigation based on the CONVERGE 3.0 simulation platform, using a validated ammonia/n-heptane combined chemical kinetic mechanism (Xu et al., 2022), to study the effects of ammonia injection control strategies—including injection timing, pressure, and injector orifice diameter—on combustion characteristics and pollutant emissions in a MAN L23/30H marine diesel engine operating in thermal atmosphere compression ignition mode. The results show that these parameters all significantly affect in-cylinder combustion processes and heat release laws: indicated thermal efficiency first increases and then decreases with advanced injection timing, reaching a maximum of 49.87% at −10°CA ATDC for 60% ammonia energy percentage (AEP); NO and NO 2 emissions rise gradually by 352% and 195% respectively as injection timing advances from 0°CA to −14°CA, while N 2 O production first decreases and then increases with a minimum of 0.47 ppm at −8°CA ATDC. Higher injection pressure (from 60 MPa to 100 MPa) reduces peak in-cylinder combustion pressure by 3.2%, improves indicated thermal efficiency by 0.12% points, increases NO and NO 2 emissions by 14.3% and 21.4% respectively, and decreases N₂O production by 9.6%. Larger orifice diameter (from 0.20 mm to 0.28 mm) enhances fuel atomization, improves indicated thermal efficiency by 0.15% points, but increases NO, NO 2 , and N 2 O emissions by 38.7%, 39.5% and 28.9% respectively. This study provides preliminary theoretical insights and quantitative data support for the optimization of injection strategies in marine ammonia engines.