Injection strategies for emission improvement in a direct-injection ammonia-diesel dual-fuel marine engine

Abstract

To achieve carbon neutrality and comply with increasingly stringent emission regulations, ammonia has emerged as a promising carbon-free fuel for the marine engine. Ammonia-diesel dual-fuel engines offer higher efficiency and reduced carbon dioxide emissions compared to conventional diesel engines. However, these engines face challenges such as high emissions of unburned NH3, N2O, and NOx. To overcome these limitations, liquid ammonia direct-injection strategies have been introduced to ammonia-diesel dual-fuel engines, necessitating optimization of combustion and emission characteristics as well as analysis. However, many studies have been limited to land-based engines, and research on large marine engines still requires further investigation. In this study, we conducted numerical simulation to investigate the combustion and emission characteristics of a directinjection ammonia-diesel dual-fuel marine engine under various injection strategies. The simulations were performed under a 50 % load condition at an engine speed of 900 rpm, with an ammonia energy fraction of 0.8. By adjusting the injection timing of ammonia and diesel to ensure proper ammonia delivery to the diesel flame region, significant reductions in unburned NH3, N2O, and NOx were achieved. The indicated mean effective pressure and emissions of unburned NH3 and N2O improved with optimized diesel injection timing, although NOx increased due to the rise in combustion temperature. Furthermore, applying a 9-hole central diesel pilot injection with advancing ammonia injection timing by 5 degrees of the crank angle from the determined optimal timing resulted in a 4.8 % increase in indicated mean effective pressure compared to the side diesel pilot injection at the same optimal timing. Unburned NH3 and N2O emissions were reduced by a factor of 3. However, NOx emissions doubled due to the increased in-cylinder temperature.