Pilot diesel-ignited ammonia dual fuel low-speed marine engines: A comparative analysis of ammonia premixed and high-pressure spray combustion modes with CFD simulation

Decarbonization of global shipping plays an important role in avoiding the worst impacts of climate change, and ammonia is a promising sustainable and zero-carbon fuel in the maritime sector. The dual-fuel mode has the potential to compensate for the poor combustion property of ammonia, but the existing studies are mainly focused on automotive medium- and high-speed engines, researches on marine low-speed engines are rarely reported. In this paper, the numerical models of pilot diesel-ignited ammonia dual fuel engine under both the premixed and high-pressure spray combustion modes are established using a commercial low-speed marine diesel engine as the prototype, and verified against the experimental data of the spray mixture formation, ignition delay, in-cylinder pressure and pollutant emissions. Then, the engine performance and emissions of both two modes are studied and optimized. The results show that under the diesel replacement ratio of the commercial methane/diesel dual-fuel marine engine (i.e. around 98%-99%), both two modes can achieve an indicated thermal efficiency (ITE) of over 50%. At the optimized ammonia premixed combustion point, the ITE is increased by 5.1% owing to the significantly reduced cooling loss, but at the expense of 140.6% higher NOx emissions compared to the diesel-only mode. The optimized ammonia high-pressure spray combustion point can achieve a similar ITE and reduce the NOx emissions by around 47% compared to the diesel-only mode. For both the optimum points, the GHG emissions (i.e. CO2 + 300 x N2O) are reduced by around 97%, while the unburned NH3 emissions are negligibly small.

Automated summary

The decarbonization of global shipping is crucial in mitigating climate change, and ammonia has been identified as a sustainable and zero-carbon fuel for the maritime industry. Despite existing studies primarily focusing on automotive engines, research on marine low-speed engines is limited. This paper establishes numerical models for a pilot diesel-ignited ammonia dual-fuel engine, utilizing a commercial low-speed marine diesel engine as a prototype. The models are validated with experimental data, and engine performance and emissions are studied and optimized under both premixed and high-pressure spray combustion modes.