Study on the impact mechanism of ammonia energy fraction on in-cylinder combustion and pollutant generation under wide operation conditions of medium-speed diesel engines

To expedite the utilization of ammonia in the domain of marine medium-speed diesel engines. The computa-tional fluid dynamics (CFD) model combined with the ammonia/n-dodecane reaction mechanism were con-ducted to explore the impact mechanism of ammonia energy ratio (AER) and diesel injection timing on in -cylinder combustion and pollutant emission at low-mid-high loads of marine medium-speed diesel engines. Further, the formation laws of Thermal and Fuel nitrogen oxides (NOX) at various AER conditions were analyzed. At 10 % load, the rising AER advanced the CA50 by 57 % and increased the in-cylinder peak pressure by 3.5 % due to the enhanced premixed combustion stage. Whereas, at higher loads (50 % & 100 %), the effect of high specific heat capacity of ammonia played a crucial role in combustion progress with the increasing AER, which reduced the in-cylinder peak pressure by 1-3 %. The sensitivity of key combustion phases (CA10-CA90) to AER also gradually decreased with the rising loads because of the inhibited diffusion combustion stage. When the IT was 6 degrees BTDC, the peak temperature in the cylinder decreased by 10 % at 10 % engine load; and the charge of excessive temperature in the cylinder was significantly reduced at 50 % and 100 % engine load. In terms of emission characteristics, HC and CO emission significantly reduced when AER was 45 %. And the proportion of Fuel NH3 in the fuel rose from 75.5 % to 91.4 % with the increasing AER. This means that the generation conditions of Thermal NOx were inhibited in the large proportion of ammonia-doped combustion conditions.