Ammonia as a hydrogen carrier in dual-fuel diesel engines: Influence of ammonia and oxygen on combustion performance and emissions

This study investigates the effects of Energy Fractions (EF) of premixed ammonia–air and ammonia–oxygen–air mixtures on in-cylinder pressure, indicated mean effective pressure (IMEP), heat release rate (HRR), mass fraction burned (MFB), indicated thermal efficiency (ITE), and emissions in a dual-fuel engine under varying loads (0 %–75 %). The key findings reveal that the peak in-cylinder pressure decreases with premixed ammonia–air, by 4.26 % and 4.53 % for EF20NH₃ and EF40NH₃, respectively, compared to conventional diesel (D100). This reduction is attributed to ammonia's higher auto-ignition temperature, lower flame temperature, and lower polytropic coefficient. Conversely, the use of a premixed ammonia–oxygen mixture (EF40NH₃+O₂) results in increased peak in-cylinder pressures by 4.5 %, 9.7 %, and 10.2 %, depending on the engine load. Furthermore, HRR, CHR, and MFB show significant improvements with EF40NH₃+O₂ across various engine loads. The IMEP for EF40NH₃+O₂ surpasses that of D100 and other premixed ammonia–air mixtures, achieving a maximum of 14.75 bar at high engine load, compared to 11.37, 11, and 10.9 bar for D100, EF20NH₃, and EF40NH₃, respectively. Emission analysis shows that CO₂ reduce by 23 %, 31.5 % and 15 % and the CO reduce by 34 %, 39.2 % and 7.7 % for EF20NH₃, EF40NH₃ and EF40NH₃+O₂ respectively, compared to D100. Premixed ammonia-oxygen significantly reduces HC emissions but increases NOx emissions due to higher combustion temperatures.