Generation characteristics of nitrogen-containing pollutants in ammonia-diesel dual-fuel engines under different operating conditions

The emission generation process in ammonia-diesel dual-fuel engines involves a series of complex physical and chemical processes influenced by numerous factors. However, the operating conditions significantly impacted the combustion state of the engine. Under different operating conditions, variations in intake and injection states resulted in noticeable differences in the combustion process and emission characteristics. To investigate the mechanisms of nitrogen-containing pollutant generation and the emission characteristics of the combustion process in ammonia-diesel dual-fuel engines under varying operating conditions, numerical simulation methods were employed to study the in-cylinder combustion temperature and the distribution of nitrogen-containing pollutants. The results indicated that the primary nitrogen-containing pollutants in ammonia-diesel dual-fuel engines included NH3, NO, NO2, and N2O. The formation of these pollutants was strongly influenced by the interactions among combustion temperature, equivalence ratio, pressure, flame conditions, residence time, and the concentrations of key reactants. As the ammonia energy ratio increased, the distribution area of the high-temperature zone in the cylinder decreased. The NH3 distribution pattern was found to be correlated with the temperature distribution, with lower NH3 concentrations in high-temperature regions and higher NO concentrations. N2O was observed to form around the NO distribution area. With the increase in ammonia energy ratio, the equivalence ratio increased while the average in-cylinder temperature decreased. Unburned NH3 was formed under low equivalence ratios and low temperatures. High NO emission regions were easily formed at equivalence ratios between 0.5 and 1 and temperatures above 1500 K, while high N2O emission regions were formed at equivalence ratios of ≤1 and temperatures between 1300 and 2100 K. Under different engine speeds, loads, and atmospheric pressures, variations in initial ammonia mass fraction and intake flow rates led to significant changes in equivalence ratios and temperatures, resulting in marked differences in pollutant distribution generated during combustion. Comparing different operating parameters, it was found that engine load had the most significant impact on ammonia combustion. As the load increased, the high-temperature region in the cylinder expanded. At an engine torque of 400 N m, the final emissions of NH3, NO, NO2, and N2O were measured at 1487, 541, 37, and 66 ppm, respectively.