Research on NOx generation mechanism of ammonia/diesel engine: Collaborative analysis of in-cylinder working process and N-element labeling

Ammonia, as a zero-carbon energy source, is a highly promising alternative fuel for engines. However, due to the fact that ammonia fuel contains over 80% nitrogen, NOx emissions have become a key challenge for ammonia fuel engines. This article mainly uses three-dimensional CFD simulation and element labeling method as research methods to jointly explore the mechanism of NOx generation in ammonia/diesel dual fuel engines. By analyzing the working process inside the cylinder of a diesel engine, the influence of oxygen concentration and temperature on the NOx generation law was explored under the conditions of 2700 r/min, 100% load, and ammonia energy ratio (AER) of 0%, 20%, 40%, and 60%. Research has shown that the overall NOx emissions decrease slightly with increasing AER, and that the oxygen concentration and temperature play important roles. Secondly, the element labeling method was used to label N in the air, and the generation amount and formation path of fuel-NOx and thermal-NOx were compared and analyzed. The results showed that as AER increased, the total NO emissions showed a monotonic decreasing trend. Fuel-NO started from scratch, with its peak first increasing and then decreasing, but its contribution to total NO gradually increased. When AER = 60%, the contribution of fuel-NO to total NO reached 68%. However, the peak value of thermal NO continued to decrease (400ppm→155ppm→150ppm→85ppm), and the contribution rate to total NO decreased significantly.