In-cylinder NOx active reduction via ammonia post-injection in ammonia direct-injection engines
摘要
High NOx emissions pose a critical challenge for ammonia engines. This study proposes ammonia post-injection as a strategy to achieve in-cylinder NOx active reduction in ammonia direct-injection engines, offering an innovative approach for NOx emission control. Computational fluid dynamics (CFD) simulation results elucidate the characteristics of ammonia combustion and NOx evolution under ammonia post-injection conditions. The post-injected ammonia can efficiently reduce the in-cylinder NOx it encounters, leading to a significant decrease in NOx concentration. Chemical kinetics analysis was conducted to reveal the underlying mechanisms and reaction pathways of the SNCR effect on NOx. The reduction of NOx primarily proceeds through the reactions between NO/NO2 and NH/NH2. NO is reduced via three pathways, yielding NNH (by NH2), N2O (by NH), and N2 (by NH and NH2), respectively. In contrast, NO2 is reduced via a single pathway that yields N2O under the action of NH and NH2. NH2 plays the overwhelmingly dominant role in reducing both NO and NO2. The effectiveness and feasibility of ammonia postinjection in reducing NOx emissions were evaluated through engine experiments. The experimental results demonstrate that the ammonia post-injection strategy enables significant NOx reduction for ammonia direct-injection engines. In the current work, a 14.4% NOx reduction was achieved without compromising combustion efficiency by appropriately delaying the ammonia post-injection timing, confirming the great potential of ammonia post-injection in NOx emission control.