Understanding of combustion characteristics and NO generation process with pure ammonia in the pre-chamber jet-induced ignition system

Abstract

As a favorable hydrogen carrier, ammonia (NH3) has great potential to achieve the goal of carbon neutrality in engines. However, it has several limits on the combustion in engines such as low laminar burning velocity (LBV), high ignition energy, and high nitrogen oxide (NOx) emission. It needs to employ an appropriate method for its utilization in internal combustion engines. The pre-chamber jet-induced ignition combustion mode which has been widely studied is an effective way to enhance the ignition and improve the combustion process of the mixture in the main chamber, especially under the lean-burn condition. To investigate the combustion effectiveness and NOx generation process of ammonia in this combustion mode, a three dimensions (3D) computational fluid dynamics (CFD) model coupled with a reduced ammonia reaction mechanism was established based on the constant volume combustion chamber (CVCC). The visualization experiments with jet-induced ignition combustion were conducted and used to validate the accuracy of the CFD model. The effect of equivalence ratio swept from 0.65 to 0.9 in the main-chamber was numerically studied in detail. The results indicated that the pre-chamber jet-induced ignition combustion mode can effectively improve the low flammability of ammonia and accelerate the combustion rate in the main chamber. The study of equivalence ratio sweeping demonstrated that a higher equivalence ratio leads to a higher combustion rate and lower NO and NO2 products, simultaneously. The increased equivalence ratio with more NH2 and NH radicals is beneficial for the NO reduction reaction and inhibits the oxidation reaction of NO → NO2. It reveals that a relatively high equivalence ratio in the lean-burn condition might be favorable for ammonia combustion.