Effect of intake oxygen concentration and ammonia energy ratio on combustion and emission characteristics of ammonia/diesel RCCI engine under low load

Abstract

The ammonia/diesel RCCI engine is one of the most promising methods for utilizing ammonia in combustion. Under high AER (Ammonia energy ratio), the ammonia combustion resistance characteristic, unburnt ammonia, and the N₂O emissions hinder the use of ammonia/diesel RCCI engines and violate the original intention of low-carbon emissions. This research investigate the effect of intake oxygen enrichment strategy on the ammonia/diesel RCCI engine through CFD simulations. The results indicate that as the IOC (intake oxygen concentration) increases, both the pressure and heat release rate in the cylinder increase, while the ignition delay time and combustion duration shorten. Under 27 % IOC, the ITE of AER80 will increase from 26 % to 27.9 %. Due to the enhancement of combustion characteristics in the cylinder, the emissions of unburnt ammonia of AER80 will significantly decrease from 13.65 g/KWh to 0.07 g/KWh and the GHG emission will also decrease 20 %, but NO_X emissions will deteriorate due to the high temperature and abundant oxygen environment. Therefore, AER50 is not suitable for intake oxygen enrichment strategy, AER60 is suitable at 23 % IOC, and AER70, and 80 are more suitable at 25 % IOC. Chemical kinetic analysis shows that increasing IOC intensifies the rate of the $\dot{OH}$ production, primarily through the reactions, 2 $\dot{OH}$(+M)=H₂O₂(+M) and $\dot{N{H}_2}$ + HO₂=H₂NO+$\dot{OH}$, which can accelerate ignition process. Additionally, it can increase the proportion of downstream reaction pathways for N₂O, N₂O(+M)=N₂+$\dot{O}$(+M) and N₂O+$\dot{H}$=N₂+$\dot{OH}$, accelerating the chain termination reaction, which results in the increase in N₂O rate of consumption and the reduction of N₂O emissions.