Experimental investigation on N 2 O emission characteristics of ammonia-diesel dual-fuel engines

Abstract

Blending ammonia in combustion is an effective approach to reducing carbon emission for diesel engines, but the combustion of ammonia may result in the generation of elevated concentrations of nitrous oxide (N₂O), potentially contributing to a new source of greenhouse gas emissions. This study investigates the impact of engine load, diesel double injection strategy, and the ammonia energy ratio (AER) on N₂O emission in ammonia-diesel dual-fuel combustion mode. The results show that N₂O emission is increased with blending ammonia compared to pure diesel combustion, while as the load increases, the higher in-cylinder combustion temperature leads to a decrease in N₂O emission. In the ammonia-diesel dual-fuel combustion mode, diesel double injection strategy is conducive to the reduction in N₂O emission compared to single injection strategy. As the pre-injection timing is delayed, N₂O emission first decreases and then increases, reaching a minimum near −40 °CA ATDC. The postponed main-injection timing leads to the decreased in-cylinder combustion temperature, deteriorating N₂O emission. In the engine with a compression ratio (CR) of 18, when AER is in the range of 20% to 65%, the volume fraction of N₂O emission remains around 20 × 10⁻⁶. However, under the condition of AER = 80%, low chemical reactivity of ammonia causes the substantially increased N₂O emission. By increasing CR to 21 and adopting optimal injection strategy, combustion in activated thermal atmosphere is achieved, resulting in a substantial reduction in the N₂O original emission concentration. Ultra-low N₂O emission (exhaust N₂O volume fraction less than 10 × 10⁻⁶) is achieved at AER = 80%.