Research on real-road NH₃ emissions of China-6 heavy-duty natural gas and diesel vehicles

In this paper, ammonia (NH₃) emissions of China-5 and three-way catalyst (TWC)-fitted China-6 heavy-duty NG vehicles and SCR-fitted China-6 diesel vehicles were characterized during real-road emission tests. The results showed that, due to side reactions within TWCs, real-world NH₃ emissions of China-6 NG vehicles were significantly higher than those of their diesel counterparts and those of China-5 NG vehicles without TWCs. For China-6 NG vehicles, NH₃ emissions occurred during the light-off of the TWCs and rich combustion. SCR-fitted China-6 heavy-duty diesel vehicles also had NH₃ emissions mainly caused by excessive SCR reductant injection and leakage from ammonia slip catalysts (ASC). China-5 heavy-duty NG vehicles emit little NH₃ for they utilized lean-combustion without SCR or TWC, where NH₃ forms. Although lean combustion NG engines hardly have NH₃ emissions, the extremely high engine-out NO_x emissions restricted its further application.