Experimental investigation of urea injection strategy for close-coupled SCR aftertreatment system to meet ultra-low NO emission regulation

• The tailpipe NOx and PM emission satisfy CARB ultra-low emission regulation under FTP cycle. • The effect of temperature and space velocity on NO x conversion is investigated. • A high NO x conversion efficiency is observed on ccSCR with air-assisted urea nozzle. • A zonal control strategy for dual urea injection is adopted to reduce tailpipe NOx emission. • The supplemental urea injection strategy is effective to reduce tailpipe NOx emission. Worldwide automotive emission regulations for heavy-duty diesel engines are increasingly stringent, especially for nitrogen oxide (NO x ) and particulate matter (PM). The development of high-efficiency aftertreatment technologies and the optimization of the urea injection strategies are key factors for the diesel engines to meet the future regulations. In this paper, the effects of temperature and space velocity on close coupled selective catalyst reduction (ccSCR) and SCR are investigated firstly. The highest NO x conversion efficiency of ccSCR and main SCR is observed at 350 °C, and an increase of space velocity leads to a more significant decrease on NO x conversion efficiency of ccSCR than that of SCR. Then three phases of urea injection state for both nozzles are determined according to the catalyst inlet temperature in the Federal Test Procedure (FTP) cycle. Urea injection strategy is optimized by urea injection ratio calibration and urea injection strategy updating based on the characteristic of three phases, as well as NH 3 sensor feedback. Finally, the composite tailpipe NO x emission under FTP cycle is reduced below 0.027 g/kW·h, satisfying California air resources board (CARB) ultra-low NO x emission regulation with a penalty of slight increase in CO 2 emissions. However, adopting a high reactivity gasoline can improve the efficiency while maintaining the low level of NO x emissions.