Experimental study of SCR DeNOX efficiency at low exhaust gas temperatures

The DeNOX efficiency of the Selective Catalyst Reduction (SCR) system relates closely to the urea injection, and the starting injection time of urea injection depends on the exhaust gas temperature. For the diesel vehicle driving on real roads, the exhaust gas temperatures change significantly. At high temperatures, the SCR system actively injects urea; whereas at low temperatures, no urea injection happens and the catalyst reaction relies solely on the stored ammonia in the catalytic container. Thus, it is important to investigate the SCR DeNOX efficiency under condition of different temperature ranging from low to high temperature. In this paper, a heavyduty diesel vehicle was tested under stationary idling condition with first rapid acceleration, then high-speed holding running and finally deceleration, and the NOX emission performance at various exhaust temperatures was acquired under two distinct operating conditions: active injection of ammonia at higher temperatures and no ammonia injection at lower temperatures. Furthermore, a deep learning model was built for predicting NOX downstream of the SCR by utilizing exhaust flow rate, average exhaust temperature, and NOX emission concentration upstream of the SCR as input parameters, and this model was verified with a model fit of 0.98. The results show that the SCR DeNOX efficiency achieves 99 % with active urea injection during high-temperature stages, and 30-40 % during low-temperature stages, relying on ammonia storage. During the ammonia storage stage, the SCR DeNOX efficiency exhibits high sensitivity to temperature, with a rate of decreases of 1.3 %/degrees C per unit temperature. In the active ammonia injection stage, the SCR DeNOX efficiency decreases at a rate of 0.5 %/degrees C per unit temperature. Various reaction stages exhibit a significant relationship between SCR DeNOX efficiency and exhaust temperature.