Sliding-mode observer for urea-selective catalytic reduction (SCR) mid-catalyst ammonia concentration estimation

This paper presents an observer design for SCR mid-catalyst ammonia concentration estimation using tailpipe NOx and ammonia sensors. Urea-SCR has been popularly used by Diesel engine powered vehicles to reduce NOx emissions in recent years. It utilizes ammonia, converted from urea injected at upstream of the catalyst, as the reductant to catalytically convert NOx emissions to nitrogen. To simultaneously achieve high SCR NOx conversion efficiency and low tailpipe ammonia slip, it is desirable to control the ammonia storage distribution along the SCR catalyst. Such a control method, however, requires a mid-catalyst ammonia sensor. The observer developed in this paper can replace such a mid-catalyst ammonia sensor and be used for SCR catalyst ammonia distribution control as well as serves for fault diagnosis purpose of the mid-catalyst ammonia sensor. The stability of the observer was shown based on the sliding mode approach and analyzed by simulations. Experimental validation of the observer was also conducted based on a medium-duty Diesel engine two-catalyst SCR system setup with emission sensors.