Date of Award


Degree Type


Degree Name

Master of Science in Mechanical Engineering (MSME)


Mechanical Engineering

Committee Chair

Peter H. Meckl

Committee Member 1

Gregory M. Shaver

Committee Member 2

Kartik B. Ariyur


The urea SCR system is the key technology to meet future NOx emissions and NH3 slip regulatory standards. Urea dosing control is the most critical part of the urea SCR system. The complexity of SCR chemical reactions and the conflicting behavior of tailpipe NOx and NH3 slip make it difficult to find the optimum amount of urea to be injected. The effective utilization of NOx and NH3 sensors is important to achieve lower NOx emission and NH3 slip. But due to the higher cost of NH3 sensors, research studies have not explored the full potential of using NH3 slip as feedback in the control strategies. This study proposes two NH3 slip reference controllers and a storage fraction reference controller that utilize the NH3 slip feedback to meet the control objectives of maximizing NOx reduction while keeping the instantaneous maximum NH3 slip below a specified limit. The significance of knowing the catalyst storage fraction was studied. Two NH3 slip reference controllers were designed, such that one control strategy used catalyst bed temperature and exhaust gas volume flow rate information, and the other used storage fraction and tailpipe NOx out information, to calculate the reference NH3 slip. The storage reference controller used catalyst bed temperature, incoming engine NOx, and NH3 slip information, to calculate the reference storage. Simulation results demonstrate the effectiveness of all the control strategies for Urban Dynamometer Driving Schedule (UDDS) operating conditions, with storage fraction reference controller resulting in better NOx reduction than NH3 slip reference controllers, assuming no error in the actual catalyst storage fraction value.