Simulation Based Control System Analysis of a Urea-SCR Aftertreatment System Based on NH3 Sensing

Maruthi N.  Devarakonda, Michigan Tech University

State-of-the-art NOx sensors are cross sensitive to NH3 and are a drawback for closed loop NOx control in urea-selective catalytic reduction (SCR) aftertreatment systems, if the cross sensitivity is not compensated. One way to overcome this limitation is to use an NH3 sensor for feedback purposes. NH3 sensors, relatively new to automotive applications, have been researched in Europe for NH3 feedback to meet the NOx emission regulations. This work presents preliminary control system simulation results in a urea-SCR catalyst based on NH3 sensor feedback. A four state control oriented lumped parameter model is used to analyze the controllability and observability properties of the urea-SCR plant. A model based estimator is designed in simulation and a control system based on sliding model control framework is developed. The control system based on NH3 sensor feedback is analyzed in simulation by comparing it to a control system based on NOx sensor feedback. Simulation results show that the NH3 sensor based strategy performs closely in comparison to a NOx sensor based strategy. The control system performance metrics in NOx index, urea index, urea usage and NH3 slip suggest that the NH3 sensor can be a potential alternative to a NOx sensor for urea-SCR control applications.

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