Improved Performance of Discrete Implementation of Switching Mode Controller for Urea-SCR
Diesel engines emit toxic gases like NOx and hydrocarbons. These gases need to be treated before they are released out the tailpipe. Thus, an aftertreatment system is installed which comprises of DOC, DPF and SCR. The DOC oxidizes the hydrocarbons and NO, the DPF traps the particulate matter and SCR reduces the NOx by reacting with NH3 at high temperatures. However, since NH3 is also a toxic gas, it cannot be released out the tailpipe in excess. It is important to inject an appropriate amount of NH3 so that it does not slip out the tailpipe. With increasingly stringent regulations on the emission limits of these toxic gases, control of SCR has become more necessary than before.
In this thesis, the work done by previous members of the lab research group was improved upon. The objective remained the same, namely, keeping the NH3 slip under 50 ppm while maximizing NOx reduction. On initial inspection, it was realized that the entire controller had been designed and implemented in continuous time. Since the controller would be implemented digitally, with limited hardware sampling time, a discrete-time implementation as done via a DCU was created. The controller switched between two controllers – slip-based and storage-based. The slip-based controller was modified to include a feedforward term in the system so that the response time could be improved along with a feedback controller to eliminate any disturbances and steady-state error, using ammonia slip feedback as measured by an NH3 sensor. It aims at keeping the maximum ammonia slip under 50 ppm. The storage controller is a feedback controller which tries to limit the ammonia storage based on the values fed by a lookup table. This lookup table is a simplified table that determines the maximum ammonia storage at any given instant based on the catalyst bed temperature. The feedback controller gains for both controllers were determined based on a linearized plant model since the initial gains were ineffective with the discretized model. The initial switching mode controller that switches between slip control and storage control switched too frequently between the controllers, thereby affecting controller performance. A switching logic was implemented to limit the number of switches. A switch will be permitted only if the previous switch occurred over a certain time. By implementing all the subparts together in the controller, incremental improvements were prominent. In the end, the performance by implementing the proposed idea was distinctly better. The metrics considered for performance comparison are the number of switches and the ability to maximize slip up to 50 ppm. Parameter error was also studied as well and its effect on the controller performance was analyzed. The data when tested against sets of underestimated, overestimated and mixed estimates for the plant parameters resulted in the underestimated parameters to work within the scope of the objective. The controller was able to compensate for the underdosing. Overestimation caused overdosing in the system which led to spikes in the NH3 slip. Thus, it is better to underestimate the plant parameters than overestimate them.
- Master of Science
- Mechanical Engineering
- West Lafayette