Urea-based selective catalytic reduction (SCR) of nitric oxides (NOx) is a key technology for heavy-duty diesel engines to achieve the increasingly stringent NOx emission standards. The aqueous urea injection control is critical for urea-SCR systems in order to achieve high NOx conversion efficiency while restricting the tailpipe ammonia (NH3) slip. For Euro VI emission regulation, an advanced control strategy is essential for SCR systems since its NOx emission limits are tighter and test procedure are more stringent compared to Euro IV and Euro V. The complex chemical kinetics of the SCR process has motivated model-based control design approaches. However, the model is too complex to allow real-time implementation. Therefore, it is very important to have a reduced order model for SCR control system.
Based on a continuous stirred tank reactor (CSTR) and mass conservation law, a one-State control-oriented SCR model was developed and validated to estimate the downstream NOx emission, NH3 slip, and NH3 storage of the SCR catalyst. A model predictive control (MPC) coupled to the control-oriented SCR model was established to control the urea injection rate and maintain the NH3 storage at a required level.
The control-oriented SCR model can capture the main dynamics of the steady state condition as well as the transient condition very well, and the NOx emissions, NH3 storage, and NH3 slip can be well predicted. With the MPC, the simulation results showed that the NH3 storage in the SCR catalyst could be well controlled, the NOx emission in the world harmonized transient cycle (WHTC) after the SCR system of a heavy-duty diesel engine with raw NOx emission at 8.66 g/kW·h was reduced to about 0.37 g/kW·h, and the averaged NOx conversion efficiency was up to 96%, while the averaged NH3 slip in the cycle was kept below 10 ppm, which met the requirements of the Euro VI emission regulation.