Ammonia Dosing Control Strategy for heavy duty automotive SCR

Abstract

Selective Catalytic Reduction (SCR) has proven to be the frontrunner for the removal of nitrogen oxides (NOX) from exhaust gas. EURO VI emission regulations present a challenge from the control point of view, so as to achieve high NOX conversion while keeping the ammonia slip under control. This thesis focuses on a model based feedforward control system to dose the urea. The SCR is modeled as a series of CSTR (continuously stirred tank reactor) with the main SCR chemical kinetics. The main reactions are ammonia adsorption, desorption, fast SCR, slow SCR, standard SCR and ammonia oxidation. The model is simplified so as to capture the important dynamics and be fast at the same time. A fast and computationally less demanding model is required from a control aspect if it has to be implemented online. Two control strategies were evaluated. The first control strategy maintains a constant ammonia slip whereas the second controller maintains a constant NOX to ammonia ratio at the outlet of the SCR. The catalyst used is an extruded vanadia based catalyst. The control strategy was tested on the engine bench with a 11.4L catalyst and an inline 6 cylinder 12.8L engine. The performance of the controller was very good compared to a at urea dosing. The control strategy achieves a higher NOX conversion, less ammonia slip and uses less urea for injection.