Nonlinear and adaptive control of NO/NO2 ratio for improving selective catalytic reduction system performance

Reducing the NO_x emissions from Diesel engines remains as a challenging issue as the emission standards for Diesel engine powered vehicles have become more stringent than ever before. Urea-based selective catalytic reduction (SCR) systems have emerged as a promising technique in addressing this issue. However, the SCR performance in terms of NO_x reduction and ammonia slip continues as an ongoing challenge due to the engine exhaust gas temperature variations, Diesel emission characteristics (especially high NO/NO₂ ratio), and immature SCR controls. The purpose of this study is to improve the SCR performance by feeding the SCR system with exhaust gas having the desired NO/NO₂ ratio. The proposed complete active NO/NO₂ ratio control consists of a low-level adaptive NO/NO₂ ratio controller and a high-level nonlinear soot mass controller. The low-level controller utilizes the pre-SCR catalysts such as Diesel oxidation catalyst (DOC) to convert part of NO into NO₂, while the high-level controller was designed and coordinated with the low-level controller to avoid NO₂ reduction through the Diesel particulate filter (DPF). Simulation and experimental results show that the proposed active NO/NO₂ ratio control has the potentials of regulating the NO/NO₂ ratio to the desired value and thus considerably improving the SCR performance. Simulation results also illustrate that the active NO/NO₂ ratio control can enable the SCR system size reduction by a half without a significant sacrifice on the overall tailpipe emission control performance. Such an integrated aftertreatment system control can be instrumental in reducing the cost and improving the performance of SCR systems, especially in low-temperature operations.