Control Challenges for Optimal NOx Conversion Efficiency from SCR Aftertreatment Systems 2009-01-0905

The regulated emissions of NO_x and particulates from diesel engines are low enough that in many cases the emissions levels cannot be met by system architectures based on improved in-cylinder combustion technologies alone. Some of the emissions reduction has to be achieved with aftertreatment systems. Thus, diesel engine architectures for meeting the regulated tailpipe emissions consist of a combination of optimized combustion technologies, and NO_x and/or particulate aftertreatment technologies. Urea-SCR is one of the NO_x aftertreatment technologies being used to meet these stringent heavy duty dyno certification and light duty chassis certification emissions. The underlying principle involved in the control strategies reported in scientific literature consists of feed forward controllers with some open loop adaptation which makes use of the NO_x reduction and ammonia storage properties of the SCR catalyst system to maximize NO_x conversion efficiency while minimizing ammonia slip and urea consumption. However, as the aftertreatment system ages with time and undergoes NO_x conversion efficiency performance degradation, feedback control strategies will be required to reduce performance variability and maintain the desired optimal performance.

This paper presents results from duty cycle analysis that show the achieveable NO_x conversion efficiency with a given selective catalytic reduction catalyst, results from error budget analysis that quantify the variation in NO_x conversion efficiency from feed forward control strategies, and the factors that have to be addressed for an effective feedback control strategy. The duty cycle analysis uses the NO_x conversion efficiency map for a typical vanadia-based SCR catalyst as well as profiles of exhaust mass flow rate, exhaust gas temperature, and engine outlet exhaust gas NO_x concentration from a diesel engine to establish the NO_x reduction trade-off for different catalyst sizes. The error budget analysis shows the variation of NO_x conversion efficiency at selected operating modes of a system with a fixed engine calibration and typical uncertainties in critical parameters of NO_x emissions, exhaust mass flow rate, catalyst bed temperature, and urea reductant delivery. Simulation results and experimental data are then used to identify the key control challenges for optimal NO_x conversion efficiency from urea based SCR aftertreatment systems.