Experimental demonstration of ammonia storage and slip modeling with control for an SCR aftertreatment system

Emission controls is currently, one of the biggest challenges and concern in the automotive industry for cleaner diesel engines. Due to the low level of NOx emission enforced by the Environmental Protection Agency (EPA), selective catalytic reduction (SCR) technology is currently, one of the most feasible solutions. SCR technology uses diesel exhaust fluid (DEF) which is a urea solution of 32.7% urea with 67.3% water to reduce NOx emission. The urea solution is converted to ammonia (NH₃) which reacts with NOx over a catalyst to form N₂ and water. In this paper, a practical and dynamic model for modeling and controlling ammonia surface coverage or storage on a selective catalytic reduction (SCR) system with a vanadia-based catalyst is implemented. It also included modeling of NH₃ slip. This work was implemented on an electronic control module and tested on a medium duty diesel engine. The tests used in this paper involve standard test and measurement procedure for engine certification by EPA. A comparison with nominal stoichiometric feedforward control was made and the results illustrate improvement in both overall urea consumption and NOx reduction efficiency.