Developing OBD monitors for targeted SCR catalyst failure modes

Michiel  van Nieuwstadt, Ford Motor Company

ORAL PRESENTATION: Selective catalytic reduction (SCR) catalysts depend on a multitude of reactions to convert Nox (Nitirc oxides) to nitrogen in the presence of NH3 (ammonia). Many of these reactions are by-products and not crucial for the Nox conversion reactions, several are undesireable by-products that cannot be avoided, and several are at the core of the desired Nox conversion behavior. A realistic model based control system can only include the key reactions in a
real time implementation. Most researchers agree that the key reactions and processes occurring are ammonia storage, ammonia desorption, reaction of
NO with NH3, reaction of NO2 with NH3 and NH3 oxidation to Nox. In this presentation we will investigate what the effect of the activity of these reactions is on the final Nox and NH3 concentrations at the outlet of the SCR. Furthermore, we will investigate whether certain properties in the SCR outlet emissions can be robustly correlated to the SCR mode of failure in an automotive production environment. By understanding the failure modes of the SCR, we aim to develop monitors that can more accurately pinpoint the nature of the failure and its associated NOx or NH3 slip behavior. This implies that we can only use readily available sensors that can be mass-produced and that we must be robust to uncontrolled transient driving behavior.