Modelling of the NOx storage behaviour during cold start of modern zeolite SCR catalysts

Abstract

Further stringent emission regulations of modern diesel engines call for a more precise prediction of NO_x emissions, thus enabling a better control of the exhaust-gas aftertreatment systems. A major part of the NO_x emissions is emitted before the light-off temperature of the selective catalytic reduction (SCR) catalyst is reached. Therefore a precise emissions prediction is necessary during the cold start phase of a diesel passenger car. Recent measurements show that NO_x emissions can be stored in the SCR catalysts during cold start. Furthermore a part of this stored NO_x can be reduced during the driving cycle.

This paper describes an empiric model predicting the NO_x storage behaviour during vehicle cold start. In a previous work the main influence parameters on the NO_x storage behaviour were investigated on a synthetic gas test bench. The knowledge gained from the previous research work defines the necessary input parameters for the NO_x storage model. These investigations showed that the NO_x storage effect strongly depends on the ammonia (NH₃-) level stored in the catalyst, exhaust-gas mass flow, the water adsorbed (H₂O) on the catalyst, and the temperature of the catalyst. The model was implemented for on-filter and flow-through SCR catalysts. There are two similar models, one for the close-coupled SCR system and the other one for the underfloor SCR system. Each NO_x storage model is split into an adsorption part and a desorption part. For both parts the pre-conditioning from the previous driving cycle is taken into account, which means that the catalyst state at the end of the last driving cycle initializes the model data for the current cycle, in consideration of the downtime between the two cycles. The desorption part calculates the NO_x conversion amount and defines the desorption mass flow of NO_x resulting from the NO_x storage effect. The developed NO_x storage model has been validated with roller dynamometer measurements and with real world driving cycles.