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 NOx emissions, thus enabling a better control of the exhaust-gas aftertreatment systems. A major part of the NOx 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 NOx emissions can be stored in the SCR catalysts during cold start. Furthermore a part of this stored NOx can be reduced during the driving cycle.This paper describes an empiric model predicting the NOx storage behaviour during vehicle cold start. In a previous work the main influence parameters on the NOx 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 NOx storage model. These investigations showed that the NOx 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 NOx 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 NOx conversion amount and defines the desorption mass flow of NOx resulting from the NOx storage effect. The developed NOx storage model has been validated with roller dynamometer measurements and with real world driving cycles.