Experimental and Numerical Analysis of Low Temperature NO _x - Conversion in Urban Busses

The selective catalytic reduction (SCR) of nitrogen oxides (NO_x) is the most promising technique to meet prospective emission regulations of diesel-driven commercial vehicles. Due to the installation of SCR- systems downstream of both the oxidation catalyst and the particulate filter, the relatively long light-off phase after engine startup limits the overall efficiency of reducing emissions. Another challenge is the fact that, once the light-off temperature is achieved it must be maintained. In particular, the operation of urban busses with highly transient load profiles involves many phases of idling with low exhaust gas temperatures (e.g., during bus stops and/or at traffic lights). In contrast to previous studies, these investigations combine properties of catalysts determined in the course of experiments with the simulation of real operational profiles. The thermal characteristics and NO_x- conversion of SCR- catalysts for three different driving cycles of urban busses - which cover typical load spectra of stop-and-go traffic, regular city traffic, and bus routes with extra-urban segments - are investigated. A comparative screening of catalysts on an engine test bench identified input parameters regarding NO_x- conversion and properties of the exhaust gas. These data were then used in a validated numerical model capable of describing the progress of temperature for each component in the exhaust gas system and the NO_x- conversion in the catalyst for several types of catalytic converters applied in various driving cycles or bus routes. The influence of start-stop operations was analyzed precisely. Furthermore, the results demonstrate significant differences of NO_x-emissions for the observed driving cycles and catalysts.