Increasing the NO2/NOX ratio in Diesel engines to enhance exhaust aftertreatment English

Modern exhaust aftertreatment systems for diesel engines use SCR catalysts to reduce oxides of nitrogen in the exhaust gas with the addition of urea. Besides the conventional SCR reaction (2NO+ ½O2+ 2NH3 → 2N2 + 3H2O), the significantly faster Fast-SCR reaction (NO + NO2 + 2NH3 → 2N2 + 3H2O) with the participation of highly reactive nitrogen dioxide plays an important role in the development of modern aftertreatment systems. For most efficient catalytic reaction NO2/NOX-rates of up to 50% are desirable. The fraction of NO2 emissions in the total NOX emissions, which usually is only about 5-10% in the raw exhaust gas, can be increased significantly by the partial oxidation in the upstream oxidation catalysts. In contrast, the targeted increase of the internal engine NO2 formation in favor of high NO2/NOx ratios is currently less considered. One reason for this is that the impacts on the formation of internal engine NO2 have been hardly investigated. This paper will deal with thermodynamic influences on the formation of NO2 in a single-cylinder commercial vehicle Diesel engine. The current study considers the effects of various engine operating parameters on NO and NO2, including the mixture formation and charge dilution. The aim of this study is to show ways of increasing the NO2/NOX ratio to enhance the efficiency of exhaust aftertreatment systems.

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