Formation of Engine Internal NO<sub>2</sub>: Measures to Control the NO<sub>2</sub>/NO<sub>X</sub> Ratio for Enhanced Exhaust After Treatment

<div class="section abstract"><div class="htmlview paragraph">The proportion of nitrogen dioxide in the engine-out emissions of a Diesel engine is of great importance for the conversion of the total oxides of nitrogen (NO<sub>X</sub>) emissions in SCR catalysts. Particularly at lower engine loads and lower exhaust temperatures an increase of the already low NO<sub>2</sub>/NO<sub>X</sub> fraction will enhance the SCR operation significantly. For this purpose, the understanding of the NO<sub>2</sub> formation during the Diesel combustion and expansion stroke is as substantial as being aware of the different thermodynamic impacts and engine operating parameters that affect the formation process.</div><div class="htmlview paragraph">To determine the influences on the NO<sub>2</sub> emission level several variation series were performed on a single-cylinder research engine. Especially the charge dilution parameters like the air-fuel ratio and the EGR rate as well as the injection parameters could be identified to be decisive for the NO<sub>2</sub> formation.</div><div class="htmlview paragraph">Empirical models were generated for different operating points by the use of statistical test plans in order to predict the impacts of those parameters on the NO<sub>2</sub> emissions. The model behavior could be validated with test data and with results from the chemical kinetic modeling of the NO<sub>2</sub> and NO<sub>X</sub> formation.</div><div class="htmlview paragraph">In general, it turned out that the NO<sub>2</sub> concentration always increases with the total NO<sub>X</sub> concentration, so the formation of NO also affects the formation of NO<sub>2</sub>. While nearly every measure that leads to an increase of NO<sub>X</sub> emissions, like earlier injection timings, higher rail pressures or lower EGR rates, leads to a disproportionately lower increase of the NO<sub>2</sub> emissions, only the dilution with additional air has a greater influence on NO<sub>2</sub> than on NO. This effect enables to adapt the NO<sub>2</sub>/NO<sub>X</sub> ratio without changing the total NO<sub>X</sub> emissions by simultaneous varying the air-fuel ratio against other parameters. The different application measures for this were mapped out and analyzed in the current work.</div></div>