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 (NOX) emissions in SCR catalysts. Particularly at lower engine loads and lower exhaust temperatures an increase of the already low NO2/NOX fraction will enhance the SCR operation significantly. For this purpose, the understanding of the NO2 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.
To determine the influences on the NO2 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 NO2 formation.
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 NO2 emissions. The model behavior could be validated with test data and with results from the chemical kinetic modeling of the NO2 and NOX formation.
In general, it turned out that the NO2 concentration always increases with the total NOX concentration, so the formation of NO also affects the formation of NO2. While nearly every measure that leads to an increase of NOX emissions, like earlier injection timings, higher rail pressures or lower EGR rates, leads to a disproportionately lower increase of the NO2 emissions, only the dilution with additional air has a greater influence on NO2 than on NO. This effect enables to adapt the NO2/NOX ratio without changing the total NOX 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.
