Investigations of NOx-reduction in the exhaust of diesel engines by selective catalytic reduction with urea

Reduction of nitric oxides in the exhaust of Diesel-engines by urea-SCR isa well known technique for stationary operating engines. Conversion rates of up to 99 % are possible without slip of ammonia. However the application of urea-SCR for vehicles like trucks or passenger cars fitted with Diesel-engines requires additional information about the dynamic behaviour of different system components under various conditions of exhaust-gas temperature, space velocity and NOx-concentration level. This paper presents results of investigations sponsored by a research organization of the German industry, FVV (Forschungsvereinigung Verbrennungskraftmaschinen e.V., Frankfurt). The results indicate that NOx-conversion is mainly dependant on exhaust-gas temperature, feed alpha (it means the mole number of ammonia formed from urea addition related to the total mole number of nitric oxides) and finally the space velocity. Conversion rates of up to 90% have been obtained at stationary engine conditions and more than 70% at dynamic engine operation starting with cold engine.