To meet the NOx limit without a penalty of fuel consumption, urea SCR system is currently regarded as promising NOx reduction technology for diesel engines. SCR system has to achieve maximal NOx conversion in combination with minimal $NH_3$ slip. In this study, as a basic research to develop an algorithm for urea injection control, the characteristics of engine out NOx emission and behavior of NOx reduction during steady-state and transient conditions were investigated using 2L DI diesel engine. Test results show that on increasing the catalyst temperature the variations in the outlet NOx concentration are faster and maximal allowable $NH_3$ storage exponentially decreases. For change from a low to high engine load, it can be seen that a few seconds after load-step is required to reach full NOx conversion and the adsorbed amount of $NH_3$ at lower temperature desorb during the next temperature increase, causing $NH_3$ slip. Engine out NOx emission needs to be corrected because NOx emissions just after step load is lower than that of steay state condition.
M. Kleemann, M. Elsener, M. Koebel and A. W okaun, "Investigation of the Ammonia Adsorption on Monolithic SCR Catalysts by Transient Response Analysis," Applied Catalysis B: Environmental, Vol.27, pp.231-242, 2000
M. Koebel, M. Elsener and M. Kleemann, "Urea-SCR: A Promising Technique to Reduce NOx Emissions from Automotive Diesel Engines," Catalysis Today, Vol.59, pp.335-345, 2000
C. Havenith and R. Verbeek, "Transient Performance of a Urea deNOx Catalyst for Low Emissions Heavy-Duty Diesel Engines," SAE 970185, 1997
T. Wang, S. Baek, M. Jung and G. Yeo, "A Study of NH$_3$ Adsorption/Desorption Characteristics in the Monolithic NH$_3$ -SCR Reactor," Transactions of KSAE, Vol.14, No.3, pp.125-132, 2006
L. Hofmann, K. Rusch, S. Fisher and B. Lemire, "Onboard Emission Monitoring on a HD Truck with an SCR System Using NOx sensors," SAE 2004-01-1290, 2004
F. Willems, R. Cloudt, E. Eijnden, M. Genderen and R. Verbeek, "Is Closed-Loop SCR Control Required to Meet Future Emission Targets?," SAE 2007-01-1574, 2007
C. Ciardelli, I. Nova, E. Tronconi, B. Konrad, D. Chatterjee, K. Ecke and M. Weibel, "SCRDeNOx for Diesel Engine Exhaust Aftectreatment: Unsteady-state Kinetic Study and Monolith Reactor Modelling," Chemical Engineering Science, Vol.59, pp.5301-5309, 2004