Optimization of urea SCR deNOx systems for HD diesel engines

In the past decade, SCR deNOx technology with urea injection has grown to maturity. European OEMs will apply SCR deNOx to meet future heavy-duty emissions legislation starting with EURO-4 (2005/2006). Numerous research programs in Europe and the US have shown a variety of system layouts and control strategies. The main differences are formed by: the engine-out NOx calibration; the application of an NO to NO2 catalyst; and open-loop or closed-loop urea dosage control. This paper gives an overview of possible SCR system configurations that are required for different stages of future emission legislation. Engine-out NOx emission is strongly influenced by ambient conditions. Projections in this study show that a combination of cold climate and a wintergrade fuel is the most severe: it may lead to 30% lower engine-out NOx emissions with respect to laboratory conditions. The varying engine-out NOx emission requires a safety margin in the case of an open-loop-controlled system in order to avoid excessive NH3 slip. Calculations in this paper show that Euro 4 limits can be met with open-loop-controlled urea dosage; an NO to NO2 catalyst is not even required. For other future emission legislation (Euro 5, US 2007 and US 2010), it becomes difficult to safely achieve the limits with open-loop control. US 2007 and 2010 legislation require an NO to NO2 catalyst because of the low average load and consequently low exhaust gas temperatures. Engines that are developed to meet US 2010 legislation need special measures to increase the exhaust gas temperature and to reduce engine-out NOx below 2 g/kWh. In all cases, optimal hydrolysis and mixing conditions are of major importance. High-speed photography and droplet size measurements are presented as methods for characterizing and modelling aqueous urea spray patterns. The information from these measurements is used as input parameters for simulation tools. These consist of a 2D/3D CFD program for urea mixing analysis and a 1D SCR system model for development of dosage control and optimization of catalyst dimension. Finally, simulations with the SCR system model are compared with measurements on the engine test bench