Kinetic Model Development for Selective Catalytic Converter Integrated Particulate Filters

To meet the stringent NO_x and particulate emissions requirements of Euro 6 and China 6 standard, Selective Catalyst Reduction (SCR) catalyst integrated with wall flow particulate filter (SCR-DPF) has been found to be an effective solution for the exhaust aftertreatment systems of diesel engines. NO_x is reduced by ammonia generated from urea injection while the filter effectively traps and burns the particulate matter periodically in a process called regeneration. The engine control unit (ECU) effectively manages urea injection quantity, timing and soot burning frequency for the stable functioning of the SCR-DPF without impacting drivability. To control the NO_x reduction and particulate regeneration process, the control unit uses lookup tables generated from extensive hardware testing to get the current soot load and NO_x slip information of SCR-DPF as a function of main exhaust state variables.

In the current work, engine dynamometer tests were conducted on a SCR-DPF at different operating conditions covering typical vehicle running conditions. The oxygen assisted and NO₂ assisted soot burning efficiency of the SCR-DPF was measured with and without urea injection at different soot loads. The impact of ammonia on soot burning at different engine operating conditions was studied. Using the test data, a physics based 1-D reaction model was developed with NO_x reduction and soot oxidation reactions. The detailed SCR chemistry includes reactions for ammonia adsorption/desorption, NO oxidation, NH₃ oxidation, standard/fast/slow NO_x reduction and N₂O formation. The soot burning reaction kinetics is described by the oxidation of soot with NO_x. The NO_x reduction and soot regeneration efficiency predictions of the model were validated with test values measured at engine dynamometer conditions under various exhaust flow rate, temperature, and soot load conditions. This 1-D kinetic model can be applied to generate calibration look up tables for the SCR-DPF control system in the vehicle to identify the right soot burning protocol to achieve the target regeneration efficiency. Few of the other areas where the model can be applied are, exhaust aftertreatment (EAT) architectural evaluation, converter sizing, wash coat loading studies, urea injection strategy development and heater element controls optimizations. Compared to the conventional hardware test-based approach, this model-based virtual approach uses less test data thus resulting in faster product development cycle and reduces the testing in engine dynamometer and vehicles.