The Study of NOx Reduction using Urea-SCR System with CPF and DOC for Light Duty Vehicle; the Diesel NOx Reduction System

Future emission limits of diesel engines require additional effort to develop advanced after-treatment devices. The Urea-SCR system is a promising device to reduce NOx emissions. The purpose of this work is to reduce NOx emissions from a Light Duty Vehicle (LDV) with a Urea-SCR (Selective Catalytic Reduction) system, a Catalytic Particulate Filter (CPF), and a Diesel Oxidation Catalyst (DOC). The purpose of the development is to analyze the atomization characteristics of the urea solution and to optimize the flow path shape in front of the SCR catalyst for uniform reductant (NH₃) distribution. Additionally, the development purpose is to determine the strategy for urea solution injection quantity and to verify the performance of the Urea-SCR system on the vehicle. The NO₂/NOx ratio plays a pivotal role in reducing NOx with Urea-SCR system. Although NO is converted into NO₂ through the DOC, the NO₂/NOx ratio is reduced due to the CRT (continuous regenerating trap) effect in the CPF. Therefore, the reduced NO₂ ratio has a negative influence on the NOx reducing efficiency of the Urea-SCR system. The negative factors associated with the reduced NO₂ amount is an increase in accumulated PM (Particulate Matter) and in the number of associated CPF regeneration times needed. Through this work the Urea-SCR system has shown an 80 percent NOx conversion during the FTP75 driving mode on a transient engine test bench.