A basic study on a urea-selective catalytic reduction system for a medium-duty diesel engine

NO_x conversion performance of a urea-selective catalytic reduction (SCR) system comprising V₂O₅/TiO₂ catalyst under steady state operating conditions of an 8-litre, common-rail turbo direct injection (TDI) diesel engine was investigated. It was shown that the urea-SCR system achieves 70-90 per cent NO_x conversion under medium and high load conditions at 1440 r/min and that NO_x conversion is low under low load conditions because of the low catalyst temperatures and the NO/NO₂ ratio being higher than unity. It was also shown that NO_x conversion exceeds 90 per cent when the catalyst temperature is higher than 530 K. To investigate the details of the chemistry and thermofluid dynamics within the urea-SCR system, a computational fluid dynamics (CFD) code that incorporates detailed surface chemistry was developed based on the modified subroutines of CHEMKIN-II. The spatial variations of chemical species including NO and NH₃ in a thin catalyst channel was calculated using the model. The calculated result of NO conversion showed relatively good agreement with experimental results.