Experimental and numerical investigations on the effect of urea pulse injection strategies to reduce NOx emission in Urea-SCR catalysts

A major challenge for auto industries is reducing NOx and other exhaust gas emissions to meet stringent Euro 7 emission regulations. A urea Selective Catalyst Reduction (SCR) after-treatment system (ATS) commonly uses upstream urea water injection to reduce NOx from the engine exhaust gas. The NOx emission conversion rate in ATSs is high for high exhaust gas temperatures but substantially low for temperatures below 200℃. This study aims to improve the NOx conversion rate using urea pulse injection in a mass-production 2.2 L diesel engine equipped with an SCR ATS operated under low exhaust gas temperature. The engine experimental results show that, under 200℃ exhaust temperature and 3.73x104 h-1 gross hourly space velocity (SV), the NOx conversion rate can be improved by 5% using 5-sec ON and 12-sec OFF (denoted as 5/12 s) urea pulse supply compared to the constant supply under time-averaged 1.0 urea equivalence ratio. It is experimentally observed that the urea pulse supply’s efficacy decreases under higher exhaust gas temperatures. The SCR model is developed with surface reactions, and the CFD results indicate that the urea pulse supply oscillates the surface reaction rates for NO and NO2, suggesting improved conversion rates. Further results on the urea pulse and constant supplies at high exhaust temperatures are reported. The NOx conversion improvement rates under various ON-OFF urea pulses are also discussed. The predicted dynamic fluctuation of the pulse supply and dithering SCR reaction is investigated.