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In order to meet the increasing pollutants discharge standard, the selective catalytic reduction (SCR) module in the diesel engine after-treatment system is an important means to reduce nitrogen oxide (NOx) emissions. SCR systems are prone to urea crystallization at lower temperatures, especially during the cold-start conditions of diesel engines. In this study, we use the diesel engine after-treatment system test bench to obtain the boundary parameter of the simulation modules, and the urea crystallization risk assessment model of the diesel SCR system is established. Comparing the computational fluid dynamics (CFD) results with the test bench results, it is shown that the predicted urea film distribution of the assessment model is in good agreement with the experimental results. In order to clarify the various factors that affect the urea crystallization risk, this paper conducts a simulation analysis on a nozzle and mixer structure and operating parameters. The CFD results indicate that the increase in urea spray time will increase the maximum urea film thickness on the SCR system mixer surface. Exhaust temperature is the most important influencing factor. When the diesel engine exhaust temperature increases from 190 °C to 300 °C, the maximum urea film thickness decreases by 32 and the urea film mass accumulation decreases by 5%. Exhaust flow has a small impact on urea crystallization risk. When the exhaust flow increases from 300 kg/h to 600 kg/h, the maximum urea film thickness decreases by 39% and the urea film mass accumulation decreases by about 1%. In addition, urea spray rate, nozzle numbers, spray angle, and spray cone angle are also factors that affect urea crystallization risk. [ABSTRACT FROM AUTHOR]

To achieve diesel engine ultra-low nitrogen oxide emission, light-off selective catalyst reduction (LO-SCR) has been suggested for better performance with lower exhaust temperature. An electric heater upstream of the exhaust aftertreatment system was applied to significantly decrease the NOx emission at a low exhaust temperature. With a 7.2 kW electric heater coupled with LO-SCR, the NOx emission during 200~500 s of the world harmonized transient cycle (WHTC) decreased from 282.6 ppm to 61.5 ppm, which is a decrease of 45%. Application of an upstream diesel oxidation catalyst (DOC) decreased the NOx emission by 63% at the same interval at the cost of worse cold-start performance. The urea input was also adjusted to avoid NOx emission during the latter part of the WHTC. [ABSTRACT FROM AUTHOR]

Among many technical routes in solving the SCR (Selective Catalyst Reduction) deposition problem, a good mixer design can greatly improve the NO x conversion efficiency of the exhaust after-treatment system while avoiding urea deposition failures. This paper presents an anti-deposition mixer with novel conception which was optimized through CFD method, the more layers of wire mesh plates, the lower the evaporation efficiency is affected by the increase in injection volume. The new mixer with three layers of wire mesh plate shows better deposition resistance performance below 250℃through urea deposits boundary experiment, and it reduces the NO x emission value by 9.5% than conventional scheme under WHTC (Word Harmonized Transient Cycle) and WHSC (Word Harmonized Steady-state Cycle) test. By using the new mixer scheme in the after-treatment system, the problem of deposition under a variety of low emission temperature can be solved. • A new conception mixer make SCR system avoiding from urea deposition. • The new mixer scheme improve the engine emission limit of NO x by 9.5%. • The new mixer has excellent urea deposition resistance performance below 250 ℃. [ABSTRACT FROM AUTHOR]