Generally, an Urea-SCR system for diesel engines sets the NOx reduction efficiency to 80-90%. The SCR system is theoretically able to achieve the NOx reduction over 95%, but, due to the combination of design factors, the result is varying. The influence factors are the exhaust gas temperature, SCR control strategy, urea dosing parts, flow uniformity, etc. Some factors depend on the exhaust system package. Thus, a system engineer must consider the relation between variable requirements and variable design factors.
The design targets for aftertreatment package in this work are following:
Improvement of NOx reduction efficiency
Improvement of noise reduction performance
To meet Exhaust back-pressure requirement
Minimization of package volume
Minimization of urea-deposit
The aftertreament strategy of the discussed subject are the SCR system for EURO V and the DPF+SCR system for Japan 2009, the developed vehicle are the heavy duty truck and the express bus.
To meet the targets, next steps are carried out:
Investigation the design factor on SCR efficiency and urea-deposit
Analysis of the correlation between SCR efficiency and exhaust basic performance (Noise and flow resistance)
Application of the TRIZ method for a physical contradiction
Optimization of the exhaust package and experimental verification
The result of packaging optimization alone improves the NOx reduction efficiency and prevents urea deposits at the urea dosing unit. By the improved urea mixing designs on the exhaust package structure, the urea-deposit can be reduced from 14.3g to 5g. At the DPF+SCR system for Japan 2009, NOx reduction rate is improved from 69.4% to 75.7% at 13-mode cycle and JE05 mode. The urea spray quantity is improved from 7.0 g/kWh to 5.7g/kWh. In addition to the improvements in emission, the exhaust package volume at the same emission level can be reduced by 10%.