## 4 Conclusions  
To sum up, it can be concluded that in contrast to the increase in catalyst volume, copper loading, or the amount of urea dosing (ANR), a high UI could be the most cost-effective approach reflecting optimized deNOx conversion with the optimized urea dosing strategy and mixing method. Hence, engine calibration, catalyst technology, emission test cycle, and converter and mixer design optimization can subsequently be carried out with the assistance of thermal fluid analytical tools and precise monitoring devices to obtain the target ANR/ammonia/urea uniformity index, especially the dynamic UI during different cycles used to assess emission compliance  
In this study, a multi-channel methodology was formulated for predicting transient SCR performance in the common 1D kinetic model throughout the WHTC cycle, in which urea maldistribution was quantified by the uniformity index with multiple channels operating in parallel to simulate the non-uniform input parameters. The final simulated results matched well with the transient data collected from engine experiments. The effect of urea maldistribution on the SCR performance and cumulative NOx emission was explored. The inlet urea maldistribution exerted a negative effect on the NOx reduction efficiency and NH\({}_{3}\) slip under transient circumstances. Up to a 9% enhancement in NOx reduction efficiency could be achieved by improving the uniformity of inlet ANR from 0.85 to 1.0. Probability density function (expressed as normal distribution)-based segment UI estimation demonstrated the most optimal correlation accuracy, with the corresponding calculated NOx efficiency within 2% for the transient cycle, which was efficient to operate and appropriate for parametric study.  
It should be mentioned that this approach could be coupled with multi-dimensional computational fluid dynamics (CFD) modelling and a detailed comprehensive measurement apparatus to determine the maldistribution factor. The accuracy of modelling, measurement and analysis tools, and the procedure's robustness had an impact on the prediction deviation, since relevant particulars of fluid, associated reactions, and the thermal field can be validated with a pre-determined accuracy. Thus, with the improvement of the measurement system, a better tool and more precise prediction could be provided for estimating real UI from any combination of catalyst technology, mixer design optimization, engine calibration, and emission test cycle. Further work could be conducted on SCR modelling and optimization, which could ultimately benefit the urea injection SCR system as well as aftertreatment system design to obtain the desired deNOx efficiency.  
Conceptualization, J.L. and K.C.; methodology, J.L.; software, K.S.; validation, J.L., G.S. and H.J.; formal analysis, H.J. and W.T.; investigation, H.J. and W.T.; resources, S.B.; data curation, H.J.; writing--original draft preparation, J.L.; writing--review and editing, K.S.; visualization, G.S.; supervision, K.S.; project administration, S.B.; funding acquisition, S.B. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
Data Availability Statement: Data is contained within the article.  
Author [PERSON] was employed by the company China National Heavy Duty Truck Group Jinan Special Vehicle Co., Ltd. Author [PERSON] was employed by the company Weichai Power Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.