Sensitivity of SCR Control Strategies to Diesel Exhaust Fluid Quality: A Simulation Study

This paper presents the evaluation of the impact of Diesel Exhaust Fluid (DEF) quality on the behavior of a controlled SCR system. Proper control of the Selective Catalytic Reduction system is crucial to fulfill NOx emissions standards of modern Diesel engines. Today, the urea concentration of DEF is not considered as a control system input. Moreover, Urea Quality Sensors (UQS) are now available to provide real time information of Diesel Exhaust Fluid quality. The impact of percent urea from 20 to 36% on the NOx emissions of a passenger car 2.2L Diesel engine is calculated using a reference SCR model and a reference SCR control tool in multiple NEDC transient conditions. Several control tunings are tested with different levels of feedback. Ammonia slip levels are also calculated. The results highlight the fact that with a standard SCR control system, a decreased urea concentration can cause a nonlinear increase of NOx emissions which is not corrected by feedback strategies that only use the exhaust gas sensor. Moreover, some scenarios are tested to capture the reactivity of the system to recover from a sudden change of urea concentration in between two transient cycles. Finally, the performance of a modified SCR control system that integrates the percent urea information as a control input is presented.