Is closed-loop SCR control required to meet future emission targets?

To meet 2010 emission targets, optimal SCR systemperformance is required. In addition, attention has to bepaid to in-use compliance requirements. Closed-loopcontrol seems an attractive option to meet the formulatedgoals. This study deals with the potential and limitationsof closed-loop SCR control.High NOx conversion in combination with acceptable NH3slip can be realized with an open-loop control strategy.However, closed-loop control is needed to make theSCR system robust for urea dosage inaccuracy, catalystageing and NOx engine-out variations. Then, the systemmeets conformity of production and in-use compliancenorms.To demonstrate the potential of closed-loop SCR control,a NOx sensor based control strategy with cross-sensitivity compensation is compared with an adaptivesurface coverage/NH3 slip control strategy and an open-loop strategy. The adaptive surface coverage/NH3 slipcontrol strategy shows best performance over simulatedESC and ETC cycles.SCR catalyst dynamics, time delay in the urea injectionand maximum NH3 slip targets limit the performance ofclosed-loop SCR control. If new reagent dosage systemsand future catalyst technology are able to relieve theselimitations, closed-loop control has the potential toreduce the calibration effort and to improve the transientcontrol performance