SCR Sensing and Estimation Systems

According to the SCR control-oriented model shown in Eq. (14.29), the key states of an SCR catalyst are SCR inlet NO(x) concentration, SCR inlet NH3 concentration, exhaust flow rate, SCR catalyst temperature, SCR-outlet NO concentration, SCR-outlet NH3 concentration, and SCR catalyst ammonia coverage ratio. Current vehicle onboard sensors are capable of measuring gas flow rate, temperature, NOx concentration, and NH3 concentration. However, the current production NOx sensors are cross-sensitive to NH3, which make the accurate measurement of SCR-outlet NOx concentration difficult. Without the information of SCR-outlet NOx concentration, closed-loop SCR control is difficult, and so is the diagnostics of SCR NOx reduction capability. Moreover, the catalyst ammonia coverage ratio ( NHa) is also hard to be directly measured. Ammonia coverage ratio is an inherent state in the SCR catalyst which directly affects the catalytic reactions. This state is 

NOx sensor ammonia cross-sensitivity and SCR catalyst ammonia coverage ratio have been the main challenges of the SCR sensing systems. The rest of this subsection provides some insight into these two problems and introduces some current solutions. 

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