Estimating tailpipe NOx concentration using a dynamic NOx/ammonia cross sensitivity model coupled to a three state control oriented SCR model

A dynamic NOx sensor model is developed to remove ammonia cross sensitivity from production NOx sensors mounted downstream of Diesel-engine selective catalytic reduction (SCR) systems. The model is validated for large amounts of ammonia slip during different engine transients. A three-state nonlinear control oriented SCR model is also developed to predict the NH3 concentration downstream of the SCR (NH3 slip). NH3 slip is then used as an input for modeling the cross sensitivity of a production NOx sensor and calculating the actual NOx concentration in the presence of NH3 contamination. The cross sensitivity is considered to be a function of temperature, normalized ammonia slip rate (NASR) and time. The validation results show that the developed model has an acceptable accuracy for the actual NOx concentration downstream of the SCR. This model should be of utility for engine emission control strategies such as SCR control.