contd...

NH₃/NO_x Ratio and Ammonia Slip

Based on the stoichiometric considerations, 90% conversion of NO_x requires the NH₃/NO_x molar ratio of about 0.9, assuming NO₂ constitutes 10% of NO_x. Concentration of NO_x in the exhaust gases varies depending upon engine operating conditions. Hence, for a vehicle continuously variable injection rate of urea is required.   If more urea than stoichiometric requirements is injected, unreacted ammonia is emitted in the exhaust which is called ‘ammonia slip’. To minimize ammonia slip, a dynamic urea dosage system governed by engine operating conditions is to be employed.  Even with the dynamic dosage system, ammonia slip occurs during transient operation. 
Typical conversion efficiency at different NH₃/NO molar ratio and ammonia slip are shown on Fig 6.12.  With increase in NH₃/NO molar ratio NO_x conversion efficiency increases and but the ammonia slip also increases. An oxidation catalyst is therefore, added to SCR system to prevent emissions of ammonia.