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3 -Way Catalytic Converters

The essential condition to simultaneously oxidize CO and HC and reduce NO_x on the same catalyst bed is to operate engine at very close to stoichiometric air-fuel ratio. Under stoichiometric engine operation, enough reducing gases CO and HC are present in the exhaust to reduce NO_x to N₂ and at the same time enough oxygen to oxidize CO and HC. Thus, all the three pollutants are removed simultaneously in a 3-way catalyst. . Dependence of conversion efficiency for the three pollutants on fuel-air equivalence ratio in a 3-way catalyst is shown in Fig. 5.14 . High conversion rates of over 80% of all the three pollutants are obtained in a small window of about 0.12 A/F unit width (0.997 < f < 1.005) around the stoichiometric air-fuel ratio.