Substoichiometric air addition to first stage of dual catalyst system

In a method for removing pollutants from the exhaust gas of an internal combustion engine in which fuel is burned with air at a substantially less than stoichiometric air-fuel ratio during cold engine operation and at a greater but substoichiometric air-fuel ratio during warm engine operation and in which the exhaust gas is contacted under net reducing conditions with a nitrogen oxide reduction catalyst in a first conversion zone and is then treated with added secondary air under net oxidizing conditions in a second conversion zone to oxidize carbon monoxide and hydrocarbons, the improvement which comprises:

• a. supplying a quantity of primary air not greater than stoichiometric to said first conversion zone and a further quantity of secondary air at least stoichiometrically equivalent to the amount of CO present, to said second conversion zone, said primary air being supplied during cold engine operation and said secondary air being supplied at all times during operation of the engine;
• b. shutting off the supply of primary air to said first conversion zone when the average air-fuel ratio reaches substantially its normal warm engine value;
• and
• c. continuing operation without substantial addition of external air to said first conversion zone.