Abstract: An engine control computer estimates the cumulative SO.sub.x stored in a NO.sub.x trap and enters a SO.sub.x purge mode of operation when a maximum storage capacity limit is exceeded. SO.sub.x accumulation is estimated based on fuel flow to the engine, A/F and trap temperature. The purge mode is terminated when the estimated SO.sub.x storage level drops below a minimum threshold level during the purging operation.

Abstract: A control method is described for an engine having a throttle body coupled to the engine intake manifold and a bypass air passageway coupled to the throttle body via an electronically controllable bypass valve. A single measurement of air inducted through both the throttle body and the bypass air passageway is provided by an air flow meter. Fuel is delivered to the intake manifold in relation to the single air measurement. An air flow correction value is generated from a functional relationship between engine torque and air/fuel ratio during lean air/fuel operating conditions. Air flow through the bypass passageway is increased by the air flow correction value to compensate for engine torque loss resulting from such lean operation.