Stoichiometric and enrichment mixture control during different split

Abstract

A control system for a multi-cylinder internal combustion engine comprises a split engine control unit for operating the engine on partial cylinders when reduced engine power can adequately operate the vehicle, and first and second upstream exhaust conduits for directing exhaust gases from first and second group cylinders respectively to a common junction to which a common downstream exhaust conduit is connected for emitting the gases to the atmosphere. A first catalytic converter is disposed in the second exhaust conduit to be exposed to the stream of gases exhausted from the second group cylinders which are activated at all times and a second catalytic converter is disposed in the common downstream conduit to be exposed to the stream of gases exhausted from the first group cylinders directed through the first conduit and to the gases passed through the first catalytic converter. An exhaust gas sensor is provided in the common exhaust conduit upstream of the downstream converter. A feedback control circuit is responsive to signals from the gas sensor to effect closed loop mixture control in which the air-fuel mixture supplied to all the cylinders during the full cylinder mode is controlled to the stoichiometric point so that both converters each promote simultaneous reduction of the NOx component and oxidation of the HC and CO components. A feedback control disabling circuit is provided which is effective during the partial cylinder mode for controlling the air-fuel ratio to the rich side of the stoichiometric point to promote reduction of the NOx component in the first converter and oxidation of the HC and CO components in the second converter.