Spark ignition engine including three-way catalyst with nox storage component

摘要

A spark ignition engine with an exhaust system having a catalyst, which includes a three-way catalyst (TWC) and a NOx storage component, and an engine control unit is provided. The engine control unit is programmed to control the air-to-fuel ratio of the engine to run at the stoichiometric air-to-fuel ratio during normal running conditions and to run lean of the stoichiometric air-to-fuel ratio during a defined portion of an engine speed/load. The engine control unit also determines the amount of NOx contacting the TWC during lean running operation in response to data input from a sensor means, thereby a remaining NOx storage capacity of the TWC is determined. The control unit is programmed to return the air-to-fuel ratio to stoichiometry when the NOx storage capacity is below a pre-determined value. The engine and its components are arranged such as to substantially prevent passing more NOx to atmosphere during an engine cycle compared with a spark ignition engine run continuously at stoichiometric conditions.