Patent ID: 6845609

Claim:
An apparatus for an exhaust gas purification in an internal combustion engine comprising: a three-way catalyst disposed in an exhaust passage in the engine; a fuel injector for fuel supply to the engine; an engine operating condition detecting unit which detects an engine operating condition; and an engine control unit which sets an air-fuel ratio in response to the engine operating condition and controls a fuel injection amount of the fuel injector based upon the air-fuel ratio, wherein the engine control unit obtains a parameter related to NOx density at an engine exit or related to a catalyst temperature, wherein the engine control unit controls the air-fuel ratio to a lean range immediately after the engine is started and thereafter, to a rich range and performs switching the air-fuel ratio from lean to rich based upon the parameter, and wherein the engine control unit sets the air-fuel ratio controlled to be in the rich range as one of a plurality of values based upon the parameter, and also richer as the parameter shows higher NOx density or higher catalyst temperature.