Exhaust gas purifying apparatus for an internal-combustion engine

摘要

An exhaust gas purifying apparatus for an internal-combustion engine comprises a three-way catalyst, nitrogen oxide purifier that is disposed downstream of the three-way catalyst, an upstream oxygen sensor that is disposed between the three-way catalyst and the nitrogen oxide purifier, and a downstream oxygen sensor that is disposed downstream of the nitrogen oxide purifier. Means is provided for setting an abnormality determination criterion in accordance with the exhaust gas amount of the internal-combustion engine wherein the criterion is equivalent to a time period until the output of the downstream oxygen density sensor is read since the output of the upstream oxygen density sensor is changed to rich after the air-fuel ratio of the exhaust gas was turned over from lean to rich. Means is provided for determining that the nitrogen oxide purifier is abnormal when the output of the downstream oxygen density sensor has been changed to rich before the said time period represented by the criterion elapses.