Patent ID: 8028517

Claim:
An exhaust gas purification device for an engine, comprising: a plurality of cylinders, the cylinders being divided into at least two cylinder groups; exhaust branch pipes connected to the cylinder groups at their upstream ends, respectively; a common exhaust pipe connected to downstream ends of the exhaust branch pipes; a NOx catalyst positioned in the common exhaust pipe; a controller configured to control an amount of purge gas such that if a sulfate contamination regeneration process for regenerating a sulfate contamination of the NOx catalyst is performed by controlling an air-fuel ratio of exhaust gas discharged from one of the cylinder groups to a rich air-fuel ratio and controlling an air-fuel ratio of exhaust gas discharged from an other cylinder group to a lean air-fuel ratio and a purge gas including fuel vapor is purged into an intake pipe, then an amount of purge gas is controlled to a target quantity for the sulfate contamination regeneration process on the basis of a concentration of fuel vapor in the purge gas determined by at least one sensor located downstream of the plurality of cylinders; exhaust branch pipe sensors provided to the exhaust branch pipes, respectively, that detect an air-fuel ratio of the exhaust branch pipes; a first NOx catalyst sensor provided in the common exhaust pipe upstream of the NOx catalyst, that detects an air-fuel ratio of the common exhaust pipe upstream of the NOx catalyst; and a second NOx catalyst sensor provided in the common exhaust pipe downstream of the NOx catalyst, that detects an air-fuel ratio of the common exhaust pipe downstream of the NOx catalyst, wherein the concentration of the fuel vapor of the purge gas is determined based on the first and second NOx catalyst sensors.