Exhaust gas purification device for an engine

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 the downstream ends of the exhaust branch pipes;
• and
• a NOx catalyst positioned in the common exhaust pipe;
• wherein when a sulfate contamination regeneration process for regenerating the sulfate contamination of the NOx catalyst is performed by controlling the air-fuel ratio of the exhaust gas discharged from one of the cylinder groups to a rich air-fuel ratio and controlling the air-fuel ratio of the exhaust gas discharged from the other cylinder group to a lean air-fuel ratio, a purge gas including fuel vapor is purged into an intake pipe and a rich degree of the mixture gas in the cylinder from which the exhaust gas having a rich air-fuel ratio is discharged, is larger than a predetermined degree, one of an amount of purge gas and a ratio of the amount of purge gas relative to an amount of fresh air flowing through the intake pipe is decreased.

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 the downstream ends of the exhaust branch pipes;
• and
• a NOx catalyst positioned in the common exhaust pipe;
• wherein when a sulfate contamination regeneration process for regenerating the sulfate contamination of the NOx catalyst is performed by controlling the air-fuel ratio of the exhaust gas discharged from one of the cylinder groups to a rich air-fuel ratio and controlling the air-fuel ratio of the exhaust gas discharged from the other cylinder group to a lean air-fuel ratio and a purge gas including fuel vapor is purged into an intake pipe, one of an amount purge gas and a ratio of the amount of purge gas relative to an amount of fresh air flowing through the intake pipe is decreased substantially in inverse proportion to a rich degree of the mixture gas in the cylinder from which the exhaust gas having a rich air-fuel ratio is discharged.

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 the downstream ends of the exhaust branch pipes;
• and
• a NOx catalyst positioned in the common exhaust pipe;
• wherein when a sulfate contamination regeneration process for regenerating the sulfate contamination of the NOx catalyst is performed by controlling the air-fuel ratio of the exhaust gas discharged from one of the cylinder groups to a rich air-fuel ratio and controlling the air-fuel ratio of the exhaust gas discharged from the other cylinder group to a lean air-fuel ratio and a purge gas including fuel vapor is purged into an intake pipe, an air-fuel ratio of the mixture gas in each cylinder is controlled on the basis of a concentration of fuel vapor in the purge gas.

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 the downstream ends of the exhaust branch pipes;
• and
• a NOx catalyst positioned in the common exhaust pipe;
• wherein when a sulfate contamination regeneration process for regenerating the sulfate contamination of the NOx catalyst is performed by controlling the air-fuel ratio of the exhaust gas discharged from one of the cylinder groups to a rich air-fuel ratio and controlling the air-fuel ratio of the exhaust gas discharged from the other cylinder group to a lean air-fuel ratio, a purge gas including fuel vapor is purged into an intake pipe and a concentration of fuel vapor in the purge gas is larger than a predetermined concentration, the sulfate contamination regeneration process is prevented from being performed.