Engine exhaust purification device

An exhaust purification device comprising:

• a first catalyst (3a) provided in an engine exhaust pipe (2),
• a second catalyst (3b) provided in downstream of the first catalyst (3a),
• a front sensor (4) which detects an exhaust characteristic flowing into the first catalyst (3a),
• a first rear sensor (5a) which detects an exhaust characteristic flowing out of the first catalyst (3a),
• a second rear sensor (5b) which detects an exhaust characteristic flowing out of the second catalyst (3b), and
• a microprocessor (6) programmed to:
• compute the oxygen storage amount stored in the first catalyst (3a) using the exhaust characteristic detected by the front sensor (4) and the exhaust characteristic detected by the first rear sensor (5a) during normal running,
• compute the oxygen storage amount stored in the first catalyst (3a) and second catalyst (3b) using the exhaust characteristic detected by the front sensor (4) and the exhaust characteristic detected by the second rear sensor (5b) after running at a lean air-fuel ratio,
• control the engine air-fuel ratio so that, during normal running, the oxygen storage amount of the first catalyst (3a) is a target amount based on the computed oxygen storage amount in the first catalyst (3a),
• control the engine air-fuel ratio so that, after running at a lean air-fuel ratio, the oxygen storage amount stored in the first catalyst (3a) and the second catalyst (3b) is a target amount based on the computed oxygen amount in the first catalyst (3a) and the second catalyst (3b)