Exhaust gas purification device for an engine

Abstract

The exhaust gas purification device according to the present invention utilizes two NO absorbents in order to remove NO from the exhaust gas of an engine operated at a lean air-fuel ratio. The NO absorbents are disposed, in series, in the exhaust passage of the engine and a nozzle for supplying a reducing agent is disposed in the exhaust passage at a position between the two NO absorbents. Further, the device includes switching valves for changing the direction of the exhaust gas flow in the exhaust gas passage. When the switching valves are set to direct the exhaust gas flow to one direction, the exhaust gas first flows through one of the NO absorbents (first NO absorbent), and after passing through the first NO absorbent, the reducing agent is supplied to the exhaust gas before it flows into the other NO absorbent (second NO absorbent). Thus, the first NO absorbent absorbs NO in the exhaust gas, and the NO absorbed by the second NOabsorbent is released from the second NO absorbent. When the switching valves are switched, the exhaust gas flows in the opposite direction, and flows through the second NO absorbent and the first NO absorbent in this order. In this case, the second NO absorbent absorbs NO in the exhaust gas, and the NO absorbed by the first NO absorbent is released and reduced to N . Therefore, by reversing the direction of the exhaust gas periodically, the absorbing operation of NO and releasing/reducing operation of NO are performed by two NO absorbents alternately. Thus, it becomes possible to remove NO in the exhaust gas continuously while the NO absorbents are prevented from being saturated by the absorbed NO .