1. Field of the Invention
The present invention relates to an exhaust gas purification device for an engine.
2. Description of the Related Art
With respect to an engine in which a lean air-fuel mixture is burned, the same applicant has proposed a new type of engine in which a NO.sub.x absorbent is arranged in the exhaust passage of the engine. This NO.sub.x absorbent absorbs the NO.sub.x when the air-fuel ratio of the exhaust gas flowing into the NO.sub.x absorbent is lean, and this NO.sub.x absorbent releases the absorbed NO.sub.x when the air-fuel ratio of the exhaust gas flowing into the NO.sub.x absorbent becomes rich. In this engine, the NO.sub.x produced when the lean air-fuel mixture is burned is absorbed by the NO.sub.x absorbent. The air-fuel ratio of the exhaust gas flowing into the NO.sub.x absorbent is temporarily made rich before the absorbing ability of the NO.sub.x absorbent is saturated, and at this time, the NO.sub.x is released from the NO.sub.x absorbent. In addition, at this time, the NO.sub.x thus released is reduced (see copending U.S. patent application No. 66,100 derived from PCT application JP92/01279).
In this engine, for example, if the air-fuel mixture fed into the engine cylinder is made rich, a large amount of unburned HC and CO etc. are discharged from the engine. In addition, at this time, since the air-fuel ratio of the exhaust gas flowing into the NO.sub.x absorbent becomes rich, the NO.sub.x is released from the NO.sub.x absorbent. At this time, a part of unburned HC and CO etc. is used for reducing the NO.sub.x discharged from the engine, and the remaining NO.sub.x is used for reducing the NO.sub.x released from the NO.sub.x absorbent. Accordingly, in this case, to prevent the NO.sub.x from being discharged into the outside air, the amount of the exhaust gas discharged from the engine must be controlled so that it becomes equal to an amount which is necessary to reduce both the NO.sub.x discharged from the engine and the NO.sub.x released from the NO.sub.x absorbent.
However, it is difficult to control the amount of the exhaust gas discharged from the engine so that it becomes equal to the minimum amount necessary to reduce the whole NO.sub.x, and actually, the amount of unburned HC and CO etc. becomes smaller or larger than the amount necessary to reduce the whole amount. In this case, a problem arises in that, if the amount of unburned HC and CO etc. becomes smaller than the amount necessary to reduce the whole NO.sub.x, since the whole NO.sub.x is not reduced, the NO.sub.x is discharged into the outside air, and that, if the amount of unburned HC and CO etc. becomes larger than the amount necessary to reduce the whole NO.sub.x, since the whole unburned HC and CO etc. is not oxidized, the unburned HC and CO etc. is discharged into the outside air.