Exhaust gas purification device for an engine

Abstract

An exhaust gas purification device of the present invention includes a three-way reducing and oxidizing catalyst disposed on an exhaust passage of an engine, and an NO.sub.x absorbent which absorbs NO.sub.x in the exhaust gas when the air-fuel ratio of the exhaust gas is lean and releases absorbed NO.sub.x when the air-fuel ratio of the exhaust gas is rich; the engine in the present invention is controlled in most of its operating region in such a manner that the operating air-fuel ratio of the engine oscillates between a rich air-fuel ratio and a lean air-fuel ratio about a center value of a stoichiometric air-fuel ratio; when the operating air-fuel ratio of the engine becomes lean, the ability of the three-way catalyst for reducing NO.sub.x in the exhaust gas becomes low, and a part of NO.sub.x in the exhaust gas passes through the three-way catalyst, and, since the air-fuel ratio of the exhaust gas is lean, this NO.sub.x is absorbed in the NO.sub.x absorbent located downstream of the three-way catalyst; when the operating air-fuel ratio of the engine becomes rich, the ability of the three-way catalyst for oxidizing HC and CO in the exhaust gas becomes low, and a part of HC and CO in the exhaust gas passes through the three-day catalyst; since the air-fuel ratio of the exhaust gas is rich, absorbed NO.sub.x is released from the NO.sub.x absorbent and this NO.sub.x is reduced as HC and CO passes through the three-way catalyst; therefore, the pollutants such as HC, CO, NO.sub.x are almost completely removed.

Dependencies are inferred from the claim text.