Exhaust gas purification device for an internal combustion engine

Abstract

In an exhaust gas purification device, a three-way catalyst, an NO.sub.x absorbing-reducing catalyst and an NH.sub.3 adsorbing-denitrating catalyst are disposed in an exhaust gas passage of the internal combustion engine. The engine is provided with direct cylinder injection valves which inject fuel directly into the respective cylinders. A control circuit controls the amount of fuel injected from the injection valve so that the air-fuel ratio of the combustion in the cylinders becomes a lean air-fuel ratio during the normal operation of the engine. Therefore, a lean air-fuel ratio exhaust gas is discharged from the cylinders during the normal operation and NO.sub.x, in the exhaust gas is absorbed by the NO.sub.x absorbing-reducing catalyst. When the amount of NO.sub.x absorbed in the NO.sub.x absorbing-reducing catalyst increases to a predetermined level, the control circuit performs an additional fuel injection during the expansion stroke or exhaust stroke of cylinders in order to adjust the air-fuel ratio of the exhaust gas leaving the cylinders to a rich air-fuel ratio. The rich air-fuel ratio exhaust gas leaving the cylinders flows into the three-way catalyst and NO.sub.x in the exhaust gas is converted into NH.sub.3 at the three-way catalyst. When the rich air-fuel ratio exhaust gas flows through the NO.sub.x absorbing-reducing catalyst, NO.sub.x is released from the NO.sub.x absorbing-reducing catalyst and is reduced to N.sub.2 by NH.sub.3 in the exhaust gas.

Dependencies are inferred from the claim text.