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An engine having in an exhaust system a lean NOx catalyst for purifying nitrogen oxide in an oxygen excessive exhaust gas by using a reducing agent, includes an intake air quantity decreasing device for decreasing a quantity of intake air flowing through an intake system when the nitrogen oxide need

An engine having in an exhaust system a lean NOx catalyst for purifying nitrogen oxide in an oxygen excessive exhaust gas by using a reducing agent, includes an intake air quantity decreasing device for decreasing a quantity of intake air flowing through an intake system when the nitrogen oxide needs to be purified by the lean NOx catalyst, and a combustion heater provided separately from the internal combustion engine also serves as a CO.sub.2 adding device for adding carbon dioxide to the intake air when a flowing quantity in the intake system decreases by operating the intake air quantity decreasing device. With this arrangement the reducing agent such as hydro carbon and the like can be supplied even when eliminating an auxiliary injection hitherto needed in the internal combustion engine having the lean NOx catalyst.

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[ What is claimed is:] [1.]1. An internal combustion engine having in an exhaust system a lean NOx catalyst for purifying nitrogen oxide in an exhaust gas in an oxygen excessive state by using a reducing agent, comprising:CO.sub.2 concentration increasing means for increasing a concentration of carb

[ What is claimed is:] [1.]1. An internal combustion engine having in an exhaust system a lean NOx catalyst for purifying nitrogen oxide in an exhaust gas in an oxygen excessive state by using a reducing agent, comprising:CO.sub.2 concentration increasing means for increasing a concentration of carbon dioxide in an intake air flowing through an air intake system of said internal combustion engine; andintake air quantity decreasing means for decreasing a quantity of the intake air flowing through the intake system when the nitrogen oxide is required to be purified by said lean NO.sub.x catalyst,wherein said CO.sub.2 concentration increasing means is a combustion device provided separately from a body of said internal combustion engine, and said combustion device functions as a CO.sub.2 adding device for adding the carbon dioxide to the intake system by introducing a combustion gas into the intake system of said internal combustion engine, andwherein said CO.sub.2 concentration increasing means functioning as the CO.sub.2 adding device adds the carbon dioxide to the intake air in the intake system when the flowing rate of the intake air in the intake system is decreased by the operation of said intake air quantity decreasing means.