Patent ID: 7802419

Claim:
An exhaust gas post treatment system for nitrogen oxide reduction of an internal combustion engine operated with excess air, comprising: an SCR catalytic converter disposed in the exhaust gas stream of the internal combustion engine for reducing nitrogen oxides, wherein a partial exhaust gas stream is branched off from said exhaust gas stream upstream of said SCR catalytic converter; a supply tank for a reduction agent; a metering device for receiving said reduction agent from said supply tank and for adding said reduction agent to said partial exhaust gas stream at a supply location, wherein said reduction agent is ammonia or a material that is adapted to release ammonia downstream of said supply location as a result of the hot exhaust gas, and wherein said partial exhaust gas stream is returned to said exhaust gas stream at a return location downstream of said supply location and upstream of said SCR catalytic converter, and wherein said SCR catalytic converter reduces the nitrogen oxides contained in said exhaust gas stream with the aid of the ammonia or released ammonia, by way of selected catalytic reduction, to nitrogen and water vapor; and an oxidation catalytic converter disposed in said partial exhaust gas stream upstream of said supply location for said reduction agent, wherein at engine operating conditions at which a reversal of said exhaust gas stream in a direction toward the internal combustion engine occurs, said oxidation catalytic converter is adapted to oxidize at least one of ammonia and reduction agent decomposition products that flow back.