Exhaust emission control system for lean engines and method for operating the system

Abstract

An emission control system for the cleaning of the exhaust gases of a lean burn engine with two or more cylinders comprises a first exhaust leg for the exhaust gases of a first group of cylinders and a second exhaust leg for the exhaust gases of a second group of cylinders. A nitrogen oxide storage catalyst is arranged in each exhaust leg. The two exhaust legs are combined downstream of the storage catalysts at a confluence to form a common exhaust leg. The common exhaust leg contains an SCR catalyst. The first and second groups of cylinders are each supplied alternately in periodic intervals with lean and rich air/fuel mixtures. Lean or rich exhaust gases are thus obtained in the combustion in the cylinders and released into the corresponding exhaust legs. Lean and rich exhaust gases are adjusted with respect to one another so as to result in a lean exhaust gas after the combination of the exhaust gases in the common exhaust leg. The regeneration of the storage catalysts may result in the formation of ammonia, which is stored by the SCR catalyst and reacted with nitrogen oxides which pass through the storage catalysts in an unwanted manner during the storage phases.

1 . Emission control system for the cleaning of the exhaust gases of a lean-burn engine with two or more cylinders, wherein a first exhaust leg accommodates the exhaust gases of a first group of cylinders and a second exhaust leg the exhaust gases of a second group of cylinders, and a nitrogen oxide storage catalyst is arranged in each exhaust leg and both exhaust legs are combined downstream of the storage catalysts at a confluence to form a common exhaust leg, characterized in that 
     an SCR catalyst is present in the common exhaust leg. 
   
   
       2 . Emission control system according to  claim 1 , wherein,
 an oxidation catalyst is arranged downstream of the SCR catalyst in the common exhaust leg.   
   
   
       3 . Emission control system according to  claim 2 , wherein
 SCR catalyst and oxidation catalyst are applied to a common honeycomb, the SCR catalyst being arranged on the inflow part of the honeycomb and the oxidation catalyst on the outflow part of the honeycomb.   
   
   
       4 . Emission control system according to  claim 1 , wherein
 an oxidation catalyst is present on a honeycomb in the form of a first layer and the SCR catalyst is applied as a second layer to this first layer.   
   
   
       5 . Method of operating the emission control system according to  claim 1 , wherein
 lean exhaust gas flows through the two nitrogen oxide storage catalysts during a storage phase, and rich exhaust gas during a regeneration phase, and storage phase and regeneration phase alternate cyclically, the regeneration phase of one of the two storage catalysts being initiated whenever the other storage catalyst is in its storage phase, and lean and rich exhaust gas being adjusted relative to one another so as to result in a lean exhaust gas after the exhaust gases have been combined in the common exhaust leg, and this exhaust gas being passed over an SCR catalyst.   
   
   
       6 . Method according to  claim 5 , wherein
 lean and rich exhaust gases are obtained by operating the cylinders assigned to the two storage catalysts with lean or rich air/fuel mixtures and released into the corresponding exhaust legs.   
   
   
       7 . Method according to  claim 5 , wherein
 the engine is operated constantly with a lean air/fuel mixture and the exhaust gas in the two exhaust legs is made richer in each case by injecting fuel or hydrocarbons for regeneration of the storage catalysts.