Method and desulfurizing nitrogen oxide storage catalysts in the exhaust gas system of a lean mix engine

Abstract

Lean burn engines require an exhaust gas system comprising nitrogen oxide storage catalysts for removal of nitrogen oxides from their exhaust gases. When the lean burn engine is operated with a sulphur-containing exhaust gas, the storage catalysts have to be desulphurized from time to time. During the desulphurization, there is the risk of high pollutant emissions. These emissions can be reduced when the cylinders of the lean burn engine are combined in two groups which release their exhaust gas to two assigned exhaust legs, in each of which is arranged at least one nitrogen oxide storage catalyst. The two exhaust legs are combined beyond the storage catalysts to form a common exhaust leg which contains a catalyst which possesses a three-way function under stoichiometric conditions. The two nitrogen oxide storage catalysts are desulphurized offset in time with respect to one another. While rich exhaust gas at a high temperature flows through one storage catalyst for desulphurization, lean exhaust gas flows through the second storage catalyst, in such a way that the combined exhaust gas is of stoichiometric composition over the entire desulphurization period. Under the stoichiometric conditions, the catalyst with three-way function is capable of converting hydrocarbons, carbon monoxide and nitrogen oxides to harmless components.

1 . Method for desulphurization nitrogen oxide storage catalysts in the exhaust gas system of a lean burn engine ( 1 ) with two or more cylinders,
 wherein   the cylinders of the lean burn engine are divided into a first group ( 2 ) and a second group of cylinders ( 2 ′) which release their exhaust gases into first ( 3 ) and second exhaust legs ( 3 ′) assigned to each, at least one nitrogen oxide storage catalyst ( 6 ) and ( 6 ′) being arranged in each exhaust leg and the two exhaust legs being combined downstream of the storage catalysts at a confluence ( 4 ) to form a common exhaust leg ( 5 ) which contains a catalyst ( 7 ) which, under stoichiometric conditions, possesses a three-way function, the first nitrogen oxide storage catalyst ( 6 ) being desulphurized by enriching the exhaust gas in the first exhaust leg ( 3 ) and raising its temperature to desulphurization temperature, while the exhaust gas in the second exhaust leg ( 3 ′) is kept constantly lean, the exhaust gases in the two exhaust legs being adjusted with respect to one another such that the exhaust gas in the common exhaust leg ( 5 ) has an air/fuel ratio of about lambda=1 over the entire desulphurization period, and the second nitrogen oxide storage catalyst ( 6 ′) being desulphurized in a corresponding manner offset in time with respect to the first nitrogen oxide storage catalyst ( 6 ).   
   
   
       2 . Method according to  claim 1 ,
 wherein,   for desulphurization of the storage catalysts, the exhaust gas is enriched by engine measures and its temperature is brought to desulphurization temperature.   
   
   
       3 . Method according to  claim 2 ,
 wherein   the engine measures are selected from the operation of the group of cylinders assigned in each case with a rich air/fuel mixture, the postinjection of fuel, a late combustion position, a multistage combustion or a combination of these measures.   
   
   
       4 . Method according to  claim 1 ,
 wherein,   for desulphurization of the storage catalysts, the exhaust gas is enriched by injecting fuel into the particular exhaust leg upstream of the nitrogen oxide storage catalyst, and its temperature is raised to desulphurization temperature by external heating.   
   
   
       5 . Method according to  claim 1 ,
 wherein,   an oxygen probe is arranged upstream and/or downstream of the catalyst with three-way function for regulation of the air/fuel ratio in the common exhaust leg.