Exhaust gas after-treatment system

Abstract

An exhaust gas after-treatment system ( 10 ) for a gasoline engine ( 12 ) has a three-way catalyst ( 14 ) close-coupled to the gasoline engine, a particulate matter control device ( 18 ) positioned downstream of the three-way catalyst, a NOx control system ( 16 ) positioned downstream of the particulate matter control device. According to one method of operation, the gasoline engine is operated in a stoichiometric condition upon start-up of the engine, the exhaust gas flow generated by the engine is conducted through the exhaust system, and then the gasoline engine ( 12 ) is operated in a lean-burn condition after the NOx control system ( 16 ) attains a minimum operating temperature.

1 . An exhaust gas after-treatment system for a gasoline engine comprising:
 a three-way catalyst close-coupled to the gasoline engine;   a particulate matter control device positioned downstream of the three-way catalyst; and   a NOx control system positioned downstream of the particulate matter control device.   
     
     
         2 . The exhaust gas after-treatment system of  claim 1 , wherein the NOx control system comprises one or more of a lean NOx trap, a lean NOx catalyst, and a selective catalytic reduction catalyst. 
     
     
         3 . The exhaust gas after-treatment system of  claim 2 , wherein the NOx control system comprises a combination of a lean NOx trap and a selective catalytic reduction catalyst. 
     
     
         4 . The exhaust gas after-treatment system of  claim 1 , wherein the particulate matter control device comprises a wall-flow filter. 
     
     
         5 . The exhaust gas after-treatment system of  claim 4 , wherein the wall-flow filter comprises a porous ceramic honeycomb body. 
     
     
         6 . The exhaust gas after-treatment system of  claim 1 , wherein the particulate matter control device comprises one of cordierite, aluminum titanate, silicon carbide, and mullite. 
     
     
         7 . The exhaust gas after-treatment system of  claim 5 , wherein the porous ceramic honeycomb body comprises a cell density in the range of about 200 to about 600 cpsi, a wall thickness in the range of about 3 mil to about 20 mil and a total wall porosity in the range of about 30% to about 60%. 
     
     
         8 . The exhaust gas after-treatment system of  claim 5 , wherein the ceramic honeycomb body is comprised of alternately plugged inlet channels and outlet channels, the inlet channels having a larger hydraulic diameter than the outlet channels. 
     
     
         9 . The method of  claim 1 , wherein the exhaust gas after-treatment system reduces particles emitted by a gasoline engine to less than about 6×10 11  particles/km when measured by the PMP protocol over the NEDC regulatory drive cycle. 
     
     
         10 . A method for cleaning an engine exhaust gas flow, comprising:
 operating a gasoline engine in a stoichiometric condition upon start-up of the engine;   conducting an exhaust gas flow generated by the engine through an exhaust system comprising, successively, a flow-through substrate having a three-way catalytic coating, a wall-flow particulate filter, and a NOx control system; and   operating the gasoline engine in a lean-burn condition after the NOx control system attains a minimum operating temperature.   
     
     
         11 . The method of  claim 10 , wherein the gasoline engine is operated at a compression ratio of less than about 15. 
     
     
         12 . The method of  claim 10 , wherein the gasoline engine is spark ignited. 
     
     
         13 . The method of  claim 10 , wherein the engine produces soot at less than about 1×10 13  particles/km when measured by the PMP protocol over the NEDC regulatory drive cycle. 
     
     
         14 . The method of  claim 10 , wherein the exhaust system reduces emitted particles to less than about 6×10 11  particles/km when measured by the PMP protocol over the NEDC regulatory drive cycle. 
     
     
         15 . The method of  claim 10 , wherein the wall-flow particulate filter has a catalytically active coating. 
     
     
         16 . The method of  claim 10 , wherein the NOx control system comprises one or more of a lean NOx trap, a lean NOx catalyst, and a selective catalytic reduction catalyst. 
     
     
         17 . The method of  claim 10 , wherein the flow-through substrate is close-coupled to the engine. 
     
     
         18 . The method of  claim 10 , wherein the wall-flow particulate filter has a cell density in the range of about 200 to about 600 cpsi, a wall thickness in the range of about 3 mil to about 20 mil, and a total wall porosity in the range of about 30% to about 60%.