Engine exhaust emission control system providing on-board ammonia generation

1. An engine exhaust emission control system providing on-board ammonia generation, said system comprising:

a NOx sensor for detecting NOx in exhaust gas from an engine;

a lean NOx trap (LNT) for removing and retaining NOx from said engine exhaust gas;

an on-board reformer for providing reformate fuel to said system;

a first conduit for conveying reformate fuel from said on-board reformer to said LNT, said reformate fuel reacting with NOx retained in said LNT, reducing said NOx and thereby forming NH 3 ;

a selective catalytic reduction (SCR) catalyst disposed downstream of said LNT for catalytically reducing NOx;

a second conduit by-passing said LNT for conveying said engine exhaust gas to said SCR catalyst;

a valve disposed upstream of said LNT and said SCR catalyst, said valve operating between a first position and a second position to selectively control the flow of said engine exhaust gas, said first position directs said engine exhaust gas through said LNT and said SCR catalyst, said second position directs said engine exhaust gas through said second conduit to by-pass said LNT; and

a controller comprising an algorithm that monitors engine operating conditions and exhaust gas conditions, estimates the cumulative amount of exhaust gas created by said engine and the amount of NOx stored in said LNT, and calculates the amount of NOx required to be converted to NH 3 ;

wherein, while reformate fuel is being conveyed from said reformer to said LNT, said valve is in said second position to cause said exhaust gas to bypass said LNT and flow directly to said SCR catalyst.

2. The engine exhaust emission control system of claim 1 , wherein said controller further operates to determine an amount of reformate fuel required to regenerate said LNT, thereby producing NH 3 , and a time interval between successive introductions of said reformate fuel into said LNT to meet specified emission requirements.

3. The engine exhaust emission control system of claim 1 , wherein said engine operating conditions are monitored by sensors.

4. The engine exhaust emission control system of claim 3 , wherein said sensors are selected from the group consisting of an engine speed sensor, a torque sensor, a pedal position sensor, a mass air flow sensor, and combinations thereof.

5. The engine exhaust emission control system of claim 1 , wherein said LNT has an operating temperature of about 225° C. to about 485° C.

6. The engine exhaust emission control system of claim 5 , wherein said LNT has an operating temperature of about 250° C. to about 375° C.

7. An engine exhaust emission control system providing on-board ammonia generation, said system comprising:

a NOx sensor for detecting NOx in exhaust gas from an engine exhaust stream;

a lean NOx trap (LNT) for removing and retaining NOx from said exhaust gas;

an on-board reformer for providing reformate fuel to said system, wherein said reformer is off-line from said engine exhaust stream;

a first conduit for conveying reformate fuel from said on-board reformer to said LNT, said reformate fuel reacting with NOx retained in said LNT, reducing said NOx and thereby forming NH 3 ;

a selective catalytic reduction (SCR) catalyst disposed downstream of said LNT for catalytically reducing NOx;

a second conduit by-passing said LNT for conveying said engine exhaust gas to said SCR catalyst;

a valve disposed upstream of said LNT and said SCR catalyst, said valve operating to selectively control the flow of said engine exhaust gas to said LNT and/or said SCR catalyst; and

a controller comprising an algorithm that monitors engine operating conditions and exhaust gas conditions, estimates the cumulative amount of exhaust gas created by said engine and the amount of NOx stored in said LNT, and calculates the amount of NOx required to be converted to NH 3 ;

wherein, while reformate fuel is being conveyed from said reformer to said LNT, said valve operates to cause said exhaust gas to bypass said LNT and flow directly to said SCR catalyst.

8. An engine exhaust emission control system providing on-board ammonia generation, said system comprising:

a NOx sensor for detecting NOx in exhaust gas from an engine;

a lean NOx trap (LNT) for removing and retaining NOx from said engine exhaust gas;

an on-board reformer for providing reformate fuel to said system;

a first conduit for conveying reformate fuel from said on-board reformer to said LNT, said reformate fuel reacting with NOx retained in said LNT, reducing said NOx and thereby forming NH 3 ;

a selective catalytic reduction (SCR) catalyst disposed downstream of said LNT for catalytically reducing NOx;

a second conduit by-passing said LNT for conveying said engine exhaust gas to said SCR catalyst;

a valve disposed upstream of said LNT and said SCR catalyst, said valve operating between a first position and a second position to selectively control the flow of said engine exhaust gas, said first position directs said engine exhaust gas through said LNT and said SCR catalyst, said second position directs all of said engine exhaust gas entering said valve through said second conduit to by-pass said LNT; and

a controller comprising an algorithm that monitors engine operating conditions and exhaust gas conditions, estimates the cumulative amount of exhaust gas created by said engine and the amount of NOx stored in said LNT, and calculates the amount of NOx required to be converted to NH 3 ;

wherein, while reformate fuel is being conveyed from said reformer to said LNT, said valve is in said second position to cause all of said exhaust gas entering said valve to bypass said LNT and flow directly to said SCR catalyst.