Method and apparatus for improving engine fuel economy

A system for controlling the air/fuel ratio (A/F) and an engine having a NO.sub.x trap located in the exhaust path, said system including a finite state machine;

• said machine increasing said A/F toward a lean A/F at a first predetermined rate in an open loop control scheme, and transitioning to a STATE 3 upon reaching a predetermined lean A/F;
• said machine exiting said STATE 3 if said NO.sub.x trap is full and decreasing said A/F toward stoichiometric at a second predetermined rate and upon reaching stoichiometric transitioning to a STATE 7 where the engine A/F is rich of stoichiometric in order to purge said trap;
• upon completion of said purge, said machine transitioning from STATE 7 to a STATE 5 where said A/F is stoichiometric;
• while in said STATE 5 said machine updating an open loop correction factor based on long term variations of a short term correction factor;
• said machine transitioning from said STATE 5 to said STATE 3 upon completion of said updating.

An article of manufacture comprising:

• a computer storage medium having a computer program encoded therein for controlling the amount of fuel supplied to an engine having an exhaust passage with a NO.sub.x trap located therein, said computer storage medium comprising:
• code for operating the engine in an open loop lean fuel control mode wherein the fuel calculation includes an open loop correction factor;
• code for calculating the NO.sub.x accumulated in said trap during said lean fuel control mode;
• code for ramping the A/F of the mixture supplied to said engine from a lean A/F to stoichiometric and switching from stoichiometric to a rich A/F for a predetermined purge time when the accumulated NO.sub.x exceeds a predetermined amount, and thereafter switching back to stoichiometric;
• code for ramping the A/F of the mixture supplied to said engine to said lean A/F from stoichiometric if the time since the last correction of said open loop correction factor is less than a predetermined update interval;
• and
• code for operating the engine in a closed loop fuel control mode at stoichiometric to update said open loop correction factor if the time since the last correction of said open loop correction factor exceeds said update interval.