method and apparatus for improving vehicle fuel economy

A system for controlling engine air/fuel ratio including at least one exhaust gas oxygen (EGO) sensor and a finite state machine;

• said machine assuming a STATE 0 in which the engine is operated at stoichiometric A/F in response to computer initialization and engine warmup;
• said machine responsive to predetermined entry conditions to transition from said STATE 0 to a STATE 1 upon the first occurrence of said entry conditions;
• said machine transitioning from said STATE 1 to a STATE 2 in response to a first predetermined number of EGO switches within a predetermined time interval and otherwise returning to STATE 0;
• said machine increasing said engine A/F toward a lean A/F at a first predetermined rate in an open loop control scheme, while in said STATE 2 and transitioning to a STATE 3 upon reaching a predetermined lean A/F;
• said machine maintaining said STATE 3 for a predetermined time interval and thereafter transitioning to a STATE 4 where said engine A/F is reduced toward stoichiometric at said first predetermined rate;
• said machine transitioning to a STATE 5 upon said engine A/F reaching stoichiometric;
• said machine while in said STATE 1 or said STATE 5 calculating fuel flow based on calibration data modified by a short term feedback correction factor, and updating an open loop correction factor based on the long term variations of said short term correction factor;
• said machine transitioning from said STATE 5 to said STATE 2 upon the first to occur of completion of said updating or upon detection of a second predetermined number of EGO switches.

A method of controlling the air/fuel ratio of an internal combustion engine comprising a sequence of the following steps:

• controlling the flow of fuel to the engine during a closed loop fuel control mode to achieve a stoichiometric A/F ratio while updating an open loop fuel correction factor,
• transitioning to an open loop fuel control mode when predetermined open loop entry conditions are met and increasing the air/fuel ratio from said stoichiometric ratio to a predetermined lean ratio using said open loop correction factor,
• periodically switching from said open loop fuel control mode to said closed loop fuel control mode to update said long term correction factor by operating said engine in said closed loop mode until an A/F is achieved within a predetermined tolerance of said stoichiometric ratio or until a predetermined number of switches between a rich A/F and a lean A/F are detected and after said update returning to said open loop fuel mode,
• whereby said long term correction factor used during the open loop fuel control mode is periodically updated under existing engine operating conditions.

A system for controlling the air/fuel ratio of an internal combustion engine comprising;

• at least one engine exhaust sensor for detecting whether the engine A/F is rich or lean of a stoichiometric A/F ratio,
• a computer for controlling the flow of fuel to said engine during a closed loop fuel control mode to achieve said stoichiometric A/F ratio while updating an open loop fuel correction factor,
• said computer transitioning to an open loop fuel control mode upon completion of said updating and increasing the A/F from said stoichiometric A/F ratio to a predetermined lean ratio using said open loop correction factor,
• said computer being programmed to exit said open loop fuel control mode and return to said closed loop mode of engine A/F control to update said long term correction factor by operating said engine in said closed loop mode until an A/F is achieved within a predetermined tolerance of said stoichiometric ratio or until a predetermined number of switches between a rich A/F and a lean A/F are detected by said EGO sensor and after said update to return to said open loop fuel mode,
• whereby said long term correction factor used during the open loop fuel control mode is updated under existing engine operating conditions.