Air/fuel ratio control apparatus

An apparatus for use with an internal combustion engine having a throttle valve located in an induction passage for controlling the amount of air supplied to the engine through the induction passage and an exhaust passage through which exhaust gases are discharged from the engine to the atmosphere, to control flow of fuel vapor purged from a fuel vapor trap into the engine through a purge passage having a purge control valve provided therein, comprising:

• means for sensing engine operating conditions;
• means for sensing an amount of air supplied into the engine;
• means for sensing an air/fuel ratio of an air/fuel mixture supplied to the engine;
• means for controlling the purge control valve to permit fuel vapor purge through the purge passage based on the sensed engine operating conditions;
• means for calculating an air/fuel ratio feedback control correction factor to correct the air/fuel ratio within a predetermined range based on the sensed air/fuel ratio;
• means for sensing an amount of fuel vapor introduced through the purge passage into the engine;
• means for calculating a purge rate of the sensed fuel vapor amount with respect to the sensed air amount;
• means for calculating a deviation between the air/fuel ratio feedback correction factor calculated with the fuel vapor purge and the air/fuel ratio feedback correction factor calculated without the fuel vapor purge;
• means for calculating a gain value based on the calculated purge rate and the calculated air/fuel ratio feedback control correction factor deviation only during the fuel vapor purge;
• a memory for storing gain values in respective memory locations addressable by different fuel vapor amounts;
• means for storing the calculated gain value in the memory in a memory location corresponding to the sensed fuel vapor amount to update a gain value stored previously in the memory location;
• means for reading a gain value stored in the memory in the memory location corresponding to the sensed fuel vapor amount;
• means for calculating a shift value by which the air/fuel ratio feedback control correction factor is to be shifted based on the read gain value and the calculated purge rate;
• means for shifting the air/fuel ratio feedback control correction factor by the calculated shift value;
• means for calculating a value for the amount of fuel supplied to the engine based on the shifted air/fuel ratio feedback control correction factor and the sensed engine operating conditions;
• and
• means for controlling the amount of fuel supplied to the engine according to the calculated value therefor.