Individual cylinder air/fuel ratio control based on a single exhaust gas sensor

A control method for fueling a plurality of individual cylinders of a multi-cylinder internal combustion engine based on an output signal of a single oxygen sensor positioned to respond to exhaust gases generated in the individual cylinders, the control method comprising the steps of:

• high-pass filtering the oxygen sensor output signal to form an imbalance signal that is a measure of air/fuel ratio imbalances among the engine cylinders;
• parsing the imbalance signal into an array of imbalance values indicative of the measured air/fuel ratio imbalance at predefined points of an engine firing cycle;
• retrieving previously stored offset values that associate each engine cylinder with an imbalance value from said array;
• developing a fuel correction amount for each engine cylinder based on the imbalance value associated with such cylinder;
• adjusting a base fuel amount by the correction amount for each cylinder to form separate fuel commands for each of said engine cylinders;
• and
• fueling each cylinder based on its respective fuel command.