System for calculating air-fuel ratio of each cylinder of multicylinder internal combustion engine

A system for calculating an air-fuel ratio of each cylinder of an internal combustion engine, in which a plurality of exhaust passages connected to the respective cylinders are merged together at an exhaust collecting portion and an air-fuel ratio sensor is arranged at the exhaust collecting portion, for calculating an air-fuel ratio of each cylinder based on a sensor detection signal from the air-fuel ratio sensor, the system comprising:

• means for calculating an air-fuel ratio at the exhaust collecting portion by receiving the sensor detection signal from the air-fuel ratio sensor;
• means for calculating a gas flow rate at the exhaust collecting portion based on a gas flow rate history of each cylinder;
• means for calculating a fuel quantity at the exhaust collecting portion from the calculated air-fuel ratio at the exhaust collecting portion, and the gas flow rate also at the exhaust collecting portion, as a burnt fuel quantity corresponding thereto;
• means for constructing an observer using a fuel quantity of each cylinder as a variable by a model in which the fuel quantity at the exhaust collecting portion is associated with the fuel quantity of each cylinder, and estimating the fuel quantity of each cylinder from the result of observation by the observer;
• and
• means for calculating an air-flow ratio of each cylinder from the fuel quantity of each cylinder thus estimated.