Patent ID: 8645046

Claim:
A controller for an internal combustion engine, comprising an air-fuel ratio estimating portion which performs a cylinder-by-cylinder air-fuel ratio estimation based on a detection value of an exhaust gas sensor arranged in a confluent portion of an exhaust gas flowed from multiple cylinders, wherein: the air-fuel ratio estimating portion defines: a first exhaust system model which outputs an air-fuel ratio at the confluent portion based on an air-fuel ratio in a cylinder; and a second exhaust system model which outputs the detection value of the exhaust gas sensor based on the air-fuel ratio at the confluent portion, the air-fuel ratio estimating portion includes: a confluent-portion-air-fuel ratio estimating portion which estimates the air-fuel ratio at the confluent portion based on the detection value of the exhaust gas sensor and the second exhaust system model; and a combust-air-fuel ratio estimating portion which estimates a combust-air-fuel ratio of each cylinder based on the air-fuel ratio at the confluent portion and the first exhaust system model.