Method for estimating richness of air-fuel mixture in a cylinder of a combustion engine

The method involves establishing a physical model (RTM) representing ejection of gas from cylinders and its movement from an exhaust circuit until it reaches a sensor. The model is coupled with an extended Kalman filter (KF) type non linear state observer, by taking into account measurement of fuel content provided by the sensor. A value of the fuel content at input of the exhaust circuit is deduced.