The optimization of fuel efficiency and the minimization of the residual gas fraction require individual cylinder control of the amounts of inducted air mass and injected fuel mass. Determination of an individual cylinder air/fuel ratio (AFR) regulator is based on the measured AFR for each cylinder, using 4 proportional UEGO sensors. The innovative character of this study describes a unified and robust individual cylinder AFR estimator, using a single measuring point: a proportional oxygen sensor located in the exhaust manifold.The model used for the estimator is a state model such that the dimension of the state and measurement matrices are unique, whatever the manifold configuration and the sensor position (confluence point or exhaust manifold: unified model), the engine speed (robust model). The study and validation of the estimator was done on an exhaust gas simulator from the exhaust valves to the catalytic converterThe nonlinear and nonstationary behaviour of the exhaust gas propagation process, and thus of the AFR estimator has been brought into evidence. Three estimators with criteria for « breakup of models » have been developped (kalman estimator, estimator with stochastic nonstationary noise state variable, state reconstructor).The estimator's quality improves with its complexity, the state reconstructor requires setting up a state matrix map of the exhaust manifold.