3 combusted at the moment k a ricostr and b ricostr are two identified coefficients which are characteristic of the engine 3 and are obtained experimentally resolving the above equation with respect to afr cil k provides afr cil k 1 b ricostr u2605 afr comp k u2212 a ricostr u2605 afr comp k u2212 1 afr cil k c 1 u2605 afr comp k u2212 c 2 u2605 afr comp k u2212 1 it has been observed that the coefficients c 1 and c 2 are not constant but depend on the operating point of the engine 3 and in particular on the number of revolutions and the torque transmitted or the quantity of air introduced by the engine 3 it is preferable therefore to implement a table which supplies the values of c 1 and c 2 corrected for the current operating point of the engine 3 in a known manner within the reconstruction device 14 it has further been observed that the coefficients a ricostr and b ricostr and therefore the coefficients c 1 and c 2 are not independent from one another but are connected by the equation it is therefore possible to reduce the mathematical model to a single coefficient it will be appreciated from the above description that it is possible to estimate the value afr cil k of the air fuel ratio of the final cylinder 3 combusted by means of a linear composition of the last measured value afr comp k and the penultimate measured value afr comp k u2212 1.