Method of estimating a combustion index and/or the indicated torque in a cylinder of a four stroke internal combustion engine with spontaneous mixture ignition

A method of estimating a combustion index (MFB50) and/or the indicated torque (T ind ) in a cylinder (2) of a four stroke internal combustion engine (1); the method of estimating presenting the steps of: determining the angular speed (É i ) of the drive shaft (4); determining, by means of a frequency analysis of the angular speed (É i ) of the drive shaft (4), at least one harmonic (© n ) of the speed signal; determining an inverse mechanical model (H(jÉ)) of the transmission; determining at least one indicated torque harmonic (T ind,n ) by applying the inverse mechanical model (H(jÉ)) of the transmission to the speed signal harmonic (© n ); and determining the combustion index (MFB50) by applying a first algebraic function (¨) to the n th indicated torque harmonic (T ind,n ) and/or determining the indicated torque (T ind ) by applying a second algebraic function (“) to the n th indicated torque harmonic (T ind,n ).