An estimation method of the crank angle at which 50% of the fuel mass has been burnt in a cylinder of an internal combustion engine with spontaneous ignition of the mixture provided with a drive shaft coupled to a phonic wheel presenting a number N of teeth; the method contemplates the steps of: reading the passage of each tooth of the phonic wheel in front of a sensor; determining the angular speed of the drive shaft at each tooth event of the phonic wheel; determining at least one harmonic of the speed signal; determining an inverse mechanical model of the transmission; determining at least one torque harmonic by applying the inverse mechanical model to the speed signal harmonic; determining an algebraic function which puts a combustion index into relation with the phase of the nth torque harmonic; and determining the combustion index by applying the algebraic function to the nth torque harmonic.