Method for controlling the combustion of a fuel-air mixture for a turbocharged internal combustion engine with controlled ignition, in particular a petrol engine

The method involves determining a crank angle value at which maximum pressure of a cylinder (12) occurs in a combustion chamber (14), during combustion of a fuel mixture. The value is compared with a maximum angle threshold value representing an abnormal combustion in the chamber. Start of the abnormal combustion is detected when the value reaches the threshold value and when a spark plug (38) is not actuated. Amount of fuel i.e. vehicular natural gas (VNG), is fed into the mixture by VNG injector (54) to vary global octane number of the mixture to reduce the angle.