Method for adapting an engine to the fuel grade by decrementing the initial octane number of the fuel

A method of adapting an engine to an octane number of fuel by decrementing a learned octane number, the method starting out from a reference setting of an ignition advance in an engine operating range for a determined octane number, the reference setting corresponding to a pinging-free operation of the engine at a determined speed and a determined torque, the operating range of the engine being broken down into plural zones, each of the zones including an anti-pinging corrective value to be applied to the ignition advance of the reference setting, the method comprising:

• detecting the presence of pinging over a time period t;
• activating an ignition advance angle correction loop if pinging is detected over the time period t;
• incrementing a zones number counter in each time period by a number of zones in which the correction of the advance of the zone concerned is greater than a first predetermined threshold specific to this zone;
• testing the correction of the advance of each zone so as to determine whether the correction of the advance exceeds a second predetermined threshold specific to this zone;
• wherein, when the zones number counter reaches or exceeds a third predetermined threshold, or when the correction of advance of at least one zone exceeds the second threshold specific to this zone, a switch to a reference setting corresponding to a lower octane number is triggered.