Abstract: The present invention pertains to the field of engine construction and may be used in the inlet systems of internal combustion engines for processing a portion of the rich air-fuel mixture fed into the main flow, wherein the liquid fuel is pulverised and converted into gas. The purpose of this invention is to increase fuel savings, to reduce the exhaust gas toxicity and to enable the use of a cheaper low-octane fuel. The method for preparing the air-fuel mixture comprises using the heat from the hot gases produced for heating the excessively rich mixture flow that contains between 10 and 100% of petrol to be used by the engine, wherein said heating is carried out before mixing the mixture with the second flow. The mixture is then submitted in one or more steps to the thermal action of activators with compensation for the air used during the reactions.