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In this paper, the performance of a spark ignition engine fuelled with lean air-gasoline-hydrogen mixtures has been evaluated by using a numerical model.A multidimensional code (KIVA-3V) has been modified in order to model the engine combustion process using a hybrid combustion model adapted for dual fueling. In particular, two one-step overall reactions in a serial mechanism have been used. Based on computed results, the heat release and the cylinder pressure have been evaluated considering an operating point of an S.I. engine designed for gasoline feeding. Varying both the equivalence ratio of the air-gasoline mixture and the percentage of supplemented hydrogen results in tune with those available from the scientific literature have been obtained. It has been found that increasing the hydrogen-gasoline ratio, at very lean mixtures, it is possible to keep the performance of the engine fuelled with pure gasoline at stoichiometric mixtures. Furthermore, it has been shown that retarding the ignition time it is possible to have acceptable pressure peaks. Obviously, the results have to be validated by means of experimental data, but they showed the possibility of optimizing a conventional spark ignition engine in order to run with very lean mixtures so to reduce pollutant emissions.