Natural gas represents today maybe the most valid alternative to conventional fuels for road vehicles propulsion. The main constituent of natural gas, methane, is characterized by a high autoignition temperature, which makes the fuel highly resistant to knocking: this allows a considerable downsizing of the engine by means of supercharging even under high compression ratio. Starting from these considerations, the authors realized a thermodynamic model of a 4-cilynder s.i. engine for the prevision of in-cylinder pressure, employing a two-zone approach for the combustion and adding sub-models to account for gas properties change and knocking occurrence. An extensive experimental campaign has been carried out on the test bed, equipped with a naturally aspirated bi-fuel s.i. engine (i.e. an engine which can run either with gasoline or with compressed natural gas), so as to set the model constants to the best matching values. Hence the model has been modified adding a turbocharger sub-model, thus allowing a prevision of the power and fuel consumption attainable by the same CNG engine when turbocharged.