Natural gas represents today a promising alternative to conventional fuels for road vehicles propulsion, since it is characterized by a relatively low cost, better geopolitical distribution than oil, and lower environmental impact. This explains the current spreading of Compressed Natural Gas (CNG) fuelled S. I. engine, above all in the bi-fuel version, i.e. capable to run either with gasoline or with natural gas. This characteristic, on the one hand, permits the vehicle to go even when natural gas is not available, on the other hand requires the engine to be designed to run safely with gasoline, i.e. with compression ratio lower than what natural gas would allow. Moreover the electronic control units are programmed to adopt rich mixture and poor spark advance when running with gasoline at medium-high loads, in order to prevent the engine from dangerous knocking phenomena: this causes an increase in fuel consumption and pollutant emissions. Starting from these considerations, the authors decided to investigate on the benefit attainable by means of a double-fuel injection, i.e. the injection of a certain amount of natural gas during the gasoline operation in order to increase the knocking resistance of the mixture and to run the engine with “overall stoichiometric” mixture even at full load, thus improving both engine efficiency and its environmental impact. To this purpose, the authors carried out an experimental campaign on the engine test bed, equipped with a fully instrumented series production bi-fuel spark ignition engine; the gasoline injection was managed by means of a real-time controlled ECU, while the simultaneous injection of natural gas was performed by means of IGBT transistors properly designed for fuel injection or spark timing control connected to a counter/timing PCI board. The results obtained fuelling the engine with both fuels in stoichiometric proportion with air show, with respect to the pure gasoline operation, considerable increase in fuel economy without remarkable power losses, while, with respect to the pure natural gas operation, only power improvements have been achieved: these advantages may lead the way to the adoption of the double-fuel injection in bi-fuel-engines.