Moreover, in recent experimental works [2, 3], the authors tested the simultaneous combustion of a gaseous fuel (compressed natural gas or LPG) and gasoline in a spark ignition engine, obtaining great improvement both in engine efficiency and pollutant emissions with respect to pure gasoline operation mode; the addition of CNG (or LPG) to the gasoline-air mixtures strongly improved the knocking resistance, thus allowing the engine to run at full load with a global stoichiometric mixture and with optimal combustion phase (spark advance). This third operating mode of bi-fuel engines, called “double fuel” combustion, requires small amounts of gaseous fuel, hence forcing the injectors to work in the non-monotonic zone of the injected mass diagram, where the control on airfuel ratio is poor. The authors hence decided to study and model the complex needle motion during the opening and closing phases so as to predict the amount of fuel injected for each injection time; the model realized has been calibrated and validated by means of experimental data collected on the test bench. Once calibrated, such a model can be effectively used to study and test injection strategies with the aim to “linearize” as much as possible the injected mass diagram, thus improving air-fuel ratio control in the lower injection times zone.