This diagram reports the measured injected mass (details on the experimental setup and method are given further in chapter 3) for each of the injection times imposed on the injector fed with air at 9 bar absolute pressure. It is worth to remark that this diagram does not represent the integral of the gas mass flow as function of time, but rather the measured injected mass for each single injection, whose duration is the injection time Dt. As can be noted, the needle bounces have a considerable influence on the total injected mass for injection duration shorter than the bounces duration (» 4 ms). The nonmonotonic behavior arises from the flow section variations caused by the needle bounces, whose intensities, as already pointed out, are related to the needle kinetic energy, which depends on the duration of the electromagnetic force applied: it may happen then that the stronger impact of the needle on the opening seat, obtained by increasing the injection time, causes a faster return of the needle towards the closed position, and hence a faster decrease of the injector outflow section, which implies a decrease on the injected mass. This conclusion, confirmed by means of oscilloscope observation of the solenoid current and armature acceleration waveforms together with mass flow data acquisition carried out for 9 bar air injection with injection duration between 1.8 and 2.5 ms, explains the parts of the injector flow chart which exhibits negative derivatives. A further increase of the injection time implies a return toward the open position of the needle, hence an increase in the injector flow section and, as a consequence, an increase in the injected mass.