As shown, the needle opening and closing impacts appear as cusps on the solenoid current and as prominent spikes on the accelerometer output signal. In particular, Fig. 3 shows that for the tested injector fed with air at 9 bar, the opening phase bounces go on for about 4 ms, after start of injection, while in the closing phase their duration is shorter, i.e., about 3 ms after stop of injection. The importance of these bounces relies on the great variations they produce on the injected mass, since the instantaneous flow section depends on the needle position; assuming a linear correlation between flow section area and needle position, it results that during the transient opening or closing phases, the injected mass depends on the integral of the needle position. When the injection time is below the opening phase transient duration (»4 ms for the injector tested), the needle transient not only is not completed, but is also influenced by the duration of the injection itself. The impact energy of the needle on the opening stop surface, in fact, depends on its kinetic energy, which is related to the duration of the electromagnetic force applied, and hence to the injection time. It results that, for injection time shorter than 4 ms, changing the injection duration modifies the needle movement and hence the integral of its position, which causes a variation in the injected mass. This introduces a non-linear dependence between the injected mass and the injection time, as is evident in the injector flow chart shown in Fig. 5.