A fuel injection system for an internal combustion engine, comprising at least one fuel electroinjector (1), and an electronic control unit (11) configured to supply the fuel electroinjector (1), in a fuel injection phase in an engine cylinder, with at least a first electrical command (Si) to cause a first fuel injection (P) to be carried out, and a second electrical command (S2) to cause a second fuel injection (M) temporally subsequent to the first fuel injection (P) to be carried out, the first and second electrical commands (Si, S2) being separated in time by an electrical dwell time (DT) such that the second fuel injection (M) starts without any discontinuity in time with respect to the first fuel injection (P).; The electronic control unit (11) is further configured to cause the first and second fuel injections (P, M) to be carried out in engine operating conditions characterized by reduced fuel ignition delays, wherein fuel combustion is prevalently diffusive and heat released during fuel combustion is sensitive to fuel injection law.