Internal Flow Characteristics and Vortex Evolution of Fuel Injector with Dynamic Needle Oscillation

To enable efficient and clean combustion in internal combustion engines, precise control over fuel injection and atomization is essential. Dynamic oscillation of the injector needle is a primary factor governing internal flow and atomization performance; however, its underlying mechanisms are substantially more complex than the commonly assumed static eccentricity model, and remain insufficiently understood. This study systematically examines this dynamic phenomenon by analyzing the impact of needle oscillation on the internal flow dynamics and vortex evolution in both single-hole and multi-hole injectors, with particular attention to the critical needle opening and closing stages. The results reveal that dynamic needle oscillation induces reverse-rotating large-scale vortices within the sac, which further interact to form small-scale vortex pairs. In the four-hole injector, flutter consistently suppresses the mass flow rate across all orifices, exhibiting pronounced asymmetry; the orifices located outside the flutter plane experience the strongest reduction, especially at low needle lifts. Moreover, under high-pressure (200 MPa) compressible conditions, the suppressive effect of flutter on mass flow rate becomes more significant, accompanied by intensified flow instability and persistent pressure oscillations. The dynamic mechanisms identified in this work overcome the limitations of conventional static simulations. These findings not only offer new insights for improving injection performance in traditional diesel engines but also provide important theoretical and engineering guidance for the design of injection systems for emerging green fuels such as liquid ammonia, methanol, and biodiesel.