Investigating hydrogen direct injection technology: A comparative analysis of nozzle geometries for enhanced mixing in internal combustion engines

Hydrogen emerges as a promising green energy carrier for heavy industry and transportation. In this context, the hydrogen internal combustion engine (ICE) represents a cost-effective and versatile solution to achieve rapid decarbonization of the transport sector. The current study investigates direct injection technology, emphasizing the need to comprehend and control hydrogen mixing inside the combustion chamber of an ICE. Two prevalent nozzle geometries were compared: the hollow-cone (HC) with outwardly opening needle and the multi-hole (MH) with inwardly opening needle. The distinctive mixing behavior of these geometries was systematically characterized experimentally and through computational fluid dynamics (CFD) simulations. Experiments were performed using commercially available gasoline direct injection (GDI) injectors operated with pure hydrogen, injecting into a constant volume chamber (CVC) pressurized with nitrogen. The high-speed Schlieren technique was employed to measure jet penetration and jet area, providing a qualitative description of the mixing behavior. CFD simulations utilized the Reynolds-averaged Navier–Stokes (RANS) turbulence model to quantitatively compare the mixing properties of both nozzle geometries. This analysis was conducted first in a quiescent environment, replicating the CVC experiments, and then in a typical light-duty pent-roof engine environment. Turbulent kinetic energy (TKE), turbulence length scale, and the hydrogen mass fraction distribution were compared in the different test cases, revealing that the multi-hole geometry holds a significant advantage, allowing faster mixing of hydrogen with ambient gases. This work demonstrates that hydrogen direct injection introduces higher turbulence in the flow patterns of the ICE charge compared to the turbulence levels developed during the intake stroke. It also highlights that careful design of injector nozzles is critical to achieving a homogeneous hydrogen/air mixture, thereby mitigating knock and promoting stable engine operation.