Computational characterization of hydrogen direct injection and nonpremixed combustion in a compression-ignition engine

With the revived interest in hydrogen (H2) as a direct combustion fuel for engine applications, a computational study is conducted to assess the characteristics of H2 direct-injection (DI) compression-ignition (CI) non-premixed combustion concept. Development of a CFD modeling using CONVERGE CFD solver focuses on hydrogen's unique characteristics by utilizing a suitable numerical method to reproduce the direct H2 injection phenomena. A grid sensitivity study is performed to ensure the fidelity of results with optimal cost, and the models are validated against constant-volume optical chamber and diesel engine experimental data. The present study aims to contribute to the future development of DICI H2 combustion engines, providing detailed characterization of the combustion cycle, and highlighting several distinct aspects of CI nonpremixed H2 versus diesel combustion. First, unlike the common description of diesel sprays, hydrogen jets do not exhibit significant flame lift-off and air entrainment near injector nozzle, and the fuel-air interface is drastically more stratified with no sign of premixing. It is also found that the DICI H2 combustion concept is governed first by a free turbulent jet mixing phase, then by an in-cylinder global mixing phase. The former is drastically more dominant with the DICI H2 engine compared to conventional diesel engines. The free-jet mixing is also found to be more effective that the global mixing, which indicates the need to completely rethink the optimization strategies for CI engines when using H2 as fuel.