Compressed Natural Gas (CNG) is regarded as a promising fuel for spark-ignited (SI) internal combustion engines (ICE) to improve engine thermal efficiency and reduce both carbon dioxide and pollutant emissions. Significant advantages of CNG are higher-octane number, higher hydrogen to carbon ratio, and lower energy-specific CO2 emissions compared with gasoline. More, it can be produced in renewable ways, and is more widespread and cheaper than conventional liquid fossil fuels. In this regard, the direct injection of CNG engines can be considered a promising technology for highly efficient and low-emission future engines. This work reports an experimental and numerical characterization of high-pressure methane jets from a multi-hole injector for direct injection engines. The tests were performed in a constant volume (CV) combustion chamber under a broad range of operating conditions in terms of injection pressure, in the range 1.0-5.0 MPa, and ambient back-pressure in between 0.05 to 1.0 MPa. The schlieren technique was employed to evaluate the effects of the injection pressure and ambient thermodynamic conditions on jet macroscopic characteristics. Then, the overall injection process has been reconstructed thanks to a CFD (computational fluid dynamic) density-based model, properly developed in OpenFOAM environment, featuring a large eddy simulation (LES) turbulence framework. The simulation reproduces the jet's transient evolution and captures its classical structures. Such model allows evaluating further parameters, not available from the experimental characterization, that provide a better knowledge of the air-fuel mixing process.
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