A study of natural gas (NG) direct injection (DI) processes has been performed using multidimensional computational fluid dynamics analysis. The purpose was to improve the understanding of mixing in DI NG engines. Calculations of injection into a constant-volume chamber were performed to document unconfined plume behavior. A full three-dimensional calculation of injection into a medium heavy-duty diesel engine cylinder was also performed to study plume behavior in engine geometries. The structure of the NG plume is characterized by a core of unmixed fuel confined to the near-field of the jet. This core contains the bulk of the unmixed fuel and is mixed by the turbulence generated by the jet shear layer. The NG plume development in the engine is dominated by combustion chamber surface interactions. A Coanda effect causes plume attachment to the cylinder head, which has a detrimental impact on mixing. Unconfined plume calculations with different nozzle hole sizes demonstrate that smaller nozzle holes are more effective at mixing the fuel and air.

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