ANALYSIS OF THE INJECTION PROCESS IN DIRECT INJECTED NATURAL-GAS ENGINES .1. STUDY OF UNCONFINED AND IN-CYLINDER PLUME BEHAVIOR

A study of natural gas (NG) direct injection (DI) processes has been p erformed using multidimensional computational fluid dynamics analysis. The purpose was to improve the understanding of mixing in DI NG engin es. Calculations of injection into a constant-volume chamber were perf ormed to document unconfined plume behavior. A full three-dimensional calculation of injection into a medium heavy-duty diesel engine cylind er was also performed to study plume behavior in engine geometries. Th e structure of the NG plume is characterized by a core of unmixed fuel confined to the near-field off the jet. This core contains the bulk o f 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 c ombustion chamber surface interactions. A Coanda effect causes plume a ttachment to the cylinder head, which has a detrimental impact on mixi ng. Unconfined plume calculations with different nozzle hole sizes dem onstrate that smaller nozzle holes are more effective at mixing the fu el and air.