1997-10-01

Comparison of Computed Spray in a Direct-Injection Spark-Ignited Engine with Planar Images 972883

ABSTRACT

Fuel spray atomization and breakup processes within a direct-injection spark-ignition (DISI) engine and outside the engine were modeled using a modified KIVA-3V code with improved spray models. The structures of the predicted sprays were qualitatively compared with planar images. The considered sprays were created by a prototype pressure-swirl injector and the planar images were obtained by laser sheet imaging in an optical DISI engine. In the out-of-engine case, the spray was injected into atmospheric air, and was modeled in a two dimensional bomb. In the engine case, the injection started from 270° ATDC, and full 3-D computations in the same engine were performed. In both cases, two liquid injection pressure conditions were applied, that is, 3.40 MPa and 6.12 MPa. The model gives good prediction of the tip penetration, and external spray shape, but the internal structure prediction has relatively lower accuracy, especially near the spray axis. The out-of-engine computation reveals that small droplets (diameter less than 10 micron) were entrained by the vortex at the spray tip, and were transported into the interior of the hollow cone by the vortex. Thus small droplets accumulate inside the spray cone. From the planar images, the evolution of the in-cylinder sprays can be summarized as occurring in three stages: immediately after the injection, the spray shows a true hollow cone structure; and then, droplets accumulate inside the spray cone, and the hollow cone structure becomes less obvious; finally, the spray appears to collapse and it loses its well organized conical structure. The model can predict the initial hollow cone structure and also predict when the collapse happens reasonably well, but the droplets near the spray axis were not predicted in the model. However, because the total contribution of these droplets is small, the model still has good overall prediction.

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