Numerical Modeling of Fuel Sprays in DISI Engines Under Early-Injection Operating Conditions

Numerical calculations of the fuel spray structure from a high-pressure swirl injector were used to enable the interpretation of experimental observations obtained in hot, hollow-cone fuel sprays issued into sub-atmospheric-pressure environments. The experiments show that the spray becomes narrower, more compact, but with a relatively long penetration depth. Model input parameters, including the droplet size distribution, early vapor production, and initial cone angle, were modified to determine which spray characteristics are important in recreating observed spray structures. A very small mean droplet diameter is needed to recreate the experimentally observed structure of the high-temperature, low-pressure sprays. Vapor addition to the emerging spray is then required to increase the axial penetration and provide the observed vapor core. Vapor addition, which was used to imitate the rapid vapor production in a flash boiling spray, produces a vapor core along the axis of the spray while increasing axial penetration and decreasing radial penetration, in agreement with experimental images. Finally, an increase of the initial cone angle is required to match the structure near the injector tip, as none of the previous modifications led to a natural change in the cone angle near the injector. The comparison of model and experiments clearly shows that the accurate simulation of evaporating sprays under realistic hot operating conditions in direct-injected stratified engines requires rethinking of the usual inputs: not only is a realistic droplet size and velocity distribution obtained under evaporating conditions required, but also the addition of locally generated vapor.