Abstract
Early direct-injection is a diesel-engine combustion strategy in which fuel is injected early in the compression stroke when in-cylinder temperature and density are both much lower than for conventional operation, increasing the potential for impingement and wall wetting due to slower vaporization of fuel. Quantitative understanding of liquid-phase fuel penetration and dispersion, therefore, is necessary for these conditions in order to avoid problems such as impingement that are counterproductive to advanced combustion strategies. This work reports liquid penetration distances and spray cone angles of n-heptane measured using high-speed imaging of elastic light scattering in a constant-pressure flow vessel. Air temperatures in the constant-pressure flow vessel included well below, near, and well above saturation temperatures of n-heptane, which should provide insight into spray behavior of diesel fuel components for early-injection conditions that also potentially range from below to above saturation. Maximum liquid penetration distances and spray angles were found to be essentially independent of injection pressure and injection duration, as well as air flow velocity, but strongly dependent on thermodynamic properties of the flowing air. Maximum liquid penetration trends with respect to temperature and spray angle trends with respect to density were in qualitative agreement with model predictions, but did not agree with expected quantitative values. In addition, trends of maximum liquid penetration versus density and spray angle versus temperature were either non-existent or in direct opposition to model predictions. Results strongly suggest that well-accepted models, based on mixing-limited vaporization of fuel and ignoring heat and mass transfer of individual droplets, are insufficient for these conditions relevant to early direct-injection.
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