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.

More precise measurements of the fuel injection process can enable better combustion control and more accurate predictions resulting in a reduction of fuel consumption and toxic emissions. Many of the current methods researchers are using to investigate the transient liquid fuel sprays are limited by cross-sensitivity when studying regions with both liquid and vapor phases present (i.e., upstream of the liquid length). The quantitative rainbow schlieren technique has been demonstrated in the past for gaseous fuel jets and is being developed here to enable study of the spray near the injector. In this work, an optically accessible constant pressure flow rig (CPFR) and a modern common rail diesel injector are used to obtain high-speed images of vaporizing fuel sprays at elevated ambient temperatures and pressures. Quantitative results of full-field equivalence ratio measurements are presented as well as more traditional measurements such as vapor penetration and angle for a single condition (13 bar, 180 °C normal air) using n-heptane injected through a single hole (0.1 mm diameter) common rail fuel injector at 1000 bar fuel injection pressure. This work serves as a proof of concept for the rainbow schlieren technique being applied to vaporizing fuel sprays, and full details of the image-processing routine are provided. The ability of the imaging technique combined with the constant pressure flow rig make this approach ideal for generating large data sets in short periods of time for a wide range of operating conditions.

More precise measurements of the fuel injection process can enable better combustion control and more accurate predictions resulting in a reduction of fuel consumption and toxic emissions. Many of the current methods researchers are using to investigate the transient liquid fuel sprays are limited by cross sensitivity when studying regions with both liquid and vapor phases present (i.e. upstream of the liquid length). The quantitative rainbow schlieren technique has been demonstrated in the past for gaseous fuel jets and is being developed here to enable study of the spray near the injector. In this work an optically accessible constant pressure flow rig and a modern common rail diesel injector are used to obtain high speed images of vaporizing fuel sprays at elevated ambient temperatures and pressures. Quantitative results of full-field equivalence ratio measurements are presented as well as more traditional measurements such as vapor penetration and angle for a single condition (13 bar, 180°C normal air) using nheptane injected through a single hole (0.1mm diameter) common rail fuel injector at 1000 bar fuel injection pressure. This work serves as a proof of concept for the rainbow schlieren technique being applied to vaporizing fuel sprays and full details of the image processing routine are provided. The ability of the imaging technique combined with the constant pressure flow rig make this approach ideal for generating large data sets in short periods of time for a wide range of operating conditions.