The effect of fuel volatility on sprays from high-pressure swirl injectors

Abstract

The effect of fuel volatility on the spray distribution of a pressure-swirl atomizer of the type used in direct-injection gasoline engines was investigated in a firing optical engine. Planar laser-induced fluorescence (PLIF) and planar Mie scattering were used to visualize the fuel spray. Experiments were performed at three loads (0.3, 0.6, and 0.9 bar intake pressure) and two head temperatures (30 °C and 90 °C). Fuel mixtures consisted of doped and undoped isooctane and indolene. Dopants were ketones of varied volatility: acetone ( T b =56°C), 2-butanone ( T b =80°C), and 3-pentanone ( T b =102°C). At high head temperature and low pressure with volatile, fuel species, the spray characteristics changed from the hollow-cone structure observed under cold conditions to a solid-cone distribution. This transition was observed with both the PLIF and Mie-scattering images, suggesting that not only vapor was being drawn to the interior of the cone, but droplets as well. The observed solid-cone structure can be explained by flash boiling of the highly volatile species, followed by entrainment of the smaller droplets into the cone center by the airflow induced by the spray. The observed transition temperatures and pressures agree well with bubble-point calculations and suggest that a superheat of about 20 K is necessary for flash boiling to be vigorous enough to noticeably affect the spray structure. Similar results were obtained with a full-boiling-range gasoline.