Quantitative vapor temperature imaging in DISI-sprays at elevated pressures and temperatures using two-line excitation laser-induced fluorescence

Abstract

Direct-injection (DI) concepts with spray-guided mixture formation are promising methods to increase the efficiency of spark ignition (SI) engines. The fuel and temperature distribution determines evaporation and ignition behavior. However, the resulting mixture and temperature are not homogeneous through the combustion chamber. Furthermore, it is strongly affected by the fuel evaporation properties, which are different for modern biofuels compared to gasoline. The present paper demonstrates the procedure and the capability of two-line excitation LIF based on 3-pentanone for accurate planar temperature determination in DISI-sprays at late injection timing conditions. The jet temperature profiles were determined for the gasoline surrogate fuel iso-octane and the biogenic component ethanol, which have a significant difference in their heat of evaporation. The spray of a 6-hole solenoid-injector in an optically accessible combustion test rig is analyzed by quasi-simultaneous excitation by two different excimer lasers. The signals are recorded with one double-shutter ICCD-camera. From the signal ratio of the images the temperature field in the fuel spray can be calculated. The tracer is calibrated with an enhanced high temperature calibration cell for temperatures up to 700K and pressures up to 1MPa, which are relevant for the studied conditions. The postprocessing strategy is optimized regarding high accuracy and precision. The fuel spray behavior is evaluated at different positions in the spray and times showing larger evaporation cooling in the jet of maximum 141K for ethanol. For a comparison of the ensemble averaged results a CFD-spray model was set up under the same boundary conditions showing very similar trends. The temperature images yielded a single-shot precision of approximately 5.6% and inaccuracies of 3.8% making this technique capable for model improvements and validation.