A multispectral UV–vis absorption technique for quantitative high-speed field-sequential imaging of fuel films and soot in combustion

Abstract

Non-evaporated liquid fuel films are the main cause of soot formation in gasoline direct-injection engines. In this study, a UV–vis absorption technique was developed to image the fuel-film thickness after direct injection in a heated constant-flow experiment. A six-hole GDI injector sprayed fuel at 100 bar onto a transparent plate 30 mm from the nozzle. The fuel consisted of a mixture of 30% toluene / 70% isooctane (boiling points 383 and 372 K, respectively). The gas and wall temperature were 376 and 352 K, respectively, and the gas pressure 1 bar. The evaporated part of the fuel was ignited, and the ensuing combustion next to the evaporating fuel film led to soot formation. Transmission of diffused backlighting from pulsed LED illumination was imaged on an intensified high-speed CMOS camera. The UV absorbance of liquid toluene was targeted with an LED at 265 nm. However, at this wavelength, absorption by toluene vapor, scattering by the liquid, extinction by soot and soot precursors, and soot incandescence all interfere with the liquid-fuel absorbance. To estimate the contributions of scattering and soot extinction, LEDs at 310, 365, and 520 nm were added to the beam path and pulsed to coincide with consecutive frames on the high-speed camera at a frame rate of 32 kHz. A dark frame was acquired to account for soot incandescence, such that the repetition rate of the resulting 5-image sequence was 6.4 kHz. Absorption by toluene vapor was estimated with a morphological image processing developed in a previous work to separate the diffuse, moving vapor cloud from the sharp, stationary features of the fuel film. The multi-spectral approach permitted obtaining spatio-temporally resolved fuel-film thickness measurements and additional information on the soot.