Fuel Film Temperature and Thickness Measurements on the Piston Crown of a Direct-Injection Spark-Ignition Engine

Fuel film temperature and thickness were measured on the piston crown of a DISI engine under both motored and fired conditions using the fiber-based laser-induced fluorescence method wherein a single fiber delivers the excitation light and collects the fluorescence. The fibers were installed in the piston crown of a Bowditch-type optical engine and exited via the mirror passage. The fuel used for the fuel film temperature measurement was a 2×10^-6 M solution of BTBP in isooctane. The ratio of the fluorescence intensity at 515 to that at 532 nm was found to be directly, but not linearly, related to temperature when excited at 488 nm. Effects related to the solvent, solution aging and bleaching were investigated. The measured fuel film temperature was found to closely follow the piston crown metal temperature, which was measured with a thermocouple. A detailed analysis of the fiber-based laser-induced fluorescence technique was used to ascertain film thickness based on a single-point calibration. The calibration methodology also accounted for the effects of fuel film temperature. A 4% by volume solution of 2,3-hexanedione in isooctane was found to be a suitable choice for fuel film thickness measurement because it was verified to be co-evaporative. The fuel film thickness was found to be quite small, less than 10 μm, for both motored and fired conditions performed at the same piston temperature. The 2,3-hexanedione was found to leave a viscous residue on the piston crown, which carried over from cycle to cycle and limited the results.