Simultaneous High-Speed Two-Color Thermometry and Laser-Induced Incandescence Soot Measurement in a Small-Bore Optical Engine Fueled With JP-8

Abstract

In-cylinder soot measurements obtained with a high-speed two-color method are compared to those simultaneously determined by the laser-induced incandescence (LII) technique in a single-cylinder, optically-accessible diesel engine fueled with JP-8. A double injection strategy was chosen to reduce pressure rise rates during operation at light load (2 bar IMEP) conditions. Injection timing was optimized for peak efficiency, at which point sufficient soot was produced to provide ample signal for both optical diagnostic techniques. Application of the two-color method to a high-speed CMOS camera allows the crank-angle-resolved observation of soot temperature and soot optical depth (KL) evolution, while LII provides soot volume fraction distribution at a known axial location in the cylinder independent of combustion gas temperature. Comparison of soot KL and LII signal at various stages of combustion shows high spatially-averaged correlation of the two signals near TDC. The degree of correlation decreases as the piston bowl descends and the line-of-sight soot KL value increasingly includes soot volumes not in the path of the laser sheet, the location of which is fixed 6.5 mm below the fire deck. The correlation between the two parameters again increases during the late cycle, indicating that in the later phases of combustion soot occurs in the squish zone above the piston bowl. Spatial cross-correlation of the two signals is weak, but increases in the highly luminous period immediately following heat release and illustrating a high degree of soot stratification. Soot KL and temperature evolution over a cycle are presented, which show no indication of being affected by the LII laser fluence.