The development of hydroxyl and soot in a methyl decanoate-fuelled automotive-size optical diesel engine

This study presents planar images of laser-induced fluorescence of fuel (fuel-PLIF) and hydroxyl (OH-PLIF) as well as incandescence of soot (soot-PLII) obtained in a small-bore optical diesel engine fuelled with methyl decanoate. The focus is on how jet-wall interaction impacts the temporal and spatial development of a reacting fuel jet, high-temperature reaction, and soot within the flame. A single-hole nozzle was used to isolate the jet-wall interaction from jet-jet interactions and to apply long injection duration corresponding to high-load engine operating conditions in which soot formation is particularly problematic. Methyl decanoate was selected as a surrogate fuel due to its low-sooting propensity and thus reduced laser attenuation, as well as the interests in oxygenated fuels, and the existence of detailed reaction kinetics in the literature. Laser-based images show that the fuel impinges on the bowl wall soon after the start of injection and then bounces off along the wall forming a wall-interacting jet. The fuel jet continues to travel along the bowl wall as well as the bottom surface of the piston bowl. During the premixed burn phase of diesel combustion, the high-temperature reaction starts to occur at the leading edge of the penetrating jet back towards the nozzle, initially near the jet axis and then spreads in the radial direction. During the mixing-controlled burn phase, the high-temperature reaction zone fills up the entire combustion chamber and the soot formation starts to occur in the rich area near the wall impingement point. The soot then flows along the bowl wall in both up-swirl and down-swirl directions. These soot pockets are surrounded by OH and they soon disappear altogether at subsequent crank angle locations, suggesting the soot oxidation by OH radicals. However, some soot pockets are transported into the centre of bowl due to the downward movement of the piston and persist for long as there are no active OH radicals.