A Study of In-Cylinder Soot Oxidation by Laser Extinction Measurements During an EGR-Sweep in an Optical Diesel Engine

Two competing in-cylinder processes, soot formation and soot oxidation, govern soot emissions from diesel engines. Previous studies have shown a lack of correlation between the soot formation rate and soot emissions. The current experiment focuses on the correlation between soot oxidation rates and soot emissions. Laser extinction is measured using a red (690nm) laser beam, which is sent vertically through the cylinder. This wavelength is long enough to minimize absorption interference from poly-aromatic hydrocarbons, while still in the visible regime. It is modulated at 72 kHz in order to produce 10 pulses per crank angle degree at an engine speed of 1200 rpm. The intake oxygen concentration is varied between 9% and 21%. The time resolved extinction measurements are used to estimate soot oxidation rates during expansion. High-speed video imaging is used in conjunction with the laser-extinction technique to indicate the location of the sooting regions, and to assess beam steering effects. The oxidation processes are described using single exponential decay fits and an attempt to correlate them with the late cycle rate of heat release was made