Combustion Studies of Soot and Fuel Based on Use of Laser Diagnostics

University dissertation from Department of Combustion Physics, Lund University

Abstract: In the work described in the present thesis, laser techniques for combustion engine diagnostics were developed and applied. The main techniques were laser-induced incandescence (LII) and laser-induced fluorescence (LIF), used for both flame and engine diagnostics. A novel technique was developed, one in which the relative fuel distribution, as measured by planar laser-induced fluorescence (PLIF), was quantified by means of Raman scattering point measurement. Although the technique showed good potential for laboratory measurements, the limited degree of optical access found in an optical engine suggested further development for engine applications to be feasible. To reduce the need of optical access through windows, an endoscopic detection system was deve-loped and was successfully implemented in an optical engine. Although the transmittance through the endoscopic system was lower than that in normal detection, PLIF measurements of fuel distribution could be made by imaging through the endoscopic detection system. A detailed flame study was performed to evaluate the possibility of using LII for the soot volume fraction and for particle size measurement. Data was compared to those being assessed by extinction-scattering measure-ments. Measured temperature was an important input for determining the primary particle size on the basis of the decay time of the LII signal. The results were encouraging, even with use of the simple decay model employed. The same flame was used to verify the possibility of making simultaneous measurements of extinction and LII, which led to an on-line extinction calibration technique. A three-dimensional representation of the soot volume fraction in a flame was constructed on the basis of eight two-dimensional LII intensity distributions obtained with the use of a Nd:YAG-laser cluster and a high-speed detection system. The LII technique was also applied to engines, where in-cylinder measurements of soot volume fraction were performed in a running direct-injection stratified-charge (DISC) engine. A technique, based on LII, for study of engine-out soot emissions allowing measurements with sub-second temporal resolution during engine transients was evaluated.

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