Two-color time-resolved LII applied to soot particle sizing in the cylinder of a Diesel engine

Abstract

Laser-induced incandescence with time-resolved detection (TR-LII) at two wavelengths was established and applied to the combustion chamber of a single-cylinder two-stroke Diesel engine. The optical system was calibrated by a black-body radiation source. TR-LII signals for different crank angles and varying engine operation conditions were measured. By fitting calculated TR-LII cooling curves to the measured signals, it was possible to determine the mean primary particle diameter and the geometric standard deviation of the in-cylinder particles for an assumed lognormal size distribution. At the beginning of the combustion process the resulting particle sizes increased with increasing crank angle (CA) according to particle formation. This was followed by a subsequent reduction of the particle sizes at later crank angles due to particle oxidation. In all examined cases the mean particle sizes increase with increasing engine load. For validation purposes, soot samples from the exhaust gas were taken onto transmission electron microscopy (TEM) grids for different engine operating conditions and were analyzed by TEM. For the obtained loose agglomerated particle structures, the particle sizes at late crank angles measured by TR-LII are in excellent agreement with the TEM-determined mean primary particle sizes in the exhaust gas. Finally, soot volume fractions were determined from the maximum TR-LII signals via calibration by in-cylinder laser-extinction measurements.