Challenges for In-Cylinder High-Speed Two-Dimensional Laser-Induced Incandescence Measurements of Soot

Laser-Induced Incandescence (LII) has traditionally been considered a straightforward and reliable optical diagnostic technique for in-cylinder soot measurements. As a result, it is nowadays even possible to buy turn-key LII measurement systems. During recent years, however, attention has been drawn to a number of unresolved challenges with LII. Many of these are relevant mostly for particle sizing using time-resolved LII, but also two dimensional soot volume fraction measurements are affected, especially in regions with high soot concentrations typically found in combustion engines. In this work the focus is on the specific challenges involved in performing high-repetition rate measurements with LII in diesel engines. All the mentioned issues might not be possible to overcome but they should nevertheless be known and their potential impact should be considered. The measurements, which, to the authors' knowledge, are the first in-cylinder high-speed LII published to date, were made using a Multi-YAG laser system, capable of producing a burst of eight high-power laser pulses during one combustion event. A high-speed framing camera was used as detector in order to match the high repetition rate of the laser. In an initial feasibility study, high-speed LII measurements were made in a laminar ethylene/air flame to test the characteristic features of high-speed LII. Excessive influence of soot sublimation was observed in the LII signals induced by later pulses in the pulse sequence for typical pulse separations of interest in the engine (139 μs). This is a result of that each laser pulse partly sublimates soot particles in the measurement volume, resulting in a lower signal yield. Analysis on the data from the in-cylinder measurements does not show signs of the same behavior. This is believed to be due to high mixing and/or very fast soot formation/oxidation in the measurement volume. This implies that time-resolved LII is a suitable technique to qualitatively follow the evolution of the soot distribution inside a reacting diesel jet.