In-cylinder sizing of diesel particles by time-resolved laser-induced incandescence (TR-LII)

Laser-induced incandescence (LII) is a technique allowing particle-size distribution measurement and quantifying in an extremely short time. It is especially attractive for rapidly varying conditions as in diesel engine combustion. A special variant is the time-resolved method (TR-LII), which yields almost instantaneous particle-size information. In the present work, the application of particle sizing in the combustion chamber of a diesel engine is demonstrated. The measuring equipment was adapted to a one-cylinder, two-stroke engine with a specially designed cylinder head, yielding highly reproducible signals for particle thermal emission after rapid heatup by a pulsed laser. The characteristic time constant tau(m) of the exponential signal decrease is a measure for the particle size. TR-LII signals for different crank angles and varying motor conditions were measured. A strong dependence of the characteristic emission time constant tau(m) during particle cooling from the measured crank angle and the engine load could be determined. Specialties in signal evaluation due to the higher pressure level during diesel combustion are discussed, and size parameters, such as the count median diameter and the geometric standard deviation of an assumed log-normal size distribution, were determined from the measured optical signals. A comparison of in situ TR-LII particle sizing with the well-established ex situ differential mobility particle-sizing technique was performed. The results are in good agreement.