The soot particle formation process inside the piston bowl of a small-bore diesel engine

Abstract

The present study unveils the soot formation processes occurring inside the piston-bowl of a small-bore diesel engine by conducting the thermophoresis-based soot sampling experiments at various locations along the flame development path. Based on planar laser-induced incandescenece (PLII) and planar laser-induced florescence of hydroxyl (OH-PLIF) imaging performed in the same optical engine previously, it was understood that the sooting flame impinges on and then flows along the bowl wall, suggesting a soot growth and persistence near the fuel-rich wall region. In the present study, a soot sampling probe is placed in five different locations including the flame–wall impingement point and four further downstream regions: two 60° and two 120° from the jet axis with two different distances from the bowl wall in each angle. Methyl decanoate is selected as a surrogate fuel due to its low-sooting propensity and thus reduced laser attenuation in the reference PLII images; however, the fuel produces high enough number of soot particles for the in-flame sampling and their statistical analysis. The transmission electron microscope (TEM) images of the sampled soot particle aggregates and their statistical analysis of sizes and fractal dimensions as well as nanoscale internal pattern of the soot primary particles show that precursor-like, small soot particles with amorphous internal carbon layer structures form in the flame–wall impingement region, which grow in size and become large soot aggregates as travelling along the bowl wall. The detailed analysis clearly indicates that the soot precursors underwent the surface growth, aggregation and coagulation to produce large, long-stretched soot aggregates during which the amorphous soot carbon layers transformed into a typical core–shell structure. At further downstream locations, the continued surface growth increases the size of soot primary particles in the core region of the soot aggregates while the oxidation of the soot primary particles located in the outer region tends to reduce the aggregate size, resulting in more compact structures. In the outer region of the flame, the intensive soot oxidation induced by the hydroxyl attack further reduces the size of large soot aggregates and at the same time, eliminates the small soot aggregates. Throughout these soot formation/oxidation processes, the soot carbon layer gaps continue to decrease, indicating more mature soot primary particles.