Soot Particle Sampling and Morphology Analysis in an Optically Accessible Diesel Engine

Abstract

A significant reduction of soot emissions from diesel engines cannot be achieved without an improved understanding of the soot processes inside the engine cylinder. While previous studies have primarily focused on exhaust soot particles, how these soot particles are formed in the first place is virtually unknown. To bridge this gap, this thesis presents a new experimental approach for collecting soot particles inside the cylinder of a single-cylinder light-duty diesel engine using a thermophoretic sampling technique. Soot samples are analysed using transmission electron microscopy (TEM) and subsequent image post-processing to obtain key parameters such as size distributions and fractal dimensions of soot particles. Results of this thesis demonstrate the successful collection of in-flame soot particles for the first time in a working diesel engine. The uncertainty analysis showed that the cyclic dispersions of engine combustion do not inflict significant impacts on particle size distribution. Parametrical studies with various injection timing and pressure revealed that the late injection timing or high injection pressure reduces the number counts, projection area, aggregate size and fractal dimension of in-flame soot particles. Increasing injection pressure also resulted in reduced primary particle size. Soot samplings were also conducted for various combustion stages by changing the sampling location with respect to the diesel flame. Reduced soot projection area and aggregate size are found for post-wall-impingement soot particles suggesting the effect of flame-wall interaction. Furthermore, late-cycle and exhaust soot particles show reduced number counts, projection area, and primary particle size. These trends suggest that small particles are easily oxidized during the combustion while large aggregates with compact structures would more likely survive the oxidation. Moreover, the wall-deposited and in-flame soot particles were compared. The results show much smaller number counts, projection area and more compacted structures for the wall-deposited soot particles. The findings of this study are expected to build a new science base needed by industry to develop improved combustion strategies that achieve further reduction in soot emissions. The data provided by this work would also help build new soot models applicable to practical diesel engine conditions.