The microstructure, dimensions, and fractal geometry of diesel particulates were investigated through morphological observations. A thermophoretic particle-sampling system was developed to collect diesel particulates directly from the high-temperature exhaust stream of a 75 hp single-cylinder direct-injection diesel engine at various engine operating conditions. The morphology of the collected diesel particulates was analyzed using a high-resolution transmission electron microscope and subsequent image processing/data acquisition system. The analysis revealed that spherical primary particles were agglomerated together to form large aggregate clusters at most engine operating conditions. The particles produced at low loading conditions displayed amorphous structures, possibly containing a significant amount of soluble organic fraction, while the soot sampled at high engine load conditions mostly exhibited graphitic structures. Measured primary particle sizes ranged from 34.4 to 28.5 nm at various engine operating conditions. The smaller primary particles observed at high engine load conditions were caused by oxidation at the encountered high temperatures. A number of small and irregularly shaped particles were captured at these conditions, which identified the existence of apparent particle oxidation. Engine-speed-dependent experiments were also performed to investigate the effects of the residence time of soot particles on particle growth. A growth mechanism of diesel particulates was suggested through analyses for fractal geometry of the cluster particles.
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