Detailed Characterization of Morphology and Dimensions of Diesel Particulates via Thermophoretic Sampling

A thermophoretic particulate sampling device was used to investigate the detailed morphology and microstructure of diesel particulates at various engine-operating conditions. A 75 HP Caterpillar single-cylinder direct-injection diesel engine was operated to sample particulate matter from the high-temperature exhaust stream. The morphology and microstructure of the collected diesel particulates were 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 for most of engine speed and load conditions. 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-operating conditions were believed to be caused by particle oxidation at the high combustion temperature. A number of small and irregularly shaped particles were captured at these conditions. Graphitic structures were observed for the soot sampled at the high engine operating conditions, while other samples from the lower engine operating conditions displayed mostly amorphous structures. Analyses for fractal geometry of soot agglomerates suggested that the diffusion-limited particulate growth mechanism, by which chain-like particle clusters form, is predominant.