Evolution in Size and Morphology of Diesel Particulates Along the Exhaust System

The physical and morphological properties of the particulate matter emitted from a 1.7-liter light-duty diesel engine were characterized by observing its evolution in size and fractal geometry along the exhaust system. A common-rail direct-injection diesel engine, the exhaust system of which was equipped with a turbocharger, EGR, and two oxidation catalysts, was powered with a California low-sulfur diesel fuel at various engine-operating conditions. A unique thermophoretic sampling system, a high-resolution transmission electron microscope (TEM), and customized image processing/data acquisition systems were key instruments that were used for the collection of particulate matter, subsequent imaging of particle morphology, and detailed analysis of particle dimensions and fractal geometry, respectively. The measurements were carried out at four different positions along the exhaust pipe. The rapid insertion of sampling probe into the exhaust stream, whose residence time resolves as short as 10 msec, provided near interference-free sampling of diesel particulates. From analyses for the soot samples collected at 2500-rpm and 25%-load, it was revealed that the primary particle diameter decreased from 28.6 nm to 19.1 nm and the corresponding radius of gyration of aggregate particles also decreased from 101.6 nm to 49.6 nm along the exhaust pipe. This reduction of particle sizes implies that particle sizes are significantly affected by aftertreatment components in the exhaust system. The detailed fractal analysis supported this finding; a fractal dimension of particles was slightly higher right after the first catalyst than those evaluated at other sampling positions, which indicates that particles became more spherical in shape during passing through the first catalyst. The fractal dimensions varied in a range of 1.5 to 1.7 along the exhaust pipe. It is speculated that the catalytic oxidation and aerodynamics of exhaust streams were major parameters influencing the distributions of measured sizes and fractal geometry. A chemical analysis using energy dispersion spectroscopy revealed that diesel particulates consist of elemental carbons in most portions.