Effects of Exhaust System Components on Particulate Morphology in a Light-duty Diesel Engine

The detailed morphological properties of diesel particulate matter were analyzed along the exhaust system at various engine operating conditions (in a range of 1000 - 2500 rpm and 10 - 75 % loads of maximum torques). A 1.7-L turbocharged light-duty diesel engine was powered with California low-sulfur diesel fuel injected by a common-rail injection system, of which particulate emissions were controlled by an exhaust gas recirculation (EGR) system and two oxidation catalysts. A unique thermophoretic sampling system first developed for internal combustion engine research, a high-resolution transmission electron microscope (TEM), and a customized image processing/data acquisition system 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 data analysis showed that the average primary particle sizes significantly changed in a range of approximately 15 nm to 30 nm as a function of engine operating condition, while overall decreasing along the exhaust pipe. Aggregate particle sizes also significantly changed in a range of approximately 50 nm to 100 nm, in terms of radius of gyration, through the entire engine operating conditions. These particle sizes were affected mainly by the oxidation catalysts and tail pipe as well as engine operating conditions. Particle geometry was changed along the exhaust pipe, most significantly by the first catalyst, while overall becoming more spherical in shape toward the tail pipe. Fractal dimensions were measured in a range of approximately 1.4 to 1.8 through the entire engine operating conditions, which identified the change of particle geometry. A distinct increase of fractal dimension appeared after the first catalyst. The spherical particle shape, represented by the larger fractal dimension, was justified by soot morphology observed at the sample collected immediately after the catalyst.