Sizes, Graphitic Structures and Fractal Geometry of Light-Duty Diesel Engine Particulates

The particulate matter of a light-duty diesel engine was characterized in its morphology, sizes, internal microstructures, and fractal geometry. A thermophoretic sampling system was employed to collect particulates directly from the exhaust manifold of a 1.7-liter turbocharged common-rail direct-injection diesel engine. The particulate samples collected at various engine-operating conditions were then analyzed by using a high-resolution transmission electron microscope (TEM) and an image processing/data acquisition system. Results showed that mean primary particle diameters (d_p), and radii of gyration (R_g), ranged from 19.4 nm to 32.5 nm and 77.4 nm to 134.1 nm, respectively, through the entire engine-operating conditions of 675 rpm (idling) to 4000 rpm and 0% to 100% loads. It was also revealed that the other important parameters sensitive to the particulate formation, such as exhaust-gas recirculation (EGR) rate, equivalence ratio, and temperature, affected particle sizes significantly. Bigger primary particles were measured at higher EGR rates, higher equivalence ratios (fuel-rich), and lower exhaust temperatures. Fractal dimensions (D_f) were measured at a range of 1.5 ∼ 1.7, which are smaller than those measured for heavy-duty direct-injection diesel engine particulates in our previous study [9, 10 and 11]. This finding implies that the light-duty diesel engine used in this study produces more stretched chain-like shape particles, while the heavy-duty diesel engine emits more spherical particles [9, 10 and 11]. The microstructures of diesel particulates were observed at high TEM magnifications and further analyzed by a Raman spectroscope. Raman spectra revealed an atomic structure of the particulates produced at high engine loads, which is similar to that of typical graphite.