Electron Microscopy Investigation of Particulate Matter from a Dual Fuel Engine

The particulate matter (PM) of a dual fuel engine was characterized in size, morphology and fractal geometry by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Particulate samples were collected from the diluted exhaust of the engine operated on diesel fuel, natural gas (NG) and synthetic biogas. The engine operating condition was kept the same to compare the results between diesel and dual fuel PM. SEM images yielded agglomerate number, size distributions and a shape descriptor. TEM was used to investigate the primary particle size distribution in agglomerates and the fractal dimensions of the sampled PM. Long chainlike PM agglomerates appeared for the diesel high load condition, whereas PM agglomerates for dual fueling were found to be smaller in size and with more spherical shapes. All of the measured PM appeared to have a bi-modal number size distribution irrespective of engine fueling condition. The average primary particle diameter increased for dual fuel PM (ranging from 26.9 to 29.5 nm) compared to diesel PM (26.4 nm). The average diameter tended to increase with the introduction of CO(2) in the gaseous fuel. PM fractal dimensions were in the range from 1.69 to 1.88 for different PM samples. Higher fractal dimensions (from 1.73 to 1.88) were obtained for dual fuel PM compared to diesel (high load) PM (1.69). This finding also implies that diesel PM are more chainlike and elongated compared to the PM measured for dual fueling conditions. The very different engine fueling conditions used here give valuable understanding of the formation processes of PM.