The scattering properties (influenced by morphology) and refractive index (dependent on microstructure) of engine-emitted soot influences its effect on climate, as well as how we interpret optical measurements of aerosols. The morphology and microstructure of soot from two different engines were studied. The soot samples were collected from a 1.9L Volkswagen TDI engine for two different fuel types (ULSD and B20) and six speed/load combinations., as well as from a Cummins ISX heavy-duty engine using the Westport pilot-ignited high-pressure direct-injection (HPDI) natural-gas fuelling system for three different speed/load combinations.
The transmission electron microscopy (TEM) was employed to investigate the soot morphology, emphasizing the fractal properties. Image processing was used to extract the geometrical properties of the thirty-five randomly chosen aggregates from each sample. The morphologies obtained from the TEM analysis were compared to the equivalent size measurements from differential mobility analyzers. The fractal dimensions (Df) were computed using the aggregate dimension and pair correlation methods. For the soot collected from VW engine, it was found that the fractal dimensions are independent of fuel type, while engine load conditions had slight influence on Df. For the soot produced by the HDPI gas engine, similar relations between fractal dimension and engine load were also observed.
Raman spectroscopy was used to discriminate the soot microstructure based on the degree of microstructural disorder. The Raman spectral analysis was done using two-band (“G” at ~1578 and “D” ~1340 cm−1) and five-band (G, D1, D2, D3, D4 at about 1580,1350, 1500, 1620 and 1200 cm−1 respectively) combinations. For the soot sampled from VW engine, the results from both methods showed that B20 soot exhibited much greater structural disorder and. Likewise, the Raman analysis of the soot from both engines also showed that the increased in engine load condition caused increases in the degree of the structural order of soot.