| Summary: | The effect of particulate matter on the climate depends on its scattering properties
(influenced by morphology) and refractive index (dependent on microstructure). The
morphology and microstructure of particulates from two different engines were studied.
The first set of soot samples was collected from a 1.9L Volkswagen Turbo Direct
Injection engine with or without a catalytic converter, using two different fuel types
(ULSD and B20) and six speed/load combinations. The second set of the samples was
taken from a Cummins ISX heavy-duty engine using the Westport pilot-ignited direct injection
natural-gas fuelling system for three different speed/load combinations.
The soot morphology was investigated using transmission electron microscopy (TEM),
emphasizing the fractal properties. A Matlab-based image processor was used to extract
geometrical properties of soot. Thirty-five aggregates were analyzed for each sample.
The fractal dimensions (Di) were computed using the aggregate dimension and pair
correlation methods. For the soot collected from VW engine, it was found that fuel type
(ULSE) and B20) and the presence of a catalytic converter did not affect the fractal
dimensions of soot aggregates, while engine load conditions had slight influence on Df.
At constant engine RPM, fractal dimensions started to decline as the engine torque was
significantly increased. For the soot produced by the natural gas engine, similar changes
in the fractal dimension with respect to the engine load were also observed. Both methods
of computing Df produced similar trends.
Raman spectroscopy was used to quantify the degree of structural disorder present in
soot. The Raman spectral analysis was done using two-band (“G” at ~1578 and “D”
~l340 cm⁻¹ and five-band (U, Dl, 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 the
degree of graphitization of the soot increased as the engine load increased. Likewise, the
Raman analysis of the soot from the Cummins engine also showed that the increased in
engine load condition caused increases in the degree of the structural order of soot.
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