  We studied engine-out soot samples collected from a heavy-duty
direct-injection diesel engine and a port-fuel injection gasoline
spark-ignition engine. The two types of soot samples were characterized using
Raman spectroscopy with different laser power. A Matlab program using
least-square-method with trust-region-reflective algorithm was developed for
curve fitting. We used a DOE (design of experiments) method to avoid local
convergence. This method was used for two-band fitting and three-band fitting.
The fitting results were used to determine the intensity ratio of D and G Raman
bands. We find that high laser power may cause oxidation of soot samples, which
gives higher D/G intensity ratio. Diesel soot has consistently higher
amorphous/graphitic carbon ratio and thus higher oxidation reactivity, in
comparison to gasoline soot, which is revealed by the higher D/G intensity
ratio in Raman spectra measured under the same laser power.
