Physical and Chemical Characterization of Gasoline Particulates and Differences Relative to Diesel Soot

Open Access
Author:
Gaddam, Chethan Kumar
Graduate Program:
Fuel Science
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
February 29, 2012
Committee Members:
  • Randy Lee Vander Wal, Thesis Advisor/Co-Advisor
Keywords:
  • HRTEM
  • nanostructure
  • tortuosity
  • SIDI engine
  • carbon lamella
  • fractal dimension
  • root form factor
Abstract:
The research reported here involves a detailed investigation of primary particle size, aggregate size and morphology, primary particle nanostructure (fringe length and tortuosity), surface and bulk chemical composition of the soot generated from a spark-ignited, direct-injection (SIDI) engine at different operating conditions and compare the TEM results to an on road medium duty diesel engine which is of different size class. The comparison is not to find or demonstrate a statistically significant difference but to identify values indicative of differences observed via TEM. The SIDI engine used in this research is a fuel-neutral, single-cylinder, four-stroke research engine which is built into a Ricardo Hydra block. The engine is a 549 [cc] with a flat-top piston, fueled by Tier II EEE. Working in collaboration with GM and PNNL, a clean baseline engine operating condition was established and repeatability at this condition was ascertained. This baseline condition is specified as 2100 rpm, 350 kPa IMEP, 280 [°bTDC] end of injection (EOI), and 25 [°bTDC] ignition timing. Engine operational conditions included lean and rich conditions, a high-load case and an advanced ignition, each relative to a reference condition. Characterization techniques involved physical structure (microscopy) and chemistry (spectroscopy), both electron and optical based. Physical size and structure characterization that include aggregate size and morphology, primary particle size and internal nanostructure, were analyzed by TEM and subsequent image analyses. Thermogravimetric analysis (TGA) partial oxidation runs were conducted with intermediate study by HRTEM to estimate the degree of burnout for but one sample. Chemical characterization included organic and elemental content quantification as analyzed by X-ray photoelectron spectroscopy (XPS), volumetric-averaged functional group identification by Fourier transform infrared spectroscopy – Attenuated total reflectance (FTIR-ATR). The TEM measurements showed that there is more primary particle size variation for SIDI samples. The aggregates formed for the conditions with lesser mixing appeared to be more compact with a higher level of tortuosity. Significantly FTIR provided a volumetric measure of the chemical composition as manifested by the functional groups. XPS showed that there are significant differences in composition between the five different engine-operating conditions. High resolution spectra over the C 1s regions showed that the soot particulates surface mostly consisted of graphitic carbon with some organic content. Corresponding relative peak intensities suggest that organics observed by FTIR-ATR are not concentrated in a surface film. Taken with FTIR-ATR measurements, this serves as strong evidence for matrix distributed organic content. A small (< 20%) surface oxygen content was observed by XPS, consistent with FTIR suggesting that the organic content is mainly alkyl hydrocarbons rather than oxygenated species. Compared to diesel engine produced soots, aggregate fractal morphologies (both fractal dimension and root form factor) were significantly lower while primary particle sizes were of similar size range. The composite of these analyses illustrates the heterogeneity of species incorporated into soot particles implies a non-uniform growth environment with respect to time, temperature, and gas-phase chemistry during the combustion cycle.