Detailed Characterization of Particulates Emitted byLean-Burn Gasoline Direct Injection Engine
Alla Zelenyuk, Pacific Northwest National Laboratory
Limited fossil fuel resources as well as upcoming U.S. fuel economy and emission standards are major challenges in current engine development. Small displacement turbocharged Gasoline Direct Injection (GDI) engines are replacing large displacement engines, particularly in light-duty trucks and sport utility vehicles, with future lean-burn GDI engines potentially offering even higher fuel economy than stoichiometric GDI engines. All GDI engines, however, suffer from higher particulate matter (PM) emissions than standard port fuel-injection engines.
We present the results of a study, in which we characterized the detailed properties of PM emissions generated by a 2.0L lean-burn GDI engine (2008 BMW 1-series 120i, 87 AKI Gasoline) operated using different combustion strategies that include lean homogeneous, lean stratified, stoichiometric, and fuel rich conditions. A three-way catalyst was installed on the engine and both engine out and catalyst out exhaust PM was characterized.
In addition to the measurements of PM number concentrations and size distributions, we characterized the size, mass, composition, and effective density of individual exhaust particles emitted under ~20 different engine operating conditions.
We find that most of the particles produced by the GDI engine are fractal agglomerates containing small amount of oxygenated organics and PAHs. These measurements are used to calculate fractal dimension, average diameter of primary spherules, and number of spherules, void fraction, and dynamic shape factors as function of particle size. The fraction of Ca-containing particles, originating from the detergent additives to lubricating oil, varies with operating conditions and has a very distinct size distribution. A large fraction of these particles is compact, with a vacuum aerodynamic diameter distribution peaking above 200 nm.
Overall, properties of GDI PM were shown to vary significantly depending on engine operation condition. Lean stratified operation yielded the most diesel-like size distributions. The vast majority of these particles are fractal soot agglomerates comprising primary spherules with an average diameter of 22 nm. They have a fractal dimension of 2.18 and are composed of elemental carbon (>80%), small amounts of organics (consisting mostly of carboxylic acids with very little contributions by hydrocarbons and PAHs), and some Ca from lube detergent additives. Depending on the size of soot agglomerates, their void fraction increased from 20% for 50 nm soot particles, to nearly 80% for particles larger than 200 nm.
Stoichiometric operation resulted in PM number concentrations an order of magnitude lower than those emitted under lean stratified operation. While fractal soot particles emitted under stoichiometric operation are very similar to those emitted under lean stratified operation, stoichiometric PM contains a higher fraction of Ca-dominated non-fractal particles. The comparison between engine out and catalyst out PM characteristics points to the removal of a volatile nuclei mode present in engine out exhaust.