Characterization of Particulate Matter Emitted by Spark Ignition Direct Injection Gasoline Engine

Alla  Zelenyuk, Pacific Northwest National Lab

Current engine development aims to reduce consumption of limited fuel resources and reduce carbon dioxide emissions by increasing fuel efficiency. While diesel engines achieve the highest fuel efficiency, they also generate the largest number of exhaust particulates known to be harmful to human health and the environment. Spark Ignition Direct Injection (SIDI) is a gasoline engine technology with potential to achieve very high fuel efficiency by operating more like diesel engines, blurring the lines that have traditionally existed between gasoline and diesel engines. At present, the studies providing detailed characterizations of particulate emission from these new engines are very sparse.
We present the results of recent studies, in which we characterized in detail the number concentration, size, composition, morphology, effective density, and mass of individual exhaust particles emitted by a single-cylinder test SIDI engine. These measurements were used to calculate particles’ fractal dimension, average diameter of primary spherules, and number of spherules, void fraction, and dynamic shape factors as function of particle size.
We find that SIDI particulate exhibited high organic content (40-60%) that is tightly bound with inorganic elemental carbon within the primary spherules, making is impossible to remove by thermos-denuder or in an evaporative chamber. The primary spherule diameter of SIDI soot varied dramatically with engine operating condition and fuel, which is a marked contrast with diesel.
Under some experimental conditions, two distinct particle modes were observed. Analysis indicates that the two particle types have nearly identical chemical composition and fractal dimension, but are constructed of primary spherules with significantly different diameters. Two particle modes of fractal soot particles were also observed when varying the end of injection timing.
Examination of the engine operating conditions under which the two modes appear suggests the presence of multiple mechanisms for in-cylinder soot formation. One possible mechanism could involve relatively rich pockets due to incomplete air/fuel mixing. Soot formation could also be facilitated by pools of liquid fuel caused by wall or piston impingement. Differences in particulate characteristics between various fuel blends also reveal the role of chemistry in soot formation. In an effort to elucidate some of these possible factors in soot formation, additional particulate characterization experiments were carried out with pre-mixed and pre-vaporized fueling.
We will present a comparison of the properties of particulate produced in the engine using direct injection and pre-mixed and pre-vaporized fuel, under otherwise identical conditions. Insight into the effect of fuel chemistry on particulate formation was derived from tests conducted with fuels that include tier II EEE gasoline and a number of ethanol blends.

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