TransForum Vol. 3, No. 4

ZEROING IN ON DIESEL PARTICULATE EMISSIONS

Thick clouds of soot particles no longer billow from new bus and truck exhaust pipes, thanks to today's advanced diesel engines, which emit fewer and smaller particles. Nevertheless, scientists and engineers need to continue studying ways of further improving diesel performance, because small soot particles may pose greater health concerns than the large particles emitted from older engines.

As a result of the relativly recent discovery that emitted particles vary greatly in shape, microstructure, and chemical composition as engine operating conditions change, researchers at Argonne have mounted a major effort to learn how particles form inside diesel engines. Their goal is to achieve a fundamental understanding of how engine speed and load conditions influence the particle formation process — which will help them develop effective ways of significantly reducing diesel particulate emissions.

With funding from the U.S. Department of Energy, Argonne used transmission electron microscopy (TEM) to inspect individual soot particles, which, at the ultrahigh magnification of TEMs, was a lot like finding a few needles in hundreds of haystacks. The researchers discovered that at low operating conditions (675 rpm/0% load to 1,400 rpm/15% load), the particles tend to stick together and have nebulous boundaries because of liquid constituents that make the particles merge with one another. At higher engine operating conditions (e.g., equal to or higher than 1,400 rpm/50% load), the particles appear more solid, and the boundaries between them become more pronounced.

Emitted diesel particles vary greatly in shape, microstructure, and chemical composition as engine operating conditions change. Understanding how speed and load influence particle formation could help researchers develop ways to reduce diesel particulate emissions (images not to same scale).

Preliminary chemical analysis showed that emitted particles contain more chemical components at lower speeds and loads because, at the low temperatures involved, several of the chemical constituents of diesel fuel don't evaporate or react completely. The particles were found to contain more potential cancer-causing pollutants; specifically, more polycyclic aromatic hydrocarbons (PAHs). At high engine operating conditions, the particles were found to consist mostly of carbon in the form of graphite. This means that when diesel trucks or buses are driven on the highway, they most likely produce graphitic particles, but in the downtown areas of major cities, they tend to produce particles that contain both graphitic and hydrocarbon components, including possibly harmful PAHs. The researchers also discovered that diesel soot particle formation is controlled by a mechanism that induces small nucleus-containing particles to join together, forming long chain-like particles that are emitted into the atmosphere.

The current research program extends this initial work to include the full range of engine operating conditions encountered in city and highway driving. Use of TEM continues because it provides the most accurate way of measuring particle size. The method is being supplemented, though, with two quantitative techniques for measuring the chemical compositions and physical phases of the emitted particles. The latter measurements are being made in partnership with researchers at the University of Illinois at Chicago and Drexel University.

The University of Illinois at Chicago has a two-laser ion trap mass spectrometer that can provide information not only about the atoms that are present, but direct information concerning the chemical compounds, too. The collaboration with Drexel University involves using Raman scattering spectroscopy to determine what portion of the particles is graphitic and what portion is in the liquid phase. The Argonne researchers are currently studying the particles that come directly from diesel engines; they also plan to study tailpipe emissions and the influence of sulfur-based impurities in diesel fuel, because there is considerable evidence that sulfur content contributes significantly to particle formation. The research could lead to important applications, given that manufacturers of emission abatement technologies need highly accurate information on particle size and chemical composition to design practical and effective aftertreatment systems.