Revealing the True Nature of Diesel Particulates

Although they produce fewer greenhouse gases than conventional gasoline vehicles, diesel-fueled vehicles are often described as sooty and dirty. The soot produced by diesel vehicles, known as particulate matter (PM), is emerging as one of the most important components of diesel emissions to be controlled.

There are many ideas about how to reduce particulate emissions from trucks, ranging from after-treatment systems (such as particulate traps) to improved combustion processes. Before any of those approaches can be implemented, however, researchers need to fully understand all of the factors influencing the nature of PM in exhaust emissions, including engine load, engine speed, and fuel composition.

Argonne's test engine with the thermophoretic sampling device attached.

Argonne researchers created a special thermophoretic sampling device to gain a better understanding of the nature of diesel PM. As particles move across a temperature spectrum, a process known as thermophoretic force causes them to move from the higher temperature range to the lower temperature range. Thermophoretic sampling works by collecting PM as it moves across the temperature spectrum. The method requires no dilution or treatment of the emissions stream bwdoew sampling, making it a novel method with which to study the true nature of the PM being collected.

Until now, the methods for evaluating PM in exhaust emissions have been largely limited to studies using impactors and dilution tunnels. These tools both work by sampling the diluted or undiluted PM in exhaust emissions to reveal its quantitative nature. The difficulty with impactors is that they are not good tools for measuring small particles; with dilution tunnels, the problem is that extra treatment is required before samples can be analyzed chemically. Neither approach tells much about the qualitative nature of the PM stream.

The transmission electron microscope can reveal the presence of graphitic structures in PM sampled under high engine loads.

Using a high-resolution transmission electron microscope (TEM) equipped with photographic and data recording capabilities, Argonne scientists can capture information about PM found in the emissions obtained with the thermophoretic sampler. They can examine individual primary-particle sizes, the sizes of massed (agglomerated) particles, the degree of oxidation shown by the particles, and the number of graphitic structures within the agglomerated PM. Oxidation and graphite content are both indicative of soot at high engine operating conditions.

Argonne researchers have thus far concluded that higher combustion temperatures and increased pressure conditions within the engine are the factors contributing most to the production of diesel exhaust emissions with PM characterized by small, agglomerated, oxidized/graphitic (sootier) particles.

Future Plans

Future research will focus on the following:

• Investigating the effects of engine rpm and load on particulate formation and oxidation,
• Examining the internal microstructure of diesel particulates, and
• Exploring the impact fuel properties (such as sulfur content) have on the size, shape, and content of diesel particulate matter.

Facilities

• Diesel engine dynamometer
• High-resolution transmission electron microscope
• Thermophoretic sampler

Research Partners

U.S. Department of Energy
Drexel University
University of Illinois at Chicago
Korea Advanced Institute of Science and Technology

August 26, 2004