Engine Nanoparticle Research: Revealing the True Nature of Diesel Particulates

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

A transmission electron microscope reveals the nanostructures of graphitic diesel soot sampled under high engine loads.

Morphology of particles collected from diesel combution with iso-paraffin-enriched fuel.

Amorphous soot particle collected from biodiesel combustion under low temperature conditions.

Researchers have many ideas about how to reduce the soot produced by diesel vehicles, known as particulate matter (PM): from after-treatment systems (such as particulate traps), to improved combustion processes, to use of alternative fuels. Before any of these approaches can be used, scientists need to fully understand all of the factors influencing the nature of PM in exhaust emissions, including engine load, engine speed and fuel composition.

Traditional methods for evaluating PM use commercial instruments, such as high-pressure impacters or scanning mobility particle sizers. The difficulty with these tools is that they are not highly accurate in measuring small particles, extra treatment is also required before analysis, and neither approach tells much about the qualitative nature of the PM stream.

Thermophoretic Sampling

In 1999, Argonne researchers were the first to create a unique 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 migrate from a higher to a 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 during sampling, making it a novel method for studying the true nature of collected PM.

Using a high-resolution transmission electron microscope equipped with photographic and data recording capabilities, Argonne scientists can now capture information about PM found in the emissions obtained with a thermophoretic sampler. They can examine:

• Individual primary-particle sizes,
• Sizes of massed (agglomerated) particles,
• Fractal geometry of complex shaped aggregates,
• Particle nanostructures,
• Degree of oxidation shown by the particles, and
• Number of graphitic structures within the agglomerated PM.

Both oxidation and graphite content can indicate soot at high engine operating conditions. Numbers of graphite structures are measured using a Raman spectroscope.

Higher Combustion Temperatures and Increased Pressure Conditions

Argonne researchers have 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.

In combustion of different fuel properties, particles from iso-paraffin-enriched fuel showed very unusual morphology, compared to normal diesel particles from typical diesel fuels which contain quite a bit of aromatic content (35 percent). Particulates from iso-paraffin-enriched fuel appeared to be much smaller in size (about 15 nm in diameter versus 30 nm on average) and contained multiple nuclei in a single primary particle. The increased curved pattern of dark fringes and increased interspacing distances between fringes (0.362 nm versus 0.348 in average) indicate a potential high degree of oxidation for these small particles.

Argonne researchers concluded that the particles presenting multiple nuclei were formed through the coalescence of smaller primary particles, where a single nucleus had already been formed prior to coalescence. With further pyrolytic reactions, these multiple-nuclei particles should have converted into single-nucleus particles and become larger primary particles with one nucleus apiece.

Scientists examined the morphology of biodiesel fuel-derived particulates collected under low temperature combustion conditions. The initial results showed that particle nanostructures were considerably different from those of conventional diesel particles (stoichiometric combustion with ultra-low sulfur fuel), such that a majority of the particles appeared to be amorphous, where no distinct fringe patterns (known as graphitic structures) were observed in primary spherules.

Another interesting observation was that the particle number density in the control volume was much greater than from conventional diesel operations. Under macroscopic examination, the morphology of these particles turned out to be quite similar to that of conventional diesel particles. The individual primary particles were nearly spherical and clustered each other to form a larger chain-like aggregate particle.

This work was performed in collaboration with the Engine Research Center of the University of Wisconsin.

Funding

This work was supported by the U.S. Department of Energy’s Vehicle Technologies Program.

Publications

• "Morphological Study of the Particulate Matter from a Light-Duty Diesel Engine," Zhu et al.
• "Characterization of Particulate Sizes, Microstructures and Fractal Geometry of a Light Duty Diesel Engine via Thermophoretic Sampling," Lee et al.
• "Detailed Characterization of Morphology and Dimensions of Diesel Particulates via Thermophoretic Sampling," SAE 2001-01-3572, Lee et al.

More

• Research Reveals the Atomic Structures of Diesel Particulates
• 3-D Animation Shows Complex Geometry of Diesel Particulates
• Development of Advanced Diesel Particulate Filtration Systems
• Evolution in Size and Morphology of Diesel Particulates along the Exhaust System
• Effects of Exhaust System Components on Particulate Morphology in a Light-duty Diesel Engine
• Fuel Property Impacts on Diesel Particulate Morphology, Nanostructures, and NOx Emissions

April 2010