First 3-D look at diesel particles
gives clues to cleaner engines
ARGONNE, Ill. (April 16, 2004) – In the first use ever of a new three-dimensional
technique to study diesel engine emissions, researchers at the U.S. Department
of Energy's Argonne National Laboratory developed information that could
lead to improved exhaust-cleaning devices, ways for industry to meet
environmental regulations, and new insights on the impact to public health
from diesel engine emissions.
Engineers at Argonne's Center for Transportation Research determined
that emission particles are not spheres, as is usually assumed, and that
shape varies depending on engine speed and load.
Emissions from diesel engines are of concern because, like their gasoline
counterparts, they contain nitrogen oxides – NOx – that contribute to
smog and global warming. Also like gasoline engines, diesel engines produce
exhaust that contains nano-sized particles, which can be inhaled and
might cause health problems.
Diesels are the most efficient heat engines ever built. They are highly
durable and 30 to 40 percent more fuel efficient than gasoline engines.
Though not currently popular with U.S. auto buyers, diesels power trucks,
trains and construction equipment in this country.
“Throughout Europe and the rest of the world, diesels are popular,” explained
mechanical engineer Raj Sekar, who leads the Engine
and Emissions Research Group doing the research. “As the cost of
fuel increases, diesel cars will look more attractive. If you have to
pay $3 a gallon to fill your car, wouldn't you rather get 40 percent
better fuel economy?”
Sekar's group studies in-cylinder combustion on engines ranging from
cars to locomotives.
Using the dynamometer at Argonne's Engine Research Facility to operate
the engines from idling to full load, engineers ran tests on a Mercedes
1.7 liter automotive diesel – commonly used in passenger cars in Europe – and
a 2.4 liter single-cylinder version of Caterpillar truck diesel using
pump-grade fuel.
Argonne mechanical engineers Kyeong Lee and Jerry Zhu, developed a bench-scale
thermophoretic sampling technique to work on internal combustion engines.
His work revealed the form and structures of diesel engine particulates
in three dimensions for the first time.
Emission samples were collected at two-foot-intervals along a nine-foot-long
exhaust pipe. The exhaust temperatures ranged from 1,200 degrees Fahrenheit
(650 degrees Celsius) near the engine to 200 degrees F (90 degrees C)
at the tailpipe.
Lee analyzed the samples in the transmission electron microscope at
Argonne's Electron Microscopy Center at 250,000 times magnification to
reveal particle shape, form and microstructure. A spectroscope associated
with the microscope revealed that the particulate matter is about 95
percent carbon, with smaller concentrations of oxygen, silicon and sulfur. “These
chemicals may come mostly from fuel or lubrication oil,” Sekar said.
“Small particles come out from the engine, but as they travel down the
exhaust pipe and they start to cool, the particles attach to one another
and may also chemically react,” Sekar said. “What comes out of the engine
may not necessarily be what comes out of the tailpipe, so this testing
allows us to track particle transformation to understand what gets into
the atmosphere.”
“We hope that these results will guide the design of an advanced particulate
trap or other mitigation devices,” said Sekar. “We are also creating
a database that will be used to study the health effects of emissions.”
Previous tests have been simpler and mainly quantitative. For example,
the Environmental Protection Agency standards
are based on simple mass analyses. Exhaust is collected on filter paper
and weighed to determine how much particulate matter is emitted.
Lee determined that temperature is the most sensitive parameter for
particulate formation and destruction. The higher temperatures caused
by carrying higher loads – 2,500 revolutions per minute at 100 percent
load – reduced the primary particle sizes and produced significant oxidation,
or combustion. He also found a broad size distribution of particles at
various engine speeds and loads.
Particulate from the truck engine was more compact; particulate from
the car engine was more chain-like.
Researchers also used an Argonne-patented technique called laser-induced
incandescence on the engines, in addition to many commercial instruments.
This technique provides real-time information about the instantaneous
mass, size and number of particles per liter of exhaust gas, even as
engine operating conditions change.
Engineers are currently studying a gasoline-powered car engine for comparison. “Both
diesel and gasoline engines,” Sekar said, “produce the greenhouse gas
carbon dioxide, but since diesel is 30 to 40 percent more efficient,
it produces 30 to 40 percent less carbon dioxide. And so many more cars
are powered by gasoline in this country.”
Future research plans include investigating the relationship between
engine operation and particulate formation and oxidation, examining particulate
microstructure and exploring the impact of fuel properties on diesel
particulate matter.
Argonne engineers worked with research partners at the University
of Illinois at Chicago, Drexel
University, the Korea
Advanced Institute of Science and Technology, Brown
University and the National Institute
of Standards and Technology.
Funding was provided by the FreedomCAR &Vehicles
Technology Program in the DOE's Energy
Efficiency and Renewable Energy Program.
The nations first national laboratory, Argonne National
Laboratory conducts basic and applied scientific research across
a wide spectrum of disciplines, ranging from high-energy physics
to climatology and biotechnology. Since 1990, Argonne has worked
with more than 600 companies and numerous federal agencies and
other organizations to help advance America's scientific leadership
and prepare the nation for the future. Argonne is operated by
the University of Chicago for
the U.S. Department of Energy's Office
of Science.
For more information, please contact Evelyn Brown (630/252-5510
or media@anl.gov) at Argonne.
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