First 3-D look at diesel particles gives clues to cleaner engines

04/15/04

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 gas counterpart, they contain nitrogen oxides NOx that contribute to smog and global warming. Diesel exhaust also contains nano-sized particles that can be inhaled and might cause health problems.

Diesels are the most efficient 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 $10 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.

Source: Eurekalert & others

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