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X-rays penetrate secrets of diesel combustion

by Katie Williams

For the first time ever, scientists are using X-rays to penetrate diesel fuel injector sprays to improve combustion. The Argonne team has uncovered an unexpected shockwave in the gas spray as well as other data that may help manufacturers build cleaner, more efficient engine-injection systems.

EYE-OPENING RESEARCH — The opening in this fuel spray injection chamber provides researchers with aneye on the inner workings of diesel engines.

The team, made up of Jin Wang of the Experimental Facilities Division and Steve Ciatti, Christopher Powell and Yong Yue of the Energy Systems Division, won the U. S. Department of Energy 2002 National Laboratory Combustion and Emissions Control R&D Award.

“This research has been full of excitement since the idea of using ‘X-ray vision’ to study high-pressure fuel sprays was first conceived three years ago,” said Wang, who is the driving force behind the experiment.

The team’s research was published in Science and named a finalist in Discover Magazine’s 2002 Innovation Awards.

Fuel factors

Diesel engines have long been the backbone of America’s transportation system. Diesel fuels the large earth-movers used to build roads, as well as cars and trucks that traverse the roads. Diesel trucks, locomotives and ships move coal, grain and consumer goods across the country and the world. Stationary diesel generators provide electricity.

Diesels are strong, reliable and efficient. They are 15 to 30 percent more thermally efficient than gasoline engines. Diesel’s trademark dark smoke has disappeared as engine cleanliness and efficiency have improved.

While much cleaner than they were 20 years ago, diesels play a major role in the transportation sector that is responsible for roughly half of the nation’s air pollutants. Global air quality is predicted to decrease and play a significant role in deteriorating health worldwide, according to the Intergovernmental Panel on Climate Change’s Third Assessment Report that was commissioned by the United Nations Environment Programme.

Argonne’s fuel-spray research team has been working to understand what happens inside a diesel engine in order to improve the process. “It’s nice to be recognized for research that is doing its part in pollution reduction,” said Powell, who was with the Experimental Facilities Division when the research was performed. “We’re working very hard at it, and it’s something no one else has been able to do.”

The payoff is not just a cleaner environment. More efficient engines would cut reliance on imported oil and associated costs. “In 2001, diesel engines in transportation consumed the equivalent of 2.56 million barrels of oil per day,” Anant Vyas of Argonne’s Center for Transportation Research explained. “Using a $25 per barrel average, even a 1 percent savings in diesel engine efficiency in the transportation sector would save about $640,000 a day.”

X-ray vision

The best way to improve diesel combustion is to watch the fuel-injection and aftermath inside a cast-metal engine.

In a diesel engine, fuel is injected as a mist into a chamber at a pressure hundreds of times higher than in a gasoline-powered engine. As a piston compresses the charge, the fuel temperature rises and the mixture burns, pushing the piston down to turn the crank shaft. This occurs in each cylinder thousands of times each minute.

Researchers built a simulated engine-combustion chamber with windows that allowed them to see inside. They used X-rays from the Advanced Photon Source (APS) to reveal details of the engine’s fuel injection systems that would not be visible with other technologies. The APS at Argonne provides the Western Hemisphere’s most brilliant source of X-rays for research.

“The X-rays allow us to track the fuel mass of a spray,” Ciatti said. “It’s unique. The standard is to use optically based techniques like lasers or photos, but with those techniques you can only see the external functions.”

Using the laser technique, laser light near the nozzle scatters a portion of droplets in the high-pressure fuel spray, limiting the quantitative evaluation. But the X-ray beams’ penetrating power allows researchers to study the spray without disrupting it.

Scientists can define the fuel-mass structure and track it over time in the APS. The X-rays also reveal the fuel spray’s structure and atomization. Researchers watch the tiny fuel particles mix with air to create combustion. They measure physical properties – such as droplet size and density, spatial distribution, air-to-fuel ratio and velocity of spray particles – that govern the spray’s combustion.

The researchers are studying a high-pressure injection system similar to that found in a passenger car. Amoco Diesel No. 2 was used as the fuel with a cerium additive to improve the image’s contrast.

The fuel is sprayed from a nozzle into a chamber filled with sulfur hexafluoride to simulate the dense interior environment of a diesel engine. In other experiments, fuel is sprayed into nitrogen at atmospheric pressure and 77 degrees Fahrenheit.

The X-ray probes the fuel spray through a window on the chamber’s side. X-rays passing through are measured by a point detector that maps the absorption image of the fuel sprays. The X-ray probes the fuel spray through a window on the chamber’s side. X-rays passing through are measured by a point detector that maps the absorption image of the fuel sprays.

X-ray intensity is recorded by the point detector and reflects the amount of fuel at any given point of the spray. As fuel mass decreases, more X-rays make it through to the detector.

Argonne’s technique allows the researchers to observe the amount of X-ray energy that is absorbed by the spray, called X-ray attenuation. The liquid fuel and cerium absorb X-rays more effectively than any gases or vapors present in the spray. Data acquisition at the detector is synchronized with the X-ray pulses within 1 nanosecond and with the fuel spray within a few microseconds. This kind of observation is not possible with optical methods.

“The picture is different than what you would get from using visible light like a camera or laser,” Powell said. “With visible light, you’re measuring how much light is scattered from the spray back to the camera. With X-rays, you’re actually measuring how many X-rays pass through the spray.”

Researchers take measurements at varying injection pressure, injection duration and gas composition in as many as 900 different positions. They examine different injector-hole sizes and hole-surface finishes. At each position, the results of 100 successive sprays are averaged to decrease statistical error.

While this technique does not allow researchers to distinguish between fuel vapor and entrained ambient gas because of their similar densities, the team detected some lower-density regions in the spray’s center that suggest the presence of gas. Determining the liquid-fuel density as a function of time and space provided valuable insights not previously possible.

Shocking results

The team found never-before-seen shockwaves in the fuel-injection system. They also saw air and fuel vapor in the diesel-spray core. The unexpected shockwaves tell researchers that there is more to learn about the fluid mechanics of fuel spray, Wang explained.

TEAM EFFORT — Members of Argonne's diesel fuel-spray research team are (from left) Jin Wang, Chris Powell, Yong Yue and Steve Ciatti.

The team will study the structure and wave-speed by varying spray conditions. They have begun using a new, highly pressurized nitrogen gas environment to see if the shockwaves appear in an environment that more closely resembles a diesel engine.

There is promise for similar studies of dense plasma and other optically dense structures using this same technique.

“This technique we have developed here is useful for more than fuel spray,” Wang said. “It can be used for other highly transient phenomena, including plasma discharge or spray coating. Most of these materials are optically opaque. They are perfect candidates for this type of application.”

The research is still in its early stages, but Powell said the team plans to continue their fuel injector and combustion research under more realistic conditions by increasing the temperature and pressure surrounding the fuel injector.

For more information, please contact Catherine Foster (630/252-5580 or cfoster@anl.gov) at Argonne.