TransForum Vol. 3, No. 1

"X-RAY VISION": TAKING A HARD LOOK AT FUEL SPRAYS AND COMBUSTION

As a child, you might have owned a see-through model engine, made mostly out of transparent plastic, that let you see what was going on inside it: with the help of a tiny electric motor, fan blades twirled, pistons moved up and down, and the crankshaft really turned! The plastic model had no fuel system or working spark plugs, so you couldn't actually watch combustion at work. Even today, engine researchers cannot directly observe all the details of critical combustion processes in a working engine. Thanks to groundbreaking research by Argonne scientists and engineers, however, the day of the "transparent" internal combustion engine is about to dawn at last. The secret? "X-ray vision!"

Teams of Argonne researchers are using high-brilliance x-rays from the Advanced Photon Source (APS) - a unique user facility dedicated to producing synchrotron x-rays for research - to shine a piercing new light on the fluid dynamics and chemistry of fuel spray behavior and combustion processes.

Seeing inside fuel sprays... a shocking discovery

A team of Argonne researchers is using APS x-rays to obtain never-before-possible quantitative data on the structure and behavior of cold gasoline and diesel sprays. Continuing and expanding on experiments reported previously [TransForum, Volume 2, No. 2], Roy Cuenca, Yong Yue, Jin Wang, and Chris Powell are using x-ray adsorption techniques to fill a critical gap in our knowledge of spray behavior and dynamics.

The gasoline spray from an injector nozzle takes the form of a hollow cone - a thin sheet of particles on the outside, surrounding an empty-air interior. Using their experimental apparatus, Cuenca and his colleagues can obtain the instantaneous local mass of fuel in any given volume element. According to Wang, "with such information, the time resolution of APS x-rays lets us measure the thickness of the gasoline spray's walls quantitatively; no one could do this before x-ray sources like the APS became available." The measurements make it possible to say exactly how well the injector is atomizing the fuel, which may provide useful insights into tip erosion, a common problem with fuel-injected engines.

Argonne's diesel spray research is also yielding some new information. Until now, it was customary to regard the dense core of the diesel spray as a liquid, but the Argonne researchers - penetrating the region within the first five millimeters of the spray nozzle, where optical laser techniques are useless - have shown that the core is no more than about 50% liquid; the rest is trapped fuel/air vapor. This is an important finding about diesel combustion, a process limited mainly by the extent of fuel-oxygen mixing.

The team's experiments also provided a surprise of major significance. Increased pressures are being used in modern injection systems to meet emissions regulations; at the higher pressures, internal cavitation can take place, disrupting the flow. In their spray experiment, the Argonne researchers were the first to detect evidence of a shock wave being formed. "In the injection cycle, the spray's trailing edge moves faster than the leading edge, catching up with it and generating shock waves," says Cuenca. "That's why it happens. The question is, does the same thing happen under actual engine operating conditions? If so, can you prevent it... or even find a way to take advantage of it?"

With data from the spray experiments, it is no longer necessary to guess at conditions close to the injector nozzle. Engine manufacturers and designers of combustion devices will benefit from Argonne's fuel-spray research. The knowledge gained could lead to higher-performance, lower-emission injectors for modern automotive engines, as well as more efficient operation.

Getting the dirt on soot

In a related effort, Argonne researchers are looking - more closely than anyone has before - at the formation of soot particles during combustion. When you look at a flame, the part that's burning reddish-orange is produced by incandescent ("red-hot") soot particles; the bluish central part of the flame is actually much hotter and contains no soot. By scattering x-rays off soot particles as they form inside a steady flame, Argonne chemists Jan Hessler, Randall Winans, and Al Wagner have determined soot particle distributions at sizes on the order of a nanometer (nm) - ten times smaller than can be achieved by optical laser techniques.

The scientists don't actually measure individual soot particles, much less get visual images of them. They measure the intensity of the x-rays as a function of scattering angle, and by analyzing these data they can characterize the distributions of soot particles in the line of the x-ray beam. The burner that produces the flame in their experiments can be moved vertically through the beam, changing the line of sight to sample the whole flame.

Mechanisms for soot growth are varied, but until now there's been little experimental evidence available for the early stages of growth. Distributions measured in laser experiments for particles in the 15-20 nm range can be calibrated with the Argonne researchers' x-ray data, filling in a vital information gap. The chemists' work opens up a region of soot growth that was previously inaccessible. "We're the first to obtain any data on soot formation at sizes of 1-2 nm and times on the order of a millisecond," says Hessler.

The nanometer-scale particles represent the earliest stage of soot formation ever seen. Models of soot formation are most sensitive to how events proceed in the earliest stages, so the researchers' experimental data provide critical information needed to validate such models. According to Wagner, "If we had a model, it would predict the distributions, which could then be compared with what we see experimentally."

As industry refiners use heavier oils (containing more soot-producing ingredients) in the fuel production process, particulate matter is likely to become more of a problem. Also, while the popular perception of air pollution is still focused on black puffs of smoke from trucks and buses, health experts are beginning to see the ultrafine, nanoscale particulates as a more serious concern. How can research on soot formation help? According to Winans, "Our research is applicable to a broad spectrum of environmental problems. Thus far, the approach to controlling emissions has been rather 'Edisonian,' but if we understand the mechanisms governing soot formation at its earliest stages, then we could control emissions much more effectively."

Taking the next step, and the next

Although the fuel-spray research has involved only cold sprays (and thus, no soot formation), it has the potential to complement the combustion studies. Winans's team found soot particles smaller than any ever observed before, and that knowledge could be helpful in future spray experiments. The researchers are planning a joint effort to study both cold and ignited sprays. The project requires that a new instrument be designed for studying spray breakup with and without ignition, in hopes of correlating soot formation and spray breakup.

"Soot formation presents both chemical and fluid mechanical problems," says Wagner. "Diesel spray breakup is also fluid mechanical. They measure density variations in the spray - we plan to coordinate our soot formation work with the spray research under more realistic combustion conditions."

The fuel-spray and soot-formation experiments at the APS have demonstrated the advantages of x-ray research at ambient temperature and pressure - the next step is to study these phenomena under more realistic engine operating conditions (temperatures of many hundreds of degrees Celsius, pressures of several atmospheres), which will require making pressurized chambers x-ray-accessible ("windows" transparent to x-rays). The new, integrated instrument being designed for both cold and ignited spray studies won't be a real engine, because it will include no piston. But a temperature-pressure environment could be achieved that approaches actual diesel engine conditions. Such a device could serve as the forerunner for an eventual x-ray engine research center based at the APS. X-rays would pass through a test engine running under realistic conditions to study oxidation, formation of NOx and particulates, etc.

"Engine researchers have been using 'paper engines' - computer simulations based on computational fluid dynamics - but the models aren't accurate enough," says Cuenca. "We've tried the 'optical engine' approach, using quartz-glass windows for laser access, but it cannot function at realistic engine temperatures or develop adequate power. But if some day we can build an 'x-ray engine,' then we can find out exactly what happens inside it under real-world conditions. That will mean a new paradigm in engine diagnostics."

Argonne's fuel-spray research is supported by the U.S. Department of Energy's Office of Transportation Technologies. The research on soot formation is supported by DOE's Office of Basic Energy Sciences (Office of Science, Division of Chemical Sciences).