Fueling the future: X-ray research improves fuel-injection efficiency, cuts emissions

To many people, the particulars of how a car's engine works are a mystery. From the gas tank to the exhaust pipe, the fuel we use makes a journey through injectors, cylinders and valves, transforming from a liquid to a gas along the way through combustion.

The automotive engineers who design the cars and trucks we drive can tinker with an engine to squeeze out a few extra horsepower or a slightly better gas mileage, but they lack a detailed picture of the structure and composition of fuel as it is sprayed into the combustion chamber. The composition of a fuel spray influences both how efficiently and how cleanly it combusts.

Because engines can cycle hundreds to thousands of times a second, and because the dynamics of the combustion itself are so difficult to control, researchers at Argonne have tried to understand and manipulate the structure and timing of the fuel spray, as the atomization of the fuel jets determines the combustion later in the cycle. By adjusting the fuel spray, scientists have the potential to significantly enhance an engine's fuel efficiency while simultaneously diminishing harmful emissions.

To do so, they need a something like a high-speed camera to capture the detail of each fuel injection. With imaging tools that use visible light, however, scientists can only trace the outline of the fuel plume. The visible light cannot penetrate the dense plume, just as a high-beam headlight cannot pierce through fog. The high-energy X-rays produced by Argonne's Advanced Photon Source, by contrast, can resolve the entire structure of the fuel spray, said Argonne researcher Jin Wang.

"It's a bit like trying to put the engine through a medical exam," he said. "It's possible your doctor might be able to find out if there's anything wrong with you just by looking at you, but he can tell a whole lot more by taking an X-ray."

Although previous studies assumed that the outline of the fuel plume "told the whole story," as Wang puts it, the latest research done with penetrating X-rays has found that most the fuel contained in the spray is concentrated in a small region around its center. "You have to have a synchrotron to do this sort of work," he said. "The highly penetrative and highly intense nature of X-rays allows us to see more than we ever could before, and allows us to take images only a nanosecond apart.”

Not all fuel sprays are the same. Even small differences in fuel droplet size, shape or structure after atomization can cause fuel to combust differently. The difference in combustion patterns not only affects the efficiency of the engine – it also generates different emission products. "For the same engine, the combustion of one type of spray might give off lots of nitrous oxides and other greenhouse gases, while a slight change to the injection parameters might greatly improve the engine's efficiency and reduce the quantity of pollutants that it produces," Wang said.

Ideally, Wang hopes to determine the mathematical and physical principles that underlie how different characteristics of fuel injection affect how the fuel spray is combusted. "The holy grail of combustion research is to come to a fundamental understanding of entire combustion process, including the sprays. This would enable us to create a set of guiding principles to design cleaner, smarter and more efficient systems," he said. "We're looking for more than just a recipe that says to tweak one parameter a certain way and another one a different way. It has to be more than merely empirical."

The deductive approach that Wang espouses has one critical advantage: with controlled modifications, researchers could likely tailor their fuel sprays to most types of engines that use several different fuels -- including biofuels. While these new and remarkable fuels themselves have drawn much of the public's attention in the race to meet the energy and environmental challenges facing the transportation industry, the advances scientists will make in green transportation over the course of the next several years will owe themselves just as much, if not more, to finding cleaner and more efficient ways to use them. — by Jared Sagoff

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For more information, please contact Jared Sagoff (630/252-5549 or jsagoff@anl.gov) at Argonne.