Maximizing the energy we extract from each gallon of oil is a powerful way to conserve energy and lower our society’s carbon footprint. Direct fuel injection engines do this by atomizing fuel into such a fine mist that it burns completely, extracting the maximum amount of energy. Diesel engines that use direct injection are well studied. But the process is less understood in engines that burn fuels with much higher vapor pressure than diesel.
These high vapor pressure fuels, such as gasoline, ethanol, and liquefied petroleum gas, spontaneously bubble and then collapse inside the injectors, a phenomenon called cavitation. Cavitation likely changes the spray dynamics and atomization downstream, but until recently it has been difficult to get precise imagery of the fuel jet as it enters the engine. Now scientists have used the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science user facility at DOE’s Argonne National Laboratory, to directly characterize the ultrafast cavitation dynamics of high vapor pressure fuels.
Even as our society electrifies, internal combustion engines for vehicles and many other devices will still be the only practical choice in many situations. Understanding the dynamics of cavitation during fuel injection will help us to use alternative fuels and our dwindling petroleum supplies as efficiently as possible.
When fuel is injected into an engine, it’s under pressure and coming out of nozzle orifices not much thicker than a hair. As it hits the air, it forms tiny droplets. Both the injection pressure and temperature affect the droplet pattern—and so the combustion efficiency—of fuels in direct injection engines. But while pressure is fairly well understood, the effect of temperature on high vapor pressure fuels is not. A team of researchers from Argonne suspected that cavitation might be a useful way to understand how the fuel’s temperature interplays with pressure to affect spray pattern and motion.
The researchers looked at the cavitation effect in n-heptane injection using X-ray Near-Field Speckle (XNFS) imaging at APS beamline 7-ID. N-heptane is a well understood hydrocarbon with fuel characteristics similar to gasoline. XNFS imaging uses the speckles, or X-rays scattered by the n-heptane ligaments and droplets, to reconstruct a motion-speed map of the fuel as it is injected and evolves within the engine. XNFS allows the researchers to “see through” the fog of atomized fuel, capturing pictures of both the surface droplets and those in the interior. The near-field imaging improves the sharpness of the image, and lets the researchers follow the motion of the fuel jet at high resolution.
Fuel injected into an engine moves fast, around 250 meters per second, close to the speed of sound. To capture still images of it without a motion blur, the researchers needed exposures as short as 1 billionth of a second. But just as a camera needs extremely bright light to capture sharp images at high shutter speeds, the researchers needed extremely bright X-ray beams to capture sharp images in 1 billionth of a second. APS beam line 7-ID allows every X-ray photon from the source to pass through to the sample. The beam is so intense and bright that it would drill a hole in most materials in a fraction of a second.
The researchers needed that brightness to get high-resolution images of the spray dynamics—but they didn’t want to damage their equipment. So they used an X-ray chopper to get brief, billionths-of-a-second-long bursts of super bright X-ray beams to illuminate the fuel jets. Using this setup, they were able to take about 350 XNFS images before, during, and after the fuel injection, over a span of 5 milliseconds.
Using the images, researchers could measure the fluid velocity. Over a series of experiments at different pressures and temperatures, they could see how the fluid velocity changed in response to changes in pressure and temperature. However, the fluid velocity, directly related to the energy transfer efficiency, did not have a simple, independent relationship to pressure or temperature.
Instead, the researchers looked at cavitation number. The cavitation number marks the likelihood of small bubbles forming in the fuel as it is injected. The APS data showed that cavitation had a simple, direct relationship to the energy efficiency of the fuel injection. The researchers also found that varying the pressure and temperature of the fuel to maximize cavitation increased the energy conversion in the process. Maximizing the cavitation effects could provide another parameter for engineers to take into consideration when they design newer, more efficient combustion engines. – Kim Krieger
See: Q. Zhang1, Y. Gao1, M. Chu1, P. Chen1, Q. Zhang1, J. Wang1, “Enhanced Energy Conversion Efficiency Promoted by Cavitation in Gasoline Direct Injection,” Energy 265 126117 (2023)
Author affiliations: 1Argonne National Laboratory.
This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science user facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. The experiment using the HSCU is partially supported by DOE Basic Energy Science Accelerator and Detector Research Program. The development of the spray structure tracking algorithm is partially supported by an ANL internal research grant (LDRD). We acknowledge Seoksu Moon and Weidi Huang for loaning us the high-speed camera and for valuable discussion. We acknowledge Donald Walko and Raymond Ziegler for their technical support at the 7-ID beamline of the APS. Donald Walko also graciously read the entire manuscript and provided numerous valuable comments.
The U.S. Department of Energy's APS at Argonne National Laboratory is one of the world’s most productive x-ray light source facilities. Each year, the APS provides high-brightness x-ray beams to a diverse community of more than 5,000 researchers in materials science, chemistry, condensed matter physics, the life and environmental sciences, and applied research. Researchers using the APS produce over 2,000 publications each year detailing impactful discoveries and solve more vital biological protein structures than users of any other x-ray light source research facility. APS x-rays are ideally suited for explorations of materials and biological structures; elemental distribution; chemical, magnetic, electronic states; and a wide range of technologically important engineering systems from batteries to fuel injector sprays, all of which are the foundations of our nation’s economic, technological, and physical well-being.
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