KEY CONCEPTS
• A technique known as operando neutron diffraction was used to analyze an operating internal combustion engine.
• A cylinder head, prepared with a new cerium aluminum alloy, was evaluated in a small-bore single cylinder internal combustion engine.
• Neutron diffraction data showed that the new alloy is stable under real engine operating conditions. 

The internal combustion engine has represented the main source of propulsion for automobiles for over a century. During this time, much of the concern with its use has centered on emissions and improving the efficacy of the three-way catalytic converter based on noble metals.

But an understanding of how the noble metal catalysts actually convert hydrocarbon combustion byproducts into carbon dioxide and water has been lacking. In a previous TLT article,¹ researchers used a combination of experiments, theoretical studies and analytical techniques to study the performance of a platinum/palladium catalyst in reacting propene with oxygen. Changes in the shape of large catalyst nanoparticles occurred during the reaction that facilitated the formation of more active catalyst sites.

A better understanding of the inner working of an internal combustion engine might also lead to the development of better materials and designs that might improve efficiency and reduce emissions. Dr. Martin Wissink, R&D associate in the Combustion and Fuel Science Research Group at Oak Ridge National Laboratory (ORNL) in Oak Ridge, Tenn., says, “The combustion chamber of an internal combustion engine is an extreme environment where metallic materials must deal with rapid changes in pressure and temperature. Cyclic loading of metals can lead to fatigue, reducing operating life.”

Wissink points out that with the life of an automobile engine projected to be 200,000 miles and even higher for a truck (500,000-1 million miles), any effort that can be done to improve efficiency will be welcomed. This issue has become more important with the downsizing of engines and the use of lighter metals such as aluminum. Wissink says, “Replacing steel or iron components with aluminum reduces the weight of the vehicle, which is an important factor in improving fuel economy. This approach is often combined with downsizing, where the engine size is reduced so that it operates at its most efficient condition more of the time. However, reducing size while maintaining the same output means that the pressure and temperature in the combustion chamber will increase, presenting a real challenge for castable aluminum alloys, which tend not to have very good high temperature properties.”

If a new material can be identified for evaluation, conventional tests are available to evaluate bulk properties by tensile and compressive loading, and crystal lattice properties such as structure and phase stability by X-ray diffraction. Wissink says, “Laboratory-scale characterization is limited in terms of the sample geometries and the operating conditions. There are extreme temperature and load gradients in a combustion chamber, both in space and time, which are almost impossible to simulate outside of an engine. It will be helpful to be able to measure what happens to a material while the engine is in operation, but putting a mechanical device, such as a strain gauge, inside a combustion chamber would be both extremely challenging and invasive and would provide limited microscopic information. A noninvasive probe, such as X-ray diffraction, can provide a wealth of information about the crystal structure and how it changes during operation, but only on the surface of a material.”

A new approach is needed not only to better understand how the inner structure of a material is impacted during combustion but also to measure how it responds over time to changing conditions. Such an approach has now been developed.

Operando neutron diffraction
Wissink and his colleagues developed a technique for analyzing an operating internal combustion engine with asynchronous neutron diffraction. With the engine in operation, the technique is known as operando neutron diffraction.

Wissink says, “One advantage in working with neutrons is their ability to penetrate deeply into structures at useful wavelengths on the order of a few angstroms, which corresponds to the scale of interatomic distances. In that wavelength range, neutrons can penetrate more than 25 centimeters into aluminum. The diffraction technique allows the user to probe interatomic distance changes on the scale of 0.0001 angstroms.” Visible light can move only a few microns into a material, and X-rays can penetrate up to a few millimeters. Neutrons have proven to be effective in penetrating centimeters into a metal such as aluminum.”

This property is very important because most engine blocks are prepared from an aluminum alloy such as A380, A383 or A319. The researchers identified a new potential aluminum cerium alloy that is a candidate for use in an internal combustion engine. This alloy is castable and exhibits dramatically improved high temperature properties² compared to conventional alloys.

In an effort to evaluate the new alloy, the researchers incorporated it into a cylinder head and placed it in a carbureted small-bore single cylinder internal combustion engine used to power an electric generator, which was modified for use in the VULCAN Engineering Materials Diffractometer^3,4 at ORNL’s Spallation Neutron Source, sponsored by the U.S. Department of Energy Office of Basic Energy Sciences. Figure 1 shows the experimental setup.


Figure 1. The experimental setup used to evaluate a new aluminum-cerium alloy in a working internal combustion engine through the use of operando neutron diffraction is shown. Figure courtesy of Oak Ridge National Laboratory.

The simplicity of this small engine was a big advantage. Neutrons are scattered strongly by hydrogen atoms, which are abundant in water-based coolant and hydrocarbon-based lubricants such as the 10W-30 motor oil used in this engine. Being air-cooled, this engine did not have coolant concerns, and because of the two-valve layout with pushrods running along the bottom of the cylinder, there were no oil passages on the top or front of the engine to interfere with the neutron beam.

The engine was operated under 0%, 55% and 92% of the rated generator load. In situ mapping of the aluminum cerium alloy in the operating engine showed good thermal stability. Wissink says, “Before starting the engine, we spatially mapped the locations of diffraction peaks corresponding to the different phases of the aluminum alloys. During engine operation, the peaks shifted in response to load and thermal expansion, and these shifts can be used to calculate stain, stress and temperature fields within the material. After the engine was shut down and cooled back to room temperature, the peaks returned to their original positions, indicating that the alloy did not undergo morphological changes—in other words, the alloy was found to be stable when faced with real engine operating conditions.”

This research should enable further work to be done to evaluate how materials perform in an operating engine. Wissink says, “We are in the process of developing a single cylinder engine purpose-built for neutron diffraction studies. While the small generator engine used in the demonstration study was chosen for ease of implementation, the new engine will be representative of a modern automotive powerplant and will be optimized for use at VULCAN.”

Additional information can be found in a recent article⁵ or by contacting Wissink at wissinkml@ornl.gov.

REFERENCES
1. Canter, N. (2020), “Determination of active vehicle exhaust catalyst sites,” TLT, 76 (11), pp. 14-15.
2. Sims, Z., et al. (2017), “High performance aluminum-cerium alloys for high-temperature applications,” Materials Horizon, 4 (6), pp. 1070-1078.
3. Further information on the VULCAN Engineering Materials Diffractometer can be found here.
4. An, K., Chen, Y. and Stoica, A. (2019), “VULCAN: A “hammer” for high-temperature materials research,” MRS Bulletin, 44 (11), pp. 878-885.
5. Wissink, M., Chen, Y., Frost, M., Curran, S., Rios, O., Sims, Z., Weiss, D., Stromme, E. and An, K. (2020), “Operando measurement of lattice strain in internal combustion engine components by neutron diffraction,” Proceedings National Academy of Sciences, 117 (52), pp. 33061-33071.