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What Exotic Material Is Critical to Optical Studies of Internal Combustion Engine Performance?

A research firm focused on improving internal combustion engines needed optically accessible cylinders and piston heads that could take the heat and pressure, knowing this material was also extremely hard to machine.

Mid Michigan Research LLC is a research firm focused on improving the efficiency of internal combustion engines. Founded by Harold Schock, a professor of mechanical engineering at Michigan State University, it has been engaged over the years to analyze information obtained by visualizing combustion in situ, i.e., within an actual pressurized cylinder with spark ignition, and created a ring-pack and piston dynamics software simulation tool, among other things.

Using a fabricated optical cylinder, matching the engine geometries under study, brings greater vision into the combustion process. Although glass might be considered a first choice for an optically accessible cylinder, it would not survive the extreme conditions of heat and pressure. Fused quartz is stronger than glass blends but difficult to fabricate in engine-cylinder sizes, and it has limited infrared transmission at higher temperatures.

Sapphire is the second hardest natural material, surpassed only by diamond. The single-crystal form of aluminum oxide possesses extreme resistance to abrasion, withstands high working temperatures (up to 1,950C), and has full-spectrum light transmission even at high temperatures. Industrial sapphire is grown commercially as a very pure crystal, but the result is one solid piece that must then be machined to the desired size and form.

A piston head with unconventional contours was precision-machined out of sapphire, the second hardest material known to man, to let researchers prove out their correlations between piston-head shape and car engine performance.
(Source: Mid Michigan Research)

Diamond grinding technology and specific handling techniques can process sapphire workpieces without damage. Mid Michigan Research turned to Insaco Inc., whose name is a contraction of Industrial Sapphire Company and history began with fabricating phonograph needles from sapphire in 1947.

Insaco is known to have pioneered many techniques for custom fabrication of precision parts from sapphire and machine unusual forms. “One of Insaco’s main specialties has been the development of techniques used for precision fabrication and polishing to optical standards of parts with IDs and ODs,” said Scott Mittl, sales engineer.

Insaco machines ceramics, sapphire, quartz, and other hardened glass or crystalline materials today for industries including aerospace, defense, semiconductor, instrumentation, and research. But in the 1980s and 1990s, Insaco used pioneering techniques for the fabrication and polishing of several designs of large sapphire cylinders for various OEMs on three continents that were conducting internal combustion engine research. The cylinders went up to 6 inches length and 4 inches in diameter with diameter tolerances and cylindricity to 0.0008 inch; a human hair, for comparison, is about 0.003 inch in diameter.

Schock and the Mid Michigan Research team had been performing in situ laser observations of fuel-air mixing and flame-front propagation in internal combustion engine cylinders for more than three decades. Having combustion chamber geometrical configurations that exactly duplicated those of metal cylinders was of primary importance, and this pushed the goal of fabricating a piston from sapphire to enable more precise measurement of the physical processes which govern the performance of a car engine.

Fabrication of a simple polished sapphire puck, representing a flat piston top, was easy to do. But what the researchers really wanted was a piston head with unique contours to test their theories on how piston head form could control and thus optimize combustion in a real environment.

Insaco again was able to pioneer new techniques, for both fabrication and polishing of these asymmetric forms in sapphire. Tolerance levels exceeded the Mid Michigan Research’s parameters. One such design continues to undergo contract testing for analysis of form versus combustion pattern under varying conditions.

“These are complicated parts, and understandably expensive to fabricate from sapphire, but [are] critical to the success of our research programs,” noted Schock, who added that Insaco has been a reliable partner in making that happen.

That research using optically accessible tools has helped the company in creating solutions for in-cylinder analysis of gasoline and diesel engines and dynamic simulation software, and in providing services for in-cylinder imaging and engine studies.

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