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Lubricating ceramic engines with exhaust.

Injecting a stream of hydrocarbon-bearing exhaust gases into a ceramic engine's bearings and friction spaces will turn the hydrocarbons in the gas into a coat of graphite-like carbon lubricant.

James L. Lauer, professor of mechanical engineering at Rensselaer Polytechnic Institute (Troy, N.Y.), has proposed an elegant solution. to the long-standing problem of finding a way to lubricate a hot-running ce- ramic engine of the future: Inject a stream of the engine's own exhaust gases into its bearing and friction spaces and the hot temperatures there will turn hydrocarbons in the gas into a thin coat of graphite-like carbon lubricant.

"The carbon lubricant is created right on the surface as long as you keep feeding in the exhaust gas," he noted.

"Though the base temperature of the engine components is, say, 500[degrees]C, contact friction causes local temperatures to reach 800[degrees]C," Lauer explained. When the exhaust gas enters these contact spaces, it is rapidly heated, which strips the hydrocarbon compounds of hydrogen atoms, yielding carbon. As the ceramic engine parts move along, the localized frictional heating at that location halts, quickly cooling the ceramic surface as well as the nearby carbon, he said. This accelerated heating and cooling process produces amorphous, or microcrystalline carbon, which plates onto the surface. The slippery carbon coating, which Lauer said is all but invisible, is only 0.5 to 1 micrometer thick. As the engine components again make contact at that point, the coating lubricates their passage by wearing rapidly. Meanwhile, the graphite-like carbon film is replenished as newly injected exhaust gas nears the once-more frictionally heated contact point.

Ceramic Engines

For decades, mechanical engineers have worked to develop lightweight fuel-efficient ceramic engines that can operate at temperatures as high as 1000[degrees]C. Research has focused on the development of advanced ceramic materials such as silicon nitride and silicon carbide, which can resist heat that would melt most conventional metal alloys.

If perennial toughness and reliability problems with structural ceramics were overcome, the material's superior resistance to elevated temperatures could obviate the need for the liquid-cooling systems used in many conventional power plants. "One of the main contributors to the weight of today's metal engine is the cooling system," Lauer explained. "An engine built of high-temperature ceramics has an advantage in that it would need no heavy cooling system, with its attendant coolant, pumps, and pipes." He added that much of the heat produced by a conventional engine merely goes to warming coolant.

A major sticking point hindering the adoption of ceramic engine technology, however, has been the problem of finding a way to lubricate bearings and other hot-contact points, Lauer said. Conventional motor oils and other liquid lubricants break down in such high heat. Alternatively, it is not easy to continuously apply temperature-resistant solid lubricants such as powdered graphite to fast-moving engine parts.

Cracking Crude

Lauer began his research on the lubrication of ceramic engines in the late 1980s. His exhaust-gas-tolubricant concept is based on his previous experience in the oil industry, where crude oil is routinely cracked by clay catalysts to produce various lower-molecular-weight hydrocarbon compounds. He suspected that under the correct conditions hot engine parts might crack (or dehydragenate) such unburned hydrocarbon compounds in exhaust as ethylene, benzene, and toluene to form a practical lubricant.

"At first, we worked with nickel-containing steel alloys," Lauer recalled. "Then we graduated to hot-pressed silicon nitride ceramic coated with nickel." Hot-pressed silicon nitride is considered a leading candidate for the structure of a ceramic engine because it has good thermal conductivity, can be formed relatively easily, and resists fracture better than most ceramics. "Then we found out that at temperatures above 400[degrees]C, you don't need the nickel. It worked fine with just silicon nitride." He characterized the thin carbon layer he deposited on the ceramic with low-energy Raman spectroscopy.

Lauer has successfully tried his lubrication scheme in laboratory tests with ethylene gas, "although acetylene, propane, and alcohols also work."

A research collaborator has demonstrated the novel system for a limited time in a diesel engine. Lauer is currently building another bearing simulator. "This concept could work in diesel engines fitted with ceramic liners, in advanced two-stroke gasoline engines, and in future aircraft turbine engines," Lauer concluded.

The ASME has recognized Lauer's work with its Prize for Innovative Research in Tribology--the study of lubrication and wear. Lauer has "a unique idea that works," said Ken Ludema, professor of mechanical engineering at the University of Michigan (Ann Arbor), who was a member of the ASME awards committee. Lauer received the award at the annual Joint Conference of ASME and the Society of Tribologists and Lubrication Engineers, which was held this month in San Diego.

Lauer's work has been supported by the New York State Energy Research and Development Authority (Albany), the National Science Foundation (Washington, D.C.), and an unnamed aerospace company. He is seeking funding for further investigations from the National Aeronautics and Space Administration's Lewis Research Center (Cleveland).
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Author:Ashley, Steven
Publication:Mechanical Engineering-CIME
Date:Oct 1, 1992
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