High Temperature Trib0l0gy

The greatest single problem facing the adiabatic engine is the high tein perature tribology problem. In the ar'ea of the topring reversal-point on the cylinder liner, temperatures approach 550C for the uncooled adiabatic engine, compared with only 200C for the conventional water-cooled engine; High temperature synthetic gas turbine lubricating oil has been used with some success at the lower outputs. However, at full loads (200 PSI BMEP), the hydrodynamic oil film protection at the topring reversal-point is significantly reduced since the temperature limit on current high temperature synthetic lubricants is about 400C. Increases in friction have been noted and scuffing may occur at these conditions (i.e., full load and uncooled conditions). High temperature lubricant research is being carried on at most petroleum and additive related companies. Liquid lubricants still offer the highest potential for high temperature tribology in environment approaching 500C. Research undferway could offer a practical solution by the early 1990s. Additive research is another area where more research is definitely needed. Additive response to acidic combustion products must be evaluated for use with advanced synthetic lubricants, additive depletion kinetics, additive effectiveness on ceramic and composite surfaces, additive toxicity, and additive deposit formation tendencies. One method of overcoming the vaporization of the oil film at the top ring reversal point is to use solid lubricants on the top piston ring (for example, LiF impregnated into etched micropockets on the ring). The principle behind this concept is that, at the lower loads and temperatures, hydrodynamic lubrication occurs with the solid lubricant impregnated ring. At high speed and load conditions, the LiF solid lubricated ring reduces friction and wear while some limited metal-to-metal contact is occurring. The design life of these solid lubricants is still being investigated. Gas lubrication for t"he adiabatic engine offers both a low friction and high temperature capability. Research performed by TACOM indicated the need for innovative methods to reduce piston side thrust (particularly for high BMEP military engines). However, because the "achilles heel" of the successful "ultimate" adiabatic engine resides within the high temperature tribology area, the pay-offs from this type of engine research remain very high. Research in this area will continue with strong long-range commitment.

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