i torque profile of a foil bear-

i ; 1ng during a ¿ingle start/stop cycle.

5 ' 'i lift-off velocity for the bearing. The bearing was subjected to 5000 start-stops at 25 °C and another 5000 at 560 °C with no problems. In other similar tests, with PS200-coated journals, the same number of test cycles were completed with bearing temperatures up to 650 °C.

High Speed Shaft Seals

PS200 has shown promise for use as a shaft seal material for turbine applications. At the very high speeds typical of such applications, friction coefficients as low as 0.1 and very low wear rates have been observed.

Stirling Engine Cylinder Liner

PS200 was also evaluated in a Stirling engine test to evaluate the concept that improved fuel efficiency could be achieved in the Stirling engine if "hot piston rings" were used. The lubrication of the piston ring/cylinder contacts in the Stirling engine is a challenging high-temperature tribological problem. Metal temperatures as high as 600 to 1000 °C occur near top dead center of the cylinder walls. The working fluid in the engine thermodynamic cycle is hydrogen. The lubricant coating, therefore, must not only provide low friction and wear, but also must be thermochemi-cally stable in a strongly reducing hydrogen atmosphere.

In current designs of the Stirling engine, the piston rings are made of reinforced polytetrafluroethylene (PTFE). They are located in ring grooves near the bottom of the piston where the temperatures are relatively low and do not degrade the PTFE. This arrangement results in a long annular gap from the top of the piston to the piston ring. This gap is known as the "appendix gap" and it is the source of parasitic energy losses (8). It therefore would be desirable to minimize the appendix gap by locating the top ring in a groove near the top of the piston. A schematic of

^-conventional / piston rings—y
mod i engine piston kith hot ring
hod i engine piston with filler ring

figure 7. - stirling engine piston configurations.

the ring locations in the baseline piston and in a piston with an added hot ring are shown in Fig. 7. It was determined by means of pin on disk tests that Stellite 6B is a good tribological material in sliding contact with PS200 in hot hydrogen.

A four cylinder automotive Stirling engine known as the Upgraded MOD I was used as the test engine. The engine was designed by Mechanical Technology Inc. (MTI) under a DOE contract managed by the Stirling engine Project Office at NASA Lewis Research Center. MTI modified the design by enlarging the cylinder bores to allow them to be coated with PS200 and by redesigning the pistons to allow them to be fitted with Stellite 6B "hot piston rings". The engine was tested for 22 hr at various speeds and a top ring reversal temperature of 700 °C. The results were compared to those obtained in baseline tests where the hot rings were removed and replaced with filler rings. This provided a direct comparison of an engine with and without a long appendix gap. Efficiency gain varied from 0 to 7 percent depending upon engine operating s. These gains were over and above any frictional losses introduced by the "hot rings." For example, Fig. 8 shows that at 5 Mpa mean operating pressure, no gain in engine efficiency was observed at 1000 rpm but up to a 7 percent gain was measured at 2000 and 4000 rpm. Over all test condition the efficiency gain averaged approximately 3 percent. Seal leakage measurements showed about a 30 percent reduction with the hot rings in place. In addition, cylinder wall temperature measurements indicated less cylinder heating in the appendix gap area. Figure 9 shows that this project moved forward from the selection of mate rials based upon their chemical and phys ical properties, through experimental research and development, to a successful engine test that verified computational analyses.

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