Hot piston ring tests

As part of the DOE/NASA Automotive Stirling Engine Project, tests were made at NASA Lewis Research Center to determine whether appendix gap losses could be reduced and Stirling engine performance increased by installing an additional piston ring near the top of each piston dome. An MTI-designed upgraded Mod I Automotive Stirling Engine was used for these tests. Unlike the conventional rings at the bottom of the piston, these rings operated in a high temperature environment (700/sup 0/C). Because of this, we called them ''hot-rings.'' It was necessary that they be made of a high temperature alloy (Stellite 6B) and that a high temperature solid lubricant coating (NASA Lewis-developed PS-200) be applied to the cylinder walls. Engine tests were run at 5, 10, and 15 MPa operating pressure over a range of operating speeds. Tests were run both with the hot-rings in place and without them to pro-side a baseline for comparison. Although budget and schedule restrictions severely limited the testing, the minimum data to assess the potential of both the hot-rings and high temperature low friction coating was obtained. Results indicated a slight increase in power and efficiency with the hot-rings over the baseline configuration. This increase was over andmore » above the friction loss introduced by the hot-rings. Seal leakage measurements showed a significant reduction in leakage with the hot-rings in place. In addition, cylinder wall temperature measurements indicated less cylinder heating in the appendix gap area - between the lower piston rings and the hot-ring. Approximately 22.4 hours of ring-on-coating operation were recorded. After the initial break-in period, wear on both the rings and the coating was low. The PS-200 coating seems to offer significant potential for long-term operation at high temperatures. Both the hot-ring and the PS-200 low friction coating show promise and should be pursued further.« less