Hot Piston Ring/Cylinder Liner Materials—Selection and Evaluation 880544
In current designs of the automotive (kinematic) Stirling engine, the piston rings are made of a reinforced polymer and are located near the bottom of the pistons because they cannot withstand the high temperatures in the upper cylinder region. Theoretically, efficiency could be improved if “hot piston rings” were located near the top of the pistons. This paper describes a program to select piston ring and cylinder coating materials to test this theory. Candidate materials were screened theoretically and in a pin on disk friction and wear test machine. Tests were performed in hydrogen at specimen temperatures up to 760°C to simulate environmental conditions in the region of “hot piston ring” reversal. Based upon the results of these tests, a cobalt based alloy, Stellite 6B, was chosen for the piston rings and PS200, which consists of a metal-bonded chromium carbide matrix with dispersed solid lubricants, was chosen as the cylinder coating. Tests of a modified engine and a baseline engine showed that the hot ring did reduce specific fuel consumption by up to 7 percent for some operating conditions and averaged about 3 percent for all conditions evaluated. Related applications of high-temperature coatings for shaft seals and as back-up lubricants for gas bearings are also described.