Development and Performance Measurements of a Beta-Type Free-Piston Stirling Engine Along With Dynamic Model Predictions

This paper presents the development and performance measurements of a beta-type free-piston Stirling engine (FPSE) along with dynamic model predictions. A test FPSE, composed of two pistons and spring elements, is designed by drawing root locus trajectories from linear eigenvalue analysis. Piston springs are developed by using finite element stress analyses and validated through static load-deflection tests. An experimental test rig for a manufactured FPSE includes ceramic heater, water cooler, and external dashpot. Tests are conducted with the atmospheric air at increasing heater temperatures under no load and external load conditions. Firstly, no load tests at increasing heater temperatures show significant increases in both piston stroke and pressure amplitude, but negligible changes in working frequency. Pressure-volume (P-V) power and thermal efficiency also increase significantly. Interestingly, dynamic discontinuities occur during continuous variations of heater temperature, demonstrating sudden increases in engine performance as well as operation uncertainty. Besides, cooling flow rate is found to have minimal influence on engine performance. Secondly, external load tests for increasing damping loads demonstrate considerable decreases in piston stroke, pressure amplitude and P-V power, but negligible changes in working frequency; while shaft power increases then decreases having an optimal operating load. Finally, the test FPSE is turned out to adapt itself to variations in heater temperatures and external loads by changing piston strokes and their phase angle.