This paper describes a method of energetically modeling a free-piston Stirling engine. This model is compared to a lower order Schmidt model, and both models are linearized from nonlinear continuous form and cast into closed-loop state space models. To validate the linear models, an experimental apparatus to simulate the performance of different engines was constructed using linear actuators, sensors, and a free-piston Stirling engine. The behavior of the experimental apparatus was compared to the behavior predicted by the linear models. The operating characteristics of the experimental apparatus are compared to the dominant poles of the closed loop models. Relations are described between the imaginary component of the dominant poles and the operating frequency of the engine and between the real component and the ability of the engine to enter sustained oscillation.
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