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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A method for studying the moisture conditions inside hardellcd concrete and cement paste is described. An electric hygrometer developed by the National Bureau of Standards was used to measure the relative hum idity in cavities at various locations in the material. The r elat ive humidity inside 12-inch concrete cubes stored at 50 percent rehtive hu midity for 300 days was found to range from 50 percent near the surface to 66 percent near the center of the cube. For cubes stored at 20-percent relative humidity the co rresponding range was from 20 t o 60 p ercent. The relative humidity over cement paste of approximately normal consistency and cured in hermetically sealed bottles fo r 28 days was found to be 88 percent for a m oderate heat-oI-hardening cement a nd 79 percent for a highearly-strength cement. CONTENTS

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