Abstract
This paper presents a linear dynamic analysis on operable charge pressure and working frequency of free-piston Stirling engines (FPSE) along with experimental verifications. The equations of motion of the FPSE are formulated as a 2-degree-of-freedom (DOF) vibration system of the power piston (PP) and displacer piston (DP), based on the state equation of ideal gas and the isothermal Stirling cycle model. The dynamic models of FPSE we considered are the 1-DOF simple vibration model of each piston and the 2-DOF root locus model of coupled pistons. We developed a test FPSE for verification of the dynamic models and conducted a series of experiments to measure the dynamic behaviors of PP and DP under varying charge pressures for various masses and stiffnesses of the PP. As a result, both prediction models showed good agreements with experimental results. The 1-DOF vibration model was found to be simple and effective for predicting the operating frequency and charge pressure of FPSE. The root locus method showed reasonable predictions with an operation criterion of the PP–DP phase angle of 90°. In addition, the FPSE was confirmed to operate in resonant oscillations when the DP–PP phase angle is 90°, based on analysis of the force vector diagram of the two pistons.
Highlights
Since modern society is mostly based on fossil fuel energy, the depletion of fossil fuels and persistent environmental pollution pose a global problem
Mou et al [15] presented a linear dynamic model to predict the frequency and phase angle of free-piston Stirling engines (FPSE) depending on charge pressure and spring stiffness, along with experimental verifications
This paper presents a linear dynamic analysis of beta-type FPSEs on charge pressure and working frequency along with experimental verification
Summary
Since modern society is mostly based on fossil fuel energy, the depletion of fossil fuels and persistent environmental pollution pose a global problem. Urieli and Berchowitz [11] developed a linear dynamics model that predicts the operating frequency, phase angle, and amplitude ratio of an engine using an isothermal model and pressure linearization They compared the model with the experimental results for RE-1000 [12] and reported prediction errors of approximately 10–40%. Mou et al [15] presented a linear dynamic model to predict the frequency and phase angle of FPSEs depending on charge pressure and spring stiffness, along with experimental verifications. We developed a test FPSE and compared the experimental results with linear dynamic model predictions, aiming to provide a simple and direct analysis of the dynamic behavior of FPSE and focus on the operable conditions of the charge pressure and the resonance phenomenon of the piston motion. The dynamic model of the FPSE we considered is (1) a 1-degree-of-freedom (DOF) simple vibration model for each piston and (2) a 2-DOF root trajectory model of two combined pistons
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