Iranian Research Organization for Science and Technology (IROST) Hydrogen, Fuel Cell & Energy Storage 2980-8537 12 3 2025 07 01 Utilization of a Fifth-Order Model for Analyzing Stirling Oscillators 169 180 1492 10.22104/hfe.2024.7207.1329 EN Shahriar Niknejad Department of Mechanical Engineering, Iranian Research Organization for Science and Technology (IROST), Tehran, Iran Amir Fassih Department of Mechanical Engineering, Iranian Research Organization for Science and Technology (IROST), Tehran, Iran Amir Mobini Department of Mechanical Engineering, Iranian Research Organization for Science and Technology (IROST), Tehran, Iran Heybatollah Jokar Department of Mechanical Engineering, Firouzabad Higher Education Center, Shiraz University of Technology, Shiraz, Iran Shahryar Zare Department of Mechanical Engineering, Iranian Research Organization for Science and Technology (IROST), Tehran, Iran Journal Article 2025 06 30 In this study, for the first time, the design of a free-piston Stirling oscillator (FPSO) using a fifth-order model is addressed. Initially, the free-piston Stirling oscillator is introduced. Then, considering the limited heat transfer coefficient, the fifth-order mechanical model of the oscillator is derived. Subsequently, the design parameters, including the stiffness and mass of the power piston and displacer piston, as well as the cross-sectional area of the rod connecting to the displacer piston, are examined. Then, the design parameters are estimated based on the objectives (a desired frequency between 70 and 100 rad/s, and the real value of the dominant closed-loop pole between 5 and 17) and the fifth-order mechanical model. Nonlinear analysis is then performed to investigate the effects of variations in frequency and the real value of the dominant closed-loop poles on the output power and phase difference between the pistons of the oscillator. The results of this study demonstrate that the use of a fifth-order model offers a more accurate evaluation and analysis of the dynamic behavior of this type of oscillator. https://hfe.irost.ir/article_1492_2fa42278c53f020f13e3f637def930d1.pdf