An experimental study of the control characteristics of a 1 kW class free‐piston stirling converter (FPSC) with alternating current (AC) bus controls is conducted. A performance test rig is constructed using a commercial beta‐type FPSC, and an AC bus controller that includes a tuning capacitor, a set of load resistors, and an AC power supply is developed. Experiment parameters include the tuning capacitance and load resistance for the AC bus circuit and the control voltage and control frequency for AC bus control under various head temperatures of the test FPSC. In addition, preliminary experiments under load resistance control scheme were conducted to identify the natural operational characteristics of the test FPSC without an external active control scheme and to determine the proper ranges of the experiment parameters for AC bus control. The experimental results are presented in terms of the output power, the power factor, and the thermal‐to‐electric efficiency of the FPSC with respect to the load resistance and the AC bus control parameters. In particular, the effect of the control frequency with respect to the FPSC operating frequency, as well as the influence of the alternator inductance on the control characteristics of FPSC are discussed in detail. Finally, it is anticipated that this research will yield a better physical understanding of comprehensive AC bus parameters related to the performance of FPSCs, will provide a simple and accurate control method for laboratory experiments on FPSCs, and will provide fundamental data for those who design advanced virtual tuning capacitor controllers.

Stirling engines are the engines that convert heat energy into mechanical work. This study models a 50 W linear generator designed for integration with a Stirling engine. To develop a model, the base design of the already developed 1 kW model was used, and its size was proportionally reduced to match the stroke of the Stirling engine. By reducing the length of the 1 kW model to a length scale factor (LSF) of 0.5, the stroke level of the engine was determined. However, the radius of the LSF 0.5 linear generator model was adjusted to match the engine. After finalizing the 50 W linear generator dimensions, the model was simulated using MAXWELL v14. software to compute output power and other electrical parameters. This study also analyzed the losses of the 50 W linear generator and its phasor diagram. Later, the output values generated using MAXWELL software were compared with the results obtained using SAGE v11. software for verification. The outcome of this study was a model that achieved an output power of 50 W with an efficiency of 90% and a generator size of 96 mm. Because of its versatility, low weight, and high efficiency, it can be used in a wide range of applications. Due to its small size, it can be utilized for empowering humanoid robots, radioisotope power, space exploration, etc.

This paper presents the development and validation of an improved quasisteady flow model (iQSFM) that applies comprehensive parasitic losses to the quasisteady flow model (QSFM) considering an oscillating flow, which is the actual type of flow occurring in a regenerator. Validation of iQSFM was evaluated by comparing it with a QSFM based on the experimental results of a RE‐1000 regenerator. Compared to QSFM, iQSFM improved the prediction accuracy by reducing the indicated power error from 66.7% to 24.9% and the efficiency error from 35.3% to 9.4%. In addition, the prediction accuracy of iQSFM was compared when the oscillating flow and the steady flow correlation were applied to a regenerator. When iQSFM applied an oscillating flow correlation to the regenerator, it predicted the experimental results of RE‐1000 slightly more accurately than in a steady flow correlation. Finally, the engine performance and parasitic losses were analyzed through a parameter study of RE‐1000 using iQSFM. Through this, it was confirmed with iQSFM that the RE‐1000 is designed to maximize the engine performance by minimizing the parasitic losses.

This paper presents the design optimization of a heat exchanger for a free-piston Stirling engine (FPSE) through an improved quasi-steady flow (iQSF) model and a central composite design. To optimize the tubular hot heat exchanger (HHX) design, a design set of central composite designs for the design factors of the HHX was constructed and the brake power and efficiency were predicted through the iQSF model. The iQSF model is improved because it adds various heat and power losses based on the QSF model and applies a heat transfer model that simulates the oscillating flow condition of an actual Stirling engine. Based on experimental results from the RE-1000, an FPSE developed by Sunpower, the iQSF model significantly improves the prediction error of the indicated power from 66.9 to 24.9% compared to the existing QSF model. For design optimization of the HHX, the inner diameter and the number of tubes with the highest brake power and efficiency were determined using a regression model, and the tube length was determined using the iQSF model. Finally, the brake output and efficiency of FPSE with the optimized HHX were predicted to be 7.4 kW and 36.4%, respectively, through the iQSF analysis results.

An experimental study of the control characteristics of a 1 kW class free‐piston stirling converter (FPSC) with alternating current (AC) bus controls is conducted. A performance test rig is constructed using a commercial beta‐type FPSC, and an AC bus controller that includes a tuning capacitor, a set of load resistors, and an AC power supply is developed. Experiment parameters include the tuning capacitance and load resistance for the AC bus circuit and the control voltage and control frequency for AC bus control under various head temperatures of the test FPSC. In addition, preliminary experiments under load resistance control scheme were conducted to identify the natural operational characteristics of the test FPSC without an external active control scheme and to determine the proper ranges of the experiment parameters for AC bus control. The experimental results are presented in terms of the output power, the power factor, and the thermal‐to‐electric efficiency of the FPSC with respect to the load resistance and the AC bus control parameters. In particular, the effect of the control frequency with respect to the FPSC operating frequency, as well as the influence of the alternator inductance on the control characteristics of FPSC are discussed in detail. Finally, it is anticipated that this research will yield a better physical understanding of comprehensive AC bus parameters related to the performance of FPSCs, will provide a simple and accurate control method for laboratory experiments on FPSCs, and will provide fundamental data for those who design advanced virtual tuning capacitor controllers.

Stirling engines are the engines that convert heat energy into mechanical work. This study models a 50 W linear generator designed for integration with a Stirling engine. To develop a model, the base design of the already developed 1 kW model was used, and its size was proportionally reduced to match the stroke of the Stirling engine. By reducing the length of the 1 kW model to a length scale factor (LSF) of 0.5, the stroke level of the engine was determined. However, the radius of the LSF 0.5 linear generator model was adjusted to match the engine. After finalizing the 50 W linear generator dimensions, the model was simulated using MAXWELL v14. software to compute output power and other electrical parameters. This study also analyzed the losses of the 50 W linear generator and its phasor diagram. Later, the output values generated using MAXWELL software were compared with the results obtained using SAGE v11. software for verification. The outcome of this study was a model that achieved an output power of 50 W with an efficiency of 90% and a generator size of 96 mm. Because of its versatility, low weight, and high efficiency, it can be used in a wide range of applications. Due to its small size, it can be utilized for empowering humanoid robots, radioisotope power, space exploration, etc.

This paper presents the development and validation of an improved quasisteady flow model (iQSFM) that applies comprehensive parasitic losses to the quasisteady flow model (QSFM) considering an oscillating flow, which is the actual type of flow occurring in a regenerator. Validation of iQSFM was evaluated by comparing it with a QSFM based on the experimental results of a RE‐1000 regenerator. Compared to QSFM, iQSFM improved the prediction accuracy by reducing the indicated power error from 66.7% to 24.9% and the efficiency error from 35.3% to 9.4%. In addition, the prediction accuracy of iQSFM was compared when the oscillating flow and the steady flow correlation were applied to a regenerator. When iQSFM applied an oscillating flow correlation to the regenerator, it predicted the experimental results of RE‐1000 slightly more accurately than in a steady flow correlation. Finally, the engine performance and parasitic losses were analyzed through a parameter study of RE‐1000 using iQSFM. Through this, it was confirmed with iQSFM that the RE‐1000 is designed to maximize the engine performance by minimizing the parasitic losses.

This paper presents the design optimization of a heat exchanger for a free-piston Stirling engine (FPSE) through an improved quasi-steady flow (iQSF) model and a central composite design. To optimize the tubular hot heat exchanger (HHX) design, a design set of central composite designs for the design factors of the HHX was constructed and the brake power and efficiency were predicted through the iQSF model. The iQSF model is improved because it adds various heat and power losses based on the QSF model and applies a heat transfer model that simulates the oscillating flow condition of an actual Stirling engine. Based on experimental results from the RE-1000, an FPSE developed by Sunpower, the iQSF model significantly improves the prediction error of the indicated power from 66.9 to 24.9% compared to the existing QSF model. For design optimization of the HHX, the inner diameter and the number of tubes with the highest brake power and efficiency were determined using a regression model, and the tube length was determined using the iQSF model. Finally, the brake output and efficiency of FPSE with the optimized HHX were predicted to be 7.4 kW and 36.4%, respectively, through the iQSF analysis results.

This study presents the development and performance measurement of a 2.5 kW class free-piston Stirling engine with detailed design and fabrication processes, considering flexure spring stiffness effects on the engine dynamics and performance. The development of the test engine includes tube type heat exchangers and planar flexure springs. In particular, the flexure springs are designed via finite element analyses and verified through stiffness measurements and fatigue life tests. For the performance measurements, an engine test rig is constructed with an LNG combustion heat supply. The thermal efficiency is evaluated in terms of combustion gas components based on the reaction formula of methane. As a result, the electric output and efficiency of the test engine were measured to be 2.46 kW and 19.46%, respectively. Furthermore, the effects of displacer spring stiffness are experimented with a stack design of flexure springs by varying the number of springs. As a result, increases in displacer stiffness increased engine operating frequency and decreased both the piston amplitudes, while power piston velocity remained relatively constant as well as the engine power output. The experimental results are analyzed through a simple linear dynamic model, which showed reasonable agreements.

<div class="section abstract"><div class="htmlview paragraph">In this study, a novel selective matching logic for a wheel/tire is proposed, to decrease the vehicle driving vibration caused by wheel/tire non-uniformity. The new logic was validated through matching simulation/in-line matching evaluation. A theoretical radial force variation model was established by considering the theoretical model of the existing references and the wheel/tire assembly mechanism. The model was validated with ZF’s high-speed uniformity equipment, which is standard in the tire industry. The validity of the new matching logic was verified through matching simulation and mass production in-line evaluation. In conclusion, the novel logic presented herein was demonstrated to effectively decrease the radial force variation caused by the wheel/tire.</div></div>

This study evaluated the effect of drum diameter on radial force variation (RFV) by comparing laboratory development equipment with a drum diameter of 240 mm and commercial equipment with 1707 mm. When measuring RFV, a large drum is suitable for imitating the flatness of the actual road surface, but it is economically advantageous to utilize a small drum. The results of the RFV measurement depend on the drum diameter and tire stiffness. Therefore, several tests have been performed to evaluate the effect of measurement conditions such as drum diameter, air pressure, and static load on RFV. The main factors of RFV are runout and tire radial stiffness; therefore, wheel samples were made by imitating wheel runout. The RFV of the same tire was measured on two different diameter equipment, and the results for the larger diameter equipment were estimated to be approximately 13.8 % greater.

The rotordynamic stability of journal gas foil–polymer bearings (GFPBs) applied to a dual-rotor bearing system was investigated. The GFPB has a high damping structure, i.e., it has an additional polymer layer, and top/bump/bottom foil structures. Test GFPBs were fabricated with nitrile butadiene rubber polymer layers with a thickness of 2 mm. Static-load-deflection tests of test gas foil bearings (GFBs) and GFPBs were performed to estimate the geometric bearing clearance (200 µm). The dual-rotor rotordynamic test rig consisted of a motor rotor, test rotor, and beam-type coupling. Two journal test bearings were installed on both the drive-end and non-drive-end sides of the test rotor. Predicting and testing the natural mode characteristics of the dual rotors revealed that the relative error between them was less than 7%, indicating that the first and second natural frequencies were 15 Hz and 160 Hz, respectively, and the third natural frequency was 1835 Hz in the tests. Based on the API 612 standard, the upper limit of the rotating speed for the test rig was limited to be approximately 92 krpm with a separation margin of 26%. Rotordynamic tests were conducted to examine the stability performance of GFBs and GFPBs, where the adjusted bearing clearance was 150 µm. The test results indicate that GFPBs have better stability performance in terms of delaying and suppressing unstable vibrations than GFBs. Specifically, GFPBs showed stable synchronous and subsynchronous responses up to a maximum rotating speed of 80 krpm. As a result, GFPB is a reliable lubricating element that can be used for vibration dampening in machines operating at relatively low temperatures.