In a free-piston engine generator (FPEG), the power of the engine can be directly regenerated by linear generators without a crank. The mover motion of this system is interrelated with engine and power generation efficiencies due to the direct connection between the mover of the generator and the piston of the engine. The generator should be designed to improve the overall energy conversion efficiency. The dimensions and mass of the mover limit its operating stroke and drive frequency. Herein, we propose a method for designing linear generators and constructing FPEG systems, considering the mover operation to improve engine efficiency. We evaluated the effect of mover operation on the engine and generation efficiencies using thermal and electromagnetic field analysis software. The proposed design method improves the overall energy conversion efficiency compared with a generator that considers only the maximization of generation efficiency. Setting the mover operation for higher engine efficiency and designing a linear generator to realize the operation can effectively improve the energy conversion efficiency of FPEGs.

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Piston motion is an important factor in improving the energy conversion efficiency of free-piston engine generators (FPEGs), and an air cylinder is an important component affecting piston motion. In this study, the effect of the air cylinder specifications on the piston drive frequency and energy conversion efficiency is clarified. By considering that the repulsion force of an air cylinder is nonlinear, the main factors that change the piston drive frequency were investigated by simulation. In addition, a piston drive frequency diagram was drawn based on the top surface area of the air cylinder and the compression ratio to discuss the effect of the air cylinder specifications on the piston drive frequency. The results indicate that the air cylinder specifications affect the piston drive frequency and generation force of a linear machine. Moreover, the structure of the air cylinder and the constraints on the maximum generation force of the linear machine narrow the possible operating range in the piston drive frequency. The air cylinder specifications based on the piston drive frequency diagram improved the FPEG energy conversion efficiency by 0.5%.

This paper describes a gamma-matched textile rectenna structure that is suitable for batteryless wearable accessories powered by microwaves using a conductive-yarn antenna. The proposed structure consists of a slidable button-shaped rectifier module with LED that allows the antenna to pass through its metal holes, thus making it possible to connect a soft textile antenna with a hard rectifier module to easily perform impedance matching by sliding the module on the antenna, while watching luminance of the LED light. To verify the effectiveness of our textile rectenna structure, we fabricated a button-shaped rectifier module 7.5 mm in diameter including an LED and a 6-cm-long bundled conductiveyarn antenna consisting of 0.5-mm-thick single yarns. The experimental results showed that, for 2.45-GHz wireless power transmission with an average transmitted power of 1 W, the LED on the rectifier module blinked while dissipating power of 0.355 mW at a distance of 70 cm from the wireless transmitter. We also fabricated a friendship bracelet and a pair of embroidered socks equipped with our rectenna as LED textile accessories.