Hybrid energy storage system (HESS) is an essential energy conversion/storage unit for the practical application of free piston engine generator (FPEG). There are few reports on the matching design of HESS applied to FPEG. In this paper, the matching design of the HESS is carried out according to the electric power demand during the startup of opposed-piston free piston engine generator (OPFPEG), and the test platform of the HESS is built to analyze the electric power supply capacity and the startup characteristics of OPFPEG under different electric power supply schemes. Under the topology of the HESS designed in this paper, the power supply can meet the instantaneous power demand and energy supply of the motor, and the engine compression ratio and peak in-cylinder pressure can meet the ignition requirements of the in-cylinder mixture. It is further found that during the startup of OPFPEG, the bus voltage and current of HESS will fluctuate violently. Under the condition of battery power supply, the bus current response is delayed and the bus voltage fluctuates violently. Under the compound power supply, the current peak is large, the bus voltage is relatively stable and the power supply is sufficient.
Wu et al. (2026) studied this question.