With the increasing focus on carbon neutrality and environmental concerns, innovations in hybrid-electric vehicles, particularly range extender electric vehicles (REEV) equipped with Free-Piston Engine Generators (FPEG), are gaining traction globally. FPEG, including single-piston and dual-piston types, offer benefits like compact design, high power density, and flexibility in energy sources such as hydrogen and natural gas. Toyota and Beijing Institute of Technology have been at the forefront of FPEG development, achieving notable advancements in performance and control accuracy. Notably, FPEG prototypes fueled by hydrogen have shown promising results, with Sandia National Laboratory and Korea Institute of Energy Research reporting enhanced efficiency and reduced emissions. However, challenges like hydrogen embrittlement, storage, and unique combustion characteristics require specialized solutions. Research teams are exploring computational simulations and experimental studies to optimize FPEG performance when using hydrogen as a fuel. Special considerations like direct injection, material compatibility, and combustion behavior are crucial for harnessing the potential of hydrogen in FPEG technology for clean energy applications.