Dynamic performance of a free piston expander-linear generator for small-scale organic rankine cycle under variable operating conditions: an experimental and numerical investigation

To tackle the critical challenge of adapting free piston expander-linear generators to the dynamic operating conditions of vehicle engine waste heat recovery Organic Rankine Cycle systems, this study introduces a comprehensive Matlab/Simulink simulation framework that integrates multi-physics coupling of thermal, mechanical, and electromagnetic dynamics. Unlike prior models focusing on steady-state performance, this work systematically quantifies the transient interactions between key design parameters—free piston assembly mass and linear generator internal resistance—and their impact on electromagnetic force generation, piston motion stability, and power output under variable intake/exhaust pressures. The study reveals that intake pressure and exhaust back pressure are the dominant dynamic factors affecting FPE-LG efficiency, with a 22% improvement in power output (96.0 W) and voltage stability (30.0 V) achieved by optimizing these pressures alongside free piston assembly mass (0.59 kg) and LG resistance (14.7 Ω). A parametric sensitivity analysis further identifies non-linear trade-offs between component sizing and operational frequency (5 Hz), providing a design map for balancing performance and durability. The methodology enables rapid prototyping of vehicle-specific free piston expander-linear generator system, with potential applications in hybrid electric vehicles, commercial trucks, and marine engines. Further work could integrate real-time control algorithms for adaptive pressure regulation and explore low global warming potential working fluids to enhance sustainability. This research lays the foundation for scalable, high-efficiency waste heat recovery technologies in mobile applications.