Temperature field analysis and thermal management optimization of the free piston engine generator based on electromagnetic-thermal coupling

The free piston engine generator is a highly promising energy conversion system characterized by high efficiency, high power density, and wide fuel adaptability. However, its compact design, extreme operating conditions, and complex multi-source thermal interactions pose serious challenges to thermal management. This study develops a multi-physics coupled temperature field model considering actual operating conditions and verifies its accuracy through experiments. Based on the validated model, the temperature characteristics and heat transfer paths of key components are analyzed. The heat dissipation performance and applicable boundaries of different cooling methods are evaluated. To address the identified thermal management limitations, optimization strategies are proposed from three aspects: heat source suppression, heat path interruption, and heat dissipation enhancement. Results indicate that during the cold start stage, the average thrust should be limited to below 280 N under natural cooling, while forced air cooling can effectively suppress temperature rise at room temperature. During the power generation stage, an inlet wind velocity of at least 5 m/s is required to maintain the maximum temperature of the linear generator below 100 °C. Although water cooling can significantly reduce the overall temperature, localized hotspots remain at the ends of the mover and connecting rod regions. The effect of the cooling water velocity on temperature exhibits a diminishing return trend, with the cooling effect approaching saturation when the velocity exceeds 3 m/s. Using low thermal conductivity connecting rod materials can effectively reduce heat conduction, but the adverse impact on piston heat dissipation should be considered. Overall thermal safety can be ensured by controlling the average piston temperature below 147 °C and enhancing surface convective heat transfer.