Thermal management optimization for cold-start preheating of a free-piston engine: simulation and experimental study of a series cooling circuit with sequential control strategy

Aiming at the cold-start challenge of a developing free piston engine generator (FPEG) under extreme environments, this study optimizes its cooling system to achieve rapid preheating. A MATLAB/Simulink-based 1D thermo-fluid coupling model was established and validated via 3D CFD analysis of the cylinder block's steady-state temperature field. To solve heat loss in the original parallel preheating circuit's radiator bypass, a “time-segmented” thermal management strategy and a new configuration with a diesel liquid heater (DLH) in series with the cooling circuit were proposed. Bench tests show the optimized series circuit increases coolant outlet temperature from 82.1 °C to 94.1 °C and cylinder block temperature from 83.3 °C to 92.5 °C, shortening preheating time by 16.7% (250 s) and supporting stable startup. Simulation confirms the optimized system's post-startup steady-state cooling capacity and reveals initial temperature's linear influence: preheating time increases with decreasing initial temperature, while steady-state coolant outlet and cylinder block temperatures rise with increasing initial temperature. This study provides key thermal management design basis for FPEG's engineering development.