Structural optimization of linear range extender: Considering thermal characteristics

The free-piston linear engine (FPE) has emerged as a promising and efficient candidate for future hybrid electric vehicles, particularly in linear range extender applications. However, escalating demands for increased power output and operational frequency have posed significant thermo-structural challenges that hinder further development. Despite its significance, existing investigations on thermal management in FPEs remain highly limited. To address this research gap, a comprehensive threedimensional, coupled multi-physics numerical model has been developed. This model was employed to investigate the influence of counterflow and co-flow cooling configurations on the thermal performance of the engine. The simulation results reveal that the counterflow configuration causes a stagnation zone in the lower cylinder block, resulting in inadequate local cooling. Conversely, the co-flow cooling configuration enhances flow uniformity and increases peak flow velocity by 18.5% (up to 1.901 m/s). And at the same section within the stagnation zone, the average flow velocity increases from 2.85e-5 m/s to 4.3e-2 m/s, effectively eliminating stagnation regions. Thermodynamic analysis reveals that the system's heat transfer coefficient improves by 6.2%, accompanied by a 17.2% increase in heat dissipation power, thereby substantially mitigating the thermal load on the engine. Although the co-flow configuration substantially enhances heat transfer efficiency, it also introduces increased hydraulic losses, as evidenced by a 39.5% rise in frictional power consumption. The study provides valuable insights for improved cooling and mechanical design for the FPE.