Leveraging thermal-fluid-solid coupling to unlock higher system performance in linear range extender

Free-piston engine generators (FPEG) are promising linear range extenders for hybrid powertrains, boasting potential gains in energy conversion over conventional crankshaft engines (CSE). However, prevailing thermal analyses rely on a uniform wall temperature assumption, failing to capture critical thermal-fluid-solid interactions under nonlinear piston dynamics and hindering cooling strategy development. This study bridges this gap with a novel explicit coupled method featuring dual-loop iteration, which for the first time resolves the spatiotemporal evolution of chamber thermal status via bidirectional combustion-conjugate heat transfer coupling. The effects of key cooling system parameters, including inlet condition, topology, and configuration, on energy conversion are subsequently evaluated. Our results demonstrate that the thermal-fluid-solid coupling effect undercuts the previous gains in heat loss and indicated thermal efficiency (ITE) for FPEG over CSE as obtained under the uniform-temperature assumption. The complex interplay between non-uniform cooling and nonlinear trajectory constrains the effective ranges of cooling solutions for simultaneous above gains, often degrading combustion quality. In response, we introduce a partitioned-optimization cooling scheme with the thermal consideration in practical cooling capacity and specific motion patterns, yielding ITE gains of 0.59% ( Ω = 0.5) and 0.65% ( Ω = 0.8) in two asymmetric trajectories. Furthermore, a general cooling design principle is also established to guide the thermal management across diverse conditions. Overall, this work provides a novel and generalized thermal analysis framework for multi-physics interactions, equipping practitioners with critical strategies for cooling design in linear range extenders with complex nonlinear dynamics.