Investigation of a Hybrid Cooling System for the Free-Double-Piston Engine Concept

Aviation propulsion is trending toward higher power density and more compact engine architectures to meet fuel-burn and emissions targets, thereby reducing available thermal margins across architectures. As a result, thermal management is a key driver of performance, reliability, and service life in modern aviation propulsion systems. An example of an innovative propulsion system is the Free Double Piston Composite-Cycle Engine (FDP-CCE), which faces these challenges: it combines piston-engine efficiency with a turbofan architecture; however, its crankshaft-free mechanism and air-lubrication system require advanced thermal management to address elevated temperatures and heat loads in critical components. This paper introduces a hybrid cooling strategy that integrates bypass-air convection with liquid-coolant passages and develops two predictive tools to assess and optimize that strategy: (i) a reduced-order analytical heat-transfer model and (ii) a corresponding computational thermal model. Using common boundary conditions and geometry, the models are cross-compared via parametric investigations of cooling configurations, material thermal properties, and the resulting temperature and heat-flux distributions across free-double piston engine layers. To ensure a fair comparison, cases are evaluated under different operating conditions. Both approaches indicate that hybrid cooling lowers peak wall temperatures and mitigates the thermal loads. The findings offer a verified in-house tool for integrating effective hybrid cooling into lightweight thermal-management solutions for next-generation composite-cycle engines.