Technological Mapping and Emerging Applications of Free-Piston Engine Systems: A Comparative Review of FPSE and FPCE Architectures

Introduction: Free-Piston Engine (FPE) technology provides flexible energy conversion for applications such as hybrid vehicles and micro-CHP systems. However, studies on Free-Piston Stirling Engines (FPSE) and Free-Piston Combustion Engines (FPCE) are still scattered, with limited analysis of their designs, subsystems, and applications. The aim is to describe recent technical gains and highlight promising avenues for deployment in energy supply and mobile applications.Method: A Systematic Literature Review (SLR) was conducted using the PRISMA-PCF protocol. Of the 263 articles identified in the Scopus database, 77 were selected based on three research questions. NVivo software supported thematic content analysis.Result: The review identified two primary FPE types: FPSE, which is suitable for external heat sources such as solar and cryogenic systems, and FPCE, which features variable compression and adaptability for hybrid vehicles. Key subsystems, such as linear alternators and return mechanisms, support distinct technical functions. Applications span energy generation, waste heat recovery, and renewable energy systems.Conclusion: This study maps the progress and application areas of FPSE and FPCE, highlighting opportunities for integration and providing direction for future development in design and performance optimization.