A Comprehensive Review of the Free Piston Engine Generator and Its Control

Review A Comprehensive Review of the Free Piston Engine Generator and Its Control Patrick Brosnan 1, Guohong Tian 1, * , Umberto Montanaro 1, and Sam Cockerill 2 1 School of Mechanical Engineering Sciences, University of Surrey, Surrey GU2 7XH, UK 2 Libertine FPE Ltd., Sheffield S9 1DA, UK * Correspondence: g.tian@surrey.ac.uk; Tel.: +44-1483-300-800 Received: 2 February 2023 Accepted: 10 March 2023 Published: 23 March 2023 Abstract: The Free Piston Engine (FPE) is a unique machine with a higher thermal efficiency than its counterpart, the Conventional Reciprocating Piston Engine (CPE). The unique piston motion of the FPE is not constrained kinematically like the CPE with its connecting crankshaft and rotational inertial masses. Moreover, when directly coupled to the Linear Electric Machine (LEM) to harness electric energy production, the Free Piston Engine Generator’s (FPEG) characteristic motion, now being dynamically constrained, permits an extensive range of piston trajectory profiles to be exploited during operation. In addition, exploring varied piston trajectories during the development stages may be vital in reducing in-cylinder combustion emissions through strategies such as Low-Temperature Combustion (LTC) and Homogeneous Charge Compression Ignition (HCCI). This review paper will focus on the key motivations and drivers for continued FPEG development. It will also highlight and review its distinct advantages and challenges in being a viable solution as a future zero-carbon engine technology. Finally, FPE fundamentals, alongside its rich history, will be introduced, clearly presenting how academia and industry have described and controlled its intrinsic non-linear dynamics.