High efficiency, fuel flexibility, and seamless integration with electrified systems are fundamental prerequisites for the next generation of internal combustion engines. In this context, the free-piston linear generator (FPLG) evolves the traditional internal combustion engine concept (ICE) by replacing the crankshaft mechanism with a linear generator, directly converting piston motion into electricity. The FPLG offers several advantages, including higher efficiency in converting mechanical energy to electricity, the ability to operate with a variable compression ratio, and reduced heat losses during the expansion stroke. Among the various tested architectures, the two-stroke, opposed-piston FPLG appears to be the most promising. However, detailed numerical and experimental investigations are necessary to fully understand how performance and efficiency are influenced by the intricate interplay of processes governing electricity generation. In particular, the significant differences between conventional crankshaft-based engines and FPLG kinematics have a profound impact on gas exchange and combustion processes. This study presents a numerical analysis of the key parameters affecting the performance and efficiency of spark-ignition opposed-piston FPLGs. Simulations were conducted using a modified 1D code, which accounts for the effects of electrical load and gas spring pressure on piston motion. Given the unconventional geometry featuring uni-flow scavenging and a side-mounted spark plug, preliminary CFD simulations were performed to develop realistic intake and exhaust system schematics and to establish an appropriate heat release rate profile. Methane was chosen as the fuel for two main reasons: it can be produced from biogenic sources and is applicable to both mobility and power generation. Additionally, its high octane number makes it particularly suitable for FPLG operation at high compression ratios. A single-cylinder unit (~250 cm3) was simulated as an initial step toward developing a small-scale prototype. Simulations examined the effects of gas spring pressure, charging pressure, electrical load, and spark timing. The results indicate that efficiency is maximized by applying the highest possible load under given operating conditions and introducing backpressure on the exhaust side to improve trapping efficiency.
Morandi et al. (Sun,) studied this question.