Parametric Analysis of Performance and Efficiency of Free-Piston Linear Generators with 1D and CFD Simulations

High efficiency, fuel-flexibility and easy integration with electrified systems are fundamental pre-requisites of the next generation of internal combustion engines. Within this context, the free-piston linear generator (FPLG) evolves the internal combustion engine (ICE) concept by replacing the crankshaft mechanism with a liner generator, directly producing electricity from the piston motion. FPLG advantages are represented by a higher efficiency conversion from mechanical energy to electricity, the possibility to operate with variable compression ratio and reduced heat losses during the expansion stroke. Among the different tested architectures, the two-stroke, opposed piston FPLG seems to be the most promising one. However, detailed numerical and experimental investigations are needed to understand how performance and efficiency are affected by the complex interplay of processes governing the electricity generation. In particular, the relevant differences existing between conventional crankshaft and FPLG kinematics drastically affects gas exchange and combustion process. Purpose of this work is a numerical study of the relevant parameters governing performance and efficiency of spark-ignition opposed-piston FPLGs: simulations were performed with a 1D code which was appropriately modified to account for the effects of electrical load and gas spring pressure on the piston motion. Considering the non-conventional geometry with uni-flow scavenging and sided spark-plug, preliminary CFD simulations were carried out to support the development of realistic intake and exhaust system schematics and provide a suitable heat release rate profile. Methane was used as fuel for two different reasons: it can have a biogenic origin and can be applied both in mobility and power generation. Moreover, its high octane number makes it particularly suitable to operate FPLG with high compression ratio. A single-cylinder ~250 cm3 unit was simulated, as a first step towards the development of a small-scale prototype. Simulations were carried out analyzing the effects of gas spring pressure, charging pressure, electrical load and spark-timing. The calculations showed that the efficiency is maximized by adopting the maximum possible load for any given working conditions, and by adopting a backpressure on the exhaust side, so as to increase the trapping efficiency.