Experimental study on free piston linear generator (FPLG) used for waste heat recovery of vehicle engine

In this study, a novel free piston expander coupling with a linear generator prototype is presented to recover exhaust waste heat efficiently from vehicle engine. The free piston linear generator (FPLG) can be used in a small-scale organic Rankine cycle (ORC) system and can directly convert the thermodynamic energy of working fluid into electricity. The effects of intake pressure, operation frequency as well as external load resistance on operation characteristics and output performance are discussed. The results show that the piston displacement profile is similar to a sinusoidal wave. The piston amplitude increases significantly with the increase of the intake pressure. It is observed that the increase in the operation frequency brings about the decrease in both piston amplitude and motion symmetry. In addition, the peak piston velocity and power output show nearly linear relation with the intake pressure, which indicates that the power output is sensitive to the piston velocity. When the operation frequency is 1.0 Hz and the external load resistance is 20 Ω, the maximum peak velocity is about 0.69 m/s and the highest power output 96 W is obtained. The conversion efficiency of the FPLG increases with the increase of the intake pressure. But the conversion efficiency increases slightly when the intake pressure is higher than 2.2 bar. When the operation frequency is 2.0 Hz, energy conversion efficiency of the FPLG can reach up to 45.82% with an intake pressure of 2.6 bar.