To improve the energy efficiency of internal combustion engines, this study develops a single-piston free piston expander-linear generator (FPE-LG) for a small scale organic Rankine cycle (ORC) in a waste heat recovery system. The prototype can operate continuously and stably, thereby indicating that the single piston FPE-LG has a feasible working principle. The operation characteristics of FPE-LG are revealed, and the effects of the intake valve opening time (IVOT) and the exhaust valve opening time (EVOT) on the output performance are investigated. The experimental results reveal that the piston can run at a relatively high velocity near the dead centers, and peak acceleration is typically achieved when the piston arrives at the operation left/right dead centers (OLDC/ORDC). The piston stroke length is decreased significantly as the operating frequency is increased. The peak power output, peak in-cylinder pressure, peak velocity of the free piston, and utilization ratio of the piston stroke (eta) are increased as the external load resistance is increased. In addition, n increases with longer IVOT and can reach up to 67.4% when the IVOT is 40 ms. The exhaust pressure decreases as the EVOT is increased, and the power output can be improved by regulating the IVOT rather than the EVOT. (C) 2018 Elsevier Ltd. All rights reserved.