A free piston engine linear generator (FPEG) with potential for compact build, high efficiency and high fuel flexibility was developed in this study. The FPEG consists of a two-stroke combustion system, a linear generator, and a gas spring chamber. There are some technical challenges in ensuring an FPEG can achieve continuous operation over a long period, including lubrication, cooling, and piston motion control. Among these technical challenges, the piston motion control is the most significant factor in improving the robustness and efficiency of the FPEG because the combustion characteristics depend strongly on the piston motion, which is controlled by the linear generator. This paper describes a novel linear generator control method which realizes the simple harmonic oscillation governed by the piston mass and the air spring pressure. In general, the generating efficiency of linear generators is low in the low-speed region. The piston speed is zero at the beginning and end of the stroke and reaches a maximum at the middle of the stroke. Thus, it is preferable for the linear generator to function only in the middle of the stroke, allowing the piston to move freely during the remainder of the stroke. To control the piston position at the beginning and end of the stroke, speed control commands for the linear generator are determined based on the position errors at the beginning and end of the stroke. To investigate the potential of this control method, a one-dimensional simulation model was developed. Simulations conducted with this model clarified that the FPEG can be operated stably and efficiently with high robustness under the conditions of this study and that the base pressure of the air spring chamber must be set in accordance with the injected fuel mass. These conclusions were confirmed through experiments with the prototype FPEG.