The FPEG consists of a two-stroke combustion chamber, a linear generator and a gas spring chamber. The piston is moved by the combustion gas, while magnets attached to the piston move within a linear coil, thereby converting kinetic energy to electrical energy. The main structural feature of the Toyota FPEG is a hollow circular step-shaped piston, which Toyota calls "W-shape". The smaller-diameter side of the piston constitutes a combustion chamber, and the larger-diameter side constitutes a gas spring chamber.
A team at Toyota Central R&D Labs Inc. is developing a prototype 10 kW Free Piston Engine Linear Generator (FPEG) featuring a thin and compact build, high efficiency and high fuel flexibility. Toyota envisions that a pair of such units (20 kW) would enable B/C-segment electric drive vehicles to cruise at 120 km/h (75 mph). The team presented two papers on the state of their work at the recent SAE 2014 World Congress in Detroit.