"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.
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"
From Road & Track Magazine:
"Toyota says this mechanically simple engine achieves a claimed thermal-efficiency rating of 42 percent in continuous use. Only the best, most complicated, and most expensive of today's gas engines can come close to that number, and only in specific circumstances. Even better, a two-cylinder FPEG is inherently balanced and would measure roughly 8 inches around and 2 feet long. An engine of that size and type could generate 15 hp, enough to move a compact electric vehicle at highway speed after its main drive battery has been depleted. That's the future."