A team at Toyota Central R&D Labs is developing a prototype Free Piston Engine Linear Generator (FPEG), which they described at the recent SAE 2014 World Congress in Detroit.

Unlike a traditional ICE, which propels a drive shaft, the FPEG is designed to generate electricity directly. It 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, converting kinetic energy to electrical energy.

The Toyota FPEG is based on a double piston system – at one end is the combustion chamber, and at the other, an adjustable gas spring chamber, which is responsible for returning the piston for the subsequent combustion event.

A portion of the kinetic energy of the piston is stored in the gas spring, and extracted on the return stroke to the combustion chamber side. A magnetic “mover” is mounted at the outer periphery of the piston. The linear generator is a permanently excited synchronous machine consisting of the stationary coil, the mover (based on neodymium-iron-boron magnets) attached to the piston, and iron-core stator. The power electronics drive the machine as both a motor and a generator.

Video: View an animation of Toyota’s FPEG here >

The researchers achieved output power of 10 kW with 42% thermal efficiency. Toyota envisions that a pair of such units (20 kW) would enable B/C-segment electric drive vehicles to cruise at 75 mph. The prototype achieved stable operation for more than 4 hours without any cooling and lubricating problems.

Source: Toyota Central R&D Labs via Green Car Congress