A turbosteamer is a BMW combined cycle engine using a waste heat recovery unit. Waste heat energy from the internal combustion engine is used to generate steam for a steam engine which creates supplemental power for the vehicle. The turbosteamer device is affixed to the exhaust and cooling system. It salvages the heat wasted in the exhaust and radiator (as much as 80% of heat energy) and uses a steam piston or turbine to relay that power to the crankshaft. The steam circuit produces 14hp and 15ft.lbf of torque at peak (for a 1.8 straight-4 engine), yielding an estimated 15% gain in fuel efficiency. Unlike gasoline-electric hybrids, these gains increase at higher, steadier speeds.[1]
BMW has been the pioneer of this concept as early as 2000 under the direction of Dr. Raymond Freymann, and while they were designing this system to fit to most current BMW models, the technology didn't reach production.