A turbosteamer is a BMW combined cycle engine using a waste heat recovery unit.
The turbosteamer device is affixed to the exhaust and cooling system.
The steam circuit produces 14 hp (10 kW) and 15 ft⋅lbf (20 N⋅m) 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.