The EPA has built a modified hybrid that uses a hydraulic system, not a battery, to store braking energy. When you press the brakes, the wheels drive a pump that compresses nitrogen gas, which is inexpensive and inert. When you accelerate again, that compressed gas runs the pump in reverse to help power the vehicle.
The hydraulic-hybrid system, scheduled to begin testing in two UPS trucks this month, with another to follow next year, promises to return at least 70 percent of the braking energy back to the wheels, which would lead to a 60 to 70 percent jump in fuel economy and a 40 percent reduction in emissions.
Perhaps that's why Charles Gray, the director of the Advanced Technology Division and one of the developers of the hydraulic hybrid, can't contain his excitement about its potential.
http://www.cnn.com/2007/TECH/03/09/cars ... index.html