Using a very elastic material, researchers have developed a hydraulic regenerative braking (HRB) system that will soon be ready for passenger vehicles. The use of hydraulic regenerative braking systems in passenger vehicles has the potential to save billions of gallons of gasoline annually in the U.S. These systems have already been successfully integrated into large vehicles such as garbage trucks and buses, resulting in fuel savings of 25-30 percent in city driving. A regenerative brake slows a vehicle by converting its kinetic energy into another form of energy, which can be used immediately or stored until needed. In conventional braking, the excess kinetic energy is converted to heat by friction in the brake linings and therefore wasted. Hybrid vehicles improve on conventional braking and most commonly rely on electrical regenerative brakes, with excess energy stored in a battery or bank of capacitors for later use. Switching from an electrical to a hydraulic system will result in even more gains in energy efficiency and power for passenger vehicles. But improving the fluid energy storage, known as an accumulator device, remains a challenge. Working at the University of Minnesota, the lead institution for the NSF-funded Engineering Research Center for Compact and Efficient Fluid Power, engineers have developed a new accumulator device. The device is connected to the drive train. When the driver brakes, it captures the kinetic energy to slow the vehicle down and route the energy into an accumulator. During acceleration, the accumulator pushes out fluid and the device acts as a hydraulic motor, assisting the engine and reducing the fuel required to accelerate. Upon reaching the maximum HRB system speed, the system will disengage, at which point the vehicle will be powered by the engine alone.