Hydraulic Hybrid Works - This is how a series hydraulic hybrid works, When the drive pedal is pressed, pressurized nitrogen, stored in the high pressure tank or accumulator, pushes hydraulic fluid through the pump/motor which then, acting as a motor, uses the pressure to turn the wheels. The now unpressurized fluid is stored in the low pressure tank or reservoir. When the vehicle has been cruising and the usable high pressure fluid drops beneath a certain level, the engine turns on. The engine is connected to a hydraulic pump, which pressurizes the low pressure fluid for the pump/motor to continue to turn the wheels. Any excess high pressure fluid, produced by the engine-pump, is stored in the high pressure accumulator.
When the brake pedal is pressed, the engine shuts off and the pump/motor slows the vehicle down by using the vehicle's kinetic energy to pressurize fluid pumping it back into the high pressure accumulator. When the driver wants to accelerate again, only the saved energy will be used to turn the wheels. This technology was created through the Clean Automotive Technology Program, by the Environmental Protection Agency.