The Hydraulic Hybrid is driven by an advanced hydrostatic drivetrain, consisting of a hydraulic free-piston engine, an accumulator and four hydraulic wheelmotors. The engine provides pressurized fluid for charging the accumulator and driving the wheelmotors. When fully charged, the engine is turned off and only fluid from the accumulator drives the car. During braking, the wheelmotors are reversed to pumps, acting as brakes, and return the entire braking energy into the accumulator. A portable electro-hydraulic module for charging can be inserted, allowing for 40 miles emission-free driving.