Hybrid vehicles use two or more sources of energy for propulsion. Typically, one source is a liquid or gaseous fuel and the other is energy stored in an energy storage system such as batteries or ultracapacitors. Other options for energy storage include compressed gas, pressurized liquids (hydraulics), and mechanical energy stored in a flywheel. The most common type of hybrid vehicle is the hybrid-electric vehicle (HEV), which typically uses a liquid fuel (gasoline, diesel, or a gasoline/ethanol blend) to power an internal combustion engine that works with batteries, or ultracapacitors, to power an electric motor.