The engine compartment of a Chevrolet Volt hybrid gas/electric car. Right side: the power inverter on top of the electric motor. Left side: the 1.4-liter gasoline-powered engine used as a generator to provide power to the electric motor or to engage mechanically to assist propulsion when the battery is depleted. The Volt is the most fuel-efficient car with an internal combustion engine sold in the United States and can travel 25 to 50 miles on a lithium-ion battery with a total range of 379 miles. Overall fuel economy is 60 miles per gallon.