You've heard of hybrid-electric, now there's hybrid-air

French car company Peugeot Citroen's latest innovation operates on compressed air, gas or a combination of the two.

At speeds above 43 mph (70 km/h), the car uses a standard 3 cylinder gas engine. But below 43 mph, such as in city driving, the car uses compressed air to power a hydraulic motor and the gas engine shuts down. When more power is needed, for hill climbing or accelerating, the two work together. This system gives 117 mpg (2l/100km) in theory. The regenerative braking stores energy as compressed air too. This hybrid is lighter than a hybrid-electric, because there are no heavy batteries. Peugeot's expects to have this car available by 2016.