With the Cactus Airflow 2L, Citroen thinks it's found a way to bring the cost of hybrid technology down to earth using nothing but air. Based on a production Cactus model, the natty, low-roofed concept car has a unique hybrid compressed air/gas engine at its core. In a regular hybrid, batteries store energy from braking and coasting downhill. It's the same idea here, but instead of storing electricity, the Airflow 2L compresses air and stocks it in a tank. It can then be released when needed, powering the car during wasteful acceleration or city driving. While that may sound wonky, the system has been tested to an rather astonishing 115 mpg or so. Still, why bother with an untested system when existing hybrid drivetrains are so mature?