Electric vehicles may be the biggest topic in alternative fuels, but they're not alone: Hydrogen-powered engines have existed for decades and are still around.

The Toyota Prius, the world's most popular hybrid car, uses a combination of an internal combustion engine and a battery electric drive system to increase fuel economy and reduce emissions.

But that difference isn't something you can ignore easily. With EPA regulations tightening for all automakers, it makes perfect sense why the rotary engine is currently dead.

But an airplane is different in that the engine typically runs at the same speed even as the throttle changes the engine’s power output. How does that work?

How air-cooled engines work, and why it doesn't matter any more From the Porsche 911 to the cheerful VW Beetle, here's why air-cooled engines once ruled.