Less engaged friends sometimes question the meaning of pushing a car to get the pistons moving - in their eyes that's pretty far an "engine". Well, here's something that is surely an engine for them too: the pneumatic engine. The input is compressed air, the output is rotating motion.
There are inline-3 and -4 versions as well as V6, V8, etc.. And what can it be used for? Well, for something like this:
If you would like to know more, I propose this YouTube channel or the http://www.lpepower.com website with a lot of pictures, video and technical data. There is no detailed description, instead you can buy a complete engine for a somewhat hefty price tag: the inline-3 starts from EUR 130.