Insert your own caffeine addiction joke here.
Stirling engines are fascinating pieces of machinery, with the potential to be far more energy efficient than the internal combustion engines we're used to. They're also way more quiet. The key difference is that gas is sealed inside a Stirling engine. The pistons move as you heat and cool that sealed gas, changing the pressure inside the system.
Stirling engines were invented in the early 19th century, but they aren't used much today, mainly because the need for an external heat source makes them a bit impractical. For instance, a car run on a Stirling engine would take a while to heat up before you could drive it. They also weigh a lot more than an equally powerful internal combustion engine, which means, in a car, you'd lose some of the efficiency you'd gained. That said, there's a lot of potential for these things. Stirling engines can run on pretty much any heat source, which makes them a great fit for alternative energy. They're already starting to show up in stationary power plants that produce both heat and electricity for universities and factory campuses.