ultra quiet engines

I'm a huge fan of the Stirling Engine.

In fact, I actually built one when I took a class during my sophomore year in college (where I learned to use a milling machine among other dangerous tools).

Unlike other engines (say, a reciprocating piston engine or a rotary engine), the Stirling Engine does not use internal combustion. The engine employs a series of pressure changes inside a sealed gas chamber to do work. Part of the chamber is heated (causing the gas to expand) and part of it is cooled down (causing the gas to compress). The series of pressure changes caused by this temperature differential results in the engine doing work. Since there are no internal explosions or venting of high-pressure gas, Stirling engines tend to be ultra-quiet and more efficient than other types of engines.

Despite how wonderful Stirling engines are, they aren't really practical (so you won't find them in cars anytime soon). First, the engines require an external energy source, so there is usually a long warm-up time before the engine starts doing useful work. Second, you can't adjust the power output very quickly; changing the temperature of the sealed gas cylinder doesn't result in an immediate change in the engine's power output.

Despite their shortcomings, the Segway folks have big plans for these engines.