It has come to my attention that some of you have never taken classes in thermodynamics. And in this day and age...
Thermodynamics covers a very broad array of topics, but the central idea is accounting for energy and how to make various types of "transactions" with it. This is important because often we find that the kind of energy we have isn't the kind we want. So we swap the chemical energy in gasoline for mechanical energy in our cars, or nuclear energy in uranium et al for electrical energy at our wall sockets.
There are various major laws of thermodynamics - a couple of them have been popularized as 1) you can't win (energy is conserved), and 2) you can't break even (entropy is always increasing). Futurists warn us of waning energy supplies and that entropy thing, leading to an eventual "heat death" of the universe. And you thought economics was the 'dismal science'.
But there's good news. There are ways to use existing energy supplies more effectively, even the dirtiest of them. One of those ways is something called a Stirling engine.
The Stirling engine is called an external combustion engine for an obvious reason - it typically uses the energy of combustion to drive it, but the combustion occurs on the outside. You don't have to atomize the fuel, and you can't foul the engine internals no matter how dirty the fuel is. In fact, you don't need combustion at all - this company will sell you a small demonstration Stirling engine that runs off the heat of your hands.
Here is an exposition on Stirling engines by HowStuffWorks, complete with animations.
This site also has a working model of a Stirling engine and a bunch of other neat stuff for your inner geek.