Hackaday has a good article on Stirling engines with animated illustrations, it’s the introduction to the subject that I wish I’d found when I was first trying to understand them long ago.
It’s really not that difficult to understand the working principle of a Stirling engine, it’s mostly just counter-intuitive that there’s enough energy there for it to really work… similar in that respect to the fire piston, or the Rüdiger roll. As described, they all seem barely plausible, you have to see it to really believe it.
Stirling engines have been an on-and-off interest of mine over a very long period, and I’ve built some “model” Stirling engines for my own learning, beta and LTD types. They continue to be controversial, but there’s little doubt in my mind that they’re under-exploited. They’ve never worked all that well for direct propulsion uses because of the time required for them to get up to working temperatures, and because they tend to be most efficient at a pretty steady RPM, not constantly accelerating and decelerating. As we’re going more and more into using series hybrid systems where the “engine” only generates electricity those problems are increasingly moot.
They can be very efficient having one run at room temperature off of the heat from the hand that it’s resting on is a memorable lesson in that respect. On the other hand they are so efficient that any tiny flame, even alcohol, is enough to drive a small model so hard that it’s trying to shake itself apart. They can also be made very quiet (one current use, last time I checked, was in submarines) and very, very reliable, since moving parts are few and they don’t generally require very tight tolerances to work- in fact, most, especially LTD designs, are so sensitive to friction that they pretty much require loose tolerances.
I’ve always thought that they’d have great potential for power generation in space, since the temperature differential to run them is essentially free, the differences between sunny and shady sides of anything in the Goldilocks Zone in a vacuum is so extreme.. and since air works as the working fluid (if you have none you have bigger problems) but is not consumed. It’s easy to imagine, say, small space stations that are powered by Stirling engines running off the temperature differential between the side facing the sun and the side facing away from the sun. Of course you have to deal with the torque as a by-product when you’re free-floating, but that’s true of most mechanical solutions.
The fact that they will work off of low temperature differentials also means that they have great potential for scavenging energy. Stirling engines have run happily for weeks off of the heat of a compost pile, and along with Peltier Junctions are a favorite way to drive a heat-circulating fan essentially free from the heat given off by a wood stove when the power is out, or in a cabin where there is none. They’ve also been built with a huge variety of materials, including tin cans.
I don’t have the engineering to be able to design Stirling engines that reach new efficiency levels, I can barely recognize the major parts in exploded diagrams of some of the more modern high-pressure variants, but it’s not that difficult to design Stirling-cycle engine variants that will at least work, and doing so mentally has been one of my stand-by mental techniques for dealing with long, boring and useless meetings at work for a very long time.
– Robert the Wombat