I am working on a stirling engine powered train for a DnD game I am a part of, and am trying to figure out how the engine works. I have the basics of the engine done, but I can not figure out how a throttle would work with this kind of engine. I know there was an actual train built using a stirling engine, but I couldn't find any information on how the engine inside works. I should also mention I am still pretty new to engineering concepts like this, so if whatever information you all can provide could be kept as simple as possible it would be much appreciated.
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4 Answers
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The general theme of a stirling engine is that you can't easily throttle it. However you can gear it instead. Continuously variable transmission was invented in 1879 for exactly this purpose, to allow an engine to operate at continuous speed while having speed control on the driven system.
answered Nov 10, 2021 at 9:27
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Shamelessly summarizing an physics SE answer on how would a Stirling Engine's throttle work?
most efficient - vary the amount of working gas. Bleed (and store gas) from the high temp side to throttle down, inject (medium temp) gas into the cold side to throttle up
less efficient - sabotage the temperature differential (like in bleed hot gas on purpose in the cold side)
If you fell the need to award ++ points, consider doing it on the original answer.
answered Nov 10, 2021 at 10:33
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Let's make it a series of steam loops that all feed into a cylinder, or a series of cylinders. Inside each steam pipe is a valve; since you don't necessarily want to be touching certain parts of this thing while it's active, each valve has magnets on its tips that can be controlled from the outside - through the wall of the pipe - with another magnet.
You can lock off steam loops by closing the valves associated with them with a powerful magnet on a stick. The more steam loops that are open, the more steam is available for the engine, and there's more of a heat difference. This means that there's more heat energy to convert to mechanical energy. The more that are closed, the less steam makes it to the cylinder/cylinders.
The train's engineer uses a "magic wand" to control the engine. In reality, it's a powerful magnet on a stick, but this is a DnD game, and the illusion of magic is an important thing for the sake of this. You can trick your players by having the engineer wave the "wand" over the Stirling engine in order to change its power output; if all you tell your group is that "the engineer waves the chunk of metal on a fancy stick over a massive, many-piped wall of glittering, shaking, steaming, dully-glowing metal", or something like that, the odds are that they'll think it's magic.
answered Nov 10, 2021 at 8:15
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1$\begingroup$ Steam? What steam? How do you use steam as a working fluid in a Stirling engine? $\endgroup$– AlexPCommented Nov 10, 2021 at 13:15
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$\begingroup$ @AlexP as I understand this answer he's using steam as a heat transfer medium and regulating the flow of steam to slow the heating of the engine. It's valid for a given value of valid, but it's going to be very slow to respond. If you're trying to stop a bicycle it's no good, if you're trying to stop a supertanker it's probably ok. Anything in between will give varied values of success. In terms of efficiency and control, a lot worse than a straight steam piston engine. $\endgroup$ Commented Nov 10, 2021 at 14:33
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1$\begingroup$ @Separatrix: Well, controlling the amount of heat heating the hot side of the engine will obviously work, and it can be done in many simple ways. But in this case I don't understand the need for complicated magnetic valves when ordinary throttle valves throttling the amount of fuel of heat carrying agent will be much easier and more effective. $\endgroup$– AlexPCommented Nov 10, 2021 at 14:37
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$\begingroup$ @AlexP Because it's cool (pardon pun) and Rube Goldberg-esque, and it integrates apparent magic into engineering. If you want practical, just teleport. $\endgroup$– DWKrausCommented Nov 11, 2021 at 4:36
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$\begingroup$ @AlexP, you're thinking too much like an engineer. This is for a D&D scenario which means if something is cool it can more effective than something ordinary. These are worlds with higher levels of narrativium than our own. $\endgroup$ Commented Nov 11, 2021 at 8:47
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Stirling engines require a very steep thermocline and good seals that don't degrade. You could do it with a sort of Ferris wheel arrangement of barrels and a heat source on one side; as the wheel rotates so do the barrels, the changing angle of the liquid relative to the barrels opening and closing gas-tight seals that don't wear.
If you're wondering where this is going, you could throttle this arrangement by adjusting the orientation of the barrels relative to the ground plane. It would give you subtle control of the length of the arc in which pressure is developed.
