What is a Stirling Engine and how does it work?
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There are a few different designs, but the general idea is super simple. When air is heated, it gets bigger, and when it cools it gets smaller.
You have two sides of a container, one hot and one cold. When the air is in the hot side it expands and pushes a piston. Then, the air is moved to the cold side where it contracts and pulls the piston. It then gets moved back to the hot side and the cycle continues.
One easy way to do this is by having a tube with a hot end and a cold end, and a solid “displacer” moves to force the air out of whichever end is not currently in use.
This whole process generates motion from the heat difference between the hot and cold sides.
The simplest form:
You have a cylinder of gas, with a insulating moving disc inside. The moving disc will act as a heat barrier, it moves through the container, through the gas, without acting on the gas, not blocking or compressing the gas, just moves to top and bottom location. Then a line connects the gas charge to a cylinder and piston in a separate place.
Top of container is cold, bottom is heated by fire or whatever is your heat source.
The gas will be cyclically heated and cooled. The hot gas expands, get from container to a cylinder and move the piston. When gas cools it will retract and suck the piston back. Piston will crank a shaft: that’s where you get the power output.
How it works:
Insulating disc is on the cold side, preventing gas to get cold. Gas gets hot.
Hot gas becomes pressurized, due to heat.
Gas goes to the piston and pushed the piston, turning the crankshaft.
The crankshaft is linked to the insulating disc, and pushes the disc down to the bottom of the gas container.
Gas now can’t be heated and is in direct contact with the cold side of the container. Gas cools down.
Cold gas has low pressure, so will suck the piston in the cylinder down.
This turns the crankshaft again. The crankshaft then pull the insulation disc up again. And we are set for another cycle.
Pros:
low operating pressures. Can be built with weak materials, way weaker than a steam engine.
No valves: it’s very simple, there’s very few moving parts. Apply heat to the container and it runs.
Silent: it spins slow, makes almost no noise compared to other engine types.
Safe: due to low operating pressures, if damaged it can’t catch fire or explode, not violently at least.
Efficiency: it can run with ridiculously low heat difference between cold and hot side. It can really do miracles if you are trying to get mechanical power from residual heat of something, like another engine. You can run a stirling engine with the heat of your car exhaust for example. That’s incredible. You can even make a desk miniature that runs on your hand’s heat.
Cons:
Low power per weight: low rpm, low force on the pistons, makes the stirling engine one (if not the) heaviest engine per horsepower produced.
Poor control: in its basic form, it can’t be throttled up and down with ease. It slowly reacts to heat coming in. Heat amount is your one and only control for the engine.
Conclusion:
It’s a very interesting engine because it’s very special in its performance range. Very different from other engines.
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