Here is a working animation of the model HK 1, a displacer-type Stirling engine in operation. The green pad moving up and down is the displacer. It moves the air back and forth between the hot and cold sides of the engine. It is made of a porous material, so as it travels it retains some of the heat, which can be used again in the next cycle. The displacer is the most important advancement in heat engines, improving their efficiency by recycling the heat (instead of throwing it away like car engines do). I wonder what the effect on global warming would be if we all used Stirling power...
When the air is displaced back and forth it becomes alternately hot and then cold and thus expands and contracts accordingly, moving the piston. This principle is known as Charles's Law. Hot air expands and cold air contracts. For illustration, try blowing up a balloon and putting it in the freezer for a few minutes. Take it out and watch it expand as it warms back up to room temperature. This is what powers the weather. Inside the engine there is only air at atmospheric pressure. There is no steam, no exotic gasses or anything else that could explode or otherwise cause harm to persons or the environment.
The orange cones (candle flames) in the above illustration represent the heat source and the ice cubes on top represent the cooling necessary for continued operation. The green pad that moves up and down is the displacer. The piston is mounted horizontally. Notice how the motion of the displacer leads that of the piston by a quarter turn. This is very close to the ideal for this type of engine.
To start the engine, heat is applied to the bottom and cooling is applied to the top surface. The crank is turned slowly counterclockwise until the engine starts.
When looking at the above animation, keep in mind that the air inside the engine is expanding and contracting, pushing and pulling on the piston. All of the work that goes into turning the engine is done by the piston. The displacer is completely passive and is moved up and down by the crank. It only acts to displace air which would ordinarily be in contact with hot and cold surfaces.
Go ahead and stop and start the presentation a few times and try to imagine the various forces at work upon the piston and linkages. The whole concept is so simple it will amaze.
Note also that the displacer is semi-porous and does not touch the sides of the can. Air can flow through it as well as around it. This causes the air to pick up some of the heat left behind from the last cycle and makes the displacer also a regenerator. A good regenerator can increase the overall thermal efficiency of an engine by as much as 50%.
The engine goes through 4 general phases of operation as outlined below:
1. Vacuum power stroke
In this frame, the crank and displacer are in their lowest position and the piston is halfway in. The bulk of the air is displaced to the top of the can where it cools. Cooler air takes up less volume so it immediately forms a slight vacuum inside the engine which tends to pull the piston inward.
2. Heating cycle
The piston tugs sharply on the piston rod and the crank turns until it is in the rightmost position. Some of this energy is stored in the flywheel, which carries the crank around until the displacer is lifted up.
3. Pressure power stroke
With the displacer in the up position, the air inside the whole engine experiences rapid heating and builds up pressure forcing the piston out to the left.
4. Cooling cycle
The crank turns and moves the displacer back down. The air rapidly cools and the cycle begins again.
More about the Stirling cycle
The theoretical limit. Any engine that converts heat into work is bounded by the Carnot efficiency: η = 1 − Tc/Th, where Tc and Th are the absolute (Kelvin) temperatures of the cold and hot ends. The larger the temperature difference, the higher the maximum possible efficiency, which is why keeping the top of the engine cool is just as important as heating the bottom. It is also why an ice cube on top of the HK-1 makes it run faster.
Why the working gas is sealed in. The HK-1 runs on ordinary air at atmospheric pressure. Because the gas is permanently contained, the working fluid can be chosen freely: modern engines use helium or hydrogen, and pressurizing the gas lets a smaller engine produce the same power. With no intake or exhaust valves, Stirling engines are practically silent, which is why they power Swedish submarines and NASA deep-space generators (source).
A quick history. Robert Stirling patented his air engine in 1816, and an engine built to his design was pumping water from a quarry in Ayrshire by 1818. His patent described the heat exchanger he called an "economiser", which later became known as the regenerator. After a century of niche use, Philips revived the design in the late 1930s, and by 1951 was building the MP1002CA "bungalow set", a quiet 180-200 watt generator (source).
Tips from the HK-1 build log. The plans reveal several practical subtleties: a little air must leak past the displacer rod seal, or the expanding air will seize the piston on warm-up; a light lubricant such as WD-40 works better than grease, which turns sticky and plugs the seals; the flywheel must spin freely; keep the heat low or the solder will melt; and the engine starts best when gently cranked counter-clockwise after a minute of heating.
See the HK-1 engine running