The Stirling engine is a closed cycle heat engine. It is typically completely sealed from the outside environment and works on the expansion and compression of the gas (typically air) that's enclosed in the sealed engine. Fuel does not move through the engine like in a conventional internal combustion engine. This means that there is no intake or exhaust.

One side of the engine is heated and the other side is cooled. This causes the gas to go through cycles of expansion and compression. This means it can produce motion by converting heat energy directly into kinetic energy or mechanical work.

An outside of view of the engine shows only it’s moving parts, a heat source and a cooling source. Remember no fuel goes through the motor because it is completely sealed from the outside.

There are many types of Stirling engines. There are high pressure engines that are used commercially. There are restored low pressure engines that are used for display. There are table top model engines built by hobbyists and students.

You can find models or kits for almost all of the configurations of Stirling engines including LTD (Low Temperature Differential) engines. An LTD can run on the heat of the palm of your hand.

The Stirling is traditionally classified as an external combustion engine. Although, properly applied, any heat source will work to power a Stirling engine. This means that the heat source is not limited to combustion only.

Here is a list of some of the possible usable heat sources:

• Solar energy
• Geothermal energy
• Nuclear energy
• Coal
• Gasoline
• Alcohol
• Natural gas
• Propane
• Wood
• Anything that will burn

A great video explaining some principles of a Gamma Stirling engine

The parts of a heat engine

Here is a short description of the parts of a Stirling engine. To help you better understand what goes into a DIY Stirling engine.

The Hot side heat exchanger

This is the hot side of the engine that the outside heat source comes into contact with. It is usually the outside wall of the expansion and compression chamber. It’s also the contact point for the cooled gas. Sometimes to collect as much heat as possible the surface area is increased using internal and or external fins. This works like a heat sink.

The Cold side heat exchanger

This is the part of the engine that the heated gas comes into contact with. It exchanges the heat in the gas to the outside air or cooling fluid. It may also have fins to add to the efficiency. Depending on the engine configuration the cold side heat exchanger can be on the opposite end of the same cylinder as the hot side heat exchanger or on another part of the engine.

The regenerator

The location of the regenerator depends on the engine configuration. Typically the regenerator is an internal heat exchanger that temporarily stores heat that would otherwise be lost between the hot and cold heat exchangers. Sometimes the displacer is made of materials that will allow it to also act as a regenerator. Regenerators are implemented in an attempt to increase efficiency.

The Displacer

This is the part of the engine that moves or displaces the gas (working fluid) from the hot heat exchanger to the cold heat exchanger.

Heat sink

Typically used on the cool side this can be as simple as the outside wall of the engine contacting the ambient air temperature. Although adding fins is more efficient. Also, a radiator can be added to involve water or a coolant.

Flywheel

Relative to the engine the flywheel is a large heavy wheel. It is mechanically connected to the piston(s) of the engine. Its job is to add to the momentum of the machine and help carry the Stirling cycle all the way through. Most heat engines use a flywheel.

The Piston

The piston is typically the same as any other piston that slides inside a cylinder. Although, there some Stirling engine designs that use a flexible membrane to act as a power piston. The piston is pushed out when the working fluid (gas) is expanded enough to exceed the outside atmospheric pressure. This action is often helped along with the use of a flywheel.

Video of a homemade DIY Stirling Engine

Ringbom engine

The Ringbom Stirling engine is a variation of the Beta Stirling. It also has two cylinders and one power piston. The power piston is located in its own cylinder that is located along side of the cylinder that houses the displacer piston. The power piston is the only piston connected to the flywheel. The displacer is not connected to the power piston or flywheel. Instead it’s free to move. The displacer piston is smoothly lifted during expansion and dropped during compression.

Free piston Stirling

The free piston Stirling engine is relatively new development. It typically follows the Beta type Stirling piston arrangement. But there is no flywheel or mechanical linkage of that type. They are more likely to be use to generate electricity or be used for refrigeration. This is because they only reciprocate. This means they are ideal for linear alternators. They are usually high pressure engines.

The thermodynamic cycle of a Stirling can be driven in reverse with the aid of an outside power source. This will cause one side to be heated and the other side to be cooled. Simply put, A Stirling engine can be a heat pump. By spinning the engine through its mechanical cycles the gas inside it is compressed and expanded, heated and cooled, respectively. Cooling with the Stirling cycle is currently used commercially for cryogenics and refrigeration.