Jun 5, 2013

What is a Stirling Engine and How Does it Work?

Model Stirling engines are fascinating and fun to build...but they can be difficult to understand. If you're interested in model Stirling engines but aren't sure exactly what they are (or why they're so great), this article will answer some of your questions.

The Stirling engine is named for its inventor, Robert Stirling. It is a closed-cycle regenerative heat engine. It works by cyclically compressing and expanding air or other gas (known as the working fluid) at different temperature levels. The heat energy produced makes the engine run.

A Stirling engine is an external combustion engine, like a steam engine. This means that all of the engine's heat goes in and out through the wall of the engine, from an external heat source. But unlike a steam engine, which uses water in both liquid and gas form, Stirling engines use only the gas forms of liquids such as hydrogen, helium or oxygen. This working fluid is compressed, heated, expanded, and then cooled again in an ongoing cycle.

Stirling engines contain a fixed amount of working fluid. The engine is sealed, so none of the gas leaves the engine, and none enters from outside. When you have a fixed amount of gas in a fixed amount of space, raising the temperature of the gas will increase its pressure. If you compress the gas, its temperature will go up.

The gas is moved back and forth between hot and cold heat exchangers. To put it simply, a basic Stirling engine works like this:

1. A hot heat exchanger (a heated cylinder) is exposed to an external heat source, which then heats the gas in that cylinder and causes the pressure of the gas to increase. This increased pressure makes the piston in the heated cylinder move down and do work.

2. The piston on the other side moves up as the other moves down, and this then pushes the heated gas into the cold heat exchanger, or cold cylinder. The cold cylinder is cooled by the ambient temperature of the surrounding environment or by an external source of cold. As the gas cools, its pressure is lowered and it becomes easier to compress.

3. The piston in the cold cylinder then moves down and compresses the cooled gas. Any heat generated by the compression is removed by the cooling source (known as the heat sink).

4. The piston in the heating cylinder moves up, and gas is forced back in to the heated cylinder. As it heats up the pressure increases, the piston moves down, and the cycle repeats over and over.

Usually a regenerator is placed in between the heated area and the cooling area. This acts as a heat store and increases the engines efficiency by retaining and recycling the heat that passes through the engine, rather than allowing it to diminish.

This is a basic overview of how Stirling engines work. There are many different types and models, but they all work according to the same theory. Model Stirling engines can be very simple, or more complex...but as long as you have a heat source, a hot air and a cool air exchanger and a heat sink and of course, all the parts, they will work.


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