Operation Simply Explained - I Hope :)

The Stirling/Carnot principal is very simple. It depends on thermodynamics and the behavior of gases. Fortunately you do not need to understand physics and chemistry to understand how the principal works.

The principal revolves around expansion (increase of volume) and contraction (decrease of volume) of gas. In all Stirling/Carnot engines the working medium is a compressible gas. Originally air was used, but today gases such as hydrogen and helium are used which give a better result.

The typical engine has a hot area and a cold(er) area. Temperature differential is the key. When a gas is heated it expands and takes up more volume. When that same gas is then cooled it contracts and takes up less volume. This is employed to do work. The expanding hot gas is used to push a piston producing power. The contraction pulls the piston (more correctly, usually another piston) and completes the cycle.

Stirling Cycle Operation - Step By Step

For this demonstration, I've drawn up some simple diagrams. The engine I'm using for the drawings is loosely based on the demonstrator from Baileycraft which I purchased a few years ago. The working medium gas in this machine is air. The engine will run from about 50RPM up to around 1500 RPM. The speed can be adjusted by "breaking" the cycle by allowing some of the working medium gas to escape and re-enter during the operation through a valve. The engine is fueled using a small alcohol burner and does not self start. On a single charge of alcohol the engine will run for about 20 minutes and is almost totally silent. The only noise comes from the motion gear. The engine is not well balanced and vibrates at high speed wasting much of its energy. The displacer piston and power piston are joined by con rods to a single eccentric flywheel. The displacer con rod and the power con rod are quartered 90 degrees to each other. You should note that there are many different kinds of Stirling engine all implementing the same principal using differing mechanical means.

Begin Power Stroke

When the cycle is about to begin, the displacer is at TDC and the power piston is at BDC. The displacer is the larger of the two pistons in the drawing. Note that the displacer is a very loose fit piston which does not actually touch the walls of its cylinder. The power piston is a tight fit, with rings or some other form of sealing the piston in the cylinder. The displacer is designed to allow some gas past the piston. Note: TDC is Top Dead Centre and BDC is Bottom Dead Centre.

The power of expansion

As the heat is applied, its energy transfers to the working medium gas causing it to expand to much larger than its original volume. The expanding hot gas starts to move into the cold side compressing the cold gas already there thus forcing the power piston towards TDC. The small amount of gas in the cold side heats both from the influx of hot gas and the compression it undergoes. The hot side is known as the heater/regenerator.

A low pressure zone forms

As the expansion continues, a low pressure zone forms behind the displacer piston. The power piston is still being forced towards TDC as the displacer continues towards BDC. This low pressure zone assists in the return stroke later in the cycle.

The mid point

At this point the gas is at its greatest expansion and compression. The pistons are moving towards their goal and the power stroke is half way through.

Equilibrium - Power Stroke Ends

At this moment the displacer has reached BDC and the power piston has reached TDC. If it was not for the stored energy in the fly-wheel and the low pressure zone which formed behind the displacer the engine would stop here.

Return Stroke begins

The rapidly cooling gas in the cold side and in the transfer pipe now contract drawing the power piston back towards BDC. The flywheel pushes the displacer and the low pressure zone formed behind it sucks it towards TDC. This helps "suck" the now cold gas back into the heater/regenerator.

Equilibrium - Return Stroke Ends

The power piston forces all the cold gas back into the heater/regenerator where it again absorbs the applied heat energy beginning the cycle over again. As with the other point of equilibrium, if it was not for the energy stored in the flywheel the engine would stop at this moment.

Operating a Stirling Engine

Operating a stirling engine is very simple, provide a heat source, such as an oil, coal, or wood fire (or anything which burns or even solar). Allow the hot side to come up to working temperature, then kick over the fly wheel(s). Done the engine will keep going as long as heat is available.

Thanks to Dan Morris for pointing out a small, but critical error in the article that may have confused people.

Thanks to Dennis Hawkins for cleaning up the images that I was too lazy to deal with.

Last modified Sunday, 20-Jul-2003 15:28:00 BST

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