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What is a Stirling engine?

A Stirling engine is a special type of 'heat engine' that converts small temperature differences into motion, and they are sometimes called 'hot-air' or 'caloric' engines.


How does an Aluminium or stainless steel low temperature Stirling engine work?

All Stirling engines are powered by a difference in temperature. In our aluminium and stainless steel low temperature engines this difference is normally achieved by warming the bottom plate to above room temperature and allowing the top plate to stay at room temperature.

In operation the engine cyclically heats and cools the air inside. This process is shown in the simplified cut-away diagrams below (It helps to remember that the large blue displacer disk just moves (displaces) the air from top to bottom and back again, and it is the small black piston that actually drives the flywheel).

Figure 1: With the large blue displacer disk at the top, most of the air inside the main chamber is at the bottom, where it is heated by the warm bottom plate. As it warms, it expands, and as it expands it pushes the small black piston upwards, which drives the flywheel round.

Figure 2: As the flywheel turns, the large blue displacer disk is moved (by means of a crank and connecting rod) to the bottom of the chamber. As the displacer moves down, the air that was at the bottom of the main chamber rushes around the outside of the displacer to the top of the chamber. So with the displacer at the bottom, most of the air is at the top, where it is cooled by the cool top plate. As it cools, it contracts, which pulls the small black piston downwards, and drives the flywheel around some more, and so the cycle continues.

figure 1 figure 2


How does an acrylic low temperature Stirling engine work?

Our acrylic low temperature engines work on exactly the same principle as our aluminium as stainless steel engines. They cyclically heat and cool (expand and contract) the air inside the main chamber, which drives a piston and in turn the flywheel. But, they differ slightly in how they use the heat supplied. A special property of the acrylic sheet we use in our acrylic engine is that is is transparent to infra-red. This means that the heat source (sunlight) can directly heat the air inside the main chamber without having to first heat the plate. On the cool side, the heat can directly radiate away into without first having to pass through the cool plate.

This might seem obvious, that a transparent plate is transparent to infra-red, but even coloured acrylic is transparent to infra-red. This means that a low temperature engine made from black, red, green (or any other colour) acrylic works just as efficiently as a transparent one.

Also, our transparent solar low temperature engines will run just as happily in the palm of your hand as they will by sunlight. The heat from your hand goes straight through the acrylic plate and directly heats the air inside the main chamber. On the top side, the heat in the air is able to directly radiate away from the engine, without having to pass through the top plate.


Reversible

All Stirling engines are fully reversible. In the Kontax Low Temperature range this means that instead of warming the bottom plate and cooling the top plate to get clockwise rotation, you can cool the bottom plate and warm the top plate to get anti-clockwise rotation.

The simplest way to get forwards (clockwise) rotation is to hold the engine in the palm of your hand, allow the bottom plate to warm up to the temparature of your hand and gently spin the flywheel clockwise. If your hand is warm enough (and the ambient room temperature cool enough) your engine should continue to run under its own power (as long as you keep it on your hand). If you have a very cold hand and the ambient room temperature is very warm there might not be enough temperature difference between the top and bottom plates for the engine to run.

The simplest way to get backwards (anti-clockwise) rotation is to place the engine over a bowl or saucer of ice, allow the bottom plate to be cooled to below room temperature and gently spin the flywheel anti-clockwise. As with conventional hand running your engine should continue to run inder its own power, for as long as the ice takes to melt and warm up to room temperature. Ice running is very reliable, because there is usually a greater temperature difference between ice and the ambient room temperature than between hand heat and ambient room temperature.

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