Stirling engines are reciprocating engines which produce mechanical energy from the expansion and contraction of a gas within the cylinders of the device. The video above is an example of a low temperature sterling engine from shinyshack.com and shows perfectly how a simple stirling works. The main feature which sets it apart from a combustion engine is that an external heat source is used to heat the gas rather than an internal combustion of fuel. The heat source can be anything from a burning flame to the heat produced by rotting plant matter, the video above used a hot liquid e.g. coffee. The theoretical efficiency of the Stirling engine is almost equal to its maximum theoretical efficiency; this is known as the Carnot cycle efficiency and is what makes the Stirling engine exceptional. However in reality this is limited by the properties of the gasses and the materials used to make it which are not perfect. As the combustion process does not require an explosion the Stirling engine is predominantly silent except for the sound of the mechanical parts moving.

Stirling engines are used mainly to produce electrical power. A company called Stirling Engine Systems have plans to supply their solar unit product to two different solar energy power stations with the intention of changing the power output from the total current output of 800 MW to a total output of 1750 MW. The solar unit is a parabolic mirror which is used to heat the hot cylinder of the engine. An obvious disadvantage here is that you would need a good level of sunlight for this to work, a dessert would be perfect. Other applications include smaller generators which run on gasses such as natural gas. These generators could be used as back up generators or on building sites with no accessible power. Another application for the Stirling engine is in submarines. It relies on the fact that the Stirling engine is almost silent as opposed to combustion engines which produce a lot of noise. The French have a research submarine called saga which uses a Stirling engine.

The first idea of an engine which used hot air to function was devised by Sir George Clay around 1807 but it wasn’t until 1816 that a hot air engine capable of work was created by Reverend Robert Stirling. The reverend created the Stirling engine and most importantly created the economiser or regenerator which is the reason his heat engine proved more successful than his predecessors. He patented both the Stirling engine and the economiser in 1816.

The image on the left shows the illustration to Robert Stirlings Engine patent in 1816.

Robert Stirling was concerned about the dangers of the steam engines of the time as they proved dangerous due to the high pressure's involved coupled with poor materials and engineering. The material used in both the Stirling engines and the steam engines was a soft iron as steel had not been invented; this was the main reason for the steam engine boiler explosions. The reverend wished to bring a degree of safety to the workplace and his Stirling engine was capable of doing this. When the Stirling engine encountered a problem it could possibly break part of itself but is unlikely to injure a nearby operator as there is no working pressure like there is in a steam engine.

Robert Stirling's brother James Stirling used a two cylinder dual heat regenerator engine to power the Dundee foundry until material failure caused the machine to be replaced with a steam engine. The Stirling Engines created by James Stirling usually lasted for around twenty years before any major repair or replacement was required.

The Stirling engines were commonly used until the 1920's where the development of internal combustion engines and the electric motor made them obsolete. This was because at the time these machines could produce more power than the Stirling engine also there was less detail required to make a combustion engine as the tolerances of the Stirling engine are much finer. This combination of cheaper manufacturing costs and greater power caused the sterling engine to disappear from the commercial market.

By the 1930 the Stirling engine was mostly forgotten until the Philips Company which wished to expand sales of they're radios to area's where there was no mains electricity decided to produce a low power generator which would aid sales. They set a group of engineers in the company research lab in Eindhoven to work on the generator. This group of engineers decided that the Stirling engine had real potential as it was both quiet and didn’t interfere with the radio signal and also that it could be run on common lamp oil. They produced an experimental engine with a bore and stroke of 30x25 mm which created 16 watts of shaft power. As a result a development program was started which continued through world war two and by the late 1940's they had an engine called the type 10 which had a bore and stroke of 55x27 mm which was then used to power a generator. The result was a portable generator capable of 200 watts of electrical power. Production was started in 1951 but it soon became clear that it could not be made at a marketable price and with the invention of the transistor the original need for the generator was gone.

Philips continued to develop experimental Stirling engines for various projects until the late 1970s but only had commercial success with the reversed sterling engine cryocooler.

The Stirling engines ability to run on any form of fuel might be its biggest attraction to modern engineers as they try to produce some form of propulsion that does not rely on petroleum product. The relative silence of the Stirling engine could also be a big factor for designers as people look for more luxury in their cars. This would also so be of interest to aeronautical engineers as a silent plane would add increased luxury for the passengers and the long servicing intervals and the fact that it does not require a lot of oil for lubrication may be of interest to airline companies who wish to reduce flying costs. Also in relation to use in an airplane, a Stirling engine is the only engine which improves performance in the cold associated with high altitude flying.

There are also many disadvantages to the Stirling engine. The main one is that they are expensive to produce due to the need for special materials and skilled workers to produce in clean environment. Stirling engines are not in great demand as a result no there are no high output production processes. Another problem that the sterling engine has is that it takes time to heat the heating cylinder sufficiently to start the engine which could cause problems with automobile applications as the customer would want the vehicle to start as soon as they turned the key. Also the Stirling engine needs to be quite big for the amount of power it produces.

At the present time the combustion engine will continue to be the dominate form of producing energy as it can still be operated at a reasonable cost. However if the production of the Stirling engine was on the scale and perfection of modern combustion engines it could possibly be produced at a reasonable price. This may become a reality as fuel costs rise and a need for cheaper fuel sources becomes more important.

The advances in material technology might someday produce a perfect Stirling engine with a perfect generator which would come close to a perfect Carnot cycle but for now the combustion engine shall reign supreme.

This piece comes from a report I created during my third year at university, I go into a bit more detail as to how it works at my site so for a bit more technical details with regards to the working of the engine have a look at Amazing Men and their Magical Machines