Stirling engines have been around for a very long time. The Stirling engine (also known as the hot-air engine) was named after from the creator of the first engine, constructed in 1816. Reverend Robert Stirling, who was a minister from the Church of Scotland, developed this engine with much success. The engine ran as a quarry pump continuously for two years until the main cylinder gave in due to over-exertion and metal fatigue (Rizzo, 1995). The limitations were not in the design, but in the poor quality of metal available in that time period.
Throughout the rest of the 19th century and well into the 20th century, many attempts to improve upon the original design have been made. Until recently, all of the Stirling engines that have been produced required a large temperature difference to function. A new type of engine is quickly becoming popular. This new breed of engine is known as the Low Temperature Differential Stirling Engine (LTD for short).
The first person to start the low temperature movement was Professor Ivo Kolin of the university of Zagreb in Croatia (Kolin, 1983). In early 1983, Kolin demonstrated his new creation to the public. The exhibit was an engine that could run on the heat of boiling water. This remarkable engine was capable of running on a temperature difference of 15°C (59°F) (Senft, 1996). Later, others picked up on the path that Kolin had started. Improvements to his original design have sparked the interests of many.
A confirmed principle of physics is that when gases are heated they expand in volume, and when allowed to cool, contract. The workings of a Stirling engine are relatively simple. The inherent cycles of operation are seen in every Stirling engine, even Low Temperature Differential Engines. These are: heating at a constant volume, expansion, cooling at a constant volume, and compression. This concept of the engine is noted as a closed cycle engine. This means that the air inside the engine never leaves the engine. In contrast, the engine found in an automobile, an internal combustion engine, incorporates the exchange of air quantities for the cycle.
The way that the engine is designed allows for the output shaft to rotate either clockwise, or counter-clockwise. If a source of heat is placed under the engine, the rotation will be counter-clockwise. If a cold source is placed underneath the bottom plate, then the rotation will be reversed to the clockwise direction. Because the engine is a closed cycle engine, the source of the heat or the cooling really does not matter. The driving energy sources could be a block of ice, boiling water, the sun (if concentrated), or even a pool of liquid nitrogen. The only stipulation is that the difference of temperature is great enough to overcome the friction generated by the motion of moving parts. The greater the change of temperature, the more the engine will react to the change in pressure inside the engine.