About Stirling Engines

Beta Type Stirling Engine.
Beta Type Stirling Engine.
Operational Principle: Stirling cycle engines operate on a closed cycle in which a quantity of gas (called the working fluid), is alternatively heated and cooled. Mechanical power is generated by the heated gas expanding against a piston. It is then moved to a cool space and contracts before being returned through the heater to repeat the cycle. Gas control is entirely by volume changes, not by valves. The traditional working fluid is air but nitrogen is less corrosive, and for high performance; helium is better, hydrogen is best.
For high specific outputs (power for size), the working fluid is pressurised- sometimes to more than 100bar.

Early History: Engines with Stirling-like features were built from around 1800 (including by Aeroplane pioneer Sir George Cayley in 1804), but their invention is generally credited to the Rev. Robert Stirling, after whom they are named. Stirling, a Scottish Presbyterian minister, developed the first practical valveless closed cycle engine and patented the regenerative heat exchanger (a key feature) in 1816.

External Combustion: Heat is applied externally to Stirling cycle engines (like for steam engines), rather than internally (as for the diesel or petrol engine and most gas turbines). They can therefore use the combustion of any solid, liquid or gaseous fuel- or a high temperature source such as solar, nuclear, or geothermal.

High Efficiency. In 1840, French scientist Sadi Carnot identified the second law of thermodynamics; which sets an upper limit for heat engines efficiency that cannot be exceeded no matter how perfectly an engine is designed or constructed. It is a function only of the maximum and minimum temperatures in a thermodynamic cycle. But engines using the Carnot cycle cannot be built (although Rudolf Diesel mistakenly thought that he could, and caused much embarrassment for himself and his licensees by claims to this effect made in his patent). Stirling engines are the only heat engines that are theoretically capable of Carnot efficiency. They do this indirectly by using Stirling's regenerative heat exchanger to store heat energy remaining in the working fluid after expansion and returning it after the "cooling" stage. Stirling engines that operate on a cycle reasonably similar to the theoretical Stirling cycle can be built.

19th Century Popularity. During the 19th century, Stirling cycle air engines (often called hot air engines) were relatively common, especially for water pumping, but also for appliances like sewing machines, dentist drills, and domestic fans. Unlike steam engines, they did not require boilers, but low power (and high cost) limited their uses. The popular Rider-Ericsson (sold from 1876 to 1920), weighs 400kgm, develops 250watts (1/3hp) in the 5" size and cost 2 years average NZ income. Production of Stirling air engines ceased when reliable and inexpensive internal combustion engines became generally available towards the end of the 19th century.

20th Century revivals: Driven by the multi-fuel advantages of the Stirling cycle and its high potential efficiency, there were many attempts to revive the Stirling engine during the 20th century. Some were aimed at mainstream applications such as cars, but Stirling engines were also designed for submarines, solar electricity generation, artificial hearts, and domestic combined heat and power (CHP) units- in which small Stirling engines generate electricity for household use with waste heat being applied to water heating.

Theory Versus Practice: With the possible exception of CHP's- which remain a promising application but are, as yet, uneconomic without "green" subsidies.- the reality after 200 years of striving by countless engineers and huge development expenditure is that Stirling engines have yet to find any long term mainstream market.

Why is this? The essential problem is that their theoretical efficiency has so far proven to be unattainable without exceeding a price that makes the engines impracticable. Except for a few designs produced by major research laboratories at enormous expense using hydrogen or helium working fluid pressurised to more than 100 atmospheres, most well designed modern Stirling engines are typically less than 15% efficient and have low specific output (power for size). They compare unfavourably with compact petrol engines which approach 30% efficiency and diesels with more than 40%.

Peter Lynn, Ashburton, New Zealand, March '12

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