Peter Lynn ARCHIVE
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. 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 airplane 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 thermodynamic which sets a limit (the Carnot cycle) for heat engine 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. Engines using the Carnot cycle have not yet proven to be practicable (Rudolf Diesel mistakenly thought that he could achieve this and caused much embarrassment to himself and his licensees by claims to this effect made in his patent). Stirling engines (and their open cycle Ericsson cousins) are the only practical heat engines that are theoretically capable of Carnot efficiency. They do this indirectly by using heat exchangers to store heat energy remaining in the working fluid after expansion and returning it after the cooling stage. Stirling engines operating 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 sewing machines, dentist drills, and domestic fans. Unlike steam engines, they did not require boilers, but low power and high cost limited their uses. An 1890’s 8” Ericsson Stirling engine weighed 350kgm, developed 200watts (1/4hp) and could cost more than a year’s average income. Production of Stirling engines tailed off as reliable and inexpensive internal combustion engines became available in the early 20th century, and as the electricity grid expanded.
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 tried for submarines, solar electricity generation, artificial hearts, and combined heat and power (CHP) units- in which small Stirling engines generate electricity for household use with waste heat being applied to water heating (eg Whispergens).
Theory Versus Practice: With the exception of submarines (because combustion at very high pressures can be used), and CHP's, which haven’t yet proved economic 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 a long-term mainstream market except as models and toys.
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. Even very expensive designs using hydrogen or helium working fluid pressurised to more than 100 atmospheres are typically less than 25% 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%.
Cryocoolers: The Stirling cycle is in good economic health however: Refrigerators that operate on the Stirling cycle are the preferred solution for many applications requiring very low temperatures- liquifying hydrogen for example.
Peter Lynn, Ashburton, New Zealand, 2025
