STIRLING’S ENGINE

 STIRLING’S ENGINE

50mm bore, 33.4mm stroke, 65 cc swept volume, gamma

Born in Perthshire, Scotland in 1790, Robert Stirling studied physics and engineering at Edinburgh University from 1805 to 1808 before studying divinity at Glasgow and then back in Edinburgh.  He was ordained as a Presbyterian minister in 1816 and moved to Kilmarnock.  In 1824 he was appointed minister for Galston in Ayrshire where he remained until his death in 1878- having been honoured with the title “Father of the Church of Scotland”.

While studying divinity he continued to apply himself to engineering, specifically to heat engines- prime movers as they were known at that time.  On appointment to Galston, he immediately set himself up with a workshop at the “Old Manse”. He’d had access to a workshop even while in Kilmarnock. 

Sometime before 1816 he developed and proved the concept of closed cycle hot air engines, the first to do so as far as is known.  Such engines have since become ubiquitously known as “Stirling Cycle Engines” or just plain “Stirling” engines.

In closed cycle engines the same body of working fluid (air in Stirling’s era) is repeatedly heated and cooled.  Open cycle engines and furnace gas engines, which were being developed as alternatives to steam engines at that time, bought in a fresh charge of air every stroke- and expelled it after expansion.

In 1816 Robert Stirling patented the “regenerator”, a heat sink that accepts heat from the working fluid (air in hot air engines) after it has been expanded to do mechanical work.  The regenerator then returns that heat to the working fluid after it has been cooled and compressed before going back to the heater at the beginning of the next cycle.  Regenerators improve efficiency- the amount of work that can be extracted from a given temperature difference.

In 1823 he built a Stirling engine to replace the steam engine in “Highland Lad”, a 52’ Clyde riverboat, and later, jointly with his younger brother James (an engineer) designed and built many improved Stirling engines including one for a Dundee foundry that used pressurised air as its working fluid for greater power.    

During the 19th century, closed cycle engines gradually proved their superiority.  Furnace gas engines lost favour because their fly ash abraded cylinders and pistons- even John Ericsson had abandoned open cycle air engines in favour of Stirling’s invention by 1860. 

Their golden age was during the late 19th century with many manufacturers offering hot air engines for various applications- especially water pumping.  In the 20th century, Stirling engines were developed further by using pressurised hydrogen and helium as the working fluid.  In this form they can be powerful and efficient. 

In about 1818 Stirling built a demonstration engine and presented it to Edinburgh University.  It remained there until 1975 when it moved to the Edinburgh Museum and is the earliest extant Stirling engine.  This is a faithful replica of that engine.

It may be noticed that it lacks a cooling system. This is because Robert Stirling did not realise until a year or two later that, for continual operation, a temperature difference needs to be maintained.  Until then he had thought it was only necessary to supply heat.

It runs great: will start a minute or two after lighting the methylated spirits burner.  It takes  20 minutes or more before the cold end gets hot and there isn’t sufficient temperature excursion in the enclosed air to make it run (no water jacket).

Peter Lynn for the Mahan Heritage Centre, June 2025