Robert Stirling ~ Clergyman and Inventor

Just over two centuries ago, Reverend Dr Robert Stirling DD (25 October 1790 – 6 June 1878), who was born at Cloag Farm, Methven, invented a simple and elegant engine that is today providing the world with an efficient solar-energy capture device, the ability to ‘effectively’ sink the pride of the US Navy and is helping to save millions of lives through its use in ultra-low temperature vaccine storing freezers. Stirling is immortalised as one of the most brilliant engineers of all time, but that was not his main occupation, he was, in fact, a minister in the Church of Scotland.

Gadget shops across the world sell models of his engines. A difference in heat is all that is required to power simple Stirling Engine models, the heat from a candle tea light, or just the steam coming from a mug of tea is sufficient.

Some fans of his amazing device even travel to Methven to see where he was born and to stay in the Cloag Farm’s holiday cottages ( From the middle of the Main Street, Methven, a turn north opposite the post office, up College Road leads to the farm, which overlooks the village.

Stirling was in 1816, a Church of Scotland Minister of the Second Charge of Laigh Kirk, Kilmarnock; and by 1824, Minister at Galston. In this area, there were hundreds of coal mines. Robert was witness to many instances of the carnage caused by early steam engines propensity to explode. Although, steam engines were vital for coal mine operation – pumping out water from mines – they were extremely dangerous and were responsible for the deaths of hundreds of miners.

Stirling remembered seeing his mother making jam and came up with the idea of making an engine that was safer. When hot jam is placed in a jar, Robert noticed that the cloth cover rose as the heated air at the top expanded. This gave him the inspiration to develop the concept of heating the air in a sealed cylinder from an external source. His invention has been described as a type of external-combustion engine. His hope was that he would eliminate the need to have any combustible source or pressure boiler within coal mines.

The Stirling Engine works by converting energy made by the expansion of a fixed amount of air or other gas within a closed (sealed) system. A difference in temperature of the air at both ends is all it requires to work. When the air is expanded by heat applied at one end, it pushes up a displacer piston in the cylinder, changing the internal volume of the air. When it reaches the top, where it is cooler, compression and de-expansion of the air makes the piston return. The top of the piston is connected via a linkage to transfer the power and drive say a flywheel, a pump or other workshop machinery. Amazingly the source of power can be either hot or cold. By applying cold to the end of the cylinder, with say ice cubes, the engine can be made to run in the opposite direction.

There are also many variations of the Stirling Engine, which are even more complex. The engine has many assets: a lack of a combustion chamber, the lack of internal heat generation and exhaust output, the ability to be driven by a clean external power source, the lack of a need to cool its cylinders as with gasoline engines, and little noise generation. The first practical 2 hp Stirling Engine was used to pump water from a quarry in Ayrshire in 1818.

Not only was Robert Stirling a world-renowned engineer, but his brother James was just as remarkable an engineer. The brothers learned their skills from their father, Michael, who was born on 18 February 1708, at Glassingall, Dunblane. He was a renowned manufacturer and inventor of a rotary threshing machine (1756-8). In addition, their descendants, Patrick, James and Matthew Stirling became legends in the railway industry throughout the world during the 1800s. The railway engine GNR 4-2-2 No. 1 (Stirling Single) by Patrick Stirling is now part of the National Collection at The National Railway Museum, York.

Subsequent development by Robert and his brother James, successfully increased power output of the Stirling Engine, enough, for example, to drive for three years all the machinery at the Dundee Iron Foundry where James worked as an engineer and manager. James Stirling (20 July 1800-1876) was also born at Cloag Farm. The Dundee Iron Foundry assisted by James Stirling built the 146,000-gallon cast-iron cistern dome water tank, engine, pump, and ball and socket pipes, for Perth’s Waterworks (established in 1832 at the corner of Marshall Place and Tay Street (today the Fergusson Gallery)). The building was designed, and the work superintended by then Rector of Perth Academy, Dr Adam Anderson.

Extensive operations were carried out at the Dundee Foundry during James Stirling’s time: castings for engines, steam boilers, textile and spinning mill machinery, railway equipment and even a steam engine for a steamship that was built by Garland & Horsburgh. This was the 100-ton steam tug, Industrien, built for William Brodie, a Scottish merchant based in Göteborg, Sweden. The Dundee Foundry built the third steam engine for the Dundee to Newtyle Railway’ Trotter, excluding the tender at a cost of £700. It was put into service on 2 March 1834. James was proud that he managed to run the Trotter up the Balbeuchly Incline (between Baldragon and Auchterhouse), 1690 yards on a rising of 1 in 25.

James Stirling further developed the second patented Stirling Hot Air Engine (1841) at the Dundee Foundry, the merit of the original idea belonging to his brother Robert. The new concept was to use compressed air in order to get more power out of the engine. The engine also used an economiser (regenerator or respirator) which reused hot air from the exhaust to increase the efficiency of his air engine – an idea which was also the invention of Robert Stirling. It is still used to this day in many industrial processes to save heat and make engines more efficient. This development may be Robert’s most important contribution to engineering.

The initial reception given to the eponymous 1816 Robert Stirling Engine was underwhelming and it was subsequently only sporadically developed. Although the engine is still used today where low to medium power is required – driving small ventilators, pumping water on farms, generating electricity, and pumping air for church organs – its demise came when it was superseded by a safer and much more powerful forms of steam engine and internal combustion engines. Stirling Engines were heavy which ruled out there use in transport applications.

Modern developments in metallurgy changed matters. In the 1950s, the Ford Motor Company looked at the possibility of Stirling Engines being used in vehicles. Since 2004, a Stirling engine has been used to generate energy from concentrated solar power. The Eurodish parabolic array in Seville, Spain, focusses the Sun’s solar energy by curved mirrors to a Stirling Engine that in turn drives an electric generator. At one time, it was the most efficient means of converting the maximum of solar energy into electricity.

In 2005, the originally diesel-electric powered, Swedish submarine HSwMS Gotland, succeeded in getting past a large US Navy battle group of ships totally undetected on multiple occasions. In a war game exercise with the $4.5 billion aircraft carrier USS Ronald Reagan and its escort fleet, the HSwMS Gotland just popped up from out of nowhere in the middle of the fleet and took multiple pictures of the carrier to prove that they had “effectively” sank it. Despite all the elaborate anti-submarine technology employed by the US fleet, the $100 million, HSwMS Gotland totally bamboozled and utterly demoralised those aboard the US ships.

Stirling Engines are used to ‘silently’ power diesel-electric submarines in use by the military navies of Sweden, India, Singapore, Japan, China, and many other countries. The Swedish (Saab) Kockums-built Stirling Engine Air Independent Propulsion (AIP) system is used to power an electrical generator for up to 20 knots propulsion and/or charging batteries. Stirling Engine powered submarines can run silent underwater for up to two weeks without the need to surface or snorkel, quietly at perhaps only a slow 5-10 knots, long enough however, to sneak right up to an enemy’s coastline without being detected. These submarines are regarded as a cheap alternative to expensive nuclear fission powered submarines.

Stirling Engine powered submarines can use the surrounding sea water as a heat sink to increase efficiency. Modern Swedish stealth submarines use GHOST (Genuine HOlistic STealth) technology. They are designed shape-wise to minimise their radar and passive sonar signature. Gotland class submarines are fitted with 27 electromagnets designed to counteract its magnetic signature to Magnetic Anomaly Detectors (MAD). Its hull has sonar-resistant coatings, and the tower is made of radar-absorbent materials. Interior machinery is coated with rubber acoustic-deadening buffers to reduce sonar detectability.

Stirling Engines have been retrofitted to many existing submarines throughout the world. China operates two types of diesel submarines powered by Stirling engines. The latest Stirling Engine powered Chinese submarine prototype has been claimed to generate 320 kilowatts power output to its electrical generators and the Type 032 Qing-class submarine is capable of submerging for 30 days. It is reported to be the world’s largest operational diesel submarine, with seven Vertical Launch System cells capable of launching cruise and ballistic missiles. Three Stirling Engines are being used as a main power source in the latest Swedish Blekinge-class (A26) submarines.

Since October 2015, the Kilopower Reactor using Stirling Technology (KRUSTY) system has been considered by NASA as a power source for interstellar travel and long-duration stays on planets. Some private enthusiasts have built Stirling Engine powered cars, boats and a miniature Stirling Engine was prototyped as a computer chip cooler, using the heat from the chips to drive a cooling fan.

Stirling Engines are used in portable refrigerators and in cryogenics. They are helping to save lives during the current pandemic. They are a component that makes the new class of ultra-low temperature freezer s (ULT freezers) reliable, stable and efficient. ULT freezers are used to store vaccines and medical samples. The Stirling Engine that has been engineered for this use is hermetically sealed, it requires no oil or lubricants and is nearly maintenance free.

A company in Gloucestershire is currently developing a new type of Stirling Engine based heat pump and refrigeration system. It is claimed to improve the efficiency of heat pumps by at least 50 percent and eliminate the need to use gasses which have a high global warming potential. The remarkable Near Isothermal Stirling heat pump uses Fluid Mechanics patented near isothermal compression and expansion technology.

On 3 October 2014, the Reverend Robert Stirling DD was posthumously inducted into the Scottish Engineering Hall of Fame. Robert Stirling’s air engine of 1827 is in the Hunterian Museum, University of Glasgow. Stirling also invented several optical and scientific instruments and was awarded an honorary degree by the University of St Andrews in 1840 in recognition of his scholarly and scientific attainments.

Stirling had seven children and a long ministerial career of 63 years. He was known as the Father of the Church of Scotland when he died in Galston on 6 June 1876. A gifted speaker, Stirling was truly beloved by his flock. He is buried in Galston Cemetery where a new gravestone was erected in December 2014 by public subscription and was rededicated in May 2015.


The Dundee Foundry also built 3 locomotives (1838 – 1840) for the Arbroath and Forfar railway, the Victoria, Britannia and the Caledonia. At the time, no less than three establishments in Dundee (the only place in Scotland) were making railway locomotives. The engines built in Dundee by James and George Carmichael, the Earl of Airlie and the Lord Whamcliffe may have been run the first regular scheduled passenger service in Scotland, in September 1833. (Robert Stephenson’s locomotive, the 29 mph Rocket at the Rainhill Trials was a few years earlier, in 1829 and three years after James Stirling died was the Tay Rail bridge disaster, 28 December 1879 at 19.16pm)

Patrick Stirling (1820-1895), born in Kilmarnock, was the son of the Reverend Robert Stirling. Patrick was Chief Mechanical Engineer of the Great Northern Railway and was well-liked by the railwaymen of the GNR. For his 70th birthday, they erected a fountain in his honour in Doncaster and 3,000 GNR railwaymen braved pouring rain to accompany his funeral procession when he died.

Patrick’s brother, James Stirling was also a locomotive engineer, and Patrick’s son, Matthew Stirling was the Locomotive Superintendent (and CME) for the Hull & Barnsley Railway. Another son of Patrick played for Doncaster Rovers and was Mayor of Doncaster. Other sons of Robert Stirling, William, born 1822 and Robert, born 1824 were engineers with the Lima to Calao railway in Peru (the oldest in South America). James, the youngest son, born in 1835, was the Locomotive Superintendent with the Glasgow and South-Western Railway Company and later, the South-Eastern Railway, he introduced many improvements to locomotive power. An attempt to shoot James was made on 29 June 1885. George Hopkins had been a footplateman and following an accident was put in charge of a stationery boiler for the forge at Ashford Works. Due to this accident and a subsequent loss of earnings, he blamed the railway company and tried to murder James Stirling.

Reverend Dr Robert Stirling DD

Eurodish system schematic

Simple gadget type – low temperature – Stirling Engine

Dundee Foundry Victoria Locomotive 2-2-2 – Arbroath and Forfar Railway- source Wiki Commons – Whishaw, F, (1840) The Railways of Great Britain and Ireland: Practically described and illustrated London: Simpkin, Marshall and Co.

GNR 4-2-2 No. 1 (Stirling Single) by Patrick Stirling. It is now part of the National Collection at The National Railway Museum in York (Image Credit)

Public Domain Illustration of Robert Stirling’s 1816 Patent Application

Robert Stirling’s air engine, 1827, Hunterian Museum, Glasgow


NASA and NNSA engineers lower the wall of the vacuum chamber around the KRUSTY system. The vacuum chamber is later evacuated to simulate the conditions of space when KRUSTY operates. Image NASA

A conceptual drawing of four Kilopower reactors on Mars’ surface.  Image NASA