Robert Stirling

Scottish clergyman

• Born: October 25, 1790
• Birthplace: Methven, Perthshire, Scotland
• Died: June 6, 1878
• Place of death: Galston, Ayrshire, Scotland

A Church of Scotland minister, Stirling invented the Stirling hot-air engine, which he and his brother, civil engineer James Stirling, refined throughout their lives. Although never widely adopted, the Stirling engine is based on remarkable thermodynamic principles that promise great efficiency and energy conservation.

Primary field: Mechanical engineering

Primary invention: Stirling engine

Early Life

Robert Stirling was born in 1790 at Cloag Farm in the parish of Methven, Perthshire, in central Scotland. His father, Patrick Stirling, was a farmer. His mother was Agnis Stirling. The couple had eight children. Robert’s grandfather, Michael Stirling, had invented a threshing machine in 1758. Robert attended Edinburgh University from 1805 to 1808, studying a wide range of classical subjects. In November, 1809, he enrolled as a divinity student at Glasgow University. On November 15, 1814, he returned to Edinburgh to continue his divinity studies. The following year, he was licensed as a minister in the Presbyterian Established Church of Scotland. On September 19, 1816, he was ordained as a minister in second charge of the Laigh Kirk parish in the prosperous town of Kilmarnock.

On July 10, Stirling married Jane Rankin, daughter of the local wine merchant. Their first child, Patrick, was born on June 29, 1820. Patrick would become an inventor and engineer in his own right. Robert and Jane would go on to have four additional sons (three of whom became engineers, and one of whom became a minister) and two daughters.

Life’s Work

Stirling came from a line of industrious farmers who diligently worked to improve their mechanical implements. Stirling apparently showed mechanical aptitude from youth and was carrying out practical experiments while studying for the ministry. On September 27, 1816, only eight days after his ordination, Stirling applied for his first patent, for a fuel-efficient air engine. This was the first version of what would become the famous Stirling hot-air engine. Stirling was motivated to develop it both to save fuel and to design a safe alternative to boiler engines, which were prone to explosions. It is also possible that he was inspired by the investigations into the transmission of heat by Edinburgh professor of mathematics John Leslie. (It is unclear how much Stirling knew of earlier proposals to obtain power from heated air such as the 1794 heated-air engine of Thomas Mead and the 1807 open-cycle engine of George Cayley.)

Stirling’s engine represented an improved method of storing and exchanging heat. It essentially worked by the alternative expansion and contraction of enclosed air acting upon a piston. To emphasize its fuel efficiency, Stirling called the central component of his engine the “economiser,” although it would soon take the name of a “regenerator.” It was the economiser that stored and released heat as the air circulated. With the promising start of his engine design, Stirling was able to acquire laboratory facilities from Thomas Morton, an inventor and the town industrialist. It would be a very productive partnership. In his Morton workshop, Stirling built numerous optical and scientific instruments, including the object glass of telescopes, which he ingeniously constructed from the bottoms of tumbler glasses. In 1818, he constructed an engine based on his economiser design to pump water from a stone quarry. The engine worked well, generating about two horsepower, until the air vessel became overheated and was crushed by the pressure of the heated air. Morton and Stirling later founded a free school in Kilmarnock to educate orphans and neglected children.

In 1823, Stirling took up a post as minister of the Galston parish. Although Stirling was involved in a dispute over the power of the established Church of Scotland over the local presbytery, his work on improving his engine designs continued unabated. In 1824, Robert and his brother James, a mechanical engineer, tried to improve the efficiency of the economiser with the use of high-pressure air by using metal sheets to subdivide the air into multiple layers. They received a patent for this improved engine in 1827. Although the patent application listed four improvements to Robert’s original design, the new hot-air engine was not a success. James built an engine at the local ironworks, but it failed to produce the more efficient use of heat that was expected.

In 1840, Robert Stirling received an honorary doctorate in divinity degree from the University of St. Andrews for his erudition in classical languages and devoted ministry. In that same year, the brothers received another patent, for a new version of their engine. The patent application again listed four improvements, including forming materials into rods, employing these rods for receiving and imparting heat, passing the air through extensive systems of surfaces, and applying cupped leathered collars around the piston rods. James Stirling built two new engines based on these improvements for the iron foundry he managed in Dundee. These engines operated on coal and proved more successful, one running continuously for a period of two years and nine months until an air vessel failed. The new engines were able to generate about forty-five horsepower. However, the cylinders were prone to burn out quickly, a problem the Stirling brothers never solved. Robert blamed the failure of the engines on the imperfections of the materials used in their construction, an unlikely explanation. He continued to express hope that his engine would become the mainstay in production of the new Bessemer iron.

James Thomson, the brother of Lord Kelvin, claimed that Robert Stirling did not fully understand the principles of his own engine. Nevertheless, the regenerative principles demonstrated in Stirling’s engine were successfully used by the inventor John Ericsson in his 1833 patented caloric engine; by Julius Jeffreys in his patented 1836 medical respirator; by industrialist brothers William and Frederick Siemens in their 1856 patented regenerative steam engine, condenser, and furnaces; by Edward Cooper in his 1857 patented hot-blast furnaces; and by F. T. Botta and G. B. Normand in their 1855 and 1856 patented chimney regenerators for use in marine boilers. For the remainder of their lives, Robert and James Stirling took out no new patents on the hot-air engine and apparently worked on it only sporadically. In 1878, after fifty-five years as minister of the Galston parish and continual refining of his hot-air engine, Stirling died on June 6.

Impact

Robert Stirling lived the life of a distinguished country parson, becoming learned in biblical languages, arranging for charitable enterprises, succoring townspeople during a cholera epidemic, and pastoring his Church of Scotland flocks in the parishes of Laigh Kirk and Galston. During this life of religious duty, he and his brother James were working ambitiously on perfecting the design of the hot-air engine that had come to Robert as a youth.

Robert Stirling’s idea for a hot-air engine had to some extent a charitable origin. He wanted to design an engine less susceptible to dangerous accidental explosions than the steam engine. With his mechanical mind, he focused on a design that centered on the efficient exchange of heat. How much of this design flowed from his understanding of thermodynamics is not certain, but the Stirling engine would in fact take advantage of scientific principles that were explained by the scientist Nicolas Léonard Sadi Carnot, who introduced the theory of the thermodynamic cycle. On April 21, 1847, William Thomson, better known as Lord Kelvin, the famed scientist who helped develop the laws of thermodynamics, delivered his first address to the Glasgow Philosophical Society on the subject of “Stirling’s Air Engine.” Thomson would also use the Stirling engine for demonstration purposes in his lectures at Edinburgh University.

The Stirling engine has intrigued engineers ever since, because of its theoretical possibilities for enormous efficiency and suggestion of a perpetual motion machine. However, no working Stirling engine has yet to come close to achieving this ideal. Although its use in industry remains limited, there are numerous researchers and Stirling engine societies convinced that its fundamental principles will one day provide a great breakthrough in the ability to generate clean, safe, and self-perpetuating power. Ingenious uses of these principles have been made over the last century. An engineer from Tyneside, England, John Malone, followed the Stirling model to invent a heat engine that used liquid at the critical point. Engineers at the Los Alamos National Laboratory have been experimenting with a Stirling engine that is powered by sound waves, and engineers from the National Aeronautics and Space Administration (NASA) have studied the use of Stirling engines with solar energy. Thus far, however, the Stirling hot-air engine remains more promise than reality.

Bibliography

Darlington, Ray, and Keith Strong. Stirling and Hot-Air Engines. Marlborough, England: Crowood Press, 2005. Darlington, an engineer, describes the history, principles, and designs of a wide variety of Stirling and hot-air engines. Many photographs and blueprints.

Organ, Allan. The Air Engine: Stirling Cycle Power for a Sustainable Future. Boca Raton, Fla.: CRC Press, 2007. Investigates modern Stirling engines as core components in new combined heat and power technology.

Sier, Robert. Hot Air Caloric and Stirling Engines. Chemsford, England: L. A. Mair, 2000. Sier is on firm ground with this encyclopedic and technical history of Stirling engines. The first of three volumes.

‗‗‗‗‗‗‗. John Fox Jennens Malone: The Liquid Stirling Engine. Chelmsford, England: L. A. Mair, 2008. Also by Sier, a short biography of English engineer John Malone, who invented a heat engine on the principles of the Stirling hot-air engine, but using liquid rather than air.

‗‗‗‗‗‗‗. Rev. Robert Stirling D.D.: A Biography of the Inventor of the Heat Economiser and Stirling Cycle Engine. Chelmsford, England: L. A. Mair, 1995. The only book-length biography of Stirling, with detailed description of the development of his air engine. Valuable appendix includes transcripts of Stirling’s sermons and his air engine patents from 1817, 1826, and 1840. However, Sier, a prolific writer on the air engine, presents the narrative portion of the biography in a disorganized fashion.