Rev. Robert Stirling

After Amontons' air engine of 1699, the engines of Cayley (1807), Stirling (1816) and later Ericsson (1833), have been the ones that gave to the hot air engine, the impulse that propelled it into History.

Three men, three different technologies, three different ventures.
Stirling being one of them.

Robert Stirling was a Scottish farmer’s son, born in 1790 in rural Perthshire, the third in what would become a family of 8 children that included 5 girls. It was an age when agricultural improvements by way of land reclamation and new machinery were in full swing in Britain. His grandfather, Michael Stirling, had invented an early rotary threshing machine driven by water power; his younger brother James Stirling (born in 1799) would make his name as an engineer and it is a fair bet that family interests in the new machinery of the age inspired the young Robert into becoming an amateur mechanic. It was said that the girls in the family were also mechanically talented.

The early 19th century was also an age when a farmer’s son attending one of the East of Scotland’s four universities was a common-place occurrence. His second year included mathematics under the aegis of John Leslie, well-known for his study of heat. It is not clear if he was later taught Natural Philosophy by the notable and enthusiastic John Playfair who was also on the staff. If so, he would have given Stirling a wide-ranging contemporary understanding of the principles behind the operation of many kinds of machine.

Robert Stirling would eventually have a long career as a Minister of the Church of Scotland, generally a graduate profession even two centuries ago. He spent additional time in divinity studies, perhaps at both the Universities of Edinburgh and Glasgow. Although he didn’t formally collect the AM (‘Artium Magister’) degree that was expected for a career in the Church, he was ordained in 1816 as Minister for Kilmarnock. He was known to maintain a workshop and a continuing interest in machinery. Indeed, even later in his career “he sometimes surprised his neighbours at midnight by the hammering resounding from the anvil in his little smithy adjoining the manse”.

Towards the end of 1816 he applied for his now famous patent (No. 4081) for an ‘Economiser’ - with application gor an air engine - that discussed various ways of exchanging heat. This was well before the time that heat was recognised as a transfer of internal energy between bodies. Heat was then considered as a substance in its own right (caloric) that could flow between bodies or simply evaporate as radiation, though Stirling himself never used the word in his patent description.

Cayley worked and improved an internal combustion engine. Ericsson developped mostly open-cycle engines. The Stirling engine, like the Amontons' engine, is an external cycle one. Like the Amontons' engine, it is also a closed-cycle engine. But the comparison with the Amontons engine stops there, as the Stirling engine is of an entire new concept.

The innovation was so much in advance of scientific knowledge at the time that at least 30 years passed before anyone was in a position to understand what made the engine work at all. Nearly 100 years later engines were still "invented".
At the time Stirling invented his engine the industry was more and more using the steam engine. Steam engine were noisy, unsafe (many of them did explode) and did consume a lot of fuel. Stirling's goal was first to develop a economic engine. The title of his first patent (#4081 of 1816) was : "Improvements for Diminishing the Consumption of Fuel, and in particular an Engine capable of being Applied to the Moving (of) Machinery on a Principle Entirely New".
This is rather a confusing title for what has been a major invention. But it shows the will of Stirling to create an engine that saves fuel, even if to do so he has to use principles that were entirely new. These new principles were: the use of air as a working fluid and the use of an economiser.
At that time the only working heat engine was the steam engine. Cayley and Stirling's idea to use air instead of water / steam as a working fluid was a quantum leap. Nowadays water is a costly commodity in many areas, and will be more and more costly in the future. They were in advanced of all steam engines / turbines that still today are used in many power stations.
Steam engines are the externally fired engines. And the genius of Stirling and previously Amontons (not followed by Caley, but copied by Ericsson) was to reproduce this same feature for air engine. Many air engines that came after Stirling are internal combustion engines (car engines, truck engines, plane engines, gas turbines). Internal combustion generate always pollution. This cannot be changed. Whereas with external combustion, pollution can be avoided.
In this respect Stirling was visionary.

Noteworhty is the 1827 patent (no. 5456) which promoted a major development in that the power piston and "plunger" were now designed to work in separate cylinders.

This was the beginning of a wide range of variants which recognised that Stirling’s original plunger did two jobs. It displaced air, or whatever gas is enclosed, between the two ends and it acted as a heat exchanger. These two jobs can be done separately by a plunger (or displacer) and a regenerator that may well be different components. Taken to extremes, the hot and cold ends can be in separate chambers connected by a tube. The displacer moves air between the tubes and the regenerator, if present in the tube, is the heat exchanger.

Whatever the configuration, by moving the air within the engine the displacer maintains the cyclic motion of the engine, which isn’t created simply by having steady hot and cold ends to a cylinder. In other engines the cycle is kept going by the engine’s valves. Like valves, the displacer is generally driven by the power train. Stirling achieved this by connecting the plunger to the engine flywheel via levers.