“As you enter the past, you willfind direction for the future” [9]

During the industrial revolution of the 18th century the steam engine was widely employed as a prime actuator for all sorts of industrial applications such as pumping, lifting heavy loads and driving rotating machinery. The technology in steam engines had developed by the turn of the 19th century to the point where huge engines were being built which could do the work of teams of horses or gigantic water wheels [10]. Despite their usefulness, steam engines had a few major failings. Firstly, they were inefficient — at the start of the 20th century typical efficiency was around 2% and peaked later at roughly 10% [11]. Even though coal was cheap at the time it still made sense to factory owners to strive for more efficient machines to maximise their profits. The main drawback of the steam engine however was their safety, or rather lack thereof. Violent boiler explosions were a matter of course, reported on a day-to-day basis. These explosions would release scalding high pressure steam and often led to fatalities, promoting engineers and inventors to search for a new alternative type of engine [11].

The Stirling Engine is a type of what is known as a hot air engine [12], which appeared early in the 19th century and has since attracted a great deal of interest from engineers, inventors, researchers, entrepreneurs and hobbyists. The basis of the Stirling Engine, or any hot air engine for that matter, is that of a gas expanding when heated and contracting when cooled. This principle has been known since ancient Greek times, when Hero of Alexandria invented a machine which used heated air to displace water whose weight was used to open the doors of a temple [13]. A hot air engine is a device that uses this principle to convert heat into mechanical work. Numerous designs have been around since as early as 1699, though it is thought that the first workable design was the open cycle gas furnace designed in about 1807 by English inventor George Cayley [12].

The invention of the Stirling Engine is widely credited in modern literature to Scottish minister Robert Stirling. Robert Stirling was a minister for 53 years, however he was not an engineer and although his name was on the original Stirling Engine patent in 1816 (British Patent No.4081) it is very doubtful that he had much to do with the invention [9]. It is much more likely that his brother James, an engineer with thermodynamic and mechanical knowledge, was responsible for the design and development of the engine.

The original patent of the Stirling brothers actually specified two separate inventions — the first and principle invention was the ‘economiser’ or what we call today a regenerator [11]. This was a hugely significant invention in its own right, and discussed in more detail in section 2.1.4. The second part to the patent was the actual engine itself, titled A Closed — Cycle Air Engine, using the economiser to “diminish the consumption of fuel”.

The Stirling brothers, regardless of who specifically was responsible, actually developed the first five working versions of the Stirling Engine from 1815 through to 1845. The first version in 1815 was very a very basic ‘gamma’ (y) design, consisting of two separated cylinders with open fire heating and air-cooling. The second version followed soon after in 1816, which had the displacer and power piston in the same cylinder in order to reduce the flow losses between the cylinders, making it a ‘beta’ (P) type engine. A description of these fundamental engine configurations is found in section 2.2.


Figure 7: First working Stirling Engine from 1815 [9]

Figure 7 depicts the Stirling brothers’ first working model of their engine. The following is a description of the engine configuration and operating principle, extracted from their 1816 patent:

“The exhibit has a long vertical cylinder containing air which is heated by an external source. A long and loosely fitting displacer works within this cylinder, it is made of thin metal and is operated by means of an overhead crankshaft with flywheel. At the opposite end of the crankshaft there is another crank which operates the working piston in a separate cylinder. The bottom of this cylinder communicates with the top of the displacer-cylinder through a pipe. Its working piston is packed with a double cup-leather and its crank is set 90° behind the displacer crank.

The included air is brought to the warm part of the cylinder has its elasticity increased and presses upon the piston with a force greater than that of atmosphere.

The piston is thus forced downwards till the pressure of the included air and that of atmosphere become equal. The impulse communicated to the fly carries the end of the crank, and the arm and bent lever are brought to such a position as to depress the rod and thus to raise the plunger from the piston. The included air is thus made to descend between the plunger and cylinder and brought to the cold part; it is cooled in its descent, has its elasticity diminished, and its pressure becomes less than that of the atmosphere, the piston is forced upwards, and the crank downwards. The revolution of the fly and crank again bring the plunger towards the piston, the air ascends through the same passage by which it descended, is heated in its ascent and forces the piston downwards and the crank upwards, and so on alternately. In this manner a rotatory motion is produced which may be applied to the moving of machinery. ”

Although the regenerator or ‘economiser’ was described and patented by the Stirling brothers in 1816, it was not put into use in a working engine until 1827. The original description of the regenerator talks about a space filled with “successive layers of plates of the thinnest iron in use, pierced with holes… .kept at a distance of two or three times their own thickness from one another.” The purpose of the regenerator is to act as a temporary heat storage element, so that as hot and cold gas is passed back and forth between the regenerator heat is removed from the gas and then put back into it. This reduces the amount of heat needed to be transferred by the heat exchangers and dramatically increases the overall efficiency of the engine.

The next two major improvements in engine design were both introduced by the Stirling brothers. In 1840 they built an engine with an external regenerator and tubular heat exchangers which increased the heat transfer area. In 1845 a pressurised cycle engine, shown in Figure 8, was built which used a separate pump to fill the engine housing with compressed air. This engine produced 2.5 HP (1.8 kW) and was used to power all the machinery at a foundry in Dundee, Scotland for many months until it was superseded by a larger version of the same engine [9].


Figure 8: Stirling brothers’ 1845 pressurised cycle engine [9]

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