[Robert Stirling]

Rev Dr Robert Stirling (1790-1878)

History and Introduction

"Van Arsdell's clever toy belongs to a diverse class of heat engines called air engines, or, more commonly, Stirling engines, after a Scots clergyman, Robert Stirling, who filed the first patent for the type in 1816. Actually, rudimentary Stirling engines existed before Stirling, going back to the late 1600's; but Stirling added a refinement that he called an 'economizer'. It is now called a 'regenerator', and it increases the engine's yield of work for a given input of heat increases, in other words, what we call its 'efficiency'. A regenerator is now a standard part of every Stirling engine design."

It runs without noise or vibration and is truly an environmentally friendly device. When one end is heated and the other kept cool, useful work can be obtained through a rotating shaft. It is a closed machine with no intake or exhaust which results in very quiet operation. Anything that gives off heat can be used to run a Stirling engine. Some common methods are burning coal, wood, straw, gasoline, kerosene, alcohol, propane, natural gas, methane and so on.But combustion is not required, only heat is required, or more accurately, a temperature difference between the hot and cold sides. This allows Stirling engines to run on solar energy, geothermal energy, or even on the surplus heat from industrial processes including cooling water from a nuclear power plant.It competed with steam engines of that time, and was even sold by Sears Roebuck to pump household water in the 1920s. Stirling engines are used today in much of the "undeveloped" world.

[Drawing of R.Stirling's engine] Robert Stirling patented his Heat Economiser in 1816. The patent described a number of applications for use in glass and other furnaces. Also described was a motive power engine incorporating his ideas and designed to work with a reduced fuel consumption compared with the designs of steam engines then available.
The drawing shown, right, is taken from his English patent. This patent is hand written, and a comparison with known examples of Stirlings own hand writing clearly shows the writing to be that of Robert Stirling and there is no reason not to believe that he also penned the drawings.
In 1818 Robert Stirling erected an air engine to his design to pump water from a stone quarry. In the 1820s he teamed up with his younger brother James who suggested that greater power output might be obtained using air at pressures greater than atmospheric pressure. Further patents were obtained in 1827 and 1840, for improvements in air engines.
Robert Stirlings patent of 1816 contained all the elements of what is now termed the STIRLING CYCLE ENGINE. That is a power piston, a displacer to move the enclosed air between the hot and cold ends and a regenerator set between the hot and cold ends of the displacer cylinder. Closed cycle operation with external heating.

[animation of R.Stirling's model]The design shown in the 1816 patent drawings is a form of inverted beam engine. Stirling states that this is preferable since the oil used to lubricate the piston can not get into the hot space, where it would be carbonized. In the animation, right, the power piston, links and crank are shown in red. The displacer, displacer beam and linkage is shown in blue. A simple Watt three link parallel motion connects the power piston to the working beam. The piston and displacer work out of phase, this is achieved by means of a bent lever.
The power piston compresses the enclosed air in the cold end of the displacer cylinder. The displacer then shifts the air from cold to hot chambers. The piston is driven back, the power stroke, by the air expanding in the hot end.

[model engine]Regrettably none of Stirling's experimental work or papers have survived except for two model engines built by him when he was a Minister at Kilmarnock. These models were donated to the Universities of Glasgow and Edinburgh some time before 1824. At Edinburgh Stirling's engine was used by Professor John Leslie, who thought practical demonstration important. In 1975 the department of Natural Philosophy at Edinburgh University donated their engine to the Royal Scottish Museum where it is now on display. At Glasgow the engine was put in to store where it lay forgotten until discovered, in 1847, by William Thomson, then professor of Natural Philosophy, who later became Lord Kelvin. Thomson used the model to show that Stirling's machine worked on a reversible cycle. The engine was in constant use by Thomson for lecture illustration. It is still at Glasgow and can be found on display with other historical artifacts in the refectory of the department of Physics.
A sectioned replica of the engine can be found on display at the London Science Museum.

Small domestic hot air engines remained in production and used into the early 1900's, at which time the internal combustion engine improved with the advancements of the electrical components. Currently they are the subject of considerable research and development efforts because of their potential for high efficiency and their clean and quiet operation. Current levels of performance are impressive and are achieved using high temperature alloys, new mechanical drives, computer designed heat exchangers, and use helium or hydrogen at high pressures for their working fluid. Modern Stirling engines can easily surpass typical gasoline and small diesel engines in efficiency and in power-to-weight ratio. They are renowned for their quiet operation and low pollution levels. This makes it ideal for future clean burning automobiles, quiet lawnmowers, and electricity production from the sun. As a final note a solar powered stirling engine coupled with a generator achieved a record solar-to-electric efficiency of 30%!

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