OLDS ENGINEERING     Mechanical Engineers & Founders

Stirling Hot Air Engines

The Huxtable Hot Air Engine

This beautifully made demonstration model is suitable for physics classes, schools, universities, science museums and collectors.

Featuring: Removable Brass Cylinder
1" (25.4 mm) bore and stroke
Ball bearing mounted crankshaft
5½" (140 mm) diameter cast iron flywheel
Cast aluminium base

The 'Stirling Hot Air Engine' was the brain-child of the Rev. Robert Stirling, who was an inventor of note as well as being a Minister of the Kirk. He was granted his first patent in 1816 at the age of twenty-six - the same year he was ordained to his first parish in the Church of Scotland.

The 'Huxtable' Engine has been devised as possibly the simplest form of 'Stirling' for the express purpose of revealing the basic functioning principle to anyone with even the slightest mechanical knowledge.

Quoting briefly from Stirling's own explanation, we find it difficult to outline the cycle more simply:-

"I employ the Expansion and Contraction (or either) of atmospheric air or any of the permanent gases by heat and cold to communicate motion to a piston or other similar contrivance. In order to produce this expansion and contraction I cause the air to pass from a cold to a hot part of the engine and the contrary alternately either in the same passage... or in different passages."

He goes on to say:-

"The passages are of course Hot at the one extremity and Cold at the other and in passing through them the air is alternately heated and cooled or expanded and contracted."

By loosening the small knurled locking screw and removing the cylinder barrel from the 'Huxtable' engine, the vital parts are revealed. Rotation of the engine allows us to observe how the 'displacer' (or 'transfer piston') causes the air to pass from the hot to the cold parts and vice-versa, thus causing expansion and contraction of the same air within the completely closed cylinder. Naturally the expansion and contraction of the air causes increase and decrease in pressure, which is applied to the working piston.

The crank driving the displacer is 'timed' to cause the rise and fall in pressure to apply power in both directions to the main driving crank.

Many variations of the 'Stirling' Engine have been produced, some of which are quite complicated to say the least. The 'Huxtable' Engine has been designed as an introduction to the 'Stirling'; as it has been found that very few people indeed can grasp the principle, even after having a full explanation. It is designed to promote thought about heat engines, especially in the younger generations. It was never intended to be a very efficient 'Stirling' engine, rather one that is simple and pleasing to the eye of those who appreciate machinery. The unusual and unique feature of this particular little Stirling' engine is that by being able to easily remove the cylinder all of the working parts are exposed.

The 'heated end' of the cylinder which is grooved to give additional heating surface, is virtually isolated from the 'cool end' by the very thin 'neck' machined in it for this purpose . The process of cooling is by conduction to the cast aluminium base of the engine. This arrangement in the 'Huxtable' engine is for simplicity of operation. Water cooling as in the case of pumping engines and air cooling fins used in certain designs have been normal practice in the past.

As in the internal combustion engine, heat is converted to mechanical work by the expansion of air in the working cylinder. In the case of the former, high grade fuel is mixed with the air which is to be expanded.
The resulting explosion is nothing more than the very rapid heating and expansion of the air-fuel charge. Like the steam Engine the Hot Air Engine is an 'external combustion' engine. The fuel is not in contact with the internal working parts as is the case of Internal Combustion engines; therefore all types and forms of fuels may be used. The working piston, a vital component in any reciprocating engine, is working in the cool area of the cylinder and needs no special lubrication.

An additional fitting has been produced for the engine in the form of a parabolic reflector to enable the model to be run on solar heat. The 24" diameter 'dish' fits directly over the cylinder and the specially designed tripod and mounting bracket allows this to be focused accurately to the sun's rays.

When properly focused, a Brilliant white 'light' will evenly surround the end of the cylinder. It is advisable to wear protective glasses to closely look at the 'heat spot' when focusing the reflector in bright sunshine. The aluminium reflector base also gives additional cooling to the cylinder, as waste heat is conducted through it to the much larger cooling area of the dish, which is in shade.

John Ericsson, born in Sweden in 1803, developed Robert Stirling's engine principle during that latter part of his life, which was spent in America. Indeed many thousands of engines bearing his name were built during the latter part of the 19th century and also in the early part of the 20th century. Few people would have realised that these engines, which were used for pumping water, were of a design originally developed to operate on the heat of the sun. The famous engineer was credited with numerous inventions and developments, but few could have excited him more than when he reported his success in a letter in 1873.
He wrote to his good friend Harry De Lamater of the De Lamater Iron Works:

"New York,
October 22, 1873, 18 PM.
DEAR HARRY . The world moves - I have this day seen a machine actuated by solar heat applied directly to atmospheric air. In less than two minutes after turning the reflector toward the sun the engine was in operation, no adjustment whatever being called for. In five minutes maximum speed was attained, the number of turns being by far too great to admit of being counted.

Having found, by long experience, that small caloric engines cannot be made to work without fail, on account of the valves getting out of order, the above solar engine is operated without valves, and is therefore absolutely reliable.

As a working model, I claim that it has never been equalled; while on account of its operating by a direct application of the sun's rays it marks an era in the worlds mechanical history. You shall see it in good time.

Yours truly, J. Ericsson"

These words are taken from a volume , 'The Life of John Ericsson', written by his secretary, William Church, in 1890. They could describe perfectly the result achieved when we apply the seemingly endless energy from the same old sun to the 'Huxtable Engine' over 100 years later.

It is interesting to note that Ericsson, who came from a humble background, devoted a deal of his time to the study of the sun and other natural phenomena. Regarding solar engines he later wrote:

"Those regions of the earth which suffer from an excess of solar heat will ultimately derive benefits resulting from an unlimited command of motive power, which will to a great extent compensate for disadvantages hitherto supposed not to he counterbalanced by any good.

There is a rainless region extending from the northwest coast of Africa to Mongolia, nine thousand miles in length, and nearly one thousand miles wide. Besides the North African deserts, this region includes the southern coast of the Mediterranean east of the Gulf of Cabes, Upper Egypt, the eastern and part of the western coast of the Red Sea, part of Syria, the eastern part of the countries watered by the Euphrates and Tigris, Eastern Arabia, the greater part of Persia, the extreme western part of China, Tibet, and, lastly, Mongolia. In the western hemisphere, Lower California, the tableland of Mexico and Guatemala, and the west coast of South America, for a distance of more than two thousand miles, suffer from continuous intense radiant hear.

We learn that 22,300,000 solar engines, each of 100 horsepower, could be kept in constant operation, nine hours a day, by utilising only that heat which is now wasted on the assumed small fraction of land extending along some of the waterfronts of the sunburnt regions of the earth. Due consideration cannot fail to convince us that the rapid exhaustion of the European coal fields will soon cause great changes with
reference to international relations, in favour of those countries which are in possession of continuous sun-power. Upper Egypt, for instance, will, in the course of a few centuries, derive signal advantage and attain a high political position on account of her perpetual sunshine, and the consequent command of unlimited motive power. The time will come when Europe must stop her mills for want of coal. Tipper Egypt, then, with her never-ceasing sun-power, will invite the European manufacturer to remove his machinery and erect his mills on the firm ground along the sides of the alluvial plain of the Nile, where an amount of motive-power may be obtained many times more than that now employed by all the manufactories of Europe."

John Ericsson would be pleased that a Swedish company Cleanergy are the world's leading supplier of sustainable energy solutions based on the Stirling engine.

Talk everywhere of the so called 'energy crisis' must surely make us think for a minute on these theories and our Creator's constant giver of life, the Sun.
The fossil fuels as we know them, and which mankind seems hell-bent on consuming in a matter of a few generations, must certainly in turn owe their origin to that great bright ball in the heavens. We are told that the very oil and coal which is used in our engines or converted to electricity was formed from the early plant and animal life Would it not then be true to say that we are burning fossilised solar energy when we burn these fuels? Even wood, which has always been an important fuel throughout the history of mankind, and the various crops proposed for conversion to liquid fuels could not develop without sunlight.

For the care and long life of your "Huxtable" engine it should be lubricated with light mineral oil before being run. Keep it covered and free from dust when In storage and avoid overspeeding when in operation. We trust that it will cause you to reflect on the wonderful things which surround our lives and which, in many cases, we take for granted. Lastly, we hope that bright young minds might be stimulated in the endeavour to make the fullest possible use of this old World's resources. This Earth with her treasures is the only one we will ever have.

Wm Olds & Sons Pty. Ltd.

"Huxtable" the Name

The original Huxtable' miniature Stirling ("Hot Air") Engine was made as a special gift by Peter Olds for Mr. Robert Huxtable of Lansing, Michigan, USA. The pair had been corresponding for a number of years prior to their first meeting when Bob visited Australia in 1978. When presented with the unusual gift his immediate reaction was to enquire if a quantity could be made to supply fellow enthusiasts he knew. Production in limited numbers has continued since that time to fill orders both locally and over seas.

Throughout his long life Bob Huxtable maintained an intense interest in antique machines of all types. His enthusiasm for Hot Air Engines was such that he reproduced a 1906 Catalogue of "Rider and Ericsson Pumping Engines".
Unfortunately the majority of these booklets were lost when the basement of his Lansing home was flooded during a very severe storm.

Wm. Olds & Sons Pty. Ltd. trading as OLDS ENGINEERING
ABN 33 009 859 250 - MECHANICAL ENGINEERING & FOUNDRY
78-80 North Street, Maryborough Q, 4650 Australia
P.O. Box 3030, Pallas St. Post Office, Maryborough Q, 4650
Phone: 07 4121 3649 Fax: 07 4123 3590 International Phone +61 7 4121 3649 Fax: +61 7 4123 3590
Innovative Engineering Excellence ~ serving over 100 years