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Stirling Hot Air Engines
The Huxtable Hot Air Engine
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This beautifully made demonstration model is suitable for physics classes, schools, universities, science museums and collectors. Click Here to Order Now
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:-
He goes on to say:-
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.
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.
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
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,
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 Qlds & 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
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Wm. Olds & Sons Pty. Ltd. trading as OLDS ENGINEERING