Hot air engines for clean energy

The engine birth

Civilization was built by man's ingenuity and labor. This was hard work. So hard that man constantly attempted to replace his physical work by other means.
At first, he used animals. But when he invented engines to power machines, civilization really expanded, and did so to such a level that any worker is nowadays much better off than a king two century ago.
The engine is born to answer man ever growing wants at the cost of least efforts.

Hard physical work has almost disappeared, machines - with their engines - do the job, and civilization expands endlessly.
The resulting well-being is incredible: man is no more obliged to work hard in order to survive. He can afford for all his needs, has time for leisure, good health and long life.
A golden future if… machines keep on going working in the place of man.

The use of engine powered machines, here taken in their broader sense (machines but also appliances, vehicles, computers, and so on) is only possible with energy.
All machines consume energy. As a result, human well-being is entirely dependent on energy generation.

A quest for clean energy

As man struggles for more well-being, energy needs will increase strongly.
As the World population grows, this increase takes place at an exponential rate. Humanity needs more and more energy. Its needs will double in thirty years, triple in fifty years.

Traditional power generation technology generates heavy pollution and the doubling or tripling of the World energy needs will only worsen our daily life and environment. A major challenge for man is to generate, as soon as possible, a clean energy.

Clean energy generation is relatively easy on a small scale, but sustainable large-scale clean power generation is another matter. Actual clean energy technologies, including the existing clean power plants, are far away from being able to fill the World growing energy demand. New ways must be found.

Humanity quest for welfare shall become a quest for clean energy.

A confusing thought suggests that an electric engine, or more generally an electric device, might be “clean” because its use does not generate pollution as it is the case of a gas engine. This is a shortcut and a more appropriate way is to look at the whole polluting chain.

Any electric device, an electric car for instance, receives its electricity from a power generation plant. As a matter of a fact, only few power generation plants deliver clean power, like solar plants and wind or water turbines. Most of the world electricity comes from power generation plants that make use of coal, oil, gas or nuclear energy that are massive polluters. Therefore, any electric device is forcibly a vector of pollution – for the time being. This can change, and has to change.

The quest for clean energy shall not be underestimated. The World desperately needs it and all ways are worth to be examined.
In this respect Hot Air Engines have undoubtedly an original and promising role to play.

Hot air engines for clean energy

The hot air engine is a technology of the past. But being compatible with a clean use, it is nowadays more and more considered as a possible clean energy technology for the future.

The hot air engine shares with the steam engine a unique feature – the external heating. This is what's make it clean energy compatible. Knowing that the theoretical efficiency of the hot air engine is well over the one of the steam engines, the former is a candidate of choice for clean energy.

The attempt to have clean energy is not new. During the 19th century solar air engines and solar steam engines have been invented. These attempts are presented in these pages.

Hot air engines also called Stirling engines, caloric engines, hot air machines or more simply air machines were invented at the beginning of the 19th century at a time when the only existing engines were the water wheels, the wind mills and the one-hundred-year-old steam engine.

At the end of that very century, all major engines had eventually been invented: the air engine, the gas engine, the hydraulic engine, the pneumatic engine, the electric engine, the gas turbine, the steam turbine, the hydraulic turbine. The 19th century has been the century that put the World into motion!
With good reason it can be called the “Engine century”.

Hot air engines have existed for 150 years, mostly during 1800 and 1950. Thousands of air engines have been built in that period and were operating for the industry. One of the most beknown one is the famous Stirling engine. But despite Stirling's failure in building a proven technology, others succeeded brilliantly.

They learnt to counter the many drawbacks and flaws, challenging their knowledge and the state of the art with passion and courage.

The technical venture

  • First external heated engines, because they use a heater, are not practical for transportation. In this area only internal fired engines can be used.
  • Second, the external heating causes a dreadful loss of energy compared to the internal fired engines. But internal fired engine is even challenging for clean technology.
  • Third, the working fluid of the external engine is by itself a limitation. Air is excellent for high temperature differences, but it is a poor heat conductor and its expansion is limited. Steam has better expansion but is limited in temperature differences.
  • Fourth, at atmospheric pressure external heated engines are of very large construction. John Ericsson’s 300 hp caloric engine had the size of a flat, which is not practical at all. In comparison a common 300 hp gas engine has a volume of less than 0.2 m3. Only high-pressure external engines can be suitable for large scale use. But this is where the practical issues start.
  • Fifth, air has a significant advantage over steam because it can reach very high temperature, and so it can push the engine performance to new level. But this advantage is also the main issue of the air engines as high temperature hurts heavily the materials used in the engines.
  • Sixth, but common to all engine technology, the engine output is physically limited as explained by the law of Carnot. At its best an engine can reach an output of 35%.

The disadvantages of the heated external engines have been bravely fought by ingenious and persevering inventors. And, except for transportation applications, they did come to perfection.
Hundreds of hot air engines were invented, most of them failed, some entered the legend. Hot air engine is a story of trials and errors, their inventors a story of bravery and perseverance.

Nonetheless their endeavors have not been enough. The history of technology shows that in the course of the 20th century all engines were gradually superseded by gas engines, turbines or other pneumatic or electric engines. Not only the hot air engine knew no exception, but during its 150 years lifetime it merely played a minor role in the engine century when compared to the other engines.

As a result of the gas engine popularity, the World is today powered by heavily polluting technologies, be it directly or indirectly.

Content of the site

These pages provides a circumstantial and detailed information about the hot air engines. Its content takes base on the reading and scrutiny of numerous archives between 1800 and 1920, most of them being books, magazines, scientific review, articles, patents.

It will present the main air engines inventors, their technology with a description of their operation. It will do it considering the technological evolution, giving a special focus to the specific technological problematic the inventors tried to answer.

Minor inventors will also be presented with a special focus on the reason of their failure.

The hot air engine existed during a century and a half and then was abandoned; a tragic destiny when considering that this engine could be a clean technology for power generation. Will it one day come back and become a technology for the future?

These pages, when presenting the hot air engine, will analyze the causes of its dismissal. They will also benchmark its position in comparison to its competitors, the steam engine and the gas engine. At last it will try to address whether this technology can be one day sustainable.

Numerous inventors started a hot air engine venture, many scientists wrote about this engine, hundreds of patents were filed, thousands of articles have been written. An enormous amount of work has been dedicated to this technology. All of this resulted in an incredible amount of learning and science.

This inquiry, by bringing most of it together in one place, will allow anyone curious about this engine to leverage on a century and a half of acquired knowledge and experience.