Interesting Facts

Interesting Facts

. . .

Thus it goes about:
Function principle of the Stirling motor

The principle of the Stirling motor is very old…and very simple! Over 2000 years ago in ancient Egypt, the expansion of heated air was used to set temple doors in motion.

The hot air engine patented in 1816 by Robert Stirling is based on a similar principle: A burner filled with fuel heats the air inside a cylinder and thereby purveys energy to the motor that activates through piecing of the flywheels.

Stirling motors are impressive machines despite their simple design!

The air in the closed operating cylinder is heated by the flame, expands, and flows past the expulsion piston to the other side of the cylinder, where it pushes the working piston towards the outside. Since the expulsion piston is now connected to the rotating flywheel it now moves back, letting the air behind the expulsion piston cool to create a vacuum.

This vacuum ensures that the working piston gets sucked back. The process then repeats from the beginning and the machine works.

Function principle of the vacuum motor

The vacuum motor patented by Henry Wood in 1758 is also commonly referred to as “flame eater” and is part of the hot air engine family along with the Stirling motor. 
 Similarly to the first Otto-motor, the atmospheric pressure performs the major part of the work.

The piston sucks the frontally positioned flame into the cylinder through a valve (the characteristic operating noise this makes is reminiscent of a Lanz tractor). The flame heats the air in the cylinder, pushing the piston back.

Thereupon the valve connected to the flywheel shuts, letting the air in the cylinder cool. Because of the atmospheric pressure, the piston moves back to its starting position. When the piston reaches its top dead center the valve opens again and the process begins anew. The flywheel supports the overcoming of both dead centers.

Functional models of vacuum motors are often constructed as stationary motors with a horizontal or standing design and one or many cylinders. One can also find vacuum motors in historical models of tractors and railcars.


Hot air engines were already in use as simple machines 2000 years ago, e.g. to open gates in temple districts. There are even documents in which famous people like Leonardo da Vinci studied hot air engines. Whether these were actually built is not known.

Also about 200 years ago, people were searching for mobile refrigeration and combustion engines. That was also the heyday of hot air engines.
In 1816, Robert Stirling applied for a patent for his first Stirling engine. This first machine was used as a coal-fired water pump to drain a quarry.

The Swede, John Ericsson, who built many technically diverse machines for use in ship engines, is closely associated with hot air engines.
There are many other manufacturers and inventors (such as Lehmann, Buchbaum,...) of hot air engines who have not been fully explored today.

The Stirlingengine

The Stirling engine was developed, because it:

  • runs without imbalance
  • was safer than internal combustion engines
  • can be reduced
  • is low in wear and friction
  • had low fuel consumption, compared to conventional steam engines
  • is characterised by silent running
  • has low heat emission
  • Is durable and robust

At the beginning of the 20th century

At the beginning of the 20th century, there were already about 250,000 Stirling engines in use in small tradesman businesses or private households, such as:

  • drives for small devices (such as sewing machines,...)
  • water pumps
  • ventilators
  • cooling machines
  • radios
  • single power sources
  • and many more ...

Electric, diesel and gasoline engines were gradually developed and increasingly displaced the Stirling engine which had thus far not been able to establish itself in the automotive industry.
Only the military still retained interest in the Stirling which they used,for example, as small generators (in the United States) or as submarine drives (in Sweden).

  • In 1850, John Ericsson developed and built solar-powered hot air engines.
  • Around 1900, they were in civilian use as toy engines in the tin toy industry (e.g. by Plank, Märklin,...).
  • From 1937-1978, the Dutch company Philips was looking for heat-powered drive units which they could use, e.g. as radios-> The research project ran until 1978
  • Almost all of the today's Stirling models have evolved from this time period.
  • After 1978, the licenses (e.g. to Ford, MAN,...) were then sold.
  • Solar-powered Stirling engines were set up from around 1990 producing solar power in sunny countries.
  • Since 1990, Böhm Stirling Technology GmbH has also developed and built its high-quality toy models based on the Stirling technique.

General information
on the Stirling engine

Normally the Stirling engines are operated with warm air, modern engines work with hydrogen or helium - Enormous improvement in performance

The work media can be metal or water as well as liquid or gaseous pistons.

The working piston pushes the air (in a closed space) back and forth - Air is quickly heated + cooled. This powers the displacing piston. The working and displacing pistons are offset by approx. 90°.

Through the heat supply from the outside, all types of fuels and heat sources can be used (e.g. through the human body), including solar radiation.:

In addition to the Stirling engine, there are 3 other types of hot air engines: the Roper, vacuum and Ericsson engine.

The Stirling engine is easily rechargeable and has the largest thermal efficiency and if the external heat source produces no emissions, there is not any material emission (it is CFC-free).
Another big advantage of Stirling engine is that it is operated with a continuous supply of heat, produces little noise, can be low in pollutants and can also be operated with radioactive decay heat, solar energy or hot water/steam.

At the same time electricity and heat can be generated with the Stirling principle.

The Future

The "electricity-generating heating systems", the microgen Stirling engines on the market now are innovative. They can be used in single and two-family dwellings, as an alternative to conventional heaters.

The Stirling engine is also used today as a CHP (combined heat and power), solar-powered engine cyrocooler and as CFC-free refrigeration equipment.

National and international Stirling conferences held annually show how simple the model engines are to build and how they can be used. No limits are set on creativity and imagination.

Stirling engines can be used in residential buildings to generate electricity and heat. If the Stirling engine is fuelled with renewable raw materials (such as pellets), the CO2 emissions are equal to the CO2 uptake during growth -> CO2 neutral

The Stirling engine can also be used for the propulsion of submarines (because the Stirling engine is an air-independent drive which only needs heat).

Solar paddles (free-piston Stirling engines) can be used in outer space.

The Stirling engine has been used to power artificial hearts since 1967 and is being further studied and developed for biomedical technology use.

Stirling cryocoolers are used both in superconductor technology and for CFC-free refrigeration equipment.
Although the Stirling engines are called the engines of the future, they are not yet widely accepted on a broad front.

They are currently used in various applications:

  • Solar-powered Stirling (solar energy is converted into mechanical energy)
  • Cogeneration units (production of heat, electricity)
  • U-boats (propulsion)
  • Space projects (generators are being developed by NASA)
  • etc.

And what else is still in the development stages?

  • Stirling engines as water pumps in the third world
  • In biomedical technology, as a pump-> drive for a blood pump which drives the heart support system
  • Research into using Stirling engines in the automotive industry has not yet been completely abandoned
  • The Sunmachine deals in depth with the Stirling technology. Here, highly efficient units are the focus

The hot-air machines inspire and are beautiful!

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