SUPERENGINE® | What is resonance anyway?
                             by Karl Obermoser

SUPERENGINE ®  | damping level

A simple mechanical resonator is defined by its mass, its constant spring rate and its damping. After initial deflection, it continues to oscillate at its natural frequency f 0 until its damping causes oscillation to cease.

If it is not deflected just once but periodically, the ratio between its amplitude and the excitation amplitude depends on the excitation frequency f and the damping, as shown in the graph on the left. At a low damping level, the maximum amplitude due to excitation is f = f0, and can be several times the excitation amplitude.

The lower the damping level, in other words the higher the resonance amplitude, the higher the energy consumed by damping.

SUPERENGINE ®  | phase position of the oscillation

The graph on the left shows the phase position of the oscillation due to excitation in relation to the oscillation causing excitation. Regardless of damping, this is always 90° at f = f0. The lower the damping level, the more rapidly the phase position changes if f deviates from f0.

It is customary in this graph to show the phase displacement itself as the y axis. Instead of this, I have shown the sine of this angle as the y axis. This reaches a maximum at 90°, which is of particular importance for Stirling engine design.

My favourite textbook of physics also tells me that the effect of resonance is of very great importance in mechanical engineering. The reason: if it cannot be ruled out, resonance can lead to damage beyond repair. If resonance cannot be entirely eliminated, its effects must always be limited.

Be that as it may, we shall not observe this recommendation here!

SUPERENGINE® | The flat-plate resonance Stirling engine
                             by Karl Obermoser

SUPERENGINE ®  | The flat-plate resonance Stirling engine

This photograph (please click on it to start the video) shows a solar engine (0.2 m² in area) using resonance as its operating principle. The video shows the full details of this small demonstration engine.

This prototype has no efficient cooling yet and therefore operates at a difference in temperature of only approximately 15 K. With more efficient cooling this could be increased to about 80 K. It starts automatically at a difference in temperature of only a few kelvins, and as the sun’s rays strike it increasingly, drives a water pump.

(The two stone blocks that can be seen lying on the device were used for provisional fine tuning of the resonant frequency.)

In mechanical engineering terms, this engine can be classified on the one hand as a free-cylinder Stirling engine and on the other as a flat-plate Stirling engine. Apart from its self-starting capability and freedom from wear, its special feature is the layout of the components, by virtue of which all the forces acting on the glass add up to zero, so that during operation the glass is exposed to no bending loads whatsoever. These enormous forces of up to one ton per square metre have so far prevented the technical application of the flat-plate principle. With this technical problem solved, flat-plate Stirling engines can now be produced in units of any desired size (see Products: SUPERENGINE ®  | Intern link  The Sunwell).

In view of the importance of the flat-plate Stirling engine because of its ability to make use of low differences in temperature, here are a few more brief details of its history:

The first flat-plate Stirling engines were built by Professor Ivo Kolin of Zagreb University. They had no regenerator, and the displacer was moved discontinuously by a mechanism with a deliberately generous amount of play.

We (that is to say Eckhart Weber in Nuremberg) soon took up this idea of a flat layout of the main components. Weber introduced the sinusoidal displacer movement and the regenerator as a flat component, together with uniformly distributed compression-and-tension tie rods passing through the regenerator in order to withstand the forces acting on the cover and base. This permitted larger engine units and acceptable levels of efficiency for the first time.

By developing and adopting an extended free-cylinder principle, I then succeeded in eliminating the entire complex mechanism and the need for the equally complex, costly tie rods.

SUPERENGINE ®  | SUNWASH - a living sculptur

Here is a picture of a “living sculpture” in which resonance is also allowed free rein. [I was commissioned by an artist to design this and carry out the necessary calculations; it is to be found in the town park of Burghausen (Bavaria, Germany)].


SUPERENGINE ®  | continue  Continue to the Stirling engine...

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