SUPERENGINE® | What’s so dead about a gas spring?
                             by Karl Obermoser

SUPERENGINE ®  | Pressure Volume Diagramm

During operation of an actual Stirling engine, only part of the enclosed mass of gas changes from one of the two predetermined temperature levels to the other. This part is described as displaced volume and, in a conventional Stirling engine, is a fixed amount determined by the crank drive and the piston dimensions.

The other part is only compressed and expanded, without playing any part in the thermodynamic cyclic process.

If this passive part of the total volume is enlarged, compression within the engine, and therefore its power output, are reduced. Stirling experts therefore refer to this passive constituent as non-constructive or “dead” volume, and there is a widely held view that the passive constituent reduces the engine’s thermodynamic efficiency.

The reason for this fallacy lies in the use of the pV graph as shown above, which is in fact not applicable to the Stirling engine and is therefore misinterpreted.

The pV graph shows the change in the condition of the gas enclosed within the engine during a working cycle. The area within the resulting curve is equivalent to the work obtained: this also applies to the Stirling engine.

It is customary in the pV graph also to show the isotherms between which the Stirling process is intended to take place. Isotherms show the pressure of the gas enclosed in the engine in relation to the volume at a constant temperature, and naturally only apply if the entire volume is at that temperature.

However, by virtue of the Stirling engine’s operating principle, precisely this can never be the case. Recording these isotherms is therefore almost certain to lead to the incorrect conclusion that the lower the proportion of passive volume to total volume, the less the actual Stirling cycle takes place in accordance with its isotherms.

SUPERENGINE ®  | self-starting solar engine

Here, for instance, are two videos showing an early demonstration model of a self-starting solar engine.

The enclosed volume is for the most part ‘dead’ volume, so that the engine ought in practice not to operate at all.

(To view the videos, click on the reduced-size pictures.)

SUPERENGINE ®  | self-starting solar engine

The volume that plays no part in the cyclic process and is allegedly “dead” is in fact full of life and has no deleterious effect whatever on the thermodynamic efficiency. Nor is it passive: on the contrary, it acts as a gas spring and its size determines the engine’s operating frequency.

What is correct, however, is that if conventional pistons that incur friction loss are used, the mechanical efficiency is lowered, because the volume that plays no part in the cyclic process also has to be compressed and expanded, although it makes no contribution to the engine’s power output. This requires an unnecessarily long piston stroke and therefore results in unnecessarily high friction losses. This aspect (too) has led to the following draft design without any dead volume.

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