SUPERENGINE® | What does a Stirling engine do?
                             by Karl Obermoser

SUPERENGINE ®  | Stirling engine

It compresses a mass of gas to a smaller volume. The increase in the heat of the gas that would normally occur is prevented by conducting heat away at the same time to the regenerator.


It then heats the gas by forcing (displacing) it at a constant volume through the cooler - regenerator - heater assembly.

Next, it expands the gas until it reaches its original volume. The gas would normally cool down, but this is prevented by supplying heat from the regenerator.

The gas is then cooled down at a constant volume by being passed through the above assembly in the opposite direction.


SUPERENGINE ®  | Stirling engine

Since the work needed for compression or the work that becomes available during expansion are proportional to the absolute temperature of the gas, the difference between them is available as useful work. Therefore, the theoretical efficiency of the Stirling engine cannot be surpassed by any other CHP machine.


However, the level of efficiency achieved by Stirling engines in practice is just as unsatisfactory as the engineering effort that has so far been devoted to putting its ideal, natural cyclic process into practice.


...What can it actually achieve?

If all loss factors are disregarded, the power available from a Stirling engine is largely determined by the displaced volume (the swept volume of the displacer in the ß-Stirling engine), the compression ratio (in the ß-Stirling engine, the ratio between the working piston’s swept volume and the mean working volume), the mean working pressure and the operating frequency or speed of rotation.

The power can be calculated as follows:


Where

Va = Swept volume of working piston in m³

Vv = Swept volume of displacer in m³

Vm = Mean working volume in m³

pm = Mean working pressure in pascal

Te = Expansion temperature in kelvins

SUPERENGINE ®  | math. symbol T = Difference in temperature in kelvins

f = Working frequency per second

K = Adiabatic exponent of working medium

Pt = Absorbed thermal power in watts and Pm = Recovered mechanical power in watts


Then Pt is:

SUPERENGINE ®  | Stirling engine thermal power

and Pm is:

SUPERENGINE ®  | Stirling engine mechanical power


Brief remarks on the current state of the art

There are innumerable different versions of the Stirling engine, and almost as many ways of classifying them.


All Stirling engines have at least two pistons. There are versions that coordinate movement of the two pistons by means of a common crank drive which may be of greater or lesser complexity, other versions in which one or both pistons oscillate freely and still others that use liquid or gaseous media as the piston.

The most fundamental and generally valid distinction, however, is according to piston function.

In this respect there are only two entirely different categories:


1. Engines with a working piston for compression and expansion, and a displacer to transfer the working gas between the heater and the cooler (referred to as beta-type Stirling engines - lower picture). These exist in all the versions referred to above, and distinctions are also made according to their proportions. If the piston and displacer cross-section is large compared with their stroke, they are described as flat-plate Stirling engines. This flat design has the advantage that elaborate heaters and coolers are not needed if the flow of heat is low, since the surface areas of the engine’s housing are sufficient to perform this function. A severe disadvantage is poor resistance to pressure, which means that this type has so far only proved practicable at reasonable effort and expense as a toy.


2. Engines with separate expansion and compression pistons, the displacer function being performed jointly by both pistons (referred to as alpha-type Stirling engines - upper picture). Until now, these have mostly been versions with a crank drive.


Stirling processes can also arise without technical aids. If an acoustic wave penetrates a fine-pore material, a Stirling-cycle process takes effect and pumps heat in the direction of the acoustic source. This relationship has been known at least since the work of Peter H. Ceperley, but until now it has not been implemented as a satisfactory mechanical engineering concept.



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