Stirling cycle

The Stirling cycle consists of two isothermal and two isochoric and is usually depicted with the pV and Ts diagram.

The Stirling cycle is realized by a machine with two pistons consisting of a displacer and power piston. The following diagram shows a Stirling engine with axial piston assembly in the cylinder ( Philips Stirling engine ).

The piston positions marked with (1,2,3,4) are the vertices of the graph Stirling comparison process in the pV and Ts diagram.

Stirling cycle (ideal )

The ideal process of the Stirling engine can be described by four changes of state and represent the following pV diagram.

The working medium is periodically expanded and compressed in a cycle of two isotherms and two isochores. The area enclosed in the PV diagram of the graph area, the work done by the machine in the work ideally.

The following process description is only valid for the uncharged Stirling engine. Charged at process pressures fluctuate around a mean pressure, which is higher than the atmospheric pressure.

In the following Figure 5, the process flow is I - IV approximately represented.

• I. Line 1 → 2: isothermal expansion, TH = constant, taken at the heat Q12 and work is delivered W12. ( This means a change of state of the trapped air is supplied in the heat without changing the temperature. The air volume is larger, the pressure decreases. Case, the gas does work by moving the working piston. )
• II line 2 → 3: isochoric cooling. V2 = constant is given in the heat Q23. ( This means that there is a change of state of the trapped air takes place, in which the volume remains constant, the pressure and temperature are small, and reach the minimum, wherein the heat is discharged from the working gas to the regenerator. )
• III. Line 3 → 4: Isothermal compression, TK = constant, made in the heat Q34 and W34 work is supplied. ( This means a change of state of the trapped air is discharged in the heat without changing the temperature. The external air pressure pushes the piston into the cylinder, the volume is smaller, the pressure in the cylinder increases. )
• IV line 4 → 1: isochoric heating, V1 = constant, is included in the heat Q41. ( This means that there is a change of state of the trapped air takes place, in which the volume remains constant, the pressure and temperature to increase and reach the maximum because of the heat passes from the regenerator to the working gas. )

Among the abbreviations used, see Glossary.

Summary

Why can deliver the Stirling engine work?

Answer in one sentence: You only need a thermal gradient

• Th = highest temperature
• Tk = lowest temperature

In Phase I of the isothermal expansion at the high temperature ( TH) of the gas takes in the closed cylinder to heat and converts completely into work. The pressure (p ) of the gas generated on the surface ( A) of the working piston ( AK), a force (F = P * A). This piston is now moved by the way? S up, so is the case given work:

In the pV diagram of the ideal Stirling process ( Fig. 4) can be seen clearly as a (red) shows the area in 1256 under the isotherms TH again.

During Phase III. the isothermal compression at low temperature ( TC) must be supplied to less work surface 4356 of the isotherms (TK ) is less. In one revolution of the engine, therefore the area enclosed by the loop area in 1234, is specifically working Wpv that will ensure total issued.

The higher the efficiency, the greater the surface shown, the more work can be produced by the motor.

Stirling process (real)

The ideal Stirling process is, like all other ideal cycle processes, can not be realized exactly. The following diagram (Figure 6) shows the surface ( yellow), the real power that remains for use in comparison with the foregoing ideal process chart.

The following list of reasons for the same time an introduction to the problems of the Stirling engine.

Reasons for efficiency losses

Some reasons why the real process deviates from the ideal:

• Mechanical friction
• A discontinuous piston control is limited only feasible

In order to improve the efficiency (the process is in the corners better extended) and to keep the dead space as small as possible, a discontinuous control piston is appropriate. The downside is increased wear caused by mechanical stress and noise.

• Due to high gas velocity isothermal changes is poorly implemented
• Regenerator is limited
• Clearance volume

Ideally, the entire working fluid (gas ) is in expansion and compression space. For even realized until 1999 engines, the dead space is about 30 to 50% of the total volume. Mostly located in these dead spaces (also called dead spaces ), the heat exchanger units such as heaters, regenerators, radiator. This revised volume ratios also bring changes in pressure conditions with it, the very unfavorable effect on the overall efficiency.

• Heat loss through the material

This heat loss is caused by the heat flow along the cylinder to the outside in the direction of the temperature gradient.

• Dissipation by working gas and pressure loss

This loss occurs at Stirling engines with a rated power speed of more than 200/min more on. The compression and expansion during these processes so quickly that the heat flux that would be required for an isothermal process, can no longer keep pace. Result is the increase in pressure in the compression or a steep drop in pressure in the expansion.

• Stirling engine
• Thermodynamic cycle