Stirling engine

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The regenerator

For most of people, Robert Stirling is the father of the engine of the same name. In fact, it would be necessary to refer, first, to the regenerator or economiser. It is probably its principal invention. As one will see it below, the overall performance of the Stirling engine is related to the temperature difference between the hot source and the cold source, but very influenced by the regenerator.

The diagram opposite shows us the 4 phases of operation:
– Heating with constant volume V_m(Gas temperature increase from T_m to T_M)). Thermal energy must be provided: Q_heat.
– Expansion at constant temperature T_M. Thermal energy must be provided to maintain gas in temperature TM: Q_exp.
– Cooling with constant volume V_M(Gas temperature decrease from T_M to T_m). Thermal energy recovered is equal to Q_cool. This one is the same one as that used to heat gas with constant volume: Q_cool = Q_heat.
– Compression at constant temperature T_m. During this phase, one provides work but one recovers thermal energy Q_comp.

Recovered mechanical energy is equal to the mechanical energy recovered during the expansion W_exp minus the mechanical energy W_comp which one must provide for compress the gas. The energy recovered during a cycle is proportional to the surface coloured in green (see the page the principles).

Now, if our accounts are made (as a Scottish or an Auvergnat):
– one spends Q_heat + Q_exp
– for mechanical energies, one recovers W_exp – W_comp

If one could recover Q_cool to reinject it at the time of the constant volume heating, one would save Q_heat. The accounts would become:
– one spends Q_exp
– for mechanical energies, one recovers W_exp – W_comp

For the same mechanical profit, one spends less calories. The regenerator invented by Robert Stirling allows this economy!

It is what one can see on the diagram opposite. On a practical level, the regenerator is an heat exchanger situed between the hot source and the cold source. See the drawings below.

Actually, it is a delicate apparatus to conceive. The heat transfers with a gas are difficult and require cumbersome devices. The regenerator constitutes a “dead” volume which thwarts the benefit of its presence. There is never absolute truth!

This site was created and is maintained by Pierre Gras. Thank you with all the people who contributed their shares: articles, photographs, vidéos, worksheets… The author is opened with any suggestion allowing to improve this site for happiness of everybody. Finally, a big thank you to Robert Stirling !

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