Working Principal of Stirling Engine

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Magneto
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Working Principal of Stirling Engine

Post by Magneto » Fri Nov 20, 2009 11:40 am

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Fig : Working principle of the Displacer type Stirling engine

Ideal working principle of Stirling engine

This figure shows the ideal working principle of Stirling engine. Gas expands, as it warms and compresses, as it cools. Stirling engines move by using this characteristic of gas. Howevernstant volume. The gas pressure inside an engine increases.

2-3 : Isothermal expansion process
The gas expands as keeping a constant temperature. The engine works to the outside.
3-4 : Constant volume cooling process
The working gas is cooled with constant volume. The gas pressure inside the engine decreeses.
4-1 : Isothermal compression process
Gas commpresses as keeping a constant temperature. The engine discharges the heat to the outside.

Working principle of the Displacer type Stirling engine

A displacer type Stirling engine makes working gas go and return between high temperature space and low temperature space by a displacer piston, and output the pressure variation that occurs by this gas movement as power by a power piston.

1 : Constant volume heating process
By the displacer piston moving to the side of compression space, the working gas in the engine becomes a higher temperature and flows to the expansion space, as passing through the heater. The gas pressure in the engine increases.
2 : Isothermal expansion process
By the pressure increased in engine, the power piston is pushed. The engine works to the outside.
3 : Constant volume cooling process
By the displacer piston moving to the side of expansion space, the working gas in the engine becomes a lower temperature and flows to compression space, as passing through a cooler. The gas pressure in the engine drops.
4 : Isothermal compression process
The power piston is pushed, and the engine is worked from the outside and discharges heat to the outside.


With an actual engine, both pistons don't move intermittently but sinusoidaly. By attaching a phase angle of 90 degree between a movement of both pistons, engines get a cycle that has such like these processes.

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Fig : Working principle of the 2-piston type Stirling engine

With 2 piston type Stirling engine, a movement of the working gas and output of power is done by two power pistons. A phase difference of 90 deg. is attached to both power piston displacement.

1 : Constant volume heating process
With this process, the power piston on the side of a compression is upper, of expansion moves under. Working gas in the engine becomes a high temperature and flows to expansion space, as passing through a heater. A gas pressure in the engine increases.
2 : Isothermal expansion process
In this process, both power pistons move to the lower side, and the working gas expands. The engine works to the outside.
3 : Constant volume cooling process
In this process, the power piston on the side of a compression is lower, and of expansion moves upward. Working gas in the engine becomes a low temperature and flows to the compression space, as passing through a cooler. The gas pressure in the engine drops.
4 : Isothermal compression process
In this process, both power pistons move upward, and the working gas is compressed. The engine is worked from the outside and discharges heat to the outside.


With an actual engine, both pistons can not move intermittently. By attaching a phase angle of 90 deg. between both pistons that move sinusoidaly, engines have above-mentioned processes approximately.

Invention of Stirling engine

Stirling engine was invented in 1816 by Robert Stirling who is Scotch clergyman closed cycle external combustion engine. It is more safe than a steam engine, as an explosion doesn't occur almost. Moreover, it attracted attention as the engine having the heat exchanger which recover wasted heat. It remain be recognized that its style locating the power piston and the displacer piston in same cylinder in order to obtain high compression ratio is good design now .


Blooming and declining in the 19th century

An bloom of Stirling engine production was brought by J. Ericsson, a Swedish inventor and engineer. He succeeded in manufacturing a practical Stirling engine and sold nearly 2000 engines of the 0.5 ~ 5 horsepower class by around 1850 in Great Britain and America. After that, many Stirling engines were made aiming at higher output and higher efficiency. However the capacity of Stirling engines in those days was inferior compared with Otto engines ( the current gasoline engine ) and diesel engines. Therefore, Stirling engines was produced poorly in a range of low outputs and have gradually been hidden in the shadow of the internal combustion engines.

Revival of the Stirling engine

The Stirling engine had attracted attention again around 1940, after Philips co. had started to develop the Stirling as a portable electric power generator. At the same time, the Stirling engine was being researched as a refrigerator. It succeeded in that it cools even 74 K. However, the transistor that needed only a small ammount of power was invented and batteries become the perferred source of power, and thus the Stirling portable generator was not made practicable as a power source.

Stirling engines of present

After that, the Stirling engine was widely researched on in Europe and North America, a policy of energy savings greatly enhanced it's development. On the other hand, research of Stirling engines in Japan was started about 20 years behind Europe and North America. After that, development of a general-purpose Stirling engine started in a 6 years plan since 1982 as a part of "Moonlight plan" of the Ministry of International Trade and the Industry. Various highly-effective engines were developed during that 6 years plan. At present, solar heat powered Stirling engines, terrestrial heat powered Stirling engines, Stirling engines for ship use, air conditioning Stirling systems, and development of Stirling refrigerators are actively being developed by people around the world. In the near future, we will be able to see various Stirling engines of low noise, long life span, reliability, multi-fuel operation, clean-exhaust, and so on.
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