Stirling engine

Principle, advantages and disadvantages

Principle

The principle is relatively simple: the principal fluid which produces a work is a Gaz (Air, Hydrogène or Hélium) with a modest pressure and is subjected to a cycle of Carnot at 4 times, separated by two phases from displacement: heating, relaxation, displacement towards the cold source, cooling, compression, return towards the hot source.

Food: The hot source of the engine can be supplied with an unspecified source: external combustion of derived from the Oil, the Natural gas, the coal, the wood, etc but also renewable energies like the solar energy or the geothermal energy.

Advantages

Producing little vibration thanks to the absence of explosion, the absence of valves which open and closes, the absence of gases which escape. That makes it quiet and reduces the mechanical constraints.
easy Maintenance : from its absence of exchange of matter with its environment and the internal absence of chemical reaction, this engine undergoes less source of deterioration than an internal combustion engine.
Good Output : it can border the 40% (that is to say 80% of the maximum of the cycle of Carnot), against approximately 35% for the spark-ignition engines: if the difference in 5 points appears weak, it means nevertheless nearly 15% (5/35) of saving energy. The electrical motors, whose output can certainly reach 95%, are not comparable, because electricity is a form of energy itself difficult to store and produce with a good output, which is a strong limit for certain applications.
Reversible . The cycle of Carnot is reversible, the Stirling engine is also: driven by another engine, it becomes a Heat pump able to cool with - 200°C or to heat with more 700°C, according to the direction of drive. This, without employing gas with special properties which confer practical or chemical disadvantages to them (like the freon of the usual refrigerating machines, destructor of the layer of ozone). In practice, moreover, it is the function of effective heat pump which makes it possible some machines to exist.
Multi-source . Because of its mode of supply heat this engine can function starting from any source of heat (combustion of a fuel unspecified, solar, nuclear, or human heat)
Faible pollution . Heat coming from outside it is easier to create this heat in a way less polluting than in many thermal engines.

Disadvantages

the sealing of the pistons is difficult to realize. The very strong temperature variations and the need for using the lightest possible gas complicate this problem.

delicate Design. Whereas the internal combustion engines produce heat directly within the fluid, very quickly and in a very homogeneous way, a Stirling system rests on heat transfers between gas and the exchangers (two sources, the recuperator), whereas the gases are heat insulators where the exchanges are very slow. Moreover, it is necessary to minimize " volume; mort" (container of the fluid which does not achieve the cycle and thus does not contribute to the output). All that poses problems of Dynamique fluids, problems difficult to solve, in connection with the exchangers, of the recuperator, the pipes or the piston which allow the displacement of gas during the cycle (problems of diameter, length, turbulences to be created or avoid, etc).

Difficult to order. The variation of mode of this engine is very difficult to realize because it can be done only while acting on the compression ratio of the working liquid.

Very bad aptitude to produce a power and a variable couple. It is a very great disadvantage for the propulsion (automobile, in particular), whereas it is precisely this application which made the fortune of the internal combustion engine. This disadvantage could be reduced a " in the case of; system hybride" (the engine works then to constant mode, the modulation of power being dealt with by the electric system), but they are still rare.

On a boat with propeller with variable step, this is however not a disadvantage.

having today only few applications in great series (see generating Whispergen), contrary to the internal combustion engine, it is much more expensive; moreover, for the same reason, the industrialists do not grant to him the same interest in term of research and development, which does not enable him to fill its delay (by supposing that possible).

Use

The Stirling engine has applications of niches, in situations where the initial costs of the system are not a serious disadvantage compared to the advantages (military applications, of research, point)

the principal commercial application of Stirling is in the field of the industrial and military refrigeration. It is used as machine for liquefaction of gases and like cooler for the guidance systems infra-red soldier.

It was used for a class of Sous-marin S Swedish, not only because of its silence, crucial property for the submarines, but also for the much weaker production of unburnt gas necessary to the contribution of a heat gradient (a difference in temperature) to a Stirling engine; indeed, a submerged submarine can evacuate gases only by compressing them with a pressure at least equal to that of the ambient conditions, requiring (and thus wasting) a considerable share of the energy supplied on board.

This engine equips also certain American classes of frigate S, and of the Drone S.

NASA studied it to provide energy to the satellite and space probes because its output is better than the solar panels. But the reliability of the solar panels (not comprising moving parts, their risks of breakdown are less than those of an engine) always made them prefer.

Three types of Stirling engine

a Stirling alpha contains two separated pistons of power, a “hot” piston, and a “cold” piston. The hot piston is located close to the exchanger at the highest temperature, and the cold piston, is located close to the terminal point of the lowest temperature. This type of engine has a ratio very high power-volume, but has engineering problems, connected (frequently) to the too high temperatures of the hot piston for its joints.

a Stirling beta has a simple piston of power placed coaxialement with a piston of “displacement”. The piston of displacement is only used to propel gas of the hot exchanger of temperature, towards the cold exchanger of temperature. On a complete cycle, this piston does not consume energy. This engine does not require a mobile “joint” in the hot part of the engine, and can reach high outputs of compression, thanks to the pistons which are able to overlap during their displacement. Small Stirling engines were developed without piston.

a Stirling gamma is Stirling beta, on which the piston of power, is not assembled coaxialement with the piston of displacement. This configuration produces a weaker compression ratio, but is generally simpler mechanically, and is often used in the Stirling engines with several cylinders.

History

Souvent the steam boilers exploded, Robert Stirling imagined an engine without boiler subjected to too strong pressures.
Stirling deposited its patent on September 27th 1816.
In 1843, his/her brother James " industrialisa" this engine, for a use in the factory where he was engineer.
1938, the Philips company invests in a " Stirling" engine; , an engine of more than 200 horses, with an output higher than 30%.

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