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What are external combustion engines?

How do they work?

What types of external combustion engines are there?

What are all the common advantages and disadvantages of external combustion engines in comparison to the much more common internal combustion engine?

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Heat engines (in thermodynamics) convert thermal (or heat) energy to mechanical energy. Heat engines typically work by utilizing the expansion of a gas and/or liquid as it is heated. External combustion engines are a type of heat engine where the heat is produced (by combustion - burning) outside of the engine itself.

From the wikipedia page:

An external combustion engine (EC engine) is a heat engine where a working fluid, contained internally, is heated by combustion in an external source, through the engine wall or a heat exchanger. The fluid then, by expanding and acting on the mechanism of the engine, produces motion and usable work. The fluid is then cooled, compressed and reused (closed cycle), or (less commonly) dumped, and cool fluid pulled in (open cycle air engine).

An example of an external combustion engine is a cylinder of a steam locomotive. In a steam locomotive, heat energy is produced by combustion from chemical energy in the fire box. This heat energy is transferred to water to produce super-heated steam which is fed to the cylinders. It is the expansion of this super-heated steam that produces the mechanical energy to power the locomotive. Using these definitions, it is important to note that the engine is just the cylinders where the expansion occurs, and not the water boiler or firebox. Thus the combustion in the firebox is external to the engine.

Conversely in an internal combustion engine, the combustion that converts chemical to heat energy occurs in the engine itself - that is inside the cylinders.

TL;DR:

  • External combustion - conversion of chemical to heat energy and conversion of heat energy to mechanical energy occur in different places.
  • Internal combustion - conversion of chemical to heat energy and conversion of heat energy to mechanical energy occur in the same place.

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