External Combustion Engine: Types & Uses

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Lesson Transcript
Instructor: Dina El Chammas Gass

Dina has taught college Environmental Studies classes and has a master's degree in Environmental and Water Resource Engineering.

In this lesson, you will learn what an external combustion engine is and how it differs from an internal combustion engine. You'll also learn about the different types of external combustion engines and what they are used for.

External Combustion Engine Meaning

An external combustion engine uses a working fluid, either a liquid or a gas or both, that is heated by a fuel burned outside the engine. The external combustion chamber is filled with a fuel and air mixture that is ignited to produce a large amount of heat. This heat is then used to heat the internal working fluid either through the engine wall or a heat exchanger. The fluid expands when heated, acting on the mechanism of the engine, thus producing motion and usable work.

External vs. Internal Combustion Engines

The difference between external and internal combustion engines is quite straightforward and is made obvious by the difference in their names. In an external combustion engine, the fuel isn't burned inside the engine. With an internal combustion engine, the combustion chamber lies right in the middle of the engine.

External engines have a working fluid that is heated by the fuel. Internal combustion engines rely on the explosive power of the fuel within the engine to produce work. In internal combustion engines, the explosion forcefully pushes pistons or expels hot high-pressure gas out of the engine at great speeds. Both moving pistons and ejected high-speed gas have the ability to do work. In external combustion engines, combustion heats a fluid which, in turn, does all the work.

External Combustion Engines Types

A steam engine is one type of external combustion engine. In a steam engine, a fuel such as coal is burned in the combustion chamber. This heat turns water in a boiler to steam. Pipes carry the steam into a turbine, which has a series of blades attached to a shaft. The high-temperature steam expands as it rushes through the turbine and so pushes on the blades and causes them to turn the shaft. The spinning shaft can drive an electric generator, move a ship's propeller, or do other useful work.

Another configuration involves injecting the high-pressure steam into a chamber with a piston. The steam pushes on the piston that is connected to a crankshaft. The crankshaft has the ability to turn the back and forth motion of the piston into rotational motion that can spin wheels or propellers.

The second type of external combustion engine is the Stirling engine. The Stirling engine differs from the steam engine in that its working fluid is always in the gaseous phase, as opposed to the steam engine, which turns liquid water into gaseous steam. Furthermore, the Stirling engine continuously recycles its working fluid, whereas steam engines dump out the condensed steam once it's been through the engine.

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