An external combustion engine is a reciprocating engine for which combustion takes place outside the cylinders. An early example is the steam engine, which typically burned coal outside of the engine to turn water into steam. The Stirling engine is an external combustion engine that uses a working fluid to move pistons in cylinders. In an internal combustion engine, on the other hand, a fuel is burned inside engine cylinders. Though steam engines have largely been replaced, Stirling engines have many potential applications.
While combustion gases do not enter the cylinders of an external combustion engine, they must be in thermal contact with the engine for it to function. In a steam engine, the heat from burning coal is transferred to water through the walls of a boiler. This heat turns water into steam, which is directed into the cylinders of the engine. At the appropriate time, the steam pushes on a piston that turns a crankshaft. In this way, a steam engine transforms the chemical energy stored in coal into the mechanical energy of a rotating crankshaft.
The Stirling engine is similar to the steam engine, except that it uses a permanently-contained gas rather than disposable steam to push pistons. It works by exchanging heat at different locations of its cylinders. Heat passes through a cylinder wall and heats up the working gas, which tends to push the piston to create power. When a piston needs to move back to its original position, a heat exchanger allows the adjacent gas to cool.
The external combustion engine has had a variety of applications over the past two centuries. Steam power was used extensively in factories as well as on ships and trains during the Industrial Revolution, largely replacing water wheels and animal muscle as sources of energy. While steam engines were eventually overtaken by internal combustion engines, they remained the primary power source into the 20th century.
The Stirling engine has been applied even more flexibly. By converting rotational energy to electricity, it can take advantage of a heat source for combined heat and power generation. It can also be used in reverse as a heat pump—taking in electrical energy and pumping away heat.
Since the Stirling engine only transfers heat, not matter, into its cylinders, it does not require combustion to be the heat source. Other non-combustion forms of heat, such as nuclear power, could work equally well with this kind of engine. In fact, it has been suggested that future nuclear power plants may incorporate the Stirling external combustion engine to simplify the design and increase efficiency.