External combustion engine

An external combustion engine (EC engine) is a heat engine where the internal working fluid is not burnt in a combustion process. The internal fluid is instead heated from an external source through a heat exchanger. Burning fuel with an oxidizer, or any other heat source can supply the external heat. An external combustion engine performs work by the engine mechanism similarly to an internal combustion engine (IC engine) in that the fluid is heated and then expanded through the internal mechanism of the engine providing motion and force becoming work. It is then cooled (closed cycle) or exchanged for cool fluid (open cycle).

There doesn't seem to be any limitation to the type of cycle that can be run in an EC engine, nor does it matter whether the cycle is open or closed. However, some cycles are more common as EC or IC engines. John Ericsson built an engine running on the diesel cycle as an external combustion engine with an open cycle.

EC engines require at least one heat exchanger and often have two, for supplying the heat into and removing the heat from the engine. The necessary temperature difference is then available to the engine. In contrast, IC engines have a heat exchanger, commonly called a cooling system, to keep the combustion chamber from overheating. Thus, the IC engine must remove heat as waste. A device called a regenerator in the EC engine saves some of that heat and is why it can be more efficient. The EC engine's heat transfer technology requires greater care during the engineering process than a cooling system. EC engines often have cooling systems, however, they may or may not be smaller than a corresponding IC engine's.

Since the combustion is external to the mechanism, they are much less particular about the type of fuel they burn. They also tend to be cleaner because the combustion is continuous and more easily regulated for temperature, oxidizers and fuel amount. If lower combustion temperatures and pressures are used it will create less exotic exhaust gases, such as nitrogen oxides (NO, NO₂ etc.).

Examples: A steam turbine is a good example of an external-combustion engine. Heat from a burning fuel/oxidizer mix, or from a nuclear reactor changes water in a device called a boiler to steam. Pipes carry the steam into the turbine, which has a series of bladed wheels 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. Steam leaving the turbine has a much lower temperature. The spinning shaft can drive an electric generator, move a ship's propeller, or do other useful work.

Some other externally heated cycles are Stirling cycle, Ericsson Cycle, Papin and Brayton cycle.

See also

• steam engines
• Stirling engines
• internal combustion engine