An external heat engine (EHE) refers to any engine that receives its heat from a source other than the fluid that makes the engine work. The most common type of EHE is the external combustion engine, which is used in many power plant designs.
External heat engines are generally steam engines, and they differ from internal combustion engines in that the heat source is separate from the fluid that does work. For example, an external combustion engine would use a flame to heat water into steam, then using the steam to turn a turbine. This is different from internal combustion, like in a car engine, where the gasoline ignites inside a piston, does work, and then is expelled.
All external combustion engines are external heat engines. There are EHEs, like solar thermal power plants, nuclear power plants, and geothermal power plants, that are not external combustion engines. Despite this, external heat engines, like nuclear reactors, are sometimes referred to as external combustion engines.
External combustion engines are the most common form of external heat engines, because of their use in power plants. An external combustion engine is unique from other EHEs because it requires a fuel to undergo combustion to create the heat that is used for work.
External combustion engines are no longer used in transportation, as mobile designs are not efficient enough, but they continue to be used in power plants. For example, a natural gas power plant boils water into steam to turn a turbine, creating electricity. The external combustion design means that the natural gas does not come in direct contact with the water, and the engine still uses the immense amount of energy emitted to do useful work. A coal-fired power plant works in much the same way, where coal is taken into the plant from the mine and burned in a boiler. Pipes send water into the boiler, and the burning coal boils the water, creating steam, which turns a turbine and creates electricity.