Final answer:
An external combustion engine is a heat engine where the working fluid is heated by combustion from an external source. Steam turbines are examples of such engines. Internal combustion engines, conversely, have the combustion process occur within the engine, such as in most cars.
Step-by-step explanation:
A heat engine where a working fluid, contained internally, is heated by combustion from an external source, is known as an external combustion engine. This definition contrasts with an internal combustion engine, where the combustion occurs within the engine itself. Steam turbines and steam engines are classic examples of external combustion engines, where heat is applied from outside the engine to produce steam which then drives the engine's mechanisms.
In an internal combustion engine, the fuel mixture is ignited inside the engine, creating high-pressure gases that do work on the pistons, thus converting chemical potential energy into mechanical work. These types of engines are commonly found in automobiles. The key difference lies in the location of the combustion process in relation to the working fluid of the engine.
Ultimately, what defines an external combustion engine is that the heat transfer originates outside the fluid or gas that does the mechanical work, as opposed to an internal combustion engine where the heat is generated within the working fluid itself.