Any combustion engine follows the basic idea of converting fossil fuels into motion. Internal combustion engines do this by converting the energy created by an explosion of the fossil fuels, into motion. External combustion engines, on the other hand, use the energy from burning fossil fuels to expand a gas or liquid and use the expansion to create motion. The addition of this intermediate step is what differentiates an internal vs. external combustion engine. External combustion engines came first, and tend to be more inefficient. Famous examples of external combustion engines are steam engines