An engine is something that produces some effect, normally called work, from a given input. In automobiles this effect is rotational torque which causes motion. The origin of engineering was the working of engines.
There is an overlap in English between two meanings of the word "engineer": 'those who operate engines' and 'those who design and construct new items'. The word "engine" is also sometimes used to refer to a device that performs a particular task (not "work" in the physics sense). This usage is particularly prevalent in computer science, where terms like search engine, "3-D graphics rendering engine" and "text-to-speech engine" are common.
Typical car engines include reciprocating piston, rotary engines, orbital, also inline, horizontally opposed, V2, V8 up to V16. All of these configurations refer to the movement of the pistons within the cylinder block.
A special case is the air engine which is driven by compressed air.