Caption: An animation of a four stroke internal combustion engine (a realization of the Otto cycle), the commonest by far kind of internal combustion engine (ICE), (which is the commonest heat engine in our time):

1. intake stroke: Air and vaporized fuel are drawn into the engine cylinder.
2. compression stroke: The air and vaporized fuel are compressed by the piston and combustion begins. Note the combustion happens inside the engine cylinder: i.e., there is internal combustion, and hence the name internal combustion engine.
3. power stroke: Combustion causes expansion of the burning gas which pushes piston downwards.
4. exhaust stroke: Exhaust is pushed out.

A key point of the engine cycle is to convert reciprocating motion into rotary motion which in a car is converted to rotation of the wheels by a gear train in the transmission.

Note that nowadays heat engines are used primarily for vehicles (which need their own internal source of mechanical energy) and thermal power plants which generate electrical power for electrical power transmission. Most other engines are electric motors that depend on electrical power transmission.

In fact, most heat engines will probably be phased out by circa year 2100 at the latest to save us from further climate change. Nuclear power (where nuclear reactions just provide heat for heat engines) may be the exception.

In particular, the internal combustion engine car will probably be phased out in 1 to 2 decades and be replaced by electric vehicles (EVs). Then the internal combustion engine will be history.

Credit/Permission: © Richard Wheeler (AKA User:Zephyris), 2010 / Creative Commons CC BY-SA 3.0.
Image link: Wikimedia Commons: File:4StrokeEngine Ortho 3D Small.gif.
Local file: local link: internal_combustion_engine.html.
File: Thermodynamics file: internal_combustion_engine.html.