Any engine powered by burning fuel inside it (for example, a standard automobile engine). Internal-combustion engines normally burn fossil fuels and therefore are a major source of air pollution. (Seesmog.)
An engine whose fuel is burned inside the engine itself rather than in an outside furnace or burner. Gasoline and diesel engines are internal-combustion engines, as are gas turbine engines such as turbojets. Compare steam engine.
cycles of a four-stroke gasoline-fueled engine:
A. intake stroke: the piston moves down, drawing air and fuel into the cylinder;
B. compression stroke: the piston moves up, compressing and heating the air and fuel mixture;
C. power stroke: the hot air and fuel mixture ignites, forcing the piston down;
D. exhaust stroke: the piston moves up, forcing the exhaust gases out of the cylinder