The engine
The engine turns fuel into motion by burning a mixture of air and fuel inside its cylinders. The spark plug ignites that mixture in a petrol engine; a diesel engine ignites it by compression alone, which is why it has no spark plug.
A two-stroke engine fires once every revolution and a four-stroke once every two. The two-stroke is simpler but burns oil with its fuel, which is why it pollutes more.
Water reaching the engine usually arrives through the air cleaner, so keep it dry and clean.
The engine is cooled by the radiator and the fan, and the fan is driven by the FAN BELT. A snapped fan belt means the engine will overheat within minutes.