When the engine is running the combustion in the cylinders heats up the engine. But if the engine gets too hot it loses power. If the engine remains hot for a long time, that causes slight distortion in the engine parts that in turn reduces the gaps between the moving parts. That is why an effective cooling system is necessary to cool down the engine parts and the lubricating oil. There are two kinds of cooling systems: air cooling, which uses the running air flow to cool the engine, and liquid cooling that circulates coolant fluid through passages around the engine.
The mechanism underlying: A liquid cooling system uses a web of passages around the cylinder to circulate coolant through. When the temperature of the coolant around the engine rises, it is sent to the radiator (a heat dissipating tank) to be cooled by running air flow or an electric fan and then circulated back to the engine. This constant circulation of coolant constantly cools the engine. Yamaha YZF-R15 uses this kind of cooling system and is expected to continue with the job in the forthcoming models of higher range mobikes.