Internal combustion engines generate considerable heat due to combustion of fuel. This process happens inside the cylinders which heats up the surrounding engine block.
Metals are susceptible to thermal expansion. So continuous heating and cooling would destroy the tight tolerances between the piston rings and the cylinder.
The cylinders are surrounded by a channel through which a fluid flows. This fluid could be either a coolant or even air.
The coolant liquid consists of ethylene glycol and water mixtures. It absorbs heat efficiently and then flows into a radiator (heat exchanger) where it gives away the heat and cools down. Then it is circulated again for maintaining the engine temperature.
This liquid cooling system is heavy and requires space. For smaller engines where not much heat is generated, it is not feasible to have a cooling system which weighs and occupies as much space as the engine itself.
In such scenario, instead of having a cooling channel surround the engine block, the engine block is surrounded by a finned type radiator (typically seen in 2 wheeler engines).
It is important to know that compared to the liquid coolant mixture, air has very poor thermal conductivity. Hence, it requires a larger heat transfer area to absorb the heat effectively.
So, in an air cooled engine, the fins help in increasing the heat transfer area and as air flows through these fins, it absorbs heat. Here the cooling is restricted by the air flowing through the fins.
Hence for larger displacement engines, liquid cooling in a closed loop is preferred.
