The function of the water jacket is to transfer the temperature of the engine combustion chamber and the inner wall of the cylinder to the coolant through heat conduction. Because the liquid is flowable, it circulates through the water pump to the radiator, and the radiator dissipates heat from the coolant through the flow of outside air. The dissipating hot coolant circulates to the engine water jacket to receive the heat generated when the engine is working, and so on. To sum up, it is the transfer of heat energy.