See Figure 2-51. The radiator holds a large volume of coolant in close contact with flowing air. This allows the radiator to transfer heat from the coolant to outside air. Many cooling systems today have a separate expansion tank outside the radiator. The expansion tank is partly filled with coolant and is connected to the radiator cap. The coolant expands in the engine as it heats up. This sends part of the coolant into the expansion tank. Then when the engine reaches operating temperature a valve in the radiator cap closes which seals the coolant system. The pressure in the cooling system increases and thus prevents boiling. This increased pressure allows a higher coolant temperature and thus a more efficient cooling system.
