Reprinted from i Saluti, June 1997,
from the Alfa Digest
Thermosyphoning and Heat Soak Effects
by Paul Rollins
On some cars the electric cooling fan continues to run on even after the engine is shut off. What's the point? If the engine isn't running, the water pump isn't circulating coolant, so the fan will only lower the temperature of coolant in the radiator and won't affect engine temperature.
This is not correct. It is indeed beneficial to have the fan continue to run after the engine is turned off. Many early automobiles did not have water pumps. They relied upon the process of thermosyphoning, where the hot coolant in the engine, being less dense than the cooler fluid in the radiator, rises to the outlet and flows into the radiator. As the coolant in the radiator gives up heat to the air and cools, it moves downward and back into the engine. This cycle continues as long as there is a temperature differential between the radiator and engine (and the thermostat is open), whether the engine is running or not. Technically, even contemporary engine cooling systems are pump-assisted thermosyphon systems.
The other phenomenon that is involved is heat soak. This occurs because there is a temperature differential (really a gradient) between the engine metal that is not in close contact with the coolant and that which is farther away. When the engine is off and there is not a high airflow through the radiator, the thermosyphon cooling effect is low. This allows heat from these remote areas to flow to areas that are much cooler when the engine is running, resulting in fuel percolation, evaporation, flooding, and coolant boiling.
To take advantage of thermosyphoning and reduce heat soak effects, many modern cars use fans that run after engine shutdown.
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