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Is Your Engine Steamed?
A common occurrence is to check the coolant level on a hardworking gas or diesel engine and to find that it is low while there appears to be no leak.
Typically, you top off the radiator and carry on—but with an uneasy feeling. Is a serious engine problem looming? The missing coolant had to go somewhere.
The answer in most instances is there’s probably not anything seriously wrong. The cooling system is working the way it should. If you work an engine hard, it uses more fuel.
What you don’t think about is the Btu of heat created by that fuel consumption. The thermal load on an engine is directly linked to the rate of fuel burned. The heat needs to be absorbed by the coolant to prevent thermal distress.
Engine Hot Spots
The job of the liquid is to remove heat from the engine, and the task of the radiator is to remove heat from the liquid. It is the liquid that cools the engine—not the radiator. The most problematic part of the cooling process is the combustion chamber, particularly around the exhaust valve. This engine hot spot is not represented by the reading on the in-cab temperature gauge.
Under high thermal loads, the coolant in the water jacket of the head undergoes a regimen identified as nucleate boil. When nucleate boiling occurs, the most effective heat transfer to the liquid happens.
When the engine is lightly loaded, the surface temperature of the combustion chamber is usually low enough that boiling (in the water jacket) does not take place. However, after the coolant boils, it is pushed from the site via the system pressure and water pump. It takes heat with it and will recondense as it cools.
Water-based antifreeze (50/50 mixture) has a low boiling point and a high surface tension. It is stubborn to release from the nucleate site. This produces an elevated metal surface temperature in the cylinder head and consumes the additive package along with the water that turned to steam. That is why supplemental coolant additives are required with traditional coolants.
There is hardly any heat transfer to water-based coolant once it becomes steam. The coolant becomes consumed over time when the engine is worked hard. Both the water and some of the additives become depleted.
The elevated temperature of the cylinder head has the potential to lead to cracking of the block. If a pressed valve seat is employed, it may drop out from hot spot coolant steaming.