Wednesday, September 5, 2012

Thermostat Myth

There is a persistent notion that if water flows through an automobile radiator too rapidly, the water will not have time to cool in the radiator and overheating will result.

This is completely false.

The water pump pumps water from the bottom of the radiator through passageways in the block and head where it ABSORBS heat produced in the combustion chambers.

The water then circulates to the top of the radiator and then down through the radiator. As the water travels through the radiator the heat is transferred to the radiator core. From the core the heat is RADIATED into the air with the help of the fan.

Every time 1000 calories of heat are ABSORBED by the water in the engine, 1000 calories of heat must be RADIATED from the radiator, or overheating will occur.

The amount of heat radiated by the radiator depends on the average temperature of the radiator which is somewhat warmer at the top than at the bottom.

If the water is flowing rapidly, the bottom and the top of the radiator are close to the same temperature and the amount of heat radiated is at a maximum.

If the flow of water is restricted, the bottom of the radiator is considerably cooler than the top. This reduces the average temperature of the radiator and therefore less heat will be radiated from the radiator and engine temperature will increase.

It is tempting to think that to cool the engine, very cool water must come from the bottom of the radiator to the engine, but if we get that very cool water, less heat will be radiated from the radiator and that is contrary to what is really needed. A larger amount of warmer water does a better job than a smaller amount of cooler water.

If water is allowed to run through the system with no limitation, the temperature will generally be too LOW, not too HIGH. This is why we use thermostats to selectively limit the flow so that the temperature in the cylinder head will reach a desired level, typically 160 or 180 degrees. The thermostat controls the flow of water through the system so that at 160 or 180 degrees the amount of heat absorbed in the engine is equal to the amount of heat radiated from the radiator, and the temperature stays quite constant.

Limiting the flow of water can only RAISE the temperature above what it would be without the limitation. Limiting the flow can never LOWER the temperature. Limiting the flow only gives the water in the engine and the engine time to become hotter.

This is not to say that thermostats do not play an important role. 
A thermostat help bring the engine temperature up fast, this helps evaporate water condensation in the engine. A warm engine is also more efficient, and It also gives you a nice warm heater in the winter.
Myths usually start from somewhere.
The only exception where a restriction of some kind may help is on a non pressurized system where the radiator is plugged enough that the water pump forces water out of the overflow tube faster than it can draw it through the plugged core. In this case a restriction keeps water from returning to the top of the radiator too fast and out the overflow.
This is not much help though as you would most likely experience overheating on hot days anyway.

Thanks Dad for this info.

No comments:

Post a Comment