Item Subject: cc:Mail Text
What's all this "Coolant Doesn't have time to cool off" stuff anyway?
It's been a bunch of years since college, but I think I remember
enough Thermodynamics to cause a little trouble. So here goes:
Is there such a thing as coolant flowing too fast? Let's not confuse
temperature and heat. Think of a baseboard heater, a long pipe with
fins down it's entire length. Hot water enters at one end, and
leaves the other somewhat cooler. The radiator is the interface
between the hot liquid and the cool air. From Physics, we know that
heat flow is proportional to the temperature difference across the
interface. The smaller the temperature difference, the less heat
flows.
If the colantflow is fast, the fluid doesn't have much of a chance to
cool, and the whole pipe stays pretty warm, heating lots of air. For
the same inlet temperature, a slower flow will make the exit end of
the pipe cooler, so less heat is being transferred to the air at that
end. Which type of flow do you want heating your house this winter?
Or cooling your car this summer?
The answer is the same. Don't slow down the coolant.
The engine is producing lots of waste heat as it works, which must be
carried away before things melt. The longer the coolant spends within
the block, the more heat it absorbs, and the higher it's temperature
gets. But as the coolant gets hotter, the temperature difference
between the block and the coolant gets smaller, reducing the flow of
heat into the coolant. This drives the block temperature higher, so
that the same amount of heat can flow into the hotter coolant. If the
coolant flow is slower, the temperature will drop more across the
radiator , but the coolant is moving more slowly through the engine
too, so the engine outlet temperature is going to be higher. This is
just what your thermostat does. It slows the flow to force the engine
to warm up quicker, then opens to maximize the flow at high
temperatures.
You wanted a new thread. Larry? Let's see what this kicks up.
Stu
|