| Thermostats revisited [message #77166] |
Thu, 18 March 2010 22:01 |
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I found a great article on how automobile thermostats work. For what it's worth:
Changing thermostats to a different rating cannot cure overheating; it can only cure overcooling. To understand why, you need to understand why a thermostat determines only the minimum operating temperature. Let me repeat that: A thermostat determines only the MINIMUM operating temperature, not the maximum temp. Keep reading to see why.
An engine takes energy stored in fuel and releases it by burning the fuel. Some of the energy (less than 25%) gets converted to kinetic energy (motion) to turn the engine and ultimately the wheels. A little bit of it gets turned into vibrations (actually, another form of motion) that we can feel and hear. The largest part of it gets turned into waste heat. This thermal energy flows into the engine parts to be carried to the radiator by the coolant (or remains in the combustion gases to be carried out the tailpipe). The radiator dumps it into the atmosphere. Any energy that makes it to the radiator is lost, having done you absolutely no good whatsoever. Under normal conditions, a cooling system in good repair can shed heat faster than the engine can produce it, so you need something to limit the capacity of the cooling system to dump heat. That's the thermostat's job.
Imagine an engine that is at normal operating temperature. Say, for instance, that you're driving along a ridge road in the mountains. At the moment you're neither climbing nor descending, there is a lot of air going through the radiator, and the thermostat is open. Now suppose you turn off the ridge road and start descending a steep hill into a valley. The cooling system is dumping heat like mad and the engine is producing very little, so the engine starts to cool down. The thermostat notices this (it sees the coolant temperature falling) and begins to close. Because there is now less coolant flowing through the radiator, there is less heat being carried to it for disposal. The thermostat keeps the engine from becoming too cold. Because the thermostat is limiting the amount of heat being carried the radiator, after a while the engine temp starts to rise again. Once it's high enough, the thermostat opens.
Now you've reached the bottom of the valley, and you start up the hill on the other side. The engine is really working hard now, and there's less air through the radiator because you're going slower. Since the engine is making heat faster than it's being carried away, it begins to get hotter. The themostat is already fully open, so there's nothing it can do. The engine gets hotter and hotter. If this carries on long enough, the radiator will boil over. It has overheated.
Another way to think about it is this: Imagine that you live in Florida in a house with no air conditioning. On a sweltering August afternoon, turning the furnace thermostat down from 68ºF to 60ºF won't make you feel any cooler!
So you see, a thermostat cannot prevent overheating. It only prevents overcooling. A thermostat can only be the cause of overheating if it is defective and doesn't open as it should.
The fix then, if your engine is truly overheating, is obviously not to put in a cooler thermostat, but to fix the fault in the cooling system. A cooler thermostat is just a band-aid that at best can only temporarily mask the problem. If the cooling system is marginal, it might buy you a little time before the engine overheats, but not much. Find the real problem -- a clogged radiator, a water pump with a defective impeller, too much scale in the engine, or a bad distributor.
A bad distributor, you say? Yes! If your timing is incorrect, then you're losing power. Power = energy. If you're losing power, then that energy has to go into the cooling system. Furthermore, the engine will have to work harder and put even more energy into the cooling system. Carl Heideman at Eclectic Motorworks says that you lose about 1 HP for every 1º your timing is off.
John Ruff
Chandler, AZ
1975 Eleganza
WA3RIG
If I use ZDDP in a new car - will the tappets go flat?
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