What's that Thermostat really doing while I'm drving around?

[DaveBoettcher]

I got into an disagreement with my father-in-law (who spent all his working life in the motor trade and therefore thinks he knows all about cars, whereas I only have a degree in engineering and experience of building nuclear power stations, and therefore know nothing) about the way a thermostat works, and I have not been able to find anything on the net to resolve it. I know that you guys in this forum know everything worth knowing about petrol engines, so I am appealing to you for help resolving the question.

My father-in-law thinks that a thermostat is closed when cold, opens as the engine warms up, and is fully open when the engine is at working temperature and stays that way; essentially that it is fully closed when cold, fully open when hot, and only in an intermediate position while the engine is warming up.

My view is that this would lead to the engine temperature swinging around as load on the engine and ambient conditions (temperature, vehicle speed) changed, whereas in practice the engine temperature stays within a tight range when driving. I think that the thermostat must be constantly moving throughout its range, modulating the flow of coolant through the radiator in response to changes in vehicle operating load, speed and external temperature, keeping the engine at its optimum operating temperature.

For example, under peak load conditions, such as labouring slowly up a steep hill whilst heavily laden on a hot day, the thermostat must be approaching fully open to provide high flow through the radiator because the engine will be producing near to maximum power and heat into the cooling system, while the cooling capability of the radiator is low, because the velocity of air flow across the radiator is low and the external air temperature is high. (The velocity of air flow across the radiator has a major effect on its ability to dissipate heat.)

Conversely, when cruising fast downhill on a motorway on a cold night on a light throttle, the thermostat will be nearly closed because the engine is producing little power, and the radiator is capable of dissipating much more heat than the engine is producing due to high air velocity and low external air temperature. Allowing too much flow of coolant to the radiator would result in the engine being over cooled and operating at lower than optimum temperature. A side effect of this would be that the passenger compartment heater would not be able to put out enough heat to keep the passengers warm.

I think that rather than being a simple open or closed device, it is quite remarkable how such a simple thing as the wax cylinder thermostat manages to keep engine temperature so constant under such widely varying conditions of vehicle operating load, speed and external temperature, and to do it so reliably. What do you guys think? And does anyone know of a proper reference paper that I can show my father-in-law to finish the argument?

[sdelasal]

My understanding is that the thermostat will vary coolant flow thru the radiator in normal driving i.e. not fully open. See here for more info for example ... http://www.eng-tips.com/viewthread.cfm?qid=229445&page=1

steve

[krystar]

my vote's on B also.

[emoore924]

Ask your father-in-law to *remove* the thermostat from his car, warm up the engine and see what happens. That's just like it being open all the time, right?

More rhetorical: If its open all the time, then why do they put it in there?

[Zero10]

The thermostat is completely closed until just a couple degrees before its "opening point", then it starts to open rather quickly (well, quickly over a small temperature range as compared to being closed over a wide one), until it is fully open at or slightly above its rated temperature.
So, this allows hot water from the engine to the rad, cold water comes back in trade, this lowers the overall temp of the engine, and as it falls below the set point of the thermostat the thermostat then begins to close. The engine temperature will vary about 2-3* from its normal operating temperature under constant conditions as a result. Far from swinging wildly.

So, mostly B.

[flash968]

well, that's closer to the reality

it does in fact begin to open just before the setting, go fully open, and then close again fairly quickly when the temp drops below the setting - this allows the heated coolant to sit in the radiator and cool off - then when the engine warms up again, it opens and lets the now cooled off coolant into the engine - then it starts the cycle all over again

it is NOT the same as running without one when it is fully open - however, it is fairy similar to running the same as a big washer in there when it's fully open - the problem with removing the thermostat is that the coolant never gets a chance to sit in the radiator and cool off, so it heats soaks, and once you get to the boiling point of your mixture, you're done

a good cap is also key to this working well - the pressure increases the boiling point of the system - if you haven't changed your cap in the last 5 years, you're overdue - check the manual for the correct rating

the temperatures in a typical system will swing 15-20 degrees, and will run about 15 degrees over the thermostat temp, or 100 degrees over ambient temp, whichever is higher

blockage due to tap water and mineral deposits will reduce the efficiency of the radiator

[DaveBoettcher]

Originally Posted by sdelasal

My understanding is that the thermostat will vary coolant flow thru the radiator in normal driving i.e. not fully open. See here for more info for example ...

http://www.eng-tips.com/viewthread.cfm?qid=229445&page=1

steve

Thanks Steve, that is an interesting thread. It confirms my view that once the engine is at operating temperature, the thermostat is constantly moving to regulate the flow of coolant from the engine to the radiator.

The thread refers to an interview with Paul Miglierina at Standard-Thomson Corporation, a division of Thomas Industrial and Automotive that make automotive thermostats sold under the Stant and Gates brand names. He said "The thermostat stays closed until the coolant temperature reaches the nominal thermostat opening temperature. After that, the thermostat regulates coolant flow to maintain the engine temperature in the optimum range. " which is pretty clear.

I didn't really conceive that it could be any other way, but my father-in-law thinks my reasoning is faulty, so I needed some more ammo, which I have now got! Thanks to everyone who replied.

[Mask of Destiny]

Originally Posted by flash968

it is NOT the same as running without one when it is fully open - however, it is fairy similar to running the same as a big washer in there when it's fully open - the problem with removing the thermostat is that the coolant never gets a chance to sit in the radiator and cool off, so it heats soaks, and once you get to the boiling point of your mixture, you're done

That doesn't make any sense. On a hot day with the car sitting still at idle, the thermostat will never close. The thermostat doesn't close to allow the coolant in the radiator a chance to cool off, but to keep the engine from running too cold.

[flash968]

yes, it does - on a hot day at idle is a perfect example of it opening and closing - you can even see it on your temp gauge (at least if you have a properly functioning system) - if the radiator is clogged, or you have a weak thermostat, or a weak water pump you may not see it, and it may stay up above the thermostat point

in fact, it does both jobs - that's why they fail as often as they do - like any metal subjected to constant heat cycles, they fatigue - they should be changed every 2 years when you flush your radiator, but definitely at least every other time

[DaveBoettcher]

Originally Posted by flash968

yes, it does - on a hot day at idle is a perfect example of it opening and closing - you can even see it on your temp gauge

Oh no it doesn't: what you see when this happens is that the water circulating through the engine and radiator is gradually getting hotter even with the thermostat fully open, because the car is stationary the lack or airflow over the radiator means that there is not enough radiator cooling to keep up with the engine heat output. When the temperature rises to the fan thermostatic switch setpoint the fan kicks in, provides the needed air flow over the radiator, and the coolant cools down. The temperature cycling you see is because of this, not because the thermostat is cycling open and shut.

at least if you have a properly functioning system - if the radiator is clogged, or you have a weak thermostat, or a weak water pump you may not see it, and it may stay up above the thermostat point

This just doesn't make any sense.

[Mask of Destiny]

Originally Posted by flash968

yes, it does - on a hot day at idle is a perfect example of it opening and closing - you can even see it on your temp gauge (at least if you have a properly functioning system) - if the radiator is clogged, or you have a weak thermostat, or a weak water pump you may not see it, and it may stay up above the thermostat point

What you'll see on a hot day at idle is the cooling fan turning on and off not the thermostat closing. The radiator doesn't work well when you're just sitting there because there's no air flowing through it so even though the engine isn't under load the temperature in the engine will slowly climb until the coolant temperature passes 90 degrees celsius in the radiator at which point the fans should kick in until the coolant temperature is < 90 degrees celsius. The thermostat opens at a cooler temperature and is located in a position that is likely to see higher coolant temperatures than the thermofan switch so if the conditions cause the fans to kick on it's unlikely that the thermostat will be closing.

Your thermostat will probably see the most action is on a cold day travelling at highway speeds. With the low ambient temperature and all the air rushing past, the radiator will do too good of a job at cooling the coolant so the thermostat will start to close.

[flash968]

sorry - that's not what i see - i have spent quite a bit of time on this car playing around with different fan temps settings, under hood airflow modifications, overflow cap pressure, and coolant mixtures, to optimize the system

the thermostat does indeed open and close at idle sitting still (if it's hot enough outside, but normally my engine does not get hot enough to open the thermostat) - the fans also come on and off if it gets hot enough - the water temp gauge reflects both of these, but at different times

my oil temp gauge also reflects the oil cooling system, which also seems to have a thermostat

the other cars i've played with over the last 30 years have been similar - if the radiator is sized correctly, and the thermostat point chosen correctly, the thermostat opens and closes at idle - how much depends on the particular condition and setup of the engine, as well as the differential between the ambient temp and the thermostat setting

for a $3 part, it sure is a complicated job to do

[DaveBoettcher]

Originally Posted by flash968

the thermostat does indeed open and close at idle sitting still (if it's hot enough outside, but normally my engine does not get hot enough to open the thermostat) - the fans also come on and off if it gets hot enough - the water temp gauge reflects both of these, but at different times

So you think that when your engine is warmed up and idling at normal operating temperature the thermostat is closed, unless it's really hot outside?

[Mask of Destiny]

Originally Posted by flash968

if it's hot enough outside, but normally my engine does not get hot enough to open the thermostat

I find this incredibly unlikely. The purpose of the thermostat is to effectively to provide the lower bound for engine temperature and the thermofan switch acts as the upper bound. If the thermostat never opens it means your engine isn't getting up to proper operating temperature.

When the thermostat is closed, there is nothing cooling your engine apart from the air in the engine compartment (unless you have the heat on anyway) which is stagnant when the car is sitting still. The coolant circulating through the system just serves to avoid local hotspots and evenly distribute heat throughout the engine block (and to the heater core if the heat is on).

my oil temp gauge also reflects the oil cooling system, which also seems to have a thermostat

I don't think the 968's have an oil thermostat, but I'm no expert. Doesn't look like Pelican sells it if they do, but I'm too lazy to go digging in the PET. Air cooled 911s do, but that's because the oil serves the role that coolant does in our cars in addition to being a lubricant.

[944Spec_bound]

Am I the only one who's put a thermostat in a pan of water and watched it open & close? Or taken off a radiator cap and monitored the coolant circulation?

It's just a 2 position switch, it hits a certain degree and pops open, cools down and pops closed. simple.

Maybe I'm not getting the question here. Are we assuming there's a smart thermostat that knows my driving conditions and rpm's?

[Schumi]

the thermostat has a small range at which the thermoreactive mechanism operates in. It, of course, does not know your driving conditions and rpms, but driving conditions and rpm have a direct correlation to horsepower, and horsepower has a direct relation to the amount of heat rejection from the block to the water.

And they're not quite $3 parts flash- Pelican here lists them at $15. If you know where to get these $3 ones you talk of, PM me. :-)

[flash968]

i test every thermostat before installing it - over the years i've had too many not operate at the indicated temperature - if you set the water temperature at just below the thermostat point BEFORE you put the thermostat in it, you can see the rate at which it opens - if you just stick it in a pot of boiling water, it will open pretty fast

no, my engine does not get up to the temp needed to open the thermostat at idle - that is a "problem" that i am still working on - i have a fairly stable temperature under load now, and never get near hot, but it runs a bit too cold most of the time

i actually have a flow when the thermostat is closed because i drilled a 1/8" hole in the thermostat, so i do get cooling - it's an old racer's trick - this maintains a more even temperature under most conditions, but can make the engine run cold when it's cold out

i'm not certain that the oil system has a thermostat or not, but it sure acts like it does, which is why i said "seems" - the oil temp seems to be independent of the water temp, or at least is much slower to respond to changes

like some other small rather generic parts, i have gotten my thermostats at a local parts house, which cross references to a different number, so they end up a lot cheaper than a porsche specific part - i'll see if i can figure out which vehicle it comes from

[Schumi]

We need to make a listing of generic parts like that to try and save us all the cost of parts stamped with porsche part numbers. I figured there would be a generic replacement for the thermostat- it practically just has to have the right diameter and seal- things that are pretty common.

Now if we can just find cheap rollers. *rollseyes*

[flash968]

lol - no kidding - stuff like that makes me crazy - things like belts, coils, thermostats, relays, yada yada - as long as you use the same manufacturer, you get the same part, often without the big price tag - porsche never made this stuff themselves, and only made a supplier deal with outside vendors - those parts have certain specs - all you have to do is meet or exceed those specs to make sure you have a good part

they usually have very competitive pricing here, and i've never heard of any issues with pelican whatsoever, and it is very convenient to have one parts source for everything, but some stuff can be found elsewhere for less, if you are willing to do the homework

sometimes it's stepping over a dollar to pick up a dime though

[944Spec_bound]

A VW rabbit thermostat is the same part, and you can get it in a lower temp (you'll want to match the Thermofan switch though). It was alot cheaper too.
I wish I had kept the part numbers.