engine thermostat 2

[thruthefence]

I have always been under the impression that the common engine cooling system thermostat, operates either "open' or "closed". Opening when the engines reaches the design operating temperature, & pretty much staying open during the time the engine is running. Only closing when the engine shuts off, & is cooling down. An associate now assures me, that the thermostat actually modulates the flow of the coolant, closing & opening to keep the coolant in a "range". Now, i am not refering to extremes of temperature, as I am sure you can get a thermostat to close on a really frigid day, but under "normal" temperate circumstances? Thanks to everyone in advance, for their input.

 

[GregLocock]

The curve I found in sae paper 2007-01-2064 is pretty much closed at 80 and open at 90. I am reasonably sure that your ECT doesn't vary by 10 degrees in normal use, so your friend is probably on the right track.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

If you are talking about a thermostat that controls water flow, they are progressive over a fairly narrow range as indicated by Greg.

If you are talking about a thermostat switch for an electric water pump or electric fan, they are the on/off type.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[RossABQ]

I've never attached anything on this forum before, but I'll try here. It is a transcript of an interview between a Stant engineer and an enthusiast who wanted similar info...

 

[GregLocock]

Just to expand on Pat's comment, on a modern engine the fans are often multispeed, and will ramp up with ECT. The ECT sensor is used for a whole bunch of diagnostics by the EEC.

Great find Ross, LPS for you.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[thruthefence]

To clarify, I was referring to the one that commonly resides under the so-called "gooseneck" that the coolant hose attaches to.

 

[DigitalGT]

In that case the thermostat you're refering to is the plain coil or wax operated type. The coil spring type do modulate as engine temperatures change. BUT the coil types that I see open at a temperature quite a bit lower than the engines standard operating temp. So even if the engine temp does suddenly drop 10° the "stat" is still wide open. They do have the abillity to adjust to temp changes but, as previously stated, that delta usually only occurs at shut down.

The wax type t-stats also technically have the abillity to modulate slightly but I don't see how it would ever happen while the engine operates. Once the wax melts and pushes on the piston to open the valve, it stays melted. A momentary drop in temp would be "ignored" by the wax type stat once it's throughly heat soaked.

"Stats will modulate but usually at a temp below which the engine generally operates."

Given that rule, I can't understand why the performance aftermarket advocates replacing the t-stat with one of a lower value like it's a magical radiator. The thermostat only promises to keep the engine at a certain minimum temp. It does nothing to keep the engine from over heating.

 

[ivymike]

I suppose it depends on what your application is. If you're going to be doing very short bursts at high power, interspersed between relatively long periods of lower power, then you need more thermal inertia - having the coolant temperature initially lower will allow you to absorb a bit more heat with a given mass.

 

[evelrod]

Given that rule, I can't understand why the performance aftermarket advocates replacing the t-stat with one of a lower value like it's a magical radiator. The thermostat only promises to keep the engine at a certain minimum temp. It does nothing to keep the engine from over heating.

A lot depends on the ambient temp and the over capacity of the heat exchanger. In the case of ECT as it relates to a race car where changing the Tstat will change the operating temp of the engine and 'overheat' is not applicable. My systems vary from engine to engine. Some like a 160f Tstat (e.g., Lotus twincam) for max hp and some like a 200f as does my little 1380 Mini Cooper. I try not to just guess at which temp the engines perform best...after seeing a NASCAR Ford pick up hp by raising the ECT to 240f...

There are other problems that mitigate my coolant temps, such as the Lotus twincam cannot handle much over 7 psi coolant pressure as it tends to loose the water pump seals if they are not installed perfectly (difficult to do consistently) but the Mini handles 15+ psi just fine.

I read the mags and the catalogs and, yes, I see the same things you express...I just don't take them all as fact.

As to the Tstat modulating the temp over a narrow range, I see this in my Lotus as the cooling capacity of the rad is excessive whereas the Mini tends to just open at +180f and stable at 200/210. In all my installations I drill several small holes around the perimeter of the Tstat to slow the incidence of overheat should it fail. Gives me a bit more time to notice the guage. I tried the so called 'fail safe' ones and I found them wanting in one fashion or another.

Rod

 

[Lou Scannon]

I agree with Evelrod. If radiator size were no object, it would have significan overcapacity and the Tstat would have full authority to regulate temperature. For a radiator that is marginally sized (under the operating condition of interest), you certainly don't want the Tstat to be offering any appreciable restriction.

 

[RossABQ]

Maybe you're saying the same thing, but my take is that in 90% of driving conditions (typical mass-produced cars), the radiator does have significant excess cooling capacity relative to cooling needs. i.e., it isn't often 120 F. ambient, and the car isn't pulling a trailer uphill with the A/C on. The thermostat does modulate in these "normal" conditions.

 

[Tmoose]

"you certainly don't want the Tstat to be offering any appreciable restriction. "

My current belief is the thermostat provides some useful restriction to ensure the pressure near the hottest portions of the coolant system (cylinder head near exhaust ports and spark plugs) are not reduced too much below the nominal pressure cap rating operating pressure by the water pump's suction head. If the pressure drops, localized boiling and very high material temps (cracked heads!) would result.

Some legit racers replace the thermostat with a restriction. Moroso and other offer restrictors with assorted hole sizes for tuning.

 

[Lou Scannon]

If the thermostatic component happens to have a secondary function of diverting or restricting flow within the water jacket as part of the overall cooling system design, that has nothing to do with thermostatic control of the coolant out temperature (to the radiator). The thermostat's one and only purpose is to regulate the temperature of the coolant being sent to the radiator (i.e. the temperature of the coolant exiting the water jacket), within it's capability under the existing conditions.

 

[DavidBoettcher]

Hemi has got it just right, the only function of the thermostat is to regulate the flow of coolant to the radiator to maintain the engine at optimum operating temperature, and to do this it is constantly moving in an analogue fashion to be slightly more open or closed within its movement range as engine heat output, vehicle speed and ambient temperature change. It does not move bang-bang between fully open or fully closed.

Any issues about which path the coolant takes through the engine have to be solved at the design and prototype stage by altering the resistance of the various paths so that balanced and even cooling is achieved throughout the engine block, whether the coolant is flowing through the radiator or recirculating.

As far as thermostats being only either "open" or "closed" as per the original question by thruthefence, this is a common misconception which I think probably stems from the experience that probably all of us have of testing a thermostat by sticking it in a pan on the stove and heating it up to see that it opens. Because it is being heated from cold to near boiling quite quickly, the thermostat does suddenly go from closed to open, which gives us the view that this is how they work.

But if we were able to vary the temperature in the pan accurately within the thermostat's operating temperature range of a few degrees, we would see it modulating within its range of movement in response to temperature changes, not just snapping open and shut as the temperature moved above and below some point.

As the guy from Standard-Thomson in RossABQ's attachement says: "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. . . The range between the minimum and maximum opening temperature is typically around 5 to 7 degrees F. . . Under normal operating conditions, the thermostat is NOT at its full open position. The thermostat is designed to keep the engine in a narrow temperature range. . . If we design a thermostat with a 0.300” full opening stroke, we may want it to operate at about 0.200” open stroke under normal conditions. If you start up a big hill, or start towing a load, the engine puts out more heat. We need to be able to open the thermostat more, say to 0.250”, so there is more coolant flow to remove the extra heat from the engine."

It is clear that we should not want the thermostat to be fully open during normal operation, say while we are cruising along, because then if we start to climb a steep hill on a hot day towing a van, and we press on the throttle to get more power, and hence more heat rejected to the cooling system, then there would be no way of increasing cooling, and the engine would overheat.

But you might think: Why don't I just make the cooling system have a big enough capacity needed to cope with this hill climbing situation? Well, because then when you were cruising downhill on a cold winters night and a light throttle, the engine would be overcooled, and your passenger would be complaining that that there was no heat coming out of the heater.

The really clever thing is that your simple wax thermostat takes care of all of this for you, making sure that the engine gets exactly the right amount of cooling as engine heat output, vehicle speed and ambient temperature change; no more, no less. For such a simple device, that's really quite remarkable.

 

[drwebb]

Boy, I have really learned a lot about thermostats from this thread and previously had no appreciation about the amount of tunability, as I also assumed they were a go/nogo valve.

My recent experience with my 2000 Celica had me confused. Last fall the T gauge started reading low- going from 1/2 bars regardless of ambient to 1/3 bars normal and much slower to heat up. The cabin heater seemed to work fine. This fall I replaced it with a name-brand aftermarket unit and it heats to 1/2 scale again, but seems to take longer to get there than the OE unit originally did. So this is all attributable to different thermostat tuning. My curiosisty now is how does a thermostat fail to allow excess cooling (weak spring, wax leakage?), and how significant might difference in tuning between the AM unit and OE be (i.e. might it reduce my gas mileage by an unnoticeable but real amount)? BTW, I went with aftermarket because it was much cheaper than from the dealer and I was a little disappointed in the OE's 75Kmi 'lifetime'.

 

[RossABQ]

Did the Aftermarket thermo have a part number specific to your car, or did it cover a bunch of different engines?

A specific instance I am aware of where the AM piece is no good is the AutoZone cheapo piece for Saturn 1.9L's. Even tho it is specifically made to fit the Saturn, it has a setpoint 12 degrees cooler than factory. It seems to open partially much earlier than setpoint, too. At any rate they behave much like you describe, take forever to heat up, gas mileage suffers. The correct AM part is sold at NAPA for about 2x the cost ($15) and duplicates the factory part's performance in every way. I gained 2 mpg after the switch, and oil consumption dropped noticeably (180k motor). The dealer piece is about $30 as I recall, sounds like Toyota is mighty proud of theirs.