Wouldn't it be easier to say that the thermostat is a valve that restricts
the flow of coolant to the radiator and thus the ability of the radiator to
cool the coolant mixture (if it can't get to the radiator, the radiator
can't cool it!), and if it won't overheat at 195, then it won't overheat
with a lower temp valve (thermostat)? The proof is that it won't even
hardly get hot without any thermostat at all if it's working properly. Try
it in an older engine (non-computer). We used 140 deg thermostats in the
old days before ethylene glycol, when we had to use alcohol as antifreeze.
The damn cars didn't even seem to give out any heat at all - because they
_were_ running cold. Cold (below 180 deg) may not be good for newer cars
"cause of the computer controls systems.
--tm
----------
> From: svieth@ameritech.net
> To: dakota-truck@buffnet.net
> Subject: DML: DO NOT change t-stat...A discussion of cooling
> Date: Friday, June 05, 1998 3:39 AM
>
> I asked my younger brother (mechanical engineer at
> a large air conditioning manufacturer) to comment
> on our recent cooling questions. Here is his reply.
> If you have any direct questions, send 'em to me
> and I'll forward them to him.
>
> -Scott
> '97 Dak
>
> p.s. Notice at the bottom where he gets a little big
> for his britches. His Jeep doesn't even have a
> Magnum engine and yet he's talking like he owns
> the street. I'm going to have to kick his
> Laredo butt with my Dak. :^) :^) :^)
>
> =======================================
>
> "... The premise of more heat
> transfer happening because of heat differential is correct, BUT that is
> not what we are really concerned about.
>
> The engine in your automobile will output a certain amount of heat
> throughout its application range. The cooling system is therefore sized
> to accommodate a range of heat dissipation rates plus some given safety
> factor. A very integral part of the cooling system is the thermostat.
> A thermostat is closed under cold (low heat load) conditions, is wide
> open under hot (high heat load) conditions, and wanders around somewhere
> in between the rest of the time (normal heat load) to keep the vehicle
> operating in the desired temperature range.
>
> By moving from closed to open and everywhere in between, the thermostat
> regulates the amount (flow) of water, during any given amount of time,
> that is exposed to the heating generating surfaces (inside of the
> engine) and the heat dissipating surfaces (inside of the radiator).
> This could be summed up by saying that the thermostat controls the heat
> transfer rate.
>
> The equation for heat transfer is Q = Mdot*Cp*dT where:
> Q = heat transfer
> Mdot = mass flow rate of transfer medium (coolant)
> Cp = heat transfer coefficient for transfer medium (coolant)
> dT = temperature difference transfer medium (coolant) and
> contact surface (either inside of engine or inside of radiator
> depending on your area of concern)
>
> The Cp is a material property of the mixture in your cooling system so
> this will only change is the mixture changes.
> The dT changes depending on ambient air temp (for radiator) or internal
> engine temp.
> The Mdot is controlled by the thermostat regulating coolant flow in the
> system and is provided by the water pump.
>
> We will start by saying that today at 2500 rpm's your engine will need
> to dissipate as much as it will need to dissipate tomorrow at 2500
> rpm's. If today with a 195 deg. F thermostat your engine dissipates
> sufficient heat to remain running we will assume that the heat transfer
> between the engine block and the coolant is sufficient so we do not need
> to look at that (design engineers should have taken care of this and
> made a recommendation of coolant properties). The mass flow of the
> coolant is regulated by the 195 deg. F thermostat but provided by the
> water pump so the water pump capacity can be ignored. The radiator is
> dissipating sufficient heat given today's ambient air conditions and the
> quality of the interior surfaces of the radiator given the appropriate
> flow rate provided by the water pump and thermostat. If you are happy
> that everything is working fine we can continue because all of these
> items will not change due to 180 deg F thermostat versus 195 deg. F
> thermostat.
>
> We will now look at the effect of the 180 deg F thermostat at 2500
> rpms (everything else should be very similar within such a short time
> period; including ambient air temp for now). The First Law of
> Thermodynamics deals with the conservation of energy within a system.
> Therefore if the same amount of energy is going into the engine
> block the same amount must be dissipated. Since "Q" (heat transfer)
> must be the same under both applications and "Cp" does not change and
> dT does not change significantly the only thing that can change is
> Mdot (mass flow rate). So to dissipate the same amount of heat, from
> the same system, under the same conditions, the 180 deg. F thermostat
> must allow more coolant to pass through the radiator to maintain the
> same system temperature (IT'S A MIRACLE - THAT'S EXACTLY WHAT IT IS
> SUPPOSED TO DO!!).
>
> Now if the "dT" changes this means that the Mdot must change
> accordingly. This is true no matter what thermostat is in the engine.
> Now you are probably wondering what happens when the ambient temperature
> changes dramatically (i.e. 120 deg F versus 70 deg F). If under the hot
> conditions, your engine did not overheat with the 195 deg. F thermostat
> it will not overheat with the 180 deg. F thermostat given that each
> thermostat allows the same maximum flow rate of the coolant.
>
> All of this comes back to how well the cooling system was designed from
> the start for the expected operating range. If the radiator is of
> sufficient size to transfer enough heat and the water pump provides
> enough mass flow and the thermostat opens and there are no other
> significant flow restrictions, the engine will not overheat whether
> there a 180 deg F, 195 deg F, 210 deg F, or no thermostat.
>
> This all has to do on whether or not you are operating in the
> anticipated operating conditions (plus a safety factor), the cleanliness
> of the cooling system, and the proper coolant mixture. If your engine
> is currently overheating or the operating temperature does not change
> after a change in the thermostat temperature you have some serious
> problems that have nothing to do with this letter.
>
> It is possible that in some applications the cooling system is too small
> to allow for a sufficient cooling to occur but this should not be the
> case in performance or truck applications otherwise you got ripped off.
>
> I hope this helped out a little bit. Sometimes a person just has to
> take some time out and help those that are less fortunate. I figured as
> the new owner of a Jeep Cherokee Laredo that I should help out the
> Daimler - Chrysler boys with their problems."
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