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."
This archive was generated by hypermail 2b29 : Fri Jun 20 2003 - 12:08:55 EDT