Radiator Cap

John Hess JohnH at ixc-comm.net
Thu Oct 16 14:38:29 GMT 1997



> ----------
> From: 	Christopher G. Moog[SMTP:cgmoog at worldnet.att.net]
> Sent: 	Thursday, October 16, 1997 12:34 AM
> To: 	diy_efi at coulomb.eng.ohio-state.edu
> Subject: 	Re: Radiator Cap
> 
> B.E. Herron wrote:
> 
> > That's exactly correct.  You have to leave the water next to the
> metal for
> > _some_ period of time in order for it to soak up some of the heat.
> Too long
> > and you over heat!
> >
> 
> Almost correct.  You can either leave the same water in contact for a
> period of
> time to absorb the heat (if the time is too long you will overheat) or
> YOU CAN
> MAINTAIN WATER IN CONTACT WITH THE ENGINE TO REMOVE THE HEAT.  It
> really doesn't
> matter if its the same water or a continuous flow (fast or slow).
> 
> > Tom wrote
> 
> > >What difference does the rate of flow have to do with heat transfer
> ??
> > >Like I said, seems to me that it's totally irrelevant as long as
> the
> > >liquid stays in contact with the metal -- in fact, seems t'me that
> the
> > >faster the better, since that keeps the delta-T at it's highest !!
> >
> 
> Correct
> 
> B.E. Herron wrote:
> 
> > Too fast and you don't take ANY heat from the metal and you over
> heat.  You
> > are correct, delta-T is required for cooling, but there is a finite
> time
> > period the liquid must be in contact with the metal for optimum
> > thermodynamic transfer of heat to take place.
> 
> Bull, think of this way, if a fast flow of water is supplied the
> engine will
> away be in contact with cool water and the jacket side surface of the
> engine
> will be in constant contact (similar to soaking in) cool water.
> 
> Try the experiment turn on the hot water faucet and wait for the water
> to get
> real hot now turn the faucet on slow so its dripping quickly.  Place
> your right
> hand under the faucet and your left hand under your right hand.  You
> feel hot
> water with the right hand and not so hot water with the left.  Your
> right will
> absorb most of the available heat from the water before it gets to
> your left but
> the overall rate of heat transferred to your hand will be small.  OK
> now do the
> same except this time have the faucet running full speed.  You will
> now scald
> both hands and the water will still be hot.  You are absorbing more
> heat even
> though the water is not staying in contact with your hand very long.
> If you
> still think the water can go too fast to absorb or give up heat try
> using a
> higher pressure water supply and let me know at what flow rate your
> hands don't
> burn (make sure your water heater is up to the task).
> 
> > Look at a Winston Cup car.  Those guys slow the pump down with the
> pulley
> > and then put restrictors in to fine-tune the cooling system based on
> the
> > track they're running (they change pulleys and restrictors for the
> different
> > tracks because of the different RPM and aero cooling, i.e. speed).
> 
> While I'm not familiar with the NASCAR setups I do know you want to
> turn the
> pump as slow as possible so you waste the least amount of power.
> 
> 
> Only possible problems I can see caused by operating without a
> thermostat are:
> 
> 1) overcooling
> 
	|
	Contradiction.  Cavitation will allow NO heat transfer.
	|

> 2) Cavitation (even in pressurized systems some areas are near enough
> to the
> boil point to cavitate).
> 
> 3) Short circuited flow.  That is high flow through one section of the
> engine
> and proportionally lower flow through another.  Lets say with the
> restriction
> (thermostat or blanking plate) the front of the engine has twice the
> flow of the
> rear (some of the flow past the front cylinders is passed to the
> heads).
> Without the restriction the flow by the front may double but the flow
> through
> the rear of the jacket might only increase 20% (these are guessed on
> my part).
> If this were to happen you could have localized overheating.
> 
	Remember that the cooling system is pressurized.  This means a
high volume pump into a low flow system is the design.  There ARE some
autos that will properly cool without a thermostat.  There are others
that will not.  Pressurizing the system means that the boiling
temperature of the system is raised and increases (no matter how
slightly) the cooling medium.  Slowing it down (within reason) allows it
to pick up more heat per volume and not only carry it away from the hot
spots, but increase the heat in lower temperature regions.  Also, it
allows the hotter coolant longer contact with the radiator with a much
greater temperature differential between the coolant and ambient, thus a
greater differential between inlet and outlet coolant.  Proper flow rate
IS necessary for optimum cooling and the thermostat of most modern cars
DOES contribute to the flow rate.  The arguments that argue that "at a
specific temperature" something does a particular thing is irrelevant,
as the temperature in a car's engine is dynamic.  As much as actually
cooling the engine, we are trying to stabilize the temperature of the
engine.  Most modern cooling systems are designed so that without
control, they will keep an engine TOO cool (assuming proper flow).  The
thermostat controls the temperature AND helps maintain that proper flow.




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