Date:         Sun, 09 Jun 1996 02:16:45 -0700
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         jwakefield@4dmg.net (john wakefield)
Subject:      Re: Heat handling design error help

When Mike West wrote:
"When you cool oil in your engine, you are cooling the oil and won't
have much affect on engine temp at all. If you all feel otherwise, I
really don't mind.
 I would also like to point out some numbers called coeffcient of
heat transfer for various materials from Kents M E Handbook:
 Water transfers heat 7 times better than oil, Air tranfers heat
8 times better than water."

  It sounds to me as though Mike thinks oil's well known and well
documented lower heat transfer characteristics compared to water, make
it somewhat like the wonderfully low values of the space shuttle's 
exterior
tiles. Perhaps you've seen photos of them heated to a glowing state, and 
then
handled by people with their bare hands while they're still glowing.
Yes, they're very hot, but they have such low heat transfer values that
the heat load they deliver to these people's hands is insufficient to 
burn their hands.
  If this effect were significant with respect to oil's ability to pull
heat from a motor, we could run a motor under load until its systems are
hot, then open a valve to supply room temperature oil from a 55 gallon
drum to the oil pump and by monitoring the returning oil's temperature,
observe only slightly warmed oil, not nearly block temperature.  Mike,
if I'm understanding his position correctly, thinks the returning oil's
temperature would not approach the motor's average temperature.  By
contrast I believe it will, even after it's first pass.  Why would I 
think
this comparitvely low heat transfer coeficient medium would have absored 
so
much heat?
1) It's effective surface area per unit oil volume flow rate is high, 
much
higher than the water based coolant's.
2) The oil's contact surfaces are intimately related to the heat
producing processes within the motor.  Water based coolant is a better
conductor, but comparitively insulated from the heat producing motor
processes.

  The SAE Journal published information years ago about the comparitive
values of pulling equal btu levels from a motor's oil compared to its 
water based coolant system, and it consistantly favors taking it from the
oiling system.  This also preserves the oil better.  An interesting 
device
recently pointed out to me, is on the driver's side of the Mack truck
diesel motor.  It's an oil to water heat exchanger.  Seems the oil's
hotter path and lower flow rate per effective contact surface area, makes
it so much hotter than the other coolant, which is the water based 
coolant,
that dumping some of its heat through that heat exchanger helps engine 
life.

  Mike went on to ask "By the way, what's an air dump?  Is it like a 
bucket
of prop-wash?"  Sorry Mike and others who may not have recognized my use 
of 
language common to the heating & air conditioning trade.  I simply meant 
the
end of an air flow path from which used air is exhaused.  On the 
Vanagons, I
believe you'll find one high on each side near the tail end, designed to 
take 
advantage of the low pressure area induced there by speedy forward 
vehicle
movement.

  I think Mike intended to indicate the comparitive heat transfer values 
to
favor water over oil, and oil over air.  Designing an adequate, good 
balanced
effective engine cooling system, leaving the normal 20% overage margin to
accomodate real world use degradation losses, isn't a trivial task.  
Since
VW failed this test in their water cooled Vanagons, we end users are left 
to
finish their design work.  They should just issue a recall and fix them.

	John Wakefield