X-Mozilla-Keys:                                                                                 
Fred wrote:

Putting cooling tubes in a passage in the pan and passing oil over the tubes
would help address the lack of velocity issue.  I could do some analysis if I
knew the cross sectional area of the oil flow passage, tube size, etc.  But I
have a big-time concern.

The heat exchanger proposed in the figure adds additional flow restriction,
as must all heat exchangers, but does so at the oil pump inlet.  A general
rule of thumb is: NEVER obstruct a pump inlet or it may cavitate (create
vapor bubbles in the flow) which could kill pump performance and cause
surging, etc.

Normally cavitation is a boiling phenomenon that occurs when the local
pressure drops to a point that the boiling temperature is lowered to the
local temperature, and local boiling occurs creating bubbles of vapor.  Oil
has a very low vapor pressure and I would not expect it to boil easily, even
at 200F.  BUT the oil has little air bubbles and possibly tiny droplets of
water from combustion.  I would be concerned that if the inlet restriction
was excessive, the oil would foam at the pump inlet.  Pumping efficiency
would go into the tank, zilch oil flow, and your imagination can take it from
there.

I have also been wary of coolant tubes in the pan because of differential
thermal expansion due to different materials or different heating rates with
same materials, (which can be addressed by putting strain-relieving bends in
the tubes) and penetrations through the pan via welding or fittings or
whatever Every one is a potential failure point.  Would you like oil in your
coolant, coolant in your oil, or oil and/or coolant all over your engine?
None seem appealing to me.

My current thinking is to combine the oil cooler and radiator in a single
package as done in some of the custom Fluidyne radiators.  Fluidyne shows
units with an aluminum radiator and a stainless steel oil cooler in a common
frame - oil cooler at the bottom or side.  I wrote them yesterday with energy
transfer requirements for a "highly modified Mazda race car"  (never say
airplane to Fluidyne - the phone goes dead) using the latest energy balance
figures for a 13B engine putting out 135 HP at a SFC of 0.5.  This yells
coolant and oil heat rejection in accordance with the guidelines Paul gave
me. I will let everyone know what I get back.

In the mean time, I think the general rule of thumb is spare no expense on
hoses and fittings in your oil system, and be sure that you have a good
strain relief where the oil line ties onto the oil cooler or engine so it
does not crack something.  Don't Don't let the lines flex where they tie to
something made of sheet metal (like a radiator or oil cooler).

Fred

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