

| Subject: | Re: Heat handling design error help |
| Date: | Sun, 09 Jun 1996 02:16:45 -0700 |
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
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