If pressure was what made the fluid hot, then the bottom of the ocean should be boiling hot instead of close to freezing.
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Right. That’s a sign that pressuring the fluid heated it.
You’re comparing apples and oranges. The ocean has massive ability to dissipate heat, beyond that generated by being heated by the sun, heated by undersea volcanic activity, and compression of the seawater.
TSM, I think you are missing the point. Pressure increase will indeed raise the temperature momentarily, but it is a fixed amount of energy. To keep the temperature raised, in view of normal energy losses due to conduction, etc., you would need a continued input of energy, or if due to pressure, a continued increase in pressure. But that has obvious limits.
Temperature increase due to turbulence and friction, on the other hand, is a continuing process, as these transfer energy from whatever caused the fluid flow into increasing the temperature of the fluid.
Consider two insulated cylinders cylinders with pistons pushing downward:
One is filled with water, the other, air. Begin with the weights supported so no pressure is applied. Release the weights. The air will compress and temperature T2 will increase according to well known thermodynamic laws. The water will compress so slightly that no movement will be seen and the temperature increase at T1 will be insignificant.
I see your point now Bill.
Nice work explaining it, by the way.
Automatic transmissions are cooled by the engine coolant (in a heat exchanger), but you are right they don’t use the engine coolant.
Re the original issue, if automatics get better mpg and therefore more efficient in the powertrain’s use of gasoline, why do they need to be cooled, while manuals don’t? From what I’ve learned here, there’s some counter acting principles at work. The automatics may produce more heat, but they have more gears and shift more efficiently, effectively making the engine more efficient at propelling the car. And another idea I came up with, automatics may not produce that much more heat, but may be more sensitive to fluid over-heating than manuals.
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I’m not 100% certain, but I believe that some class 8 trucks which use the Eaton-Fuller heavy duty manual have transmission fluid coolers
danke …
They are cooled to prolong life because they get hot, Jalopnik says
“The transmission fluid flows in a loop between the impeller to the turbine. The fluid coupling in the video above suffers from severe churning losses (and consequent heat buildup) as the fluid returning from the turbine has a component of its velocity that opposes the rotation of the impeller. That is, the fluid returning from the turbine works against the impeller’s rotation and thus against the engine.”
Good link Barky!
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It seems to me that some automatic transmissions were air cooled and didn’t have a heat exchanger built into the radiator. The VW 411 with an automatic had an air cooled engine and obviously no radiator. I suppose the transmission was similarly cooled.
The VW Automaitic Stick had no fluid cooler as the transmission was a manual 3 speed. There was a torque converter which allowed the car to sit at a stop light in gear but there were no internal clutches, valve body, and so on inside the transmission.
Some of the early Powerglide transmissions were air cooled (which essentially meant no cooling at all) but you’re going back to the late 50’s when it didn’t matter if the transmission lasted past 100K miles because the rest of the car wouldn’t either.
Modern automatic transmissions with locking torque converters are more efficient than the old automatic transmissions that always had torque converter slip and thus don’t heat up the automatic transmission fluid as much.
It is torque converter slip that caused the lion’s share of the heat in an automatic transmission. Oil pump losses and clutch slip is nearly insignificant in comparison unless you have a really badly slipping clutch or band in which case you have bigger problems anyway.
Every BTU of heat that friction creates in a transmission, automatic or not, is 778 ft-lb of energy that didn’t make it to the output shaft of the transmission.
I seem to recall that a light would come on the dash indicating “overheating transmission…shift to lower gear.”
They probably meant the torque converter temperature. A slipping torque converter gets hot just like a slipping clutch. It’s just that the slip doesn’t wear anything out like on a normal clutch. Then again, the torque converter might also have shared its oil with the transmission so it could have been the entire transmission that overheated.
When they finally came out with locking torque converters on automatic transmissions, my reaction was “what took them so long?”
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I am surprised that Chevrolet got away with the air cooled PowerGlide. On the 1950-1952 the transmission depended entirely on the torque converter when the car took off from a dead stop in “Drive”. One had to start in “Low” to use the lower speed of the two speed transmission. In 1953 the PowerGlide was revamped so that it started off in “Low” and shifted to direct when in “Drive”.
I know the VW Beetle had the “automatic stick shift”, but as I recall, the VW Fastback and Squareback of the latr 1960s and early 1970s offered a fully automatic transmission as ab option. The VW 411 also was equipped with an automatic transmission. These cars all had air cooled engines, so I assume the transmission was air cooled.
All designs seem to have what we would now consider common sense problems in their first iteration. These were no exception.
Hopefully self-driving cars will advance as much between now and their future as automatic transmissions have over these past generations.
Since the subject came up here…My mom had a VW Bug with the the 3-speed auto-stick or whatever the heck they called it. I remember there was both a fluid drive and a clutch and the clutch automatically disengaged as the shift lever was moved. Was the torque converter an actual torque converter with a stator or was it just a fluid drive?
I think it was a torque converter, which let it get away with 3 speeds.