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Why are automatic transmissions cooled?

Manual transmission aren’t cooled except by airflow, but automatics seem like they always have some kind of liquid cooling system, usually the engine coolant. Why is that? The reason I’m asking is that newer automatic yield better mpg’s than corresponding manual transmissions, so it doesn’t make sense that the automatic version would produce more heat than the manual. If the automatic gets better mpg, that makes it more energy efficient, and it should be cooler than a manual, not hotter. So why is there a need to liquid cool a newer automatic transmission?


Hydraulic pressure over an extended period of time requires a means of cooling.


If the transmission fluid in an automatic gets hotter than a manual transmission, then shouldn’t a car equipped with an automatic version get worse mpg? The heat has to be coming from the energy in the gasoline, right?

The heat comes from pressure. But in theory, if an automatic transmission and a manual transmission have the same number of gears, same gear ratios and are shifted at the same time under identical circumstances, the manual will be more efficient and get better gas mileage.

But todays latest automatic transmissions have more gears and are shifted by a computer that hits the ideal shift points better than most human drivers.

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I’ll guess that most manual transmissions are driven either over revved in every gear or lugged in every gear the vast majority of the time @GeorgeSanJose. Very few drivers would get a vacuum gauge and do all the shifting that automatics do “automatically” to remain in the best performance range.

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I like Tester’s answer. You are hung up on efficiency. Take an old fashioned tire pump, and pump up a small tire, and the pump gets hot. That is because increasing pressure of a substance makes it hotter, has nothing directly with the transfer of energy. I might be wrong, but that is my theory subject to correction by men with more knowledge on this topic.

Also, each 1000 feet you go up, average temp drops around 3 degrees, strictly due to thinner air.

I am told there is a place on the west coast of Mexico where rich people have two houses. One down on the beach by the ocean, and another way up the mountain which is adjacent to that beach. In the winter they hang out down there because it is warmer. In the summer when the beach is very hot, they go up the mountiain for the cool environment.

I’d guess that the energy that gets the pump hotter comes from the person doing the pumping? They ate their Wheaties for breakfast, , and now the energy from the Wheaties is transferred to heat the pump.

Just to be complete… SOME manual transmissions have oil pumps (my old Honda S2000 did), and some have pumps and coolers - Some variants of the C7 Corvette and certain Camaros have transmission AND differential oil pumps and coolers for track-play as do some Mustangs like the GT350R for the same reason. Big horsepower applied means big heat to both differentials and manual transmissions even if their efficiency is 95%.

A great deal of the heat from compressing air is the result of the same phenomena as occurs with an AC compressor. When a given volume of gas is highly compressed the heat energy in the gas is compressed also and as pressure rises the temperature rises. The evaporator on a car heats the refrigerant to ambient temperature ~90* F but once compressed the refrigerant at the condenser is approaching 200*. Then when cooled to ~100* it is forced through the expansion valve where the pressure drops to 30psi and the temperature drops to 40*. The compressor likely adds some heat but it’s negligible I would guess. It was my understanding that the greatest heat produced by an automatic transmission is in the torque converter when accelerating. Some old Powerglides had impellers welded on the leading edge and there were large ports in the bell housing for air to enter and exit.

Energy has to be transferred for something to get hotter, unless the increase in temperature is due to a pressure or volume change. (or due to chemical action) But those are totally reversible. That is, if you change the pressure or volume back to where it was, the temp change will exactly reverse.

Hydraulic pressure over an extended period of time requires a means of cooling. No, pressure alone does not cause thermal energy. There has to be another factor.

I seem to recall an equation from chemistry class

P* V = n * R * T

So if the volume and number of molecules contained in that volume (the n* R part) are constant, increasing pressure would cause a rise in temperature, right? I think that’s for gasses though, may not apply to liquids.

I’ve never seen an automatic that uses engine coolant. Trannys have very different needs that engines. It may be confusing because some manufacturers use a separate section of the radiator to cool the tranny fluid, but they’re never mixed unless there’s an unwanted leak between the sections. Some manufacturers use separate heat exchangers to dissipate the fluid, such that even in the event of a leak it cannot mix with engine coolant.

Matter when compressed dispenses heat, matter expanded absorbs heat.
Automatic transmissions are hydraulic machines, they use hydraulic pressure to operate the valves that operate the mechanical devices that change gears. The compressed fluid dispenses heat energy.

They also use their fluid in the torque converter to absorb energy from the engine spinning to allow the car to idle without moving the car forward. In that case, the heat is created by fluid turbulence in the torque converter. That turbulence generates heat. In both cases, the heat dispensed by the fluid must be released. It’s all done through the tranny cooler, the very same heat exchanger used by the above paragraph.

Measure the temperature of any hydraulic system after it’s been at rest for a few days. Work the system and measure it again. You’ll find it’s heated up. In larger systems, such as an articulated loader, it will have heated up considerably. You’ll be able to readily feel it with your hand.

Yep, modern automatics work best at a “warm” temperature, so one function of the “cooler” is to more quickly warm the trans fluid up after start-up.

Some Hondas (this for a Civic) even have a separate trans warmer:


It’s a hydraulic motor that creates heat as the fluid is put under high pressure to create the friction necessary to create the drive. One of the problems with the hydro drives on lawn mowers is that the little fan they use cannot dissipate the heat enough and the transmissions fail due to heat.

First, all liquids are incompressible, so they do not heat up from rising pressure (and dropping volume) like a gas does.
What does most of the heating of auto transmission fluid is fluid friction as it swirls around in the torque converter.
Plus, a little heat from friction in the gears and bearings, same as in a manual.
Once the torque converter clutch locks the heating in the converter stops.

If you have liquid (or gas) in a container, under no pressure; and insert a propeller, like the paint mixer that attaches to a drill, then swirl the liquid (or gas) around there will be heating.
The amount of heat will be equal to the energy put into the propeller.

Fluid friction also accounts for a substantial portion of the heating of motor oil as it moves around in bearings and between the piston and cylinder.

Automatics can get better MPGs because they can do a better job of keeping the engine in an efficient speed and throttle setting; and typically have lower RPMs in top gear, along with locking up the torque converter.

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yes, but it’s not an increase in total energy, ie, if you reduce the pressure, the temperature will decrease.

Not quite. They can be compressed or expanded, as can solids and, of course gasses. The amount of energy required is much different for each, but they can all be compressed. The change in heat capacity is what causes the heat to be released. You might want to start your search here:

Turbulence is, however, also a significant contributor to tranny fluid heat. I’m on board with that one.

Th engine coolant on a warmed up engine is cooler than the transmission fluid so it acts as a heat exchanger.

Same principle behind Ford’s TFI ignition module which was mounted on the distributor near the thermostat outlet. The idea was that engine coolant would pull heat away from the module as the module ran much hotter than the engine coolant.
Too hot as it worked out… :-(a

Every automatic car I have owned is “cooled” by a heat exchanger located in the bottom tank of the radiator. The engine coolant doesn’t go to the transmission but it’s temperature does. If you get a towing package on your car or truck a separate cooler is added using air to cool the fluid before it goes to the radiator.

I do not believe it’s pressure that causes the transmission fluid to get hot and to need cooling. As previously noted, if you pressurize the fluid in a transmission and measure the temperature immediately and again hours later, you’ll see the fluid has cooled in the subsequent reading.

I always understood it’s the friction created by the fluid as it circulates and operates, (acts on) the components in the transmission, with the biggest producer of friction being the torque converter.

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