Trans convertor slippage question

[supton]

Let's say an engine makes enough torque in a particular gear to get the job done, and let's say that the transmission is reluctant to lock up the convertor. Would less heat be generated with a downshift and the engine lazily spinning away? Same hp through the convertor, albeit less torque, and presumably less slippage due to higher rotational speed.

This weekend I got the chance to play with this in the truck, and found that it seemed to run cooler in 2nd than in 3rd. I'm guessing it's largely because it's still around rated stall speed in either gear this particular speed (20mph). I haven't a clue how the 1,800 rpm stall speed was determined, I'm just going off memory on that, and I do understand that "stall" is impacted by power into and resistance to acceleration. [For a given convertor, increasing hp in raises stall speed, raising vehicle weight and/or using numerically lower gearing also raises stall speed. The stall speed is not a hard on/off.]

For the record, I was driving up Mt Cadillac in the fog. 25mph road but I was basically loafing up it at 20mph. I have my SG watching ATF temp, not sure what sensor but I'm guessing it's the convertor outlet, given how fast it responds. If I let it, the truck would climb ok in 3rd but I could watch the temp drop if I dropped it into 2nd on the same grade.

 

[IndyIan]

At high enough engine rpm the torque converter becomes very efficient, even if its not locked mechanically. At lower engine rpms the TC absorbs some energy(hp) in "slippage". I read that the output torque is actually higher than the engines torque when slippage occurs at low rpm, but as you saw this costs hp showing up as heat in the trans.

 

[meep]

not sure about your 2010 tundra, but in my '06 I suspect there is a thermostat on the trans cooler. this may make temps harder to read. Mine seems to keep it right around 160. I might get the TC outlet temp to come above 160, but not for long--- as it will spike then get brough down to 160, even if the conditions that caused the 180f spike are still happening.

I would expect the same as you, though-- that the lower gear with higher rpm would yield less loss.

 

[440Magnum]

You got it. You could NEVER run the low highway RPMs that modern 5 to 9-speed automatics run if you didn't have locking convertors. And intuitively you'd think the old non-lockup automatics would have roasted themselves... but spinning >3500 RPM on the highway let the convertors be well into their "tightly coupled" mode with minimal slippage. People learn that when they stick a Gear Vendors O/D behind a vintage non-lockup transmission without changing the rear gears and try to lug the convertor at 1500 RPM. Going to a higher (numerical) ratio rear end so that you run ~2500 on the highway is an OK way to split the difference.

 

[supton]

Yeah, the stator, somehow, magically, redirects oil off the (compressor? the side being driven as opposed to the driving side) and directs it back at the driven side. I guess during slippage the oil coming off is moving with some velocity, as opposed to lockup where it is not; this "firing" it back at the driven element it acts as a gear ratio. Of sorts, since it's kinda lossy. During 100% slippage (no vehicle movement) one can get upwards of 2:1 gain in torque; but once the vehicle starts moving, well, can't get something for nothing. That's why automatics, for years, had one gear less than stickshifts: at takeoff it "had" another gear, for the required torque multiplication.

[During lockup, the oil coming off the driven element can't possibly have much energy left in it, otherwise it wouldn't be locked up--there would be loss of energy somehow. During high slip though, the initial hit of oil is reflected, and the stator catches that, and gives it another go. Somehow this works, and it's important that the stator have a one-way clutch, I guess it spins during slippage? Never haven owned an automatic before, I never did much reading up on it.]

Edit: sorry, was directing this at Indylan, didn't see the other posts until now.

 

[440Magnum]

Originally Posted By: IndyIan
I read that the output torque is actually higher than the engines torque when slippage occurs at low rpm, but as you saw this costs hp showing up as heat in the trans.


That's correct. Mind-boggling at first, but that's the very definition of a "torque convertor" as opposed to a "fluid clutch" which doesn't amplify torque. The torque amplification is made possible by a sprag-clutched reaction element (stator) inside the torque convertor.

 

[supton]

Thanks 440Magnum, that makes sense.

meep, on my transmission, possibly yours, there are two internal sensors for reading temperature: it's surmised (but nobody seems to know) that one is convertor outlet and the other is pan.

http://www.tacomaworld.com/forum/technic...tml#post3938462

When the convertor locks, or unlocks, I can usually see the difference after a couple seconds, what I believe is the update rate on the SG. I know on mine I do have a thermostat, and I've contemplated locking it into full bypass, so as to get cooling all the time--just haven't yet. I've also meant to enter in both sensor codes to try out in the SG, but never manage to remember to do so. By way of reference, whichever sensor it is I have programmed in, it will slowly come up to 195F or so on a cold start. On the highway it was staying at 197-199F in full lockup, even though the water temp was lower. Climbing Cadillac it would not lock up the convertor at all, and was running about 208F in 50F weather, running 2100rpm. Finally, if I'm not paying attention it will spike to 230F in hot weather on a hill--but you can watch it drop stupid fast once the convertor locks up.

 

[supton]

Hmm, maybe the stator doesn't redirect the oil, maybe when it gets hit by the oil it tries to drive the driven side, hence the additional force... This will bug me until I wiki this...

 

[supton]

Ah, found it:

http://en.wikipedia.org/wiki/Torque_converter#Operational_phases

Quote:
The key to the torque converter's ability to multiply torque lies in the stator. In the classic fluid coupling design, periods of high slippage cause the fluid flow returning from the turbine to the impeller to oppose the direction of impeller rotation, leading to a significant loss of efficiency and the generation of considerable waste heat. Under the same condition in a torque converter, the returning fluid will be redirected by the stator so that it aids the rotation of the impeller, instead of impeding it. The result is that much of the energy in the returning fluid is recovered and added to the energy being applied to the impeller by the prime mover. This action causes a substantial increase in the mass of fluid being directed to the turbine, producing an increase in output torque. Since the returning fluid is initially traveling in a direction opposite to impeller rotation, the stator will likewise attempt to counter-rotate as it forces the fluid to change direction, an effect that is prevented by the one-way stator clutch.


So, I was wrong; the fluid hitting the driven element is not directed back at the driven element, but directed so as to hit the driving element in a way so as to not impede it.

I had to look outside that link to find that the stator one-way clutch is referenced to the transmission case--under high slippage the stator doesn't spin at all, relative to the case; under low slippage it spins with the convertor.

http://auto.howstuffworks.com/auto-parts/towing/towing-capacity/information/torque-converter3.htm

Quote:
The stator has a very aggressive blade design that almost completely reverses the direction of the fluid. A one-way clutch (inside the stator) connects the stator to a fixed shaft in the transmission (the direction that the clutch allows the stator to spin is noted in the figure above). Because of this arrangement, the stator cannot spin with the fluid -- it can spin only in the opposite direction, forcing the fluid to change direction as it hits the stator blades.

 

[meep]

Originally Posted By: supton
Thanks 440Magnum, that makes sense.

meep, on my transmission, possibly yours, there are two internal sensors for reading temperature: it's surmised (but nobody seems to know) that one is convertor outlet and the other is pan.

http://www.tacomaworld.com/forum/technic...tml#post3938462

When the convertor locks, or unlocks, I can usually see the difference after a couple seconds, what I believe is the update rate on the SG. I know on mine I do have a thermostat, and I've contemplated locking it into full bypass, so as to get cooling all the time--just haven't yet. I've also meant to enter in both sensor codes to try out in the SG, but never manage to remember to do so. By way of reference, whichever sensor it is I have programmed in, it will slowly come up to 195F or so on a cold start. On the highway it was staying at 197-199F in full lockup, even though the water temp was lower. Climbing Cadillac it would not lock up the convertor at all, and was running about 208F in 50F weather, running 2100rpm. Finally, if I'm not paying attention it will spike to 230F in hot weather on a hill--but you can watch it drop stupid fast once the convertor locks up.


I've got both PIDs entered into the Torque app for android os, as well as an honest gauge. the sender for the gauge is coupled to the housing just above the pan and reflects sump temp.

My tundra has the tow package with aux AT cooler and oil-to-water cooler, as well as the gearing in the tail. I load it on trips with a 6,000lb travel trailer, which works the truck pretty hard.

Sump climbs to 160 per the ecu and stays there. pretty much no matter what. If I'm not towing, it takes an hour to reach that temp. Per the real gauge, it's the same, -10degrees. Gauge commutes around 120 and tows at 150. shifts and TC activity don't phase the sump temps much. they do, but its soft and slow.

The TC output updates very quickly on the torque app. temps swing immediate with lockup/unlockup and hills. still, it's very hard to get it to break 180. Requires a hill climb while towing. and it comes back to 160 in under 30 seconds after getting off the hill.

THIS WAS INTERESTING: I tow with O/D off, and it pretty much just sits at 160. TC stays locked. However, I let it into OD on level ground at 60 mph for a stretch. TC stayed locked. temp immediately started to climb. yep, with the TC still good and locked. climbed all the way up to 180 in about 4 minutes, then once it hit 182, it quickly dropped to 160 and stayed there. I locked it back out of OD. I've always read that top gear creates more heat since the driven gear has such a small outer dia that it gets beaten by the powered gear, not enough surface area on that driven gear and the friction + compression of oil ends up creating a lot of heat... and hence why OD when towing is a no-no. I figure it heated up and the t-stat was probably not quick to open, but once it did, the temps immediately came under control.

 

[440Magnum]

Originally Posted By: supton
Ah, found it:

http://en.wikipedia.org/wiki/Torque_converter#Operational_phases



Slicker than owl snot, isn't it?

I still think automatic transmissions are all designed and built by the Vorlons on thier homeworld and left in unmarked wooden crates at an obscure shipping dock outside Kokomo...

 

[rslifkin]

There's a reason they're called "automagics". They've got stuff to make gears from and tools to make gears with and then magic puts them together to make gears and transmit power (ok, so it's not quite that mystical, but autos do pretty much work on hopes, dreams and magic).

 

[Duffman77]

Originally Posted By: 440Magnum
Originally Posted By: IndyIan
I read that the output torque is actually higher than the engines torque when slippage occurs at low rpm, but as you saw this costs hp showing up as heat in the trans.


That's correct. Mind-boggling at first, but that's the very definition of a "torque convertor" as opposed to a "fluid clutch" which doesn't amplify torque. The torque amplification is made possible by a sprag-clutched reaction element (stator) inside the torque convertor.



It's easiest to wrap your head around the following equation:
HP(in)=HP(out)-Heat generated(in the torque converter)

And since the engine makes HP as a function of RPM, you get more torque when the engine speed is faster than the converter output speed.

 

[meep]

Originally Posted By: supton
Ah, found it:

http://en.wikipedia.org/wiki/Torque_converter#Operational_phases



very cool. I never understood the value of the stator. I'm not sure if the TC in the old '57 I grew up with even had that element. I just remember the pump and the turbine. wonder if TCs have had incremental designs.