Torque Convertors: A Physics Puzzle

[Rx7man]

I have a question about torque convertors... They just aren't making any sense to me because they don't seem to follow laws of physics.

I don't see how they can increase torque AT ALL without having something stationary somewhere against which to apply force.. I can only see them as a fluid coupling that allows the engine to rev to a point where it's in it's torque band, but in no instance can the torque out be more than the engine is capable of producing.

The outer part of the torque convertor rotates with the engine, and the outer output sleeve drives the oil pump in the transmission..
The inner part is acted on by the fluid and is where your power is taken from.. Without something stationary somewhere, can you somehow get more torque from the output shaft than the engine produces? Am I missing something? Even if the output shaft turns at a lower speed than the engine it doesn't make sense.

 

[anorlunda]

A torque converter can't amplify power (torque*RPM).

If it conserves power, then TORQUE1*RPM1=TORQUE2*RPM2 or TORQUE2=TORQUE1*RPM1/RPM2.

Even if it is only 50% efficient, and 50% of the power is turned to heating the fluid, TORQUE2=-0.5*TORQUE1*RPM1/RPM2.

 

[xxChrisxx]

A torque converter has a stator that redirects the flow. This is the stationary element in the circuit.
The stator is housed on a one way clutch so it only reacts when there is a speed differential between the pump and the turbine.

It amplifies torque at the expense of speed.

Maximum torque ratio occurs at stall. (input rotating output stationary).
At slip reduces the torque ratio reduces.
When output = input speed torque ratio = 1 (ie no torque advantage)


http://www.volkspage.net/technik/ma..._fundamentalsselectiondesignandaplication.pdf
See chapter 10

 

[Rx7man]

That's a really excellent read, definitely saving it for future reference as well for the other chapters!

However, it hasn't clarified anything for me.. can anyone draw a free body diagram or anything that might help me? There's an element called a stator, but it's not fixed.. There's only and input shaft and an output shaft (if you ignore the oil pump drive).. torque on one must be equal to the torque on the other, so in any case, the engine has to produce that torque.

 

[xxChrisxx]

It'd be a bit silly to call it a stator if it never operated in a fixed stated.

It sits on a one way clutch that is grounded to the transmisson case via a splined sleeve.

 

[Rx7man]

I must have missed seeing the two splined shafts.. if that's the case, then it all makes sense again.

 

[jrmichler]

The last time I pulled an automatic transmission, there was one splined shaft. The torque converter was bolted to the flexplate. The transmission input shaft was inserted into the torque converter. The transmission input shaft had a bearing pilot at the tip, then splines to engage the torque converter. No other connections. One input, one output.

And I agree completely with the OP. Input torque equals output torque, but the RPM's can be different. In order for output torque to be different from input torque, there must be a reaction torque through a third member. The sum of moments must equal zero.

 

[xxChrisxx]

It's all in the book chaps.

 

[Rx7man]

The last time I pulled an automatic transmission, there was one splined shaft. The torque converter was bolted to the flexplate. The transmission input shaft was inserted into the torque converter. The transmission input shaft had a bearing pilot at the tip, then splines to engage the torque converter. No other connections. One input, one output.

And I agree completely with the OP. Input torque equals output torque, but the RPM's can be different. In order for output torque to be different from input torque, there must be a reaction torque through a third member. The sum of moments must equal zero.

That's what I always saw, though looking at pictures of newer transmissions it seems like they do have another splined shaft, which would then make it possible

 

[Averagesupernova]

In an automobile transmission there is more than one output. Something needs to drive the oil pump. It may appear to be just one output but on the convertors that I last had my hands on there is a notched sleeve around the splined shaft. The sleeve is solid to the outer housing of the convertor, which then engages inside the transmission.

 

[Rx7man]

yes.. and I just looked at an old 727 chrysler tranny, and indeed it does have two splined shafts, plus the notched sleeve for the oil pump, so that can work, there's something against which to put force against!.. Don't know if I was blind now or my memory is failing.. thought I saw some with only one splined shaft, hence my confusion.

I'm reading the other chapters of that book now.

 

[Averagesupernova]

I didn't recall more than one splined shaft so I didn't mention it, but I thought there had to be something else. Thanks for looking.

 

[OldYat47]

That's an excellent explanation of the components and operation of a modern torque converter. One video is worth a thousand words?