Torque multiplication versus slip

[MarkZE]

Can anyone shed some light on the concept of torque multiplcation through an automatic transmission torque converter?

The best I can come up with is the idea that torque multiplication is proportional to the ratio of flexplate RPM and input shaft RPM much like it would be for a chain or a set of gears.

So for example engine at 5000 rpm and the input shaft is doing 2500 rpm in a perfect world the torque at the input shaft is twice that which the engine is creating.

Of course there is some significant losses at the extreme ends of the RPM ranges here otherwise torque would tend towards infinity when gearbox input shaft speed was zero.

When it comes to something viscous like a torque converter I just see the speed differential is a slippage which to me registers as an outright loss of energy.

Is there such a thing as a torque converter dyno that can show hard evidence of torque multiplcation?

 

[GregLocock]

"Is there such a thing as a torque converter dyno that can show hard evidence of torque multiplcation?
"

Yes there is. OEMs have maps of torque converter performance. They are complex, but you have already identified the basics.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ForestGump]

"The best I can come up with is the idea that torque multiplication is proportional to the ratio of flexplate RPM and input shaft RPM much like it would be for a chain or a set of gears.

So for example engine at 5000 rpm and the input shaft is doing 2500 rpm in a perfect world the torque at the input shaft is twice that which the engine is creating."

Unlike direct mechanical transmission of chain or gear, torque converter transmits torque by fluid. A change of fluid moment generates torque multiplication. There is indeed energy loss in the fluid. For this reason, the torque multiplication ratio cannot be compared with gear sets. For instance, the maximum torque ration (in stall state when the input shaft of transmission stands still) is around 2.0.

 

[TheBlacksmith]

Some dozers and loaders with automatic transmissions had torque multiplication approaching a claimed 3:1, but as a price, they also had a transmission cooler about the same size as the engine radiator to deal with the tremendous heat generated at maximum slip. This was in the '70's, I don't know the current state of the art.

Blacksmith

 

[GregLocock]

Yeah, significantly more than 2 even for automotive. In theory the upper limit would be quite high (ultimately limited by friction), and you could go multistage.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Warpspeed]

A torque converter is not just a wet clutch, think of it more as a hydraulic pump driving a separate hydraulic motor.

If the pump and motor are made to displace different amounts of fluid per revolution, it is possible by design to get speed or torque multiplication.

 

[Metalguy]

Here's what's missing-the way a TC can actually multiply torque. Slippage by itself merely means that heat is being generated and wasted. What happens is that the stator blades REDIRECT the oil flow AFTER it passes thru the driven turbine such that the oil is moving in the same direction as the pump--at least near the inner area of the pump section. The return flow acts to help the engine rotate the pump--more torque. There is a HP loss but a torque gain.

 

[Fabrico]

Good explanation MG. Typical automotive TQ's multiply torque up to 2-1/2 times. It is a ratio of input speed vs output speed minus a bit of oil heating. But, the more it multiplies, the more heat is generated. Gearing being only close to equal, as long as the engine can cause the TQ to multiply torque, significantly more power will be applied to the rear wheels than a mechanical drive, even with the waste. With a lockup clutch, it is, for all practical purposes, 100% efficient when locked. Cars with automatic transmissions from the 50's used fluid couplings.

 

[Metalguy]

The 1950's fluid couplings were mostly used by Pontiac and Olds. They were called "Hydramatics", and had low first gears (~3.97) and 4 gears to compensate for the lack of torque mult. Chevy and Buick used a 1.82 low gear and 1-1 high with their TQ's. I forget what Cadillacs had back then.

 

[Fabrico]

Chrysler had them too. It was the venerable "Clutchflight".

 

[curlyhiro]

Stall speed is controlled by size and distance between driven turbine and drive turbine. We've used torque conveters with 5,000rpm stalls behind the high reving small block Chevrolet engines but, these engines were turning upwards of 9,000rpm's. Almost anything is possible.

 

[mloew]

How Torque Converters Work

Best regards,

Matthew Ian Loew


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