Auto gearbox efficiency

[Rovinghawk]

I'm curious- what is the most efficient speed for a torque converter gearbox?

At very low engine speed the car doesn't move if there's even a little braking force. Also at high speeds there will be appreciable friction losses.

What sort of speed is the most efficient? Would an efficiency graph be similar to a car's power graph where it rises steadily to a peak then drops off a bit faster than it rose?

No particular reason for asking other than idle curiosity.

[Rovinghawk]

kambites said:

I think a simple fluid coupling gains significant efficiency with speed, so the 'box is probably at its most efficient at peak RPM.

Does that compensate for increased frictional losses?

[Rovinghawk]

jkh112 said:

At certain speeds torque converter boxes lock up and hence gain in efficiency. The exact speed at which this happens varies across a range of factors including gear, revs, box type, car manufacturer.

Specifically ZF 5HP24 but curious as a general concept.

Is the lock-up a mechanical device or how does it work?

[Rovinghawk]

Thank you.

[Rovinghawk]

Max_Torque said:

As mentioned, modern epicyclic wet disc clutch automatic gearboxes now have a 'fully rated' lock up clutch on the torque convertor.

When you say modern, since what sort of date?