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If you drive a (modern or old) manual transmission car for 5000 km deliberately at 1 km/hr, you will burn through the clutch (long before you reach your destination).

The idea is this. If you attempt to drive at the lowest possible RPM at which the engine will not stall, say 800 rpm, the gear ratio on 1st means your speed will be a large multiple of 1 km/hr.

Hence you'd have to drive with the clutch partially depressed. This is clearly a recipe for wearing out your clutch.

What happens if you do the same (drive 5000 km at 1 km/hr) on a modern automatic transmission (such as two-clutch DSG, but not CVT)?

Would you wear out the/a clutch in a modern automatic transmission if you drove 5000 km at 1 km/hr?

I'm asking here after this question failed to get an answer on mechanics.SE.

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Most modern automatic transmissions contain a torque converter which is a special kind of fluid clutch filled with oil which allows slippage between the input and output shafts. And while slipping, the torque converter multiplies the input torque by a factor equal to the percent slippage and thereby acts like another set of transmission gears. (Dual-clutch automatics do not have torque converters.)

At slow speeds, the torque converter is slipping all the time, furnishing an extra-low gear ratio so the engine can comfortably idle along as the vehicle creeps forward. Thus there is no clutch slippage in an automatic transmission; the first gear clutch pack is "hard engaged" at the lowest speed while the torque converter slips.

This means slow travel will not wear out any of the clutch packs in an automatic transmission.

FYI at top speed, the torque converter is locked out so it cannot slip, because slippage consumes about 10% of the power flowing through it. The lockout feature improves gas mileage during cruise conditions by preventing slippage losses.

The slippage losses show up as heat generated by fluid friction in the torque converter's oil and must be carried away by a heat exchanger which either dumps the waste heat into the air or into the engine's liquid cooling system.

Since there are no sliding parts or friction clutches inside a torque converter (just spinning oil), it never wears out unless overloaded, which causes it to overheat and ruins the oil seals and whatnot inside.

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  • $\begingroup$ Thank you for this enlightening answer. Now I'm wondering why a torque converter has never been implemented as a clutch-replacement device in manual transmission cars. Does a torque converter not enable, for example, uniform (from 0% to 100%) torque conversion? I'll study this a bit more and perhaps ask a sequel. $\endgroup$
    – Sam7919
    Commented Apr 22 at 18:04
  • $\begingroup$ @Sam7919 Manual transmissions cannot be shifted under load, that's why they have the clutch to disengage the engine. Torque converters don't disengage the engine, so you can only use them in automatic transmissions that can be shifted under load $\endgroup$ Commented Apr 22 at 18:36
  • $\begingroup$ DCT autos do not have a torque converter, That's why the mpg and performance of modern autos is better than that of manuals. The original question is a good one that I don't know the answer to, but your answer is definitively wrong in some (many) cases. $\endgroup$ Commented Apr 22 at 21:44
  • $\begingroup$ @GregLocock Non-DCT automatics also get better mpg then manuals now a days. This is partly due to smarter shifting at lower RPMs and more gear ratios then on a manual. Also modern torque converters lock up when not shifting eliminating the slip that occurred in the old days. $\endgroup$
    – Eric S
    Commented Apr 22 at 21:51
  • $\begingroup$ OK, none the less DCTs do not have tcs. $\endgroup$ Commented Apr 22 at 21:59

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