Under the skin: how torque converters are needed

Audi torque converter-based gearbox

Torque converter based gearboxes are the original type of automatic transmission and they are developing rapidly

Original type automatic transmission, torque converter can provide the most satisfying acceleration curve

A large number of different automatic transmissions have been tried over the years, but the most successful and enduring has to be the planetary torque converter. New kids on the block like the DCT (Dual Clutch) and even the grown-up variants of the CVT (Continuously Variable Transmission) system have threatened to knock the world of any car off its pedestal, but have failed so far.

That said, the classic automatic transmission has earned the nickname 'box slush' for nothing. The name comes from the 'wet' response at the beginning of the transmission, which was received by people A to B, but hardly useful for the drive. Torque converters are fluid clutches that connect the engine to the gearbox, not the clutch. They look like big metal donuts, but inside they contain three main components plus automatic transmission fluid. The side impeller motor and the transmission side is the turbine. Both contain blades and are similar to the ones you see in a jet engine when you climb the stairs to an airplane.

The turbine throws the transmission fluid outward through centrifugal force, while the engine revs build up and into the turbine, which is forced to spin, driving the transmission. The liquid is pushed back to the center of the impeller in a continuous cycle.

That's not the whole story, although there is a third component that turns what would be an inefficient fluid clutch into a more efficient torque converter. This is called the stator (because it still remains) and sits between the impeller and the turbine. The stator diverts the fluid back to the impeller, slowing it down and thus multiplying the torque between the motor and gearbox.

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So far so good: when the car accelerates from rest, the torque converter provides a satisfying slingshot feeling when you put your foot down. Once at cruising speed though, the turbines (gearbox side) can keep up with the rotation of the impeller (engine side), increasing fuel consumption and emissions. Once those things became more important, the transmission designers added a torque converter lock-up clutch to mechanically lock the two halves together at cruising speed.

While the gear layout in a DCT gearbox resembles a manual, inside a traditional automatic transmission it is quite different. Instead of gears arranged one above the other shafts, cars traditionally use epicyclic (sun and planet) gearboxes arranged one after the other in a line. Using clutches to control how torque is transmitted through each gearbox creates different gear ratios. Adding more gearboxes creates even more gear ratios, so three gearboxes can deliver six speeds.

Torque-converter gearboxes are smart and maybe a little brutal, but while handling has improved over the years and they've always been refined, they should get more efficient. More on how transmission engineers achieve next week.

Why are they so amazing

Torque converters may not look, but they are one of the most amazing devices ever to grace a powertrain. They take up a lot of clutch space, they multiply torque between the engine and gears and they are still the choice when refining is the most important thing.


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