Torque converters and automatic transmission

What is a torque converter?

A fluid coupling device that transmits and multiplies torque between the engine and gear train.

What are the two major functions of a torque converter?

• Functions as a clutch (couples/uncouples engine & gear train)

• Functions as a torque multiplier (improves acceleration & pulling power)

How does a torque converter transmit power?

By generating and directing hydraulic force (fluid in motion and under pressure).

At low fluid velocity, how much torque is transmitted?

Little to none — so engagement is low.

What controls engagement of a torque converter?

The velocity of the fluid. Higher fluid velocity → more torque transmitted.

What are the 5 main components of a torque converter?

Pump, turbine, stator, housing, lock-up clutch (some converters).

What is the pump in a torque converter?

The drive member fixed to the housing that generates fluid velocity using fins

What is the turbine?

The driven member splined to the transmission input shaft; it receives fluid from the pump

What is the stator and what does it do?

A reaction member on a one-way clutch that redirects returning fluid and increases torque

What does the torque converter housing do?

Retains fluid and attaches converter to the flex plate; must be rigid and strong

What is the purpose of a lock-up clutch?

Mechanically locks pump and turbine to eliminate slip and improve fuel efficiency

What are the two types of oil flow in a torque converter?

Vortex flow and rotary flow

What does vortex flow dominate?

When the pump spins faster than the turbine

When does rotary flow dominate?

When turbine speed approaches pump speed

What happens to the stator at higher speeds?

It freewheels on its one-way clutch to reduce drag as rotary flow increases

Why is a lock-up clutch needed?

Even when "coupled," the pump spins 3-6% faster than the turbine, causing energy loss

What does lock-up do?

Eliminates slip by mechanically connecting the pump and turbine

What is the key difference between manual and automatic transmissions?

Manuals lock/unlock different gear sets; automatics use planetary gear sets for all gear ratios

What are the three main components of a planetary gear set?

Sun gear, planet/carrier pinions, ring gear

Why are planetary gears used?

They provide large gear ratios and can change rotation direction

What happens if two of the three planetary gear members are held stationary?

The gear set locks and gives a 1:1 ratio

What happens if none of the planetary gear members are stationary?

No torque is transmitted (neutral)

Driving the carrier with the ring stationary causes what?

The sun gear spins in the same direction at increased speed

Driving the sun gear with the ring stationary produces what?

Very reduced output speed at the carrier

Driving the ring with the carrier stationary results in what?

The sun gear turns in the opposite direction at high speed

What engages or restrains components in an automatic transmission?

Clutches (multi-disc wet clutches and band clutches)

What does a band clutch do?

Tightens around a rotating drum to hold or transmit power

How are clutches actuated in the transmission?

By hydraulic pistons controlled by the valves in the valve body

What supplied hydraulic pressure in an automatic transmission?

The internal "front pump"