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Automatic transmission and torque converter

Short answer

A three-element torque converter uses an impeller, turbine, and stator (with one-way clutch). At low turbine speed, the stator redirects oil to multiply torque up to ~2.5 times. A lock-up clutch eliminates slip at cruising speed. Automatic shifts are achieved by a planetary gearset controlled by a hydraulic valve body sensing speed and load.

Explain the working of a three-element torque converter. How does it multiply torque, and what is the role of the lock-up clutch? Briefly describe how a planetary gearset and hydraulic control give automatic gear changes.

Torque converter

A torque converter is a fluid coupling that also multiplies torque; it replaces the clutch in an automatic transmission.

 Engine side      Turbine       Stator
 Impeller (pump)  (to gearbox)  (one-way clutch)
  )) vanes ((  ->  )) vanes ((  -> )) vanes ((
  <------------------ oil circulates ------------>

Three elements

  1. Impeller (pump): welded to the converter housing and driven by the engine; its vanes throw the oil outward.
  2. Turbine: connected to the gearbox input shaft; oil leaving the impeller strikes its vanes and drives it.
  3. Stator: placed between the turbine and the impeller, and mounted through a one-way clutch on a stationary shaft fixed to the transmission case, so it can turn only in the engine's direction.

Torque multiplication: at low turbine speed (when the car is starting), the oil leaving the turbine moves against the impeller's direction of rotation. The stator vanes redirect this oil to flow in the impeller's direction, so the oil returning to the impeller helps it, and the turbine torque becomes higher than the engine torque (about 2 to 2.5 times at stall). As the turbine speed approaches the impeller speed (the coupling point), the oil leaves the turbine in the same direction as the impeller, the one-way clutch frees the stator, and the unit acts as a plain fluid coupling (torque ratio of 1).

Lock-up clutch: at cruising speed, a friction clutch locks the turbine to the impeller housing. This removes the slip (about 2 to 5 %), improving fuel economy and reducing heat.

Gear changes

  • A planetary gearset (sun gear, planet carrier, ring gear) gives different ratios when one member is held by a brake band or multi-plate clutch and another is driven, or when two members are locked together. By changing which members are held, several forward ratios and reverse are obtained.
  • A hydraulic control unit (valve body) uses oil pressure from the pump. A governor supplies a pressure that depends on road speed, and the throttle (TV) valve supplies a pressure that depends on engine load. The shift valves compare these pressures and apply or release the clutches and bands, so up-shifts and down-shifts happen automatically.
  • Modern units use an ECU and solenoid valves instead of the governor.

Advantages are smooth driving and easy operation; disadvantages are lower efficiency than a manual gearbox, high cost and complex servicing.

✓ Answer checked by Claude Opus 5.5 (high effort)

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