--- title: "Automatic transmission and torque converter" subject: "Automotive Technology" chapter: 5 key: "semester_5/automotive_technology:automatic-transmission-torque-converter" source: https://www.bctengineer.com/q/semester_5/automotive_technology/automatic-transmission-torque-converter site: BCT Engineering --- # Automatic transmission and torque converter Automotive Technology · Chapter 5: Transmission Systems - Asked in: year not given, 8 marks - Video: [Torque Converter, How does it work?](https://www.youtube.com/watch?v=bRcDvCj_JPs) — Sabin Civil Engineering - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/automotive_technology/automatic-transmission-torque-converter - Whole chapter: https://www.bctengineer.com/md/semester_5/automotive_technology/chapter-5 ## Question 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. ## 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. ## Answer #### Torque converter A torque converter is a **fluid coupling** that also multiplies torque; it replaces the clutch in an automatic transmission. ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which are the three elements of a torque converter? A. Sun, planet carrier, ring B. Impeller, turbine, stator C. Clutch, piston, band D. Pump, valve, governor Answer: B. Impeller (driven by engine), turbine (to gearbox) and stator (one-way clutch). (Key point: Three elements) 2. How does torque multiplication occur at stall? A. Stator redirects turbine oil in the impeller's direction, adding energy to the impeller B. Lock-up clutch engages C. Impeller speeds up D. Turbine becomes fixed Answer: A. The stator turns the returning oil, so turbine torque becomes about 2 to 2.5 times the engine torque. (Key point: Torque multiplication) 3. What happens at the coupling point? A. Torque multiplication increases B. One-way clutch frees the stator and the unit acts as a fluid coupling with ratio 1 C. Stator locks D. Lock-up releases Answer: B. Oil leaves the turbine in the correct direction, so the stator freewheels and no multiplication occurs. (Key point: Coupling point) 4. What is the role of the lock-up clutch? A. Multiplies torque at start B. Locks turbine to housing at cruise, removing slip and improving economy C. Holds the planetary ring D. Selects reverse Answer: B. It removes the 2 to 5 % slip, improving fuel economy and reducing heat. (Key point: Lock-up clutch) 5. How do shift valves decide to change gear? A. Compare governor (speed) pressure with throttle (load) pressure B. Driver's pedal only C. Compare oil colour D. Fixed timer Answer: A. Road-speed and engine-load pressures are compared and they apply or release clutches and bands; modern units use ECU and solenoids. 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