Transmission
Transmission
This domain tests why a vehicle needs a transmission, how manual and automatic transmissions differ, and how gear ratios multiply torque. Questions use ratio calculations and ask about the function of the torque converter, CVT, AMT and DCT.
Core rules
- Purpose: match engine speed and torque to road speed and load; allow the engine to run near its efficient range; provide reverse and neutral.
- Manual gearbox: the driver selects gears through a clutch. Automatic: hydraulic torque converter or clutch packs, planetary gearsets, controlled by a TCU.
- Torque converter: fluid coupling with a stator; it multiplies torque at low speed and slips, so no clutch pedal is needed. A lock-up clutch reduces slip at cruising speed.
- CVT: stepless ratio through a belt and pulleys; AMT: automated manual with a robotised clutch; DCT: two clutches and two input shafts for fast shifts.
- Overall ratio = gearbox ratio × final drive ratio. A lower numerical ratio gives higher speed and lower torque.
Worked example
A car has a second-gear ratio of 2.50:1 and a final drive of 4.10:1. Overall ratio = 2.50 × 4.10 = 10.25:1. If engine torque is 150 N·m, ideal wheel torque = 150 × 10.25 = 1537.5 N·m before transmission losses. With 8% loss, about 1414.5 N·m reaches the wheels. In top gear at 0.80:1, overall ratio = 0.80 × 4.10 = 3.28:1, so wheel torque falls but wheel speed rises - the reason a lower gear climbs better.
Traps
- Multiplying engine speed by the overall ratio instead of dividing.
- Confusing a torque converter with a fluid flywheel; only the converter has a stator that multiplies torque.
- Forgetting the final drive when calculating overall ratio.
- Checking automatic transmission fluid with the engine off when the dipstick requires the engine running and the fluid warm.