An automatic transmission lets a car change gear ratios without a clutch pedal or manual gear lever movement. It matters because an engine only works well over a limited range of speeds, while a car must start from rest, climb hills, cruise, and accelerate. The transmission matches engine speed and torque to road conditions so the vehicle can move smoothly.
In a cutaway view, it sits between the engine and the driveshaft, acting as a controlled power path.
Key Facts
- Gear ratio = input speed / output speed
- Torque multiplication in a low gear helps the vehicle start moving from rest.
- Vehicle speed relation: wheel rpm = vehicle speed / tire circumference
- Power flow: engine to torque converter to planetary gearset to output shaft to driveshaft.
- Torque converter slip allows the engine to keep running when the vehicle is stopped.
- Hydraulic pressure and electronic solenoids apply clutches and bands to choose each gear.
Vocabulary
- Torque converter
- A fluid coupling between the engine and transmission that transfers torque and allows the engine to idle while the car is stopped.
- Planetary gearset
- A compact gear system with a sun gear, planet gears, and a ring gear that can create several gear ratios.
- Clutch pack
- A stack of friction plates inside the transmission that locks selected parts together when hydraulic pressure is applied.
- Valve body
- A network of passages and valves that directs transmission fluid pressure to control gear changes.
- Transmission control module
- An electronic controller that uses sensor data to decide when and how the transmission should shift.
Common Mistakes to Avoid
- Thinking an automatic transmission has no clutches is wrong because it uses internal clutch packs instead of a driver-operated clutch pedal.
- Assuming the torque converter is just a simple connector is wrong because it can multiply torque at low speed and allow controlled slip.
- Using one gear ratio for every driving condition is wrong because starting, accelerating, climbing, and cruising require different torque and speed relationships.
- Ignoring transmission fluid is wrong because the fluid transfers force, cools parts, lubricates gears, and provides hydraulic pressure for shifting.
Practice Questions
- 1 A transmission input shaft spins at 2400 rpm in a gear with a ratio of 3.00:1. What is the output shaft speed in rpm?
- 2 A car has tires with a circumference of 2.0 m and is traveling at 20 m/s. What is the wheel speed in revolutions per second, and what is the wheel speed in rpm?
- 3 Explain why an automatic transmission needs either a torque converter or another slipping device when the car is stopped in Drive with the engine idling.