Automatic Transmission : Definition, Working, Types, Pros & Cons [PDF]

What is Automatic Transmission?

Cars, while seemingly simple to use, are intricate machines. Among the most complex parts is the automatic transmission. This multi-speed system in vehicles doesn’t need the driver’s input to shift gears during normal driving, adjusting the gear ratio as the car moves.

Automatic transmissions use sensors and internal oil pressure to shift gears at the right time, with the torque converter managing the gear shifts when the transmission and engine are disconnected.

You can identify an automatic car by its pedals – if it has only two, it’s automatic. The third pedal, the clutch, is found in manual cars.

Automatic cars are also accessible for people with disabilities, requiring only two functional limbs to operate. The lack of a clutch or gear stick allows the driver to concentrate more on the road.

Automatic Transmission Parts

Following are the main parts of an automatic transmission and their functions:

#1 Oil Pump

Indeed, an oil pump is crucial in automatic transmissions. It circulates transmission fluid throughout the system, ensuring that necessary lubrication and cooling fluid reaches key components like the torque converter and clutches. This helps in the smooth and efficient functioning of the transmission.

#2 Clutch Packs

Imagine a planetary gear set is like a complex jungle gym for gears. To get different gears working together, we use special tools called clutch packs. These clutch packs act like sticky hands – when we squeeze them with hydraulic pressure (like using a water hose), they clamp down and connect certain gears in the jungle gym. This lets the gears work together and transfer power.

#3 Torque Converter

The torque converter is a fluid link between the engine and the transmission. It has two main parts: the turbine and the impeller, both sitting in transmission fluid. When the engine spins the impeller, it moves the fluid to drive the turbine, which then powers the gearbox.

#4 Planetary Gear Set

The main part of an automatic gearbox is the planetary gear set. It includes a sun gear, a planet carrier with several planet gears, and a ring gear. By using different combinations of these gears, the car can shift smoothly and move forward or backward with different gear ratios.

#5 Output Shaft

The output shaft transfers power from the transmission to the car’s wheels. It connects to the planetary gear set and uses the engine’s rotational torque to move the car forward or backward.

#6 Brake Band

Some automatic transmissions use brake bands to apply resistance or hold specific components. They control gear engagement by wrapping around a drum connected to a particular gear or shaft and are activated hydraulically.

#7 Oil Pan

Transmission fluid is kept in the oil pan at the bottom of the gearbox. There’s also a transmission filter that helps remove impurities from the fluid.

#8 Valve Body

In an automatic transmission, the valve body acts as the control center. It has channels and valves that manage the clutch packs, bands, and flow of transmission fluid. To change gears smoothly and effectively, the valve body adjusts hydraulic pressure based on signals from the vehicle’s computer (ECU).

Automatic Transmission Functions

  • The torque converter connects the engine and transmission, transferring power between them.
  • Transmission fluid circulates to keep the system lubricated and cool.
  • Planetary gear sets provide variable gear ratios for forward and reverse.
  • Clutch packs use hydraulic pressure to engage and disengage gears.
  • The output shaft transfers power to the driving wheels.
  • Band brakes manage gear engagement.
  • The valve body controls fluid flow to automatically shift gears.
  • Sensors and the ECU coordinate gear changes based on the driving situation.
  • This system works together to deliver smooth and effective gear changes, allowing the vehicle to move and accelerate.

Automatic Transmission Working

Most automatic transmissions use hydraulic power to shift gears, combining a torque converter with gear sets to provide different ratios for the vehicle’s speed.

The engine is linked to the transmission through the torque converter, which sends power to the gears using pressurized fluid. This setup lets the car come to a stop without stalling, replacing the need for a manual friction clutch.

The pump changes engine power into transmission fluid, which drives the torque converter’s turbine. This boosts the fluid’s power, giving extra energy to the turbine.

As a result, a swirling power is created, spinning the turbine and the central shaft it’s connected to. This energy moves from the shaft to the transmission’s first set of planetary gears.

Automatic Transmission Types

Automatic transmissions come in various types, each offering its own advantages and features.

#1 Automated Manual Transmission (AMT)

The Automated Manual Transmission (AMT) is like an automatic gearbox, but it uses a computer and actuator to decide when to change gears based on how fast the engine is running. With an AMT, you need to use the selection stick to pick the driving mode.

Automated Manual Transmission (AMT)

Once it’s set, you can just relax and drive; the AMT handles the clutch and gear changes for you. It shifts gears automatically when going up or down, which might feel a bit jerky sometimes, but overall, it’s pretty smooth.

The AMT system is cheaper and can be found in cars like the Tata Nexon, Maruti Suzuki Wagon R, Hyundai Grand i10 Nios, and many others.

#2 Continuously Variable Transmission (CVT)

CVT, or continuously variable transmission, is a newer type of automatic gearbox. Unlike traditional transmissions with fixed gear ratios, a CVT constantly adjusts its gear ratios based on the vehicle’s speed.

Continuously Variable Transmission (CVT)

It achieves this by using two conical pulleys connected by a belt that changes diameter as needed, depending on factors like speed, weight, and incline.

Because the car moves at various speeds, the CVT is highly efficient, striving to maintain a constant engine speed, measured in revolutions per minute (RPM).

#3 Dual Clutch Transmission (DCT)

The Direct Shift Gearbox, also called Dual Clutch Transmission (DCT), uses two clutches to switch gears instantly. It operates with two clutches, each handling either the odd or even gear groups: 1, 3, 5 and 2, 4, 6 respectively.

Dual Clutch Transmission (DCT)

Unlike Automated Manual Transmission (AMT) and manual transmissions, this design prevents power loss because the gears are always engaged and ready to switch as soon as the engine reaches the right speed.

A DCT transmission offers a smooth and efficient driving experience and is commonly found in luxury vehicles.

#4 Intelligent Manual Transmission (IMT)

The iMT transmission stands out from others because it combines manual gear shifting with automatic clutch control. When you move the gear stick, sensors send signals to an actuator, which engages the clutch for you.

Intelligent Manual Transmission (IMT)

This setup lets you control fuel efficiency and gear changes just like in a manual transmission, but without needing to use a clutch pedal.

#5 Torque Converter

The oldest form of automatic gearbox is the torque converter, a marvel of engineering. Instead of a clutch, it employs two turbines—one connected to the engine and the other to the transmission.

Torque Converter

Power from the engine turbine moves through a fluid between the two turbines to the transmission turbine. Despite being more expensive, torque converters are just as efficient as manual transmissions.

Cars equipped with torque converters include the Ford EcoSport, Mahindra Thar, Tata Safari, and Kia Seltos.

Pros and Cons of Automatic Transmission

Advantages of Automatic TransmissionDisadvantages of Automatic Transmission
Easier operation in high-traffic areasHigher initial cost
Simple start and stop with one pedalLess driver concentration
Quick and smooth transmission shiftsPotential for distracted driving
Easier to learn compared to manualLower fuel efficiency
Less likely to stall, especially on hillsHigher fuel consumption compared to manual
Good performance in hilly terrain


That’s all I’ve got on “Automatic Transmission.” Thanks for sticking with it. If you think I missed something or if you’re not sure about what I said, feel free to ask!

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