How Does an Automatic Transmission Work?

In recent years there has been a massive increase in the number of automatic cars on Irish roads. As most people know, the main difference between manual transmission cars and automatic transmission cars is the lack of the clutch pedal and the gearshift. 

An automatic gearbox does not need the driver to use the clutch pedal or gear shift to change gears. Instead, the transmission automatically changes gears in relation to the speed of the vehicle. But how does the gearbox know to do this?

Bell Housing

The bell housing is the place where the automatic transmission connects to the engine. In the bell housing, there is a torque converter and the planetary gear sets.
The torque converter is used in place of a clutch as found in manual transmission vehicles. The planetary gear sets are responsible for providing different gear ratios, which is how the automatic transmission changes gears.

Torque Converter

The torque converter is how the engine sends power to the automatic gearbox. There are three main parts of the torque converter.

1. The Impaller: This is the part of the torque converter that connects directly to the engine and drives the turbine.
2. The Turbine: Driven by the impaller, this connects to the transmission input shaft and provides torque.
3. Transmission Fluid: This flows in a loop between the impaller and the turbine. A central reactor redirects the flow of the transmission fluid as it moves from the turbine to the impaller increasing the torque output.

Increased torque output assists with accelerating and prevent the car from stalling when stationary. 

Planetary Gear Set 

The planetary gear set is how the automatic transmission changes gears. The planetary gear set uses multiple parts to create gear ratios.

1. The sun gear
2. The planet gears
3. The ring gear

The ring gear and the planet gears rotate around the sun gear. All three parts of the planetary gear set can be the input, output or remain stationary. As you use the accelerator and the brakes the input and output of this system changes which in turn changes the gear ratio.

In Simple Terms

When the automatic vehicle is stationary, the torque converter is not creating enough torque to power the turbine so it does not move. When you use the accelerator the torque converter spins faster, delivering more transmission fluid and thus more torque to the planetary gear set. This gear set then adjusts. As all of the parts of the gear set change the car automatically changes gear rations.