How Does Automatic Transmission Change Gears

As drivers, we know how the transmission works: we push the clutch in, change the gear and then continue driving normally. Manual transmission also teaches us how to perform clutch control without stalling or rolling on an incline. So how does an automatic car change gears?

There are more parts in this system that makes it a more complicated system. An automatic car has something called a torque converter. For the car to move forward, whether manual or automatic, torque is needed for the initial momentum.

The torque converter is a fluid link where its job is to connect the engine to the transmission and the transmission to the driven wheels (Front Wheel Drive or Rear Wheel Drive).

The engine is connected to the transmission in a bell housing. This is where the torque converter is. The transmission also contains something called planetary gear sets which provide different gear ratios.

The engine’s flex plate is also connected to this torque converter. So the torque converter basically takes the place of a clutch in an automatic car. When the crankshaft rotates it also rotates the converter. This is how the torque converter will disconnect and connect the engine’s power to the load being driven.

The torque converter has main components that make automatic transmission successful. These include:

  • The impeller – connected to the engine which is responsible for driving the turbine using viscous forces on the transmission fluid.
  • The turbine – connected to the transmission input shaft that sends the torque force to the transmission.
  • The stator – which sits between the impeller and the turbine. It lessens churning losses.
    The lock up clutch.

During the compression of the fluid it returns from the turbine that works against the impeller and its rotating movement that also acts on the engine.

The stator redirects the fluid so that the majority of the velocity gets driven towards the impeller which then adds to the torque produced by the motor. It can only rotate in one direction if the impeller and turbine are moving at the same speed. Stators do not apply torque when on a highway, only when coming to a stop or accelerating.

A planetary gear set consists of a sun gear and planet gears which rotate around the central sun gear, like our solar system. A planet carrier gear connects the planet gears with a ring gear that meshes them. The planetary gear set prevents certain components from moving when using clutches and brakes. This alters the input and the output of the system that changes the overall gear ratio.

Depending on which component is fixed it will determine the final gear ratio. If a ring gear is stationery or fixed, the ratio will be shorter than if the sun gear is fixed or stationery.