Most often than not we think of a clutch in the context of a manual transmission. An automatic transmission does actually have a clutch system, but often only a mechanic would refer to it as such. Your automatic transmission does the same function as a manual transmission – it just does it a bit differently.
The automatic transmission basics
The automatic transmission has an amazing amount of parts, all of which come together in what’s referred to as a torque converter system. Your engine and your transmission meet at the bell housing, which has a torque converter for automatic cars, as opposed to a clutch for a manual transmission. The torque converter links the engine and the transmission so that the wheels will turn. Planetary gear sets offer the different gear ratios.
The torque converter has a turbine, an impeller, a stator, and a lock-up clutch (so there you go – your automatic transmission DOES have a clutch). The impeller is part of the housing of the torque converter, and is attached to the engine. It drives the turbine by means of transmission fluid, and then returns the fluid from the turbine.
Meanwhile, the stator is resting between the turbine and the impeller, reducing churning loss and increasing torque output by redirecting the fluid from the turbine to the impeller, helping it to move and adding to motor torque. The stator can only rotate one way, so it either rotates with the impeller or does not move at all. The stator offers you more torque when you’re at idle, but not when you’re at highway speed.
Planetary gear set
Now that you know how the engine gives power to the transmission. But how does your automatic transmission change gears? It does it by means of a planetary gear set. It gets its name since it consists of a main, or sun, gear in the middle. There are some other gears that rotate around the sun, a planet carrier that connects them, and a ring gear that meshes with the planet gears. The whole system functions on the principle of using clutches and brakes to keep various parts from moving, so you can change your gear ratio without needing to engage different gears, as you would with a manual transmission – they’re already engaged for you. We could go into much greater detail here, but you have the basics at this stage.
The fluid pump
Now, how precisely do clutches and brakes in an automatic vehicle work to change the gears? The key is the fluid pump, working togather with the torque converter. The torque converter essentially drives the fluid pump, and the pressure coming from the pump activates the brakes and clutches in the planetary gear set. A rotor spins in the pump housing and produces chambers that create a change in volume. As the volume decreases, the fluid is either pumped or compressed as the gears mesh. Then hydraulic signals are transferred to your automatic transmission, by means of clutches and band brakes, locking the torque converter.
Mechanics devote several weeks in class learning about automatic transmissions. This is essentially a brief overview to outline a bit about how the automatic transmission functions and to answer the question “does an automatic transmission have a clutch?” as Yes, it does!