The Clutch: Every transmission needs a method to disconnect it from the engine in certain situations – largely because when vehicles stop, the engine would also stop, or stall, if still connected directly to the wheels.

Manual transmissions use a clutch to connect and disconnect the engine from the transmission. When the driver presses the clutch pedal, the engine and transmission are physically separated. When the driver releases the clutch pedal, he applies the engine’s power to the transmission, and therefore, to the wheels, once more.

As the vehicle is stopping, the clutch needs to be disengaged, separating the engine from the transmission and wheels. If not, the engine will stop when the wheels stop, causing a stall and making you look like a total noob.

A clutch works on the principle of friction: when engaged, the clutch, which is attached to the transmission, clamps solidly to the flywheel, which is attached to the engine. Power is transmitted, and the vehicle is moving.

The Torque Converter: Automatic transmissions need a way to disconnect the engine from the wheels, just like a manual. In an automatic, the torque converter does this job. It occupies the same location in the vehicle’s driveline as a clutch, and has the same job, though it works in a different way.

A torque converter is a type of fluid coupling which looks like a big metal donut. A rotating component called the housing is attached to the engine, and therefore spins at engine speed. A component called the turbine is attached to the transmission.

The turbine and housing spin very closely to one another, but never touch. Instead, force is transferred between the two components via a special fluid circulating between them. Imagine you place a bowl full of syrup on a turntable, and place a small dish on top of the liquid in the bowl. When the bowl spins, it imparts a rotating force on the fluid, which turns the smaller dish, too. At some point, the bowl, liquid and dish will spin as a single unit. Same deal.

When the torque converter housing spins fast enough, it transmits a rotational force on the fluid, which in turn, rotates the turbine and sends power to the transmission.

Here’s why there’s no clutch: the fluid inside the torque converter requires a certain rotational force before it effectively binds the turbine and housing, causing them to rotate as a single unit. This rotational force exists only above a certain speed between the turbine and housing. At very low speeds, or when the engine is idling, this force doesn’t exist, and the components can spin freely, preventing a stall.

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