In an automatic transmission, what replaces the clutch?

In an automatic transmission, the torque converter plays a crucial role analogous to that of a clutch in a manual transmission. It enables smooth engagement and disengagement of the engine from the transmission, allowing the vehicle to stop without stalling while still in gear.

The torque converter serves several functions: it multiplies engine torque when the vehicle is accelerating, it allows for variable speed changes, and importantly, it provides a fluid coupling between the engine and the transmission. This fluid coupling permits the engine to continue running while the vehicle is stationary, which is essential in automatic vehicles where a driver may come to a stop without needing to disengage the transmission manually.

Other options mentioned serve different functions: the differential is involved in distributing engine power to the wheels and allowing them to rotate at different speeds; the master cylinder plays a role in the braking system; and the power booster enhances braking power. Each of these components is important in automotive function, but they do not replace the clutch's role in an automatic transmission like the torque converter does.