Why does a car move when the gear is in “D[rive]” but the accelerator is not being pressed?
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Instead of a clutch, an automatic transmission receives engine torque through a torque converter. A torque converter is a fluid-filled chamber that holds two fan-like rotors. Each rotor turns with its own shaft, one connected to the engine’s crankshaft, and the other to the transmission gearbox.
As the crankshaft spins, its rotor drags fluid along with it which turns the gearbox rotor. When the car is stopped with brakes applied, oil can flow around the gearbox rotor, allowing the engine to keep running despite the gearbox’s inability to turn.
Once you release the brakes, the gearbox rotor can turn again, causing the constant engine torque to make the car roll forward.
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