Eli5: How come when I’m stopped in a car and I let go of the brakes, it moves forward without touching the accelerator?

Question

Eli5: How come when I’m stopped in a car and I let go of the brakes, it moves forward without touching the accelerator?

In: Engineering

Answer 1

In an automatic transmission, as long as you’re in Drive the transmission is in gear. There’s a special fluid coupling (called a “torque converter”) between the engine and transmission that lets the engine keep spinning when you stop the car with the brakes, but as soon as you release the brakes the torque coming through the torque converter is enough to start you rolling.