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

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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
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Firing the ignition (turning the key) starts the engine’s motion. As long as there is fuel to burn, the engine will slowly move. This is why a car will move forward as long as the brakes aren’t locked, at about 2 mph

Probably because you have an automatic transmission and are still in drive. So naturally it will move forward.

I’m no mechanic, I may be using the wrong terms, but manuals don’t do this. Do Automatic transmissions never fully disengage the clutch?

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.

Two things:

1. I don’t know the exact details of why this happens, but if a car’s engine turns any slower than about 1,000 RPM, it will stall. So, the car is designed so that the engine is always turning at at least 1,000 RPM even when you’re not touching the accelerator.
2. A car with an “automatic” transmissions always has its engine powering its wheels as long as you’re not braking. So, that leisurely little 1,000 RPM makes the car move without any accelerator input.

It’s your torque converter. It’s filled with fluid and is between your engine and transmission.

Some cars have a lower idle RPM like my wife’s Buick and won’t move when you let off the brakes. Others have a higher rpm and can get the vehicle significantly fast with the foot off the brakes.