Eli5 Why do cars move at all when in drive without the accelerator being used. I’m not talking cruise control.

Question

Eli5 Why do cars move at all when in drive without the accelerator being used. I’m not talking cruise control.

In: 42

Answer 1

Not all cars do this. An example is a BMW M-car that has the M-DCT. it’s a type of automatic transmission. in drive, if you let off the brake but don’t touch the accelerator yet, it will hold perfectly still (or even roll backwards if you’re on an inline). it won’t move until you touch the accelerator.

but most cars move forwards when you aren’t touching the accelerator because of their torque converter. your transmission connects your engine to your wheels. if the engine spins faster, so do the wheels. if the engine slows down, so do the wheels. if the wheels come to a complete stop, so does the engine, because they’re directly connected. except that would suck to have your engine die every time you stop. so a torque converter is a thing that goes between your engine and transmission that lets both sides spin at different speeds. it uses fluid to do this. picture a mixing bowl, like one used for cooking. imagine it’s filled with water, and you use your hand to spin the water inside of the bowl around in a circle. now if you take a plastic strainer and drop it in, it would spin for a little bit, because of the water spinning, right? now imagine the bowl side also having some “fins” to help spin the fluid (transmission fluid), and the plastic strainer side being connected to the wheels. when you spin the bowl side, the water speeds up. the water moving pushes on the plastic strainer side and wants to make the wheels move too. but you can come to a complete stop. the water is moving and will try to make the wheels move but it’s not directly connected so it’s not a problem. that’s why when you let off the brake, it starts to creep forward

manual transmission cars literally die at a stop if you don’t push in the clutch. in a manual car, the torque converter is replaced with a manual clutch. the clutch is to disconnect the engine and transmission so you can come to a complete stop without the engine dying.