Eli5 Why do cars move at all when in drive without the accelerator being used. I’m not talking cruise control.
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The engine has to be running at all times. When you put it in gear, that energy is going to go to the drive shaft.
Automatic cars have a torque converter. It’s like a fan facing another fan. The first is connected to the engine, the second to the wheels. Provided the engine is on, the wheels experience a bit of force.
In the old days when you wanted to stop, you’d press in the clutch or shift to neutral.
When automatic transmissions were invented they needed an automatic way to stop without shifting.
The torque converter was invented to fill this need. It’s basically a slip coupling between the engine and transmission. This way the engine can always spin, even if you’re in drive and the car is stopped.
The consequence is that it always pushes the car forward a little, but you can easily overcome it with the brakes.
I’ve only driven Manuel cars, but I think the principle is the same for Automatics too. When you start the engine in a car, the engine keeps running whether you’re moving or not. It’s whole job is to spin a rod that leads to the wheels and makes them spin too.
When the car isn’t in gear or is in park, the car doesn’t move because the engine isn’t connected to the wheels at that moment. The engine is still spinning, but it’s not attached to anything right now.
Once you put the car in gear (or drive) you’re connecting the already spinning engine to the wheels, causing them to spin and the car to move.
So if you put the car in drive and leave it like that, the car will crawl forward slowly until you stop it or it hits something.
What the accelerator does is make the engine spin faster, thus making the car go faster. The engine will spin at a base rate without having to touch the accelerator at all, which is just enough to make the car move, albeit very slowly.
Does that help?
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.
Cars with manual transmissions don’t do this. Rather than creep, they roll freely.
Cars with automatic transmissions have a thing called a torque converter that is part used to switch gears. It’s kind of like a propeller attached to the engine facing another propeller attached to the gears, with fluid between them. When you’re stopped, there’s enough force from spinning one to move the other and make the car creep forward.
In electric cars, there’s no engine, and there’s nothing like the transmission of the ICE car. The makers of EVs then decide how it’s going to behave. My car actually has a configuration setting for this: roll like a manual, creep like an automatic, or hold still (which I use).
It’s called creep and is designed to assist with pulling away smoothly on hill starts, junctions etc where you may roll back a little. It’s configurable on teslas for example, if you don’t like it.
The engine is always turning in a petrol/diesel car. If the clutch is engaged, i.e. the pedal is not pressed, then the engine is connected to the wheels and thus they will also turn. (If the wheels cannot turn due to a steep slope or something then the engine starts having problems and will very quickly stall).
Automatic cars have a slightly different clutch mechanism which others have already explained well but you can treat it as if the clutch is always engaged.
So in summary: the car moves because the engine has to keep turning to be on, even in an idle state with no acceleration, and the wheels are connected to the engine.
With manual transmission:
When you start the car and stop pressing the accelerator the car will automatically maintain minimum revolutions of motor, so that it doesn’t ‘die’ when you are waiting for red light or something. So when you shift into the first gear, start moving and then release the accelerator the engine will try to maintain the idling rpm by automatically maintaining the flow of fuel and air necessary.
In order to keep the engine running, the belts have to always be spinning, therefore the pistons are constantly firing, and whenever you idle that doesn’t stop the belts or else you would kill the engine, so with the belts constantly spinning the engine is constantly trying to get those tires to spin, only reason why it doesn’t in park is because the brake is on which stops the tires and neutral disengages the tires from the belt
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