Why do automatic vehicles coast when on flat ground?

https://youtu.be/leCEmJA0WsI

this video explains the crucial part required for a basic automatic transmission.

as you can see, the constant feeding of power from the engine with the non direct connection of the fluid coupling means that there is always power being transmitted to the wheels.

if a car with a manual gearbox is left in first gear with the clutch pedal released, it too will moveeven with no gas applied.

now recently there has been an increase in popularity for automated manual gearboxes and dual clutch gearboxes (often called direct shift gearbox, or seamless shift and other similar names).

these gearboxes have a clutch just like a manual gearbox but unlike a manual, a computer controls this clutch. when you are stopped. the computer controls the clutch to release the engine from the gearbox giving you no movement at all while keeping the engine from stalling. once you give the car enough gas to get moving the computer will slowly engage the clutch to get the car moving.

when slowing down these will have engine braking just like a manual because the clutch is engaged.

in modern automatics, there has been a development to have locking sleeves go through the fluid coupling to provide a direct connection from the engine to the gearbox. this improves efficiency and speed. it also provides engine braking meaning the car will slow down.

based on the programming of these locking sleeves, the car will or wont coast well. for the most part, i would figure it coasts well as this helps fuel economy as less speed is lost to friction.

In a manual (stick) shift car, when left in gear and throttle released, the idling engine provides resistance and thus reduces speed of vehicle ("engine braking") ........there is a continuous mechanical link from the tires thru the differential, thru the drive shafts, transmission, clutch, finally to engine until the clutch is disengaged.

There are many different automatic transmission designs. Most employ some sort of a hydraulic turbine between the engine and transmission - usually a "torque converter" which doesn't work in reverse, has a "stall speed" under which it cannot transmit motion. Most modern transmissions have an automatic clutch between torque converter and engine that releases when throttle is released. Most have internal clutches (or "bands") that release in conjunction with throttle position. A CVT transmission immediately goes to a ratio below the "stall speed" when throttle is released. In the past, automatic transmissions were actuated by internal hydraulics, engine and driveshaft speed and throttle position. Now, transmission are electronically/computer controlled and changes are instantaneous.

The reason why an automatic vehicle will coast will in drive and on perfectly flat ground is because the car is constantly in gear it doesnt shift into neutral every time you stop. Whe n you are sitting there the car is in gear and the little bit of idle the car has pushes the car just a little bit with not much force.

Yes they can feel like they do coast because of the fact they have up to 8 gears CVT dont even have gears like a typical trans does So yes at the right speed it wont have a negative effect ( engine braking due to the speed and gear ratio) on your engine and it'll feel like its"coasting " but it will slow down pretty quick and not "hold" that coasting feeling it'll be short if at all

It's not coasting I mean not how I drive. I always have my foot on the gas pedal slightly but my car stays the same constant speed and if I take my foot off then I slow down.

That's not coasting, it's in gear.

Modern ones do not, there is always engine braking.