Eli5: Why does a car keep moving when turned off in “D” instead of “P”?


I realize the locking mechanism in “P” prevents the car from moving but how does it move in “D” in the first place? The car is turned off! There is no ignition to the engine!

In: Engineering

Because there is nothing physically locking the transmission from moving. Fluid pressure inside an automatic along with clutches is what keeps it locked and going forward when it’s running in D or R When it’s not running and there is no pressure it’s mostly free.

P and D are connected to the part of the car that controls how much power is transmitted to the wheels. This part that transmits power is called the transmission and is made up of a number of spinning gears. When you put a car in P, a lever is inserted between the teeth of one of the gears, stopping the transmission from spinning. This lever is called the “pawl”. Since the transmission is what turns the wheels, the wheels cannot move while the pawl is in this locked position. When the car is in D, the pawl is not stuck between the teeth of the gears, so the transmission is free to turn. The wheels can then also rotate and move the car.

Some good answers here, let’s get a tiny bit more specific and ELI5.5 Your engine is a bunch of mechanically moving parts. If you yanked your engine out of the car and put a long bar connected to the back or front of the engine, you could manually make the internals move. (Breaker bar on the harmonic balancer or flywheel) From experience, a smaller engine is easier to do this with than a larger one because more surface area = more friction.

So… back to your question. If a car is in “D” then the transmission is in gear, meaning the wheels are connected to the engine internals physically through the transmission. When you are in P or N, the transmission separates the engine from the wheels physically. So if you are in D with the car off and you want the car to move – push it hard enough and it will. Or… use gravity by letting a few thousand pounds and an angle to work out the math for you. 😉
Now… ELI6, you may not actually be making the engine move as other people noted. An automatic car has a torque converter which is a fancy device filled with fluid. This plays the role of a clutch via fliusld pressure. At low speeds with the car off and in D, you are likely to only be spinning the fluid a bit. Either way – this isn’t particularly healthy for the car, so please don’t go around doing this intentionally unless you enjoy repair bills.

“D” is pretty much “N” just with a bit more force required to make it move. But we’re talking up to 2 tons of metal and plastic here, sitting on 4 things specifically invented for the purpose of moving. Add in gravity and boom you now have a moving car.

Also your street may seem level, it’s probably not as level as you imagine. A 10 degree incline as huge as your whole neighborhood probably just looks flat to you, it does not look flat to physics.