How does a car stay still when the engine is on in park?



When a car has its engine on, the engine is working. But if its in park, the car doesn’t move. How? The engine is still running like if the car was driving, but what mechanism prevents the car from moving?

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On a manual car you have to either press the clutch, which physically disconnects the engine from the gears, or shift into neutral, which also disconnects the wheels from the engine.

Automatic cars generally have a hydraulic coupling, so the engine spins a turbine that makes a bunch of liquid go around, and then a second turbine is spun up by that moving liquid, and turns the wheels, but because it’s a liquid, you can just fix the wheels, so they can’t turn, and the first turbine can still just spin freely in the liquid. Imagine holding one of those small windmills under a flowing tap, if you let it turn freely, it spins really fast, but you can also just hold on to it so it can’t move, and the water can just flow around it

The engine turns a gear in the transmission, that in turn rotates the wheels.

If you put the transmission in park or neutral, the gear is decoupled so it just spins in space but doesn’t connect to anything.

The park gear locks the transmission in place so the car can’t roll. Neutral just decouples the engine but lets the wheels turn.

The parking gear (actually called a pawl). Shifting into “park” connects the driveshaft directly to an unpowered gear that does not turn easily. That gear holds the wheels from turning, but is not a perfect brake so it won’t hold a car on a steep hill and will either snap or slide free and let the car roll away. Set your parking brake on hills!

On an automatic car, once you shift into park or neutral, every clutch inside the transmission disconnects, so the engine/flywheel/torque converter output shaft turns nothing. Plus the parking pawl locks the output shaft of the transmission to prevent the car from moving.

For stopping at a red light shifted into Drive, the transmission is in gear so any transmission input shaft rotation will result in car rolling forward. At this time the slipping part is the “torque converter” upstream of the transmission.

Torque converter takes the role of the foot operated clutch and lower gear ratios on an automatic car. If you hold the car still while in gear, the converter slips so the engine can keep turning without moving the car even if you are in gear.

On older generation cars, the TC is always slipping because it’s a fluid coupling. This creates heat and fuel efficiency problems. Newer cars will lock up the TC once you are speeding up eliminating this friction loss.

A car has a “transmission”, something between the engine and wheels that uses a system of gears to change how fast the engine is trying to spin the wheels. When you shift into park, the transmission disconnects from the wheels altogether. It also drops a little pin into the linkage on the wheel side of things, so the car won’t even roll on its own from gravity on a gentle incline. Mind you, you should always use the parking brake when you park, for peace of mind against that little pin failing – that can happen on a less gentle incline, or if someone crashes into your car.