Submitted by neonpablo t3_z6484n in explainlikeimfive
In a car with an automatic transmission, you start creeping forward as soon as you lift your foot off the brake. When you’re in Drive, but standing still with the brakes stopping the car, how does the engine avoid wearing down the first gear?
Thank you!
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Specifically the torque converter uses a fluid coupling: https://en.wikipedia.org/wiki/Fluid_coupling
It's a good read!
In addition to torque converter, many higher end sport cars use Dual Clutch Transmissions. In that case there is an actual clutch that engages/slips into 1st gear, just like you would do in a manual.
Interesting, thanks!
The gear itself doesn't wear at all from static force, since it's just pieces of metal meshing with each other but not moving. They can wear a bit from turning under power, though this is pretty negligible in vehicle transmissions that are working properly.
I think you're thinking of clutches. Manual transmission cars use clutches which physically slip to start from a stop. If you left one partially engaged while braked at a full stop, it would indeed wear very fast, so don't do that. Automatic transmission cars use torque converters, which as described in the other posts is a fluid coupling instead of a mechanical slippage, so it doesn't wear significantly from exerting force while the car is held at a stop by the brakes.
I've asked people before if the brake in automatic vehicles disengages the clutch and I never got an answer.
there is a slip mechanism called a torque converter, that at low speeds, works like a fluid coupling. that's why the vehicle creeps forwards when you ease off the brake.
think of a fluid filled donut with an impeller driven by the engine, and another impeller driving the transmission. at idle speeds the system has some slip. at higher speed differential there is torque multiplication due to a stator inside redirecting fluid flow. but that's well beyond ELI5 without diagrams and videos.
manual transmission cars don't have a fluid coupling, but rely on a friction clutch that works much like a brake pedal but inversely. stepping on the pedal releases the clutch into false neutral. and releasing the pedal engages the clutch and lets you bark the tires and get going.
No, that wouldn’t be necessary.
Automatics don't have a clutch, they have a fluid coupler, torque converter. Think of it like 2 fans facing each other. The fan attached to the engine is always spinning and blowing on the second fan which makes it spin. Since there is no direct connection you can hold the brake and stop one of the fans.
These old Army Corp film reels are great for learning about mechanical systems. Here's a film about torque converters. https://youtu.be/leCEmJA0WsI
griz75 t1_ixzad8t wrote
Its called a torque converter. Simplest way to explain it is that its an automatic clutch that engages/disengages the transmission. At low rpms with the brakes pressed it slips so the transmission sits idle. They are designed to require certain amounts of torgue to engage the transmission to make it work.