Eli5 what does the transmission on a car do and how does it work?
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The transmission on a car uses gears to reduce the number of full turns of the engine per minute to a good number of full turns of the wheels per minute. The engine works best going really fast, but the wheels don’t. It turns out that the amount of rotations of the engine works best at pretty much one value, but the wheels need to go at different speeds, so the transmission switches the gears so the wheels go the right speed and the engine stays in the same range all the time.
You know how, on a ten speed bike, you need to change gears to optimize the amount of work you’re doing to the speed you’re going. The transmission in your car does approximately the same thing, but using weights and fluids and gear rotational speed.
It changes the ratio at which the engine spins its shaft compared to the speed the wheels spin.
The more times the engine’s shaft spins per times the wheels spin, the harder the engine can push the car up a hill or get it moving from being stopped.
Once the car is moving then the engine doesn’t have to push the car as hard to keep it moving. So the transmission makes the number times the engine shaft spins closer to the same as the number of times the wheels spin.
The real name of the shaft coming out of the engine is, “crank shaft,” and the transmission doesn’t actually go from the engine to the wheels, but this is a simple way to visualize it.
For more information, look up diagrams that show the crank shaft, the gears inside the transmission, the various kinds of transmissions, and how it connects to the power train.
If someone argues with you about what the “power train” is, be aware that I am using the phrase in its original sense. Some people prefer to call everything from the engine to the tires by that name.
So don’t let that confuse you.
Automatic transmissions were invented by a mad scientist who basically made the worlds most popular mechanical computer. It used pressure from the front and rear pump of the transmission along with a governor (weights on the end of a valve that are opened/closed based on speed) along with a vacuum operated valve (to help determine engine load), and, a valve connected to the throttle linkage to determine throttle position. The valve body directs fluid on what clutch pack or packs to activate (via hydraulic pressure) based on gear selector position, throttle position, and, engine load.
You have a torque converter that is like two fans facing each other, except it’s all submerged in fluid. This lets you idle and take off with minimal heat to the clutch packs and lets you come to a complete stop without stalling the engine. It allows some slipping to help you take off from a stop faster and it’s the reason why automatics will sometimes run a less aggressive rear differential ratio than a manual version of the same car.
Modern automatics are pretty much the same except computers and electronic solenoid valves and sensors took over most of the mechanical stuff except for the shift lever, and even the shift lever is done by wire these days.
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