How does an automatic transmission work



How does an automatic transmission work

In: Engineering

In a modern automatic, a computer monitors road speed and engine speed (and also how hard you’re pressing on the accelerator pedal). It will then activate [electric solenoids]( in the transmission to turn on or off certain [hydraulic circuits](, which then release or apply particular [clutches and bands]( to obtain the optimum gear ratio (1st for slow speeds, 2nd for faster, etc) in the [planetary gearset](

You can see the hydraulic circuits, clutches, bands, and solenoids in most of the transmission rebuild videos on this YT channel:


Before we had computers, we had machines that worked with electronic switches called “relays” and a handful of other devices. For example, we could make jukeboxes before we had computers out of this stuff.

Before we had relays and other electronics, we could build computer-like machines out of gears that behaved like switches.

An automatic transmission is sort of like one of those no-computer machines (though modern ones might use computers.) It’s filled with a fluid, and that fluid is what behaves like electricity or gears. There is a very complicated series of tubes and valves that make up the “program” for the transmission.

When the car’s engine turns, it turns gears inside the transmission. So does the motion of the wheels the transmission is connected to. These two sets of gears create pressure in the fluid inside the transmission.

When the engine is turning really fast compared to the wheels, the pressure causes a certain valve to open. This sends pressurized fluid to a part of the transmission that causes the gears to move, making different gears engage with the engine. That changes the difference in speed between the wheels and engine, which changes the pressure, so the valve closes and the “shifting” to a higher gear is complete.

If instead the wheels are moving very fast compared to the engine, it makes a different kind of pressure that opens a different valve. That also causes fluid to go to a part that changes which gears engage with the engine, which changes the difference in gear speeds, which changes the pressure and shuts the valve again. That’s shifting to a lower gear.

That’s just like if a computer were watching the speeds of the different gears and choosing when to shift, it’s just not always done using computers.

This is useful for the car because the difference in size between two gears (called the “gear ratio”) affects a lot of things that determine how much power the gear has when it turns. For example, if you connect a very tiny gear to a very big gear, you have to spin that tiny gear very many times to turn the big gear once. *However*, the way gears work, if that big gear were connected to something very heavy, you’d find it is easier to turn the little gear than it would be to turn the big gear by itself.

The opposite is also true: if you connect a very big gear to a very small gear, it’s hard to turn the big gear, but turning the big gear very slowly makes the small gear turn very quickly. However, if the small gear is connected to something heavy you’ll find it is harder to turn the big gear than it would be to turn the thing connected to the small gear!

So a transmission has a lot of different gear sizes on the inside. Some sizes are better at providing a lot of pulling power but can’t make the car go very fast. Others are very good at making the car go very fast but can’t pull very hard. The “low gears” tend to be the ones that pull very hard, because making a car go from a full stop takes a lot of power. The next higher gears can’t pull as hard, but that doesn’t matter so much because the car is already moving. The highest gears are very bad at pulling, but if the car is already going fast they use very little engine power to *keep* it going very fast.

There are also lots of other interesting situations transmissions have to handle. For example, usually if you floor the accelerator you’ll feel it shift. This is a “passing shift”. When you jam the accelerator, the car is probably in one of those fast but not powerful high gears. To give you more power so you can accelerate, pushing the accelerator in this state opens the “shift down” valve. This puts you in a lower gear so you can pull harder at the cost of burning more fuel. Once you let the accelerator rest, the transmission fluid opens the “shift up” valve and moves back to the “right” gear for the job of maintaining speed.