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Comment by bprater | Hacker News Reader

Quickly clicking thru the options on this webpage will give you an idea on how automatic transmissions work in automobiles -- using the planetary gearset.

The core mechanic is the locking of different combinations of the gears. This is a completely different mechanic than in the standard 'stick shift' manual transmission, which uses traditional toothed gears meshing together.

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I (along with nearly everybody I knew) had a bicycle with that gearbox in it back in the '70s.

http://sheldonbrown.com/internal-gears.html

(And I was _so_ jealous of the few kids with the "10 speed racers"!)

Oh, that's how they work! I hadn't looked into it but I have one sitting around at home waiting for a spare weekend so I can dissect it to figure it out.

I think you're thinking of an electronic CVT, like the Prius uses.

Wikipedia tells me that there are some CVTs that use planetary gears, but a traditional automatic uses them too: http://auto.howstuffworks.com/automatic-transmission2.htm

Both "normal" automatic transmissions and the Prius use epicyclic gears. The main difference between the two would be that an automatic transmission in normal operation has one power input (the engine) and one output. As bprater alluded to, any variation in gearing is achieved through fixing different parts of the transmission.

The Prius has two electric motors and a gas engine. It's not a CVT in the traditional sense though, this simulator does a good job of demonstrating how it works(implemented in Flash, not D3...) http://eahart.com/prius/psd/

That simulation is absolutely fascinating. I knew hybrid cars were packed with loads of advanced electronic tech, but the use of epicyclic gears as the PSD is pretty cool.

Toyota's HSD system uses such a scheme, but mechanically, Honda's IMA replaces the flywheel with an electric generator/motor, the remainder of the drivetrain being your typical car (they use either a CVT or a manual stick, depending on models and regions). The key then is entirely in the logic driving the electric motor and the VTEC system.

[0] http://www.blogcdn.com/www.autoblog.com/media/2010/06/2011cr...

No, he's not. A CVT doesn't have fixed gear ratios (that's where the continuous comes in). A traditional automatic transmission uses planetary gears to have a fixed number of gear ratios.

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