Continuously Variable Transmissions
Page talks about frictional, electrical, hydraulic, and other types of CVTs.
As usual, someone has taken another advanced concept and implemented it in Lego. Don't miss the guy's other Lego creations, including a pneumatic V4 engine. I tried to build a pneumatic Lego engine when I was a kid, but when I applied the compressed air source, i.e. my mouth, it just whistled.