Originally Posted By: Louie's gone fishing
People I've talked to think the Prius transmission is a CVT. It's not. It's a one speed tranny with no reverse, no neutral and one speed forward. The only constant is that it goes at the same speed as the vehicle, always, because it's connected to the bigger of two electric motors which is connected to the wheels. It's a planetary transmission.
Having given this "issue" a great deal of thought, I've concluded it's really a moot point. Yes, the design is fundamentally and totally different, mechanically, than the rest of the CVTs on the market. Yes, in effect, it's a "one speed" transmission. It just happens that the one speed is itself continuously variable.
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The only constant is that it goes at the same speed as the vehicle, always, because it's connected to the bigger of two electric motors which is connected to the wheels.
This is only partially correct. The outer ring is directly connected to the drive wheels, and the large MG is connected to that. The ICE, however, is connected to the ring that carries the planet gears inside the outer ring (the smaller MG is connected to the center shaft). Thus, the ICE rpms are free to vary continuously, even at a constant vehicle speed. This happens commonly as the computer decides to alter the balance between electric and gas drive.
So, although the outer ring remains "fixed" to the drive wheels, and always turns in relation to them in the same ratio, the rest of the trans certainly is variable. Perhaps a better way describe this transmission is a "one side CVT". That would distinguish between the belt-and-cone designs that vary at both ends, whereas this design is only variable on the "input" side.