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Discussion Starter · #1 ·
Hi All,

I am curious about the CVT in the Prius. Looking at the engine/motor/generator/transmission/axle cross section in the TMC Special Report on the THS II, it looks like the coupling from the power split device to the axle is either a belt or chain drive. In a van Doorne transmission the pulleys change in diameter to cause the variable transmission ratio. I believe most other cars that have a CVT have this configuration.

From the cross section the Prius does not have look to have the variable pulleys. Since first reading about the THS system I have wondered if the power split device had enough RPM range so that the variable pulley system could be eliminated. Does anybody know how that connection from the power split device to the axle is configured?

Thanks, Don...
 

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Donee,

THS (and THSII) do not have the variable ratio pully system. In the Prius, the power split device (PSD) is connected via chain to reduction gears, differential to the axels. The PSD which is a planetary gear set, has the sun gear connected to MG1 (alternater and starter), the planet carrier is connected to the engine, and the ring gear is connected to MG2 (main electric) and the wheels.

In a few postings on this and on priuschat, there is a link to someones web site that has a simulator of the PSD that is interactive. It allows you to see the interaction of the PSD components as you accelerate and slow down. Sorry I don't have the link, hopefully someone else knows it and can post.
 

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And, according to the account in "The Prius That Shook The World", the only reason the chain is in there at all is because the original design was a few centimeters too long to fit into the engine compartment, so they split the system in two and connected them via the chain..
 

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Also, although Toyota refers to the Prius 'transmission' as an eCVT...it really isn't and doesn't operate at all like a CVT. Check out Graham's site for a clearer explaination of what's really going on.
 

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Discussion Starter · #7 ·
Hi All,


Thank you for the replies. I found the graham1 website, and it was very explanatory. :)

I guess a better description of the Prius transmission is that it is an EMCVT, for Electro-Magnetic Continuously Variable Transmission. In its simplest mode, the back driving of the MG1 (Sun Gear coupled) provides the energy for MG2, allowing the engine (driving the planet carrier) to operate at a much higher RPM. How much the MG1 is back driven is related to the difference between the engine and the Ring Gear (MG2, transmission output) RPM. Its the electrical flow of energy that allows for the variability in the transfer ratio. The power waveform from MG1 is normalized and then turned into the needed power waveform for the torque and RPM needed for MG2.

Don...
 
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