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My understanding is that the Prius uses a CVT transmission.
You can't get added torque from an engine turning at a low speed by placing more load on it to turn a generator to turn an electric motor. If you disconnect the engine from the trans and use it to supply electricity to turn an emotor, your acceleration is then limited to the emotor's modest horsepower.
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The system uses a a sun-and-planet gearset to split the engine's load between a mechanical connection, and a generator, which converts those extra RPMs into electric current. The engine is never disconnected from the transmission.
This site explains how it works, here's an excerpt:
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Always in Top Gear
We have discovered above how the Prius transmission uses an epicyclic gear, a generator and control electronics to adjust the spin rate of the engine without the use of either a step transmission or a conventional CVT. We also discovered that a fixed proportion (about 72%) of the ICE torque is sent mechanically to the wheels. Without the ability to change the gear ratio between the ICE and the wheels, we cannot multiply up the ICE torque to get high acceleration at low speed. Although we've solved the problem of letting the ICE spin at a suitable rate when the car is moving slowly, the torque coupling is equivalent to always being in top gear!
This problem is partly solved already by passing generated electricity from MG1 to MG2 which adds its torque to that of the ICE. Electric motors do not share with an ICE the problem of not generating torque at low speed. In fact, this is where they generate the most torque. If we run the ICE at, say, 2000 r.p.m., with the car barely moving, a lot of power passes from the ICE to MG1. In fact, until the car starts moving, all the power goes to MG1. Even though MG1 gets only 28% of the torque, it gets all of the movement! Since MG2, connected to the wheels, is not rotating very fast, it doesn't take much power to generate its maximum torque. The Prius can launch from a standstill at respectable acceleration up to about 15 k.p.h. using ICE power passing primarily through MG1 and MG2. Only about a fifth of the torque comes directly through the mechanical path during this initial acceleration.
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Though this system could run without battery power theoretically, it can't start if the high voltage battery pack is dead, since it uses one of the motors to start the engine. It doesn't have a backup starter like the Hondas do, and I dont believe there's any convenient place to attach one. However, this is not a mechanical CVT, as it never changes the actual gear ratios at all, it simply "blends" the torque between the directly mechanical, and the electrical pathways.
Also, the fact that this system cannot run wihtout electric motors is relevent, even though electric motors are likely the last thing that will fail in normal driving -- the electric motors run the risk of overheating in offroad situations where they have a chance of being stuck in the mud for example. While this would not be a problem for a normal car, it could fry an electric motor trying to produce all that torque. That's why you won't be seeing any offroad-capable hybrid trucks using Toyota's system. However, Honda's system would potneitally work in a truck for going offroad as even without electric motors running, it can gear down the engine.