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emissions. 1. Its engine can accelerate to 15 mph before switching on the gas engine. Once the engine starts, it operates in a narrow speed band. 2. Its uses a unique power split device which keeps the engine in its most efficient load and speed range most of its driving time. The 1.5-liter engine runs at a maximum speed of 5,000 rpm making 76 horsepower. Keeping the maximum speed of the engine low allows for the use of lighter component that improve efficiency. The electric motor on the Prius is rated at 67 horse power from 1,200 to 1,540 rpm. It produces 295 pound-feet of torgue from 0 to 1,200 rpm without using the gas engine.(http://auto.howstuffwo rks.com/hybrid-car7.htm)

The power split device is a gearbox that hooks the gas engine, generator and the electric motor together. It allows the car to operate like a parallel hybrid (the electric motor or the gas engine can power the car by itself, or they can operate together). The power split device can also operate like a serice hybrid (the gas engine can operate no matter how fast you are driving and charges the batteries or provides power to the wheels when needed.) It also acts as a continuously variable transmission (CVT) which eliminates the need for a manual or automatic transmission. And since the generator starts the engine, the car doesn't need a starter.

"The power split device is a planetary gear set. The electric motor is connected to the ring gear of the gear set. It is also directly connected to the differential, which drives the wheels. So, whatever speed the electric motor and ring gear spin at determines the speed of the car. The generator is connected to the sun gear of the gear set, and the engine is connected to the planet carrier. The speed of the ring gear depends on all three components, so they all have to work together at all times to control the output speed.
When you accelerate, initially the electric motor and batteries provide all of the power. The ring gear of the power split device is connected to the electric motor, so it starts to spin with the motor. The planet carrier, which is connected to the engine, is stationary because the engine is not running. Since the ring gear is spinning, the planets have to spin, which causes the sun gear and generator to spin. As the car accelerates, the generator spins at whatever speed it needs to in order for the engine to remain off." (http://auto.howstuffworks.com /hybrid-car7.htm)

"The generator is connected to the sun gear of the gear set,

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