Hearted Youtube comments on The Car Care Nut (@TheCarCareNut) channel.
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Seeing you discuss the Prius CVT made me want to share my thoughts on it.
What's called a planetary in the video is a differential, a differential is only planetary gears. Differentials are used in all cars to keep power applied to the drive wheels equally while turning corners. You may remember a time when one drive wheel slips in mud or snow and the other wheel goes nowhere.
So imagine a rear wheel drive car with one rear tire removed and pretend the car magically stays suspended. Then disconnect the hand brake from the wheel thats on the ground and leave it attached only to the floating brake hub.
Put the car in gear and try to drive it.
It won't go anywhere. Like in the snow, the hub will only just spin because of the differential.
Then start applying the hand brake to that hub, slowing it down, and the differential will put torque on the other wheel. The car will go and you can control the engine rpm and vehicle speed with the combined use of the gas pedal and the handbrake. If you put enough pressure on the handbrake so that the hub completely stops, the differential will pass all the power to the drive wheel.
That's the principle behind the Prius CVT.
Now replace the handbrake with a generator and instead of creating a lot of heat with the handbrake you create electricity with the generator. Increasing the field current of the generator produces more electricity and puts more of a load on the differential. Consequently more torque is applied to the other wheel. Of course, the electricity needs a place to go so put a motor directly on the other wheel, a battery to store fluctuations of demand, a computer to control the whole system, and you have a Prius!
No friction parts in the CVT since it's only basically a differential.
When slowing, the engine stops, the generator just spins with no field current, and the motor acts as a generator, charging the battery while slowing the car. Finally the brakes bring the car to a stop.
So the Prius actually has two differentials (planataries), one is the CVT and the other is just the usual one on the wheels. All elegantly put into the same shell with the motor and generator.
To recap, the CVT differential has three shafts, the engine, the generator, and the motor which is always connected directly to the usual wheel differential.
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