Why Do we Use the Automatic Transmissions in cars?

Q: Why not use Continuos Variable Transmissions or electric drive transmissions how is the Automatic Transmission better?
(2010-08-17 00:14:27)

A: CV transmissions are more difficult to implement. Anything that is difficult is more expensive. Also there are longevity considerations i.e. will the transmission have to be replaced every 10,000 miles?

The Toyota Prius does use a CVT of sorts. The engine and the small motor/generator share the input to the gearbox. The gearbox doesn't actually change gear ratios like a CVT would. When the car is stopped the power of the engine is being turned into electricity with the motor and stored in the battery. When the car needs to move the motor is slowed down so that the engine's power is directed to wheels. It looks like a CVT but in reality is a standard gearbox that has a component to send unused power to another place.

The electric drive you refer to (I assume you're talking about the type use in locomotives) has a lower efficiency an higher weight than the automatic. In a locomotive the weight is of no consequence in fact the fact that it is heavy is a bonus, improves traction. The simplicity of the drive and that it can easily be split between each axle far out weighs the efficiency loss.

Source: http://questionabouttoyota.blogspot.com/2010/08/why-do-we-use-automatic-transmissions.html