What is a continuously variable transmission?
A continuously variable transmission, or CVT, is a type of automatic transmission that provides more useable power, better fuel economy and a smoother driving experience than a traditional automatic transmission.
A Continuous Variable Transmission
How CVT works
To understand this better, we must make comparisons with a conventional automatic transmission.
Conventional automatic transmissions use a set of gears that provides a given number of ratios (or speeds). The transmission shifts gears to provide the most appropriate ratio for a given situation: Lowest gears for starting out, middle gears for acceleration and passing, and higher gears for fuel-efficient cruising.
A conventional automatic transmission
The CVT replaces the gears with two variable-diameter pulleys, each shaped like a pair of opposing cones, with a metal belt or chain running between them. One pulley is connected to the engine (input shaft) and the other to the drive wheels (output shaft). The halves of each pulley are movable; as the pulley halves come closer together the belt is forced to ride higher on the pulley, effectively making the pulley's diameter larger.
A continuous variable transmission
Changing the diameter of the pulleys varies the transmission's ratio (the number of times the output shaft spins for each revolution of the engine), in the same way, that a 10-speed bike routes the chain over larger or smaller gears to change the ratio. Making the input pulley smaller and the output pulley larger gives a low ratio (a large number of engine revolutions producing a small number of output revolutions) for better low-speed acceleration. As the car accelerates, the pulleys vary their diameter to lower the engine speed as car speed rises. This is the same thing a conventional transmission does, but instead of changing the ratio in stages by shifting gears, the CVT continuously varies the ratio -- hence its name.
A CVT uses pulleys in place of conventional gears
Driving a car with a CVT
The controls for a CVT are the same as an automatic: Two pedals (accelerator and brake) and a P-R-N-D-L-style shift pattern. When driving a car with a CVT, you won't hear or feel the transmission shift -- it simply raises and lowers the engine speed as needed, calling up higher engine speeds (or RPMs) for better acceleration and lower RPMs for better fuel economy while cruising.
Many people find the CVT disconcerting at first because of the way cars with CVTs sound. When you step hard on the accelerator, the engine races as it would with a slipping clutch or a failing automatic transmission. This is normal -- the CVT is adjusting the engine speed to provide optimal power for acceleration. Some CVTs are programmed to change ratios in steps so that they feel more like a conventional automatic transmission.
Most new economy sedans and crossovers use this type of transmission. It is not widely used on heavy duty vehicles as they are not as reliable as conventional automatic transmissions in high power/torque applications.
A CVT Transmission
Advantages of CVTs
Engines do not develop constant power at all speeds; they have specific speeds where torque (pulling power), horsepower (speed power) or fuel efficiency are at their highest levels. Because there are no gears to tie a given road speed directly to a given engine speed, the CVT can vary the engine speed as needed to access maximum power as well as maximum fuel efficiency. This allows the CVT to provide quicker acceleration than a conventional automatic or manual transmission while delivering superior fuel economy.
Disadvantages of CVTs
The CVT's biggest problem has been user acceptance. Because the CVT allows the engine to rev at any speed, the noises coming from under the hood sound odd to ears accustomed to conventional manual and automatic transmissions. The gradual changes in engine note sounds like a sliding transmission or a slipping clutch -- signs of trouble with a conventional transmission, but perfectly normal for a CVT. Flooring an automatic car brings a lurch and a sudden burst of power, whereas CVTs provide a smooth, rapid increase in maximum power. To some drivers this makes the car feel slower; in fact, a CVT will generally out-accelerate an automatic.
Automakers have gone to great lengths to make the CVT feel more like a conventional transmission. Many CVTs are programmed to simulate the "kick-down" feel of a regular automatic when the pedal is floored. Some CVTs offer a "manual" mode with steering-wheel-mounted paddle shifters that allows the CVT to simulate a conventional stepped transmission.
Because early automotive CVTs were limited as to how much horsepower they could handle, there has been some concern about the long-term reliability of the CVT. Advanced technology has made the CVT much more robust. Nissan has more than a million CVTs in service around the world and says their long-term reliability is comparable to conventional transmissions. Several have however failed.. So far Toyota's transmissions have held up better.
I personally like to stay away rom new tech till it's been proven. It's your choice.
The pulley system in a CVT
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