Engineers Corner The CVT

Some of us might not have heard about the CVT “Continuously Variable Transmission.  The CVT is a type of transmission which like the conventional automatic-manual gearbox is used to vary engine speed with driving speed but which unlike the conventional gearbox has no gears. It’s not exactly new in motoring but it’s believed to be the next step forward in Modern automotive technology. 

Nowadays it is believed that to improve performance and fuel efficiency in cars, the best option is to add more gears to the gearbox which is why some of the more modern cars have up to seven gears. That’s where the CVT comes in; because unlike regular transmissions where there’s a power drop between transmission as the car shifts gears, the CVT just provides one smooth rapid acceleration with no drop in power as the transmission pulleys rise in diameter. When driving a car with a CVT, you never 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. The only disadvantage of this is that it is a bit disorientating especially for people who are used to the jerks and jolts associated with gear shifts in conventional automatics. 

How the CVT works

Traditional transmissions use a gearset that provides a given number of ratios (or speeds). The transmission (or the driver) 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.

Though there are several types of CVTs, most cars use a pair of 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), the other to the drive wheels (output shaft). The halves of each pulley are moveable; as the pulley halves come closer together the belt is forced to ride higher on the pulley, effectively making the pulleys diameter larger. 

Changing the diameter of the pulleys varies the transmissions ratio (the number of times the output shaft revolves 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 automatic or manual transmission does, but while a conventional transmission changes the ratio in stages by shifting gears, the CVT continuously varies the ratio -- hence its name.

 I’ve included a little 3D view of how the CVT works below

Advantages of the CVT

The advantages of the CVT are countless especially when compared to conventional transmissions. Its more fuel efficient/economic especially since you can get higher cruising speeds at lower engine RPM’s, it provides quicker acceleration off the line since there are no gear changes to cause any drop in engine power, and it also lasts longer since there are fewer moving parts likely to get damaged “issues like grinding of gears in the gear box and blunt toothed gears cant occur in a CVT”

Disadvantages of the CVT

The CVTs 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. Also as most people are used to the jerks that occur in gear change, driving a car using a CVT can be a bit disorientating as there is no jerk or jolt as the car accelerates. Another funny thing about cars using CVT’s is that, it is possible for the engine to be dropping in speed  and the car would still be accelerating; this can be quite confusing for people who aren’t used to it.   

All in all, the CVT is still a wonderful transmission choice and even the best compared with what is available today. In fact the CVT is so good, the regulating body that governs F1 racing actually banned the use of it; they saw it as cheating because they were aware F1 cars using this transmission would out-accelerate those who weren’t.  

CVTs can now be found in cars from Nissan, Audi, Honda, Ford, GM, and other automakers. Why not try one when getting your next car?.... I should also note that the CVT was first sketched by Leonardo Da Vinci in 1490.

Heres a video of Nissan bragging about the beauty of the CVT enjoy!

Download