The transmission in your car is the device used to change gears. With a standard transmission, you control what gear a car is in with a gear shift. With an automatic transmission, it does it for you. From an engineering point of view, the basic design of these two types of transmissions has been the same for decades.
About a decade ago, Audi introduced a new type of automatic transmission called a continuously variable transmission (CVT). CVTs have been around for years in applications such as snowmobiles and off-road recreational vehicles but they were new to the automobile marketplace. According to Audi of Gainsville (Gainesville, FL), you can find CVTs in Audi A4, A5, A7 and A8 vehicles.
How they work
Conventional automatic transmissions use sets of gears inside that provide different ratios or speeds. Control units sense speeds and shift a transmission’s gears automatically to provide the most appropriate ratio for a given driving situation: lowest gears for low speeds, middle gears for acceleration and passing, and higher gears for efficient cruising.
A CVT replaces the complex gears sets common in standard transmissions with two variable-diameter pulleys, each shaped like a pair of opposing cones, with a metal belt running between them. One pulley is connected to the engine, the other to the drive wheels. The way it works is that during operation the pulleys move in and out such that a drive belt between moves higher on one pulley, lower on the other and this provides a numerical gear reduction. You can picture this roughly as the way a 10-speed bike works, by routing the chain over smaller or larger gears to change the gear ratios.
What they are like to drive
The controls for a CVT-based car are the same as an automatic-based car: Two pedals (gas 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, though. 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.
Engines do not develop constant power at all speeds; they have specific speeds where either torque, horsepower or fuel efficiency are at their optimum levels. Because a CVT can theoretically adjust to any gear ratio, engineers can program it to give maximum performance during all these situations.
Disadvantages of the CVT
They sound different. Because the CVT allows the engine to rotate at any speed, the noises coming from under the hood can sound odd to ears accustomed to conventional automatic transmissions. Automakers like Audi have gone to great lengths to make the CVT feel and sound more like a conventional transmission.
CVTs have been built for over a decade now and the technology has proven itself to be robust and reliable. As we mentioned, essentially the only disadvantage to a CVT-equipped car is that they sound and feel a little different than conventional automatic transmissions. A first this caused some concerns but CVTs are almost at the threshold of being “established technology” now.