You probably know that the transmission in your car is the device used to change gears. With a standard transmission, you control what gear a car is in by shifting a gear shift, with an automatic transmission, the transmisson does it for you. From an engineering point of view, the basic designs of these two types of transmissions has been the same for many, many years.
About a decade ago, Audi introduced a new type of automatic transmission called a continuously variable transmission (CVT). The operating concept had been around for years in recreational applications such as snowmobiles and off-road vehicles but they were relatively new to the automobile marketplace. Today, CVTs are pretty common and according to Audi of Gainsville (Wilmington, DE), you can find CVTs in a good part of the product line, as in the Audi A4, A5, A7 and A8 vehicles.
How they work
Your standard automatic transmissions use multiple sets of gears inside that provide different ratios or speeds. Control units, electronic ones these days, sense speeds and load and shift a transmission’s gears automatically for you. They chose 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, and shift to that range.
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 drive wheels, the other to the engine. 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 providing 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.
Driving with CVTs
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. Drivers that are new to the CVT experience note that CVTs have a difference “feel.”
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.
They “feel” different. Because the CVT allows the engine to rotate at optimal speeds, the sound coming from under the hood can sound odd to ears accustomed to conventional automatic transmissions. This fact has not been disregarded by automakers like Audi, though. They 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 at the threshold of being “standard technology” now.