As true as this may be, it doesn't mean that traditional drivetrain designs can't be improved upon for increased efficiency, and one of the methods for reducing fuel consumption and emissions in our modern cars and trucks is the continuously variable transmission. Sounds complicated, eh? Like many things mechanical, though, it's not impossible to understand at a basic level, so let's dive right in (after the break), shall we?
So, uh, what's a CVT?
Continuously variable transmissions (CVTs) come in many shapes and sizes. At its most basic level, any transmission that doesn't rely on fixed gear ratios could be considered a CVT. The most common type of CVT used for transportation purposes feature variable-diameter pulleys connected with either a rubber belt or a metal chain. Automakers like Honda, Nissan and Audi all use metal chains while the vast majority of scooters, along with a few motorcycles and garden tractors, use V-shaped rubber belts.
The ends of the belt are wrapped around V-shaped pulleys that are constructed of two cones that face one another, creating a groove where the belt can sit. One pulley is connected to the engine and the opposite pulley is connected to either a driveshaft or, in the case of scooters, the drive wheel itself. Both pulleys work together to continually alter the gear ratio. As they say, a picture (or, in this case, an animation) is worth a thousand words. See here, here and here.
OK, fine. Why should I care?
So, now that we have a basic understanding of how one type of CVT works, the next logical question is: "why is it beneficial to use a CVT in the first place?" Because there are no fixed gear ratios in a CVT transmission, there's little need to compromise on either frugality or performance, meaning the car can have both good acceleration and high fuel economy. Further, because the transmission is constantly changing its ratio, the engine can be kept at its most efficient speed, which has desirable effects on overall vehicle emissions. Lastly, cars equipped with CVT transmissions don't jerk back and forth as the transmission switches ratios, creating a smooth ride.
In 2007, Nissan announced that it had sold over a million vehicles equipped with CVT transmissions. At the time, Nissan's data suggested that a million cars equipped with CVT's should be equal to the carbon reduction of 200,000 hybrid vehicles.
Sounds great! Any drawbacks?
Depending on your point of view, the smoothness of a CVT transmission may not be desirable. Many drivers accustomed to the feel of a traditional transmission that changes gears as the car accelerates and decelerates don't like the smooth feeling of a CVT transmission.
Also, the belt or chain of a CVT transmission is a potential wear item and must be able to withstand a great deal of torque from the engine, which is a bit of an engineering problem. For this reason, many CVT transmissions are mated up with relatively small engines that don't generate a ton of power. Recently, modern advances in high strength materials and high-tech lubricants have allowed more powerful engines to be mated up with CVT transmissions.
Got it... anything else I should know?
Lately, a number of hybrid vehicles have appeared on the market that use new kinds of CVT transmissions. For instance, Toyota uses a type of CVT in the Hybrid Synergy Drive technology that's used on every hybrid vehicle from the Japanese automaker. These systems allow multiple input sources for power – generally, a gasoline-powered engine along with an electric motor – and are much more complex creatures that take all kinds of computing power to operate... plus degrees in engineering and computer sciences to fully understand. Still, for all intents and purposes, these are continuously variable transmissions as well.