LinhPTK | May 29, 2017
With the push toward fuel efficiency in today’s vehicles, increasing numbers of them are being equipped with a continuously variable transmission (CVT). These transmissions function in a way similar to an automatic transmission, but have a simpler build, lower cost, and greater efficiency.
In general terms, a CVT functions by allowing continual variance in ratios between the engine’s output from the crankshaft and the turning of the driveshaft or axles. This ratio allows the engine to push the car forward at various speeds without working as hard as it would otherwise. Unlike a geared transmission, which has set ratios from one gear to the next, a CVT has infinite ratios between its minimum and maximum set. So the driver of a vehicle equipped with a CVT will not feel gear shifts or changes.
There are limitations to a CVT, such as the amount of torque (engine power output) it can handle and how much it can send to the driveshaft. These limits have so far meant that CVTs are found primarily in small- and medium-sized cars and crossovers.
The most common type used in automotive is the Reeve’s Drive (or variable-diameter pulley) system. Its basic components are two hourglass-shaped pulleys, between which a flexible metallic belt is suspended. The pulleys can move in or out, changing the diameter of the center of their “hourglass,” and they oppose one another, so when one expands, the other contracts. The metallic belt is constructed as two small-link chains between which thin metal strips are suspended. The chain is usually made of one harder alloy, while the strips are of a softer and more pliable alloy.
The strips grip the center “hourglass” portion of the pulleys, flexing to get maximum hold. The drive pulley receives power from the engine and turns the belt, which rotates the output pulley, which itself turns the drive shaft or axle, and thus the wheels. Because the pulleys can expand and contract, the ratio of one side versus the other can range from 1:1 to much higher, depending on the application. Like a bicycle’s pulleys, when the drive pulley in a CVT contracts to become larger, the opposing output pulley expands, becoming smaller. This makes the ratio higher, turning the wheels more often for every turn of the engine. The opposite can also be true, requiring more engine turns to turn the wheels fewer times. These ratios can be minutely changed almost on demand, allowing for maximum engine efficiency for the amount of power and speed given to the wheels by allowing the car’s engine to run at optimum rotations per minute (RPM) more often.
In a car, this maximizing of engine rotation rates means that peak efficiency can be held most of the time. Because the belt in a CVT is metal rather than rubber, it has a long lifespan. The metallurgy required to construct a belt durable enough to be used in an automotive transmission was only recently achieved, which is why the CVT did not appear earlier despite having been invented several decades ago. A CVT is also cheaper and simpler to build, making it lower cost to the consumer as well. They are also smaller and lighter than a conventional geared transmission.
Today, many vehicles from Nissan, Subaru, Toyota, and others utilize a CVT as their “automatic transmission” option. Fuel efficiencies in these vehicles have improved greatly thanks to this new technology.