Transmission Basics

If you've read about the structure and function of automatic transmissions in How Automatic Transmissions Work, then you know that the job of the transmission is to change the speed ratio between the engine and the wheels of an automobile. In other words, without a transmission, cars would only have one gear -- the gear that would allow the car to travel at the desired top speed. Imagine for a moment driving a car that only had first gear or a car that only had third gear. The former car would accelerate well from a complete stop and would be able to climb a steep hill, but its top speed would be limited to just a few miles an hour. The latter car, on the other hand, would fly at 80 mph down the highway, but it would have almost no acceleration when starting out and wouldn't be able to climb hills.

So the transmission uses a range of gears -- from low to high -- to make more effective use of the engine's torque as driving conditions change. The gears can be engaged manually or automatically.

Photo courtesy DaimlerChrysler Mercedes-Benz CLK automatic transmission.  See more CVT pictures.

In a traditional automatic transmission, the gears are literally gears -- interlocking, toothed wheels that help transmit and modify rotary motion and torque. A combination of planetary gears creates all of the different gear ratios that the transmission can produce, typically four forward gears and one reverse gear. When this type of transmission cycles through its gears, the driver can feel jolts as each gear is engaged.

CVT Basics
Unlike traditional automatic transmissions, continuously variable transmissions don't have a gearbox with a set number of gears, which means they don't have interlocking toothed wheels. The most common type of CVT operates on an ingenious pulley system that allows an infinite variability between highest and lowest gears with no discrete steps or shifts.

Photo courtesy Ford Motor Company Ford Freestyle Duratec engine with CVT

If you're wondering why the word "gear" still appears in the explanation of a CVT, remember that, broadly speaking, a gear refers to a ratio of engine shaft speed to driveshaft speed. Although CVTs change this ratio without using a set of planetary gears, they are still described as having low and high "gears" for the sake of convention.

Next, we'll look at the different types of CVTs: pulley-based, toroidal and hydrostatic.