ÂIf you’ve read about the structure and function of CVT automatic transmissions in Part-1: CVT Transmissions, then you know that the job of the transmission is to change the speed ratios between the engine and the appropriate (front or rear) 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.
After reading the introductory post-1 on CVT transmissions, introduced by Saturn in 1987, imagine for a moment driving a car that only had first gear or a car that only had third gear.
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.
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.
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.
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 drive-shaft 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. Stay tuned to this story in GotTransmissions.com blog. I have a short and interesting vibe on a lawnmower we had when I was kid (late ’50’s) that had a form of a CVT transmission in it..
