Describing Continuously Variable Transmission Operation

It’s important to know how the components of EV/HEV interact with each other and to identify the components and describe their functions. Continuously Variable Transmission (CVT) have become the transmission of choice for electric and hybrid vehicles. They are smaller and lighter than the geared transmission/transaxle which makes them more useful as engineers continue to trim vehicle weight.

You should already be familiar with the functions of the geared automatic transmission and transaxles, but here is a quick review. An automobile transmission couples engine rotational speed to wheel rotational speed. The ratio between the two speeds is varied to accommodate driving needs. For example, at a low speed ratio, the wheels turn slower than the engine, providing more torque and less speed. At a high speed ratio, the wheels turn at the same rate or slightly faster than the engine, to provide more speed and less torque. For the remainder of this lesson, references to transmissions include transaxles.

CVT Operation

Before CVT production, changing speed ratios were done by changing gears. Transmissions used clutches and bands to shift gears up and down according to driving conditions. CVTs change speed ratios without the stepwise shift characteristic of geared transmissions, varying ratios seamlessly. The speed ratio is altered hydraulically using computer controls. Instead of gears, the CVT uses a pulley system with two pulleys and a belt which allows for more variability between the high and low gears. One pulley, known as the driver or primary pulley, is connected to the engine’s output shaft and the transmission’s input shaft. The other is the driven pulley and is connected to the transmission output shaft which turns the wheels.

Review this link to see how CVT components work. Identify the driver pulley and driven pulley, reverse and forward clutches, steel belt and the input shaft from engine and output shaft that control the wheels. Watch this video showing a steel belt CVT and how it works.

Notice in this video how the rubber belt CVT seamlessly varies the gear ratios in response to vehicle load.

A photograph of a Nissan HR15DE engine with Xtronic CVT.

Copyright Nissan Global. Used with permission
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