How does a (pulley based) continuous variable transmission (CVT) work?

Question

How does the (pulley based) CVT achieve the following?

• no requirement for a clutch
• fuel economy
• smooth gear transition

Answer 1

Review of geared transmissions

In general, the idea of a geared transmission is depended on the gear ratio. The gear ratio $i$ affects many aspects of a gear transmission, and the following equations apply:

$$i=\frac{n_1}{n_2} = \frac{z_2}{z_1}= \frac{d_2}{d_1}= \frac{M_2}{M_1}$$

where:

• $n_i$ is the rpm in gear i
• $z_i$ is the number of teeth in gear i
• $d_i$ is the diameter of gear i
• $M_i$ is the torque in gear i

Basically, what this gear ratio mandates is that a gear with a greater number of teeth will rotate slower than another with a less teeth.

The fact that the ratio of teeth is the same with the ratio of diameters, $$i=\frac{z_2}{z_1}= \frac{d_2}{d_1}$$

essentially allows us to abstract the geared transmission with the idea of two circles that are rotating in contact (without slipping).

Figure 1: abstraction of operating pitch circles (source:Tec-science

A manual shift gearbox works by having (typically 4 to 6) different pair combination of gear diameters, thus changing the gear ratio by "shifting" to another pair.

However, during shifting, you need to use the clutch in order to decouple gear transmission. The activation of the clutch is what reduces the acceleration.

How the concept of pulley based continuous variable transmission (CVT) works.

The idea behind the continuous variable transmission is very simple, yet powerful because it achieves the following things.

• there is an infinite number of gear ratios (which enable the car engine to operate at its optimal point)
• there is no need for a clutch (no need to "shift" gears)

The simplest way ( Evans Friction Cone - circa 1903) to achieve that is the following:

Figure 2: CVT concept (source:autotrader.ca

Basically, what happens is that the position of the pulley in the image above can be moved to any position, thus changing the diameters involved on each rotating shaft. Further the pulleys are designed to move according to the power demand of the car, always ensuring that the engine operates at its optimal range. Technically, this is possible because, CVTs have an infinite number of drive ratios.

Modern implementations of pulley based CVTs

The first pulley-belt CVTs is probably the Variomatic. It was developed by Hub van Doorne for DAF and introduced around 1958.

Currently, the prevailing implementation of the CVT is different than the above illustration.

Figure 3: CVT concept (source:rapidrepairautocenter

In this implementation, each pulley (primary and secondary) concurrently change their effective diameter. This is achieved by moving their side walls in opposite direction (see above).

This effectively changes the gear ratio, without the need to decouple the two shafts.

Other types of CVTs

Apart from pulley based CVT's there are other implementations. Namely:

• Toroidal CVTs (which replace pulleys and belts with discs and power rollers) (e.g. used by the 1999 Nissan Cedric (Y34),
• Hydrostatic CVTs (which involve converting fluid flow to rotational motion).
• Electrical (e.g. used in Toyota e-CVT in 1997 Toyota Prius)
• Epicyclic (or planetary CVT), uses planetary gears. (e.g. used in Toyota e-CVT in 1997 Toyota Prius)
• Ratcheting
• Magnetic
• Friction-disk transmissions (e.g. used in Plymouth locomotive)