Continuously Variable Transmissions

The Continuously Variable Transmission (CVT) was first patented in Europe in 1886 and a US patent was taken out in 1935. Automotive Engineering Online is quoted as saying that Leonardo Da Vinci drew a sketch of a CVT in 1490 and a rudimentary version appeared on the Clyno (a British car) of 1923.

The concept of CVT is to make an infinite number of gear speeds by using a pulley or belt. Normal transmission has steps when you change gear so a typical car might have 5 or 6 steps as the revs change and the driver selects a gear.

It was DAF, the Dutch car company that first brought CVT to the general car market – it had been used on other machines and agricultural equipment for years. The DAF version was basically a belt attached to cones that enabled the car to adjust the gearing according to the road speed thus enabling better economy and smoothness. The DAF 600 was a small car that started a series of vehicles using this technology. When DAF was bought by Volvo, the transmission department was separated and Volvo developed a number of models with a revised CVT layout.

Being separate, the new company (which was founded by the creator of DAF) could sell the system to other companies. Other manufacturers to sell CVT cars include Subaru, Toyota, Nissan, Honda, BMW, Ford and Chrysler. Even Audi sold a chain version on some models.

There are quite a few different types of CVT. Firstly there is the Variable Diameter Pulley. This uses a V shaped belt with two pulleys. As the speed of the engine increases, it moves the belt up one and down the other thus providing a smooth gear change. Toroidal or roller based CVTs use rollers to transmit the power through the gearbox. The rollers touch two conical discs and it is the movement of the rollers up and down the discs that transmits the power to the wheels.

There are several other types that use a mix of rollers, pulleys, cones and even hydraulics. The concept is the same for all types – make the system as smooth as possible so that they driver doesn’t need to change gear themselves. The image below is the “lineartronic” version from Subaru. It uses a pulley to help change the ratios so that the engine is always providing the power at the best speed and this then flows to the wheels finding the right ration for the conditions – and they have designed it so that the gearbox can run in all-wheel-drive mode as smoothly as possible.

You can always tell when a car has CVT as it will have a transmission called Synchrotronic, Torotrak, Infinitran, Multitronic or Variomatic. Great sounding names for a piece of technology!

Most drivers wouldn’t consider the type of gearbox that their car has – they just need it to work, however for many manufacturers, a CVT is a way of finding the best economy for the road conditions which is a great selling point.