Continuously Variable Transmissions

The concept of a Continuously Variable Transmission, CVT, is not new and has been credited to Leonardo Da Vinci in the late 1400’s. Dutch vehicle maker, DAF, used a CVT in the late fifties.

CVT’s aren’t currently the dominant transmission design in our cars. But they are becoming more common with a number of makers including Audi, Nissan, Mitsubishi and Honda offering models with a CVT.

The following is an explanation of their basic operating principles.

A normal transmission uses a fixed number of gears to transmit the engine power to the wheels. At low road speeds these gear ratios also multiply the available engine torque to allow acceleration. To allow road speed to rise further with lower engine RPM, higher gears are necessary. Each gear ratio is a function of the relative diameters of the mated gears.

The design of a CVT provides the ability to constantly vary the available ratios between an upper and lower limit, effectively offering an infinite number of ratios. A common design does this with two metal cone pulleys that expand or contract under hydraulic pressure, with drive transmitted via a high strength steel belt. One pulley inputs power from the engine and the other provides output to the driving wheels. As one pulley contracts the other expands to keep belt tension correct. At the same time the contraction or expansion moves the belt contact point up or down the conical drive faces of the pulleys, effectively providing a change of pulley diameters. The ratio thus provided is also changed. A computer controls the system.

A CVT is able to offer a wider spread of gear ratios between top and low gear and can better match the actual gear ratio to the operating conditions of speed and load, keeping the engine revs in just the right spot for optimum torque or fuel economy. From the driver’s point-of-view a CVT’s operation is similar to a normal auto, except there are no discreet shift points felt by the driver.

Earlier CVT’s weren’t too impressive, but most newer examples are very smooth and some can handle the power of bigger engines.