Although it is a technology that is over 100 years old, the continuously variable transmission (CVT) just keeps coming back to re-invent itself. Dave Wilson checks out some CVT developments.
The best way to match the speed of the engine to the speed of the vehicle would be to offer the driver an infinite number of ratios. That’s the idea behind the CVT – the continuously variable transmission. It’s not a new idea though. In fact, the first patent on the subject goes back to 1877. But it’s an idea that has taken some time to bring to fruition because the necessary mechanical technology hasn’t been in place to make it a commercial reality.
Perhaps the best known CVT is the Van Doorne belt design. The Van Doorne brothers started business in 1928, but only diversified into DAF cars in the late 1950s. The heart of their cars was the CVT, which they called Variomatic, and all DAF cars had it.
Following DAF’s lead, Honda developed a CVT in the 1990s and installed it into the 1996 Civic series. The Honda CVT, called the Honda Multi Matic also used the push-type metal belt similar to the Van Doorne Transmission. The latest CVT design to emerge is the Powertoros from NSK-RHP which has been installed in the Nissan Gloria Sedan in Japan. The design itself comprises power rollers sandwiched between input and output disks (see Figure1).
The action of changing the angles of the power rollers changes the rotational radius at the contact area, and with it the relative speed. When the input disk and the first power roller make contact at an area with a small radius, and the output disk and the second power roller make contact at an area with a large radius, deceleration results. Reversing those relationships produces acceleration. In all cases, the smooth changing of the power roller angle yields a smooth continuous change in speed. The sensitivity of the system means that moving a power roller within just a 1mm range in relation to the input and output disks generates enough force to make the power roller turn on its own and change its angle of orientation with the disks. The result is a transmission system that allows rapid speed changes and sporty driving.
Under heavy loads, a lubricating film one thousandth of a millimetre thick is formed at the contact points between the disks and power rollers of the Powertoros unit.
The special oil used in the traction drive becomes glass like under high pressure at the contact points, enabling the transmission of power from disk to oil and from oil to power roller.
Contrast the CVT approach with the IVT (Infinitely Variable Trans-mission) from Torotrak.
While the term CVT is used to describe all transmissions with the ability to sweep through a continuous range of ratios rather than be dependent upon the limited number of pre-fixed ratios found in conventional transmissions, the IVT produces a spread of ratios from full reverse to high overdrive, without the need for a starting device or reversing mechanism.
Here, a variator forms the heart of the design. Inside the variator are two pairs of discs. There is a cavity (or toroid) between each pair, and in each cavity there are three rollers. These rollers transmit drive from the peripheral, engine driven, input discs to the output discs located in the centre.
Each roller is attached to a hydraulic piston. The pressure in the pistons can be increased or decreased to move the rollers backwards or forwards within the variator. As the roller moves it generates forces which are seen as torques at the road wheels, resulting in a change of vehicle speed.
There is no metal to metal contact between the discs and rollers, traction being transmitted through a film of traction fluid.
It may have taken over a hundred years, but it looks as if the evolution of the CVT has not ended yet…
INFORMATION: NSK-RHP Tel: 0115 936 6470; Torotrak Tel: 01772 900900
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