The reciprocating internal combustion engines which replaced the steam engines raised a problem in road and railway traction: they develop no torque at rest. From then on, a constant obsession of engineers and inventors was to conceive a continuously variable (CVT) or even infinitely variable (IVT) transmission system. Today, Torotrak's IVT bears without problems the 475 Nm of the 5,4 liters Ford V8. It was also set, amongst others, on some Ford Mondeo and on a 17 ton lorry. Who will be the first OEM to market it?
The most common of the variators (CVT) consists of two pulleys of variable rolling diameters connected by a belt. The Audi Multitronic in which the metal push belt was replaced by a chain represents the last evolution of this concept. Its overall ratio span reaches the value of 6.05, value remarkable though nowhere near infinity.
A great number of competitor systems exist. One of them is made of two concentric discs facing each other and presenting a toroidal cavity. They revolve in opposite direction, connected by rollers the angle of which determines the ratio. Patented by Charles Hunt in 1877, improved by Frank Hayes in the 20s, marketed by Austin around 1930 for its model Seven, improved by Perbury between 1960 and 1980, giving entire satisfaction in the NATO's vertical takeoff Hawker-Siddeley Harrier for the drive of a 25 kW generator between 7000 and 17000 rpm, developed by Leyland Trucks, resumed in 1986 by the BTG group from whom the branch Torotrak demerged in 1998, the toroïdal system is henceforth reliable and able to transmit high torques in a competitive weight and volume. This is due, on one hand, to traction oils whose viscosity increases strongly under pressure and which pass on the torque without any contact of the metal parts, by a film of 0,05 to 0,4 microns thickness (the diameter of a hair is of the order of 100 µ). On the other hand, the mastery of the precision machining and the grades of steels bearing height surface stresses – grades similar to those used for the roller bearings – by the Japanese firms NSK and Koyo-Seiko allows them to produce durable toroidal discs and rollers. Koyo-Seiko supplies Torotrak while NSK delivers these components for the semi-toroïdal "Extroïd" CVT of Jatco. In Japan, the Nissan Cedric and Gloria of 280 hp are available with the Extroïd, which has a limited ratio span of 4,36 and which requires a hydrodynamic torque converter – locked up immediately after the start up.
Image above : The double cavity variator includes 2x3 rollers. Confined in the torus, they don't produce any reaction thrust on their bearings. The axial loading is hydraulic, regulated electronically according to the tranmitted torque.