What is a Toroidal Transmission

Nissan produced Cedric/Gloria model with integrated CVT modification with the torque up to 370 Nm and the engine with a power of 280 hp for the domestic market of Japan. The fact is that Extroid CVT of Nissan fundamentally differs from CVTs of the Multitronic type. Extroid CVT is not a V-belt transmission, but a friction drive CVT. Nowadays Extroid CVTs are commonly known as “toroidal” due to the fact that the working surface of driving and driven discs in this transmission has the form of a torus. The toroidal type of transmission is not so popular in modern cars as V-belt transmissions. This technology is used only for RWD and AWD vehicles with a longitudinal engine positioning.

The technology for a toroidal transmission was patented in 1877 by Charles Hunt. The British company Austin was the first among automakers to consider this technology as a viable option for cars and introduced the mini-car Austin 7 equipped with a toroidal gearbox.

Apart from that, all efforts of car manufacturers to upgrade and apply toroidal transmissions in vehicles were in vain, because used materials did not allow engineers to produce rollers of sufficient durability. For a long time this technology was considered to be “dead” until Jatco and NSK, the company producing bearings, decided to accept a challenging task and bring toroidal CVTs back to life. More than 20 years of hard work and experiments were crowned with success. In 1999 Nissan started using toroidal CVTs in Cedric/Gloria models, and a couple of years later in legendary Skyline model. Mazda also decided to give a try to toroidal transmission and presented the concept of Nextourer crossover equipped with a 3-liter engine V6 and the toroidal CVT produced by Jatco. The British company Torotrack experimented with toroidal solutions for Ford Expedition. However, after a period (until 2000) of strong research and development activities, major automobile companies again lost interest in this technology.

Learn more about Torotrack transmission

Principle of Operation

The main feature of this CVT is the usage of discs and rollers instead of belts and pulleys. The design of this gearbox includes 2 discs (drive and driven). One disc is attached to the engine and the other one – to the drive shaft. The rollers are positioned between the discs. During operation the rollers rotate along 2 axes. They rotate about the horizontal axis and tilt in or out around the vertical axis, which allows the wheels to touch the discs in various parts. The pressure in the contact pattern between the discs and the rollers may reach up to 10 tons.

The gear ratio varies as the rollers rotate along the vertical axis and the torque is transmitted due to the frictional force of the rollers. If the rollers and driving disc interact near the rim, then the speed grows and the torque gets reduced – the maximum gear. If the rollers interact with the driven disc on the edge of the rim, then the speed slows down and the torque increases – the minimum gear. Thanks to this principle this type of gearbox can have almost an infinite number of gears.

More informative videos about toroidal CVTs

Peculiarities

• Standard types of oil should not be used for toroidal transmissions. Special friction oil used for these CVTs turns into a film at high speeds, which contributes to the increase of friction interactions.
• Rollers are made of expensive carbon steel which endures high loads.
• Repair services for toroidal CVTs are very costly. It’s hard to find spare parts for this transmission and in case of serious failures transmission repair specialists very likely will recommend transmission replacement.

Without regard to the auto business, toroidal transmissions acquired distinction in the aircraft industry, when this CVT was used to drive a 25-kilowatt generator in the jet-fighter Harrier.

Jet-fighter Harrier in action

KERS system for Formula One

The toroidal transmission technology also captured the attention of Formula One participants. This type of transmission was selected for realization of the mechanical KERS system, which should correspond to functional requirements of F1 teams.

The mechanical KERS (Kinetic Energy Recovery System) was developed thanks to integrated efforts of Xtrac, Flybrid and Torotrack companies. This system includes the Flybrid Systems’ flywheel, Xtrac’s transmission solution, and Torotrack’s CVT technology. The toroidal CVT is mounted between the flywheel and the engine shaft. The operation principle of KERS system consists of transmitting the part of accumulated energy from the flywheel to the engine shaft during accelerations and under increased loads (standard internal combustion engines, in turn, are not very efficient and consume much fuel in the acceleration mode). When the vehicle slows down, kinetic energy of the engine shaft rotation is transmitted to the flywheel via the toroidal CVT, while in normal conditions this energy is simply wasted. In the process of subsequent acceleration, accumulated energy recuperates via the toroidal CVT, helping the engine to spin the shaft. The flywheel, made of carbon fiber and steel, is capable of accelerating up to 60 thousand RPM (revolutions per minute). The flywheel is located in a vacuum chamber and is connected to the transmission with the help of several gears. Thanks to the mentioned vacuum chamber and a unique system of seals, it is now possible to implement the mechanical KERS system in practice.

All in all, high production cost of toroidal transmissions allows to install these transmissions only on luxury cars. Engineers often call this gearbox extravagant and regard it as a desire of some companies to stand out of the pack. The efficiency of this CVT is not much higher if compared to other CVTs. Therefore, application of toroidal transmissions is the question of financial capacities and viability.