INTRODUCTIONTOROIDAL VARIATOR TRANSMISSIONADVANTAGESLIMITATIONSAPPLICATIONHYBRID HOPES AND FUTURE PLANSCONCLUSIONREFERENCE

Research and development of the internal combustion (IC) engine is nearing both perfection and obsolescence. But advancements in fuel economy and emissions have effectively stalled as many IC vehicles meet Low Emissions Vehicle standards.Todays vehicle manufacturers face a dilemma: how to cut fuel consumption and carbon emissions while preserving the performance characteristics that consumers have become accustomed to. Instead of concentrating on fuel and engine a new generation transmission system that replaces conventional toothed gears found in traditional transmissions claims not only to slash CO2 emissions and improve fuel efficiency, but also boost performance.

One potential solution to this fuel economy dilemma is the toroidal variable transmission (TVT), an old idea that has only recently become a bastion of hope to automakers.Instead of the conventional toothed gears found in traditional transmissions, the Toroidal Variable Transmissions (TVTs) use innovative steel discs and rollers.When used in place of a conventional gearbox the new transmission offers seamless acceleration from standstill to cruising speed and back again, thereby reducing harmful exhaust gases by eliminating 'emission spikes' caused by gear changes, thus improves both mileage and acceleration compared to traditional transmissions. Today, however, ongoing TVT research has led to ever-more robust transmissions, and thus ever-more-diverse automotive applications.

Varies the transmission ratio continuously Shifts automatically with an infinite number of ratios. Seamless power deliveryConstantly changes its gear ratio to optimize engine efficiencyAllows the engine to rev almost immediately to deliver maximum torque

Traction drives are fixed-ratio mechanical drives. They transmit power through the traction forces on metal rollers that are arranged on loaded in a planetary configuration. These rollers can be cones, cylinders, discs, rings, spheres, or toroids. The speed ratio is determined by the radius of rotation of the driver roller on the driven member.These drives use synthetic fluids called traction fluids which reduce the contact pressure between the rolling members to increase life and power capacities.Under high pressure, viscosity of the fluid increases dramatically so the fluid behaves more like a plastic material.This plastic-like material enables the drive to transmit power without appreciable metal to metal contact.

Traction fluidOutput discInput discRoller

Uses rollers to transmit torque between the input disc (which connects the crankshaft) and output disc (which connects the driveshaft). A viscous fluid with high shear strength properties is fundamental in transmitting torque between rollers and discs.Rollers and discs never touch.The angle of the rollers changes relative to shaft position resulting in a change in gear ratio.The change in angle by a roller must be mirrored by the opposing roller.

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The rolling elements are not in direct contact but are separated by the now highly compressed, thin film of lubricant.The traction performance of the lubricant is crucial to the efficiency of traction drive.Standard mineral oils can be used but durable and moisture-resistant lubricants known as cycloaliphatic based fluids have been specifically designed for traction drives.These enable significant power to be transferred efficiently across a traction drive contact.For example, some 60 kW, or the power of a small family car, can be passed across a 20 mm, 2 x 1 micron contact patch, with a volume of only 0.00002 cc.

Mechanism of interaction of the roller position is done by the change in variator ratio which produces pressure perturbation in the hydraulic circuit.

The side view shows the positioning of the rollers on the toroidal cavity along the axis of the variator.The top view shows the castor angle provided for the stability of the roller.While the front view shows the roller tilt along the toroidal centre.

Traction fluid is a transmission fluid with properties specifically designed for transmitting torque between two smooth rolling elements. The ability of a traction fluid to transfer torque is measured by its Coefficient of Traction, or Ct.The synthetic base oil used in a traction fluid is tailored to give them high traction coefficients over wide operating ranges.In the TVT, the lubricant is required not only to transmit torque between the variators disks and rollers, but also to have all the usual characteristics of an automatic transmission fluid.The high friction requirements have resulted in the development of traction fluids such as Santotrac-50.Unfortunately, it has been shown that lubricants formulated to give high traction properties tend to exhibit poor low temperature viscosity.

Absolute progressiveness because of variety in gear ratiosEliminates 'emission spikes' on the engine caused by conventional gear box, thus reducing emission of harmful gases Outstanding creeping ability up and downhill Lightning fast transient response Relaxed driving as here is not much requirements of gear changesLow revolutions in cruising Optimal engine use control line ensures that the engine always operates at minimum specific fuel consumption, i.e. at maximum efficiency.Mileage improved by 20%

Not yet in production as there is a lack of demandThere is a lack of direct drive, instead a chain or belt drive has to be usedTVTs require far more complex algorithms to accommodate an infinite division of speeds and gear ratios which is simple in regular automatic transmission.

It can be sized to work efficiently in the main drive transmissions of everything from lawn mowers to 40-tonne trucks.Toroidal traction drive systems are already being used in a unique zero-turn ride-on lawn mower.The system is also soon to be available in a new range of tractor transmissions that offer high levels of speed control, fuel savings and class-leading efficiency.On a larger scale, main drive transmissions capable of handling 350 kW are currently under development for larger vehicles including trucks and busesThese drives could help the automotive industry reduce engine size without compromising performance and power.

The hybrid system comprises a flywheel connected via the traction drive variator to the base power train. When a vehicle is slowing, kinetic energy is transferred through the traction drive variator and into the flywheel, maintaining the energy in its kinetic state.To recover this energy, the power flow in the variator is reversed and the kinetic energy is released to the vehicles road wheels. During this process energy remains in a kinetic state.In contrast, during electrical regeneration, the kinetic energy is converted to electrical energy and then to chemical energy as it is stored in the vehicle battery pack; to recover the energy, this process is then reversed.

Be it in a conventional transmission system delivering significant fuel savings or part of a hybrid system that can recover otherwise wasted energy, the traction drive will prove crucial to the challenges facing engineering communities.The drives are already in use in the highly cost-competitive lawn and garden market and are poised to penetrate the commercial vehicle market as a main drive transmission technology.The technology could also bring benefits to the construction and material handling markets, and in each case significant carbon dioxide savings could be made.Small traction drives for superchargers and air-conditioning compressors will help to improve the efficiency of conventional internal combustion engines as well as reduce carbon emissions.As well as having great potential in the kinetic energy recovery systems of mechanical hybrid vehicles, the technology could be used in electric vehicles, enabling electric motors to run at steady state conditions and reduce current draw.

The toroidal variator transmission that is competitive in package size, weight and cost with the latest stepped automatic transmission designs, whilst delivering excellent vehicle performance and fuel economy gains in excess of 15%.Today, only a handful of cars worldwide make use of TVTs, but the applications and benefits of toroidal variator transmissions can only increase based on todays research and development.As automakers continue to develop TVTs, more and more vehicle lines will begin to use them.As development continues, fuel efficiency and performance benefits will inevitably increase; this will lead to increased sales of TVT-equipped vehicles.Increased sales will prompt further development and implementation, and the cycle will repeat ad infinitum.