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Traction Drives

A new design of transmission drive is set to help vehicle manufacturers reduce carbon dioxide emissions. Dick Elsy is CEO of Torotrak, a UK developer of transmission systems based on traction drive. He explains how his company has developed a range of drives for lawn mowers, cars, tractors, trucks and buses.

A typical roller and disc from a traction drive variator used in a high power application © Torotrak

A typical roller and disc from a traction drive variator used in a high power application © Torotrak

Today’s vehicle manufacturers face a dilemma: how to cut fuel consumption and carbon emissions while preserving the performance characteristics that consumers have become accustomed to. A new breed of variable transmission known as traction drive could have the answer.

UK company Torotrak has developed a transmission system based on a variable traction drive that boosts fuel efficiency and delivers improvements in acceleration over conventional step-ratio automatic transmissions. It does this by enabling the engine to operate close to optimum efficiency, regardless of vehicle road speed, which conventional transmissions do not.

To demonstrate the advantages, Torotrak recently fitted its transmission system into a mid-sized city bus and ran the vehicle on the industry-benchmark London fuel economy test cycle; the result was a 19% improvement in fuel economy. The variable drive technology is flexible enough to also be used for applications such as variable fan drives and supercharger drives which can improve the operating efficiency of internal combustion engines and yield further fuel economy benefit.

Figure 2a: Torotrak’s zero-turn ride-on lawn mower transmission is already in production in the US

Figure 2a: Torotrak’s zero-turn ride-on lawn mower transmission is already in production in the US

The principles of traction drive

The management of friction between moving metal surfaces through the use of fluid lubricant is at the heart of the field of engineering known as tribology.

In this field of work, engineers focus almost entirely on the task to minimise the levels of friction between moving surfaces.

Another branch of this field of engineering applies the same technical principles but uses them to transmit power through the lubricating medium in a process called ‘traction drive’.

Torotrak have put these principles of traction drive into practical use by developing a variable drive device known as a ‘variator’. The drive transmits torque, using specially developed traction fluid, between loaded rollers and discs (see The contact patch). This gives a simple and robust continuously variable transmission (CVT) that changes ratio continuously, unlike a conventional transmission that changes its gear ratio in steps.

When configuring the variator, the development team adopted a ‘full-toroidal’ variator layout as this represented the most stable and simple geometry for practical application (figure 1).

Inside the variator are two pairs of discs. The space between each pair of discs forms a hollow doughnut
shape or ‘toroid’. Within each toroidal space, three rollers transmit drive from the outer engine-driven discs to the output discs located in the centre. The rollers transfer power between the input discs and the output discs via the traction fluid.

Each roller is attached to a hydraulic piston (or control rod). The pressure in the pistons can be increased or decreased to create a range of reaction torque within the variator.

Figure 2b: A cutaway of the lawn mower’s twin toroidal transmission

Figure 2b: A cutaway of the lawn mower’s twin toroidal transmission

Practical results

For a vehicle manufacturer, the full-toroidal variator has two key advantages. First, it is inherently stable, and second, the device is torque-controlled rather than ratio-controlled. This means the torque at the output is directly proportional to the amount of force applied to the control rods. The ratios, and therefore the roller positions, effectively self-compensate to deliver the required torque. The end result is a traction drive transmission that can deliver 94% mechanical efficiency in terms of power in and power out.

In the continuously variable transmission drive, the input of the variator is driven by a power source, such as an engine, and the output speed is determined by the position of the rollers in the toroid. This direct connection between input and output would be suitable for a continuous drive system such as a fan drive or, with the use of a clutch or other decoupling device, in a vehicle drive transmission.

The CVT can also be configured with an epicyclic gear train to give an infinitely variable transmission, which provides a smooth spread of ratio from forward to reverse. From a vehicle manufacturer’s point of view, this is beneficial as it eliminates the need for a vehicle to have a clutch or torque converter to decouple the transmission from the engine before the vehicle can stop. This so-called ‘geared neutral’ condition also enables a seamless shuttling effect, sweeping the transmission from forwards to reverse, ideal for use in vehicles such as fork-lift trucks and wheeled loaders.

Why use traction drives?

A number of types of variable drive technology are used today, the main incumbents are hydrostatic drives and belt or chain continuously variable drives.

Hydrostatic drives make use of hydraulic pumps and motors and are typically found in agriculture, construction and industrial products. These drives have been in use for at least 50years, are durable and offer high torque and power. However, they are expensive and mechanically inefficient, making them unsuitable for use as the main transmission in road transport such as cars and trucks.

A number of car manufacturers offer belt or chain continuously variable transmissions, which comprise metallic belts running on a pair of conical drums. While reliable and well-established, having been developed over the last 30 years, these transmission drives are relatively expensive and difficult to package due to the spacing of the two conical drums. The systems are also limited in power and torque capacity and can’t be scaled up for use in heavier vehicles such as trucks.

In contrast, the full-toroidal variable traction drive is mechanically efficient. A unique feature of traction drive is the way it maintains high levels of efficiency with scale. Unlike other forms of variable drive technology, it can be sized to work efficiently in the main drive transmissions of everything from lawn mowers to 40-tonne trucks.

Full-toroidal traction drive systems are already being used in a unique zero-turn ride-on lawn mower. Here, a novel steering system is linked to a pair of toroidal transmissions to allow the mower to turn through 360degrees within its own length. 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 (figures2a and 2b).

On a larger scale, main drive transmissions capable of handling 350 kW are currently under development for larger vehicles including trucks and buses.

The Torotrak engineering team is also developing very small variable traction drives for automotive engine auxiliaries such as air-conditioning compressors and superchargers. These drives could help the automotive industry reduce engine size without compromising performance and power.

Hybrid hopes

A further potential use of traction drive is in the kinetic energy recovery systems (KERS) of mechanical hybrid vehicles. Here, the traction drive is being developed to work in conjunction with lightweight, high-speed flywheels to store and recover braking energy in a range of vehicles from small cars to buses. It could provide a cheaper and more energy efficient alternative to the conventional hybrid-electric power train.

The hybrid system comprises a flywheel connected via the traction drive variator to the base powertrain.

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 vehicle’s 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. Efficiency losses take place at each energy state change.

As a result, the purely mechanical flywheel configuration has much greater efficiency than the electrical systems, which spells good news for manufacturers striving to cut CO2emissions from buses and urban commercial vehicles. The first buses and commercial vehicles to have been developed with conventional hybrid-electric powertrains with regenerative braking are heavy and costly.
In some cases, incorporating the powertrain can double the cost of the base vehicle.

However, a recent programme of work started with automotive technology developer Ricardo and coach and bus manufacturer Optare on a demonstrator bus as part of the Flybus project looks promising (figure 4). Mathematical modelling indicates that enough braking energy can be recovered to cut fuel use by more than 20% on a typical city route.

Making plans

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. Already, two-speed (ratio) gearboxes are being used to enhance electric vehicle performance – imagine what can be done with an infinite number of ratios.

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