Continuously variable transmissions have been used in niche vehicle and machinery applications for decades, but have never become as popular as their benefits might suggest. Paul Stevens reports
For a given prime mover, typically an electric motor or internal combustion engine, it is usually necessary to use a gearbox or other means of converting the prime mover’s output to the required speed and torque. In applications such as conveyors, it is likely that a fixed-speed gearbox will be employed, possibly with a variable-frequency drive to enable the conveyor speed to be adjusted. Automotive applications, on the other hand, tend to use either a manually operated gearbox or an automatic gearbox with a torque converter. However, there is an alternative that offers advantages in both types of application, namely the continuously variable transmission (CVT), which is available in various different formats.
Compact mechanical speed variators can be used in machinery and, when properly specified, operated and maintained, offer a long, trouble-free life. Likewise, CVT units have been widely used in automotive applications for decades and, as we will see later, may soon also find their way into production vehicles as part of kinetic energy recovery systems that help to cut fuel consumption.
Bonfiglioli of Italy is an established manufacturer of mechanical speed variators for use with electric motors. The latest units feature an improved speed setting mechanism that helps to extend the lifetime of the core components, as well as ensuring that operating temperatures are lower and the output is smoother. Typical applications for this type of product include conveyors, food processing machinery, pharmaceutical processing machinery and machine tools. Mechanical speed variators can be attractive to users who want to avoid the use of electrical speed controls, and who appreciate the energy-efficiency benefits compared with using, say, throttles to control the flow through pumps and fans.
For automotive applications, DAF was the first manufacturer to make a real success of CVT technology in the late 1950s, with its small passenger car that utilised the company’s Variomatic CVT. This transmission was based on a rubber belt running between variable diameter pulleys. CVTs with more robust steel belts have subsequently been used in vehicles such as the Ford Fiesta and Fiat Uno. Other automotive manufacturers that have launched models with a CVT of one type or another include Subaru, Nissan, Honda, Audi and BMW.
Automotive manufacturers in the past have mainly used CVTs as a more compact, lightweight alternative to conventional automatic transmissions for smaller cars. In theory, the CVT also contains a smaller number of parts and therefore offers the potential to be more reliable and have lower maintenance costs. In reality, however, conventional automotive transmissions are now so highly developed that their reliability is generally excellent.
A further advantage of the CVT is that it is smoother and therefore improves comfort levels for the driver and passengers. But the smooth acceleration – with little or no change in engine note - has led some drivers to complain that CVT cars are slow. In fact the acceleration is still brisk, but the usual sensory indicators (noise and vibration) are more subdued, giving a false sense of poor performance. Another misconception among drivers is that CVTs are only suitable for small, low-powered cars. While this may have been the way CVTs were introduced to the market in vehicles such as the early DAFs, today there are CVTs available for trucks, buses and off-highway vehicles. Electronically controlled CVTs can also be used to give paddle- or lever-operated gear changing between preset ‘gear ratios’ in sportscars, as was the case with the Steptronic transmission in the MG F.
Beyond the mainstream automotive markets, CVTs are also popular for small vehicles such as motorscooters, snowmobiles and off-road vehicles such as the John Deere Gator. Large vehicles, including agricultural machinery and construction equipment, can also benefit. ZF, for example, has recently launched a CVT to handle input powers of up to 500 HP (506 PS or 373kW). This unit, based on the proven ZF-Eccom transmission system, is aimed at delivering improved performance for articulated agricultural vehicles.
Today there is renewed interest in CVTs, driven by concerns over fuel consumption and emissions. One company operating in this field, Torotrak, claims that the technology offers a 10–20 per cent improvement in fuel economy compared with conventional automatic transmissions in automotive applications. Ford Motor Company says its 2008 Lincoln Mercury Mariner Hybrid reduces fuel consumption by 14 per cent compared with the 2007 model (Fig. 3). The Mariner Hybrid features a 2.3-litre, four-cylinder, 16-valve Atkinson cycle petrol engine that produces 133 horsepower (135 PS or 99kW) at 6000 rpm, assisted by a permanent-magnet electric motor that produces 70kW at 5000 rpm. Transmission is via an electronically controlled CVT and customers have a choice of front-wheel drive or four-wheel drive models.
Another area of interest currently lies in the Formula 1 (F1) race car industry. Perhaps unusually, the CVT-related technologies being developed by F1 teams have the potential to be rolled out into production vehicles relatively quickly. Formula 1’s governing body, FIA (Federation International de l’Automobile), has decreed that from the start of the 2009 season teams will be permitted to use kinetic energy recovery systems (KERS) for the first time. And one way to implement a KERS system is to connect a flywheel-based energy storage unit via a CVT, as is the case with the system being developed by transmission specialist Xtrac. The Xtrac system utilises a Torotrak full-toroidal traction-drive infinitely variable transmission (IVT) technology, which is a variation on the CVT concept.
Xtrac says the flywheel KERS currently being developed for F1 could be applied to road vehicles in a number of ways. By providing an additional boost of power, the KERS technology is particularly relevant to the trend to fit cars with smaller engines in pursuit of better fuel efficiency and lower emissions. Applying a flywheel KERS could overcome the problems of a loss of engine torque and driveability that would otherwise result from engine downsizing. It is also possible that a mechanical KERS could be used to power vehicle auxiliary systems or extend the range of electric hybrid vehicles.
Xtrac technical director Adrian Moore comments: “All are potential applications of the technology, subject to the size of the flywheel, compactness of the system and vehicle packaging requirements. These are all resolvable technical issues. The intent of the KERS technology in F1 is to consider energy recovery, storage and discharge, and to demonstrate that technology in a novel and effective way.”
Torotrak and Xtrac believe that the combination of a CVT and flywheel is potentially significantly more compact, efficient, lighter and environmentally-friendly than the traditional alternative of electrical-battery systems – which some F1 teams are developing and which are commonly used in production hybrid vehicles.
Depending on the application, a KERS can be more efficient in recovering energy during braking and should be substantially cheaper to produce than electric hybrid systems. The flywheel is also better at ‘deep cycle’ charging and recharging, wherein all the energy is either released or recovered from the unit, with no loss in performance over the life of the vehicle – as can be the case with electric systems in which batteries can lose their ability to charge and discharge fully.
Xtrac has played a key role in designing, developing and integrating a mechanical KERS system for F1 with partners Flybrid and Torotrak. Xtrac’s role is to provide the toroidal CVT between the flywheel and the vehicle powertrain. The three F1 project collaborators all believe that the mechanics of this system can be transferred to road vehicles and other technology areas.
Moore states: “There are some peculiarities that are distinct to F1 due to the regulations, such as the control system, integration of the ECU and the operating parameters, but as the original Torotrak CVT concept was intended for road vehicle transmissions, the flywheel KERS opportunity readily allows for the use of toroidal CVTs in cars.
“Historically, there are many instances where a new technology initially looks challenging to install in certain applications only to find a few years later that it is smaller, lighter and performs better than anyone ever envisaged. F1 will certainly help advance this process."
The energy recovery rate and storage requirements of a flywheel for a road car could be considerably less than that required in F1, where the energy that could be recovered from 5 ‘g’ braking is significant. As the flywheel is required to be charged, some preliminary motion may be required, though road cars could store energy directly from the engine, which is not currently permissible under F1 regulations. Road driving conditions also vary from the stop-start conditions experienced in a town environment to constant-speed cruising on motorways.
Complementing its work in F1, Xtrac is also an active partner in a new project to develop a flywheel hybrid system for premium cars. The company is part of a consortium supported by the UK government's Technology Strategy Board, which recently announced funding of £23million (around €29million) for 16 innovative low-carbon vehicle development projects. The flywheel hybrid project concerns the design and development of a KERS for use in a premium segment passenger car as an alternative to other hybrid systems, and to prove its effectiveness and viability for production. Jaguar is leading the project, with other members of the consortium being Prodrive, Xtrac, Torotrak, Flybrid Systems, Ford and Ricardo.
Clearly there is plenty of interest in CVT technologies within the broad-based automotive and motorsport industry, but there are other sectors that could also benefit. For example, wind turbines might be a potential application, with some studies reportedly predicting that a CVT could reduce the cost of generating electricity by up to 11 per cent. In this application, the CVT eliminates the generator speed variation that is caused by variable wind speeds and, hence, turbine blades. The CVT avoids the need for complex and inefficient electrical systems that are required in conventional wind turbines. In April 2008, GCI (Gear Chain Industrial) of the Netherlands announced that it was starting development work on a high-power chain for wind turbine CVT applications.
It seems likely that the use of CVTs will become increasingly widespread in the near future, especially in applications where even a small improvement in energy efficiency is sufficient to offset any additional costs compared with traditional technologies. They will not be used as a universal replacement for fixed-ratio gearboxes, but they may, at last, cease to be viewed as a technological curiosity.
Torotrak’s toroidal variable drive technology
Torotrak develops CVT technology for a wide variety of applications, including passenger vehicles, motorsports, tracks, buses, off-highway and auxiliary drive markets. It is Torotrak’s technology that Xtrac has licensed for use in a Formula 1 kinetic energy recovery system.
With Torotrak’s technology, each CVT unit is based around a variator. Within the variator there are an input disc and an opposing output disc, with the cavity between the opposing disc surfaces being toroidal. Two or three rollers are located inside the toroidal cavity and positioned so that the outer edge of each roller is in contact with the toroidal surfaces of the input disc and output disc.
As the input disc rotates, power is transferred via the rollers to the output disc, which rotates in the opposite direction to the input disc. The angle of the roller determines the ratio of the variator; changing the angle of the roller therefore results in a change of ratio. With the roller at a small radius (near the centre) on the input disc and at a large radius (near the edge) on the output disc the variator produces a low ratio. Moving the roller across the discs to a large radius at the input disc and a low radius at the output disc produces a high ratio. The movement between the ratios is smooth and continuous, and any ratio between the maximum and minimum can be obtained.
Power is transferred through the contacting surfaces of the discs and rollers via a microscopic film of specially developed long-molecule traction fluid. This fluid separates the rolling surfaces of the discs and rollers at their contact points, avoid metal-to-metal contact.
The input and output discs are clamped together within each variator unit. The traction fluid in the contact points between the discs and rollers become highly viscous under this clamping pressure, increasing its ‘stickiness’ and creating an efficient mechanism for transferring power between the discs and rollers.