Continuously variable transmissions (CVTs) have been used in mass-produced vehicles since the 1950s, but the technology has enjoyed a steady increase in popularity over the past 10 years. Paul Stevens reports on some of the latest developments and applications, including pressure-charging systems and hybrid power trains.
Continuously variable transmission (CVT) technology still accounts for only a small proportion of all transmissions installed in cars and commercial vehicles, but CVTs are steadily growing in popularity, largely due to the fuel savings that can be achieved. Already used for a number of cars and commercial vehicles, CVTs are now being investigated by manufacturers of diverse vehicle types, from trucks to micro-cars, as well as for indirect applications.
Rather than offering a set of fixed gear ratios, CVTs can change steplessly through an infinite range of ratios. One advantage of a CVT is that it enables the engine to run at a wider range of speeds for a given road speed; for example, the engine speed might be set to deliver peak efficiency or maximum power (which is usually higher than the speed at which peak efficiency is achieved).
CVT - and its close relative the infinitely variable transmission (IVT) - is not new to the industry: DAF introduced the first CVT-equipped vehicle in 1958; more recently, Nissan boasted about selling one million cars with CVT in 2007, while Audi (with some A4 models), BMW (in some Minis) and Ford are also major users.
But while the technology has been used in various cars - including a number of sports cars - it has potential applications beyond being simply a gearbox replacement. In the UK, a consortium of automotive companies has developed a prototype Flywheel Hybrid System for Premium Vehicles (FHSPV), which adds up to 60kW of recovered energy to the engine's output. The consortium, which is currently testing prototypes, anticipates that future models will reduce fuel consumption by 20 per cent.
Compared with conventional hybrid systems, flywheel hybrids reduce the number of energy conversions, thereby helping to improve the overall efficiency of the regenerative braking system. Rather than converting kinetic energy into electricity for storage in a battery, a small CVT, connected to the car's rear differential, transfers the energy directly into a compact, high-speed flywheel. When the driver reapplies the accelerator, the CVT smoothly transfers the energy back to the wheels.
This project is part-funded by the Technology Strategy Board, the UK's national innovation agency. Industrial partners include Jaguar Land Rover, Flybrid Systems, Ford, engineering consultancies Prodrive and Ricardo, and transmission experts Torotrak and Xtrac.
Based on a flywheel developed by Flybrid Systems, the FHSPV spins at speeds of up to 60000rpm so that it achieves a high energy density, which makes it smaller and easier to package. The CVT, which manages the flywheel's speed and the flow of kinetic energy, was built by motorsport firm Xtrac using Torotrak's traction drive technology. Ford Motor Company is examining the potential for secondary applications for flywheel-CVT systems.
Separate to this, a Volvo-led project to evaluate flywheel technology will incorporate Torotrak's CVT technology with the aim of boosting fuel economy (Fig. 1). The project, part-funded by the Swedish Energy Agency, will use Torotrak's variable drive technology with flywheel technology from Flybrid Systems. Other partners in the kinetic energy recovery system (Kers) project include SKF and Volvo Powertrain.[Page Break]
Relatively cheap technology
Torotrak says that this project will demonstrate how mechanical hybrids could reduce emissions and improve fuel economy more cost-effectively than electrical hybrid systems. Torotrak chief executive Dick Elsy comments: "We sense real momentum in the growing markets for efficiency-enhancing devices to reduce CO2 emissions. The industry needs cost-effective hybrid solutions: using a Torotrak variable drive transmission, with a mechanical flywheel, has demonstrated the capability for double-digit improvements in fuel economy."
Another way of using flywheels is seen in the UK Flybus project that is investigating alternatives to battery-hybrid buses (Fig. 2). The first prototype has now been built, based on an Optare Solo midibus with an Allison automatic transmission. The flywheel-hybrid unit attaches to an unused power take-off shaft, with Torotrak's traction drive managing the flow of energy in and out of Ricardo's high-speed carbon composite Kinergy flywheel.
As well as being suitable for use on buses, the system could be equally effective on commercial vehicles such as delivery vans and trucks operating stop-start schedules. With over 2.5 million medium and heavy commercial vehicles manufactured worldwide each year, the technology could make a significant contribution to reducing vehicle emissions globally.[Page Break]
But CVT/IVT technology could capture more market share if it starts to be adopted for more small cars - which tend to be produced in higher volumes. Tata Motors has used a Torotrak prototype transmission in the Tata Pixel concept car that was revealed at this year's Geneva Motor Show (Fig.3). This four-seat urban vehicle uses an IVT to boost fuel economy and manoeuvrability. Rob Oliver, Torotrak's director of product development, explains: "The Tata Pixel is an example of how IVT technology addresses the needs of small and city-cars. Features such as the IVT's zero-turn capability provide tremendous opportunity to deliver new technology solutions to urban drivers."
The rear-engined Tata Pixel uses Torotrak's traction-drive and epicyclic technology to create an integrated IVT. In normal driving, the transmission provides seamless ratio changes. During tight manoeuvring, or parking, the transmission's ability to control each rear wheel independently means that the wheels can be rotated in opposite directions. By linking the vehicle's steering system with the IVT's control mechanism, the car can pivot around its rear axle.
Automotive components supplier Bosch also believes that CVT technology will become more widely accepted: in April 2011 it opened a new factory in Vietnam to make CVT push belts. This is Bosch's second push belt plant, the first being located in the Netherlands. The new facility is scheduled to produce 1.6 million belts in its first year.
"With most of our customers for CVT technology located in Asia, we expect production volume in the new facility to rise to 2.3 million units by 2015 - to support, in particular, the Japanese and Chinese automotive markets," said Rolf Bulander, president of the company's gasoline systems division.
A Bosch CVT automatic transmission is used in a hybrid petrol-electric Honda Insight - which is described as "the world's greenest rally car" - though this car was banned from rallying because it was said to be "too fast". The rally Insight, which can achieve 100mpg when driving slowly, was developed by Oaktec Hybrids (Fig. 4).
With CVTs becoming more commonly encountered on high-volume vehicles as gearbox replacements, as well as finding applications in hybrid vehicles, motorsport and indirectly within vehicle power trains, it appears that this technology is finally becoming mainstream. It is difficult to predict whether CVTs will become as commonplace as, say, turbochargers or conventional automatic gearboxes, but the technology is certainly gathering momentum.