RESEARCH ANALYSIS: Review of dual-clutch transmissions

By Matthew Beecham | 25 September 2008

Europe may have long resisted the automatic gearbox, but it remains a fact that using a manual in stop-start city conditions is no longer a sensible option. Dual-clutch transmissions offer an increasingly popular way forward for the European driver, writes Matthew Beecham.

Gearing-up for new transmissions

New environmental and fuel efficiency legislation combined with major advances in electronics and manufacturing techniques have paved the way for a flood of new automated transmission technologies, including high-performance AMTs, dual-clutch transmissions (DCTs), CVTs for smaller vehicles and the highly innovative new technologies such as Torotrak and the Antonov Automatic Drive. The most likely winner (that will replace traditional automatics and boost market penetration of automated transmissions generally) will be the dual-clutch, pre-select DCT concept, for its combination of refinement, efficiency, low cost and driving satisfaction.

The direct shift gearbox, powershift gearbox, twin-clutch gearbox are just a few of the terms used to describe the dual-clutch gearbox. The gearbox concept combines the advantages of a conventional manual shift with the qualities of a modern automatic transmission. It originated in motor sport and is characterised by having two part gearbox units, which operate the odd and even gears, respectively. It means that when changing gear, the engine torque is transferred continuously from one gear to the other. The result is gentle, jerk-free gear changes with the same relaxed driving style found in an automatic combined with the efficiency of a manual transmission.

For some time, BorgWarner has been working on dual-clutch systems. The VW Group pioneered this technology and continues to push dual-clutch technology in Europe. VW co-developed the six-speed twin-clutch gearboxes (branded DSG) with BorgWarner, making use of the supplier's DualTronic wet-clutch and control-system technology. In addition to its performance and fuel economy benefits, one of the interesting things about this dual-clutch is the fact that it can be "tuned" via software thereby changing the personality from sporty to limo-comfortable.

Earlier this year, Volkswagen updated its DSG, adding an extra speed and swapping units immersed in oil for a new pair of dry clutches. VW said the switch to dry clutches saves weight and improves the efficiency of the system while making the new gearbox more compact. Available in addition to, rather than replacing, the existing six-speed unit, the new seven-speeder is designed to operate under moderate power and torque loadings. It will be launched with a new 122ps 1.4-litre petrol TSI engine and will subsequently be offered with the 1.9-litre Tdi turbodiesel with particulate filter. In the Golf hatchback, compared with the six-speed manual transmission, the new seven-speed DSG auto brings a 10g/km CO2 saving (down from 149g/km to 139) and a fuel economy improvement of over 3mpg (combined 44.8 mpg for manual and 47.9 for seven-speed DSG). Compared with the six-speed DSG, the new unit's lower gears are more closely spaced, giving improved in-gear acceleration to aid overtaking while the higher gears are lengthened to reduce load on the engine and maximise economy. As with the six-speeder, of which over a million have been produced since launch (it is also offered by other VW group brands Audi, Skoda and Seat), the new seven-speed system features a hill-hold function to aid starts when the vehicle is on an incline.

BorgWarner says it has a 'unique new dual-clutch transmission for developing markets and small cars' in the final development stages. Working in conjunction with several Asian automakers, BorgWarner says it will be providing patent-pending dual-clutch transmission architecture that delivers fuel efficiency in an affordable package ideal for the growing small car demand in developing markets. "Drivers in developing markets like China and India are looking for the ease of using an automatic transmission in an affordable vehicle," said Dr. Bernd Matthes, president and general manager, BorgWarner Transmission Systems. "Up until now, the complexity and cost of automatic transmissions made it impractical to package this option in smaller cars."

BorgWarner says it is significantly reducing the complexity of the transmission hardware. "The competition in this fast-growing and high-potential market segment is fierce and extremely motivating," Matthes added. "Together with our OEM partners, we are working to craft affordable, differentiating systems that distinguish our customers' products from their competition and keep BorgWarner at the forefront of transmission technology." Working with these partners, BorgWarner is currently testing and refining its first phase transmission hardware and controls software. BorgWarner expects production implementation of its new compact dual-clutch transmission design within two to three years. The patent-pending "power split" design of the new transmission uses a large number of common and simplified parts and modules which enhance the affordability of the transmission, the ease of packaging in small vehicles and the manufacturability of the transmission in emerging markets. The small vehicle segment is expected to grow 30% over the next five years from about 19 million to almost 25 million cars.

Germany's LuK is also working on its core components, the dual clutch, actuation system and relevant software. "The main advantage of a dual clutch transmission," said Norbert Indlekofer, president and CEO of LuK Group and president of Schaeffler Group Automotive's Transmission Systems division, "is that changing gear is possible without torque interruption. This creates a very dynamic yet economical driving experience. The LuK double clutch has been delivered to Volkswagen in series production since January 2008."

LuK's double clutch is made up of two clutches located on two drive shafts. One clutch operates the uneven gears - first, third, fifth and seventh - whilst the second controls the engagement and disengagement of the even gears - second, fourth and sixth - together with reverse. To take an acceleration manoeuvre in second gear as an example, the third gear is already pre-selected in the other sub-gearbox so that changing gear now becomes a particularly speedy, sporty manoeuvre that does not involve any loss of traction. The gear change is triggered electronically and implemented by means of hydraulic actuators. Torque superimpositions on gear changing and the fact that the individual clutches open and close within a fraction of a second mean that the driver is hardly aware of the gear change process.

Last June, Ford's European unit became the latest to adopt a twin-clutch automatic transmission which it is branding PowerShift. The new six-speed box is available initially for the Focus and C-Max ranges coupled with the 2.0-litre TDCi diesel engine in 110 and 136ps tunes. The system uses a standard automatic gear selector in the centre console with the usual P, R, N and D settings and a manual-shift option. Volkswagen pioneered six-speed twin-clutch gearboxes (branded DSG) and group stablemate Audi is now launching new seven-speed versions for both transverse and longitudinal mounting. Mitsubishi is also adopting the technology and will soon launch the first SUV with this type of transmission.

Chrysler's new dual-clutch transmission, developed with Getrag, made its debut in international markets outside North America last spring in the 2009 Dodge Journey and Avenger and 2009 Chrysler Sebring, delivering claimed fuel economy improvements and a CO2 emissions reduction of 6%. A commitment to dual-clutch transmission technology is part of the company's powertrain offensive, announced last year. This technology improves fuel economy and CO2 emissions by reducing parasitic losses by eliminating the torque converter and using synchronisers instead of shift clutches. The dual-clutch transmission will be mated to a [bought-in Volkswagen] two-litre turbo diesel engine. VW offers this engine its own models with a similar dual-clutch gearbox it calls DSG for direct shift gearbox.

Prospects for DCT shifting up a gear

Manufacturers are predicting steady growth for DCT over the next few years. BorgWarner says it expects production of its dual-clutch transmission modules to increase 500% over the next six years, a key driver of the company's growth. At full-launch of announced programmes under contract in 2012-2013, the company will be providing its DualTronic technology to an expected 2.3 million dual-clutch transmissions annually. That's up from 450,000 dual-clutch transmissions produced this year. BorgWarner reckons that its "dramatic growth" would be driven by Bits position in dual-clutch transmission technology with awarded business from transmission and automakers around the world. These include programs with VW, Audi, Bugatti, SAIC, and a Japanese automaker.

"With legislation driving issues like fuel economy and emissions reduction, and consumers looking for performance and drivability enhancement, the demand for dual-clutch transmissions is increasing," said BorgWarner transmission systems president and general manager Bernd Matthes.

Ralph Bast, vice president, passenger cars, Powertrain Division, ZF Sachs AG, told us: "In Europe, we expect a rising demand for the DCT and a total market volume of 1.4 to 2 million units per year. In North America, we also expect a wider penetration but only up to a maximum volume of approximately 1 million units per year." As far as the Asian markets are concerned, Bast added: "Especially in Japan the DCT concept will probably be rather limited to the sports cars segment due to the wide acceptance of CVT transmissions in the Japanese market. Our expectations forecast a market volume of up to 100,000 units per year until 2015. In China, we also expect a rising demand but with a certain delay."

Matthew Beecham


See also: Global market review of OE gearbox clutches - forecasts to 2014 (download)