<<BACK TO HOME

Dual clutch transmissions are getting ready to enter the mass market, but the gearbox technology will have to keep pace with other developments in the drivetrain. By Tristan Honeywill

BorgWarner has been central to the introduction of dual clutch transmissions (DCTs). It worked with Volkswagen on the first versions launched in 2003. VW now plans to introduce its own seven-speed DCT for the mass market. The experts say the technology will achieve significant market penetration in the next few years. The experts aren’t always right.

BorgWarner has so far announced five programmes for its Dualtronic DCT modules, with Volkswagen Group, Getrag, SAIC of China and one unnamed European OEM. If these deals hit capacity, the firm will be building 1.5 million transmissions a year.

More programmes are expected to start later this year. Development and the market move fast these days, but the stakes, and expectations, get proportionally higher. A few years ago, continuously variable transmissions (CVTs) caused similar excitement, but they failed to win over buyers.

The quality of the first DCTs produced in series volumes will be critical to the public’s perception of the technology. Customers will expect reliable shift quality and visible improvements in fuel efficiency.

“Producing prototypes is not difficult,” says Frank Guenter, Dualtronic systems general manager. “Providing consistent precision and performance is far more challenging in mass production.”

As with conventional step automatic transmissions, clean assembly of the hydraulic control systems is key to this. The friction system is trickier, however. Failure to get uniform torque and stable friction characteristics over a system’s lifetime will lead to vibrations, juddering clutches and uncomfortable launches. If the shift quality suffers, so will demand for DCTs.

“For shift quality in a DCT, you need low hysteresis on the shift actuation to ensure all the functions in the software transfer into the hydraulics,” says Guenter.

For BorgWarner’s modules to be cost-competitive, it needs big volumes through having applications on a number of platforms.

“This means tighter packaging constraints,” says Guenter. “The modules must be as small as possible but still provide the full performance range. This creates areas with tight tolerances, which impact on the assembly procedures.”

The welding of sheet metal parts is particularly important, as are the tolerances in the friction material. Poor thermal performance would result in larger contact areas or more friction plates.

This is why BorgWarner has so far focused on mid-sized or small vehicles with gasoline engines. The clutch system can be sized for large-displacement diesels with higher low-end torque, but, besides some fairly simple tribological issues, packaging them for a number of platforms is not easy. The firm is working on dry clutch DCTs with 600Nm torque capacity for luxury diesel vehicles.

So far the systems in production are hydraulically actuated with an integrated torsion vibration pad. BorgWarner doesn’t make the complete transmission, nor does it produce the electronics.

That should simplify things, but DCTs’ introduction coincides with several new directions for drivetrains.

Hybridisation means that just as the company starts to accrue experience in manufacturing its modules, it will have to introduce electro-mechanical and high-pressure electro-hydraulic systems. Guenter will not name customers, but says such systems will be in the market in 2009 or 2010.

Integrating ESC into transmission control strategies is becoming a factor too. BorgWarner is already working on one such project, using an intelligent torque transfer system it has fitted for Porsche.

Compounding this, most next-generation DCTs will have seven speeds, to compete with conventional automatics, which are getting more efficient – two systems on the market now have eight speeds.

Some DCTs might also go to eight speeds, but this is likely to be the limit, says Guenter. A typical DCT has one actuator for two synchronisers. Add more gears and the gearbox gets heavier and would be shifting constantly, compromising fuel efficiency.

Europe will be the main market for DCTs. Japan is still “CVT country”, says Guenter. “In North America in the past we expected only niche applications. Fuel prices have changed that.

We’ll see mass volume applications there too soon.”
There is growing interest from OEMs in China and India. This could be the turning point for the technology. The manufacturing will require a lot of automation, but BorgWarner would be crazy not to do more assembly by hand there.

If the modules can be built and perform reliably on SAIC cars in China, they ought to thrive in Europe.