Transmission Design for Lower CO2
Led by Europe, where legislation threatens to impose heavy financial penalties on
carmakers with high emissions, concern about CO2 is driving a
thorough review of every aspect of vehicle design.
Slippery aerodynamics have been a feature of mainstream cars for a decade or more: engines, too, have been under the microscope for some years. As a result, major economy gains are now starting to show through in vehicles that people can actually drive and buy. There's still more to come, however, and now it's the turn of other areas of vehicle design to make their contribution in the search for further fuel savings.
As early as summer 2006 Ford of Europe executive vice president Lewis Booth cited weight reduction and advanced transmissions as potential economy drivers; a year later, Ford announced its Powershift series of dual clutch transmissions offering CO2 reductions of over 10 percent.
Dual clutch transmissions (DCT) are able to save fuel as they do not have the
energy-sapping torque converter of a conventional automatic and are able to
shift ratios on an automated program designed to keep the engine in its most
efficient rev range. The same unit can also provide a very sporty drive, should
the driver so desire, with gear changes taking place within a fraction of a
second.
Volkswagen, pioneer of the DCT in the volume market in 2003, has long since passed the million sales mark with its six-speed DCT-equipped mid-range models in the Golf class, despite an option price for the system in excess of $2300. More recently, it has launched a simpler, lower-cost seven-speed transmission with dry rather than wet clutches, aimed at a broader audience of mainstream drivers with more modest, lower-torque vehicles.
But while driving fun, rather than CO2 minimization, is the most likely motivation for the bulk of those buying decisions, it is the CO2-saving potential of the system that is the true focus of the carmakers' interest.
Automated manual transmissions (AMTs) are essentially robotized versions of standard five-or six-speed gearboxes. For a variety of reasons, in particular some unsatisfactory early applications, they have been less popular with consumers. Among the complaints have been slow and jerky shifts and unresponsive performance.
A cheaper alternative to DCTs, AMTs again offer programmed economy modes giving good CO2 emissions in the statutory tests and also give perceived consumer benefits of clutchless operation and by-wire control allowing the elimination of the conventional gear lever in the cabin. Nevertheless, the consensus appears to be that this type of transmission will disappear on all but the smallest cars once DCTs grow in volume and come down in price.
Getrag, which supplies Ford and Volvo, predicts it will be supplying half a million DCTs a year by 2010 and two million by 2014. Analysts CSM Worldwide forecast that the market size will be two million units per year by 2013.
However, those assumptions do not allow for other breakthrough technologies, such as Torotrak, Antonov or Zeroshift. While industry heavyweights may be inclined to view these with some suspicion as risky and lacking in proven credibility, the time is certainly ripe for a step-change in transmission design.
The Zeroshift development, in particular, appears to offer the advantages of DCT but at substantially lower cost and with minimal manufacturing change. No OEM deals have yet been forthcoming, however. Its secret is low shift effort and precise matching of gear engagement, eliminating the need for synchromesh.
Continuously variable transmissions (CVTs) remain popular in Japan, largely on the strength of their smoothness of operation and their very good CO2 performance. European buyers remain to be convinced, but some Japanese manufacturers are expected to fit this system to their European models as an interim measure pending the development of more cost-effective DCTs.
Speakers and delegates at recent transmissions conferences have tended to echo the consensus that CO2 has become the dominant priority in transmission design; most are agreed that the best solution is indeed the DCT, but with the central proviso – critical in the volume segments -- that the high costs of current systems are an important barrier to consumer acceptance.
In that context several encouraging developments are already under way, such as Ricardo's simplified eDCT control system and BorgWarner's Micro DCT project aimed at low-cost markets.