NEW DCTS AIM TO RESTORE APPEAL
Honda and Volkswagen have fired the opening salvos in a gearbox war which aims to restore the customer appeal of dual clutch transmissions. This move is especially timely in North America, where a new generation of super-smooth eight- and nine-speed planetary units from suppliers such as ZF have eroded the perceived advantages of DCTs in terms of sophisticated shifting. DCTs have also suffered from bad press and negative customer feedback, with drivers accustomed to silky-smooth torque converter automatics complaining that DCTs are unacceptably jerky in low-speed maneuvering and slow off the line when accelerating from rest.
Volkswagen, with less of a strict focus on the North American market, has chosen to stay with its established dual wet clutch formula and add a further three speeds to its DQ500 DCT to bring the total ratio count to ten — the first production passenger car transmission of any kind to feature so many ratios. Cleverly, the new DQ511 retains the same dimensions and practically the same weight as its seven-speed companion — essential if it is to fit into VW’s modular component matrix — and, remarkably, the extra ratios have been achieved without having to increase the number of gear pairs.
“Reversing the power flow within the gearbox was enabled without additional pairs of gearwheels by employing an entirely new shifting strategy and adding two shifting elements and a reversing wheel,” says VW’s briefing. “This enables the implementation of ten progressively stepped gears.”
Significantly, in view of Honda’s similar comments on DCT launch performance, VW has taken the opportunity to make the first three gears low (short) to further improve drive-off, and the “finely graduated” steps right up to ninth gear enable efficiency gains. Tenth is a long gear, significantly reducing rpm and thus CO2 emissions during cruise. VW also points to the possibility of gears for special functions, such as an ultra-low crawler ratio for off-road work, or a special super-overdrive.
Like Honda, VW has developed a special oil for this transmission, and special coatings on the gearwheels and low-friction bearings help further improve efficiency. As with all of VW’s latest DCTs, the DQ511 is equipped to permit engine-off coasting, fully decoupling the engine.
Honda brings the torque converter to DCTs
The six engineers authoring Honda’s technical paper on the new eight-speed DCT list sporty driving feel, top-level fuel economy and compatibility of installation as the principal priorities in the design of their new transmission. Among the positive points about current DCTs, they cite “the balance of environmental performance with the elements that make driving fun,” but acknowledge the problems of clutch-based systems in standing starts and low-speed jerkiness, a very sensitive point in the US.
The addition of a torque converter, says Honda, will “achieve a balance between product appeal for standing starts and a sporty gearshift feeling in a midsize vehicle.” The three-shaft design places the slim torque converter at the inner end, with the even and odd clutches alongside one another at the opposite (outer) side of the transmission at the end of the main and secondary shafts, respectively. Honda wanted to avoid the larger-diameter concentric clutches of most competitor transmissions, but found it necessary to devise a twin torsional damper to iron out engine speed fluctuations and the gear rattle they were found to cause.
The twin clutches could be kept small and low in inertia, explains Honda, thanks to new friction materials and a specially-developed DCT fluid giving a 30 percent improvement in static friction. The fluid benefits from new friction modifiers tuned for the characteristics of the wet clutches, and a sulfur EP additive helps to protect the gears. With a low molecular weight VI improver, the fluid’s viscosity has been kept low in the interest of reducing agitation resistance, with a kinematic viscosity of 5.5 mm2/s at 100°C. “This places the fluid at the top level for DCT-compatible fluids,” say the Honda engineers.
Gear selector operation is hydraulic, though Honda does not explain whether the torque converter lock-up clutch is released during gearshifts to bring the torque converter into play. Yet, the influence of the torque converter in improving launch performance is clear: the zero to 100 km/h acceleration performance is trimmed by 1.3 seconds and the time to peak acceleration — which translates into the off-the-line responsiveness that the customer feels – is reduced by a full second, compared with other companies’ DCTs.
Torque converters are often associated with a penalty in efficiency, but again Honda claims a 28 percent reduction in friction torque compared with the outgoing five-speed AT. This is shown in an 8 percent improvement in fuel efficiency in the US combined cycle.
In June 2013, we reported on a GM study for a torque converter-equipped seven-speed DCT, presented at the SAE of that year. The GM paper suggested that “extreme” final drive ratios could be made possible through the smoothing effect of the torque converter, and that fuel consumption could, in some cases, be better than that of a dual dry clutch unit. “The DCTC concept or use of a torque converter with shifting clutches internal to the transmission structure has the potential to be an enabler for DCT technology to achieve improved fuel economy and drive quality,” concluded the GM paper, though it is not known whether the company plans to pursue the idea further.