Inside Audi's premium DCT
Having pioneered mainstream dual clutch transmission applications in 2003 with the six-speed wet-clutch DQ250 on the Audi TT and A3 as well as the VW Golf, engineering planners in the Volkswagen group were quick to realize the significant potential of the system for other sizes and types of vehicle. For premium producer Audi, however, this posed a problem: configured only for transverse installation, the DQ250 could only be used on the brand's smaller TT and A3 models; all of its larger, more luxurious and higher-margin models, which employ longitudinal engine architectures, would effectively be excluded from dual clutch applications.
That is why the decision was taken, relatively early on in Audi's DCT experience, to extend the transmission range with a dual clutch gearbox oriented for the inline configuration and, frequently, the quattro All-wheel Drive (AWD) system, of its larger models. Mindful of the rapid proliferation of new model lines that were being developed in Audi's bid to become the world's most successful premium car manufacturer, the new transmission could look forward to substantial production volumes within the brand and would therefore be not just a worthwhile investment but also a very useful advantage in a marketplace concerned with CO2 and fuel consumption as much as performance and smoothness.
Shortly after the new seven-speed unit went into production in the third quarter of 2008, its principal architect, Mario Schenker, told an engineering conference that the goals of the team were excellent shift dynamics, comfortable and smooth start-off and shifting, and low fuel consumption. Additional requirements, said Schenker, now head of dual clutch transmission design at Audi, were to fit in with the VW group's system of modular components and architectures, including quattro All-wheel Drive (AWD), and to be compatible with future shift-by-wire and stop-start functionality requirements,
Packaging
The longitudinal placement of the transmission and the requirement to incorporate the center differential and power take-off for four wheel drive dictated a fundamentally different internal layout to that used for earlier transverse gearboxes. Equipped with seven forward speeds, the transmission is a two-shaft design: the twin concentric clutches are at the front, just behind the front axle line; the larger ten-plate outer clutch serves the odd-numbered gears via the solid inner shaft. The inner clutch, with 12 plates, drives the even-numbered gears and reverse via a short hollow shaft, with the self-locking center differential at the rear of the casing dividing the output torque between front and rear axles. From this differential a driveshaft runs obliquely forward to the front differential, mounted just below and ahead of the clutches, while drive to the rear axle is taken conventionally from the tail of the casing.
The mechatronic block, with its ten solenoid valves is positioned low down at the front for easy access. The whole assembly, says Schenker, is designed to keep packaging width to a minimum, thus allowing for a more compact transmission tunnel within the vehicle. A center-to-center distance between the shafts of 89 mm allows a wide 8:1 spread of gear ratios, enabling both a low (short) first gear and a high (tall) seventh ratio for low cruising consumption and CO2 emissions.
Particular concern surrounded the lubrication of the transmission, explained Schenker. "The dual clutch, for instance, requires a lubricant that maintains consistently good friction coefficient ratios at the contact surfaces of the plates throughout the transmission's service life. The mechatronics and oil pump need a fluid with a viscosity that is as low as possible, even at low temperatures. And the gearwheels, synchromeshes and bearings call for a lubricant that guarantees good lubrication even at high temperatures and when subject to high compression forces. It must not foam, either."
Audi's solution was to avoid uneasy compromises and to create two completely separate oil circuits. The mechatronics and hydraulics use 7.5 liters ( 1.9 gallons) of synthetic ATF, while the gear systems are protected by 4.3 liters of hypoid oil (1.1 gallons). Yet the circuitry is more complex than it sounds: while the transmission oil chamber for the ATF is confined to the clutches, the lower mechatronic block and the oil pump, the hypoid oil must circulate around all the gearsets as well as the center and front differentials.
"This division ensures that any deposits of dirt in the gearset chamber and the abrasion debris produced during operation cannot enter the mechatronics' sensitive hydraulic circuit," Schenker told conference delegates. "This significantly reduces the likelihood of particles penetrating the hydraulic system, one of the principal causes of failure in automatic transmissions."
A further benefit of running the gearsets in hypoid oil rather than ATF is that the gear wheel thicknesses could be reduced by around 2mm per pair, reducing not only overall length (by 16mm) but also weight, by 2kg.
The cooling of the clutches was best accomplished by a large volume of oil rather than a high-pressure supply, asserted Schenker. "Especially when pulling away under high loads, very high quantities of oil are required which cannot be supplied by the oil pump alone. When starting off, an ejector pump cuts in which provides extra cooling oil at low pressure. This enables the quantity of oil for cooling the clutches to be approximately doubled if required, without the need for additional pump capacity. This concept has already proven itself at Audi in the multitronic (CVT) transmission."
Application
The outcome of Audi's program is the seven-speed S tronic, capable of handling engine speeds of up to 8,500 rpm, and input torques of up to 600 Nm and power of 330kW. Its weight, including lubricants and two-mass flywheel, is 141.5 kg.
The new transmission launched commercially in the Q5 medium SUV in the summer of 2008 and is now also available in versions of the A4 sedan, and A5 and S5 sports coupé and convertibles. The next-generation A6 and its likely Sportback derivative will share the same architecture and thus the same transmission options. Further developments to the transmission itself are likely to include the provision of stop-start functionality.
Story Filed: 5/8/2009
By Tony Lewin, managing editor DCTfacts.com