New-Generation Priorities
With the existing generation of its compact front-drive cars coming up for replacement from 2012, market forces encouraged Mercedes to take a very different approach to the vehicles’ design, engineering and manufacture. The A- and B-Class ranges had been very advanced in their configuration, with a sophisticated double-floor construction sandwiching the near-horizontal engine and transmission below the floor to allow exceptional interior space within a compact format. This unusual arrangement meant that the driveshafts to the front wheels were actually positioned ahead of the engine/gearbox unit.
The downside of this unique layout came in the form of high manufacturing costs, which hampered the profitability of the models and restricted the number of variants that could be made. In addition, commonality with other Mercedes models was limited, reducing the opportunity for economies of scale in parts production.
For the new generation, Mercedes elected to adopt a more conventional approach, with a standard single floor, a more upright engine and lower all-up weight. An associated advantage is the ability to spin off a wide variety of variants from this New Generation Compact Car (NGCC) platform. The B-Class, revealed in September 2011, is the first of five different models on an architecture which will later include the smaller A-Class, now much more compact and sporty, a compact SUV and two further derivatives yet to be announced.
The wholesale repackaging of the platform, especially the drivetrain, allowed Mercedes a free approach to engines and transmissions. This, said Gerhard Henning, director of transmission and driveline, passenger car development at Daimler AG in Stuttgart, allowed the company to develop entirely new gearboxes. As he explained in a paper delivered at the 2011 CTI Innovative Transmissions Symposium in Berlin, carrying out the work in-house provided maximum benefit all around:
“The decision to develop and produce both gearboxes in-house opened up new possibilities for an advanced gearbox technology, something previously not available on the market.”
Furthermore, continued Henning, synergy effects could be achieved in production as individual assemblies (including the shafts, idler gears, sliding sleeves, differential gear system and production of housing parts for both manual and DCT gearboxes) could be produced within the coordinated production system of Daimler AG.
Efficiency is Paramount
Among the design priorities listed by Henning for the new transmission family are compact packaging and low weight, modular componentry and the realization of an efficient overall concept in order to fulfill current and future CO2 emission requirements.
The latter condition was a key one, necessitating the inclusion of intelligent stop-start and, crucially, ruling out any further evolution of the existing belt CVT on the grounds of efficiency. The decision was made to go for an all-new DCT, with some components and processes shared with the related six-speed manual, also freshly developed. In addition to the efficiency advantages offered by the dual clutch system, Henning told the Symposium that DCT was also selected as a transmission that offered a sportier and more emotional driving experience – in contrast to the previous CVT, even though that had enjoyed an unusually high take-up in the model ranges on which it was offered.
The new vehicle platform dictated a compact axial length for the new transmissions of 360 mm. This necessitated a space-saving three-shaft design and, in the case of the dual clutch 7G-DCT, an innovative gearset design which, says Henning, is in the process of being patented. Common to both is a reverse gear mechanism that avoids having to have its own shaft, acting on first gear on the SG6-310 manual and, on the DCT, through a combination of gears to reverse the direction of rotation. This, says Mercedes, gives the advantage of a free choice of ratios – often, in conventional designs, reverse has to be set higher than ideal. Multiple use of gear pairs further aids compactness and low weight, resulting in an installed length of 357 mm and a dry mass of 46 kg.
Two overall ratios are offered at market launch: one with a ratio spread of 6.17 for gasoline engines and 6,66 for diesels. Maximum torque capacity is 310 Nm and all higher-torque variants of the NGCC range will only be offered with the DCT option.
Dual Clutch Option
The decision to go against the small car trend and equip the 7G-DCT with a wet clutch system was a fundamental one and, says Henning, was based on two important requirements – those for thermal robustness and low rotational inertia.
Henning’s team felt that achieving the target torque capacity of 350 Nm with a dry clutch system would result in a larger and heavier package with higher moments of inertia, pointing toward inferior shift quality. An oil-cooled multi-plate clutch design would, on the other hand, allow greater torque capacity and a subsequent upgrading to 450 Nm for future and more powerful all-wheel drive derivatives. Specifically, said Henning, the wet clutch system is10 kg lighter and has 50 percent less inertia than a dry counterpart would have been.
Perhaps even more importantly, Henning felt that a wet clutch arrangement was essential in order to ensure thermal robustness in “critical situations” such as when towing a trailer, giving an all-up train weight of over 3 tonnes, and when stopping or moving off on steep inclines. The likelihood of SUV-type derivatives, currently in the planning stage, was further reason to use a wet clutch to meet these particularly high demands.
Once again, gear pairs are combined so as to reduce complexity; gears five and seven, as well as four and six, each need only a single drive gear. Two gearset variants are offered, with the small gasoline models geared for a ratio spread of 7.16 and the larger diesel and gasoline versions giving a spread of 7.99.
Efficiency is improved by reducing churning losses in the gearbox via a baffle in the differential area. This also cuts down on foaming and, says Henning, leads to improved acoustics and a higher gearshift quality.
Hydraulic Actuators
While innovative hydraulic shift actuators act on the sliding sleeves for each individual gear, another consideration during the development of the 7G-DCT was to minimize the number of electrical interfaces. Fully integrated are five position sensors, three rpm sensors and two pressure and temperature sensors. Also integrated into the control unit are connections for the electric oil pump, nine solenoid valves and the lift solenoid.
The electric oil pump is an auxiliary device. It not only maintains pressure in the gearbox during engine stop-start interludes, but also boosts the main mechanical pump when extra cooling oil flow to the clutches is required at low speeds. This feature enables a reduction in size of the main pump, thus improving overall efficiency. The auxiliary pump also serves as an independent energy source for the park-by-wire system to allow the lock to be engaged and released even when the engine is off. Again, this arrangement avoids the need for a separate actuator.
“We’ve come up with something very clever,” comments Henning.
Results
Model for model, the new DCT-equipped Mercedes B-Class has been measured on the European NEDC drive cycle as being 21 percent more economical than its predecessor. Of this 36 g/km reduction in CO2, some 16 g are due to the transmission, says Henning, with the DCT element accounting for a 9 percent fuel saving and other transmission features, such as the stop-start, contributing the remainder. “The new DCT transmission is thus at the level of the manual gearbox for all the NGCC applications,” he told the Symposium.
Those applications stand to be many and varied. And, given that a significant proportion of the future NGCC-based model palette is likely to be of the higher-powered and four-wheel drive versions, where the 7G-DCT is a compulsory standard fitment, Henning expects a 250,000-unit annual production rate. “Along with the new A-Class, we are expecting 50 percent higher sales, and we are assuming the take-up rate for automatic transmissions will increase still further from our already high levels. This is truly a new milestone in transmission technology.”
Story Filed: 1/23/2012
By Tony Lewin, managing editor DCTfacts.com