New Development of Highly Efficient Front-Wheel Drive Transmissions in the Compact Vehicle Segment Introduction Dr. Ing. Ansgar Damm, Dipl.-Ing. Tobias Gödecke, Dr. Ing. Ralf Wörner, Dipl.-Ing. Gerhard Henning Daimler AG, Stuttgart Mercedes-Benz will introduce the first vehicle model of the new generation of compact cars (NGCC), the B-Class, on the Chinese market in August 2012. The design change, based on a new vehicle architecture, was rolled out on the German market in 2011. In line with this concept, the second model of the compact vehicle family, the new A-Class, will appear on the market in Germany in September 2012. Mercedes-Benz developed two new transmissions from scratch according to the new vehicle concept and layout: The 7-speed dual clutch transmission, known as the 7G-DCT, and the 6- speed manual transmission, known as the SG6-310. The following objectives were of primary importance as these new transmissions were being developed: Space-saving design as well as a lower gross weight in line with the general packaging concept requirements Precise shift response and a high degree of gear change smoothness Fulfillment of current and future requirements with regard to CO 2 directives through a highly efficient overall design Integration of additional functions, such as the stop/start function and park-bywire, along with provision for additional drive system technology Flexibility through the use of a modular gear set Use of shared components Together with the launch of the B-Class, the 7G-DCT dual clutch transmission with be offered on the Chinese market beginning in August 2012. Initial Situation Both newly developed transmissions will be introduced in connection with various output levels of the M270 4-cylinder gasoline engine as well as the OM651 diesel engine. For the time being, the B-Class is equipped with the fully newly developed M270 4-cylinder gasoline engine for the Chinese market. The customer can select between two M270 variants: 90 kw with 200 Nm as well as 115 kw with 250 Nm. An additional variant with 155 kw and 350 Nm will be available in April 2013. The range of engines available may be expanded to include a diesel engine in the future. New transmissions had to be developed for the NGCC vehicles due the use of these 4- cylinder engines in the vehicles having front transverse and rear north-south architecture. This was mainly due to the associated package requirements for the vehicle front end.
The decision to develop and produce the 7G-DCT dual clutch transmission and SG6-310 manual transmission in-house opened up new possibilities for an advanced transmission technology that is currently not available on the market. It is possible to realize synergy effects in production at Daimler AG as the individual assemblies, such as shafts, idler gears, sliding sleeves, the differential and the housing parts, are produced in the coordinated production system. The New 7G-DCT 7-Speed Dual Clutch Transmission Technical Design and Layout The new 7G-DCT replaces the previously used Autotronic continuously variable automatic transmission. Mercedes-Benz has over five decades of experience when it comes to developing automatic and manual transmissions. Thus, it was possible to implement the product concept quickly. The innovative transmission gear set concept allows for a very efficient and compact design. Figure 1 shows the transmission layout. Figure 1:7G-DCT layout The outer of the two concentric clutches is connected to the odd gears 1, 3, 5 and 7, while the inner clutch is connected to gears R, 2, 4 and 6. Other transmissions available on the market also have this layout. The difference lies in the decision in favor of an oil-cooled multidisk clutch (see figure 2) instead of the more established dry dual clutch.
The advantages of this variant are: Thermal robustness: The driver can depend on the high reliability of NGCC vehicles in critical situations also in the case of a permanent vehicle/trailer weight of over 3 tons or when maneuvering, stopping and moving off on inclines. SUV vehicles, which are currently in the planning stage as part of the NGCC, make particularly high demands on the thermal robustness of the starting device. Low moment of inertia: In the case of dry clutches, the demand for a torque capacity of 350 Nm (with an option to be increased) has resulted in significantly higher weights and moments of inertia than for the wet multidisk clutch the result being lower driving dynamics. The oil-cooled multidisk clutch can be modified easily in order to increase the torque by changing the mounting of the friction disks. Figure 2: Dual clutch The new gear set significantly distinguishes itself via the high integration of reverse gear (R) using the existing gear ratios as well as the use of two gear chains. The 7G-DCT can thus have a very short layout in comparison to competitor gearboxes with comparable torque capacity. Both 5th and 7th gears as well as 4th and 6th gears require only one drive gear each. A smooth, progressive gear ratio step could be achieved in conjunction with the final drive design in spite of the compulsory combination in these gears. The reverse gear now has a completely new design and no longer requires the additional reverse gear shaft that is used in many other transmissions available on the market. The advantage of this design is the gear ratio freedom. This results, however, in higher costs due to the additional weight and additionally required installation space. The reversal in the direction of rotation is facilitated in other systems via the combination of 2nd gear and subsequent transmission to the drive shaft, which is not directly connected with 2nd gear. Depending on the layout of the gear set, this design can lead to a relatively long transmission of reverse gear. In order to reverse the direction of rotation in the 7G-DCT, power is transmitted from 2nd gear to the idler gear of reverse gear, which is located on drive shaft 2. The idler gear of reverse gear is connected to the idler gear of 3rd gear, and the power is transmitted via this to the drive shaft. The power flow to the output gear (see figure 3) is ultimately realized via
1st gear, which is also engaged. This allows for a more favorable gear ratio in reverse gear in spite of the resulting constraints. It can also be reversed by simply switching the clutches between 1st gear and R. Figure 3: Power flow in reverse gear All the NGCC requirements can be fulfilled via the combination of two gear set variants with two different final drive gear ratios. Two different gear sets with a gear ratio spread of 7.14 and 8 are used depending on the engine variant. Gear set 1 Gear set 2 1st gear 15.943 15.943 2nd gear 10.038 10.038 3rd gear 6.927 6.359 4th gear 4.915 4.335 5th gear 3.606 3.205 6th gear 2.771 2.501 7th gear 2.232 1.995 Reverse gear 12.807 13.950 Gear ratio spread 7.142 7.990 Figure 4: Gear ratio and gear ratio spread The newly developed gearshift actuators, which are installed as modules and integrate the hydraulic functions, actuate the sliding sleeves for shifting the individual gears. Efficiency is improved by reducing the churning losses in the transmission via an enclosure that is attached to the differential. Furthermore, high gearshift quality and better acoustics are achieved due to reduced oil foaming, even in high load scenarios. Mechatronics The key component of the fully integrated 7G-DCT control unit is a 32 bit microcontroller with integrated flash memory and RAM. The basis consists of a thick-film substrate, which acts as a circuit carrier. The non-enclosed components are secured to this using a conductive adhesive. It was also possible to integrate the actuation and interference suppression of the electrical oil pump on the circuit carrier in addition to the solenoid control system output
stages and the sensor evaluation electronics. A very wide temperature range can thus be covered by this assembly technology. Particular attention was focused on reducing the electrical interfaces during the development of the 7G-DCT. In this connection, five position sensors, three rpm sensor and two pressure and temperature sensors were integrated into the control unit. The connection to the circuit carrier is realized using flex foil and aluminum wire. The electrical oil pump, nine solenoid valves and lift solenoids were similarly integrated into the control unit. A complex wiring harness could thus be dispensed with, and the connection between the 7G-DCT and the vehicle could be reduced to a 5-pin plug. Due to numerous advantages, an electrical auxiliary oil pump was also integrated in the 7G- DCT, similar to previously used planetary gear systems. In comparison to earlier applications, this is highly integrated into the 7G-DCT, both from a design and functional perspective. The auxiliary oil pump ensures start/stop operation by maintaining the oil pressure when the engine is at standstill. It is also used to boost the mechanical oil pump during normal driving at low engine speeds. The auxiliary oil pump also regulates the required cooling flow rate for the clutch at higher loads. The otherwise-required control valve can thus be dispensed with. The overall efficiency of this pump concept is significantly increased through the use of the mechanical vane-type pump together with the electrical auxiliary oil pump. A further function of the 7G-DCT is linked to the park-by-wire system. An additional energy source, which is independent of the engine speed, is required if a P-b-W function is connected to a stop/start-capable drive. The parking lock can thus be engaged and released, also when the engine is at standstill, without the use of an additional actuator. Thanks to the integration of the auxiliary oil pump, only eight control solenoid valves, one control valve and one shift valve have to be installed in spite of the wide range of functions offered by the 7G-DCT (seven gears, wet dual clutch, park-by-wire). In a further developed version, the 7G-DCT will be offered with AWD and with higher power and torque outputs. In comparison to its predecessor, the Autotronic, the new overall concept of the 7G-DCT features fuel consumption savings of 9% in the NEDC. The 7G-DCT is thus at least at the level of the manual transmission for all NGCC applications. The New SG6-310 6-Speed Front-Wheel Drive Manual Transmission Technical Design and Layout The SG6-310 was developed together with the 7G-DCT. The SG6-310, which has an installed length of 357 mm, is a 3-shaft transmission with a very compact design. Its dry weight is 46 kg. The drive shaft features a fixed-floating bearing, while both output shafts and the differential have tapered roller bearings. All tapered roller bearings are installed according to the "set-right" principle. No separate setting is required. Figure 5 shows the transmission layout.
Figure 5: SG6-310 layout 1st, 2nd, 5th and 6th gears are shifted on the lower output shaft, while 3rd, 4th and R are shifted on the upper one. A space-saving and comparably weight-reducing design could be achieved via the multiple usage of gear pairs. On the one hand, one shared fixed gear could be used for 4th and 5th gears, while a reverse shaft or reverse idler gear could be dispensed with due to the direct meshing. The synchronization of 1st and 2nd gears is designed as a triple-cone synchromesh system in order to meet the high requirements with regard to gear change smoothness. Double-cone synchromesh systems are used for all remaining forward gears. Reverse gear is designed as a single-cone system. Four weight-optimized aluminum cast shift forks are installed in the SG6-310 to optimize the overall weight. These are guided by means of sliding bearings on just two shift rods. It must be possible to detect when the transmission is in neutral in order to ensure the stop/start function. This is done by sensing a magnet that is secured on the selector shaft; this also triggers the activation of the reversing lamp. The torque capacity of the SG6-310 is currently 310 Nm. 330 Nm can even be temporarily achieved in the overtorque function. The offer for the market launch consists of two gear ratio series. Both were optimized during development with regard to fuel consumption and gear change smoothness as well as pulling-away and acceleration response. Figure 6 shows two versions of the SG6-310 gear ratios, which are used depending on the engine variant. Gear set 1 Gear set 2 1st gear 14.438 13.182 2nd gear 8.169 7.339 3rd gear 5.203 4.695 4th gear 3.614 3.322 5th gear 2.733 2.429 6th gear 2.337 1.976 Reverse gear 12.994 12.031 Gear ratio spread 6.177 6.672 Figure 6: SG6-310 gear ratio and gear ratio spread
The gearshift lever is mounted in a plastic shift block with mechanical lift lock for reverse gear. It is connected to the shift module at the transmission side via two Bowden cables for the gearshift and selection movement. A very harmonious and precise shift sequence is possible for the driver due to a finely coordinated engagement of the internal gearshift in connection with the carefully coordinated coupling elements on the cables. The high shift comfort is also supplemented by the specially designed clutch with low pedal force, optimal modulation and very good NVH characteristics. Summary The newly developed transmission range is an important part of the technological leap achieved by the new vehicle architecture of the new generation of compact cars (NGCC). Additional technology modules could be successfully integrated with the 7G-DCT. A highly efficient electrohydraulic control unit is thus combined with a wet starting device. Optimal pulling away comfort and robustness is thus ensured. The additional function modules "Parkby-Wire" and "Start/stop" were also integrated for the first time. The 7G-DCT sets additional standards with regard to efficiency due to the combination of the electrical and mechanical oil pumps. The SG6-310 predecessor already featured a high degree of gear change smoothness and precision. The focus during the development of the SG6-310 manual transmission was thus firmly on further optimization. Improvements were achieved in the shift quality with an additional reduction in the shift force as well as with regard to the friction loss and weight. To sum up, both transmissions will contribute to fulfilling current and future CO 2 emission requirements. The future viability of the concept is ensured by the variable gear set modules, which can also be installed in subsequent NGCC drive variants.