AAM, Schaeffler launch AWD disconnect systems
June 20, 2013
Leading driveline suppliers are working fast to develop innovative transmission architectures that will enable automakers to address two parallel, but normally conflicting, developments in passenger car demand. Increasingly stringent CO2-based vehicle taxation regulations are focusing customer preferences on lighter, more efficient and more economical vehicles. However, another group of buyers is now beginning to demand the safety and year-round reassurance of all-wheel drive – which tends to boost total weight, increase overall driveline friction and thus raise fuel consumption. Given the overlap between these two groups and the realization that a growing percentage of buyers are likely to want both AWD and lower consumption on the same vehicle, a top priority for driveline engineers is to eliminate, or at least minimize, the fuel consumption penalty of ticking the “AWD” option box on the order form.
American Axle & Manufacturing (AAM) is claiming an industry first with its EcoTracTM AWD system, which disconnects drive to the secondary axle when it is not needed. The system was presented by product engineering director Christopher Phelps at the 2013 CTI Innovative Automotive Transmissions, Hybrid & Electric Drives Symposium and Exhibition in Rochester, Michigan, in May of this year.
Current AWD systems are compromised by the presence of many components which “go along for the ride” in normal driving where drive is only needed to the front wheels, stated Phelps. “These components are accelerated to and from high rotational velocities, and maintained at those velocities, throughout the vehicle drive cycle. The components, including seals, bearings, and geartrains, represent a considerable amount of rotational inertia and drag on the vehicle.” AAM’s EcoTrac, he said, minimizes the effect of this inertia and drag by disconnecting as many of the moving components as possible.
In its default front-drive mode, the EcoTrac vehicle powers only its front differential and driveshafts. The power take-off unit (PTU) taking the drive to the rear is isolated, and a multi-plate clutch on the rear axle interrupts the torque path to the differential, ensuring that only the half shafts rotate while the vehicle is in motion. When either the driver or the automatic sensing systems call for AWD to be selected, the rear-axle clutch engages, and the differential, propeller shaft and PTU gears begin to speed up. Once the propeller shaft is revolving at the correct speed, the transfer case gears engage smoothly with the gearbox’s output shaft.
AAM says that at 65 mph (103 km/h) the system saves 0.75 hp of engine power, claiming that EcoTrac “provides a fully commandable power delivery without degradation of fuel economy when compared with a Front Drive vehicle.”
Similar principles are employed by a new AWD system from Schaeffler, with twin disconnect clutches used to isolate rotating elements of the driveline that are not needed under two-wheel drive operation. Again, the system claims to minimize frictional losses in order to reduce fuel consumption by up to six percent.
“A conventional AWD system can increase the fuel consumption of a vehicle by up to ten percent if the friction losses in the drive train and the mass of the components are taken into consideration,” explained Prof. Peter Gutzmer, CTO of Schaeffler AG.
“Our AWD disconnect clutch reduces this additional friction by more than half by decoupling the secondary drive train and improving its energy efficiency. Suppliers and the automotive industry have been working to uncover the smallest potential for optimization for many years. The AWD disconnect clutch makes a significant contribution to optimizing the drive train, which, in turn, helps automotive manufacturers meet the increasingly stringent fuel consumption standards.”
In the Schaeffler system, the multi-plate disconnect clutch in the front power take-off unit decouples drive to the propeller shaft and the rear axle; a second arrangement in the rear axle isolates the rear wheels from driving the propeller shaft. The re-engagement process to reinstate AWD, however, is subtly different than AAM’s. On the rear axle, electrically-powered dog clutches re-engage the drive. At the front PTU, a hydraulic two-stage system ensures a smooth take-up of torque. First, twin axial disc surfaces with a high coefficient of friction are used to absorb synchronization energy, and then a self-energizing, bi-directional wedge clutch takes care of high drivetrain torques.
Like AAM, Schaeffler claims its disconnect system will allow AWD vehicles to achieve fuel consumption figures similar to their front-drive counterparts, without impairing driving behavior.
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