April 18, 2012
Article by Jim Kerr; photo courtesy ZF.com
Some people confuse all-wheel drive (AWD) with four-wheel drive (4WD), but 4WD vehicles have been built for much longer than AWD cars and trucks. So what is the difference? AWD vehicles are designed to operate with all wheels propelling the vehicle under all driving conditions and the system is designed to allow different wheel speeds between the front and rear axles. 4WD vehicles drive both axles at the same speed all the time and should only be operated in 4WD when driven on poor traction surfaces. Otherwise, everything from tires to drive axles to transfer cases can be damaged by the binding of the drivetrain.
In corners, 4WD systems can get out of control easily if you are on hard surfaces or ice, because one axle has to slip to allow the turn. AWD systems allow the front tires to turn at different speeds from the rear tires, so traction is maintained and vehicle handling is stable.
Many of the newer AWD systems are based on front-wheel drivetrains, with auxiliary rear-drive axles. Take Ford’s 2013 Taurus AWD models for example. Here, the AWD system aids in balancing the handling of the vehicle on all road surfaces. The intelligent AWD system computer constantly monitors and predicts traction, delivering torque to all four wheels, even before wheels begin to slip. The on-demand electronic centre coupler is capable of adjusting torque from front to rear (up to 100 percent to either axle). Combined with the Ford’s AdvanceTrac traction control and Roll Stability Control, which use brake intervention as part of their control strategy, the intelligent system is able to transfer torque not only from front to rear, but from side to side as well.
Acura takes a different approach to AWD by combining vehicle stability control into the operation of their AWD system. Instead of cutting power or braking wheels, they add performance while controlling the vehicle direction. It’s called Super-Handling All-Wheel Drive (SH-AWD). Instead of braking a wheel on one side of the vehicle to make it turn, the SH-AWD system accelerates the opposite rear wheel to push the car back into line, all without a decrease in vehicle speed or performance.
To achieve this, two things happen. First, the rear wheels accelerate speed faster than the front wheels. Second, the speed increase is directed to each rear wheel independently. A carbon-fibre driveshaft transmits the front driveline speed into a planetary gearset located in front of the rear axle. The unit drives at a 1:1 ratio but if the vehicle starts to oversteer or understeer, the computer will use oil pressure from an internal rear axle pump to apply disc clutches, causing the planetary gearset to operate in overdrive. The rear axle can turn up to five percent faster than the front axle.
There are also planetary gearsets to drive each rear wheel. Each planetary gear unit is connected between the differential ring gear and the axle shafts. An electromagnetic clutch can apply the clutch and gearset progressively, controlling torque output to each axle shaft. Up to 70% of the total engine torque can be applied to one rear wheel. The Acura system not only aids in traction but adds stability control without any decrease in vehicle performance. Just like Ford’s AWD system, the GM and Acura AWD units operate seamlessly without any input needed from the driver.
All-wheel drive provides many benefits in handling and safety. New technologies and materials have made the units smaller, lighter, and more responsive. Subaru and Audi may have the most experience with all-wheel drive, but manufacturers such as Acura, Porsche, Suzuki, GM, BMW, and Mercedes-Benz are but a few of the manufacturers making it a common technology on the road today.