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An integral element of the Magna FLEX4 AWD disconnect system is the power take-off unit (PTU), which allows a vehicle to run in 2WD for the majority of driving conditions, and switch to AWD only when slip events are anticipated.
Magna (magna.com) didn’t need to load us onto a bus and drive 350 miles north up I-75 to show off its latest all-wheel drive technology. During the blustery winter of 2014, the snow-covered roads of metro Detroit would have sufficed just fine. Nevertheless, we cross the Mackinac Bridge over the frozen straits, and drive farther north into the Upper Peninsula to a snow testing facility in Brimley, MI—not too far from where the Edmund Fitzgerald met its doom in Lake Superior when those “gales of November came early.” In this case it was January and those gales had dumped a considerable amount of snow on the UP.
Upon arrival, we hop in a Land Rover LR2 equipped with Magna’s FLEX4 all-wheel-drive (AWD) disconnect system that shuts off torque to the rear wheels (in the case of a front-wheel drive vehicle) when it isn’t required. There are a number of these systems on the road now described with adjectives like “adaptive” and “agile,” but Magna engineers say the prototype FLEX4 system is unlike any other because it is “intuitive.” Unlike existing systems, which send a torque signal to engage AWD after detecting a wheel slip event, the Magna Flex4 system anticipates potentially slick conditions and engages AWD before any slippage occurs. To do this effectively, the system leverages Magna’s “Climate Trac” technology, taking inputs from the windshield wipers and rear-window defroster—in addition to wheel, body and powertrain sensors—to determine if AWD should be engaged.
“Most of the time you don’t need AWD in normal city or highway driving,” explains Ajit Khatra, product engineer for AWD systems. He suggests that “normal” accounts for as much as 80%
of the time behind the wheel. So the objective is to seamlessly engage AWD only when needed so as to minimize friction-related and other losses associated with unnecessary engage-ment and then—again seamlessly and within milliseconds—to reengage it when conditions warrant.
When a vehicle is at a stop, AWD is engaged to offer maximum traction upon takeoff, but once it reaches 20 mph or more, provided traction is sufficient, the FLEX4 system disconnects the rear wheels. If a road is snow covered but smooth, the AWD remains disengaged until road conditions deteriorate or a hard turn, or yaw event, is being made. Snow or ice isn’t required; FLEX4 will automatically engage on dry pavement depending on surface conditions or driver inputs.
The architecture of the FLEX4 system consists of a power take-off unit (PTU) with a Magna-patented disconnect system in the front axle and couplings on the rear axle with twin dog clutches. Instead of using friction, clutches use teeth cut into one shaft to fit into recesses on the other, allowing them to turn at the same speed without slipping. This allows for greater durability and fuel efficiency, Magna engineers say, because slippage causes parasitic losses and wears the clutch out faster. A single clutch design is also feasible, the engineers add, though the demonstration vehicle has two clutches.
“FLEX4 functions like a CVT in terms of front output torque,” says Ron Frawley, the global product manager for transfer cases/4WD systems. “This also gives OEMs a chance to vary torque distribution,” he adds. This additional benefit allows OEMs to “tune to the brand,” or code the software accordingly to have the torque distribution reflect the desired driving dynamics of a particular vehicle make.
The FLEX4 system remains a prototype for now. However, Magna has a production award with an unnamed European OEM, and start of serial production is anticipated in 2016.