More and more automakers are offering all-wheel-drive than in the past. This extra choice is great news for consumers but it can lead to confusion; the number of technologies on the market is practically vertigo-inducing. Each manufacturer’s system is unique, and a lot of times the underlying hardware is different. Not every all-wheel-drive setup was created equally. Which one is best for you? Here’s an overview of what some major manufacturers offer.
“There are two basic vehicle architectures out there today,” said Mark Barrett, the Group Vice President of Engineering, Product Development and Procurement at American Axle (AAM). How the engine is mounted in a vehicle is “really what defines that high-level architecture.”
Engines can be positioned transversely (side to side) like they are in most front-wheel-drive vehicles or longitudinally (front to back) as in rear-wheel-drive cars and trucks. Basically it’s how the power flows through a vehicle. Barrett said this is important because a, “transverse engine configuration means you have to go 90 degrees to transfer the torque to the rear.”
A power transfer unit (PTU) is needed to turn that corner in the driveline. According to Barrett, a PTU is “essentially a gearbox” that sends torque rearward. It’s the foundation of an all-wheel-drive configuration used by many manufacturers today. Vehicles like the Honda CR-V and Ford Fusion feature systems like this.
All-wheel drive that’s paired with a longitudinally mounted engine tends to be more traditional, like something you’d find on a full-size SUV. These systems usually feature a transfer case to send power to the front axle and a defined torque-split front to rear. But this layout is not only for trucks. The new Cadillac ATS sports sedan has a longitudinally mounted engine and an available all-wheel-drive system provided by AAM. Also, the Chrysler 300 and Dodge Charger full-size sedans offer a very similar part-time all-wheel-drive setup.
AAM provides individual components to automakers, but in most cases “we do the whole driveline system,” Barrett said. This is advantageous compared to the piecemeal approach because, “sometimes the parts don’t function as well as they should.” There can be noise or vibration issues and doing it separately can actually cost more.
“We supply a lot to GM, on their Opel Insignia, Cadillac SRX all-wheel-drive [and] Buick Regal all-wheel-drive,” Barrett said, but Detroit is not their only customer. AAM’s European operations provide a torque-transferring center differential to Audi for its advanced, driver-focused quattro system.
‘ROUND THE CLOCK, AROUND THE BLOCK
Aside from how the engine is mounted in a vehicle there are other ways to classify all-wheel-drive layouts. According to Barrett, full-time systems feature a defined torque-split front to rear and are always on and available to work.
“The driver doesn’t have to do anything,” he said, which is an advantage of this layout. One downside though is that constantly spinning all of those extra components creates friction and reduces fuel economy. Generally, full-time systems seem to be more performance oriented, or found on trucks. Mitsubishi’s small and speedy Lancer Evolution uses this technology as does the ferocious Nissan GT-R.
Unlike its full-time brother, part-time all-wheel drive only powers four wheels when slippage is detected. This reduces parasitic losses and boosts fuel economy. “Every tenth of a mile per gallon is critical,” Barrett said, noting that OEMs are battling to minimize consumption. He also said when you go from front-wheel drive to all-wheel drive you lose about two miles per gallon, which is a big deal.
These systems seem a lot more common than their always-on siblings. The Ford Explorer features a part-time system as does the Nissan Murano and Rogue. In the name of fuel economy, Acura switched to a part-time setup when it redesigned the RDX crossover for 2013.