The advent of all-wheel drive (AWD) has given drivers more control, safety, reliability and performance — and for off-road enthusiasts, they have opened up all kinds of possibilities. But there are different types of AWD, and with manufacturers using different names to describe the same technology and sometimes designing slightly different takes on the same idea, the topic can be confusing. Here’s a quick rundown to help you weed through some of the most common options available today and understand what each variant has to offer.
Mostly seen on trucks, this setup allows a driver to select when an AWD system is engaged, essentially turning it from a two-wheel-drive vehicle (2WD) into a four-wheel-drive vehicle (4WD) through the transfer case. It’s a manual choice that is mechanically engaged, and it doesn’t use a center differential, which means the wheels will rotate at the same speed. This is no good going around corners and a vehicle should never be driven in this mode under normal conditions or it will cause transmission windup and damage. Still, it’s good for someone who expects to sometimes find themselves in difficult driving conditions and who has a good grasp on how to engage and drive the system.
Full-time AWD means all wheels are always driving, always getting torque. Different manufacturers solve the windup problem in different ways, but the general idea is that wheels must be able to spin at different speeds and maintain traction as needed. One method is to use a center differential with a traction control device, such as a multi-plate hydraulic clutch or viscous coupling. The Haldex and Torsen are also well-known systems. Haldex is electronically controlled, whereas Torsen is mechanical and uses a pre-calculated torsion force to determine torque transfer.
Another solution is creative braking. When one wheel begins to slip, the ECU recognizes it and uses the ABS to control the system, sending more torque to the wheels that have traction. Full-time AWD is beneficial because it’s responsive and always on.
Automatic AWD is similar to part-time in that it defaults to 2WD, but the key difference here is that the driver is not in charge of selecting when the AWD system kicks in — the vehicle’s computer is. The benefit of this system is that it’s more efficient than full-time, and it eliminates the need for a center differential. If there is a sudden demand for traction in one of the drive wheels, a traction control device (noted earlier) automatically kicks into action and sends torque to the other axle and wheels as needed. When that demand disappears, the system reverts to 2WD. This can happen very quickly, and with the technology and algorithms being developed today, the process is even becoming predictive using driving patterns and sensors.
Electric and Hybrid
There are a few vehicles coming out with electric solutions, but the number is sure to expand in the future. These vehicles use electromagnetic couplings and electric motors with sensor systems to send torque where it is needed. Some vehicles even have an electric motor powering one set of wheels and a gasoline engine powering the other. The electric-driven set can kick in whenever the car’s computer system instructs it to do so based on any number of factors.
This is by no means an exhaustive list, and manufacturers get pretty creative when they market their products, but the most common systems remain largely the same. Die-hard off-roaders will want to do a deeper dive into all that 4WD has to offer, but this should be a good start for everyone else.
Check out all the drivetrain components available on NAPA Online, or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on different types of all-wheel drive, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.
Photos courtesy of Blair Lampe.
Blair Lampe is a New York-based professional mechanic, blogger, theater technician, and speechwriter. In her downtime she enjoys backpacking wherever her boots will carry her, rock climbing, experimental theatre, a crisp rosé , and showering love on her 2001 Sierra truck.