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Modern Automotive Drivetrains

Modern Automotive Drivetrains

By Jeff Youngs, February 24, 2012
Should I choose front- or rear-wheel drive for my next vehicle? Do I need four-wheel or all-wheel drive? With so many drivetrain choices, today's automotive marketplace can be confusing for many buyers. Since each drivetrain (the system connecting the engine to the wheels) configuration is designed for different driving conditions, choosing which wheels are driven, or powered by the engine, is an important decision when selecting your next vehicle.

Making sense of it all is a bit easier when you understand the six types of drivetrain technologies and how each works:

Front-wheel drive (FWD) _ A front-wheel-drive vehicle sends all the engine power to the two front wheels; in effect, FWD "pulls" the vehicle down the road. With the engine, transmission, and powertrain located in the front of the vehicle, there is more passenger space in the cabin and reduced drivetrain complexity because all the components are in close proximity (possibly reducing maintenance costs). Handling of FWD vehicles is generally considered stable, predictable, and good for winter weather, as the majority of the vehicle's weight is located over the driven wheels, which increases traction. Also, some FWD vehicles exhibit "torque steer" (the steering wheel twists or resists the driver under hard acceleration) and sluggish handling can result. Some examples of front-wheel-drive vehicles include the Honda Accord, Toyota Camry, and Nissan Altima.

Rear-wheel drive (RWD) _ Once the standard driveline for automobiles, a rear-wheel-drive vehicle sends engine power to the two rear wheels. In effect, the rear wheels push the vehicle down the road. Because a front-engine, rear-wheel-drive vehicle doesn't benefit from the weight of the engine over the driven wheels, the RWD configuration is not the best design for winter traction, although most rear-wheel-drive vehicles feature some type of traction or stability control to help overcome this disadvantage. RWD vehicles are typically well-balanced and offer superior handling and braking for enthusiast drivers. Some examples of rear-wheel drive vehicles include the BMW 3 Series, Lexus IS Series, and the Infiniti G35.

All-wheel drive (AWD) _ With all-wheel drive, one of the most sophisticated drivelines available today, the engine sends power to all four wheels. Advanced electronics, gears, or fluid-filled differentials can send power to all wheels equally, or transfer torque to the wheels (or wheel) with the most traction. Modern engineering has advanced AWD systems to the point where they are nearly seamless to the driver. In fact, AWD systems can be designed with a front- or rear-wheel bias (such as on various Infiniti and Audi models, among others), allowing engineers to improve driving dynamics. Unlike 4WD systems, AWD systems do not usually have a low range for serious off-road use, as this is not the intended use of the vehicle. Instead, AWD systems offer vastly improved capabilities for winter driving or on wet slippery roads. Some examples of all-wheel-drive vehicles include the Subaru Legacy, BMW X5, and Acura RL.

Part-time all-wheel drive (Part-time AWD) _ Essentially a two-wheel-drive vehicle, this driveline configuration sends power to just two wheels (front or rear) until additional traction is needed. The system is designed to automatically transfer power to the other two wheels once the driven wheels slip, effectively becoming all-wheel drive for a limited period of time. Because some loss of traction must occur before the system intervenes, this type of driveline isn't completely seamless to the driver. Very effective in preventing complete traction loss, this type of drivetrain is not recommended for heavy off-road use. Examples of part-time all-wheel-drive vehicles include the Honda CR-V AWD and Toyota RAV4 AWD.

Full-time four-wheel drive (4WD) _ Operating much like all-wheel drive, this type of driveline is usually more robust because it is designed for serious off-road use. Engine torque is usually evenly distributed to each wheel on dry surfaces, and varies automatically according to conditions in low-traction situations. Like part-time four-wheel drive, these vehicles are equipped with a "low" range, and many feature locking differentials (the ability to make all wheels rotate at the same speed) for added traction. Extremely capable off-road and in winter weather, these full-time 4WD systems add weight, at the expense of fuel economy. Some examples of full-time four-wheel-drive vehicles include the Land Rover Range Rover, Lexus LX Series, and Hummer H2.

Part-time four-wheel drive (Part-time 4WD) _ This drivetrain configuration typically sends power to the rear wheels all the time. If traction is lost, power can be sent through an electric, mechanical, or hydraulic switching system to all four wheels. In many cases, driver intervention is required to change into 4WD. This drivetrain is considered part-time because it often can't be used on dry pavement for fear of damaging the mechanicals. Part-time four-wheel-drive vehicles are equipped with a low range for off-road use, and many have locking differentials for even more grip in extremely slippery conditions. Some examples of part-time 4WD vehicles include the Jeep Wrangler, Chevrolet Avalanche 4WD, and Nissan Titan 4WD.
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