Four-wheel Drive (4WD) and All-Wheel Drive (AWD)

A vehicle that provides power to all four wheels has some key advantages in slippery or rough terrain. First and foremost, four-wheel drive (4WD) enables the vehicle to move under conditions of reduced traction. What is lost on many drivers is that four-wheel drive does not enable the vehicle to stop more rapidly, evidence of this being commonly seen in the mountainous states of the western United States. Nevertheless, the U.S. market has seen an explosion in the sale of four-wheel-drive sport utility vehicles (SUVs). Several automakers also have successfully marketed all-wheel-drive (AWD) vehicles, most notably Subaru and Audi. Furthermore, there is a vast array of adjectives used to define the systems, including part-time four-wheel drive, full-time four-wheel drive, all-wheel drive, and so forth. The differences among these systems often owe more to marketing than engineering. Adding to the confusion is the fact that the automakers themselves use various terms for their systems, often meaning something quite different to a competitor. In short, any attempt to classify four- or all-wheel drive systems invariably will meet with exceptions to the classification. This work will classify these systems into three broad categories: part-time four-wheel drive, full-time fourwheeldrive. and all-wheel drive.

Part-Time Four-wheel Drive (4WD)

The key feature of a part-time four-wheel-drive system is the inclusion of a separate transfer case aft of the transmission. This is the lowest cost option and can be considered the first-generation option. It is called part-time because it can be used only in conditions that will allow for wheel slip, such as dirt roads, full snow coverage, and so forth. The reason for this is that there is no mechanism to eliminate driveline wind-up. Recall that a differential is used at the rear axle to allow differences in wheel rotation while the vehicle is cornering. With four-wheel drive, the same thing is happening with the front axle and the rear axle. One is traveling faster than the other; therefore, something must allow for the speed difference. In the absence of a center differential, the only mechanism allowing wheel speed variation is for the wheel to break free at the contact patch. Because this requires large forces on dry pavement, the part-time system cannot be used on dry pavement without serious drivetrain damage.

The transfer case also incorporates two selectable gear ratios-low and high. In four-wheel drive low, the vehicle has a limited top speed. However, because of the large gear ratio in low, the vehicle has a large amount of torque available at the drive wheels to enable the driver to extricate the vehicle from difficult situations. Transmissions and Driveline The other feature of this system is that'the front hubs usually are locked manually. In twowheel drive, the front wheels spin freely around the spindles. When the driver desires fourwheel drive, the hubs must be locked manually onto the drive spindles for torque to be applied through the front wheels. The Isuzu Rodeo, Ford Bronco, and Dodge Ram all have part-time four-wheel drive.

On-Demand Four-Wheel Drive (4WD)

This is the next option in terms of increasing convenience and cost. An open differential is incorporated between the front and rear axles. The open differential absorbs shaft speed variations between the front and rear output shafts. However, being an open differential, it sends torque to the axle with least resistance. This system allows driving in four-wheel drive on dry pavement, but this will decrease the fuel economy of the vehicle. For this reason, it is referred to as on-demand. The driver may use two-wheel drive when there is no need for fourwheel drive. This system also will have automatic locking hubs that are either vacuum operated or electrically operated, saving the driver from a trip out of the cab in inclement weather to lock the hubs. Often "on-demand" (a configuration) is confused with "shift-on-the-fly" (an engagement method). The Chevrolet Blazer is an example of a vehicle with on-demand fourwheel drive.

Full-Time Four-Wheel Drive (4WD)

This is the highest cost option. This system has differentials everywhere, at both front and rear axles and in the transfer case. This allows the vehicle to be in four-wheel drive on dry pavement. The system allows for slip, but something had to be done about situations of very low traction-that is, the open differentials would send torque to the wheel with the least traction. Some vehicles, most notably the AM General Hummer, can lock all of the differentials. Other vehicles, such as the 1995 Jeep Grand Cherokee, have a viscous coupling that transmits power from the wheels that slip to the wheels that grip. In this category, the distinction between four-wheel drive and all-wheel drive begins to blur. For the purposes of this work, full-time four-wheel drive is applied to vehicles that still require the driver to select the four-wheel-drive option.

All-Wheel Drive (AWD)

For the purposes of this work, an all-wheel drive vehicle does not have a selectable transfer case. Generally, these vehicles are not intended for off-road use, but use four-wheel drive for inherent stability. Usually, they use viscous couplings to send power from the spinning wheels to the gripping wheels. The system operates automatically and requires no driver intervention. This system also is used on high-performance cars to eliminate wheel spin caused by the enormous torque generated at the rear wheels.