In general, the terms 4WD (Four-Wheel Drive) and AWD (All-Wheel Drive) describe vehicles in which engine power is transmitted to all four wheels, as opposed to normal vehicles in which only two wheels receive torque from the engine.
Although 4WD and AWD sound similar, there are some functional and design differences between the two systems. The term 4WD describes a transmission system in which the power of the engine can be switched between two speed ranges and transmitted to all four wheels. The AWD system lacks the two-speed transfer feature. Having this feature, 4WD is considered superior to AWD.
Another key difference found between today’s AWD and 4WD vehicles is how engine power is shared between the front and rear wheels. Under normal driving conditions, AWD vehicles deliver 90 percent of their engine power to the front wheels. Additional power from the engine is diverted to the rear wheels via viscous coupling, only when the front wheels begin to slip. On the other hand, generally a 4WD transmits almost all the power of its engine to the rear wheels. The second gear option available in 4WD allows engine power to be divided equally between the front and rear wheels.
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