FULL-TIME 4-WHEEL DRIVE
Full time 4-Wheel Drive is the traditional four-wheel drive system that was introduced almost a century ago. It is a primitive four-wheel drive system that splits the torque generated equally to all the four wheels. This four-wheel drive system is operational in full time and hence these are dedicated systems for an off-road system. The system uses a secondary special low gear ratio in order to increase the torque (The pulling capacity of the engine). Speed is dependent on the gearing ratio and since this system uses a low gearing ratio, the vehicle moves slowly. This speed is suitable only for off-road conditions and not the city driving conditions. They are equipped with differential systems to prevent slipping of wheels. Though this system makes the vehicle highly capable of wading through difficult terrains and harsh weather conditions, the weight of the system makes it less fuel efficient.
PART-TIME 4-WHEEL DRIVE
The part-time 4-wheel drive system is a slight variation of the full-time 4-wheel drive system that addresses the fuel economy problem of the full-time system. In the Part-time 4WD system, under normal conditions, the torque is transferred equally only to the rear wheels of the vehicle. When these is less traction in the rear wheels then the vehicle acts like the full time 4WD system. This actuation is done manually by the driver by mechanical, electronic or hydraulic systems. A lever or a switch generally activates one of these mechanisms. The option of either activate or not engage all the four wheels greatly reduces the drag and hence fuel economy is better. However, it is costlier than its parent system.
PART-TIME ALL WHEEL DRIVE SYSTEMS
This system is very much suitable for compact vehicles being used today. Mainly because, this system by default engages the front wheel drive just like the modern front wheel drive used in commercial compact vehicles. When the traction in the front wheels are not enough, by means of a lever or a switch, the torque is transmitted to the rear wheels also. The system unlike the above-mentioned systems, does not utilize the special low gearing ratio. So, this system is not suitable for complete off-road conditions. This is however cheap and efficient and can be used in entry level compact vehicles. These systems are a right back up during inclement weather conditions.
ALL-WHEEL DRIVE SYSTEM
Out of the four types, the AWD system is the technically advanced four-wheel drive system. Here the transfer of torque is controlled by an electronic system and it is intelligently programmed to determine when to transfer how much torque to each wheel. The system is even capable of transferring the whole torque to a single wheel incase the traction is needed in only one wheel. This sophistication comes with complexity. Complex mechanical systems are needed in order to transfer the torque efficiently and quickly and hence maintenance cost increases. This system however is advantageous than the full-time four-wheel drive system because of the flexibility.