Four wheel drive (4WD) vehicle is a vehicle that is capable of transferring engine power to all the four wheels. More often, people call these vehicles as 4X4 cars. In this terminology, the first digit represents number of axle ends on a vehicle while the second digit represents the number of wheels which directly receive engine power.
Thus, 4X4 means:
4: Number of axle ends on a vehicle (2 ends of front axle + 2 ends of rear axle)
4: Number of wheels that directly receive power from engine
The wheels which receive power from the engine are also called driving wheels. Therefore, one can say that a four wheel drive (4WD) or 4X4 vehicle contains 4 driving wheels.
According to the way torque is transferred between axles, four wheel drive systems are of following types:
Part time four wheel drive: This is the first generation of four wheel drive systems. Usually, such vehicles are rear wheel driven and they can be converted to four wheel drive vehicles with the help of a component called 'Transfer case'. When driver engages transfer case, engine power gets transferred to front wheels also, alongwith the rear wheels. It is the most simple and cheap 4WD system.
On demand four wheel drive: Under the normal operating condition, this system operates as two wheel drive. However, it can switch over to 4WD mode whenever needed. For doing this, it employs an open differential and the operation is either vacuum or electric controlled.
Full time four wheel drive (4WD): This system works as all time 4WD. Its' operation is fully automatic and it does not need any input from driver. It uses viscous coupling for torque transfer and thus, can even adjust to individual wheel slips.
What is the difference between 4WD and AWD?
AWD stands for All Wheel Drive. Technically, an AWD vehicle can be either a full time or part time four wheel drive vehicle. However, according to the recent nomenclature followed in the automotive industry, this term is used to refer to full time 4WD vehicles only. That means in an AWD vehicle, all the wheels will directly receive engine power all the time. Thus, one can say that AWD is synonymous to full time 4WD.
Advantages of Four Wheel Drive (4WD) system:
- It improves overall stability and grip of the vehicle.
- It helps to ride on slippery road conditions like mud or snow covered roads.
- 4x4 systems are useful for off-roading.
Disadvantages of four wheel drive (4WD) system:
- The fuel economy or mileage of 4X4 vehicles is generally lower as compared to equivalent two wheel drive vehicles.
- As all the wheels are driven wheels, rate of tyre wear in case of 4WD cars is higher compared to two wheel drive vehicles.