4-Wheel vs All-Wheel drive platforms - Benefits and limitations

Originally published at: https://www.pakwheels.com/blog/4-wheel-vs-all-wheel-drive/

Many of us when hearing the question of what is the difference between all-wheel (AWD) drive and four-wheel (4WD) drive become confused as to whether aren’t these two the same? Cars contain four wheels, and hence surely, they should be identical terms, no? The short answer is no! Although they might seem to mean the same, they are not, and there is sufficient difference between the two. Let us look at the major differences between an AWD and 4WD vehicle.

Four Wheel Drive (4WD):

A 4WD is mainly attributed to trucks and off-roading – in general. When we hear the term four by four, i.e., 4x4, it basically is a reference to 4WD. These have all-terrain capabilities.

Working:

As we already know, power from the engine is transferred to the transmission which passes it forward to the front and rear axles where each wheel is then given power evenly. Although a common process, it causes sufficient problems because while turning the inside wheel would need to spin at a different speed than the outer one whereas the engine power causes all the wheels to have the same angular velocity. This means that the wheels on the inside would slip allowing them to cover a shorter radius at the same time as the outer wheels covering a longer arc. This is where the 4WD system comes into play which can be turned on and off both electronically and through levers. It has two gears:

1. 4WD high – it splits the engine power less evenly between the wheels causing what is known as a limited slip hence providing power to the wheel with more traction


2. 4WD low – it limits the wheel speed for most of the available power helping in harsh terrains

As it can be turned on and off, a 4WD, when used as a 2WD, saves fuel because of fewer restrictions and fewer moving parts (that means less moving friction among parts as well).

All Wheel Drive (AWD):

An all-wheel drive is a recent innovation that is generally attributed to cars, although trucks do use the system too, somewhat.

Working:

An AWD system remains on all the time. It is operated either electronically or mechanically. It utilizes three differentials, overcoming the problem in 4WD vehicles. A differential helps split the engine power between the wheels. A central differential is used in an AWD vehicle which splits power between the front and rear axles and provides more power to the axle have more traction. Another differential between the wheels then transfers the distributed power further to the wheel having the most traction and grip, splitting it out. Computer systems with sensors and real-time data are used to read the wheels speed etc. and an ECU helps to decide how much power to be sent to which wheel depending on the grip. Hybrid vehicles use electric motors on two or all four of the wheels which work in pretty much the same way and help to provide instantaneous torque to the wheel with most traction.

AWD systems, however, are not as robust and tough as 4WD systems and cannot deliver power as accurately as them for low-speed off-roading. And since it cannot be turned off, fuel consumption is an issue.

Summary:

To summarize is as in the diagram below: