More specifically, what are the pros/cons of each? Why do some cars use one system over the others and which is “better”?
EDIT: Thanks everyone for the explanations. It’s been very useful.
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Front wheel vs. rear wheel drive are easy to understand. With front wheel drive (FWD), the motor is connected to the front wheels through the transmission (the gears that control how much power goes from the engine to the wheels). In rear wheel drive (RWD), the motor is connected to the rear wheels through the transmission and then a long drive shaft to the rear axle.
There are pros and cons to each of these. Generally, it is considered that FWD is better for traction and RWD is better for power basically because FWD is pulling you forward and RWD is pushing you forward.
4WD vs. AWD is a little more fuzzy. 4WD came first. Both, as their names imply, allow any or all of the four wheels to be powered by the motor. It’s the implementation of how this is done that differs. To go back to the original versions of these systems that were purely mechanical is the best way to understand them.
In a vehicle, the axles that are powered must have something called a differential. This is a set of gears between the two wheels on that axle that is connected to the drive shaft (the shaft that the motor spins). It is called a differential because the wheels on a vehicle must be able to spin at different speeds. This is because when you go around a curve, the inner tires travel a shorter distance than the outer wheels. A solid axle between each side wouldn’t allow for this and the wheels would wear unevenly or could even break at the hub from the forced use of power on the inner wheel.
Now, with a modern 4WD vehicle, there is also a center differential between the front and rear axles. As you might guess, this is because the front and rear axles need to be able to spin at different speeds for similar reasons that there are differentials between the wheels. Additionally, since both the front and rear axles can receive power from the motor, both of them have differentials. Early 4WD systems lacked a center differential and the front and rear differentials were manually locked. This meant they could only be used off-road and in some cases you had to exit your vehicle to switch modes.
Now, you have a vehicle that can receive power from the motor at any wheel. But you need to add some other stuff to make it practical. It’s no good having a 4WD vehicle if one wheel that loses traction starts spinning and hogging all the power since it will follow the path of least resistance through the differential gears. So you need something called a limited slip differential that can detect a spinning wheel that has lost traction and send power to the ones that haven’t. Also, 4WD uses more energy. It’s just not very efficient to power all four wheels all the time. So, you add in something called a transfer case that lets you control whether the center differential should be engaged with both the front and rear wheels or just one axle. This will let you pick between 2WD (typically the rear wheels) or 4WD. So your car has two shifters – one for the transmission gear and one for the transfer case.
Then we come to all wheel drive (AWD). AWD is a form of 4WD. But it is generally simplified for the operator. Typically, AWD is automated and using an automatic transfer case to select which wheels need power. Sometimes you may have some choice over high or low gears, but usually not. Think of this like the difference between manual transmissions and automatic transmissions. Modern AWD systems can offset the balance of power to the front and rear axles which can allow for better fuel economy when running in 4WD such that there is minimal loss of efficiency and the vehicle can run in 4WD at all times.
Nowadays, fans of either system generally consider manually-operated 4WD systems to be superior for off-road driving and automatic AWD systems to be optimized for on-road driving. High-end 4WD systems feature things like locking differentials which does force the wheels to spin at the same rate which is better in low-traction conditions like snow, sand, or mud. Meanwhile, high-end AWD systems optimize traction for slippery road conditions like rain, snow, or ice. They use forms of traction control to limit the speed of a slipping wheel through the brakes or the differentials to prevent it from losing traction. They can also allow for more output from the motor to the wheels since there are more paths for that power to travel from the motor to the wheels which is why some performance vehicles have AWD systems.
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