Dear Mods
I started off writing down a small description with required links as there is a lot of confusion amongst people about when 4WD should be engaged or not and the difference between different iterations of the same. I then expanded this to help non-technical people, who are interested in automotive stuff, in understanding how an automobile functions! Hope it is OK.
An
internal combustion engine burns fuel to push
pistons. They convert chemical energy into mechanical energy. The piston(s) move up and down due to expansion of gases when fuel is brunt.
The reciprocating motion of the pistons is converted to rotational motion with the help of a
crankshaft, different from a camshaft. Multiple pistons help keep the crankshaft moving continuously in a much smoother way. Imagine when you were pushing or being pushed on a merry-go-round at the local kids’ area. If you push it once every-time it turns one full circle then you have the sensation of the carousel speeding-up and then slowing down, again and again. This is jerky movement and is exemplified by a single cylinder engine like the Royal Enfield thump that people like

! If you do it 4 or 6 times, it is much smoother as the power input comes in quick succession.
The crankshaft is connected to a
flywheel. The job of the flywheel is to absorb these jerks of the pistons transmitted through the crankshaft and it thus also acts as a reservoir of rotational energy. The flywheel is a heavy object and its size and weight depend on the
torque being generated.
Now the engine is switched on and the pistons start reciprocating, so the crankshaft turns and the flywheel starts spinning. The
tachometer gives the revolutions per minute of the crankshaft/flywheel.
The
clutch, in default position is pressed tight against the flywheel with the help of springs. It acts as the interface between the engine and the
gearbox so that gears can be changed without having to stop the flywheel from rotating. When we press the clutch pedal, it moves the clutch away from the flywheel/pressure plate thereby disengaging the gearbox from the engine so that the gearbox does not have to synchronise so many different rotational devices. The gearbox is an extremely complicated piece of engineering!
In a rear-wheel drive vehicle, the gearbox then transmits this rotational motion, as per your desired torque and power characteristics (selected through the gearbox), through a
propeller shaft to the rear
differential .
Now this is another complex piece of engineering. The name itself is an indicator of its role. A differential plays two roles. First it must change the direction of the rotational motion by 90 degrees. Secondly, it has an arrangement inside that lets the two rear-wheels to rotate at different speeds. This is required as the wheels describe different curves when the vehicle is turning. The outer wheel describes a larger circle and the inner wheel a smaller circle in the same time. The differential is designed to accommodate this difference in speeds. The same rotation of the propeller shaft helps in turning the two wheels at different rotational speeds as and when required. Because of this necessary capability, a drawback is introduced. When one wheel loses traction, as on a slippery surface, it simply starts spinning at a faster speed and the wheel which has got traction is not able to move the vehicle as all the energy has been acquired by the spinning wheel. Now, this can be overcome if we have a system of locking the differential i.e. we ensure that the left and right wheels operate at the same rotational speed as if they are attached directly to each other on a single shaft.
Locking differentials allow this to happen. But the downside is that while turning, especially tight turns, the wheels are not allowed to turn at different speeds and so get dragged, resulting in wear.
There are many versions of differentials which are there. It makes for highly technical but engaging reading!
This is just the complexity that is there on a rear-wheel-drive vehicle. Imagine the complexity involved if you have to drive all four wheels together. In RWD vehicles, the front wheels are being rotated due to the friction of the ground. The rear wheels push the vehicle and the front wheels have no choice but to start rotating. When we steer a vehicle, the
steering is also a gear and not just a rod that connects the steering wheel to the front wheels’ axle! You must have noticed that when you turn the steering wheel, the front wheels do not turn parallel to each other. The steering gear is designed in such a way that it turns the left and right wheels through different angles depending on the direction in which you are turning. This is again because when you are turning, the outer and inner wheels describe different circles. It is even more pronounced at the front as these wheels provide direction to the vehicle. The outer wheel will be less ‘turned’ towards the direction of the turn compared to the inner wheel.
When the wheels are out of
alignment the angles that the wheels turn changes from what is specified and so the wheels get dragged, resulting in uneven wear.
Now if we want to drive all four wheels together, as in four-wheel drive, we have to ensure that all four wheels rotate together. They are joined to the same crankshaft-flywheel-gearbox. So they are rotating due to input from the gearbox and they are also experiencing the force of the other wheels which are also pushing the vehicle. If you are going in a straight line, there is not much of a problem. But roads are not straight; they are not even 2-dimensional! Left, right, up, down; the wheels travel where you take them. Imagine participating in a 3-legged race. Your legs are trying to stride at your own pace but the leg that is tied to the other person is experiencing 2 forces; it is experiencing your applied force and the applied force of your partner.
In
4WD vehicles like Gypsy and Vcross, we must be very careful where we engage 4H or 4L. We have to apply it only on low friction surfaces like sand, mud, slush, etc. This is because they do not have any mechanical device that helps in dividing the differential in rotational requirements of the four wheels. The front and rear propeller shafts are rigidly coupled. Based on conditions of turn, etc. what ends up happening is that the actual translation of the force onto the ground, of the propeller shafts of front and rear, results in the front and rear wheels trying to drive the vehicle at different speeds! If we are on a low friction surface, the wheels slip so that an average of the two speeds is obtained. If we are on a high friction surface, like a normal tarred or concrete road, the wheels cannot spin.
So driveline windup happens. This can massively damage the whole drivetrain right up-to the gearbox. Everything is connected to each other and trying to turn at different speeds and also forcing each other to do the same!
For the same reason, it is not a good idea to put tyres of varying diameters in the same vehicle. Do not go just by the rim diameter but also factor in the sidewall height to get the true picture.
Now add all the other complex bits and the electronics and you realize that the modern automobile is an engineering marvel and, thus, justifying its cost!