2 days ago
Torque Shop: The difference between four-wheel and all-wheel drives
The Denza Z9 GT has three electric motors to power all four wheels. PHOTO: BYD
Do the terms 'four-wheel drive' and 'all-wheel drive' have the same meaning? And what is front-wheel drive and rear-wheel drive?
Logically, four-wheel drive (4WD) describes any four-wheeled vehicle with a drivetrain that sends torque and power to all four wheels. Technically, so does the term all-wheel drive (AWD).
The main reason for conventional cars to have such a system is to get better traction.
However, there are some who argue that AWD is reserved for describing drivetrains that permanently power all four wheels, and, for this reason, differs from 4WD, which could be primarily two-wheel drive (2WD) with selectable or automatic 4WD.
Before the confusion deepens, here is what the different vehicle drivetrains are all about.
A 2WD car has only two of its wheels powered by the engine. It could be the front or rear wheels. In either configuration, there is no possibility of the other two wheels being driven, as there is no power transmission component to facilitate it.
There are two categories of 4WD systems. The more common type is found mostly in passenger cars and often referred to as 'part-time 4WD', because the drivetrain is designed to transmit torque to only one axle – the front or the rear.
B ut it is still able to divert torque to the otherwise non-driven pair. This occurs mostly on slippery roads, when the driven pair begins to lose traction and spin. The system then switches from 2WD t o 4WD.
In most such drivetrains, the torque transfer to the second pair of wheels occurs via some form of electronically controlled or mechanical slip-induced multi-plate clutch arrangement. Torque transfer takes place automatically when skidding or wheel spin is detected.
In the other version of 4WD, torque is constantly transmitted to all four wheels. Under normal driving conditions, torque split between the front and rear axles is usually 50:50 .
A centre differential, a device that could be mechanical or electronic, c an effectively vary torque split continuously to optimise traction.
Some 4WD passenger cars are designed to be rear-biased, but are still able to vary the split to achieve front bias and get as much as 70 per cent of the drive, for example.
Hybridisation in vehicles, where an electric motor is combined with an internal combustion engine, has introduced yet another variation of the 4WD.
On the Lexus RX350h, for example, the petrol engine, aided by an electric motor, drives the front wheels, while a second electric motor mounted at the rear provides power to the back wheels only.
There are also mid-engine cars like the BMW i8, where the rear wheels are powered by the internal combustion engine and the front wheels by an electric motor.
Such arrangements do not require mechanical power transmission assemblies to transmit torque from the engine to the second pair of wheels, because the battery-powered electric motors function independently. They are controlled by a sophisticated high-voltage power-control module.
These cars are 2WD under most road conditions, while the motor-driven wheels assist in situations where more grip and traction are necessary, as well as to boost total power when quicker acceleration is demanded.
For a fully electric vehicle (EV), having a motor to power each of the two axles enables it to be 4WD.
Some EVs, such as the Denza Z9 GT, even have three motors – usually one at the front and two at the back – to achieve 4WD.
While there is no mechanical connection between the front and rear axles of EVs, the central power control module apportions torque more precisely than on 4WDs in conventional engine-powered cars and petrol-hybrid cars.
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