If two driven wheels are good, four must be better, right? Well, not always. Most regular sedans and hatchbacks have, for the last 3-4 decades, been front-wheel-drive only and for good reason. On uniformly smooth, dry tarmac, four-wheel-drive (4WD) or all-wheel-drive (AWD) has little or no benefits over front-wheel-drive and no advantages in terms of added traction or improved handling. It’s only when the going gets really rough and the vehicle in question has to perform over broken surfaces that offer poor grip and traction that 4WD really comes into its own. Well, either that, or in the case of very high performance vehicles, with very high power outputs – 4WD/AWD can also help with such ‘supercars’ by maximising traction, which ultimately improves driving dynamics. Here, we take an in-depth look at 4WD/AWD systems and how the latest developments in electronics are helping these systems evolve further.
BASIC CHARACTERISTICS OF RWD, FWD
Up until the 1970s-80s, rear-wheel-drive (RWD) was quite common and popular with car manufacturers. With RWD, the engine’s power is sent to the rear wheels, which provide propulsion, leaving the front wheels to handle the steering function alone. Purists prefer RWD, especially in the context of high-performance cars, because of the way it allows the vehicle to be driven at the limit. With RWD, the throttle pedal can be used to adjust the car’s stance and since the front wheels are only handling the steering part (without being burdened with the responsibility of also transferring the engine’s power to the tarmac), the overall driving experience can often be more ‘pure.’
However, especially in the pre-electronics era, powerful RWD cars tended to ‘oversteer,’ which could be potentially dangerous on the road. With oversteer, steering input is amplified and because of power going to the rear wheels, the back end of the vehicle can slide out (with the rear wheels spinning faster than the front wheels, either due to loss of grip or due to excessive power transfer), which can be difficult to control, especially for inexperienced drivers. With the advent of modern electronics like traction control systems, the tendency to oversteer can be tightly controlled or even altogether eliminated, though this can dilute the driving experience by taking control away from the driver.
On the other end of the spectrum are front-wheel-drive (FWD) cars, which usually have transversely mounted engines (as opposed to longitudinally mounted engines on most RWD cars), with the power going to the front wheels only. In this case, the front wheels are responsible for propulsion as well as steering, which can sometimes lead to ‘torque steer’ (where, due to the use of unequal length driveshafts, the vehicle pulls to one side during hard acceleration, along with excessive front wheel hop in some cases), which drivers can find disconcerting. On the other hand, FWD vehicles are much more likely to display ‘understeer’ characteristics during hard cornering, which is in most cases safer than a RWD vehicle’s oversteer. Also, a FWD layout can free up more cabin space, since engines mounted transversely take up less space than longitudinally mounted engines. This in turn allow engineers to provide more space inside the cabin, which is especially important with compact and medium-sized cars.
4WD vs. AWD
With both 4WD and AWD, the engine drives all four instead of just the front or the rear wheels. And yet, the two are different. With AWD, all four wheels are driven all of the time, regardless of the grip/traction situation at each individual wheel. With 4WD, depending on the type of system employed, the driver might either be able to manually select 4WD mode for improved traction (with engine power going to either only the front or only the rear wheels during normal operation), or, in some more modern, electronically controlled systems, the ECU might automatically engage 4WD when it detects loss of grip at the front or rear.
We’ll note here that older, simpler part-time 4WD systems, which employ a direct mechanical link between front and rear axles (that does not allow the front and rear axles to rotate at different speeds), can be operated in 4x4 mode only in off-road conditions. This is because while turning, the radius of turn is different for front and rear axles, and hence the wheels on the axle with the smaller radius of turn must be able to ‘slip’ in order to complete the turn. This kind of slip is possible only on loose/dirt surfaces, and in the absence of this slip, the entire driveline, including the axles and the propeller shaft, can come under heavy stress. Hence, with some older 4WD vehicles, 4x4 must be deselected when the vehicle is being driven on smooth tarmac.
With most of the older part-time 4WD systems, as mentioned above, the driver could engage 4WD manually, by engaging a 4x4 transfer case that would transform a RWD vehicle into a 4WD. In many cases, this setup included a lower gear ratio as well, providing three different settings – 2WD High, 4WD High and 4WD Low, the last being for severely challenging off-road terrain only.
More modern systems, which allow pushbutton selection of various 4WD modes and can be used on any kind of terrain, operate via multiple differentials. There’s a centre differential that allows the front and rear axles to operate at different rotational speeds, while an additional set of front and rear differentials allow each wheel to rotate at different speeds if required – this is useful when the vehicle is being driven on slippery, muddy, badly broken terrain, where the grip level for each wheel might be different. Unlike AWDs, some 4WD systems that are fully optimised for hard-core off-road use allow the centre differential to be ‘locked’ when required, which ensures equal apportioning of torque between the front and rear wheels, allowing the vehicle to be driven over extremely challenging terrain. On the other hand, AWD systems (as used on luxury cars like the Audi A8, or supercars like the Nissan GT-R and Lamborghini Huracan) are optimised for maximising grip on regular tarmac, thereby boosting traction and handling prowess at high speeds and improving safety.
ELECTRONICALLY CONTROLLED 4WD, AWD
Electronics are an important component of modern 4WD and AWD systems, which need minimal human intervention and yet deliver best possible performance across all kinds of terrain and under extremely demanding circumstances. Such systems are able to detect loss of grip at any wheel and automatically juggle the correct apportioning of torque to the most appropriate wheel(s), for maximising traction. Electronically controlled 4WD/AWD systems constantly monitor all four wheels for any undue slippage – situations where any of the four wheels begins to spin at a different rate as compared to other wheels, implying possible loss of traction – and automatically take corrective action.
Under ‘normal’ driving conditions, vehicles equipped with such smart 4WD systems can operate in 2WD mode (which reduces power losses in the drivetrain and improves fuel economy) most of the time, and yet go to 4WD mode in the blink of an eye, as soon as on-board computer senses loss of grip in any of the driven wheels. Electronic 4WD systems also offer various operating modes, which optimise power delivery and traction as per driving conditions. There could be, for example, driving modes for normal driving, and driving on snow, gravel and mud/slush etc. Such systems also have settings for hill ascent and descent, whereby the vehicle is able to traverse extreme angles with minimal driver input, at a steady pace, without loss of control. Depending on the setting chosen, the onboard computers automatically send torque to the wheels in a manner that maximises traction, and even brake each individual wheel as required, with little or no input from the driver.
While various OEMs have developed their own versions of electronically controlled 4WD/AWD systems, one of the oldest and best performing AWD systems remains Audi’s ‘quattro’ AWD, the latest iteration of which is ‘quattro with ultra technology.’ With the latest in predictive technology, quattro ultra can keep a vehicle operating in FWD mode most of time, switching to AWD in milliseconds as soon as the system detects a requirement for AWD. Its torque distribution system directs power to the front/rear wheels as required and even brakes inner or outer wheels independently, in order to maximise traction and high-speed cornering performance. Quattro ultra achieves its highs levels of dynamic performance via the use of two clutches, arranged in a way that one of the clutches can completely decouple the propeller shaft from the rear wheels, allowing the vehicle to operate in FWD mode (when AWD is not required), for maximum efficiency.
While other on-demand 4WD systems also disengage at the centre differential, they leave the rear differential and propeller shaft spinning with the rear wheels even in FWD mode. The Audi system, by decoupling the rear part completely, gains added efficiency and fuel economy. And yet, when AWD needs to cut in, that propeller shaft can be accelerated to the required speed within fractions of a second, engaging AWD in an instant. The shift from FWD to AWD and back is seamless, and the driver cannot feel the shift happening.
The linking up and decoupling, as required, happens on the basis of data that the onboard computers gather via many sensors – these sensors ‘read’ as many as 100 parameters, including driver behaviour (how aggressive he is with throttle and brake controls), road surfaces and weather conditions, gear position, throttle position, braking and steering effort, and many others. Two vehicles already equipped with quattro ultra technology are the current-model Audi A4 Allroad and Audi Q5, and these are expected to have best-in-class driving dynamics.
4WD/AWD ON HYBRIDS, EVs
Plug-in hybrid electric vehicles and pure EVs can be considerably heavier (due to the added weight of batteries and electric motors) than regular IC-engined vehicles and AWD torque vectoring systems can sometimes help with improving driving dynamics despite the increased weight. At lower speeds, electric motors transmit much higher levels of torque as compared to IC engines, and torque vectoring can help manage that factor more efficiently. One example in this case would be GKN’s ‘eTwinster’ prototype, which integrates torque vectoring AWD into an eAxle module, which may be used in the near future on production-spec high-performance PHEVs and pure EVs.
Another noteworthy example is Mitsubishi’s twin-motor 4WD system for PHEVs, which uses separately mounted electric motors at both the front and rear axles to drive all four wheels. As used on the Mitsubishi Outlander PHEV, the twin-motor 4WD system can be used in either hybrid or full EV mode, with claimed benefits including optimal torque delivery to each wheel, maximised traction, improved acceleration and better overall handling. Integrated with the company’s S-AWC (super all wheel control) system, the twin-motor AWD is fully integrated with the vehicle’s active yaw control, ABS, stability control and traction control systems, and even features a 4WD Lock button to simulate the locking of the centre differential on a more conventional 4WD vehicle.
Going forward, active, electronically controlled 4WD systems as well as electric motor-operated AWD systems for PHEVs and EVs are likely to gain further traction, with industry projections pegging China as one of the biggest potential markets for such systems. Even as 4WD and AWD systems continue to contribute in a big way towards boosting the dynamic performance of conventional IC-engined vehicles, with ongoing development and innovation in this space, they will also be similarly useful on high-performance electric vehicles in the near future.
TEXT: Sameer Kumar