Though American drivers will not lightly abandon their beloved large cars, they face three crucial pressures to shift their ground, writes Anthony Howard.

The fallout from disarray in global financial markets has created a climate of uncertainty about employment and other prospects. Rapid rises in energy have affected not only vehicle running costs but also the cost of living in general. And there is always the concern that the price of crude will begin another climb.

Then are the implications of the Energy Bill, passed by Congress in December 2007. This mandates a 40% improvement in car and truck fuel economy to an average of 35mpg by 2020.

The process gets off to a brisk start, demanding a 25% advance through model years 2011 to 2015. In other words: tomorrow. The 2015 targets are expected to be averages of 37.5mpg for passenger cars and 28.6mpg for light trucks. This will take fleet averages from 26.7mpg in 2007 to 31.8mpg in 2015, representing a 4.6% annual change in place of the expected 4%.

Large SUVs in the 8,500-10,000lb range, once immune to CAFÉ rules, are now also caught in the net.

This all adds up to a tall order, even in less elaborate US-style vehicles. It appears even more challenging when you add on the complexities, weight and friction losses inherent in traditional 4WD systems that appeal so strongly to Americans.

This is likely to accelerate development and introduction of ‘active’ or ‘on demand’ 4WD systems, making use of increasingly clever electronic controls.

The benefits sought, of course, include reduced weight and friction losses, contributing to economy improvements.

The picture is much the same in Europe and elsewhere, with ups and downs in prices at the pump and constant regulatory pressure for reductions in fuel consumption and emissions.

Though subject to similar considerations, 4WD is much more of a niche attribute in Europe than in the US. Large 4×4 vehicles such as those produced by Land Rover have found success as luxury cars, a far cry from the vision of the company’s founding fathers.

And, in its quest to dance on the top of the pinhead along with BMW and Mercedes-Benz, Audi pioneered its quattro 4WD system for road cars, appealing to drivers seeking a distinct performance advantage and/or the ability to move around the Alps unhindered during winter.

BMW and Mercedes have since responded to the challenge, seeing commercial advantage in the added value that 4WD implies, and marketing it not only for SUVs but also as a handling and safety enhancement for conventional passenger cars.

Back at VW Group, 4WD may be used increasingly as part of deliberate moves to shift the Volkswagen brand upmarket, above its SEAT and Skoda stablemates.

Also running constantly in the background to the 4WD scenario, is the increasing adoption of hybrid vehicles. Already, part of the hybrid plot is the use of conventional mechanical/internal combustion drive of one axle, supplemented by electrical drive at the other. If taking this logic forward to embrace driving each wheel with its own electric motor were to prove feasible, then this might be the beginning of the end for 4WD as we know it.

Hybrids winning friends
Eco-friendly hybrids are on the march, winning friends with their combination of both internal combustion and electric propulsion, operating independently or together depending on circumstances. For example, the AWD Lexus RX400h SUV has a 208bhp 3.3-litre V6 petrol engine and two electric motors: 123kW at the front and 50kW at the rear, delivering a combined maximum output of 269bhp.

The high torque of the electric motors helps deliver 0-100km/h (0-62mph) acceleration in 7.6 seconds, while top speed is 200km/h (124mph). The electric motors take care of stop-start progress up to 50km/h, when the V6 comes into its own, charging the battery while cruising. Regenerative braking is also used.

Electric Motor Assisted 4WD
A GKN Driveline variation on this theme, for example, provides on-demand rear drive by electric motor. This simplifies adaptation of FWD vehicles to on-demand
4WD because it is compact and eliminates the need for a transfer case, PTU or prop-shaft. A light modular unit with two-stage reduction gears, it offers a wide range of ratios and output torque capacity up to 1,200Nm.

Adaptable to a variety of E-motor technologies, it will have a differential to distribute torque evenly between the wheels, while its active controlled connect/disconnect clutch allows seamless switching between 2WD and 4WD modes. This can be in automatic and/or driver selectable. When the electric motor reaches its speed limit – or the driver selects 2WD – the clutch disengages the motor from the differential, and the resultant low drag torque helps improve fuel consumption.

The Detroit Three
Chrysler promises a wide range of hybrid electric-drive vehicles…more

See also: Global market review of automotive 4WD transmissions – forecasts to 2014