BMW took the automotive industry by surprise this week by using the 2001 North American International Auto Show in Detroit to unveil an exceptionally well-kept secret: a front-engined X4 coupe concept car, built on the chassis of the U.S.-built X5 SUV.

BMW reckons its impressive new vehicle with aluminum skin, displayed in Highland Silver, shows that it is opening a new chapter in vehicle design.

Certainly it is dramatically different from anything the Germans have ever created and introduces a new phrase to car styling terminology: 'Flame Surfacing'.

Surfaces developed by this technique are described by BMW as "reminiscent of the forms of energetic flames, such as those from the burning of gas under pressure".

BMW says that Flame Surfacing "gives body surfaces the freedom to turn in on themselves, lending a high degree of visual tension. At the same time, this is a controlled tension whose energy pervades the entire visual image without in any way diminishing the vehicle's overall harmony."

Translated, this apparently means that, on the X4, a horizontal surface atop each front fender begins concave upward. Continuing on into the door, it remains concave upward, gently falling along the way. Then, in the transition to the rear flank, it begins to climb, switching its orientation to convex outward; and continues to the rounded edges of the tall tail.

If it goes into production, the X4's unique selling point would be a combination of the best features (especially styling) of a sporty coupe with the four-wheel drive platform and off-road abilities of the X5 SUV.

Clearly, this mix of purpose is going to feature on a near-future generation of vehicles aimed mostly at affluent but younger buyers. Also at Detroit, General Motors has this week proudly wheeled out such hybrids as the Buick Bengal, which joins elements of a station wagon and two-seat sports coupe together, and the Cadillac Vizon, which blends SUV and station wagon with Cadillac house-style luxury.

Perforated aluminum gives the X4's traditional BMW "two-kidneys" grille a sporty look and the large quad headlights impart a traditional BMW "face", which the PR blurb describes as "full of attention and readiness to spring into action".

In an unusual touch, the stylists have introduced asymmetry into the usually highly symmetrical world of motor vehicles.

On the X4, for instance, the two taillight assemblies are not symmetrical. Their inboard edges are parallel, both pointing downward to the right. That of the left lighting cluster continues downward through the rear vehicle surface.

Also, on the right side, there is no C-pillar: the right-side door window continues smoothly into the rear window. Opening the rear hatch reveals more: From that extended taillight line rightward - this turns out to be the lid's left edge - almost the entire rear section opens and tilts rearward, revealing the cargo space and rear seats. The hatch includes the entire rear window, creating an immense opening that makes loading cargo and entering/exiting the rear passenger space, from the curb side, very convenient.

Inside, the X4 coupé is just as innovative. Now becoming common in the U.S. market, adjustable pedals are standard as are 'sports' front seats. Because of the high ground clearance, the seats are relatively high to ease entry and exit, and their prominent side bolsters are cut out deeply in the pelvis area.

This is more a '1+3'-seater than a traditional '2+2'. Everything near the driver is oriented to him or her, and the "working space" is clearly distinguished from the front passenger's space in traditional coupe fashion. A gap has been left between the dash and engine compartment, letting natural light into the front footwells.

The show X4 coupé is is driveable, and based on the chassis of BMW's X5. Unusually, it is powered by BMW's latest three-litre, six-cylinder engine with common-rail direct fuel injection, developing 184 bhp but modified for extra torque.

A five-speed Steptronic automatic transmission, with manual-shift paddles on the steering wheel, takes the drive to all four wheels.

Traction and stability control are both standard, along with Hill Descent Control (HDC) (either borrowed or adapted from Land Rover which BMW recently sold to Ford). Like the larger X5, the X4 can be taken off-road, subject to certain limits.

Another innovation, likely to be the only view most road users get of this car if it's built, is the brake lights, carried in the dual rear lighting clusters under glass covers.

They indicate to following drivers how hard the X4 coupé's driver is braking: On the basis of an electronic signal of deceleration, under light braking only the lights' outer rings illuminate; as deceleration increases, the illumination spreads inward until, under full braking, their entire area is illuminated.